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High-efficient thermoelectric materials: The case of orthorhombic IV-VI
compounds: Improving the thermoelectric efficiency is one of the greatest challenges in
materials science. The recent discovery of excellent thermoelectric performance
in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao
et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of
orthorhombic IV-VI compounds GeS,GeSe,SnS,and SnSe based on the
first-principles combined with the Boltzmann transport theory, we show that the
Seebeck coefficient, electrical conductivity, and thermal conductivity of
orthorhombic SnSe are in agreement with the recent experiment. Importantly,
GeS,GeSe,and SnS exhibit comparative thermoelectric performance compared to
SnSe. Especially, the Seebeck coefficients of GeS,GeSe,and SnS are even larger
than that of SnSe under the studied carrier concentration and temperature
region. We also use the Cahill's model to estimate the lattice thermal
conductivities at the room temperature. The large Seebeck coefficients, high
power factors, and low thermal conductivities make these four orthorhombic
IV-VI compounds promising candidates for high-efficient thermoelectric
materials. | cond-mat |
Dielectric function of CuBr$_\mathrm{x}$I$_{1-\mathrm{x}}$ alloy thin
films: We study the dielectric function of CuBr$_\mathrm{x}$I$_{1-\mathrm{x}}$ thin
film alloys using spectroscopic ellipsometry in the spectral range between 0.7
eV to 6.4 eV, in combination with first-principles calculations based on
density functional theory. Through the comparison of theory and experiment, we
attribute features in the dielectric function to electronic transitions at
specific k-points in the Brillouin zone. The observed bandgap bowing as a
function of alloy composition is discussed in terms of different physical and
chemical contributions. The band splitting at the top of the valence band due
to spin-orbit coupling is found to decrease with increasing Br-concentration,
from a value of 660 meV for CuI to 150 meV for CuBr. This result can be
understood considering the contribution of copper d-orbitals to the valence
band maximum as a function of the alloy composition. | cond-mat |
Thermoelectric response near a quantum critical point: the case of
CeCoIn5: We present a study of thermoelectric coefficients in CeCoIn_5 down to 0.1 K
and up to 16 T in order to probe the thermoelectric signatures of quantum
criticality. In the vicinity of the field-induced quantum critical point, the
Nernst coefficient nu exhibits a dramatic enhancement without saturation down
to lowest measured temperature. The dimensionless ratio of Seebeck coefficient
to electronic specific heat shows a minimum at a temperature close to threshold
of the quasiparticle formation. Close to T_c(H), in the vortex-liquid state,
the Nernst coefficient behaves anomalously in puzzling contrast with other
superconductors and standard vortex dynamics. | cond-mat |
Impurity electrons in narrow electric field-biased armchair graphene
nanoribbons: We present an analytical investigation of the quasi-Coulomb impurity states
in a narrow gapped armchair graphene nanoribbon (GNR) in the presence of a
uniform external electric field directed parallel to the ribbon axis. The
effect of the ribbon confinement is taken to be much greater than that of the
impurity electric field, which in turn considerably exceeds the external
electric field. Under these conditions we employ the adiabatic approximation
assuming that the motion parallel ("slow") and perpendicular ("fast") to the
ribbon axis are separated adiabatically. In the approximation of the isolated
size-quantized subbands induced by the "fast" motion the complex energies of
the impurity electron are calculated in explicit form. The real and imaginary
parts of these energies determine the binding energy and width of the
quasi-discrete state, respectively. The energy width increases with increasing
the electric field and ribbon width. The latter forms the background of the
mechanism of dimensional ionization. The S-matrix - the basic tool of study of
the transport problems can be trivially derived from the phases of the wave
functions of the continuous spectrum presented in explicit form. In the
double-subband approximation we calculate the complete widths of the impurity
states caused by the combined effect of the electric field and the Fano
resonant coupling between the impurity states of the discrete and continuous
spectra associated with the ground and first excited size-quantized subbands. | cond-mat |
Universal Dynamic Magnetism in Yb-Pyrochlores with Disparate Ground
States: The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described
by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra
and interacting via anisotropic exchange. Structurally, only the non-magnetic
B-site cation, and hence, primarily the lattice parameter, is changing across
the series. Nonetheless, a range of magnetic behaviors are observed: the low
temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character,
while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low
temperatures. While the static properties of the ytterbium pyrochlores are
distinct, inelastic neutron scattering measurements reveal a common character
to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless
continuum of spin excitations, resembling over-damped ferromagnetic spin waves
at low Q. Furthermore, the specific heat of the series also follows a common
form, with a broad, high-temperature anomaly followed by a sharp
low-temperature anomaly at T_C or T_N. The novel spin dynamics we report
correlate strongly with the broad specific heat anomaly only, remaining
unchanged across the sharp anomaly. This result suggests that the primary order
parameter in the ytterbium pyrochlores associated with the sharp anomaly is
"hidden" and not simple magnetic dipole order. | cond-mat |
Relaxation of frustration and gap enhancement by the lattice distortion
in the $Δ$ chain: We clarify an instability of the ground state of the $\Delta$ chain against
the lattice distortion that increases a strength $(\lambda)$ of a bond in each
triangle. It relaxes the frustration and causes a remarkable gap enhancement;
only a $6\%$ increase of $\lambda$ causes the gap doubled from the
fully-frustrated case $(\lambda=1)$. The lowest excitation is revealed to be a
kink-antikink bound state whose correlation length decreases drastically with
$\lambda$ increase. The enhancement follows a power law, $\Delta E_{\rm
gap}\sim (\lambda-1) + 1.44 (\lambda -1)^{\frac{2}{3}}$, which can be obtained
from the exact result of the continuous model. This model describes a spin gap
behavior of the delafossite YCuO$_{2.5}$. | cond-mat |
Direct visualization of phase separation between superconducting and
nematic domains in Co-doped CaFe2As2 close to a first order phase transition: We show that biaxial strain induces alternating tetragonal superconducting
and orthorhombic nematic domains in Co substituted CaFe2As2. We use Atomic
Force, Magnetic Force and Scanning Tunneling Microscopy (AFM, MFM and STM) to
identify the domains and characterize their properties, finding in particular
that tetragonal superconducting domains are very elongated, more than several
tens of micron long and about 30 nm wide, have the same Tc than unstrained
samples and hold vortices in a magnetic field. Thus, biaxial strain produces a
phase separated state, where each phase is equivalent to what is found at
either side of the first order phase transition between antiferromagnetic
orthorhombic and superconducting tetragonal phases found in unstrained samples
when changing Co concentration. Having such alternating superconducting domains
separated by normal conducting domains with sizes of order of the coherence
length opens opportunities to build Josephson junction networks or vortex
pinning arrays and suggests that first order quantum phase transitions lead to
nanometric size phase separation under the influence of strain. | cond-mat |
Revealing frustrated local moment model for pressurized hyperhoneycomb
iridate: paving a way toward quantum spin liquid: There have been tremendous experimental and theoretical efforts toward
discovery of quantum spin liquid phase in honeycomb-based-lattice materials
with strong spin-orbit coupling. Here the bond-dependent Kitaev interaction
between local moments provides strong magnetic frustration and if it is the
only interaction present in the system, it will lead to an exactly solvable
quantum spin liquid ground state. In all of these materials, however, the
ground state is in a magnetically ordered phase due to additional interactions
between local moments. Recently, it has been reported that the magnetic order
in hyperhoneycomb material, $\beta$-Li$_2$IrO$_3$, is suppressed upon applying
hydrostatic pressure and the resulting state becomes a quantum paramagnet or
possibly a quantum spin liquid. Using ab-initio computations and strong
coupling expansion, we investigate the lattice structure and resulting local
moment model in pressurized $\beta$-Li$_2$IrO$_3$. Remarkably, the dominant
interaction under high pressure is not the Kitaev interaction nor further
neighbor interactions, but a different kind of bond-dependent interaction. This
leads to strong magnetic frustration and may provide a platform for discovery
of a new kind of quantum spin liquid ground state. | cond-mat |
Comment on "Theoretical analysis of quantum turbulence using the Onsager
ideal turbulence theory'': In a recent paper [T. Tanogami Phys. Rev. E 103, 023106 ] proposes a scenario
for quantum turbulence where the energy spectrum at scales smaller than the
inter-vortex distance is dominated by a quantum stress cascade, in opposition
to Kelvin wave cascade predictions. The purpose of the present comment is to
highlight some physical issues in the derivation of the quantum stress cascade,
in particular to stress that quantization of circulation has been ignored. | cond-mat |
Spectral and localization properties of random bipartite graphs: Bipartite graphs are often found to represent the connectivity between the
components of many systems such as ecosystems. A bipartite graph is a set of
$n$ nodes that is decomposed into two disjoint subsets, having $m$ and $n-m$
vertices each, such that there are no adjacent vertices within the same set.
The connectivity between both sets, which is the relevant quantity in terms of
connections, can be quantified by a parameter $\alpha\in[0,1]$ that equals the
ratio of existent adjacent pairs over the total number of possible adjacent
pairs. Here, we study the spectral and localization properties of such random
bipartite graphs. Specifically, within a Random Matrix Theory (RMT) approach,
we identify a scaling parameter $\xi\equiv\xi(n,m,\alpha)$ that fixes the
localization properties of the eigenvectors of the adjacency matrices of random
bipartite graphs. We also show that, when $\xi<1/10$ ($\xi>10$) the
eigenvectors are localized (extended), whereas the
localization--to--delocalization transition occurs in the interval
$1/10<\xi<10$. Finally, given the potential applications of our findings, we
round off the study by demonstrating that for fixed $\xi$, the spectral
properties of our graph model are also universal. | cond-mat |
Assembly of hard spheres in a cylinder: a computational and experimental
study: Hard spheres are an important benchmark of our understanding of natural and
synthetic systems. In this work, colloidal experiments and Monte Carlo
simulations examine the equilibrium and out-of-equilibrium assembly of hard
spheres of diameter $\sigma$ within cylinders of diameter $\sigma\leq D\leq
2.82\sigma$. Although in such a system phase transitions formally do not exist,
marked structural crossovers are observed. In simulations, we find that the
resulting pressure-diameter structural diagram echoes the densest packing
sequence obtained at infinite pressure in this range of $D$. We also observe
that the out-of-equilibrium self-assembly depends on the compression rate. Slow
compression approximates equilibrium results, while fast compression can skip
intermediate structures. Crossovers for which no continuous line-slip exists
are found to be dynamically unfavorable, which is the source of this
difference. Results from colloidal sedimentation experiments at high P\'eclet
number are found to be consistent with the results of fast compressions, as
long as appropriate boundary conditions are used. The similitude between
compression and sedimentation results suggests that the assembly pathway does
not here sensitively depend on the nature of the out-of-equilibrium dynamics. | cond-mat |
Time Constants of Spin-Dependent Recombination Processes: We present experiments to systematically study the time constants of
spin-dependent recombination processes in semiconductors using pulsed
electrically detected magnetic resonance (EDMR). The combination of
time-programmed optical excitation and pulsed spin manipulation allows us to
directly measure the recombination time constants of electrons via localized
spin pairs and the time constant of spin pair formation as a function of the
optical excitation intensity. Using electron nuclear double resonance, we show
that the time constant of spin pair formation is determined by an electron
capture process. Based on these time constants we devise a set of rate
equations to calculate the current transient after a resonant microwave pulse
and compare the results with experimental data. Finally, we critically discuss
the effects of different boxcar integration time intervals typically used to
analyze pulsed EDMR experiments on the determination of the time constants. The
experiments are performed on phosphorus-doped silicon, where EDMR via spin
pairs formed by phosphorus donors and Si/SiO2 interface dangling bond defects
is detected. | cond-mat |
The Tin Pest Problem as a Test of Density Functionals Using
High-Throughput Calculations: At ambient pressure tin transforms from its ground-state semi-metal
$\alpha$-Sn (diamond structure) phase to the compact metallic $\beta$-Sn phase
at 13$^\circ$C (286K). There may be a further transition to the simple
hexagonal $\gamma$-Sn above 450K. These relatively low transition temperatures
are due to the small energy differences between the structures, $\approx
20$\,meV/atom between $\alpha$- and $\beta$-Sn. This makes tin an exceptionally
sensitive test of the accuracy of density functionals and computational
methods. Here we use the high-throughput Automatic-FLOW (AFLOW) method to study
the energetics of tin in multiple structures using a variety of density
functionals. We look at the successes and deficiencies of each functional. As
no functional is completely satisfactory, we look Hubbard U corrections and
show that the Coulomb interaction can be chosen to predict the correct phase
transition temperature. We also discuss the necessity of testing
high-throughput calculations for convergence for systems with small energy
differences. | cond-mat |
Field-induced reorientation of helimagnetic order in Cu$_2$OSeO$_3$
probed by magnetic force microscopy: Cu$_2$OSeO$_3$ is an insulating skyrmion-host material with a magnetoelectric
coupling giving rise to an electric polarization with a characteristic
dependence on the magnetic field $\vec H$. We report magnetic force microscopy
imaging of the helical real-space spin structure on the surface of a bulk
single crystal of Cu$_2$OSeO$_3$. In the presence of a magnetic field, the
helimagnetic order in general reorients and acquires a homogeneous component of
the magnetization, resulting in a conical arrangement at larger fields. We
investigate this reorientation process at a temperature of 10~K for fields
close to the crystallographic $\langle 110\rangle$ direction that involves a
phase transition at $H_{c1}$. Experimental evidence is presented for the
formation of magnetic domains in real space as well as for the microscopic
origin of relaxation events that accompany the reorientation process. In
addition, the electric polarization is measured by means of Kelvin-probe force
microscopy. We show that the characteristic field dependency of the electric
polarization originates in this helimagnetic reorientation process. Our
experimental results are well described by an effective Landau theory
previously invoked for MnSi, that captures the competition between
magnetocrystalline anisotropies and Zeeman energy. | cond-mat |
Excitonic giant Zeeman effect in GaN:Mn^3+: We describe a direct observation of the excitonic giant Zeeman splitting in
(Ga,Mn)N, a wide-gap III-V diluted magnetic semiconductor. Reflectivity and
absorption spectra measured at low temperatures display the A and B excitons,
with a shift under magnetic field due to s,p-d exchange interactions. Using an
excitonic model, we determine the difference of exchange integrals between
Mn^3+ and free carriers in GaN, N_0(alpha-beta)=-1.2 +/- 0.2 eV. Assuming a
reasonable value of alpha, this implies a positive sign of beta which
corresponds to a rarely observed ferromagnetic interaction between the magnetic
ions and the holes. | cond-mat |
Temperature-induced nanostructural evolution of hydrogen-rich voids in
amorphous silicon: A first-principles study: The paper presents an $ab$ $initio$ study of temperature-induced
nanostructural evolution of hydrogen-rich voids in amorphous silicon. By using
large $a$-Si models, obtained from classical molecular-dynamics simulations,
with a realistic void-volume density of 0.2%, the dynamics of Si and H atoms on
the surface of the nanometer-size cavities were studied and their effects on
the shape and size of the voids were examined using first-principles
density-functional simulations. The results from $ab$ $initio$ calculations
were compared with those obtained from using the modified Stillinger-Weber
potential. The temperature-induced nanostructural evolution of the voids was
examined by analyzing the three-dimensional distribution of Si and H atoms
on/near void surfaces using the convex-hull approximation, and computing the
radius of gyration of the corresponding convex hulls. A comparison of the
results with those from the simulated values of the intensity in small-angle
X-ray scattering of $a$-Si/$a$-Si:H in the Guinier approximation is also
provided, along with a discussion on the dynamics of bonded and non-bonded
hydrogen in the vicinity of voids. | cond-mat |
Two-Step Discontinuous Shear Thickening of Dilute Inertial Suspensions
Having Soft-Core Potential: Kinetic theory for dilute inertial suspension having soft-core potential is
theoretically investigated. From the analysis of the scattering process, the
expression of the scattering angle is analytically obtained. We derive the flow
curve between the viscosity and the shear rate, which shows two-step
discontinuous shear thickening when we change the softness of the particles.
The molecular dynamics simulation shows that our theoretical results are
consistent with the numerical ones. | cond-mat |
Field theory of absorbing phase transitions with a non-diffusive
conserved field: We investigate the critical behavior of a reaction-diffusion system
exhibiting a continuous absorbing-state phase transition. The
reaction-diffusion system strictly conserves the total density of particles,
represented as a non-diffusive conserved field, and allows an infinite number
of absorbing configurations. Numerical results show that it belongs to a wide
universality class that also includes stochastic sandpile models. We derive
microscopically the field theory representing this universality class. | cond-mat |
Topological Insulator VxBi1.08-xSn0.02Sb0.9Te2S as a Promising n-type
Thermoelectric Material: As one of the most important n-type thermoelectric (TE) materials, Bi2Te3 has
been studied for decades, with efforts to enhance the thermoelectric
performance based on element doping, band engineering, etc. In this study, we
report a novel bulk-insulating topological material system as a replacement for
n-type Bi2Te3 materials: V doped Bi1.08Sn0.02Sb0.9Te2S (V:BSSTS) . The V:BSSTS
is a bulk insulator with robust metallic topological surface states.
Furthermore, the bulk band gap can be tuned by the doping level of V, which is
verified by magnetotransport measurements. Large linear magnetoresistance is
observed in all samples. Excellent thermoelectric performance is obtained in
the V:BSSTS samples, e.g., the highest figure of merit ZT of ~ 0.8 is achieved
in the 2% V doped sample (denoted as V0.02) at 550 K. The high thermoelectric
performance of V:BSSTS can be attributed to two synergistic effects: (1) the
low conductive secondary phases Sb2S3, and V2S3 are believed to be important
scattering centers for phonons, leading to lower lattice thermal conductivity;
and (2) the electrical conductivity is increased due to the high-mobility
topological surface states at the boundaries. In addition, by replacing one
third of costly tellurium with abundant, low-cost, and less-toxic sulfur
element, the newly produced BSSTS material is inexpensive but still has
comparable TE performance to the traditional Bi2Te3-based materials, which
offers a cheaper plan for the electronics and thermoelectric industries. Our
results demonstrate that topological materials with unique band structures can
provide a new platform in the search for new high performance TE materials. | cond-mat |
Relativistic Nonextensive Thermodynamics: Starting from the basic prescriptions of the Tsallis' nonextensive
thermostatistics, i.e. generalized entropy and normalized q-expectation values,
we study the relativistic nonextensive thermodynamics and derive a Boltzmann
transport equation that implies the validity of the H-theorem where a local
nonextensive four-entropy density is considered. Macroscopic thermodynamic
functions and the equation of state for a perfect gas are derived at the
equilibrium. | cond-mat |
NMR of liquid 3He in clay pores at 1.5 K: In the present work a new method for studying porous media by nuclear
magnetic resonance of liquid 3He has been proposed. This method has been
demonstrated in an example of a clay mineral sample. For the first time the
integral porosity of clay sample has been measured. For investigated samples
the value of integral porosity is in the range of 10-30%. Inverse Laplace
transform of 3He longitudinal magnetization recovery curve has been carried
out, thus distribution of relaxation times T1 has been obtained. | cond-mat |
Collusion of Interactions and Disorder at the Superfluid-Insulator
Transition: A Dirty 2d Quantum Critical Point: We study the stability of the Wilson-Fisher fixed point of the quantum
$\mathrm{O}(2N)$ vector model to quenched disorder in the large-$N$ limit.
While a random mass is strongly relevant at the Gaussian fixed point, its
effect is screened by the strong interactions of the Wilson-Fisher fixed point.
This enables a perturbative renormalization group study of the interplay of
disorder and interactions about this fixed point. We show that, in contrast to
the spiralling flows obtained in earlier double-$\epsilon$ expansions, the
theory flows directly to a quantum critical point characterized by finite
disorder and interactions. The critical exponents we obtain for this transition
are in remarkable agreement with numerical studies of the superfluid-Mott glass
transition. We additionally discuss the stability of this fixed point to scalar
and vector potential disorder and use proposed boson-fermion dualities to make
conjectures regarding the effects of weak disorder on dual Abelian Higgs and
Chern-Simons-Dirac fermion theories when $N=1$. | cond-mat |
Oscillatory Thickness Dependence of the Coercive Field in Magnetic 3D
Anti-Dot Arrays: We present studies on magnetic nano-structures with 3D architectures,
fabricated using electrodeposition in the pores of well-ordered templates
prepared by self-assembly of polystyrene latex spheres. The coercive field is
found to demonstrate an oscillatory dependence on film thickness reflecting the
patterning transverse to the film plane. Our results demonstrate that 3D
patterned magnetic materials are prototypes of a new class of geometrical
multilayer structures in which the layering is due to local shape effects
rather then compositional differences. | cond-mat |
On the elastic-wave imaging and characterization of fractures with
specific stiffness: The concept of topological sensitivity (TS) is extended to enable
simultaneous 3D reconstruction of fractures with unknown boundary condition and
characterization of their interface by way of elastic waves. Interactions
between the two surfaces of a fracture, due to e.g. presence of asperities,
fluid, or proppant, are described via the Schoenberg's linear slip model. The
proposed TS sensing platform is formulated in the frequency domain, and entails
point-wise interrogation of the subsurface volume by infinitesimal fissures
endowed with interfacial stiffness. For completeness, the featured elastic
polarization tensor - central to the TS formula - is mathematically described
in terms of the shear and normal specific stiffness (ks,kn) of a vanishing
fracture. Simulations demonstrate that, irrespective of the contact condition
between the faces of a hidden fracture, the TS (used as a waveform imaging
tool) is capable of reconstructing its geometry and identifying the normal
vector to the fracture surface without iterations. On the basis of such
geometrical information, it is further shown via asymptotic analysis --
assuming "low frequency" elastic-wave illumination, that by certain choices of
(ks,kn) characterizing the trial (infinitesimal) fracture, the ratio between
the shear and normal specific stiffness along the surface of a nearly-panar
(finite) fracture can be qualitatively identified. This, in turn, provides a
valuable insight into the interfacial condition of a fracture at virtually no
surcharge -- beyond the computational effort required for its imaging. The
proposed developments are integrated into a computational platform based on a
regularized boundary integral equation (BIE) method for 3D elastodynamics, and
illustrated via a set of numerical experiments. | cond-mat |
Quantitatively consistent, scale-spanning model for same-material
tribocharging: By rigorously accounting for mesoscale spatial correlations in donor/acceptor
surface properties, we develop a scale-spanning model for same-material
tribocharging. We find that mesoscale correlations affect not only the
magnitude of charge transfer but also the fluctuations-suppressing otherwise
overwhelming charge-transfer variability that is not observed experimentally.
We furthermore propose a generic theoretical mechanism by which the mesoscale
features might emerge, which is qualitatively consistent with other proposals
in the literature. | cond-mat |
Effect of the iron valence in the two types of layers in
LiFeO$_2$Fe$_2$Se$_2$: We perform electronic structure calculations for the recently synthesized
iron-based superconductor LiFeO$_2$Fe$_2$Se$_2$. In contrast to other
iron-based superconductors, this material comprises two different iron atoms in
3$d^5$ and 3$d^6$ configurations. In band theory, both contribute to the
low-energy electronic structure. Spin-polarized density functional theory
calculations predict an antiferromagnetic metallic ground state with different
moments on the two Fe sites. However, several other almost degenerate magnetic
configurations exist. Due to their different valences, the two iron atoms
behave very differently when local quantum correlations are included through
the dynamical mean-field theory. The contributions from the half-filled 3$d^5$
atoms in the LiFeO$_2$ layer are suppressed and the 3$d^6$ states from the FeSe
layer restore the standard iron-based superconductor fermiology. | cond-mat |
A threshold model of plastic waste fragmentation: New insights into the
distribution of microplastics in the ocean and its evolution over time: Plastic pollution in the aquatic environment has been assessed for many years
by ocean waste collection expeditions around the globe or by river sampling.
While the total amount of plastic produced worldwide is well documented, the
amount of plastic found in the ocean, the distribution of particles on its
surface and its evolution over time are still the subject of much debate. In
this article, we propose a general fragmentation model, postulating the
existence of a critical size below which particle fragmentation becomes
extremely unlikely. In the frame of this model, an abundance peak appears for
sizes around 1mm, in agreement with real environmental data. Using, in
addition, a realistic exponential waste feed to the ocean, we discuss the
relative impact of fragmentation and feed rates, and the temporal evolution of
microplastics (MP) distribution. New conclusions on the temporal trend of MP
pollution are drawn. | cond-mat |
The influence of structural variations on the constitutive response and
strain variations in thin fibrous materials: The stochastic variations in the structural properties of thin fiber networks
govern to a great extent their mechanical performance. To assess the influence
of local structural variability on the local strain and mechanical response of
the network, we propose a multiscale approach combining modeling, numerical
simulation and experimental measurements. Based on micro-mechanical fiber
network simulations, a continuum model describing the response at the mesoscale
level is first developed. Experimentally measured spatial fields of thickness,
density, fiber orientation and anisotropy are thereafter used as input to a
macroscale finite-element model. The latter is used to simulate the impact of
spatial variability of each of the studied structural properties. In addition,
this work brings novelty by including the influence of the drying condition
during the production process on the fiber properties. The proposed approach is
experimentally validated by comparison to measured strain fields and uniaxial
responses. The results suggest that the spatial variability in density presents
the highest impact on the local strain field followed by thickness and fiber
orientation. Meanwhile, for the mechanical response, the fiber orientation
angle with respect to the drying restraints is the key influencer and its
contribution to the anisotropy of the mechanical properties is greater than the
contribution of the fiber anisotropy developed during the fiber sheet-making. | cond-mat |
Reversible Diffusion-Limited Reactions: "Chemical Equilibrium" State and
the Law of Mass Action Revisited: The validity of two fundamental concepts of classical chemical kinetics - the
notion of "Chemical Equilibrium" and the "Law of Mass Action" - are re-examined
for reversible \textit{diffusion-limited} reactions (DLR), as exemplified here
by association/dissociation $A+A \rightleftharpoons B$ reactions. We consider a
general model of long-ranged reactions, such that any pair of $A$ particles,
separated by distance $\mu$, may react with probability $\omega_+(\mu)$, and
any $B$ may dissociate with probability $\omega_-(\lambda)$ into a geminate
pair of $A$s separated by distance $\lambda$. Within an exact analytical
approach, we show that the asymptotic state attained by reversible DLR at $t =
\infty$ is generally \textit{not a true thermodynamic equilibrium}, but rather
a non-equilibrium steady-state, and that the Law of Mass Action is invalid. The
classical picture holds \text{only} in physically unrealistic case when
$\omega_+(\mu) \equiv \omega_-(\mu)$ for any value of $\mu$. | cond-mat |
Spin-Valley Coherent Phases of the $ν=0$ Quantum Hall State in Bilayer
Graphene: Bilayer graphene (BLG) offers a rich platform for broken symmetry states
stabilized by interactions. In this work we study the phase diagram of BLG in
the quantum Hall regime at filling factor $\nu=0$ within the Hartree-Fock
approximation. In the simplest non-interacting situation this system has eight
(nearly) degenerate Landau levels near the Fermi energy, characterized by spin,
valley, and orbital quantum numbers. We incorporate in our study two effects
not previously considered: (i) the nonperturbative effect of trigonal warping
in the single-particle Hamiltonian, and (ii) short-range SU(4)
symmetry-breaking interactions that distinguish the energetics of the orbitals.
We find within this model a rich set of phases, including ferromagnetic,
layer-polarized, canted antiferromagnetic, Kekul\'e, a "spin-valley entangled"
state, and a "broken U(1) $\times$ U(1)" phase. This last state involves
independent spontaneous symmetry breaking in the layer and valley degrees of
freedom, and has not been previously identified. We present phase diagrams as a
function of interlayer bias $D$ and perpendicular magnetic field $B_{\perp}$
for various interaction and Zeeman couplings, and discuss which are likely to
be relevant to BLG in recent measurements. Experimental properties of the
various phases and transitions among them are also discussed. | cond-mat |
Classification of Abelian and Non-Abelian Multilayer Fractional Quantum
Hall States Through the Pattern of Zeros: A large class of fractional quantum Hall (FQH) states can be classified
according to their pattern of zeros, which describes the way ideal ground state
wave functions go to zero as various clusters of electrons are brought
together. In this paper we generalize this approach to classify multilayer FQH
states. Such a classification leads to the construction of a class of
non-Abelian multilayer FQH states that are closely related to $\hat{g}_k$
parafermion conformal field theories, where $\hat{g}_k$ is an affine simple Lie
algebra. We discuss the possibility of some of the simplest of these
non-Abelian states occuring in experiments on bilayer FQH systems at $\nu =
2/3$, 4/5, 4/7, etc. | cond-mat |
Temperature Induced Shifts of Yu-Shiba-Rusinov Resonances in
Nanowire-Based Hybrid Quantum Dots: The strong coupling of a superconductor to a spinful quantum dot results in
Yu-Shiba-Rusinov (YSR) discrete subgap excitations. In isolation and at zero
temperature, the excitations are $\delta$ resonances. In transport experiments,
however, they show as broad differential conductance peaks. We obtain the
lineshape of the peaks and their temperature dependence in
superconductor-quantum-dot-metal (S-QD-N) nanowire-based devices. Unexpectedly,
we find that the peaks shift in energy with temperature, with the shift
magnitude and sign depending on ground state parity and bias voltage.
Additionally, we empirically find a power-law scaling of the peak area versus
temperature. These observations are not explained by current models. | cond-mat |
Duality in quantum transport models: We develop the `duality approach', that has been extensively studied for
classical models of transport, for quantum systems in contact with a thermal
`Lindbladian' bath. The method provides (a) a mapping of the original model to
a simpler one, containing only a few particles and (b) shows that any dynamic
process of this kind with generic baths may be mapped onto one with equilibrium
baths. We exemplify this through the study of a particular model: the quantum
symmetric exclusion process introduced in [D. Bernard, T. Jin, Phys. Rev. Lett.
123, 080601 (2019)]. As in the classical case, the whole construction becomes
intelligible by considering the dynamical symmetries of the problem. | cond-mat |
Entangled quantum currents in distant mesoscopic Josephson junctions: Two mesoscopic SQUID rings which are far from each other, are considered. A
source of two-mode nonclassical microwaves irradiates the two rings with
correlated photons. The Josephson currents are in this case quantum mechanical
operators, and their expectation values with respect to the density matrix of
the microwaves, yield the experimentally observed currents. Classically
correlated (separable) and quantum mechanically correlated (entangled)
microwaves are considered, and their effect on the Josephson currents is
quantified. Results for two different examples that involve microwaves in
number states and coherent states are derived. It is shown that the quantum
statistics of the tunnelling electron pairs through the Josephson junctions in
the two rings, are correlated. | cond-mat |
Hot Brownian Motion of thermoresponsive microgels in optical tweezers
shows discontinuous volume phase transition and bistability: Microgels are soft microparticles that often exhibit thermoresponsiveness and
feature a transformation at a critical temperature, referred to as the volume
phase transition temperature. The question of whether this transformation
occurs as a smooth or as a discontinuous one is still a matter of debate. This
question can be addressed by studying individual microgels trapped in optical
tweezers. For this aim, composite particles were obtained by decorating pNIPAM
microgels with iron oxide nanocubes. These composites become self-heating when
illuminated by the infrared trapping laser, featuring Hot Brownian Motion
within the trap. Above a certain laser power, a single decorated microgel
features a volume phase transition that is discontinuous, while the usual
continuous sigmoidal-like dependence is recovered after averaging over
different microgels. The collective sigmoidal behavior enables the application
of a power-to-temperature calibration and provides the effective drag
coefficient of the self-heating microgels, thus establishing these composite
particles as potential micro-thermometers and micro-heaters. Moreover, the
self-heating microgels also exhibit an unexpected and intriguing bistability
behavior above the critical temperature, probably due to partial collapses of
the microgel. These results set the stage for further studies and the
development of applications based on the Hot Brownian Motion of soft particles. | cond-mat |
Hydration at highly crowded interfaces: Understanding the molecular and electronic structure of electrolytes at
interfaces requires an analysis of the interactions between the electrode
surface, the ions, and the solvent environment on equal footing. Here, we
tackle this challenge by exploring the initial stages of Cs+ hydration on a
Cu(111) surface by combining experiment and theory. Remarkably, we observe
"inside out" solvation of Cs ions, i.e, their preferential location at the
perimeter of the water clusters on the metal surface. In addition, water-Cs
complexes containing multiple Cs+ ions are observed to form at these surfaces.
Established models based on maximum ion-water coordination and the double layer
notion cannot account for this situation and the complex interplay of
microscopic interactions is the key to a fundamental understanding. | cond-mat |
Stiffening graphene by controlled defect creation: Graphene extraordinary strength, stiffness and lightness have generated great
expectations towards its application in flexible electronics and as mechanical
reinforcement agent. However, the presence of lattice defects, unavoidable in
sheets obtained by scalable routes, might degrade its mechanical properties.
Here we report a systematic study on the elastic modulus and strength of
graphene with controlled density of defects. Counter intuitively, the in-plane
Young modulus increases with increasing defect density up to almost twice the
initial value for vacancy content of ~0.2%, turning it into the stiffest
material ever reported. For higher density of vacancies, elastic modulus
decreases with defect inclusion. The initial increase in Young modulus is
explained in terms of a dependence of the elastic coefficients with the
momentum of flexural modes predicted for 2D membranes. In contrast, the
fracture strength decreases with defect density according to standard fracture
continuum models. These quantitative structure-property relationships, measured
in atmospheric conditions, are of fundamental and technological relevance and
provide guidance for applications in which graphene mechanics represents a
disruptive improvement. | cond-mat |
FFLO or Majorana superfluids: The fate of fermionic cold atoms in
spin-orbit coupled optical lattices: The recent experimental realization of spin-orbit coupling (SOC) for
ultra-cold atoms opens a completely new avenue for exploring new quantum
matter. In experiments, the SOC is implemented simultaneously with a Zeeman
field. Such spin-orbit coupled Fermi gases are predicted to support Majorana
fermions with non-Abelian exchange statistics in one dimension (1D). However,
as shown in recent theory and experiments for 1D spin-imbalanced Fermi gases,
the Zeeman field can lead to the long-sought Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superfluids with non-zero momentum Cooper pairings, in contrast to the
zero momentum pairing in Majorana superfluids. Therefore a natural question to
ask is which phase, FFLO or Majorana superfluids, will survive in spin-orbit
coupled Fermi gases in the presence of a large Zeeman field. In this paper, we
address this question by studying the mean field quantum phases of 1D
(quasi-1D) spin-orbit coupled fermionic cold atom optical lattices. | cond-mat |
Thermodynamics of volume collapse transitions in cerium and related
compounds: We present a non-linear elastic model of a coherent transition with
discontinuous volume change in an isotropic solid. The model reproduces the
anomalous thermodynamics typical of coherent equilibrium including intrinsic
hysteresis (for a pressure driven experiment) and a negative bulk modulus. The
novelty of the model is that the statistical mechanics solution can be easily
worked out. We find that coherency leads to an infinite-range density--density
interaction, which drives classical critical behavior. The pressure width of
the hysteresis loop shrinks with increasing temperature, ending at a critical
point at a temperature related to the shear modulus. The bulk modulus softens
with a 1/2 exponent at the transition even far from the critical point. Many
well known features of the phase diagram of Ce and related systems are
explained by the model. | cond-mat |
Superconductivity in layered Zintl phase LiSn2As2: We report the superconductivity in the layered Zintl phase LiSn$_2$As$_2$,
which is isostructural to NaSn$_2$As$_2$ and has a transition temperature
($T_{\mathrm{c}}$) of 1.6 K. Despite similar $T_{\mathrm{c}}$ and Debye
temperatures, substituting of Na with Li considerably increases the upper
critical field. Based on a systematically comparation of Sn$_4$As$_3$, NaSnAs,
NaSn$_2$As$_2$,Na$_{1-x}$Sn$_2$P$_2$, SrSn$_2$As$_2$, and LiSn$_2$As$_2$, we
propose that carrier doping, intimately related to the formation of lone-pair
electrons, controls superconductivity in layered SnAs-based compounds rather
than chemical pressure. The current findings provide a thorough and
comprehensive understanding of Sn-based Zintl phase. | cond-mat |
Topological d-wave pairing structures in Jain states: We discuss d-wave topological (broken time reversal symmetry) pairing
structures in unpolarized and polarized Jain states. We demonstrate pairing in
the Jain spin singlet state by rewriting it in an explicit pairing form, in
which we can recognize d-wave weak pairing of underlying quasiparticles -
neutral fermions. We find and describe the root configuration of the Jain spin
singlet state and its connection with neutral excitations of the Haldane-Rezayi
state, and study the transition between these states via exact diagonalization.
We find high overlaps with the Jain spin singlet state upon a departure from
the hollow core model for which the Haldane-Rezayi state is the exact ground
state. Due to a proven algebraic identity we were able to extend the analysis
of topological d-wave pairing structures to polarized Jain states and integer
quantum Hall states, and discuss its consequences. | cond-mat |
Topological and holonomic quantum computation based on second-order
topological superconductors: Majorana fermions feature non-Abelian exchange statistics and promise
fascinating applications in topological quantum computation. Recently,
second-order topological superconductors (SOTSs) have been proposed to host
Majorana fermions as localized quasiparticles with zero excitation energy,
pointing out a new avenue to facilitate topological quantum computation. We
provide a minimal model for SOTSs and systematically analyze the features of
Majorana zero modes with analytical and numerical methods. We further construct
the fundamental fusion principles of zero modes stemming from a single or
multiple SOTS islands. Finally, we propose concrete schemes in different setups
formed by SOTSs, enabling us to exchange and fuse the zero modes for
non-Abelian braiding and holonomic quantum gate operations. | cond-mat |
A new model to describe the physics of VOPO: In the past different models for the magnetic salt vanadyl pyrophosphate
(VOPO) were discussed. Neither a spin ladder nor an alternating chain are
capable to describe recently measured magnetic excitations. In this paper we
propose a 2D model that fits better to experimental observations. | cond-mat |
Spin-ladders with spin gaps: A description of a class of cuprates: We investigate the magnetic properties of the Cu-O planes in stoichiometric
Sr$_{n-1}$Cu$_{n+1}$O$_{2n}$ (n=3,5,7,...) which consist of CuO double chains
periodically intergrown within the CuO$_2$ planes. The double chains break up
the two-dimensional antiferromagnetic planes into Heisenberg spin ladders with
$n_r=\frac{1}{2}(n-1)$ rungs and $n_l=\frac{1}{2}(n+1)$ legs and described by
the usual antiferromagnetic coupling J inside each ladder and a weak and
frustrated interladder coupling J$^\prime$. The resulting lattice is a new
two-dimensional trellis lattice. We first examine the spin excitation spectra
of isolated quasi one dimensional Heisenberg ladders which exhibit a gapless
spectra when $n_r$ is even and $n_l$ is odd ( corresponding to n=5,9,...) and a
gapped spectra when $n_r$ is odd and $n_l$ is even (corresponding to
n=3,7,...). We use the bond operator representation of quantum $S=\frac{1}{2}$
spins in a mean field treatment with self-energy corrections and obtain a spin
gap of $\approx \frac{1}{2} J$ for the simplest single rung ladder (n=3), in
agreement with numerical estimates. | cond-mat |
In-situ measurements of the optical absorption of dioxythiophene-based
conjugated polymers: Conjugated polymers can be reversibly doped by electrochemical means. This
doping introduces new sub-bandgap optical absorption bands in the polymer while
decreasing the bandgap absorption. To study this behavior, we have prepared an
electrochemical cell allowing measurements of the optical properties of the
polymer. The cell consists of a thin polymer film deposited on gold-coated
Mylar behind which is another polymer that serves as a counterelectrode. An
infrared transparent window protects the upper polymer from ambient air. By
adding a gel electrolyte and making electrical connections to the
polymer-on-gold films, one may study electrochromism in a wide spectral range.
As the cell voltage (the potential difference between the two electrodes)
changes, the doping level of the conjugated polymer films is changed
reversibly. Our experiments address electrochromism in
poly(3,4-ethylene-dioxy-thiophene) (PEDOT) and
poly(3,4-dimethyl-propylene-dioxy-thiophene) (PProDOT-Me$_2$). This closed
electrochemical cell allows the study of the doping induced sub-bandgap
features (polaronic and bipolaronic modes) in these easily oxidized and highly
redox switchable polymers. We also study the changes in cell spectra as a
function of polymer thickness and investigate strategies to obtain cleaner
spectra, minimizing the contributions of water and gel electrolyte features. | cond-mat |
Octonacci Photonic Crystals with Negative Refraction Index Materials: We investigate the optical transmission spectra for $s$-polarized (TE) and
$p$-polarized (TM) waves in one-dimensional photonic quasicrystals on a
quasiperiodic multilayer structure made up by alternate layers of SiO$_{2}$ and
\textit{metamaterials}, organized by following the Octonacci sequence.
Maxwell's equations and the transfer-matrix technique are used to derive the
transmission spectra for the propagation of normaly and obliquely incident
optical fields. We assume Drude-Lorentz-type dispersive response for the
dielectric permittivity and magnetic permeability of the metamaterials. For
normally incident waves, we observe that the spectra does not have self-similar
behavior or mirror symmetry and it also features the absence of optical band
gap. Also for normally incident waves, we show regions of full transmittance
when the incident angle $\theta_{C} = 0^{\circ}$ in a particular frequency
range. | cond-mat |
Scale invariance and superfluid turbulence: We construct a Schroedinger field theory invariant under local spatial
scaling. It is shown to provide an effective theory of superfluid turbulence by
deriving, analytically, the observed Kolmogorov 5/3 law and to lead to a
Biot-Savart interaction between the observed filament excitations of the system
as well. | cond-mat |
Surface criticality in random field magnets: The boundary-induced scaling of three-dimensional random field Ising magnets
is investigated close to the bulk critical point by exact combinatorial
optimization methods. We measure several exponents describing surface
criticality: $\beta_1$ for the surface layer magnetization and the surface
excess exponents for the magnetization and the specific heat, $\beta_s$ and
$\alpha_s$. The latter ones are related to the bulk phase transition by the
same scaling laws as in pure systems, but only with the same violation of
hyperscaling exponent $\theta$ as in the bulk. The boundary disorders faster
than the bulk, and the experimental and theoretical implications are discussed. | cond-mat |
Spin Glass in a Field: a New Zero-Temperature Fixed Point in Finite
Dimensions: By using real space renormalisation group (RG) methods we show that
spin-glasses in a field display a new kind of transition in high dimensions.
The corresponding critical properties and the spin-glass phase are governed by
two non-perturbative zero temperature fixed points of the RG flow. We compute
the critical exponents, discuss the RG flow and its relevance for three
dimensional systems. The new spin-glass phase we discovered has unusual
properties, which are intermediate between the ones conjectured by droplet and
full replica symmetry breaking theories. These results provide a new
perspective on the long-standing debate about the behaviour of spin-glasses in
a field. | cond-mat |
Structure and magnetism in $\rm LaCoO_3$: The temperature dependence of the hexagonal lattice parameter $c$ of single
crystal $\rm LaCoO_3$ (LCO) with $H=0$ and $800$Oe, as well as LCO bulk powders
with $H=0$, was measured using high-resolution x-ray scattering near the
transition temperature $T_o\approx 35$K. The change of $c(T)$ is well
characterized by a power law in $T-T_o$ for $T>T_o$ and by a temperature
independent constant for $T<T_o$ when convoluted with a Gaussian function of
width $8.5$K. This behavior is discussed in the context of the unusual magnetic
behavior observed in LCO as well as recent generalized gradient approximation
calculations. | cond-mat |
Single-dot spectroscopy via elastic single-electron tunneling through a
pair of coupled quantum dots: We study the electronic structure of a single self-assembled InAs quantum dot
by probing elastic single-electron tunneling through a single pair of weakly
coupled dots. In the region below pinch-off voltage, the non-linear threshold
voltage behavior provides electronic addition energies exactly as the linear,
Coulomb blockade oscillation does. By analyzing it, we identify the s and p
shell addition spectrum for up to six electrons in the single InAs dot, i.e.
one of the coupled dots. The evolution of shell addition spectrum with magnetic
field provides Fock-Darwin spectra of s and p shell. | cond-mat |
Singular low-energy states of tilted Dirac semimetals induced by the
fermion-fermion interactions: We attentively investigate the effects of short-range fermion-fermion
interactions on the low-energy properties of both two-dimensional type-I and
type-II tilted Dirac semimetals by means of the renormalization group
framework. Practicing the standard renormalization group procedures via taking
into account all one-loop corrections gives rise to the coupled
energy-dependent evolutions of all interaction parameters, which are adopted to
carefully examine whether and how the fermion-fermion interactions influence
the low-energy physical behaviors of tilted Dirac fermions. After carrying out
the detailed analysis of coupled flows, we figure out the tilting parameter
dictates the low-energy states of tilted Dirac fermions in conjunction with
starting values of fermion-fermion couplings. With proper variations of these
two kinds of parameters, the tilted Dirac fermions can either flow towards the
Gaussian fixed point or undergo certain instability that is conventionally
accompanied by a phase transition in the low-energy regime. In addition, all
potential instabilities can be clustered into five distinct classes owing to
the competitions between the tilting parameter and initial fermionic
interactions. Moreover, the dominant phases accompanied by the instabilities
are determined via computing and comparing the susceptibilities of eight
potential phases. | cond-mat |
Magnetic structure and exchange interactions in the layered
semiconductor CrPS4: Compounds with two-dimensional (2D) layers of magnetic ions weakly connected
by van der Waals bonding offer routes to enhance quantum behavior, stimulating
both fundamental and applied interest. CrPS4 is one such magnetic van der Waals
material, however, it has undergone only limited investigation. Here we present
a comprehensive series of neutron scattering measurements to determine the
magnetic structure and exchange interactions. The observed magnetic excitations
allow a high degree of constraint on the model parameters not normally
associated with measurements on a powder sample. The results demonstrate the 2D
nature of the magnetic interactions, while also revealing the importance of
interactions along 1D chains within the layers. The subtle role of competing
interactions is observed, which manifest in a non-trivial magnetic transition
and a tunable magnetic structure in a small applied magnetic field through a
spin-flop transition. Our results on the bulk compound provide insights that
can be applied to an understanding of the behavior of reduced layer CrPS4. | cond-mat |
The angular momentum of a magnetically trapped atomic condensate: For an atomic condensate in an axially symmetric magnetic trap, the sum of
the axial components of the orbital angular momentum and the hyperfine spin is
conserved. Inside an Ioffe-Pritchard trap (IPT) whose magnetic field (B-field)
is not axially symmetric, the difference of the two becomes surprisingly
conserved. In this paper we investigate the relationship between the values of
the sum/difference angular momentums for an atomic condensate inside a magnetic
trap and the associated gauge potential induced by the adiabatic approximation.
Our result provides significant new insight into the vorticity of magnetically
trapped atomic quantum gases. | cond-mat |
Hybrid density functional theory study on zinc blende GaN and diamond
surfaces and interfaces: Effects of size, hydrogen passivation and dipole
corrections: GaN based high electron mobility transistors show promise in numerous device
applications which elicits the need for accurate models of bulk, surface, and
interface electronic properties. We detail here a hybrid density functional
theory study of zinc blende (zb) GaN and diamond bulk and surface properties,
and zb GaN on diamond interfaces using slab supercell models. Details are
provided on the dependence of electronic properties with respect to supercell
size, the use of pseudo-hydrogen to passivate the bottom GaN layer, and dipole
corrections. The large bulk modulus of diamond provides a templating structure
for GaN to grow upon, where a large lattice mismatch is accounted for through
the inclusion of a cationic Ga adlayer. Looking at both type I and II surfaces
and interfaces of GaN shows the instability of zb GaN without an adlayer (type
II), where increased size, pseudo-hydrogen passivation and dipole corrections
do not remove the spurious interaction between the top and bottom layers in
type II GaN. Layer dependent density of states, local potential differences,
and charge density differences show that the type I interface (with a Ga
adlayer) is stable with an adhesion energy of 0.704 eV/{\AA}2 (4.346 J/m2);
interestingly, the diamond charge density intercalates into the first layer of
GaN, which was seen experimentally for wurtzite GaN grown over diamond. The
type II interface is shown to be unstable which implies that, to form a stable,
thin-film zb interface between GaN and diamond, the partial pressure of
trimethylgallium must be controlled to ensure a Ga layer exists both on the top
and bottom layer of the GaN thin film atop the diamond. We believe our results
can shed light towards a better understanding of the GaN/diamond multifaceted
interface present in the GaN overgrowth on diamond samples. | cond-mat |
Scaling of the conductance in gold nanotubes: A new form of gold nanobridges has been recently observed in ultrahigh-vacuum
experiments, where the gold atoms rearrange to build helical nanotubes, akin in
some respects to carbon nanotubes. The good reproducibility of these wires and
their unexpected stability will allow for conductance measurements and make
them promising candidates for future applications . We present here a study of
the transport properties of these nanotubes in order to understand the role of
chirality and of the different orbitals in quantum transport observables. The
conductance per atomic row shows a light decreasing trend as the diameter
grows, which is also shown through an analytical formula based on a one-orbital
model. | cond-mat |
Microstructure and Fe-vacancy ordering in the KFexSe2 superconducting
system: Structural investigations by means of transmission electron microscopy (TEM)
on KFexSe2 with 1.5 \leq x \leq 1.8 have revealed a rich variety of
microstructure phenomena, the KFe1.5Se2 crystal often shows a superstructure
modulation along the [310] zone-axis direction, this superstructure can be well
interpreted by the Fe-vacancy order within the a-b plane. Increase of
Fe-concentration in the KFexSe2 materials could not only result in the
appearance of superconductivity but also yield clear alternations of
microstructure. Structural inhomogeneity, the complex superstructures and
defect structures in the superconducting KFe1.8Se2 sample have been
investigated based on the high-resolution TEM. | cond-mat |
Magneto-Seebeck effect in bismuth: Thermoelectricity was discovered almost two centuries ago in bismuth. The
large and negative Seebeck coefficient of this semimetal remains almost flat
between 300 K and 100 K. This striking feature can be understood by considering
the ratio of electron and hole mobilities and the evolution of their equal
densities with temperature. The large and anisotropic magneto-Seebeck effect in
bismuth, on the other hand, has not been understood up to the present day.
Here, we report on a systematic study of the thermopower of bismuth from room
temperature down to 20 K upon application of a magnetic field of 13.8 T in the
binary-bisectrix plane. The amplitude of the Seebeck coefficient depends on the
orientation of the magnetic field and the anisotropy changes sign with
decreasing temperature. The magneto-Seebeck effect becomes non-monotonic at low
temperatures. When the magnetic field is oriented along the binary axis, the
Seebeck coefficient is not the same for positive and negative fields. This
so-called Umkehr effect arises because the high symmetry axes of the Fermi
surface ellipsoids are neither parallel to each other nor to the high symmetry
axes of the lattice. The complex evolution of thermopower can be accounted for
in a large part of the ($T,B,\Theta$)-space by a model based on semiclassical
transport theory and incorporating Landau quantization. The employed energy
dependence of the scattering time is compatible with electron-acoustic phonon
scattering. We find that the transverse Nernst response plays an important role
in setting the amplitude of the longitudinal magneto-Seebeck effect.
Furthermore, Landau quantization significantly affects thermoelectricity up to
temperatures as high as 120 K. | cond-mat |
Size quantization of an exciton: A toy model of the "dead layer": Size-quantization levels of an exciton in large nanocrystals is studied
theoretically. For the nanocrystal size, $L$, much bigger than the Bohr radius,
$a_B$, the level positions do not depend on $a_B$. The correction to the levels
in a small parameter $a_B/L$ depends on the reflection phase of the exciton
from the boundary. Calculation of this phase constitutes a three-body problem:
electron, hole, and the boundary. This calculation can be performed
analytically in the limit when the hole is much heavier than the electron.
Physically, a slow motion of the hole towards the boundary takes place in the
effective potential created by the fast motion of the electron orbiting the
hole and touching the boundary. As a result, the hole is reflected before
reaching the boundary. The distance of the closest approach of the hole to the
boundary (the dead layer) exceeds $a_B$ parametrically. | cond-mat |
Internal Space Renormalization Group Methods for Atomic and Condensed
Matter Physics: The functional renormalization group method is used to take into account the
vacuum polarization around localized bound states generated by external
potential. The application to Atomic Physics leads to improved Hartree-Fock and
Kohn-Sham equations in a systematic manner within the framework of the Density
Functional Theory. Another application to Condensed Matter Physics consists of
an algorithm to compute quenched averages with or without Coulomb interaction
in a non-perturbative manner. | cond-mat |
A note on generalized hydrodynamics: inhomogeneous fields and other
concepts: Generalized hydrodynamics (GHD) was proposed recently as a formulation of
hydrodynamics for integrable systems, taking into account infinitely-many
conservation laws. In this note we further develop the theory in various
directions. By extending GHD to all commuting flows of the integrable model, we
provide a full description of how to take into account weakly varying force
fields, temperature fields and other inhomogeneous external fields within GHD.
We expect this can be used, for instance, to characterize the non-equilibrium
dynamics of one-dimensional Bose gases in trap potentials. We further show how
the equations of state at the core of GHD follow from the continuity relation
for entropy, and we show how to recover Euler-like equations and discuss
possible viscosity terms. | cond-mat |
A kinetic model for the finite-time thermodynamics of small heat engines: We study a molecular engine constituted by a gas of $N \sim 10^2$ molecules
enclosed between a massive piston and a thermostat. The force acting on the
piston and the temperature of the thermostat are cyclically changed with a
finite period $\tau$. In the adiabatic limit $\tau \to \infty$, even for finite
size $N$, the average work and heats reproduce the thermodynamic values,
recovering the Carnot result for the efficiency. The system exhibits a stall
time $\tau^*$ where net work is zero: for $\tau<\tau^*$ it consumes work
instead of producing it, acting as a refrigerator or as a heat sink. At
$\tau>\tau^*$ the efficiency at maximum power is close to the Curzorn-Ahlborn
limit. The fluctuations of work and heat display approximatively a Gaussian
behavior. Based upon kinetic theory, we develop a three-variables Langevin
model where the piston's position and velocity are linearly coupled together
with the internal energy of the gas. The model reproduces many of the system's
features, such as the inversion of the work's sign, the efficiency at maximum
power and the approximate shape of fluctuations. A further simplification in
the model allows to compute analytically the average work, explaining its
non-trivial dependence on $\tau$. | cond-mat |
Flow can order: Phases of live XY spins in two dimensions: We present the hydrodynamic theory of active XY spins coupled with flow
fields, for systems both having and or lacking number conservation in two
dimensions (2D). For the latter, with strong activity or nonequilibrium drive,
the system can synchronize, or be phase-ordered with various types of order,
e.g., quasi long range order (QLRO) or new kind of order weaker or stronger
than QLRO for sufficiently strong active flow-phase couplings. For the number
conserving case, the system can show QLRO or order weaker than QLRO, again for
sufficiently strong active flow-phase couplings. For other choices of the model
parameters, the system necessarily disorders in a manner similar to immobile
but active XY spins, or 2D Kardar-Parisi-Zhang surfaces. | cond-mat |
The electronic and transport properties of a molecular junction studied
by an integrated piecewise thermal equilibrium approach: An integrated piecewise thermal equilibrium approach based on the
first-principles calculation method has been developed to calculate bias
dependent electronic structures and current- and differential
conductance-voltage characteristics of the gold-benzene-1,4-dithiol-gold
molecular junction. The calculated currents and differential conductance have
the same order of magnitude as experimental ones. An electron transfer was
found between the two electrodes when a bias is applied, which renders the two
electrodes to have different local electronic structures. It was also found
that when Au 5d electrons were treated as core electrons the calculated
currents were overestimated, which can be understood as an underestimate of the
Au-S covalent bonding and consequently the contact potential barrier and the
replacement of delocalized Au 5d carriers by more itinerant delocalized Au 6sp
carriers in the electrodes. | cond-mat |
Hopf-link multi-Weyl-loop topological semimetals: We construct a generic two-band model which can describe topological Weyl
semimetals with multiple closed Weyl loops. All the existing multi-Weyl-loop
semimetals including the nodal-net, or nodal-chain and Hopf-link states can be
examined within one same framework. Based on a two-loop model, the
corresponding drum-head surface states for these topologically different bulk
states are studied and compared with each other. The connection of our model
with Hopf insulators is also discussed. Furthermore, to identify experimentally
these topologically different Weyl semimetal states, especially distinguish the
Hopf-link from unlinked ones, we also investigate their Landau levels. It is
found that the Hopf-link state can be characterized by the existence of a
quadruply degenerate zero-energy Landau band, regardless of the direction of
the magnetic field. | cond-mat |
Spontaneous symmetry breaking: exact results for a biased random walk
model of an exclusion process: It has been recently suggested that a totally asymmetric exclusion process
with two species on an open chain could exhibit spontaneous symmetry breaking
in some range of the parameters defining its dynamics. The symmetry breaking is
manifested by the existence of a phase in which the densities of the two
species are not equal. In order to provide a more rigorous basis to these
observations we consider the limit of the process when the rate at which
particles leave the system goes to zero. In this limit the process reduces to a
biased random walk in the positive quarter plane, with specific boundary
conditions. The stationary probability measure of the position of the walker in
the plane is shown to be concentrated around two symmetrically located points,
one on each axis, corresponding to the fact that the system is typically in one
of the two states of broken symmetry in the exclusion process. We compute the
average time for the walker to traverse the quarter plane from one axis to the
other, which corresponds to the average time separating two flips between
states of broken symmetry in the exclusion process. This time is shown to
diverge exponentially with the size of the chain. | cond-mat |
Delay and distortion of slow light pulses by excitons in ZnO: Light pulses propagating through ZnO undergo distortions caused by both bound
and free excitons. Numerous lines of bound excitons dissect the pulse and
induce slowing of light around them, to the extend dependent on their nature.
Exciton-polariton resonances determine the overall pulse delay and attenuation.
The delay time of the higher-energy edge of a strongly curved light stripe
approaches 1.6 ns at 3.374 eV with a 0.3 mm propagation length. Modelling the
data of cw and time-of-flight spectroscopies has enabled us to determine the
excitonic parameters, inherent for bulk ZnO. We reveal the restrictions on
these parameters induced by the light attenuation, as well as a discrepancy
between the parameters characterizing the surface and internal regions of the
crystal. | cond-mat |
Optical orientation of nuclei in nitrogen alloys GaAsN at room
temperature: The intensity and the giant circular polarization of edge luminescence in a
longitudinal magnetic field have been measured in nitrogen alloys GaAsN under
circularly polarized pumping. It has been found that these dependences are
shifted with respect to zero field by a value Beff. The magnitude of the
internal field Beff increases with increase in pumping intensity and reaches
saturation (~250 Gauss) at great densities of excitation. The saturation of the
Beff field with growth of pumping indicates that this is a field of nuclei,
polarized dynamically due to hyperfine interaction with optically oriented deep
paramagnetic centers, rather than a field of exchange interaction created on
the center by spin-polarized photo-excited conduction electrons. The short time
of nuclear polarization by electrons (<15 mks), measured under modulation of
circular polarization of the exciting light with high frequency, points to a
small number of nuclei undergoing hyperfine interaction with an electron
localized at a center. | cond-mat |
Self-assembly of multicomponent structures in and out of equilibrium: Theories of phase change and self-assembly often invoke the idea of a
`quasiequilibrium', a regime in which the nonequilibrium association of
building blocks results nonetheless in a structure whose properties are
determined solely by an underlying free energy landscape. Here we study a
prototypical example of multicomponent self-assembly, a one-dimensional fiber
grown from red and blue blocks. If the equilibrium structure possesses
compositional correlations different from those characteristic of random
mixing, then it cannot be generated without error at any finite growth rate:
there is no quasiequilibrium regime. However, by exploiting dynamic scaling,
structures characteristic of equilibrium at one point in phase space can be
generated, without error, arbitrarily far from equilibrium. Our results thus
suggest a `nonperturbative' strategy for multicomponent self-assembly in which
the target structure is, by design, not the equilibrium one. | cond-mat |
Clusters in a magnetic toy model for binary granular piles: Results on a generalized magnetically controlled ballistic deposition (MBD)
model of granular piles are reported in order to search for the effect of "spin
flip" probability q in building a granular pile. Two different regimes of spin
cluster site distributions have been identified, a border line $q_c(\beta J)$
where J is the interaction potential strength. | cond-mat |
Quantum body in uniform magnetic fields: In this article it will be presented the first attempt made in order to
perform gauge invariant calculations of eigenstates of a quantum body in its
condensed phase, the latter reacting to an external uniform magnetic field. The
target is achieved introducing a new unitary translation operator transforming
eigenstates into a new set of eigenstates having different total linear
momentum. This new quantum representation solves the problem of calculating the
magnetic response of quantum eigenstates of finite or either infinite periodic
systems to uniform magnetic fields, where equivalence between the customarily
used representation and the new representation has been made. | cond-mat |
Tunable-range, photon-mediated atomic interactions in multimode cavity
QED: Optical cavity QED provides a platform with which to explore quantum
many-body physics in driven-dissipative systems. Single-mode cavities provide
strong, infinite-range photon-mediated interactions among intracavity atoms.
However, these global all-to-all couplings are limiting from the perspective of
exploring quantum many-body physics beyond the mean-field approximation. The
present work demonstrates that local couplings can be created using multimode
cavity QED. This is established through measurements of the threshold of a
superradiant, self-organization phase transition versus atomic position.
Specifically, we experimentally show that the interference of near-degenerate
cavity modes leads to both a strong and tunable-range interaction between
Bose-Einstein condensates (BECs) trapped within the cavity. We exploit the
symmetry of a confocal cavity to measure the interaction between real BECs and
their virtual images without unwanted contributions arising from the merger of
real BECs. Atom-atom coupling may be tuned from short range to long range. This
capability paves the way toward future explorations of exotic, strongly
correlated systems such as quantum liquid crystals and driven-dissipative spin
glasses. | cond-mat |
Thin film modeling of crystal dissolution and growth in confinement: We present a continuum model describing dissolution and growth of a crystal
contact confined against a substrate. Diffusion and hydrodynamics in the liquid
film separating the crystal and the substrate are modeled within the
lubrication approximation. The model also accounts for the disjoining pressure
and surface tension. Within this framework, we obtain evolution equations which
govern the non-equilibrium dynamics of the crystal interface. Based on this
model, we explore the problem of dissolution under an external load, known as
pressure solution. We find that in steady-state, diverging (power-law)
crystal-surface repulsions lead to flat contacts with a monotonic increase of
the dissolution rate as a function of the load. Forces induced by viscous
dissipation then surpass those due to disjoining pressure at large enough
loads. In contrast, finite repulsions (exponential) lead to sharp pointy
contacts with a dissolution rate independent on the load and on the liquid
viscosity. Ultimately, in steady-state the crystal never touches the substrate
when pressed against it, independently from the nature of the crystal-surface
interaction due to the combined effects of viscosity and surface tension. | cond-mat |
Real spin glasses relax slowly in the shade of hierarchical trees: The Parisi solution of the mean-field spin glass has been widely accepted and
celebrated. Its marginal stability in 3d and its complexity however raised the
question of its relevance to real spin glasses. This paper gives a short
overview of the important experimental results which could be understood within
the mean-field solution. The existence of a true phase transition and the
particular behaviour of the susceptibility below the freezing temperature,
predicted by the theory, are clearly confirmed by the experimental results. The
behaviour of the complex order parameter and of the Fluctuation Dissipation
ratio are in good agreement with results of spontaneous noise measurements. The
very particular ultrametric symmetry, the key feature of the theory, provided
us with a simple description of the rejuvenation and memory effects observed in
experiment. Finally, going a step beyond mean-field, the paper shortly
discusses new analyses in terms of correlated domains characterized by their
length scales, as well as new experiments on superspin glasses which compare
well with recent theoretical simulations. | cond-mat |
Frustration -- Exactly Solved Frustrated Models: After a short introduction on frustrated spin systems, we study in this
chapter several two-dimensional frustrated Ising spin systems which can be
exactly solved by using vertex models. We show that these systems contain most
of the spectacular effects due to the frustration: high ground-state
degeneracy, existence of several phases in the ground-state phase diagram,
multiple phase transitions with increasing temperature, reentrance, disorder
lines, partial disorder at equilibrium. Evidences of such effects in non
solvable models are also shown and discussed. | cond-mat |
Higher-Order Results for the Relation between Channel Conductance and
the Coulomb Blockade for Two Tunnel-Coupled Quantum Dots: We extend earlier results on the relation between the dimensionless tunneling
channel conductance $g$ and the fractional Coulomb blockade peak splitting $f$
for two electrostatically equivalent dots connected by an arbitrary number
$N_{\text{ch}}$ of tunneling channels with bandwidths $W$ much larger than the
two-dot differential charging energy $U_{2}$. By calculating $f$ through second
order in $g$ in the limit of weak coupling ($g \rightarrow 0$), we illuminate
the difference in behavior of the large-$N_{\text{ch}}$ and
small-$N_{\text{ch}}$ regimes and make more plausible extrapolation to the
strong-coupling ($g \rightarrow 1$) limit. For the special case of
$N_{\text{ch}}=2$ and strong coupling, we eliminate an apparent ultraviolet
divergence and obtain the next leading term of an expansion in $(1-g)$. We show
that the results we calculate are independent of such band structure details as
the fraction of occupied fermionic single-particle states in the weak-coupling
theory and the nature of the cut-off in the bosonized strong-coupling theory.
The results agree with calculations for metallic junctions in the
$N_{\text{ch}} \rightarrow \infty$ limit and improve the previous good
agreement with recent two-channel experiments. | cond-mat |
Thermal generation, manipulation and detection of skyrmions: Recent years have witnessed significant progresses in realizing skyrmions in
chiral magnets1-4 and asymmetric magnetic multilayers5-13, as well as their
electrical manipulation2,7,8,10. Equally important, thermal generation,
manipulation and detection of skyrmions can be exploited for prototypical new
architecture with integrated computation14 and energy harvesting15. It has yet
to verify if skyrmions can be purely generated by heating16,17, and if their
resultant direction of motion driven by temperature gradients follows the
diffusion or, oppositely, the magnonic spin torque17-21. Here, we address these
important issues in microstructured devices made of multilayers:
(Ta_CoFeB_MgO)15, (Pt_CoFeB_MgO_Ta)15 and (Pt_Co_Ta)15 integrated with on-chip
heaters, by using a full-field soft X-ray microscopy. The thermal generation of
densely packed skyrmions is attributed to the low energy barrier at the device
edge, together with the thermally induced morphological transition from stripe
domains to skyrmions. The unidirectional diffusion of skyrmions from the hot
region towards the cold region is experimentally observed. It can be
theoretically explained by the combined contribution from repulsive forces
between skyrmions, and thermal spin-orbit torques in competing with magnonic
spin torques17,18,20,21 and entropic forces22. These thermally generated
skyrmions can be further electrically detected by measuring the accompanied
anomalous Nernst voltages23. The on-chip thermoelectric generation,
manipulation and detection of skyrmions could open another exciting avenue for
enabling skyrmionics, and promote interdisciplinary studies among spin
caloritronics15, magnonics24 and skyrmionics3,4,12. | cond-mat |
Bound on annealing performance from stochastic thermodynamics, with
application to simulated annealing: Annealing is the process of gradually lowering the temperature of a system to
guide it towards its lowest energy states. In an accompanying paper [Luo et al.
Phys. Rev. E 108, L052105 (2023)], we derived a general bound on annealing
performance by connecting annealing with stochastic thermodynamics tools,
including a speed-limit on state transformation from entropy production. We
here describe the derivation of the general bound in detail. In addition, we
analyze the case of simulated annealing with Glauber dynamics in depth. We show
how to bound the two case-specific quantities appearing in the bound, namely
the activity, a measure of the number of microstate jumps, and the change in
relative entropy between the state and the instantaneous thermal state, which
is due to temperature variation. We exemplify the arguments by numerical
simulations on the SK model of spin-glasses. | cond-mat |
Transient excitonic states in optically-pumped Dirac materials: overview
of recent work: Driven and non-equilibrium quantum states of matter have attracted growing
interest in both theoretical and experimental studies in condensed matter
physics. We review recent progress in realizing transient collective states in
driven or pumped Dirac materials (DMs). In particular, we focus on
optically-pumped DMs which have been theoretically proposed as a promising
platform for observation of a transient excitonic instability.
Optical pumping combined with the linear (Dirac) dispersion of the electronic
spectrum offers a knob for tuning the effective interaction between the
photoexcited electrons and holes, and thus provides a way of reducing the
critical coupling for excitonic instability. As a result, a transient excitonic
condensate could be achieved in a pumped DM while it is not feasible in
equilibrium. We provide a unifying theoretical framework for describing
transient collective states in two- and three-dimensional DMs. We describe
experimental signatures of the transient excitonic state and summarize
numerical estimates of the magnitude of the effect, namely the size of the
dynamically-induced excitonic gaps and the values of the critical temperatures
for several specific systems. We also discuss general guidelines for
identifying promising material candidates.Finally, we comment recent
experimental efforts in realizing transient excitonic condensate in pumped DMs
and outline outstanding issues and possible future directions. | cond-mat |
Tunnel-Junction Thermometry Down to Millikelvin Temperatures: We present a simple on-chip electronic thermometer with the potential to
operate down to 1 mK. It is based on transport through a single normal-metal -
superconductor tunnel junction with rapidly widening leads. The current through
the junction is determined by the temperature of the normal electrode that is
efficiently thermalized to the phonon bath, and it is virtually insensitive to
the temperature of the superconductor, even when the latter is relatively far
from equilibrium. We demonstrate here the operation of the device down to 7 mK
and present a systematic thermal analysis. | cond-mat |
Chemically Functionalized Semiconducting Carbon Nanotubes: Limits for
High Conductance Performance: We present a first-principles study of the electronic transport properties of
micrometer long semiconducting CNTs randomly covered with carbene functional
groups. Whereas prior studies suggested that metallic tubes are hardly affected
by such addends, we show here that the conductance of semiconducting tubes with
standard diameter is on the contrary severely damaged. The configurational
averaged conductance as a function of tube diameter and with a coverage of up
to one hundred functional groups is extracted. Our results indicate that the
search for a conductance-preserving covalent functionalization route remains a
challenging issue. | cond-mat |
$α$-FeSe as an orbital-selective incoherent metal: An LDA+DMFT
study: $\alpha$-FeSe, a prototype iron-chalcogenide superconductor, shows clear
signatures of a strange incoherent normal state. Motivated thereby, we use
LDA+DMFT to show how strong multi-band correlations generate a low-energy
pseudogap in the normal state, giving an incoherent metal in very good
semi-quantitative agreement with observations. We interpret our results in
terms of $\alpha$-FeSe being close to Mottness. A wide range of anomalous
responses in the "normal" state are consistently explained, lending strong
support for this view. Implications for superconductivity arising from such an
anomalous state are touched upon. | cond-mat |
Huge (but finite) time scales in slow relaxations: beyond simple aging: Experiments performed in the last years demonstrated slow relaxations and
aging in the conductance of a large variety of materials. Here, we present
experimental and theoretical results for conductance relaxation and aging for
the case-study example of porous silicon. The relaxations are experimentally
observed even at room temperature over timescales of hours, and when a strong
electric field is applied for a time $t_w$, the ensuing relaxation depends on
$t_w$. We derive a theoretical curve and show that all experimental data
collapse onto it with a single timescale as a fitting parameter. This timescale
is found to be of the order of thousands of seconds at room temperature. The
generic theory suggested is not fine-tuned to porous silicon, and thus we
believe the results should be universal, and the presented method should be
applicable for many other systems manifesting memory and other glassy effects. | cond-mat |
Limitations of the two-media approach in calculating magneto-optical
properties of layered systems: It is shown that in polar geometry and normal incidence the 2x2 matrix
technique - as discussed in detail in a preceeding paper [Phys. Rev. B 65,
144448 (2002)] - accounts correctly for multiple reflections and optical
interferences, and reduces only in the case of a periodic sequence of identical
layers to the Fresnel formula of reflectivity, which in turn is the theoretical
basis of the two-media approach, widely used in the literature to compute
magneto-optical Kerr spectra. As a numerical example ab-initio calculations of
the optical constants for an fcc Pt semi-infinite bulk using the spin-polarized
relativistic screened Korringa-Kohn-Rostoker method show very good agreement
with experimental data. | cond-mat |
Ferromagnetic exchange, spin-orbit coupling and spiral magnetism at the
LaAlO_3/SrTiO_3 interface: The electronic properties of the polar interface between insulating oxides is
a subject of great current interest. An exciting new development is the
observation of robust magnetism at the interface of two non-magnetic materials
LaAlO_3 (LAO) and SrTiO_3 (STO). Here we present a microscopic theory for the
formation and interaction of local moments, which depends on essential features
of the LAO/STO interface. We show that correlation-induced moments arise due to
interfacial splitting of orbital degeneracy. We find that gate-tunable Rashba
spin-orbit coupling at the interface influences the exchange interaction
mediated by conduction electrons. We predict that the zero-field ground state
is a long-wavelength spiral and show that its evolution in an external field
accounts semi-quantitatively for torque magnetometry data. Our theory describes
qualitative aspects of the scanning SQUID measurements and makes several
testable predictions for future experiments. | cond-mat |
Modelling a suspended nanotube oscillator: We present a general study of oscillations in suspended one-dimensional
elastic systems clamped at each end, exploring a wide range of slack (excess
length) and downward external forces. Our results apply directly to recent
experiments in nanotube and silicon nanowire oscillators. We find the behavior
to simplify in three well-defined regimes which we present in a dimensionless
phase diagram. The frequencies of vibration of such systems are found to be
extremely sensitive to slack. | cond-mat |
Discovering Strongly Correlated Quantum Spin Liquid: Strongly correlated Fermi systems are among the most intriguing and
fundamental systems in physics. We show that the herbertsmithite ZnCu3(OH)6Cl2
can be viewed as a new type of strongly correlated electrical insulator that
possesses properties of heavy-fermion metals with one exception: it resists the
flow of electric charge. We demonstrate that herbertsmithite's low temperature
properties are defined by a strongly correlated quantum spin liquid made with
such hypothetic particles as fermionic spinons which carry spin 1/2 and no
charge. Our calculations of its thermodynamic and relaxation properties are in
good agreement with recent experimental facts and allow us to reveal their
scaling behavior which strongly resembles that observed in heavy-fermion
metals. Analysis of the dynamic magnetic susceptibility of strongly correlated
Fermi systems suggests that there exist at least two types of its scaling. | cond-mat |
Magnon Wave-function and Impurity Effects in S=1 Antiferromagnetic
Chains: A Large-n Approach: A large-n approximation to the S=1 antiferromagnetic chain, using the
symmetric tensor representation and its conjugate, is developed to order 1/n in
order to calculate the magnon wave-function and to study the effect of
modifying the exchange coupling from J to J' on a single link. It is shown that
a magnon boundstate exists below the Haldane gap for arbitrarily small negative
J'-J but only above a certain critical value of J'-J for positive values. In
the former case the binding energy vanishes as the square of (J-J'). | cond-mat |
Absence of disordered Thouless pumps at finite frequency: A Thouless pump is a slowly driven one-dimensional band insulator which pumps
charge at a quantised rate. Previous work showed that pumping persists in
weakly disordered chains, and separately in clean chains at finite drive
frequency. We study the interplay of disorder and finite frequency, and show
that the pump rate always decays to zero due to non-adiabatic transitions
between the instantaneous eigenstates. However, the decay is slow, occurring on
a time-scale that is exponentially large in the period of the drive. In the
adiabatic limit, the band gap in the instantaneous spectrum closes at a
critical disorder strength above which pumping ceases. We predict the scaling
of the pump rate around this transition from a model of scattering between rare
states near the band edges. Our predictions can be experimentally tested in
ultracold atomic and photonic platforms. | cond-mat |
Learning and predicting time series by neural networks: Artificial neural networks which are trained on a time series are supposed to
achieve two abilities: firstly to predict the series many time steps ahead and
secondly to learn the rule which has produced the series. It is shown that
prediction and learning are not necessarily related to each other. Chaotic
sequences can be learned but not predicted while quasiperiodic sequences can be
well predicted but not learned. | cond-mat |
Improved Thin Film Quality and Photoluminescence of N-Doped Epitaxial
Germanium-on-Silicon using MOCVD: Ge-on-Si structures in-situ doped with phosphorus or arsenic via metal
organic chemical vapor deposition (MOCVD) were investigated. Surface roughness,
strain, threading dislocation desnity, Si-Ge interdiffusion, dopant diffusion,
and photoluminescence were characterized to study the impacts of defect
annealing and Si substrate offcut effects on the Ge film quality and most
importantly, the light emission properties. All samples have a smooth surface
(roughness < 1.5 nm), and the Ge films have a small tensile strain of 0.2%.
As-grown P and As-doped Ge films have threading dislocaiton densities from
2.8e8 to 1.1e9 cm^(-2) without defect annealing. With thermal cycling, these
values reduced to 1-1.5e8 cm^(-2). The six degree offcut of the Si substrate
was shown to have little impact. In contrast to delta doping, the out-diffusion
of dopants has been successfully suppressed to retain the doping concentration
upon defect annealing. However, the photoluminescence intensity decreases
mostly due to Si-Ge interdiffusion, which also causes a blue-shift in the
emission wavelength. Compared to a benckmarking sample from the first Ge laser
work doped by delta doping method in 2012, the as-grown P or As-doped Ge films
have similar photoluminescence intensity at a 25% doping concentration and
smoother surface, which are promising for Ge lasers with better light emission
efficiencies. | cond-mat |
Nonlinear electric transport in graphene with magnetic disorder: The influence of magnetic impurities on the transport properties of graphene
is investigated in the regime of strong applied electric fields. As a result of
electron-hole pair creation, the response becomes nonlinear and dependent on
the magnetic polarization. In the paramagnetic phase, time reversal symmetry is
statistically preserved, and transport properties are similar to the clean
case. At variance, in the antiferromagnetic phase, the system undergoes a
transition between a superdiffusive to a subdiffusive spreading of a wave
packet, signaling the development of localized states. This critical regime is
characterized by the appearance of electronic states with a multifractal
geometry near the gap. The local density of states concentrates in large
patches having a definite charge-spin correlation. In this state, the
conductivity tends to half the minimum conductivity of clean graphene. | cond-mat |
Rigorous proof of a phase transition of parallelizability in a
one-dimensional structure assembly: In this paper, we prove the existence of a phase transition of
parallelizability in the assembly of one-dimensional chains. By introducing the
parallel efficiency that measures how efficiently the parallel assembly works,
the parallelizable phase is defined by its positive value. The
parallelizable/unparallelizable transition is then identified by the
non-analytic change in the parallel efficiency from a positive value to zero.
By evaluating the parallel efficiency on each side of the transition point, we
show the existence of a phase transition in this system. | cond-mat |
Thermodynamics of Antiferromagnetic Solids in Magnetic Fields: We analyze the thermodynamic properties of antiferromagnetic solids subjected
to a combination of mutually orthogonal uniform magnetic and staggered fields.
Low-temperature series for the pressure, order parameter and magnetization up
to two-loop order in the effective expansion are established. We evaluate the
self-energy and the dispersion relation of the dressed magnons in order to
discuss the impact of spin-wave interactions on thermodynamic observables. | cond-mat |
Remanence effects in the electrical resistivity of spin glasses: We have measured the low temperature electrical resistivity of Ag : Mn
mesoscopic spin glasses prepared by ion implantation with a concentration of
700 ppm. As expected, we observe a clear maximum in the resistivity (T ) at a
temperature in good agreement with theoretical predictions. Moreover, we
observe remanence effects at very weak magnetic fields for the resistivity
below the freezing temperature Tsg: upon Field Cooling (fc), we observe clear
deviations of (T ) as compared with the Zero Field Cooling (zfc); such
deviations appear even for very small magnetic fields, typically in the Gauss
range. This onset of remanence for very weak magnetic fields is reminiscent of
the typical signature on magnetic susceptibility measurements of the spin glass
transition for this generic glassy system. | cond-mat |
Anisotropy of graphite optical conductivity: The graphite conductivity is evaluated for frequencies between
0.1 eV, the energy of the order of the electron-hole overlap, and 1.5 eV, the
electron nearest hopping energy. The in-plane conductivity per single atomic
sheet is close to the universal graphene conductivity $e^2/4\hbar$ and,
however, contains a singularity conditioned by peculiarities of the electron
dispersion. The conductivity is less in the $c-$direction by the factor of the
order of 0.01 governed by electron hopping in this direction. | cond-mat |
Persistent photovoltage in methylammonium lead iodide perovskite solar
cells: Open circuit voltage decay measurements are performed on methylammonium lead
iodide (CH3NH3PbI3) perovskite solar cells to investigate the charge carrier
recombination dynamics. The measurements are compared to the two reference
polymer-fullerene bulk heterojunction solar cells based on P3HT:PC60BM and
PTB7:PC70BM blends. In the perovskite devices, two very different time domains
of the voltage decay are found, with a first drop on a short time scale that is
similar to the organic solar cells. However, two major differences are also
observed. 65-70% of the maximum photovoltage persists on much longer
timescales, and the recombination dynamics are dependent on the illumination
intensity. | cond-mat |
Ultrafast Charge Migration in XUV Photoexcited Phenylalanine: a
First-Principles Study Based on Real-Time Nonequilibrium Green's Functions: The early stage density oscillations of the electronic charge in molecules
irradiated by an attosecond XUV pulse takes place on femto- or subfemtosecond
timescales. This ultrafast charge migration process is a central topic in
attoscience as it dictates the relaxation pathways of the molecular structure.
A predictive quantum theory of ultrafast charge migration should incorporate
the atomistic details of the molecule, electronic correlations and the
multitude of ionization channels activated by the broad-bandwidth XUV pulse. In
this work we propose a first-principles Non Equilibrium Green's Function method
fulfilling all three requirements, and apply it to a recent experiment on the
photoexcited phenylalanine aminoacid. Our results show that dynamical
correlations are necessary for a quantitative overall agreement with the
experimental data. In particular, we are able to capture the transient
oscillations at frequencies 0.15PHz and 0.30PHz in the hole density of the
amine group, as well as their suppression and the concomitant development of a
new oscillation at frequency 0.25PHz after about 14 femtoseconds. | cond-mat |
Noise Measurements of High-Speed, Light-Emitting GaN Resonant-Tunneling
Diodes: We report here the first RF noise measurements on two designs of n-doped
GaN/AlN double-barrier resonant tunneling diodes (RTDs), each having a
room-temperature negative differential resistance (NDR) and also strong near-UV
light emission. The measurements are made with a standard, un-isolated RF
receiver and calibration is made using a substitution-resistor/hot-cold
radiometric technique which works in the positive differential resistance (PDR)
region but not the NDR region. A high-quality InGaAs/AlAs double-barrier RTD is
used as a control sample and displays shot noise suppression down to
$\Gamma\approx$0.5 in the PDR region, as expected. The GaN/AlN RTDs display
both shot-noise enhancement and suppression in the PDR regions, but no obvious
sign of sudden shot-noise enhancement in the threshold bias region of light
emission. This supports the hypothesis that the holes required for light
emission are created by electronic (Zener) interband tunneling, not impact
ionization. Further the minimum shot-noise factor of $\Gamma\sim$ 0.34 suggests
that the GaN/AlN RTDs are acting like triple-barrier devices. | cond-mat |
Quantum chaos on a critical Fermi surface: We compute parameters characterizing many-body quantum chaos for a critical
Fermi surface without quasiparticle excitations. We examine a theory of $N$
species of fermions at non-zero density coupled to a $U(1)$ gauge field in two
spatial dimensions, and determine the Lyapunov rate and the butterfly velocity
in an extended random-phase approximation. The thermal diffusivity is found to
be universally related to these chaos parameters i.e. the relationship is
independent of $N$, the gauge coupling constant, the Fermi velocity, the Fermi
surface curvature, and high energy details. | cond-mat |
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