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Magnetic models on various topologies: A brief review is given on the study of the thermodynamic properties of spin
models defined on different topologies like small-world, scale-free networks,
random graphs and regular and random lattices. Ising, Potts and Blume-Capel
models are considered. They are defined on complex lattices comprising
Appolonian, Barab\'asi-Albert, Voronoi-Delauny and small-world networks. The
main emphasis is given on the corresponding phase transitions, transition
temperatures, critical exponents and universality, compared to those obtained
by the same models on regular Bravais lattices. | cond-mat |
Quantum Monte Carlo study of ultracold gases (PhD thesis): This Dissertation presents results of a thorough study of ultracold bosonic
and fermionic gases in three-dimensional and quasi-one-dimensional systems.
Although the analyses are carried out within various theoretical frameworks
(Gross-Pitaevskii, Bethe ansatz, local density approximation, etc.) the main
tool of the study is the Quantum Monte Carlo method in different modifications
(variational Monte Carlo, diffusion Monte Carlo, fixed-node Monte Carlo
methods). We benchmark our Monte Carlo calculations by recovering known
analytical results (perturbative theories in dilute limits, exactly solvable
models, etc.) and extend calculations to regimes, where the results are so far
unknown. In particular we calculate the equation of state and correlation
functions for gases in various geometries and with various interatomic
interactions. | cond-mat |
Anomalous Hall effect in van der Waals bonded ferromagnet
Fe$_{3-x}$GeTe$_2$: We report anomalous Hall effect (AHE) in single crystals of
quasi-two-dimensional Fe$_{3-x}$GeTe$_2$ ($x \approx 0.36$) ferromagnet grown
by the flux method which induces defects on Fe site and bad metallic
resistivity. Fe K-edge x-ray absorption spectroscopy was measured to provide
information on local atomic environment in such crystals. The dc and ac
magnetic susceptibility measurements indicate a second-stage transition below
119 K in addition to the paramagnetic to ferromagnetic transition at 153 K. A
linear scaling behavior between the modified anomalous Hall resistivity
$\rho_{xy}/\mu_0H_{eff}$ and longitudinal resistivity
$\rho_{xx}^2M/\mu_0H_{eff}$ implies that the AHE in Fe$_{3-x}$GeTe$_2$ should
be dominated by the intrinsic Karplus-Luttinger mechanism rather than the
extrinsic skew-scattering and side-jump mechanisms. The observed deviation in
the linear-M Hall conductivity $\sigma_{xy}^A$ below 30 K is in line with its
transport characteristic at low temperatures, implying the scattering of
conduction electrons due to magnetic disorder and the evolution of the Fermi
surface induced by possible spin-reorientation transition. | cond-mat |
Measuring Dirac Cones in a Sub-Wavelength Metamaterial: The exciting discovery of bi-dimensional systems in condensed matter physics
has triggered the search of their photonic analogues. In this letter, we
describe a general scheme to reproduce some of the systems ruled by a
tight-binding Hamiltonian in a locally resonant metamaterial: by specifically
controlling the structure and the composition it is possible to engineer the
band structure at will. We numerically and experimentally demonstrate this
assertion in the microwave domain by reproducing the band structure of
graphene, the most famous example of those 2D-systems, and by accurately
extracting the Dirac cones. This is a direct evidence that opting for a
crystalline description of those sub-wavelength scaled systems, as opposed to
the usual description in terms of effective parameters, makes them a really
convenient tabletop platform to investigate the tantalizing challenges that
solid-state physics offer. | cond-mat |
On form-factor expansions for the XXZ chain in the massive regime: We study the large-volume-$L$ limit of form factors of the longitudinal spin
operators for the XXZ spin-$1/2$ chain in the massive regime. We find that the
individual form factors decay as $L^{-n}$, $n$ being an even integer counting
the number of physical excitations -- the holes -- that constitute the excited
state. Our expression allows us to derive the form-factor expansion of
two-point spin-spin correlation functions in the thermodynamic limit
$L\rightarrow +\infty$. The staggered magnetisation appears naturally as the
first term in this expansion. We show that all other contributions to the
two-point correlation function are exponentially small in the large-distance
regime. | cond-mat |
Memory effects in nonlinear transport: kinetic equations and ratchet
devices: We present a new method to derive kinetic equations for systems undergoing
non-linear transport in the presence of memory effects. In the framework of
mesoscopic nonequilibrium thermodynamics, we derive a generalized Fokker-Planck
equation incorporating memory effects through time-dependent coefficients. As
applications, we first discuss the non-Markovian dynamics of anomalous
diffusion in a potential, analyzing the validity of the fluctuation-dissipation
theorem. In a second application, we propose a new ratchet mechanism in which
the periodic driving acting on the particle is induced by the Onsager coupling
of the diffusion current with an oscillating thermodynamic force. | cond-mat |
Magnetic phase diagram and transport properties of FeGe_2: We have used resistivity measurements to study the magnetic phase diagram of
the itinerant antiferromagnet FeGe_2 in the temperature range from 0.3->300 K
in magnetic fields up to 16 T. In contrast to theoretical predictions, the
incommensurate spin density wave phase is found to be stable at least up to 16
T, with an estimated critical field \mu _0H_c of ~ 30 T. We have also studied
the low temperature magnetoresistance in the [100], [110], and [001]
directions. The transverse magnetoresistance is well described by a power law
for magnetic fields above 1 T with no saturation observed at high fields. We
discuss our results in terms of the magnetic structure and the calculated
electronic bandstructure of FeGe_2. We have also observed, for the first time
in this compound, Shubnikov-de Haas oscillations in the transverse
magnetoresistance with a frequency of 190 +- 10 T for a magnetic field along
[001]. | cond-mat |
Topological Valley Currents in Bilayer Graphene/Hexagonal Boron Nitride
Superlattices: Graphene superlattices have recently been attracting growing interest as an
emergent class of quantum metamaterials. In this paper, we report the
observation of nonlocal transport in bilayer graphene (BLG) superlattices
encapsulated between two hexagonal boron nitride (hBN) layers, which formed
hBN/BLG/hBN moir\'e superlattices. We then employed these superlattices to
detect a long-range charge-neutral valley current using an all-electrical
method. The moir\'e superlattice with broken inversion symmetry leads to a hot
spot with Berry curvature accumulating at the charge neutral point (CNP), and
it harbors satellites of the CNP. We observed nonlocal resistance on the order
of 1 $\text{k}\Omega$, which obeys a scaling relation. This nonlocal resistance
evolves from the quantum Hall effect but without magnetic field/time-reversal
symmetry breaking, which is associated with a hot-spot-induced topological
valley current. This study should pave the way to developing a
Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with
inversion-symmetry breaking. | cond-mat |
Continuum versus discrete flux behaviour in large mesoscopic
Bi(2)Sr(2)CaCu(2)O(8+delta) disks: Scanning Hall probe and local Hall magnetometry measurements have been used
to investigate flux distributions in large mesoscopic superconducting disks
with sizes that lie near the crossover between the bulk and mesoscopic vortex
regimes. Results obtained by directly mapping the magnetic induction profiles
of the disks at different applied fields can be quite successfully fitted to
analytic models which assume a continuous distribution of flux in the sample.
At low fields, however, we do observe clear signatures of the underlying
discrete vortex structure and can resolve the characteristic mesoscopic
compression of vortex clusters in increasing magnetic fields. Even at higher
fields, where single vortex resolution is lost, we are still able to track
configurational changes in the vortex patterns, since competing vortex orders
impose unmistakable signatures on "local" magnetisation curves as a function of
the applied field. Our observations are in excellent agreement with molecular
dynamics numerical simulations which lead us to a natural definition of the
lengthscale for the crossover between discrete and continuum behaviours in our
system. | cond-mat |
Hydrogenated Amorphous Silicon Carbide: A Low-loss Deposited Dielectric
for Microwave to Submillimeter Wave Superconducting Circuits: Low-loss deposited dielectrics will benefit superconducting devices such as
integrated superconducting spectrometers, superconducting qubits and kinetic
inductance parametric amplifiers. Compared with planar structures, multi-layer
structures such as microstrips are more compact and eliminate radiation loss at
high frequencies. Multi-layer structures are most easily fabricated with
deposited dielectrics, which typically exhibit higher dielectric loss than
crystalline dielectrics. We measured the sub-kelvin and low-power microwave and
mm-submm wave dielectric loss of hydrogenated amorphous silicon carbide
(a-SiC:H), using a superconducting chip with NbTiN/a-SiC:H/NbTiN microstrip
resonators. We deposited the a-SiC:H by plasma-enhanced chemical vapor
deposition at a substrate temperature of 400{\deg}C. The a-SiC:H has a mm-submm
loss tangent ranging from $0.80 \pm 0.01 \times 10^{-4}$ to $1.43 \pm 0.04
\times 10^{-4}$ in the range of 270 to 385 GHz. The microwave loss tangent is
$3.2 \pm 0.2 \times 10^{-5}$. These are the lowest low-power sub-kelvin loss
tangents that have been reported for microstrip resonators at mm-submm and
microwave frequencies. We observe that the loss tangent increases with
frequency. The a-SiC:H films are free of blisters and have low stress: $-$20
MPa compressive at 200 nm thickness to 60 MPa tensile at 1000 nm thickness. | cond-mat |
Characterization of Electron Pair Velocity in
YBa$_{2}$Cu$_{3}$O$_{7-\textit{$δ$}}$ Thin Films: The superconducting phase transition in
YBa$_{2}$Cu$_{3}$O$_{7-\textit{$\delta $}}$(YBCO) thin film samples doped with
non-superconducting nanodot impurities of CeO$_{2}$ are the focus of recent
high-temperature superconductor studies. Non-superconducting holes of the
superconducting lattice induce a bound-state of circulating paired electrons.
This creates a magnetic flux vortex state. Examining the flow of free-electrons
shows that these quantized magnetic flux vortices arrange themselves in a
self-assembled lattice. The nanodots serve to present structural properties to
constrict the "creep" of these flux vorticies under a field response in the
form of a pinning-force enhancing the critical current density after phase
transition. In this work, a model for characterizing the superconducting phase
by the work done on electron pairs and chemical potential, following the
well-known theories of Superconductivity (Bardeen-Cooper-Scheifer \&
Ginzburg-Landau), is formulated and tested.A solution to the expression for the
magnetic flux, zero net force and pair velocity will generate a setting for the
optimal deposition parameters of number density, growth geometry and mass
density of these nanodot structures. | cond-mat |
Effects of liquid fraction and contact angle on structure and coarsening
in two-dimensional foams: Aqueous foams coarsen with time due to gas diffusion through the liquid. The
mean bubble size grows, and small bubbles vanish. However, coarsening is little
understood for foams with an intermediate liquid content, particularly in the
presence of surfactant-induced attractive forces between the bubbles, measured
by the contact angle. Rigorous bubble growth laws have yet to be developed, and
the evolution of bulk foam properties is unclear. We present a quasi-static
numerical model for coarsening in two-dimensional wet foams, focusing on growth
laws and related bubble properties. The deformation of bubbles is modelled
using a finite-element approach, and the gas flow through both films and
Plateau borders is approximated. We give results for disordered two-dimensional
wet foams with 256 to 1024 bubbles, at liquid fractions from $2\%$ to beyond
the zero-contact-angle jamming transition, and with contact angles up to
$10^\circ$. Simple analytical models are developed to aid interpretation. We
find that nonzero contact angle causes a proxy of the initial coarsening rate
to plateau at large liquid fractions, and that the individual bubble growth
rates are closely related to their effective number of neighbours. | cond-mat |
Evolution of electronic structure of Ru-doped single-crystal iridiates,
Sr$_2$Ir$_{1-x}$Ru$_x$O$_4$: We investigated Ru-doped single-crystal 5$d$ iridiates,
Sr$_2$Ir$_{1-x}$Ru$_x$O$_{4}$, at three different doping concentrations ($x =$
0.01, 0.07 and 0.10) using optical spectroscopy. The undoped pristine compound
(Sr$_2$IrO$_{4}$) is known as a novel $J_{eff}$ = 1/2 Mott insulator.
Remarkably, the optical conductivity spectra of all three samples exhibited the
insulating behavior, although we observed weak Drude components in the optical
conductivity spectra down to the lowest temperature of 30 K. The charge-carrier
densities of the Ru-doped iridiates estimated from the Drude components are
significantly smaller than the expected values estimated from the nominal
Ru-doping concentrations. Herein, we provide temperature- and doping-dependent
electronic structure evolution of Ru-doped iridiates. We expect that our
results will be useful for understanding the intriguing Ru-doping-dependent
properties of 5$d$ iridiate Sr$_2$IrO$_{4}$. | cond-mat |
Room-Temperature Superconductivity in Boron-Nitrogen Doped Lanthanum
Superhydride: Recent theoretical and experimental studies of hydrogen-rich materials at
megabar pressures (i.e., >100 GPa) have led to the discovery of very
high-temperature superconductivity in these materials. Lanthanum superhydride
LaH$_{10}$ has been of particular focus as the first material to exhibit a
superconducting critical temperature (T$_c$) near room temperature. Experiments
indicate that the use of ammonia borane as the hydrogen source can increase the
conductivity onset temperatures of lanthanum superhydride to as high as 290 K.
Here we examine the doping effects of B and N atoms on the superconductivity of
LaH$_{10}$ in its fcc (Fm-3m) clathrate structure at megabar pressures. Doping
at H atomic positions strengthens the H$_{32}$ cages of the structure to give
higher phonon frequencies that enhance the Debye frequency and thus the
calculated T$_c$. The predicted T$_c$ can reach 288 K in
LaH$_{9.985}$N$_{0.015}$ within the average high-symmetry structure at 240 GPa. | cond-mat |
Segregated quantum phases of dipolar bosonic mixtures in two-dimensional
optical lattices: We identify the quantum phases in a binary mixture of dipolar bosons in
two-dimensional optical lattices. Our study is motivated by the recent
experimental realization of binary dipolar condensate mixtures of Er-Dy [Phys.
Rev. Lett. 121, 213601 (2018)]. We model the system by using the extended
two-species Bose-Hubbard model and calculate the ground-state phase diagrams by
using mean-field theory. For selected cases we also obtain analytical phase
boundaries by using the site-decoupled mean-field theory. For comparison we
also examine the phase diagram of two-species Bose-Hubbard model. Our results
show that the quantum phases with the long-range intraspecies interaction phase
separate with no phase ordering. The introduction of the long-range
interspecies interaction modifies the quantum phases of the system. It leads to
the emergence of phase-separated quantum phases with phase ordering. The
transition from the phase-separated quantum phases without phase ordering to
phase ordered ones breaks the inversion symmetry. | cond-mat |
Structure of inactive states of a binary Lennard-Jones mixture: We study the structure of inactive states in a prototypical model glass, the
Kob-Andersen binary Lennard-Jones mixture. These inactive states are obtained
by transition path sampling and are at dynamical phase coexistence with an
active equilibrium state. Configurations in the inactive states are kinetically
stable and are located in deeper basins of the energy landscape than their
active counterparts. By analyzing trajectory-to-trajectory fluctuations within
the inactive state, we assess correlations between kinetic stability, energy
and other structural properties. We show that measures of local order
associated to stable local packings and bond-orientational order are weakly
correlated with energy and kinetic stability. We discuss what kinds of
structural measurement might capture the relevant dynamical features of the
inactive state. | cond-mat |
A general moment NRIXS approach to the determination of equilibrium Fe
isotopic fractionation factors: application to goethite and jarosite: We measured the reduced partition function ratios for iron isotopes in
goethite FeO(OH), potassium-jarosite KFe3(SO4)2(OH)6, and hydronium-jarosite
(H3O)Fe3(SO4)2(OH)6, by Nuclear Resonant Inelastic X-Ray Scattering (NRIXS,
also known as Nuclear Resonance Vibrational Spectroscopy -NRVS- or Nuclear
Inelastic Scattering -NIS) at the Advanced Photon Source. These measurements
were made on synthetic minerals enriched in 57Fe. A new method (i.e., the
general moment approach) is presented to calculate {\beta}-factors from the
moments of the NRIXS spectrum S(E). The first term in the moment expansion
controls iron isotopic fractionation at high temperature and corresponds to the
mean force constant of the iron bonds, a quantity that is readily measured and
often reported in NRIXS studies. | cond-mat |
Femtosecond optical breakdown in silicon: We investigate photoinization, energy deposition, plasma formation and the
ultrafast optical breakdown in crystalline silicon irradiated by intense
near-infrared laser pulses with pulse duration $\tau \le $ 100 fs. The
occurrence of high-intensity breakdown was established by the sudden increase
of the absorbed laser energy inside the bulk, which corresponds to threshold
energy fluence $\Phi_{th} > $ 1 J/cm$^2$. The optical breakdown is accompanied
by severe spectral broadening of the transmitted pulse. For the studied
irradiation conditions, we find that the threshold fluence increases linearly
with the increase of the pulse duration, while the corresponding laser
intensity threshold decreases. The effect of the high plasma density on the
stability of diamond lattice is also examined. For near threshold fluences,
when about 5 \% of valence electrons are promoted into the conduction band, the
Si-Si bonds are softened and large Fermi degeneracy pressure arises (with
pressure up to 100 kbar). The mechanical instability of the diamond lattice
suggests that the large number of electron-hole pairs leads directly to
ultrafast melting of the crystal structure. | cond-mat |
Classifying transport behavior via current fluctuations in open quantum
systems: There are two standard ways of classifying transport behavior of systems. The
first is via time scaling of spread of correlations in the isolated system in
thermodynamic limit. The second is via system size scaling of conductance in
the steady state of the open system. We show here that these correspond to
taking the thermodynamic limit and the long time limit of the integrated
equilibrium current-current correlations of the open system in different order.
In general, the limits may not commute leading to a conflict between the two
standard ways of transport classification. Nevertheless, the full information
is contained in the equilibrium current-current correlations of the open
system. We show this analytically by rigorously deriving the open-system
current fluctuation dissipation relations (OCFDR) starting from an extremely
general open quantum set-up and then carefully taking the proper limits. We
test our theory numerically on the non-trivial example of the critical
Aubry-Andr\'e-Harper (AAH) model, where, it has been recently shown that, the
two standard classifications indeed give different results. We find that both
the total current autocorrelation and the long-range local current correlations
of the open system in equilibrium show signatures of diffusive transport up to
a time scale. This time scale grows as square of system size. Beyond this time
scale a steady state value is reached. The steady state value is conductance,
which shows sub-diffusive scaling with system size. | cond-mat |
Wood compression in four-dimensional in situ tomography: Wood deformation, in particular when subject to compression, exhibits
scale-free avalanche-like behavior as well as structure-dependent localization
of deformation. We have taken three-dimensional (3D) x-ray tomographs during
compression with constant stress rate loading. Using digital volume
correlation, we obtain the local total strain during the experiment and compare
it to the global strain and acoustic emission. The wood cells collapse layer by
layer throughout the sample starting from the softest parts, i.e., the spring
wood. As the damage progresses, more and more of the softwood layers throughout
the sample collapse, which indicates damage spreading instead of localization.
In 3D, one can see a fat-tailed local strain rate distribution, indicating that
inside the softwood layers, the damage occurs in localized spots. The observed
log-normal strain distribution is in agreement with this view of the
development of independent local collapses or irreversible deformation events.
A key feature in the mechanical behavior of wood is then in the complex
interaction of localized deformation between or among the annual rings. | cond-mat |
Tomonaga-Luttinger liquid parameters of magnetic waveguides in graphene: Electronic waveguides in graphene formed by counterpropagating snake states
in suitable inhomogeneous magnetic fields are shown to constitute a realization
of a Tomonaga-Luttinger liquid. Due to the spatial separation of the right- and
left-moving snake states, this non-Fermi liquid state induced by
electron-electron interactions is essentially unaffected by disorder. We
calculate the interaction parameters accounting for the absence of Galilei
invariance in this system, and thereby demonstrate that non-Fermi liquid
effects are significant and tunable in realistic geometries. | cond-mat |
Condensation of classical nonlinear waves: We study the formation of a large-scale coherent structure (a condensate) in
classical wave equations by considering the defocusing nonlinear Schr\"odinger
equation as a representative model. We formulate a thermodynamic description of
the condensation process by using a wave turbulence theory with ultraviolet
cut-off. In 3 dimensions the equilibrium state undergoes a phase transition for
sufficiently low energy density, while no transition occurs in 2 dimensions, in
analogy with standard Bose-Einstein condensation in quantum systems. Numerical
simulations show that the thermodynamic limit is reached for systems with
$16^3$ computational modes and greater. On the basis of a modified wave
turbulence theory, we show that the nonlinear interaction makes the transition
to condensation subcritical. The theory is in quantitative agreement with the
simulations. | cond-mat |
Aging-induced continuous phase transition: Aging is considered as the property of the elements of a system to be less
prone to change states as they get older. We incorporate aging into the noisy
voter model, a stochastic model in which the agents modify their binary state
by means of noise and pair-wise interactions. Interestingly, due to aging the
system passes from a finite-size discontinuous transition between ordered
(ferromagnetic) and disordered (paramagnetic) phases to a second order phase
transition, well defined in the thermodynamic limit, belonging to the Ising
universality class. We characterize it analytically by finding the stationary
solution of an infinite set of mean field equations. The theoretical
predictions are tested with extensive numerical simulations in low dimensional
lattices and complex networks. We finally employ the aging properties to
understand the symmetries broken in the phase transition. | cond-mat |
Fluctuations and scaling in creep deformation: The spatial fluctuations of deformation are studied in creep in the Andrade's
power-law and the logarithmic phases, using paper samples. Measurements by the
Digital Image Correlation technique show that the relative strength of the
strain rate fluctuations increases with time, in both creep regimes. In the
Andrade creep phase characterized by a power law decay of the strain rate
$\epsilon_t \sim t^{-\theta}$, with $\theta \approx 0.7$, the fluctuations obey
$\Delta \epsilon_t \sim t^{-\gamma}$, with $\gamma \approx 0.5$. The local
deformation follows a data collapse appropriate for an absorbing
state/depinning transition. Similar behavior is found in a crystal plasticity
model, with a jamming or yielding phase transition. | cond-mat |
Neural Network Analytic Continuation for Monte Carlo: Improvement by
Statistical Errors: This study explores the use of neural network-based analytic continuation to
extract spectra from Monte Carlo data. We apply this technique to both
synthetic and Monte Carlo-generated data. The training sets for neural networks
are carefully synthesized without ``data leakage". We found that the training
set should match the input correlation functions in terms of statistical error
properties, such as noise level, noise dependence on imaginary time, and
imaginary time-displaced correlations. We have developed a systematic method to
synthesize such training datasets. Our improved algorithm outperform the widely
used maximum entropy method in highly noisy situations. As an example, our
method successfully extracted the dynamic structure factor of the spin-1/2
Heisenberg chain from quantum Monte Carlo simulations. | cond-mat |
The height distribution of the KPZ equation with sharp wedge initial
condition: numerical evaluations: The time-dependent probability distribution function of the height for the
Kardar-Parisi-Zhang equation with sharp wedge initial conditions has been
obtained recently as a convolution between the Gumbel distribution and a
difference of two Fredholm determinants. We evaluate numerically this
distribution over the whole time span. The crossover from the short time
behavior, which is Gaussian, to the long time behavior, which is governed by
the GUE Tracy-Widom distribution, is clearly visible. | cond-mat |
Improving Electric Contacts to Two-Dimensional Semiconductors: Electrical contact resistance to two-dimensional (2D) semiconductors such as
monolayer MoS_{2} is a key bottleneck in scaling the 2D field effect
transistors (FETs). The 2D semiconductor in contact with three-dimensional
metal creates unique current crowding that leads to increased contact
resistance. We developed a model to separate the contribution of the current
crowding from the intrinsic contact resistivity. We show that current crowding
can be alleviated by doping and contact patterning. Using Landauer-B\"uttiker
formalism, we show that van der Waals (vdW) gap at the interface will
ultimately limit the electrical contact resistance. We compare our models with
experimental data for doped and undoped MoS_{2} FETs. Even with heavy
charge-transfer doping of > 2x10^{13} cm^{-2}, we show that the
state-of-the-art contact resistance is 100 times larger than the ballistic
limit. Our study highlights the need to develop efficient interface to achieve
contact resistance of < 10 {\Omega}.{\mu}m, which will be ideal for extremely
scaled devices. | cond-mat |
Monolayer MoS$_2$ Strained to 1.3\% with a Microelectromechanical System: We report on a modified transfer technique for atomically thin materials
integrated onto microelectromechanical systems (MEMS) for studying strain
physics and creating strain-based devices. Our method tolerates the non-planar
structures and fragility of MEMS, while still providing precise positioning and
crack free transfer of flakes. Further, our method used the transfer polymer to
anchor the 2D crystal to the MEMS, which reduces the fabrication time,
increases the yield, and allowed us to exploit the strong mechanical coupling
between 2D crystal and polymer to strain the atomically thin system. We
successfully strained single atomic layers of molybdenum disulfide (MoS$_2$)
with MEMS devices for the first time and achieved greater than 1.3\% strain,
marking a major milestone for incorporating 2D materials with MEMS We used the
established strain response of MoS$_2$ Raman and Photoluminescence spectra to
deduce the strain in our crystals and provide a consistency check. We found
good comparison between our experiment and literature. | cond-mat |
Emergent Spacetime in Quantum Lattice Models: Many quantum lattice models have an emergent relativistic description in
their continuum limit. The celebrated example is graphene, whose continuum
limit is described by the Dirac equation on a Minkowski spacetime. Not only
does the continuum limit provide us with a dictionary of geometric observables
to describe the models with, but it also allows one to solve models that were
otherwise analytically intractable. In this thesis, we investigate novel
features of this relativistic description for a range of quantum lattice
models. In particular, we demonstrate how to generate emergent curved
spacetimes and identify observables at the lattice level which reveal this
emergent behaviour, allowing one to simulate relativistic effects in the
laboratory. We first study carbon nanotubes, a system with an edge, which
allows us to test the interesting feature of the Dirac equation that it allows
for bulk states with support on the edges of the system. We then study Kitaev's
honeycomb model which has a continuum limit describing Majorana spinors on a
Minkowski spacetime. We show how to generate a non-trivial metric in the
continuum limit of this model and how to observe the effects of this metric and
its corresponding curvature in the lattice observables, such as Majorana
correlators, Majorana zero modes and the spin densities. We also discuss how
lattice defects and $\mathbb{Z}_2$ gauge fields at the lattice level can
generate chiral gauge fields in the continuum limit and we reveal their
adiabatic equivalence. Finally, we discuss a chiral modification of the 1D XY
model which makes the model interacting and introduces a non-trivial phase
diagram. We see that this generates a black hole metric in the continuum limit,
where the inside and outside of the black hole are in different phases. We then
demonstrate that by quenching this model we can simulate Hawking radiation. | cond-mat |
Conventional Superconductivity properties of the ternary boron-nitride
Nb2BN: Superconducting bulk properties of ternary Nb2 BN are confirmed and are
described by means of magnetization, electronic transport and specific-heat
measurements. BCS conventional super- conductivity is found with Tc = 4.4 K.
Critical fields Hc1 (0)= 93 Oe and Hc2 (0)= 2082 Oe are extrapolated by
magnetic and resistivity measurements. The specific heat data reveals {\gamma}
= 6.3 mJ/mol K2 and {\beta} = 0.293 mJ/mol K4 in good agreement with the BCS
Theory. | cond-mat |
Influence of electric fields on dielectric properties of GPI
ferroelectric: Using modified microscopic model of GPI by taking into account the
piezoelectric coupling with strains $\varepsilon_i$ in the frames of
two-particle cluster approximation, the components of polarization vector and
static dielectric permittivity tensor of the crystal at applying the external
transverse electric fields $E_1$ and $E_3$ are calculated. An analysis of the
influence of these fields on thermodynamic characteristics of GPI is carried
out. A satisfactory quantitative description of the available experimental data
for these characteristics has been obtained at a proper choice of the model
parameters. | cond-mat |
Current-induced birefringent absorption and non-reciprocal plasmons in
graphene: We present extensive calculations of the optical and plasmonic properties of
a graphene sheet carrying a dc current. By calculating analytically the
density-density response function of current-carrying states at finite
temperature, we demonstrate that an applied dc current modifies the Pauli
blocking mechanism and that absorption acquires a birefringent character with
respect to the angle between the in-plane light polarization and current flow.
Employing the random phase approximation at finite temperature, we show that
graphene plasmons display a degree of non-reciprocity and collimation that can
be tuned with the applied current. We discuss the possibility to measure these
effects. | cond-mat |
Contraction and expansion effects on the substitution-defect properties
of thirteen alloying elements in bcc Fe: Proposed as blanket structural materials for fusion power reactors, reduced
activation ferritic/martensitic (RAFM) steel undergoes volume expanding and
contracting in a cyclic mode under service environment. Particularly, being
subjected to significant fluxes of fusion neutrons RAFM steel suffers
considerable local volume variations in the radiation damage involved regions.
It is necessary to study the structure properties of the alloying elements in
contraction and expansion states. In this paper we studied local substitution
structures of thirteen alloying elements Al, Co, Cr, Cu, Mn, Mo, Nb, Ni, Si,
Ta, Ti, V, and W in bcc Fe and calculated their substitutional energies in the
volume variation range from -1.0% to 1.0%. From the structure relaxation
results of the first five neighbor shells around the substitutional atom we
find the relaxation in each neighbor shell keeps approximately uniform within
the volume variation from -1.0% to 1.0% except those of Mn and the relaxation
of the fifth neighbor shell is stronger than that of the third and forth,
indicating that the lattice distortion due to the substitution atom is easier
to spread in <111> direction than in other direction. The relaxation pattern
and intensity are related to the size and electron structure of the
substitutional atom. For some alloying elements, such as Mo, Nb, Ni, Ta, Ti and
W, the substitutional energy decreases noticeably when the volume increases.
Further analysis show that the substitutional energy comprises the energy
variation originated from local structure relaxation and the chemical potential
difference of the substitutional atom between its elemental crystalline state
and the solid solution phase in bcc Fe. We think the approximately uniform
relaxation of each neighbor shell around a substitutional atom give rise to a
linear decrease in the substitutional energy with the increasing volume. | cond-mat |
Symmetry-enforced planar nodal chain phonons in non-symmorphic materials: Topological semimetal states which are constrained by symmetries and give
birth to innovative excitations are the frontiers of topological quantum
matter. Nodal chains in which two nodal rings connect at one point were first
discovered in non-symmorphic electronic systems and then generalized to
symmorphic phononic systems. In this work, we identify a new class of planar
nodal chains in non-symmorphic phononic systems, where the connecting rings lie
in the same plane. The constituting nodal rings are protected by mirror
symmetry, their intersection is guaranteed by the combination of time-reversal
and non-symmorphic two-fold screw symmetry. In addition, the connecting points
are four-fold degenerate while those in previous works are two-fold degenerate.
We searched all 230 space groups and found 8 space groups that can host the
proposed planar nodal chain phonons. Taking wurtzite GaN (space group No.186)
as an example, the planar nodal chain is confirmed by first-principles
calculations. The planar nodal chains result in two distinct classes of
drumhead surface. The first category lies on the [10(-1)0] surface Brillouin
zone and the second lies on the [0001] surface Brillouin zone. Our finding
reveals a class of planar nodal chains in non-symmorphic phononic systems,
expands the catalog of topological nodal chains, and enriches the family of
topological surface states. | cond-mat |
Chiral hedgehog textures in 2D XY-like ordered domains: The textures associated with a point defect centered in a circular domain of
a thin film with XY-like ordering have been analyzed. The family of equilibrium
textures, both stable and metastable, can be classified by a new radial
topological number in addition to the winding number of the defect. Chiral
textures are supported in an achiral system as a result of spontaneously broken
chiral symmetry. Among these chiral textures, our theoretical analysis
accurately describes two categories of recently discovered ``reversing spiral''
textures, ones that are energetically stable and metastable. | cond-mat |
Hall Conductivity in the presence of repulsive magnetic impurities: The Hall conductivity of disordered magnetic systems consisting of hard-core
point vortices randomly dropped on the plane with a Poissonian distribution,
has a behavior analogous to the one observed experimentally by R.~J.~Haug,
R.~R.~Gerhardts, K.~v.~Klitzling and K.~Ploog, with repulsive scatterers \cite
{1}. We also argue that models of homogeneous magnetic field with disordered
potential, have necessarily vanishing Hall conductivities when their Hilbert
space is restricted to a given Landau level subspace. | cond-mat |
Parametric statistics of the scattering matrix: From metallic to
insulating quasi-unidimensional disordered systems: We investigate the statistical properties of the scattering matrix $S$
describing the electron transport through quasi-one dimensional disordered
systems. For weak disorder (metallic regime), the energy dependence of the
phase shifts of $S$ is found to yield the same universal parametric
correlations as those characterizing chaotic Hamiltonian eigenvalues driven by
an external parameter. This is analyzed within a Brownian-motion model for $S$,
which is directly related to the distribution of the Wigner-Smith delay time
matrix. For large disorder (localized regime), transport is dominated by
resonant tunneling and the universal behavior disappears. A model based on a
simplified description of the localized wave functions qualitatively explains
our numerical results. In the insulator, the parametric correlation of the
phase shift velocities follows the energy-dependent autocorrelator of the
Wigner time. The Wigner time and the conductance are correlated in the metal
and in the insulator. | cond-mat |
Crow instability in trapped Bose-Einstein condensates: We show theoretically that elongated vortex-antivortex dipoles can be created
controllably in trapped Bose-Einstein condensates, using known experimental
techniques. Vortex dipoles of sufficient length are unstable and cascade into
slow vortex rings which ultimately decay via sound emission. This instability
of antiparallel vortex line elements, which self-generates Kelvin waves on
vortex loops and in trapped atomic gases, may play a role in bridging the
Kelvin-wave and Kolmogorov-Richardson cascades of quantum turbulence. | cond-mat |
Devil's staircases, quantum dimer models, and stripe formation in strong
coupling models of quantum frustration: We construct a two-dimensional microscopic model of interacting quantum
dimers that displays an infinite number of periodic striped phases in its T=0
phase diagram. The phases form an incomplete devil's staircase and the period
becomes arbitrarily large as the staircase is traversed. The Hamiltonian has
purely short-range interactions, does not break any symmetries of the
underlying square lattice, and is generic in that it does not involve the
fine-tuning of a large number of parameters. Our model, a quantum mechanical
analog of the Pokrovsky-Talapov model of fluctuating domain walls in two
dimensional classical statistical mechanics, provides a mechanism by which
striped phases with periods large compared to the lattice spacing can, in
principle, form in frustrated quantum magnetic systems with only short-ranged
interactions and no explicitly broken symmetries. | cond-mat |
Robustness and observability of rotating vortex-lattices in an
exciton-polariton condensate: Exciton-polariton condensates display a variety of intriguing pattern-forming
behaviors, particularly when confined in potential traps. It has previously
been predicted that triangular lattices of vortices of the same sign will form
spontaneously as the result of surface instabilities in a harmonic trap.
However, natural disorder, deviation of the external potential from circular
symmetry, or higher-order terms modifying the dynamical equations may all have
detrimental effects and destabilize the circular trajectories of vortices. Here
we address these issues, by characterizing the robustness of the vortex lattice
against disorder and deformations of the trapping potential. Since most
experiments use time integrated measurements it would be hard to observe
directly the rotating vortex lattices or distinguish them from vortex-free
states. We suggest how these difficulties can be overcome and present an
experimentally viable interference-imaging scheme that would allow the
detection of rotating vortex lattices. | cond-mat |
Measurement of the ν= 1/3 fractional quantum Hall energy gap in
suspended graphene: We report on magnetotransport measurements of multi-terminal suspended
graphene devices. Fully developed integer quantum Hall states appear in
magnetic fields as low as 2 T. At higher fields the formation of longitudinal
resistance minima and transverse resistance plateaus are seen corresponding to
fractional quantum Hall states, most strongly for {\nu}= 1/3. By measuring the
temperature dependence of these resistance minima, the energy gap for the 1/3
fractional state in graphene is determined to be at ~20 K at 14 T. | cond-mat |
Spin-controlled Mott-Hubbard bands in LaMnO_3 probed by optical
ellipsometry: Spectral ellipsometry has been used to determine the dielectric function of
an untwinned crystal of LaMnO_3 in the spectral range 0.5-5.6 eV at
temperatures 50 K < T < 300 K. A pronounced redistribution of spectral weight
is found at the Neel temperature T_N = 140 K. The anisotropy of the spectral
weight transfer matches the magnetic ordering pattern. A superexchange model
quantitatively describes spectral weight transfer induced by spin correlations.
This analysis implies that the lowest-energy transitions around 2 eV are
intersite d-d transitions, and that LaMnO_3 is a Mott-Hubbard insulator. | cond-mat |
First order character and observable signatures of topological quantum
phase transitions: Topological quantum phase transitions are characterised by changes in global
topological invariants. These invariants classify many body systems beyond the
conventional paradigm of local order parameters describing spontaneous symmetry
breaking. For non-interacting electrons, it is well understood that such
transitions are continuous and always accompanied by a gap-closing in the
energy spectrum, given that the symmetries protecting the topological phase are
maintained. Here, we demonstrate that sufficiently strong electron-electron
interaction can fundamentally change the situation: we discover a topological
quantum phase transition of first order character in the genuine thermodynamic
sense, that occurs without gap closing. Our theoretical study reveals the
existence of a quantum critical endpoint associated with an orbital instability
on the transition line between a 2D topological insulator and a trivial band
insulator. Remarkably, this phenomenon entails unambiguous signatures
associated to the orbital occupations that can be detected experimentally. | cond-mat |
Jerk current: A novel bulk photovoltaic effect: We investigate a physical divergence of the third order polarization
susceptibility representing a photoinduced current in biased crystalline
insulators. This current grows quadratically with illumination time in the
absence of momentum relaxation and saturation; we refer to it as the
\textit{jerk current}. Two contributions to the current are identified. The
first is a hydrodynamic acceleration of optically injected carriers by the
static electric field, and the second is the change in the carrier injection
rate in the presence of the static electric field. The jerk current can have a
component perpendicular to the static field, a feature not captured by standard
hydrodynamic descriptions of carriers in electric fields. We suggest an
experiment to detect the jerk current and some of its interesting features. | cond-mat |
Rotation-induced macromolecular spooling of DNA: Genetic information is stored in a linear sequence of base-pairs; however,
thermal fluctuations and complex DNA conformations such as folds and loops make
it challenging to order genomic material for in vitro analysis. In this work,
we discover that rotation-induced macromolecular spooling of DNA around a
rotating microwire can monotonically order genomic bases, overcoming this
challenge. We use single-molecule fluorescence microscopy to directly visualize
long DNA strands deforming and elongating in shear flow near a rotating
microwire, in agreement with numerical simulations. While untethered DNA is
observed to elongate substantially, in agreement with our theory and numerical
simulations, strong extension of DNA becomes possible by introducing tethering.
For the case of tethered polymers, we show that increasing the rotation rate
can deterministically spool a substantial portion of the chain into a fully
stretched, single-file conformation. When applied to DNA, the fraction of
genetic information sequentially ordered on the microwire surface will increase
with the contour length, despite the increased entropy. This ability to handle
long strands of DNA is in contrast to modern DNA sample preparation
technologies for sequencing and mapping, which are typically restricted to
comparatively short strands resulting in challenges in reconstructing the
genome. Thus, in addition to discovering new rotation-induced macromolecular
dynamics, this work inspires new approaches to handling genomic-length DNA
strands. | cond-mat |
Quantum contact process: The contact process is a paradigmatic classical stochastic system displaying
critical behavior even in one dimension. It features a non-equilibrium phase
transition into an absorbing state that has been widely investigated and shown
to belong to the directed percolation universality class. When the same process
is considered in a quantum setting much less is known. So far mainly
semi-classical studies have been conducted and the nature of the transition in
low dimensions is still a matter of debate. Also from a numerical point of
view, from which the system may look fairly simple --- especially in one
dimension --- results are lacking. In particular the presence of the absorbing
state poses a substantial challenge which appears to affect the reliability of
algorithms targeting directly the steady-state. Here we perform real-time
numerical simulations of the open dynamics of the quantum contact process and
shed light on the existence and on the nature of an absorbing state phase
transition in one dimension. We find evidence for the transition being
continuous and provide first estimates for the critical exponents. Beyond the
conceptual interest, the simplicity of the quantum contact process makes it an
ideal benchmark problem for scrutinizing numerical methods for open quantum
non-equilibrium systems. | cond-mat |
Weyl semimetals and superconductors designed in an orbital selective
superlattice: We propose two complementary design principles for engineering
three-dimensional (3D) Weyl semimetals and superconductors in a layer-by-layer
setup which includes even and odd parity orbitals in alternating layers -
dubbed orbital selective superlattice. Such structure breaks mirror symmetry
along the superlattice growth axis which, with the help of either a basal plane
spin-orbit coupling or a spinless p+ip superconductivity, stabilizes a 3D Dirac
node. To explore this idea, we develop a 3D generalization of Haldane model and
a Bogoliubov-de-Gennes (BdG) Hamiltonian for the two cases, respectively, and
show that a tunable single or multiple Weyl nodes with linear dispersion in all
spatial directions can be engineered desirably in a widespread parameter space.
We also demonstrate that a single helical Weyl band can be created at the
$\Gamma$-point at the Fermi level in the superconducting case via gapping out
either of the orbital state by violating its particle-hole symmetry but not any
other symmetries. Finally, implications of our results for the realization of
anomalous Hall effect and Majorana bound state are discussed. | cond-mat |
Correlations between mechanical, structural, and dynamical properties of
polymer nanocomposites: We study the structural and dynamical mechanisms of reinforcement of a
polymer nanocomposite (PNC) via coarse-grained molecular dynamics simulations.
In a regime of strong polymer-filler interactions, the stress at failure of the
PNC is clearly correlated to structural quantities, such as the filler loading,
the surface area of the polymer-filler interface, and the network structure.
Additionally, we find that small fillers, of the size of the polymer monomers,
are the most effective at reinforcing the matrix by surrounding the polymer
chains and maximizing the number of strong polymer-filler interactions. Such a
structural configuration is correlated to a dynamical feature, namely, the
minimization of the relative mobility of the fillers with respect to the
polymer matrix. | cond-mat |
Emergent channel over a pair of pockets in strong density waves: Different channels over which electrons scatter between parts of the Fermi
surface are the key to various electronic quantum matters, such as
superconductivity and density waves. We consider an effective model in higher
dimensions where each of the two pockets in the original model maps to (the
Landau levels of) two Dirac fermions. We discover an emergent channel when two
Dirac fermions from different pairs annihilate, where the presence of a strong
density wave is essential. We support our analysis with numerical calculations
on model examples in the vicinity of ferromagnetic and antiferromagnetic
orders. We also discuss interesting consequences on electron interaction
channels that beyond-mean-field fluctuations may induce. | cond-mat |
Coupled Effects in Quantum Dot Nanostructures with Nonlinear Strain and
Bridging Modelling Scales: We demonstrate that the conventional application of linear models to the
analysis of optoelectromechanical properties of nanostructures in bandstructure
engineering could be inadequate. The focus of the present paper is on a model
based on the coupled Schrodinger-Poisson system where we account consistently
for the piezoelectric effect and analyze the influence of different nonlinear
terms in strain components. The examples given in this paper show that the
piezoelectric effect contributions are essential and have to be accounted for
with fully coupled models. While in structural applications of piezoelectric
materials at larger scales, the minimization of the full electromechanical
energy is now a routine in many engineering applications, in bandstructure
engineering conventional approaches are still based on linear models with
minimization of uncoupled, purely elastic energy functionals with respect to
displacements. Generalizations of the existing models for bandstructure
calculations are presented in this paper in the context of coupled effects. | cond-mat |
Structural reconstruction and anisotropic conductance in
$4f$-ferromagnetic monolayer: Two-dimensional magnets are promising for nanoscale spintronic applications.
Currently, most available candidates are based on $3d$ transition metal
compounds, with hexagonal or honeycomb lattice geometry. Here, a GdCl$_3$
monolayer with $4f$ moments is theoretically studied, which can be exfoliated
from its existing bulk. Its orthorhombic structure and hendecahedral ion cages
are unique in two-dimensional. Furthermore, a significant structural
reconstruction is caused by the implantation of Li atoms into its interstitial
position, which also lead to ferromagnetism via a double-exchange-like process.
Its highly anisotropic conductance may be peculiarly useful for
nanoelectronics. | cond-mat |
Finite-temperature critical point of a glass transition: We generalize the simplest kinetically constrained model of a glass-forming
liquid by softening kinetic constraints, allowing them to be violated with a
small finite rate. We demonstrate that this model supports a first-order
dynamical (space-time) phase transition, similar to those observed with hard
constraints. In addition, we find that the first-order phase boundary in this
softened model ends in a finite-temperature dynamical critical point, which we
expect to be present in natural systems. We discuss links between this critical
point and quantum phase transitions, showing that dynamical phase transitions
in $d$ dimensions map to quantum transitions in the same dimension, and hence
to classical thermodynamic phase transitions in $d+1$ dimensions. We make these
links explicit through exact mappings between master operators, transfer
matrices, and Hamiltonians for quantum spin chains. | cond-mat |
Pressure consistency for binary hard-sphere mixtures from an integral
equation approach: The site-site Ornstein-Zernike equation combined with the Verlet-modified
bridge function has been applied to the binary hard sphere mixtures and
pressure consistency has been tested. An equation of state has been computed
for the case where a packing fraction is $\eta = 0.49$, diameter ratios are
$\sigma_{2}/\sigma_{1} = 0.3$ and $0.6$, and the mole fractions are $x_{1} =
0.125, 0.5, 0.75$, and $1$. An excess chemical potential for each component has
been obtained as well. Our findings for thermodynamic properties are in good
agreement with available data in literature. | cond-mat |
Effect of Succinonitrile on Ion Transport in PEO-based Lithium Ion
Battery Electrolytes: We report the ion transport mechanisms in succinonitrile (SN) loaded solid
polymer electrolytes containing polyethylene oxide (PEO) and dissolved lithium
bis(trifluoromethane)sulphonamide (LiTFSI) salt using molecular dynamics
simulations. We investigated the effect of temperature and loading of SN on ion
transport and relaxation phenomenon in PEO-LiTFSI electrolytes. It is observed
that SN increases the ionic diffusivities in PEO-based solid polymer
electrolytes and makes them suitable for battery applications. Interestingly,
the diffusion coefficient of TFSI ions is an order of magnitude higher than the
diffusion coefficient of lithium ions across the range of temperatures and
loadings integrated. By analyzing different relaxation timescales and examining
the underlying transport mechanisms in SN-loaded systems, we find that the
diffusivity of TFSI ions correlates excellently with the Li-TFSI ion-pair
relaxation timescales. In contrast, our simulations predict distinct transport
mechanisms for Li-ions in SN-loaded PEO-LiTFSI electrolytes. Explicitly, the
diffusivity of lithium ions cannot be uniquely determined by the ion-pair
relaxation timescales but additionally depends on the polymer segmental
dynamics. On the other hand, the SN loading induced diffusion coefficient at a
given temperature does not correlate with either the ion-pair relaxation
timescales or the polymer segmental relaxation timescales. | cond-mat |
Rethinking mean-field glassy dynamics and its relation with the energy
landscape: the awkward case of the spherical mixed p-spin model: The spherical p-spin model is not only a fundamental model in statistical
mechanics of disordered system, but has recently gained popularity since many
hard problems in machine learning can be mapped on it. Thus the study of the
out of equilibrium dynamics in this model is interesting both for the glass
physics and for its implications on algorithms solving NP-hard problems. We
revisit the long-time limit of the out of equilibrium dynamics of mean-field
spherical mixed p-spin models. We consider quenches (gradient descent dynamics)
starting from initial conditions thermalized at some temperature in the ergodic
phase. We perform numerical integration of the dynamical mean-field equations
of the model and we find an unexpected dynamical phase transition. Below an
onset temperature, higher than the dynamical transition temperature, the
asymptotic energy goes below the "threshold energy" of the dominant marginal
minima of the energy function and memory of the initial condition is kept. This
behavior, not present in the pure spherical p-spin model, resembles closely the
one observed in simulations of glass-forming liquids. We then investigate the
nature of the asymptotic dynamics, finding an aging solution that relaxes
towards deep marginal minima, evolving on a restricted marginal manifold.
Careful analysis, however, rules out simple aging solutions. We compute the
constrained complexity in the aim of connecting the asymptotic solution to the
energy landscape. | cond-mat |
Connectedness percolation of hard deformed rods: Nanofiller particles, such as carbon nanotubes or metal wires, are used in
functional polymer composites to make them conduct electricity. They are often
not perfectly straight cylinders, but may be tortuous or exhibit kinks.
Therefore we investigate the effect of shape deformations of the rodlike
nanofillers on the geometric percolation threshold of the dispersion. We do
this by using connectedness percolation theory within a Parsons-Lee type of
approximation, in combination with Monte Carlo integration for the average
overlap volume in the isotropic fluid phase. We find that a deviation from a
perfect rodlike shape has very little effect on the percolation threshold,
unless the particles are strongly deformed. This demonstrates that idealized
rod models are useful even for nanofillers that superficially seem imperfect.
In addition, we show that for small or moderate rod deformations, the universal
scaling of the percolation threshold is only weakly affected by the precise
particle shape. | cond-mat |
Optical Black-hole Analog Created by Topological Phase Transition with a
Long-lived Horizon: Hawking radiation, a manifestation of quantum field theory in curved
spacetime, has stimulated extensive theoretical and experimental studies of
various black-hole (BH) analogs. However, an undisputed confirmation of Hawking
radiation remains elusive. One challenge is BH analog structures with
long-lived horizons are difficult to achieve. Here, we theoretically
demonstrate a new type of optical BH analog based on light cone evolution
associated with topological phase transition of Dirac cones. The transition
from a type-II to type-I Dirac/Weyl cone creates an analogous curved spacetime
that crosses a type-III Dirac/Weyl cone, which affords a stationary
configuration of long-lived event horizon. Photons tunneling through the
horizon emit a spectrum of Hawking radiation. As an example, we design a
laboratory version in an inhomogeneous two-dimensional graphyne-like
topological photonic lattice with a Hawking temperature of 0.14 mK.
Understanding Hawking-like radiation in this unique topological BH is not only
of fundamental interest in its own right but may also provide new hints to
gravitational physics. | cond-mat |
Defining Temperatures of Granular Powders Analogously with
Thermodynamics to Understand the Jamming Phenomena: For the purpose of applying laws or principles originated from thermal
systems to granular athermal systems, we may need to properly define the
critical temperature concept in granular powders. The conventional
environmental temperature in thermal systems is too weak to drive movements of
particles in granular powders and cannot function as a thermal energy
indicator. For maintaining the same functionality as in thermal systems, the
temperature in granular powders is defined analogously and uniformly in this
article. The newly defined granular temperature is utilized to describe and
explain one of the most important phenomena observed in granular powders, the
jamming transition, by introducing jamming temperature and jamming volume
fraction concepts. The predictions from the equations of the jamming volume
fractions for several cases like granular powders under shear or vibration are
in line with experimental observations and empirical solutions in powder
handlings. The goal of this article is to establish similar concepts in
granular powders, allowing granular powders to be described with common laws or
principles we are familiar with in thermal systems. Our intention is to build a
bridge between thermal systems and granular powders to account for many
similarities already found between these two systems. | cond-mat |
Hyperuniformity of Maximally Random Jammed Packings of Hyperspheres
Across Spatial Dimensions: The maximally random jammed (MRJ) state is the most random configuration of
strictly jammed (mechanically rigid) nonoverlapping objects. MRJ packings are
hyperuniform, meaning their long-wavelength density fluctuations are
anomalously suppressed compared to typical disordered systems, i.e., their
structure factors $S(\mathbf{k})$ tend to zero as the wavenumber $|\mathbf{k}|$
tends to zero. Here, we show that generating high-quality strictly jammed
states for space dimensions $d = 3,4,$ and $5$ is of paramount importance in
ensuring hyperuniformity and extracting precise values of the hyperuniformity
exponent $\alpha > 0$ for MRJ states, defined by the power-law behavior of
$S(\mathbf{k})\sim|\mathbf{k}|^{\alpha}$ in the limit
$|\mathbf{k}|\rightarrow0$. Moreover, we show that for fixed $d$ it is more
difficult to ensure jamming as the particle number $N$ increases, which results
in packings that are nonhyperuniform. Free-volume theory arguments suggest that
the ideal MRJ state does not contain rattlers, which act as defects in
numerically generated packings. As $d$ increases, we find that the fraction of
rattlers decreases substantially. Our analysis of the largest truly jammed
packings suggests that the ideal MRJ packings for all dimensions $d\geq3$ are
hyperuniform with $\alpha = d - 2$, implying the packings become more
hyperuniform as $d$ increases. The differences in $\alpha$ between MRJ packings
and recently proposed Manna-class random close packed (RCP) states, which were
reported to have $\alpha = 0.25$ in $d=3$ and be nonhyperuniform ($\alpha = 0$)
for $d = 4$ and $d = 5$, demonstrate the vivid distinctions between the
large-scale structure of RCP and MRJ states in these dimensions. Our work
clarifies the importance of the link between true jamming and hyperuniformity
and motivates the development of an algorithm to produce rattler-free
three-dimensional MRJ packings. | cond-mat |
Transport Properties of Multiple Quantum Dots Arranged in Parallel:
Results from the Bethe Ansatz: In this paper we analyze transport through a double dot system connected to
two external leads. Imagining each dot possessing a single active level, we
model the system through a generalization of the Anderson model. We argue that
this model is exactly solvable when certain constraints are placed upon the dot
Coulomb charging energy, the dot-lead hybridization, and the value of the
applied gate voltage. Using this exact solvability, we access the zero
temperature linear response conductance both in and out of the presence of a
Zeeman field. We are also able to study the finite temperature linear response
conductance. We focus on universal behaviour and identify three primary
features in the transport of the dots: i) a so-called RKKY Kondo effect; ii) a
standard Kondo effect; and iii) interference phenomena leading to sharp
variations in the conductance including conductance zeros. We are able to use
the exact solvability of the dot model to characterize these phenomena
quantitatively. While here we primarily consider a double dot system, the
approach adopted applies equally well to N-dot systems. | cond-mat |
Examination of the tradeoff between intrinsic and extrinsic properties
in the optimization of a modern internal tin Nb3Sn conductor: In modern Nb3Sn wires there is a fundamental compromise to be made between
optimizing the intrinsic properties associated with the superfluid density in
the A15 phase (e.g. Tc, Hc, Hc2, all of which are composition dependent),
maximizing the quantity of A15 that can be formed from a given mixture of Nb,
Sn and Cu, minimizing the A15 composition gradients within each sub-element,
while at the same time generating a high vortex pinning critical current
density, Jc, by maximizing the grain boundary density with the additional
constraint of maintaining the RRR of the Cu stabilizer above 100. Here we study
these factors in a Ta-alloyed Restacked-Rod-Process (RRP) wire with ~70 microns
diameter sub-elements. Consistent with many earlier studies, maximum non-Cu
Jc(12T,4.2K) requires preventing A15 grain growth, rather than by optimizing
the superfluid density. In wires optimized for 12T, 4.2K performance, about 60%
of the non-Cu cross-section is A15, 35% residual Cu and Sn core, and only 5% a
residual Nb7.5wt.%Ta diffusion barrier. The specific heat and chemical analyses
show that in this 60% A15 fraction there is a wide range of Tc and chemical
composition that does diminish for higher heat treatment temperatures, which,
however, are impractical because of the strong RRR degradation that occurs when
only about 2% of the A15 reaction front breaches the diffusion barrier. As this
kind of Nb3Sn conductor design is being developed for sub-elements 1/2 the
present size, it is clear that better barriers are essential to allowing higher
temperature reactions with better intrinsic A15 properties. We present here
multiple and detailed intrinsic and extrinsic evaluations because we believe
that only such broad and quantitative descriptions are capable of accurately
tracking the limitations of individual conductor designs where optimization
will always be a compromise between inherently conflicting goals | cond-mat |
Orbital magnetization of correlated electrons with arbitrary band
topology: Spin-orbit coupling introduces chirality into electronic structure. This can
have profound effects on the magnetization induced by orbital motion of
electrons. Here we derive a formula for the orbital magnetization of
interacting electrons in terms of the full Green's function and vertex
functions. The formula is applied within dynamical mean-field theory to the
Kane-Mele-Hubbard model that allows both topological and trivial insulating
phases. We study the insulating and metallic phases in the presence of an
exchange magnetic field. In the presence of interactions, the orbital
magnetization of the quantum spin Hall insulating phase with inversion symmetry
is renormalized by the bulk quasi-particle weight. The orbital magnetization
vanishes for the in-plane antiferromagnetic phase with trivial topology. In the
metallic phase, the enhanced effective spin-orbit coupling due to the
interaction sometimes leads to an enhancement of the orbital magnetization.
However, at low doping, magnetization is suppressed at large interaction
strengths. | cond-mat |
Magnetic molecular orbitals in MnSi: A large body of knowledge about magnetism is attained from models of
interacting spins, which usually reside on magnetic ions. Proposals beyond the
ionic picture are uncommon and seldom verified by direct observations in
conjunction with microscopic theory. Here, using inelastic neutron scattering
to study the itinerant near-ferromagnet MnSi, we find that the system's
fundamental magnetic units are interconnected, extended molecular orbitals
consisting of three Mn atoms each, rather than individual Mn atoms. This result
is further corroborated by magnetic Wannier orbitals obtained by ab initio
calculations. It contrasts the ionic picture with a concrete example, and
presents a novel regime of the spin waves where the wavelength is comparable to
the spatial extent of the molecular orbitals. Our discovery brings important
insights into not only the magnetism of MnSi, but also a broad range of
magnetic quantum materials where structural symmetry, electron itinerancy and
correlations act in concert. | cond-mat |
Self-avoiding walks subject to a force: We prove some theorems about self-avoiding walks attached to an impenetrable
surface (i.e. positive walks) and subject to a force. Specifically we show the
force dependence of the free energy is identical when the force is applied at
the last vertex or at the top (confining) plane. We discuss the relevance of
this result to numerical results and to a recent result about convergence rates
when the walk is being pushed towards the surface. | cond-mat |
Spin-polarized transport in II-VI magnetic resonant tunneling devices: We investigate electronic transport through II-VI semiconductor resonant
tunneling structures containing diluted magnetic impurities. Due to the
exchange interaction between the conduction electrons and the impurities, there
arises a giant Zeeman splitting in the presence of a moderately low magnetic
field. As a consequence, when the quantum well is magnetically doped the
current-voltage characteristics shows two peaks corresponding to transport for
each spin channel. This behavior is experimentally observed and can be
reproduced with a simple tunneling model. The model thus allows to analyze
other configurations. First, we further increase the magnetic field, which
leads to a spin polarization of the electronic current injected from the leads,
thus giving rise to a relative change in the current amplitude. We demonstrate
that the spin polarization in the emitter can be determined from such a change.
Furthermore, in the case of a magnetically doped injector our model shows a
large increase in peak amplitude and a shift of the resonance to higher
voltages as the external field increases. We find that this effect arises from
a combination of giant Zeeman splitting, 3-D incident distribution and broad
resonance linewidth. | cond-mat |
Connection between matrix-product states and superposition of Bernoulli
shock measures: We consider a generalized coagulation-decoagulation system on a
one-dimensional discrete lattice with reflecting boundaries. It is known that a
Bernoulli shock measure with two shock fronts might have a simple random-walk
dynamics, provided that some constraints on the microscopic reaction rates of
this system are fulfilled. Under these constraints the steady-state of the
system can be written as a linear superposition of such shock measures. We show
that the coefficients of this expansion can be calculated using the
finite-dimensional representation of the quadratic algebra of the system
obtained from a matrix-product approach. | cond-mat |
eQE 2.0: Subsystem DFT Beyond GGA Functionals: By adopting a divide-and-conquer strategy, subsystem-DFT (sDFT) can
dramatically reduce the computational cost of large-scale electronic structure
calculations. The key ingredients of sDFT are the nonadditive kinetic energy
and exchange-correlation functionals which dominate it's accuracy. Even though,
semilocal nonadditive functionals find a broad range of applications, their
accuracy is somewhat limited especially for those systems where achieving
balance between exchange-correlation interactions on one side and nonadditive
kinetic energy on the other is crucial. In eQE 2.0, we improve dramatically the
accuracy of sDFT simulations by (1) implementing nonlocal nonadditive kinetic
energy functionals based on the LMGP family of functionals; (2) adapting
Quantum ESPRESSO's implementation of rVV10 and vdW-DF nonlocal
exchange-correlation functionals to be employed in sDFT simulations; (3)
implementing "deorbitalized" meta GGA functionals (e.g., SCAN-L). We carefully
assess the performance of the newly implemented tools on the S22-5 test set.
eQE 2.0 delivers excellent interaction energies compared to conventional
Kohn-Sham DFT and CCSD(T). The improved performance does not come at a loss of
computational efficiency. We show that eQE 2.0 with nonlocal nonadditive
functionals retains the same linear scaling behavior achieved in eQE 1.0 with
semilocal nonadditive functionals. | cond-mat |
The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory
Results: The high-pressure dynamics of a computer-modeled silica melt is studied in
the framework of the mode-coupling theory of the glass transition (MCT) using
static-structure input from molecular-dynamics (MD) computer simulation. The
theory reproduces the experimentally known viscosity minimum (diffusivity
maximum) as a function of density or pressure and explains it in terms of a
corresponding minimum in its critical temperature. This minimum arises from a
gradual change in the equilibrium static structure which shifts from being
dominated by tetrahedral ordering to showing the cageing known from
high-density liquids. The theory is in qualitative agreement with computer
simulation results. | cond-mat |
Capillary imbibition in a square tube: When a square tube is brought in contact with bulk liquid, the liquid wets
the corners of the tube, and creates finger-like wetted region. The wetting of
the liquid then takes place with the growth of two parts, the bulk part where
the cross section is entirely filled with the liquid and the finger part where
the cross section of the tube is partially filled. In the previous works, the
growth of these two parts has been discussed separately. Here we conduct the
analysis by explicitly accounting for the coupling of the two parts. We propose
coupled equations for the liquid imbibition in both parts and show that (a) the
length of each part, $h_0$ and $h_1$, both increases in time $t$ following the
Lucas-Washburn's law, $h_0 \sim t^{1/2}$ and $h_1 \sim t^{1/2}$, but that (b)
the coefficients are different from those obtained in the previous analysis
which ignored the coupling. | cond-mat |
Evidence for 4e charge of Cooper quartets in a biased multi-terminal
graphene-based Josephson junction: In a Josephson junction (JJ), Cooper pairs are transported via Andreev bound
states (ABSs) between superconductors. The ABSs in the weak link of
multi-terminal (MT) JJs can coherently hybridize two Cooper pairs among
different superconducting electrodes, resulting in the Cooper quartet (CQ)
involving four fermions entanglement. The energy spectrum of these CQ-ABS can
be controlled by biasing MT-JJs due to the AC Josephson effect. Here, using
gate tunable four-terminal graphene JJs complemented with a flux loop, we
construct CQs with a tunable spectrum. The critical quartet supercurrent
exhibits magneto-oscillation associated with a charge of 4e; thereby presenting
the evidence for interference between entangled CQ-ABS. At a finite bias
voltage, we find the DC quartet supercurrent shows non-monotonic bias dependent
behavior, attributed to Landau-Zener transitions between different Floquet
bands. Our experimental demonstration of coherent non-equilibrium CQ-ABS sets a
path for design of artificial topological materials based on MT-JJs. | cond-mat |
Quantum Monte Carlo Study on the Spin-1/2 Honeycomb Heisenberg Model
with Mixing Antiferromagnetic and Ferromagnetic Interactions in External
Magnetic Fields: The continuous imaginary-time quantum Monte Carlo method with the worm update
algorithm is applied to explore the ground state properties of the spin-1/2
Heisenberg model with antiferromagnetic (AF) coupling $J>0$ and ferromagnetic
(F) coupling $J^{\prime}<0$ along zigzag and armchair directions, respectively,
on honeycomb lattice. It is found that by enhancing the F coupling $J^{\prime}$
between zigzag AF chains, the system is smoothly crossover from one-dimensional
zigzag spin chains to a two-dimensional magnetic ordered state. In absence of
an external field, the system is in a stripe order phase. In presence of
uniform and staggered fields, the uniform and staggered out-of-plane
magnetizations appear while the stripe order keeps in $xy$ plane, and a
second-order quantum phase transition (QPT) at a critical staggered field is
observed. The critical exponents of correlation length for QPTs induced by a
staggered field for the cases with $J>0$, $J^{\prime}<0$ and $J<0$,
$J^{\prime}>0$ are obtained to be $\nu=0.677(2)$ and $0.693(0)$, respectively,
indicating that both cases belong to O(3) universality. The scaling behavior in
a staggered field is analyzed, and the ground state phase diagrams in the plane
of coupling ratio and staggered field are presented for two cases. The
temperature dependence of susceptibility and specific heat of both systems in
external magnetic fields is also discussed. | cond-mat |
Operator-valued Riemann-Hilbert problem for correlation functions of the
XXZ spin chain: The generating functional of correlation functions for the XXZ spin chain is
considered in the thermodynamic limit. We derive a system of integro-difference
equations that prescribe this functional. On the basis of this system we
establish the operator-valued Riemann-Hilbert problem for correlation functions
of the XXZ spin chain. | cond-mat |
Decoding the Mechanisms of Reversibility Loss in Rechargeable Zinc-Air
Batteries: Attaining high reversibility of electrodes and electrolyte is essential for
the longevity of secondary batteries. Rechargeable zinc-air batteries (RZABs),
however, encounter drastic irreversible changes in the zinc anodes and air
cathodes during cycling. To uncover the mechanisms of reversibility loss in
RZABs, we investigate the evolution of zinc anode, alkaline electrolyte, and
air electrode through experiments and first-principles calculations. Morphology
diagrams of zinc anodes under versatile operating conditions reveal that the
nano-sized mossy zinc dominates the later cycling stage. Such anodic change is
induced by the increased zincate concentration due to hydrogen evolution, which
is catalyzed by the mossy structure and results in oxide passivation on
electrodes, and eventually leads to low true Coulombic efficiencies and short
lifespans of batteries. Inspired by these findings, we finally present a novel
overcharge-cycling protocol to compensate the Coulombic efficiency loss caused
by hydrogen evolution and significantly extend the battery life. | cond-mat |
High-pressure synthesis and the enhancement of the superconducting
properties of FeSe0.5Te0.5: A series of FeSe0.5Te0.5 bulk samples have been prepared through the high gas
pressure and high-temperature synthesis (HP-HTS) method to optimize the growth
conditions, for the first time and investigated for their superconducting
properties using structural, microstructure, transport, and magnetic
measurements to reach the final conclusions. Ex-situ and in-situ processes are
used to prepare bulk samples under a range of growth pressures using Ta-tube
and without Tatube. The parent compound synthesized by convenient synthesis
method at ambient pressure (CSP) exhibits a superconducting transition
temperature of 14.8 K. Our data demonstrate that the prepared FeSe0.5Te0.5
sealed in a Ta-tube is of better quality than the samples without a Ta-tube,
and the optimum growth conditions (500 MPa, 600{\deg}C for 1 h) are favourable
for the development of the tetragonal FeSe0.5Te0.5 phase. The optimum bulk
FeSe0.5Te0.5 depicts a higher transition temperature of 17.3 K and a high
critical current density of the order of >10^4 A/cm^2 at 0 T, which is improved
over the entire magnetic field range and almost twice higher than the parent
compound prepared through CSP. Our studies confirm that the high-pressure
synthesis method is a highly efficient way to improve the superconducting
transition, grain connectivity, sample density, and also pinning properties of
a superconductor. | cond-mat |
Doping Effect and Flux Pinning Mechanism of Nano-SiC Additions in MgB2
Strands: Superconducting MgB2 strands with nanometer-scale SiC additions have been
investigated systematically using transport and magnetic measurements. A
comparative study of MgB2 strands with different nano-SiC addition levels has
shown C-doping-enhanced critical current density Jc through enhancements in the
upper critical field, Hc2, and decreased anisotropy. The critical current
density and flux pinning force density obtained from magnetic measurements were
found to greatly differ from the values obtained through transport
measurements, particularly with regards to magnetic field dependence. The
differences in magnetic and transport results are largely attributed to
connectivity related effects. On the other hand, based on the scaling behavior
of flux pinning force, there may be other effective pinning centers in MgB2
strands in addition to grain boundary pinning. | cond-mat |
Geometric theory on the elasticity of bio-membranes: The purpose of this paper is to study the shapes and stabilities of
bio-membranes within the framework of exterior differential forms. After a
brief review of the current status in theoretical and experimental studies on
the shapes of bio-membranes, a geometric scheme is proposed to discuss the
shape equation of closed lipid bilayers, the shape equation and boundary
conditions of open lipid bilayers and two-component membranes, the shape
equation and in-plane strain equations of cell membranes with cross-linking
structures, and the stabilities of closed lipid bilayers and cell membranes.
The key point of this scheme is to deal with the variational problems on the
surfaces embedded in three-dimensional Euclidean space by using exterior
differential forms. | cond-mat |
Steady-state and quench dependent relaxation of a quantum dot coupled to
one-dimensional leads: We study the time evolution and steady state of the charge current in a
single-impurity Anderson model, using matrix product states techniques. A
nonequilibrium situation is imposed by applying a bias voltage across
one-dimensional tight-binding leads. Focusing on particle-hole symmetry, we
extract current-voltage characteristics from universal low-bias up to high-bias
regimes, where band effects start to play a dominant role. We discuss three
quenches, which after strongly quench-dependent transients yield the same
steady-state current. Among these quenches we identify those favorable for
extracting steady-state observables. The period of short-time oscillations is
shown to compare well to real-time renormalization group results for a simpler
model of spinless fermions. We find indications that many-body effects play an
important role at high-bias voltage and finite bandwidth of the metallic leads.
The growth of entanglement entropy after a certain time scale (proportional to
the inverse of Delta) is the major limiting factor for calculating the time
evolution. We show that the magnitude of the steady-state current positively
correlates with entanglement entropy. The role of high-energy states for the
steady-state current is explored by considering a damping term in the time
evolution. | cond-mat |
Kinetics of inherent processes counteracting crystallization in
supercooled monatomic liquid: Crystallization of supercooled liquids is mainly determined by two competing
processes associated with the transition of particles (atoms) from liquid phase
to crystalline one and, vice versa, with the return of particles from
crystalline phase to liquid one. The quantitative characteristics of these
processes are the so-called attachment rate $g^{+}$ and the detachment rate
$g^{-}$, which determine how particles change their belonging from one phase to
another. In the present study, a {\it correspondence rule} between the rates
$g^{+}$ and $g^{-}$ as functions of the size $N$ of growing crystalline nuclei
is defined for the first time. In contrast to the well-known detailed balance
condition, which relates $g^{+}(N)$ and $g^{-}(N)$ at $N=n_c$ (where $n_c$ is
the critical nucleus size) and is satisfied only at the beginning of the
nucleation regime, the found {\it correspondence rule} is fulfilled at all the
main stages of crystallization kinetics (crystal nucleation, growth and
coalescence). On the example of crystallizing supercooled Lennard-Jones liquid,
the rate $g^{-}$ was calculated for the first time at different supercooling
levels and for the wide range of nucleus sizes $N\in[n_c;\,40\,n_c]$. It was
found that for the whole range of nucleus sizes, the detachment rate $g^{-}$ is
only $\approx2$\% less than the attachment rate $g^{+}$. This is direct
evidence that the role of the processes that counteract crystallization remains
significant at all the stages of crystallization. Based on the obtained
results, a kinetic equation was formulated for the time-dependent distribution
function of the nucleus sizes, that is an alternative to the well-known kinetic
Becker-D\"{o}ring-Zeldovich-Frenkel equation. | cond-mat |
Exceptional point description of one-dimensional chiral topological
superconductors/superfluids in BDI class: We show that certain singularities of the Hamiltonian in the complex wave
vector space can be used to identify topological quantum phase transitions for
$1D$ chiral topological superconductors/superfluids in the BDI class. These
singularities fall into the category of the so-called exceptional points
($EP$'s) studied in the context of non-Hermitian Hamiltonians describing open
quantum systems. We also propose a generic formula in terms of the properties
of the $EP$'s to quantify the exact number of Majorana zero modes in a
particular chiral topological superconducting phase, given the values of the
parameters appearing in the Hamiltonian. This formula serves as an alternative
to the familiar integer ($\mathbb{Z}$) winding number invariant characterizing
topological superconductor/superfluid phases in the chiral BDI class. | cond-mat |
Twisted bilayer blue phosphorene: A direct band gap semiconductor: We report that two rotated layers of blue phosphorene behave as a direct band
gap semiconductor. The optical spectrum shows absorption peaks in the visible
region of the spectrum and in addition the energy of these peaks can be tuned
with the rotational angle. These findings makes twisted bilayer blue
phosphorene a strong candidate as a solar cell or photodetection device. Our
results are based on ab initio calculations of several rotated blue phosphorene
layers. | cond-mat |
Bulk Geometry of the Many Body Localized Phase from Wilson-Wegner Flow: Tensor networks are a powerful formalism for transforming one set of degrees
of freedom to another. They have been heavily used in analyzing the geometry of
bulk/boundary correspondence in conformal field theories. Here we develop a
tensor-network version of the Wilson-Wegner Renormalization Group Flow
equations to efficiently generate a unitary tensor network which diagonalizes
many-body localized Hamiltonians. Treating this unitary tensor network as a
bulk geometry, we find this emergent geometry corresponds to the shredded
horizon picture: the circumference of the network shrinks exponentially with
distance into the bulk, with spatially distant points being largely
disconnected. | cond-mat |
Observation of an optical non-Fermi-liquid behavior in the heavy fermion
state of YbRh$_{2}$Si$_{2}$: We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon
energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy
quasiparticle state, a linear dependence of the low-energy scattering rate on
both temperature and photon energy was found. We relate this distinct dynamical
behavior different from that of Fermi liquid materials to the non-Fermi liquid
nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an
antiferromagnetic quantum critical point. | cond-mat |
Faulty evidence for superconductivity in ac magnetic susceptibility of
sulfur hydride under pressure: It is generally believed that sulfur hydride under high pressure is a high
temperature superconductor. In National Science Review 6, 713 (2019) Huang and
coworkers reported detection of superconductivity in sulfur hydride through a
highly sensitive ac magnetic susceptibility technique and an unambiguous
determination of the superconducting phase diagram. In this paper we present
evidence showing that the experimental results reported in that paper do not
support the conclusion that sulfur hydride is a superconductor. | cond-mat |
Theory of Superfluids with Population Imbalance: Finite Temperature and
BCS-BEC Crossover Effects: In this paper we present a very general theoretical framework for addressing
fermionic superfluids over the entire range of BCS to Bose Einstein
condensation (BEC) crossover in the presence of population imbalance or spin
polarization. Our emphasis is on providing a theory which reduces to the
standard zero temperature mean field theories in the literature, but
necessarily includes pairing fluctuation effects at non-zero temperature within
a consistent framework. Physically, these effects are associated with the
presence of pre-formed pairs (or a fermionic pseudogap) in the normal phase,
and pair excitations of the condensate, in the superfluid phase. We show how
this finite $T$ theory of fermionic pair condensates bears many similarities to
the condensation of point bosons. In the process we examine three different
types of condensate: the usual breached pair or Sarma phase and both the one
and two plane wave Larkin- Ovchinnikov, Fulde-Ferrell (LOFF) states. The last
of these has been discussed in the literature albeit only within a
Landau-Ginzburg formalism, generally valid near $T_c$. Here we show how to
arrive at the two plane wave LOFF state in the ground state as well as at
general temperature $T$. | cond-mat |
Conductance in strongly correlated 1D systems: Real-Time Dynamics in
DMRG: A new method to perform linear and finite bias conductance calculations in
one dimensional systems based on the calculation of real time evolution within
the Density Matrix Renormalization Group (DMRG) is presented. We consider a
system of spinless fermions consisting of an extended interacting nanostructure
attached to non-interacting leads. Results for the linear and finite bias
conductance through a seven site structure with weak and strong
nearest-neighbor interactions are presented. Comparison with exact
diagonalization results in the non-interacting limit serve as verification of
the accuracy of our approach. Our results show that interaction effects lead to
an energy dependent self energy in the differential conductance. | cond-mat |
Thermally Activated Motion of Sodium Cations in Insulating Parent
Low-Silica X Zeolite: We report a $^{23}$Na spin-lattice relaxation rate, $T_1^{-1}$, in low-silica
X zeolite. $T_1^{-1}$ follows multiple BPP-type behavior as a result of thermal
motion of sodium cations in insulating material. The estimated lowest
activation energy of 15~meV is much lower than 100~meV observed previously for
sodium motion in heavily Na-loaded samples and is most likely attributed to
short-distance jumps of sodium cations between sites within the same supercage. | cond-mat |
Nature of order from random two-body interactions: We investigate the origin of order in the low-lying spectra of many-body
systems with random two-body interactions. Our study based both on analytical
as well as on numerical arguments shows that except for the most $J$-stretched
states, the ground states in the higher $J$-sectors are more orderly and
develop larger energy gaps than the ones in the J=0-sector. Due to different
characteristic energy scales in different $J$-sectors the J=0 ground states may
predominate only when all the states are taken together. | cond-mat |
Detailed magneto-heat capacity analysis of SnAs topological
superconductor: In this article, we report magneto-heat capacity analysis of superconducting
SnAs, which is characterized through X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), and magneto-transport measurements. The
studied SnAs superconductor evidenced the presence of superconductivity at
around 4K, and the same is seen to persist up to an applied field of 250Oe. The
bulk nature of superconductivity is determined through AC susceptibility along
with heat capacity measurements. Magneto-heat capacity measurements show SnAs
to be a fully gapped s wave superconductor. This finding is well supported by
calculated superconducting physical parameters. Further, the calculation of the
residual Sommerfeld coefficient at different fields confirms node-less
superconductivity in SnAs. | cond-mat |
Dynamical generation of skyrmion and bimeron crystals by a circularly
polarized electric field in frustrated magnets: A skyrmion crystal (SkX) has attracted much attention in condensed matter
physics, since topologically nontrivial structures induce fascinating physical
phenomena. The SkXs have been experimentally observed in a variety of
materials, where the Zeeman coupling to the static magnetic field plays an
important role in the formation of the SkXs. In this study, we theoretically
propose another route to generate the SkXs by using a circularly polarized
electric field. We investigate a non-equilibrium steady state in a classical
frustrated Heisenberg magnet under the circularly polarized electric field,
where the electric field is coupled to the electric polarization via the
spin-current mechanism. By numerically solving the Landau-Lifshitz-Gilbert
equation at zero temperature, we show that the electric field radiation
generates a SkX with a high topological number in the high-frequency regime,
where the sign of the skyrmion number is fixed to be negative (positive) under
the left (right) circularly polarized field. The intense electric field melts
these SkXs and generates isolated skyrmions. We clarify that the microscopic
origin is effective electric-field-induced three-spin interactions by adopting
the high-frequency expansion in the Floquet formalism. Furthermore, we find
that the electric field radiation generates another type of SkXs, a bimeron
crystal, in the low-frequency regime. Our results provide a way to generate the
SkXs and control the topology by the circularly polarized electric field. | cond-mat |
Second sound resonators and tweezers as vorticity or velocity probes :
fabrication, model and method: An analytical model of open-cavity second sound resonators is presented and
validated against simulations and experiments in superfluid helium using a new
resonator design that achieves unprecedented resolution. The model incorporates
diffraction, geometrical misalignments, and flow through the cavity, and is
validated using cavities with aspect ratios close to unity, operated up to
their 20th resonance in superfluid helium.An important finding of this study is
that resonators can be optimized to selectively sense either the quantum vortex
density carried by the throughflow -- as typically done in the literature -- or
the mean velocity of the throughflow. We propose two velocity probing methods:
one that takes advantage of geometrical misalignments between the tweezers
plates, and another that drives the resonator non-linearly, beyond a threshold
that results in the self-sustainment of a vortex tangle within the cavity.A new
mathematical treatment of the resonant signal is proposed to adequately filter
out parasitic signals, such as temperature and pressure drift, and accurately
separate the quantum vorticity signal. This elliptic method consists in a
geometrical projection of the resonance in the inverse complex plane. Its
effectiveness is demonstrated over a wide range of operating conditions.The
resonator model and elliptic method are being utilized to characterize a new
design of second-sound resonator with high resolution thanks to miniaturization
and design optimization. These second-sound tweezers are capable of providing
time-space resolved information similar to classical local probes in
turbulence, down to sub-millimeter and sub-millisecond scales. The principle,
design, and micro-fabrication of second sound tweezers are being presented in
detail, along with their potential for exploring quantum turbulence. | cond-mat |
Large fluctuations of the KPZ equation in a half-space: We investigate the short-time regime of the KPZ equation in $1+1$ dimensions
and develop a unifying method to obtain the height distribution in this regime,
valid whenever an exact solution exists in the form of a Fredholm Pfaffian or
determinant. These include the droplet and stationary initial conditions in
full space, previously obtained by a different method. The novel results
concern the droplet initial condition in a half space for several Neumann
boundary conditions: hard wall, symmetric, and critical. In all cases, the
height probability distribution takes the large deviation form $P(H,t) \sim
\exp( - \Phi(H)/\sqrt{t})$ for small time. We obtain the rate function
$\Phi(H)$ analytically for the above cases. It has a Gaussian form in the
center with asymmetric tails, $|H|^{5/2}$ on the negative side, and $H^{3/2}$
on the positive side. The amplitude of the left tail for the half-space is
found to be half the one of the full space. As in the full space case, we find
that these left tails remain valid at all times. In addition, we present here
(i) a new Fredholm Pfaffian formula for the solution of the hard wall boundary
condition and (ii) two Fredholm determinant representations for the solutions
of the hard wall and the symmetric boundary respectively. | cond-mat |
Probing superfluid $^4\mathrm{He}$ with high-frequency nanomechanical
resonators down to $\mathrm{mK}$ temperatures: Superfluids, such as superfluid $^3\mathrm{He}$ and $^4\mathrm{He}$, exhibit
a broad range of quantum phenomena and excitations which are unique to these
systems. Nanoscale mechanical resonators are sensitive and versatile force
detectors with the ability to operate over many orders of magnitude in damping.
Using nanomechanical-doubly clamped beams of extremely high quality factors
($Q>10^6$), we probe superfluid $^4\mathrm{He}$ from the superfluid transition
temperature down to $\mathrm{mK}$ temperatures at frequencies up to $11.6 \,
\mathrm{MHz}$. Our studies show that nanobeam damping is dominated by
hydrodynamic viscosity of the normal component of $^4\mathrm{He}$ above
$1\,\mathrm{K}$. In the temperature range $0.3-0.8\,\mathrm{K}$, the ballistic
quasiparticles (phonons and rotons) determine the beams' behavior. At lower
temperatures, damping saturates and is determined either by magnetomotive
losses or acoustic emission into helium. It is remarkable that all these
distinct regimes can be extracted with just a single device, despite damping
changing over six orders of magnitude. | cond-mat |
KMC-MD Investigations of Hyperthermal Copper Deposition on Cu(111): Detailed KMC-MD (kinetic Monte Carlo-molecular dynamics) simulations of
hyperthermal energy (10-100 eV) copper homoepitaxy have revealed a re-entrant
layer-by-layer growth mode at low temperatures (50K) and reasonable fluxes (1
ML/s). This growth mode is the result of atoms with hyperthermal kinetic
energies becoming inserted into islands when the impact site is near a step
edge. The yield for atomic insertion as calculated with molecular dynamics near
(111) step edges reaches a maxima near 18 eV. KMC-MD simulations of growing
films and a minima in the RMS roughness as a function of energy near 25 eV. We
find that the RMS roughness saturates just beyond 0.5 ML of coverage in films
grown with energies greater than 25 eV due to the onset of adatom-vacancy
formation near 20 eV. Adatom-vacancy pairs increase the island nuclei density
and the step edge density which increases the number of sites available to
insert atoms. Smoothest growth in this regime is achieved by maximizing island
and step edge densities, which consequently reverses the traditional roles of
temperature and flux: low temperatures and high fluxes produce the smoothest
surfaces in these films. Dramatic increases in island densities are found to
persist at room temperature,where island densities increase an order of
magnitude from 20 to 150 eV. | cond-mat |
Significant enhancement of ferromagnetism in Zn$_{1-x}$Cr$_{x}$Te doped
with iodine as an n-type dopant: The effect of additional doping of charge impurities was investigated in a
ferromagnetic semiconductor Zn$_{1-x}$Cr$_{x}$Te. It was found that the doping
of iodine, which is expected to act as an n-type dopant in ZnTe, brought about
a drastic enhancement of the ferromagnetism in Zn$_{1-x}$Cr$_{x}$Te while the
grown films remained electrically insulating. In particular, at a fixed Cr
composition of x = 0.05, the ferromagnetic transition temperature Tc increased
up to 300K at maximum due to the iodine doping from Tc = 30K of the undoped
counterpart, while the ferromagnetism disappeared due to the doping of nitrogen
as a p-type dopant. The observed systematic correlation of ferromagnetism with
the doping of charge impurities of both p- and n-type, suggesting a key role of
the position of Fermi level within the impurity d-state, is discussed on the
basis of the double exchange interaction as a mechanism of ferromagnetism in
this material. | cond-mat |
Proposal for a two-channel quantum dot setup: Prediction for the
capacitance lineshape: We have made a detailed proposal for a two-channel quantum dot setup. The
energy scales in the problem are such that we are able to make connection with
the two-channel Anderson model, which, in spite of being well-known in the
context of heavy-Fermion systems remained theoretically elusive until recently
and lacked a mesoscopic realization. Verification of our precise and robust
predictions for the differential capacitance lineshape of the dot will provide
an experimental signature of the two-channel behavior. | cond-mat |
Universal Behavior of Entanglement in 2D Quantum Critical Dimer Models: We examine the scaling behavior of the entanglement entropy for the 2D
quantum dimer model (QDM) at criticality and derive the universal finite
sub-leading correction $\gamma_{QCP}$. We compute the value of $\gamma_{QCP}$
without approximation working directly with the wave function of a generalized
2D QDM at the Rokhsar-Kivelson QCP in the continuum limit. Using the replica
approach, we construct the conformal boundary state corresponding to the cyclic
identification of $n$-copies along the boundary of the observed region. We find
that the universal finite term is $\gamma_{QCP}=\ln R-1/2$ where $R$ is the
compactification radius of the bose field theory quantum Lifshitz model, the
effective field theory of the 2D QDM at quantum criticality. We also
demonstrated that the entanglement spectrum of the critical wave function on a
large but finite region is described by the characters of the underlying
conformal field theory. It is shown that this is formally related to the
problems of quantum Brownian motion on $n$-dimensional lattices or equivalently
a system of strings interacting with a brane containing a background
electromagnetic field and can be written as an expectation value of a vertex
operator. | cond-mat |
Semiclassical analysis of edge state energies in the integer quantum
Hall effect: Analysis of edge-state energies in the integer quantum Hall effect is carried
out within the semiclassical approximation. When the system is wide so that
each edge can be considered separatly, this problem is equivalent to that of a
one dimensional harmonic oscillator centered at x=x_c and an infinite wall at
x=0, and appears in numerous physical contexts. The eigenvalues E_n(x_c) for a
given quantum number n are solutions of the equation S(E,x_c)=\pi [n+
\gamma(E,x_c)] where S is the WKB action and 0<\gamma<1 encodes all the
information on the connection procedure at the turning points.
A careful implication of the WKB connection formulae results in an excellent
approximation to the exact energy eigenvalues. The dependence of \gamma
[E_n(x_c),x_c] \equiv \gamma_c (x_c) on x_c is analyzed between its two extreme
values 1/2 as x_c goes to -infinity far inside the sample and 3/4 as x_c goes
to infinity far outside the sample. The edge-state energies E_n(x_c) obey an
almost exact scaling law of the form E_n(x_c)=4 [n+\gamma_n(x_c)] f(x_c/4 n +3)
and the scaling function f(y) is explicitly elucidated | cond-mat |
Thermodynamics and criticality of su($m$) spin chains of Haldane-Shastry
type: We study the thermodynamics and critical behavior of su($m$) spin chains of
Haldane-Shastry type at zero chemical potential, both in the $A_{N-1}$ and
$BC_N$ cases. We evaluate in closed form the free energy per spin for arbitrary
values of $m$, from which we derive explicit formulas for the energy, entropy
and specific heat per spin. In particular, we find that the specific heat
features a single Schottky peak, whose temperature is well approximated for
$m\lesssim10$ by the corresponding temperature for an $m$-level system with
uniformly spaced levels. We show that at low temperatures the free energy per
spin of the models under study behaves as that of a one-dimensional conformal
field theory with central charge $c=m-1$ (with the only exception of the
Frahm-Inozemtsev chain with zero value of its parameter). However, from a
detailed study of the ground state degeneracy and the low-energy excitations,
we conclude that these models are only critical in the antiferromagnetic case,
with a few exceptions that we fully specify. | cond-mat |
Damage in porous media due to salt crystallization: We investigate the origins of salt damage in sandstones for the two most
common salts: sodium chloride and sulfate. The results show that the observed
difference in damage between the two salts is directly related to the kinetics
of crystallization and the interfacial properties of the salt solutions and
crystals with respect to the stone. We show that, for sodium sulfate, the
existence of hydrated and anhydrous crystals and specifically their dissolution
and crystallization kinetics are responsible for the damage. Using magnetic
resonance imaging and optical microscopy we show that when water imbibes sodium
sulfate contaminated sandstones, followed by drying at room temperature, large
damage occurs in regions where pores are fully filled with salts. After partial
dissolution, anhydrous sodium sulfate salt present in these regions gives rise
to a very rapid growth of the hydrated phase of sulfate in the form of clusters
that form on or close to the remaining anhydrous microcrystals. The rapid
growth of these clusters generates stresses in excess of the tensile strength
of the stone leading to the damage. Sodium chloride only forms anhydrous
crystals that consequently do not cause damage in the experiments. | cond-mat |
Interstitial Transition Metal Doping in Hydrogen Saturated Silicon
Nanowires: We report a first principles systematic study of atomic, electronic, and
magnetic properties of hydrogen saturated silicon nanowires (H-SiNW) which are
doped by transition metal (TM) atoms placed at various interstitial sites. Our
results obtained within the conventional GGA+U approach have been confirmed
using an hybrid functional. In order to reveal the surface effects we examined
three different possible facets of H-SiNW along [001] direction with a diameter
of ~2nm. The energetics of doping and resulting electronic and magnetic
properties are examined for all alternative configurations. We found that
except Ti, the resulting systems have magnetic ground state with a varying
magnetic moment. While H-SiNWs are initially non-magnetic semiconductor, they
generally become ferromagnetic metal upon TM doping. Even they posses
half-metallic behavior for specific cases. Our results suggest that H-SiNWs can
be functionalized by TM impurities which would lead to new electronic and
spintronic devices at nanoscale. | cond-mat |
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