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Combined effects of local and nonlocal hybridization on formation and
condensation of excitons in the extended Falicov-Kimball model: We study the combined effects of local and nonlocal hybridization on the
formation and condensation of the excitonic bound states in the extended
Falicov-Kimball model by the density-matrix-renormalization-group (DMRG)
method. Analysing the resultant behaviours of the excitonic momentum
distribution $N(q)$ we found, that unlike the local hybridization $V$, which
supports the formation of the $q=0$ momentum condensate, the nonlocal
hybridization $V_n$ supports the formation of the $q=\pi$ momentum condensate.
The combined effect of local and nonlocal hybridization further enhances the
excitonic correlations in $q=0$ as well as $q=\pi$ state, especially for $V$
and $V_n$ values from the charge-density-wave (CDW) region. Strong effects of
local and nonlocal hybridization are observed also for other ground-state
quantities of the model such as the $f$-electron density, or the density of
unbound $d$-electrons, which are generally enhanced with increasing $V$ and
$V_n$. The same calculations performed for nonzero values of $f$-level energy
$E_f$ revealed that this model can yield a reasonable explanation for the
pressure-induced resistivity anomaly observed experimentally in
$TmSe_{0.45}Te_{0.55}$ compound. | cond-mat_str-el |
Universal scaling of Klein bottle entropy near conformal critical points: We show that the Klein bottle entropy [Phys. Rev. Lett. 119, 261603 (2017)]
for conformal field theories (CFTs) perturbed by a relevant operator is a
universal function of the dimensionless coupling constant. The universal
scaling of the Klein bottle entropy near criticality provides an efficient
approach to extract the scaling dimension of lattice operators via data
collapse. As paradigmatic examples, we validate the universal scaling of the
Klein bottle entropy for Ising and Z3 parafermion CFTs with various
perturbations using numerical simulation with continuous matrix product
operator approach. | cond-mat_str-el |
Quantum phases of a frustrated four-leg spin tube: We study the ground state phase diagram of a frustrated spin-1/2 four-leg
tube. Using a variety of complementary techniques, namely density matrix
renormalization group, exact diagonalization, Schwinger boson mean field
theory, quantum Monte-Carlo and series expansion, we explore the parameter
space of this model in the regime of all-antiferromagnetic exchange. In
contrast to unfrustrated four-leg tubes we uncover a rich phase diagram. Apart
from the Luttinger liquid fixed point in the limit of decoupled legs, this
comprises several gapped ground states, namely a plaquette, an incommensurate,
and an antiferromagnetic quasi spin-2 chain phase. The transitions between
these phases are analyzed in terms of total energy and static structure factor
calculations and are found to be of (weak) first order. Despite the absence of
long range order in the quantum case, remarkable similarities to the classical
phase diagram are uncovered, with the exception of the icommensurate regime,
which is strongly renormalized by quantum fluctuations. In the limit of large
leg exchange the tube exhibits a deconfinement cross-over from gapped magnon
like excitations to spinons. | cond-mat_str-el |
Exchange renormalized crystal field excitation in a quantum Ising magnet
KTmSe$_2$: Rare-earth delafossite compounds, ARCh$_2$ (A = alkali or monovalent ion, R =
rare earth, Ch = chalcogen), have been proposed for a range of novel quantum
phenomena. Particularly, the Tm series, ATmCh$_2$, featuring Tm ions on a
triangular lattice, serves as a representative group of compounds to illustrate
the interplay and competition between spin-orbit coupling, crystal fields, and
exchange couplings in the presence of geometric frustration. Here we report the
thermodynamic and inelastic neutron scattering studies on the newly discovered
triangular-lattice magnet KTmSe$_2$. Both heat capacity and neutron diffraction
reveal the absence of long-range magnetic order. Magnetic susceptibility shows
strong Ising-like interactions with antiferromagnetic correlations.
Furthermore, inelastic neutron scattering measurements reveal a branch of
dispersive crystal field excitations. To analyze these observations, we employ
both the transverse field Ising model and the full crystal field scheme, along
with exchange interactions. Our results suggest a strong competition between
spin exchange interactions and crystal field effects. This work is expected to
offer a valuable framework for understanding low-temperature magnetism in
KTmSe$_2$ and similar materials. | cond-mat_str-el |
Elementary excitations in the ordered phase of spin-1/2 J1-J2 model on
square lattice: We use recently proposed four-spin bond-operator technique (BOT) to discuss
spectral properties of frustrated spin-$\frac12$ $J_1$--$J_2$ Heisenberg
antiferromagnet on square lattice at $J_2<0.4J_1$ (i.e., in the N\'eel ordered
phase). This formalism is convenient for the consideration of low-lying
excitations which appear in conventional approaches as multi-magnon bound
states (e.g., the Higgs excitation) because separate bosons describe them in
BOT. At $J_2=0$, the obtained magnon spectrum describes accurately available
experimental data. However, calculated one-magnon spectral weights and the
transverse dynamical structure factor (DSF) do not reproduce experimental
findings quantitatively around the momentum ${\bf k}=(\pi,0)$. Then, we do not
support the conjecture that the continuum of excitations observed
experimentally and numerically near ${\bf k}=(\pi,0)$ is of the Higgs-magnon
origin. Upon $J_2$ increasing, one-magnon spectral weights decrease and spectra
of high-energy spin-0 and spin-1 excitations move down. One of spin-0
quasiparticles becomes long-lived and its spectrum merges with the magnon
spectrum in the most part of the Brillouin zone at $J_2\approx0.3J_1$. We
predict that the Higgs excitation and another spin-0 quasiparticle become
long-lived around ${\bf k}=(\pi/2,\pi/2)$ at $J_2\agt0.3J_1$ and produce sharp
anomalies in the longitudinal DSF. | cond-mat_str-el |
Interplay of disorder and spin fluctuations in the resistivity near a
quantum critical point: The resistivity in metals near an antiferromagnetic quantum critical point
(QCP) is strongly affected by small amounts of disorder. In a quasi-classical
treatment, we show that an interplay of strongly anisotropic scattering due to
spin fluctuations and isotropic impurity scattering leads to a large regime
where the resistivity varies as T^alpha, with an anomalous exponent, alpha, 1
<= alpha <= 1.5, depending on the amount of disorder. I argue that this
mechanism explains in some detail the anomalous temperature dependence of the
resistivity observed in CePd_2Si_2, CeNi_2Ge_2 and CeIn_3 near the QCP. | cond-mat_str-el |
Specific Heat Study of Magnetic and Superconducting Transitions in
CePt3Si: Measurements of specific heat between 80 mK to 4 K and electrical resistivity
between 80 mK to 10 K were carried out for polycrystalline CePt3Si samples cut
into small pieces (typically $\sim $10 mg). In the specific heat measurements,
we observed an antiferromagnetic transition jump at TN = 2.2 K for all the
samples, while the heights have large variations. As regards superconductivity,
we observed two distinct transition jumps at Tcl $\sim$ 0.45 K and Tch $\sim$
0.75 K, which were the same for all the samples. From the measurements of
specific heat and resistivity, systematic relations were found between
antiferromagnetic and superconducting transitions. We conclude that
antiferromagnetism, whose transition temperature is 2.2 K, coexists with
superconductivity, whose transition temperature is Tcl. In this sample,
residual electronic specific heat coefficient in the superconducting state
$\gamma_{\rm s}$ was quite small, and specific heat divided by temperature
below Tcl decreased almost linearly with decreasing temperature. In order to
reveal the characteristic properties of the magnetism and superconductivity of
the CePt3Si system, it is important to study the two superconducting phases
with Tcl and Tch, respectively. | cond-mat_str-el |
Electronic structure of RE1-xAxMnO3 manganite films investigated by
magnetic circular dichroism spectroscopy: Magnetic circular dichroism (MCD) spectroscopy was used to study the features
of the electronic structure of an epitaxial La0.7Ca0.3MnO3 film in the range of
1.2 - 4 eV. The study of the temperature behavior of the MCD spectra made it
possible to establish a correlation between the magnetooptical and transport
properties of the sample. The data obtained were analyzed in comparison with
MCD data for polycrystalline manganite films of various RE1-xAxMnO3
compositions. The MCD spectra of the films were compared with the spectra of
the off-diagonal component of the permittivity tensor calculated from the data
of the magneto-optical Kerr effect for films of the same composition. A unified
set of ground and excited electronic states characteristic of RE1-xAxMnO3
manganites in the visible and near infrared ranges has been identified. These
results are important for a qualitative theoretical description of the
electronic structure of strongly correlated magnetic oxides. | cond-mat_str-el |
Tomonaga-Luttinger Liquid in a Quasi-One-Dimensional S=1 Antiferromagnet
Observed by the Specific Heat: Specific heat experiments on single crystals of the S=1 quasi-one-dimensional
bond-alternating antiferromagnet Ni(C_9H_24N_4)(NO_2)ClO_4, alias NTENP, have
been performed in magnetic fields applied both parallel and perpendicular to
the spin chains. We have found for the parallel field configuration that the
magnetic specific heat (C_mag) is proportional to temperature (T) above a
critical field H_c, at which the energy gap vanishes, in a temperature region
above that of the long-range ordered state. The ratio C_mag/T increases as the
magnetic field approaches H_c from above. The data are in good quantitative
agreement with the prediction of the c=1 conformal field theory in conjunction
with the velocity of the excitations calculated by a numerical diagonalization,
providing a conclusive evidence for a Tomonaga-Luttinger liquid. | cond-mat_str-el |
Low-energy scales and temperature-dependent photoemission of heavy
fermions: We solve the $S=1/2$ Kondo lattice model within the dynamical mean field
theory. Detailed predictions are made for the dependence of the lattice Kondo
resonance and the conduction electron spectral density on temperature and band
filling $n_{c}$. Two low-energy scales are identified in the spectra: a
renormalized hybridization pseudogap scale $T^{*}$, which correlates with the
single-ion Kondo scale, and a lattice Kondo scale $T_{0} < T^{*}$, which acts
as the Fermi-liquid coherence scale. The lattice Kondo resonance is split into
a main branch, which is pinned at the Fermi level, and whose width is set by
$T_{0}$, and an upper branch at $\omega\approx T^{*}$. The weight of the upper
branch decreases rapidly away from $n_{c}=1$ and vanishes for $n_{c}\lesssim
0.7$. In contrast, the pseudogap in the conduction electron spectral density
persists for all $n_{c}$. On increasing temperature, the lattice Kondo
resonance at the Fermi level vanishes on a temperature scale of order $10
T_{0}$, as in impurity model calculations. In contrast to impurity model
spectra, however, the position of the lattice Kondo resonance depends strongly
on temperature, particularly close to the Kondo insulating state. The results
are used to make predictions on the temperature dependence of the low-energy
photoemission and inverse photoemission spectra of metallic heavy fermions and
doped Kondo insulators. We compare our results with available high-resolution
measurements on YbInCu$_4$ and YbAgCu$_4$. The loss in intensity with
increasing temperature, and the asymmetric lineshape of the low-energy spectra
are well accounted for by our model. More detailed agreement with experiment
would require including the $f$-orbital degeneracy and crystal-field excited
states. | cond-mat_str-el |
Critical properties of S=1/2 Heisenberg ladders in magnetic fields: The critical properties of the $S=1/2$ Heisenberg two-leg ladders are
investigated in a magnetic field. Combining the exact diagonalization method
and the finite-size-scaling analysis based on conformal field theory, we
calculate the critical exponents of spin correlation functions numerically. For
a strong interchain coupling, magnetization dependence of the critical
exponents shows characteristic behavior depending on the sign of the interchain
coupling. We also calculate the critical exponents for the $S=1/2$ Heisenberg
two-leg ladder with a diagonal interaction, which is thought as a model
Hamiltonian of the organic spin ladder compound
${Cu}_2({1,4-diazacycloheptane})_2{Cl}_4$. Numerical results are compared with
experimental results of temperature dependence of the NMR relaxation rate
$1/T_1$. | cond-mat_str-el |
Quantum walk versus classical wave: Distinguishing ground states of
quantum magnets by spacetime dynamics: We investigate the wavepacket spreading after a single spin flip in
prototypical two-dimensional ferromagnetic and antiferromagnetic quantum spin
systems. We find characteristic spatial magnon density profiles: While the
ferromagnet shows a square-shaped pattern reflecting the underlying lattice
structure, as exhibited by quantum walkers, the antiferromagnet shows a
circular-shaped pattern which hides the lattice structure and instead resembles
a classical wave pattern. We trace these fundamentally different behaviors back
to the distinctly different magnon energy-momentum dispersion relations and
also provide a real-space interpretation. Our findings point to new
opportunities for real-time, real-space imaging of quantum magnets both in
materials science and in quantum simulators. | cond-mat_str-el |
Vertical Boundary at x ~ 0.11 in the Structural Phase Diagram of the
La1-xSrxMnO3 System (0.08 <= x <= 0.125): The structural phase diagram of the La1-xSrxMnO3 system in the compositional
range 0.08 <= x <= 0.125 has been investigated by high-resolution synchrotron
x-ray powder diffraction techniques between 20-600 K. Recent studies have
reported that there is an unusual rentrant-type phase transition in this range
involving an abrupt change in lattice parameters but no change in the crystal
symmetry, which remains orthorhombic Pbnm. The transition to the reentrant
phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with
some unusual properties. Our results demonstrate that for samples with x
=0.11-0.125 there exist two lower-symmetry structural regions having monoclinic
and triclinic symmetry respectively. There is a sharp first-order transition
from the monoclinic to the triclinic phase coinciding with the transition to
the ferromagnetic insulating phase, and an abrupt crossover from the
orthorhombic Pbnm region with a near-vertical phase boundary just below x =
0.11. The new phases indicate the presence of some novel type of orbital
ordering unlike that found in LaMnO3. | cond-mat_str-el |
Spin polarization of strongly interacting 2D electrons: the role of
disorder: In high-mobility silicon MOSFET's, the $g^*m^*$ inferred indirectly from
magnetoconductance and magnetoresistance measurements with the assumption that
$g^*\mu_BH_s=2E_F$ are in surprisingly good agreement with $g^*m^*$ obtained by
direct measurement of Shubnikov-de Haas oscillations. The enhanced
susceptibility $\chi^* \propto (g^*m^*)$ exhibits critical behavior of the form
$\chi^* \propto (n - n_0)^{-\alpha}$. We examine the significance of the field
scale $H_s$ derived from transport measurements, and show that this field
signals the onset of full spin polarization only in the absence of disorder.
Our results suggest that disorder becomes increasingly important as the
electron density is reduced toward the transition. | cond-mat_str-el |
Path-integral Monte Carlo study of electronic states in quantum dots in
an external magnetic field: We explore correlated electron states in harmonically confined few-electron
quantum dots in an external magnetic field by the path-integral Monte Carlo
method for a wide range of the field and the Coulomb interaction strength.
Using the phase structure of a preceding unrestricted Hartree-Fock calculation
for phase fixing, we find a rich variety of correlated states, often completely
different from the prediction of mean-field theory. These are finite
temperature results, but sometimes the correlations saturate with decreasing
temperature, providing insight into the ground-state properties. | cond-mat_str-el |
P-wave pairing and ferromagnetism in the metal-insulator transition in
two dimensions: Based on recent experimental evidence for a spin polarized ground state in
the insulating phase of the two-dimensional electron system, we propose that
ferromagnetic spin fluctuations lead to an attractive interaction in the
triplet channel and cause p-wave pairing in the conducting phase. We use the
Landau Fermi liquid phenomenology to explain how the enhanced spin
susceptibility near the critical density yields an attractive potential, in a
similar mechanism to superfluidity in $^3$He. As the density is decreased, the
p-wave order parameter undergoes a transition from a unitary to a nonunitary
state, in which it coexists with ferromagnetism for a range of densities. As
the density is further reduced, the pairing amplitude vanishes and the system
is described by a ferromagnetic insulator. Thus, we find two quantum critical
points as a function of density associated with the polarization of the paired
state and ferromagnetism. We explain the magnetotransport measurements in
parallel and perpendicular magnetic fields and propose a shot noise experiment
to measure the pair charge. | cond-mat_str-el |
Origin of Biquadratic Exchange Interactions in a Mott Insulator as a
Driving Force of Spin Nematic Order: We consider a series of Mott insulators in unit of two orbitals each hosting
spin-1/2 electron, and by pairing two spin-1/2 into spin-1 triplet, derive the
effective exchange interaction between the adjacent units via fourth order
perturbation theory. It turns out that the biquadratic exchange interaction
between spin-1, which is one of the origins of the nematic order, arises only
in processes where the four different electrons exchange cyclically along the
twisted loop, which we call "twisted ring exchange" processes. We show that the
term becomes the same order with the Heisenberg exchange interactions when the
on-orbital Coulomb interaction is not too large. Whereas, the inter-orbital
Coulomb interactions give rise to additional processes that cancel the twisted
ring exchange, and strongly suppresses the biquadratic term. The Mott insulator
with two electrons on degenerate two orbitals is thus not an ideal platform to
study such nematic orders. | cond-mat_str-el |
Tunable Weyl Semi-metal and its Possible Realization in Optical Lattice: Weyl semimetal (WSM) is an exotic topological state in condensed matter
physics. In this paper, based on a two-band cubic lattice model, we studied
WSMs with a pair of tunable Weyl nodes. It is pointed out that there exist
three types of WSMs with different tilt strengths: type-I WSM, type-II WSM and
type-1.5 WSM. In particular, type-1.5 WSM has one type-I node and one type-II
node. We studied chiral modes, surface Fermi arcs and quantum anomalous Hall
effect in different types of WSMs. In addition, we give an experimental setup
to realize the different types of WSMs based on timely technique. | cond-mat_str-el |
Quench Dynamics Across Topological Quantum Phase Transitions: We study the dynamics of systems quenched through topological quantum phase
transitions and investigate the behavior of the bulk and edge excitations with
various quench rates. Specifically, we consider the Haldane model and
checkerboard model in slow quench processes with distinct band-touching
structures leading to topology changes. The generation of bulk excitations is
found to obey the power-law relation Kibble-Zurek and Landau-Zener theories
predict. However, an anti-Kibble-Zurek behavior is observed in the edge
excitations. The mechanism of excitation generation on edge states is revealed,
which explains the anti-Kibble-Zurek behavior. | cond-mat_str-el |
In pursuit of deconfined quantum criticality in Ising gauge theory
entangled with single-component fermions: We highlight exotic quantum criticality of quasi-two-dimensional
single-component fermions at half-filling that are minimally coupled to a
dynamical Ising gauge theory. With the numerical matrix product state based
iDMRG method, we discover a robust quantum critical line in the infinite
cylinder geometry, where gauge confinement and dimerized translation symmetry
breaking emerge simultaneously. We investigate how the transition can be split
by a $\mathbb{Z}_2$ topologically ordered dimerized phase that is stabilized by
additional short-range repulsive interactions. We conjecture a $u(1)$
deconfined criticality scenario, propose a corresponding low-energy effective
field theory of the exotic quantum critical point in the two-dimensional limit
and identify its shortcomings | cond-mat_str-el |
Unconventional superconductivity in the layered iron germanide
YFe$_2$Ge$_2$: Since the discovery of superconductivity in LaFePO in 2006, numerous
iron-based superconductors have been identified within diverse structure
families, all of which combine iron with a group-V (pnictogen) or group-VI
(chalco- gen) element. Unconventional superconductivity is extremely rare among
transition metal compounds outside these layered iron systems and the cuprates,
and it is almost universally associated with highly anisotropic electronic
properties and nearly 2D Fermi surface geometries. The iron-based intermetallic
YFe$_2$Ge$_2$ features a 3D Fermi surface and a strongly enhanced low
temperature heat capacity, which signals strong electronic correlations. We
present data from a new generation of high quality samples of YFe$_2$Ge$_2$,
which show superconducting transition anomalies below 1.8 K in thermodynamic as
well as transport measurements, establishing that superconductivity is
intrinsic in this layered iron compound outside the known superconducting iron
pnictide or chalcogenide families. The Fermi surface geometry of YFe$_2$Ge$_2$
resembles that of KFe$_2$As$_2$ in the high pressure collapsed tetragonal
phase, in which superconductivity at temperatures as high as 10 K has recently
been reported, suggesting an underlying connection between the two systems. | cond-mat_str-el |
Single-crystal investigations on the multiferroic material
LiFe(WO$_4$)$_2$: The crystal and magnetic structure of multiferroic LiFe(WO$_4$)$_2$ were
investigated by temperature and magnetic-field dependent specific heat,
susceptibility and neutron diffraction experiments on single crystals.
Considering only the two nearest-neighbour magnetic interactions, the system
forms a $J_1$, $J_2$ magnetic chain but more extended interactions are
sizeable. Two different magnetic phases exhibiting long-range incommensurate
order evolve at $T_{\text{N}1}\approx 22.2 \text{ K}$ and $T_{\text{N}2}\approx
19 \text{ K}$. First, a spin-density wave develops with moments lying in the
$ac$ plane. In its multiferroic phase below $T_{\text{N}2}$, LiFe(WO$_4$)$_2$
exhibits a spiral arrangement with an additional spin-component along $b$.
Therefore, the inverse Dzyaloshinskii-Moriya mechanism fully explains the
multiferroic behavior in this material. A partially unbalanced multiferroic
domain distribution was observed even in the absence of an applied electric
field. For both phases only a slight temperature dependence of the
incommensurability was observed and there is no commensurate phase emerging at
low temperature or at finite magnetic fields up to $6\text{ T}$.
LiFe(WO$_4$)$_2$ thus exhibits a simple phase diagram with the typical sequence
of transitions for a type-II multiferroic material. | cond-mat_str-el |
Nonequilibrium self-energy functional approach to the dynamical Mott
transition: The real-time dynamics of the Fermi-Hubbard model, driven out of equilibrium
by quenching or ramping the interaction parameter, is studied within the
framework of the nonequilibrium self-energy functional theory. A dynamical
impurity approximation with a single auxiliary bath site is considered as a
reference system and the time-dependent hybridization is optimized as
prescribed by the variational principle. The dynamical two-site approximation
turns out to be useful to study the real-time dynamics on short and
intermediate time scales. Depending on the strength of the interaction in the
final state, two qualitatively different response regimes are observed. For
both weak and strong couplings, qualitative agreement with previous results of
nonequilibrium dynamical mean-field theory is found. The two regimes are
sharply separated by a critical point at which the low-energy bath degree of
freedom decouples in the course of time. We trace the dependence of the
critical interaction of the dynamical Mott transition on the duration of the
interaction ramp from sudden quenches to adiabatic dynamics, and therewith link
the dynamical to the equilibrium Mott transition. | cond-mat_str-el |
Cluster Monte Carlo Algorithm for the Quantum Rotor Model: We propose a highly efficient "worm" like cluster Monte Carlo algorithm for
the quantum rotor model in the link-current representation. We explicitly prove
detailed balance for the new algorithm even in the presence of disorder. For
the pure quantum rotor model with $\mu=0$ the new algorithm yields high
precision estimates for the critical point $K_c=0.33305(5)$ and the correlation
length exponent $\nu=0.670(3)$. For the disordered case, $\mu=1/2 \pm 1/2$, we
find $\nu=1.15(10)$. | cond-mat_str-el |
Generalized Lifshitz-Kosevich scaling at quantum criticality from the
holographic correspondence: We characterize quantum oscillations in the magnetic susceptibility of a
quantum critical non-Fermi liquid. The computation is performed in a strongly
interacting regime using the nonperturbative holographic correspondence. The
temperature dependence of the amplitude of the oscillations is shown to depend
on a critical exponent nu. For general nu the temperature scaling is distinct
from the textbook Lifshitz-Kosevich formula. At the `marginal' value nu = 1/2,
the Lifshitz-Kosevich formula is recovered despite strong interactions. As a
by-product of our analysis we present a formalism for computing the amplitude
of quantum oscillations for general fermionic theories very efficiently. | cond-mat_str-el |
Single hole dynamics in the t-J model on a square lattice: We present quantum Monte Carlo (QMC) simulations for a single hole in a t-J
model from J=0.4t to J=4t on square lattices with up to 24 x 24 sites. The
lower edge of the spectrum is directly extracted from the imaginary time
Green's function. In agreement with earlier calculations, we find flat bands
around $(0,\pm\pi)$, $(\pm\pi,0)$ and the minimum of the dispersion at
$(\pm\pi/2,\pm\pi/2)$. For small J both self-consistent Born approximation and
series expansions give a bandwidth for the lower edge of the spectrum in
agreement with the simulations, whereas for J/t > 1, only series expansions
agree quantitatively with our QMC results. This band corresponds to a coherent
quasiparticle. This is shown by a finite size scaling of the quasiparticle
weight $Z(\vec k)$ that leads to a finite result in the thermodynamic limit for
the considered values of $J/t$. The spectral function $A(\vec k, \omega)$ is
obtained from the imaginary time Green's function via the maximum entropy
method. Resonances above the lowest edge of the spectrum are identified, whose
J-dependence is quantitatively described by string excitations up to J/t=2. | cond-mat_str-el |
Parametric pumping and kinetics of magnons in dipolar ferromagnets: The time evolution of magnons subject to a time-dependent microwave field is
usually described within the so-called "S-theory", where kinetic equations for
the distribution function are obtained within the time-dependent Hartree-Fock
approximation. To explain the recent observation of "Bose-Einstein condensation
of magnons" in an external microwave field [Demokritov et al., Nature 443, 430
(2006)], we extend the "S-theory" to include the Gross-Pitaevskii equation for
the time-dependent expectation values of the magnon creation and annihilation
operators. We explicitly solve the resulting coupled equations within a simple
approximation where only a single condensed mode is retained. We also
re-examine the usual derivation of an effective boson model from a realistic
spin model for yttrium-iron garnet films and argue that in the parallel pumping
geometry (where both the static and the time-dependent magnetic field are
parallel to the macroscopic magnetization) the time-dependent Zeemann energy
cannot give rise to magnon condensation. | cond-mat_str-el |
Enhanced skyrmion metastability under applied strain in FeGe: Mechanical straining of skyrmion hosting materials has previously
demonstrated increased phase stability through the expansion of the skyrmion
equilibrium pocket. Additionally, metastable skyrmions can be generated via
rapid field-cooling to form significant skyrmion populations at low
temperatures. Using small-angle x-ray scattering and x-ray holographic imaging
on a thermally strained 200 nm thick FeGe lamella, we observe
temperature-induced strain effects on the structure and metastability of the
skyrmion lattice. We find that in this sample orientation (H || [1 1 0]) with
no strain, metastable skyrmions produced by field cooling through the
equilibrium skyrmion pocket vanish from the sample upon dropping below the well
known helical reorientation temperature. However, when strain is applied along
[110] axis, and this procedure is repeated, a substantial volume fraction of
metastable skyrmions persist upon cooling below this temperature down to 100 K.
Additionally, we observe a large number of skyrmions retained after a complete
magnetic field polarity reversal, implying that the metastable energy barrier
protecting skyrmions from decay is enhanced. | cond-mat_str-el |
Shubnikov-de Haas oscillations spectrum of the strongly correlated
quasi-2D organic metal (ET)8[Hg4Cl12(C6H5Br)]2 under pressure: Pressure dependence of the Shubnikov-de Haas (SdH) oscillations spectra of
the quasi-two di- mensional organic metal (ET)8[Hg4Cl12(C6H5Br)]2 have been
studied up to 1.1 GPa in pulsed magnetic fields of up to 54 T. According to
band structure calculations, its Fermi surface can be regarded as a network of
compensated orbits. The SdH spectra exhibit many Fourier components typical of
such a network, most of them being forbidden in the framework of the
semiclassical model. Their amplitude remains large in all the pressure range
studied which likely rules out chemical potential oscillation as a dominant
contribution to their origin, in agreement with recent calculations relevant to
compensated Fermi liquids. In addition to a strong decrease of the magnetic
breakdown field and effective masses, the latter being likely due to a
reduction of the strength of electron correlations, a sizeable increase of the
scattering rate is observed as the applied pressure increases. This latter
point, which is at variance with data of most charge transfer salts is
discussed in connection with pressure-induced features of the temperature
dependence of the zero-field interlayer resistance | cond-mat_str-el |
Valence skipping, internal doping and site-selective Mott transition in
PbCoO$_3$ under pressure: We present a computational study of PbCoO$_3$ at ambient and elevated
pressure. We employ the static and dynamic treatment of local correlation in
form of density functional theory + $U$ (DFT+$U$) and + dynamical mean-field
theory (DFT+DMFT). Our results capture the experimentally observed crystal
structures and identify the unsaturated Pb $6s$ - O $2p$ bonds as the driving
force beyond the complex physics of PbCoO$_3$. We provide a geometrical
analysis of the structural distortions and we discuss their implications, in
particular, the internal doping, which triggers transition between phases with
and without local moments and a site selective Mott transition in the
low-pressure phase. | cond-mat_str-el |
Fractional quantum Hall states at zero magnetic field: We present a simple prescription to flatten isolated Bloch bands with
non-zero Chern number. We first show that approximate flattening of bands with
non-zero Chern number is possible by tuning ratios of nearest-neighbor and
next-nearest neighbor hoppings in the Haldane model and, similarly, in the
chiral-pi-flux square lattice model. Then we show that perfect flattening can
be attained with further range hoppings that decrease exponentially with
distance. Finally, we add interactions to the model and present exact
diagonalization results for a small system at 1/3 filling that support (i) the
existence of a spectral gap, (ii) that the ground state is a topological state,
and (iii) that the Hall conductance is quantized. | cond-mat_str-el |
Nonperturbative Dynamical Theory and A Scheme for Nonequilibrium
Transport: We develop a nonperturbative dynamical theory (NDT) that is useful for
treating nonequilibrium transport in a system with strong correlation. We apply
our NDT to the single-impurity Anderson model in equilibrium to check its
reliability by comparing with the results of numerical renormalization group
method (NRG). We finally suggest a self-consistent loop to calculate the
current in a lead-dot-lead system with Kondo coupling. | cond-mat_str-el |
Interacting Fermions Picture for Dimer Models: Recent numerical results on classical dimers with weak aligning interactions
have been theoretically justified via a Coulomb Gas representation of the
height random variable. Here we propose a completely different representation,
the Interacting Fermions Picture, which avoids some difficulties of the Coulomb
Gas approach and provides a better account of the numerical findings. Besides,
we observe that Peierls' argument explains the behavior of the system in the
strong interaction case. | cond-mat_str-el |
Role of exciton screening in the 7/3 fractional quantum Hall effect: The excitations of the 7/3 fractional Hall state, one of the most prominent
states in the second Landau level, are not understood. We study the effect of
screening by composite fermion excitons and find that it causes a strong
renormalization at 7/3, thanks to a relatively small exciton gap and a
relatively large residual interaction between composite fermions. The
excitations of the 7/3 state are to be viewed as composite fermions dressed by
a large exciton cloud. Their wide extent has implications for experiments as
well as for analysis of finite system exact diagonalization studies. | cond-mat_str-el |
'Kondo state' and Kondo resonance in a two-dimensional electron gas: The delicate balance of spin-screening and spin-aligning interactions
determines many of the peculiar properties of dilute magnetic systems. We study
a surface-supported all-organic multi-impurity Kondo spin system at the atomic
scale by low-temperature scanning tunnelling microscopy and -spectroscopy. The
model system consists of spin-1/2 radicals that are aligned in one-dimensional
chains and interact via a ferromagnetic RKKY interaction mediated by the 2DEG
of the supporting substrate. Due to the RKKY-induced enhanced depopulation of
one spin-subband in the 2DEG, we finally succeeded to detect the so far
unobserved 'Kondo state' as opposed to the well-established Kondo resonance.
Its cloud of screening electrons, that are virtually bound to the radicals
below the Kondo temperature, represents the extended exchange hole of the
ferromagnetically polarized spin chain imaged here in real space. | cond-mat_str-el |
Persistence of singlet fluctuations in the coupled spin tetrahedra
system Cu2Te2O5Br2 revealed by high-field magnetization and 79Br NQR - 125Te
NMR: We present high-field magnetization and $^{79}$Br nuclear quadrupole
resonance (NQR) and $^{125}$Te nuclear magnetic resonance (NMR) studies in the
weakly coupled Cu$^{2+}$ ($S=1/2$) tetrahedral system Cu$_2$Te$_2$O$_5$Br$_2$.
The field-induced level crossing effects were observed by the magnetization
measurements in a long-ranged magnetically ordered state which was confirmed by
a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In
the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much
larger than that observed by static bulk measurements. Our results imply that
the inter- and the intra-tetrahedral interactions compete, but at the same time
they cooperate strengthening effectively the local intratetrahedral exchange
couplings. We discuss that the unusual feature originates from the frustrated
intertetrahedral interactions. | cond-mat_str-el |
Critical phase induced by Berry phase and dissipation in a spin chain: Motivated by experiments on spin chains embedded in a metallic bath, as well
as closed quantum systems described by long-range interacting Hamiltonians, we
study a critical SU(N) spin chain perturbed by dissipation, or equivalently,
after space-time rotation, long-range spatial interactions. The interplay of
dissipation and the Wess-Zumino (Berry phase) term results in a rich phase
diagram with multiple renormalization-group fixed points. For a range of the
exponent that characterizes the dissipative bath, we find a second-order phase
transition between the fixed point that describes an isolated critical spin
chain and a dissipation-induced-ordered phase. More interestingly, for a
different range of the exponent, we find a stable, gapless, nonrelativistic
phase of matter whose existence necessarily requires coupling to the
dissipative bath. Upon tuning the exponent, we find that the fixed point
corresponding to this gapless, stable phase "annihilates" the fixed point that
describes the transition out of this phase to the ordered phase. We also study
a relativistic version of our model, and we identify a new critical point. We
discuss the implications of our work for Kondo lattice systems and engineered
long-range interacting quantum systems. | cond-mat_str-el |
Evidence for strong 5d electron correlations and electron-magnon
coupling in a pyrochlore, Y2Ir2O7: We report the observation of an unusual behavior of highly extended 5d
electrons in Y2Ir2O7 belonging to pyrochlore family of great current interest
using high resolution photoemission spectroscopy. The experimental bulk spectra
reveal an intense lower Hubbard band in addition to weak intensities in the
vicinity of the Fermi level, e_F. This provides a direct evidence for strong
electron correlation among the 5d electrons, despite their highly extended
nature. The high resolution spectrum at room temperature exhibits a pseudogap
at e_F and |e - e_F|^2 dependence demonstrating the importance of electron
correlation in this system. Remarkably, in the magnetically ordered phase (T <
150 K), the spectral lineshape evolves to a |e - e_F|^1.5 dependence
emphasizing the dominant role of electron-magnon coupling. | cond-mat_str-el |
Dielectric signature of charge order in lanthanum nickelates: Three charge-ordering lanthanum nickelates La2-xAxNiO4, substituted with
specific amounts of A = Sr, Ca, and Ba to achieve commensurate charge order,
are investigated using broadband dielectric spectroscopy up to GHz frequencies.
The transition temperatures of the samples are characterized by additional
specific heat and magnetic susceptibility measurements. We find colossal
magnitudes of the dielectric constant for all three compounds and strong
relaxation features, which partly are of Maxwell-Wagner type arising from
electrode polarization. Quite unexpectedly, the temperature-dependent colossal
dielectric constants of these materials exhibit distinct anomalies at the
charge-order transitions. | cond-mat_str-el |
Fragile magnetic order in metallic quasicrystals: Inspired by recent experimental studies of local magnetic moments interacting
with a metallic quasicrystal, we study the low-temperature fate of spins placed
in the two-dimensional Ammann-Beenker tiling. In the diluted local moment
limit, we calculate the spin relaxation rate $1/T_{1}$, as measured by electron
spin resonance, and show that it displays a marked dependence on the system
size $N$ and the Fermi energy of the electronic bath. For a finite
concentration of spins, we integrate out the conduction electrons and generate
an effective magnetic coupling between the local moments, which we treat as
Ising spins. Despite the strongly frustrating nature of the magnetic couplings
and the lack of periodicity in the problem, we find long-range orders for
finite $N$ in our large-scale Monte Carlo simulations. However, the resulting
magnetically state is fragile, as clusters of essentially free spins fluctuate
down to very low temperatures. | cond-mat_str-el |
Family of Sachdev-Ye-Kitaev models motivated by experimental
considerations: Several condensed-matter platforms have been proposed recently to realize the
Sachdev-Ye-Kitaev (SYK) model in their low-energy limit. In these proposed
realizations, the characteristic SYK behavior is expected to occur under
certain assumptions about the underlying physical system that (i) render all
bilinear terms small compared to four-fermion interactions and (ii) ensure that
the coupling constants are approximately all-to-all and independent random
variables. In this work we explore, both analytically and numerically, the
family of models that arises when we relax these assumptions in ways motivated
by real physical systems. By relaxing (i) and allowing large bilinear terms, we
obtain a novel, exactly-solvable cousin of the SYK model. It exhibits two
distinct phases separated by a quantum phase transition characterized by a
power-law, $\sim |\omega|^{-1/3}$ scaling of the low-energy spectral density,
despite being a non-interacting model. By relaxing (ii), we obtain close
relatives of the SYK model which exhibit interesting behaviors, including a
chaotic non-Fermi liquid phase with continuously varying fermion scaling
dimension, and a phase transition to a disordered Fermi liquid as a function of
interaction range and disorder length scale. | cond-mat_str-el |
Spin textures on general surfaces of the correlated topological
insulator SmB6: Employing the $\mathbf{k}\cdot\mathbf{p}$ expansion for a family of
tight-binding models for SmB$_6$, we analytically compute topological surface
states on a generic $(lmn)$ surface. We show how the Dirac-cone spin structure
depends on model ingredients and on the angle $\theta$ between the surface
normal and the main crystal axes. We apply the general theory to $(001)$,
$(110)$, $(111)$, and $(210)$ surfaces, for which we provide concrete
predictions for the spin pattern of surface states which we also compare with
tight-binding results. As shown in previous work, the spin pattern on a $(001)$
surface can be related to the value of mirror Chern numbers, and we explore the
possibility of topological phase transitions between states with different
mirror Chern numbers and the associated change of the spin structure of surface
states. Such transitions may be accessed by varying either the hybridization
term in the Hamiltonian or the crystal-field splitting of the low-energy $f$
multiplets, and we compute corresponding phase diagrams. | cond-mat_str-el |
Supercurrent and multiple singlet-doublet phase transitions of a quantum
dot Josephson junction inside an Aharonov-Bohm ring: We study a quantum dot Josephson junction inside an Aharonov-Bohm
environment. The geometry is modeled by an Anderson impurity coupled to two
directly-linked BCS leads. We illustrate that the well-established picture of
the low-energy physics being governed by an interplay of two distinct (singlet
and doublet) phases is still valid for this interferometric setup. The phase
boundary depends, however, non-monotonically on the coupling strength between
the superconductors, causing the system to exhibit re-entrance behavior and
multiple phase transitions. We compute the zero-temperature Josephson current
and demonstrate that it can become negative in the singlet phase by virtue of
the Coulomb interaction U. As a starting point, the limit of large
superconducting energy gaps \Delta=\infty is solved analytically. In order to
tackle arbitrary \Delta<\infty and U>0, we employ a truncated functional
renormalization group scheme which was previously demonstrated to give
quantitatively reliable results for the quantum dot Josephson problem. | cond-mat_str-el |
Permutation Group Symmetry and Correlations: Correlation factors are constructed that are consistent with the permutation
symmetry group of N Fermions at given value of the filling factor. | cond-mat_str-el |
Hybrid excitations due to crystal-field, spin-orbit coupling and
spin-waves in LiFePO$_4$: We report on the spin waves and crystal field excitations in single crystal
LiFePO$_4$ by inelastic neutron scattering over a wide range of temperatures,
below and above the antiferromagnetic transition of this system. In particular,
we find extra excitations below $T_N=50$ K that are nearly dispersionless and
are most intense around magnetic zone centers. We show that these excitations
correspond to transitions between thermally occupied excited states of
Fe$^{2+}$ due to splitting of the $S=2$ levels that arise from crystal field
and spin-orbit interaction. These excitations are further amplified by the
highly distorted nature of the oxygen octahedron surrounding the iron atoms.
Above $T_N$, magnetic fluctuations are observed up to at least 720~K, with
additional excitation around 4 meV, likely caused by single-ion splittings
through spin-orbit and crystal field interactions. The latter weakens slightly
at 720~K compared to 100~K, which is consistent with calculated cross-sections
using a single-ion model. Our theoretical analysis, using the MF-RPA model,
provides both detailed spectra of the Fe $d-$ shell and estimates of the
average ordered magnetic moment and $T_N$. By applying the MF-RPA model to a
number of existing spin-wave results from other Li$M$PO$_4$ ($M=$ Mn, Co, and
Ni), we are able to obtain reasonable predictions for the moment sizes and
transition temperatures. | cond-mat_str-el |
Antiferromagnetism in the magnetoelectric effect single crystal
LiMnPO$_4$: Elastic and inelastic neutron scattering studies reveal details of the
antiferromagnetic tansition and intriguing spin-dynamics in the
magneto-electric effect single crystal LiMnPO$_4$. The elastic scattering
studies confirm the system is antiferromagnetic (AFM) below $T_N$=33.75 K with
local magnetic moments (Mn$^{2+}$; $S = 5/2$) that are aligned along the
crystallographic a-axis. The spin-wave dispersion curves propagating along the
three principal axes, determined by inelastic scattering, are adequately
modeled in the linear spin-wave framework assuming a spin-Hamiltonian that is
parameterized by inter- and in-plane nearest- and next-nearest-neighbor
interactions, and by easy-plane anisotropy. The temperature dependence of the
spin dynamics makes this an excellent model many-body spin system to address
the question of the relationship between spin-wave excitations and the order
parameter. | cond-mat_str-el |
Classical spin dynamics based on SU($N$) coherent states: We introduce a classical limit of the dynamics of quantum spin systems based
on coherent states of SU($N$), where $N$ is the dimension of the local Hilbert
space. This approach, that generalizes the well-known Landau-Lifshitz dynamics
from SU(2) to SU($N$), provides a better approximation to the exact quantum
dynamics for a large class of realistic spin Hamiltonians, including $S \geq 1$
systems with large single-ion anisotropy and weakly-coupled multi-spin units,
such as dimers or trimers. We illustrate this idea by comparing the spin
structure factors of a single-ion $S=1$ model that are obtained with the SU(2)
and SU(3) classical spin dynamics against the exact solution. | cond-mat_str-el |
Orbital-singlet pairing and order parameter symmetry in Sr_2RuO_4: Based on the degeneracy of the d_{zx} and d_{yz} orbitals in Sr_2RuO_4 it is
argued that the Cooper pairs condense in orbital singlets. Together with the
spin-triplet wave functions the real-space wave function then is symmetric.
Considering interaction effects the order parameter is found to have A_{1g}
symmetry consistent with a number of experimental observations. The sensitivity
of the material on non-magnetic impurities follows in a straightforward manner
from the orbital-singlet configuration. | cond-mat_str-el |
Cycloidal Magnetic Ordering in Noncentrosymmetric EuIrGe$_3$: Successive magnetic phase transitions at $T_{\text{N}}$=12.2 K,
$T_{\text{N}}^{\;\prime}$=7.0 K, and $T_{\text{N}}^{\;*}$=5.0 K in EuIrGe$_3$,
an intermetallic compound with a body centered tetragonal lattice belonging to
a polar space group $I4mm$, has been investigated by neutron diffraction and
resonant X-ray diffraction. It is shown that EuIrGe$_3$ exhibits an
incommensurate longitudinal sinusoidal order with $q\sim (0, 0, 0.792)$ and
$m_{q} \parallel c\text{-axis}$ in the high temperature phase
($T_{\text{N}}^{\;\prime}< T < T_{\text{N}}$), which changes to a cycloidal
order with $q=(\delta', 0, 0.8)$ ($\delta'\sim 0.017$) and $m_{q} \parallel
ac\text{-plane}$ in the intermediate phase ($T_{\text{N}}^{\;*} < T <
T_{\text{N}}^{\;\prime}$). In the low temperature phase ($T <
T_{\text{N}}^{\;*}$), the cycloidal plane rotates by $45^{\circ}$ to have
$q=(\delta, \delta, 0.8)$ ($\delta\sim 0.012$). It is also pointed out that the
X-ray scattering amplitude from odd-parity magnetic quadrupole due to the polar
environment interfere with that from normal even-parity magnetic dipole in the
magnetic ordered phase. | cond-mat_str-el |
Photoinduced charge carrier dynamics in Hubbard two-leg ladders and
chains: The charge carrier dynamics of doped electronic correlated systems on ladders
and chains, subject to ultrafast photoirradiation, is investigated using the
time-dependent Lanczos method. The time-resolved optical conductivity and the
temporal profiles of other relevant quantities, including the doublon number,
the kinetic energy, and the interaction energy, are calculated. Two competitive
factors that can influence the transient charge carrier dynamics are identified
as the thermal effect and the charge effect. We demonstrate that the analysis
of their interplay can provide an intuitive way to understand the numerical
results and the recent optical pump-probe experiment on a two-leg ladder
cuprate. | cond-mat_str-el |
Massive CP$^1$ theory from a microscopic model for doped
antiferromagnets: A path-integral for the t-J model in two dimensions is constructed based on
Dirac quantization, with an action found originally by Wiegmann (Phys. Rev.
Lett. {\bf 60}, 821 (1988); Nucl. Phys. B323, 311 (1989)). Concentrating on the
low doping limit, we assume short range antiferromagnetic order of the spin
degrees of freedom. Going over to a local spin quantization axis of the dopant
fermions, that follows the spin degree of freedom, staggered CP$^1$ fields
result and the constraint against double occupancy can be resolved. The
staggered CP$^1$ fields are split into slow and fast modes, such that after a
gradient expansion, and after integrating out the fast modes and the dopant
fermions, a CP$^1$ field-theory with a massive gauge field is obtained that
describes generically incommensurate coplanar magnetic structures, as discussed
previously in the context of frustrated quantum antiferromagnets. Hence, the
possibility of deconfined spinons is opened by doping a colinear
antiferromagnet. | cond-mat_str-el |
Short-Range Entangled Bosonic States with Chiral Edge Modes and
$T$-duality of Heterotic Strings: We consider states of bosons in two dimensions that do not support anyons in
the bulk, but nevertheless have stable chiral edge modes that are protected
even without any symmetry. Such states must have edge modes with central charge
$c=8k$ for integer $k$. While there is a single such state with $c=8$, there
are, naively, two such states with $c=16$, corresponding to the two distinct
even unimodular lattices in 16 dimensions. However, we show that these two
phases are the same in the bulk, which is a consequence of the uniqueness of
signature $(8k +n, n)$ even unimodular lattices. The bulk phases are stably
equivalent, in a sense that we make precise. However, there are two different
phases of the edge corresponding to these two lattices, thereby realizing a
novel form of the bulk-edge correspondence. Two distinct fully chiral edge
phases are associated with the same bulk phase, which is consistent with the
uniqueness of the bulk since the transition between them, which is generically
first-order, can occur purely at the edge. Our construction is closely related
to $T$-duality of toroidally compactified heterotic strings. We discuss
generalizations of these results. | cond-mat_str-el |
DC and optical signatures of the topological reconstruction of the Fermi
surface for electrons with parabolic band dispersion: We study the main intra-band and inter-band transport properties at zero
temperature of free electron-like system undergoing a topological
reconstruction of the Fermi surface for the two-dimensional and
three-dimensional case. The calculated intra-band properties include the
single-particle density of states, the total and the effective concentrations
of electrons and the thermopower. As for the inter-band case, the real part of
the conductivity has been calculated within the vanishing inter-band relaxation
approximation as a function of the incident photon energy. Within this
approach, it is shown that the optical conductivity has a nonvanishing
component parallel to the reconstruction wave vector and the shape which
depends on the value of the Fermi energy. Each dimensionality has its
particular features in the transport quantities presented in the paper, which
are discussed and compared with those in the free electron scenario. Finally,
we identify the signature of the topological reconstruction of the Fermi
surface in the intra-band and inter-band transport functions. | cond-mat_str-el |
Destruction of Neel order and local spin spirals in insulating
La_{2-x}Sr_xCuO_4: Starting from the t-J model, we derive an effective field theory describing
the spin dynamics in the insulating phase of La_{2-x}Sr_xCuO_4, x < 0.055, at
low temperature. Using Monte Carlo simulations, we show that the destruction of
Neel order is driven by the single-hole localization length kappa. A phase
transition at 2% doping is consistent with the value of kappa known from the
variable range hopping conductivity. The static spin structure factor obtained
in our calculations is in perfect agreement with neutron scattering data over
the whole range of doping. We also demonstrate that topological defects (spin
vortex-antivortex pairs) are an intrinsic property of the spin-glass ground
state. | cond-mat_str-el |
Evidence for Charge Glass-like Behavior in Lightly Doped
La_{2-x}Sr_{x}CuO_{4} at Low Temperatures: A c-axis magnetotransport and resistance noise study in
La_{1.97}Sr_{0.03}CuO_{4} reveals clear signatures of glassiness, such as
hysteresis, memory, and slow, correlated dynamics, but only at temperatures (T)
well below the spin glass transition temperature T_{sg}. The results strongly
suggest the emergence of charge glassiness, or dynamic charge ordering, as a
result of Coulomb interactions. | cond-mat_str-el |
Jahn-Teller systems at half filling: crossover from Heisenberg to Ising
behavior: The Jahn-Teller model with $E\otimes\beta$ electron-phonon coupling and local
(Hubbard-like) Coulomb interaction is considered to describe a lattice system
with two orbitals per site at half filling. Starting from a state with one
electron per site, we follow the tunneling of the electrons and the associated
creation of an arbitrary number of phonons due to electron-phonon interaction.
For this purpose we apply a recursive method which allows us to organize
systematically the number of pairs of empty/doubly occupied sites and to
include infinitely many phonons which are induced by electronic tunneling. In
lowest order of the recursion (i.e. for all processes with only one pairs of
empty/doubly occupied sites) we obtain an effective anisotropic pseudospin 1/2
Heisenberg Hamiltonian $H_{eff}$ as a description of the orbital degrees of
freedom. The pseudospin coupling depends on the physical parameters and the
energy. This implies that the resulting resolvent $(z-H_{eff}(z))^{-1}$ has an
infinite number of poles, even for a single site. $H_{eff}$ is subject to a
crossover from an isotropic Heisenberg model (weak electron-phonon coupling) to
an Ising model (strong electron-phonon coupling). | cond-mat_str-el |
Magnetic phase diagram and structure of the magnetic phases in the
quasi-one-dimensional antiferromagnet BaCu_2Si_2O_7: symmetry analysis: We have performed a symmetry analysis of the properties of the recently
discovered quasi-one-dimensional compound BaCu_2Si_2O_7. The existence of the
unusual spin-reorientation transitions is explained as an effect of the
unusually strong relativistic interactions. The possible connection between the
magnitude of the relativistic interactions and the low-dimensional structure of
the BaCu_2Si_2O_7 is discussed. The structure of the magnetic phases is
determined. | cond-mat_str-el |
Quantum criticality with two length scales: The theory of deconfined quantum critical points describes phase transitions
at temperature T = 0 outside the standard paradigm, predicting continuous
transformations between certain ordered states where conventional theory
requires discontinuities. Numerous computer simulations have offered no proof
of such transitions, however, instead finding deviations from expected scaling
relations that were neither predicted by the DQC theory nor conform to standard
scenarios. Here we show that this enigma can be resolved by introducing a
critical scaling form with two divergent length scales. Simulations of a
quantum magnet with antiferromagnetic and dimerized ground states confirm the
form, proving a continuous transition with deconfined excitations and also
explaining anomalous scaling at T > 0. Our findings revise prevailing paradigms
for quantum criticality, with potentially far-reaching implications for many
strongly-correlated materials. | cond-mat_str-el |
Magnetic Single-Electron Transistor as a Tunable Model System for
Kondo-Destroying Quantum Criticality: Single-electron transistors attached to ferromagnetic leads can undergo a
continuous quantum phase transition as their gate voltage is tuned. The
corresponding quantum critical point separates a Fermi liquid phase from a
non-Fermi liquid one. Here, we expound on the physical idea proposed earlier.
The key physics is the critical destruction of the Kondo effect, which
underlies a new class of quantum criticality that has been argued to apply to
heavy fermion metals. Its manifestation in the transport properties is studied
through an effective Bose-Fermi Kondo model; the bosonic bath, corresponding to
the spin waves of the ferromagnetic leads, describes a particular type of
sub-Ohmic dissipation. We also present results for general forms of sub-Ohmic
dissipative bath, and consider in some detail the case with critical paramagons
replacing spin waves. Finally, we discuss some delicate aspects in the
theoretical treatment of the effect of a local magnetic field, particularly in
connection with the frequently employed Non-Crossing Approximation. | cond-mat_str-el |
Orbital ordering transition in Ca$_2$RuO$_4$ observed with resonant
x-ray diffraction: Resonant x-ray diffraction performed at the $\rm L_{II}$ and $\rm L_{III}$
absorption edges of Ru has been used to investigate the magnetic and orbital
ordering in Ca$_2$RuO$_4$ single crystals. A large resonant enhancement due to
electric dipole $2p\to 4d$ transitions is observed at the wave-vector
characteristic of antiferromagnetic ordering. Besides the previously known
antiferromagnetic phase transition at $\rm T_{N}=110$ K, an additional phase
transition, between two paramagnetic phases, is observed around 260 K. Based on
the polarization and azimuthal angle dependence of the diffraction signal, this
transition can be attributed to orbital ordering of the Ru $t_{2g}$ electrons.
The propagation vector of the orbital order is inconsistent with some
theoretical predictions for the orbital state of Ca$_2$RuO$_4$. | cond-mat_str-el |
Weyl Fermion Magneto-Electrodynamics and Ultra-low Field Quantum Limit
in TaAs: Topological semimetals are predicted to exhibit unconventional
electrodynamics, but a central experimental challenge is singling out the
contributions from the topological bands. TaAs is the prototypical example,
where 24 Weyl points and 8 trivial Fermi surfaces make the interpretation of
any experiment in terms of band topology ambiguous. We report magneto-infrared
reflection spectroscopy measurements on TaAs. We observed sharp inter-Landau
level transitions from a single pocket of Weyl Fermions in magnetic fields as
low as 0.4 tesla. We determine the W2 Weyl point to be 8.3 meV below the Fermi
energy, corresponding to a quantum limit - the field required to reach the
lowest LL - of 0.8 Tesla - unprecedentedly low for Weyl Fermions. LL
spectroscopy allows us to isolate these Weyl Fermions from all other carriers
in TaAs and our result provides a new way for directly exploring the more
exotic quantum phenomena in Weyl semimetals, such as the chiral anomaly. | cond-mat_str-el |
The electronic structure and magnetic phase transition of hexagonal FeSe
thin films studied by photoemission spectroscopy: Hexagonal FeSe thin films were grown on SrTiO3 substrates and the temperature
and thickness dependence of their electronic structures were studied. The
hexagonal FeSe is found to be metallic and electron doped, whose Fermi surface
consists of six elliptical electron pockets. With decreased temperature, parts
of the bands shift downward to high binding energy while some bands shift
upwards to EF. The shifts of these bands begin around 300 K and saturate at low
temperature, indicating a magnetic phase transition temperature of about 300 K.
With increased film thickness, the Fermi surface topology and band structure
show no obvious change except some minor quantum size effect. Our paper reports
the first electronic structure of hexagonal FeSe, and shows that the possible
magnetic transition is driven by large scale electronic structure
reconstruction. | cond-mat_str-el |
Ground state of Ce$_{3}$Bi$_{4}$Pd$_{3}$ unraveled by hydrostatic
pressure: Noncentrosymmetric Ce$_{3}$Bi$_{4}$Pd$_{3}$ has attracted a lot of attention
as a candidate for strongly correlated topological material, yet its
experimental ground state remains a matter of contention. Two conflicting
scenarios have emerged from a comparison to prototypical Kondo insulator
Ce$_{3}$Bi$_{4}$Pt$_{3}$: either Ce$_{3}$Bi$_{4}$Pd$_{3}$ is a
spin-orbit-driven topological semimetal or a Kondo insulator with smaller Kondo
coupling than its Pt counterpart. Here we determine the ground state of
Ce$_{3}$Bi$_{4}$Pd$_{3}$ via electrical resistivity measurements under
hydrostatic pressure, which is a clean symmetry-preserving tuning parameter
that increases hybridization but virtually preserves spin-orbit coupling.
Ce$_{3}$Bi$_{4}$Pd$_{3}$ becomes more insulating under pressure, which is a
signature of Ce-based Kondo insulating materials. Its small zero-pressure gap
increases quadratically with pressure, similar to the behavior observed in the
series Ce$_{3}$Bi$_{4}$(Pt$_{1-x}$Pd$_{x}$)$_{3}$, which indicates that Pt
substitution and applied pressure have a similar effect. Our result not only
demonstrates that Kondo coupling, rather than spin-orbit coupling, is the main
tuning parameter in this class of materials, but it also establishes that
Ce$_{3}$Bi$_{4}$Pd$_{3}$ has a narrow-gap Kondo insulating ground state. | cond-mat_str-el |
The Role of the Exchange Interaction in the One-Dimensional
$n$-Component Hubbard Model: The commensurate $p/q$-filled $n$-component Hubbard chain was investigated by
bosonization and high-precision density-matrix renormalization-group analysis.
It was found that depending on the relation between the number of components
$n$, and the filling parameter $q$, the system shows metallic or insulating
behavior, and for special fillings bond-ordered (dimerized, trimerized,
tetramerized etc.) ground state develops in the insulating phase. A mean-field
analysis shows that this bond ordering is a direct consequence of the
spin-exchange interaction, which plays a crucial role in the one-parameter
Hubbard model -- not only for infinite Coulomb repulsion, but for intermediate
values as well. | cond-mat_str-el |
Honeycomb rare-earth magnets with anisotropic exchange interactions: We study the rare-earth magnets on a honeycomb lattice, and are particularly
interested in the experimental consequences of the highly anisotropic spin
interaction due to the spin-orbit entanglement. We perform a high-temperature
series expansion using a generic nearest-neighbor Hamiltonian with anisotropic
interactions, and obtain the heat capacity, the parallel and perpendicular spin
susceptibilities, and the magnetic torque coefficients. We further examine the
electron spin resonance linewidth as an important signature of the anisotropic
spin interactions. Due to the small interaction energy scale of the rare-earth
moments, it is experimentally feasible to realize the strong-field regime.
Therefore, we perform the spin-wave analysis and study the possibility of
topological magnons when a strong field is applied to the system. The
application and relevance to the rare-earth Kitaev materials are discussed. | cond-mat_str-el |
Negative Josephson coupling in the Kondo strong coupling limit: We Show that pair hopping through a Kondo singlet give rise to a negative
Josephson coupling. Thus, our calculation supports the existance of staggered
pair correlations in the strong coupling limit of a one dimensional Kondo
lattice. | cond-mat_str-el |
Crystal Field and Magnetoelastic Interactions in Tb2Ti2O7: In terms of a semi-phenomenological exchange charge model, we have obtained
estimates of parameters of the crystal field and parameters of the
electron-deformation interaction in terbium titanate Tb2Ti2O7 with a pyrochlore
structure. The obtained set of parameters has been refined based on the
analysis of spectra of neutron inelastic scattering and Raman light scattering,
field dependences of the forced magnetostriction, and temperature dependences
of elastic constants. | cond-mat_str-el |
Unconventional resistivity at the border of metallic antiferromagnetism
in NiS2: We report low-temperature and high-pressure measurements of the electrical
resistivity \rho(T) of the antiferromagnetic compound NiS_2 in its
high-pressure metallic state. The form of \rho(T) suggests that metallic
antiferromagnetism in NiS_2 is quenched at a critical pressure p_c=76+-5 kbar.
Near p_c the temperature variation of \rho(T) is similar to that observed in
NiS_{2-x}Se_x near the critical composition x=1 where the Neel temperature
vanishes at ambient pressure. In both cases \rho(T) varies approximately as
T^{1.5} over a wide range below 100 K. However, on closer analysis the
resistivity exponent in NiS_2 exhibits an undulating variation with temperature
not seen in NiSSe (x=1). This difference in behaviour may be due to the effects
of spin-fluctuation scattering of charge carriers on cold and hot spots of the
Fermi surface in the presence of quenched disorder, which is higher in NiSSe
than in stoichiometric NiS_2. | cond-mat_str-el |
Scalar Chiral Spin-1/2 Order on Kagome Lattices in Nd3Sb3Mg2O14: We introduce $\mathrm{Nd_{3}Sb_{3}Mg_{2}O_{14}}$ with ideal kagome lattices
of neodymium ions in ABC stacking. Thermodynamic measurements show a
Curie-Weiss temperature of $\Theta_{CW}=-0.12~$K, a Nd$^{3+}$ spin-1/2 Kramers
doublet ground state, and a second order phase transition at $T_N=0.56(2)~$K.
Neutron scattering reveals non-coplanar scalar chiral ${\bf k} =0$ magnetic
order with a correlation length exceeding 400 \AA = 55 $a$ and an ordered
moment of $1.79(5)~\mu_B$. This order includes a canted ferromagnetic component
perpendicular to the kagome planes favored by Dzyaloshinskii-Moriya
interactions. | cond-mat_str-el |
Strain induced edge magnetism at zigzag edge in graphene quantum dot: We study the temperature dependent magnetic susceptibility of a strained
graphene quantum dot by using the determinant quantum Monte Carlo method.
Within the Hubbard model on a honeycomb lattice, our unbiased numerical results
show that a relative small interaction $U$ may lead to a edge ferromagnetic
like behavior in the strained graphene quantum dot, and a possible room
temperature transition is suggested. Around half filling, the ferromagnetic
fluctuations at the zigzag edge is strengthened both markedly by the on-site
Coulomb interaction and the strain, especially in low temperature region. The
resultant strongly enhanced ferromagnetic like behavior may be important for
the development of many applications. | cond-mat_str-el |
Emergent SU(3) symmetry in a four leg spin tube: We consider an antiferromagnetic four leg spin-1/2 tube using abelian and
non-abelian bosonization. We show that in the limit of weak interchain
coupling, the most relevant interaction gives rise to an emergent SU(3)}
symmetry, broken only by marginal interactions that can be canceled by diagonal
interchain couplings. We discuss the low energy spectrum in the semiclassical
limit and using a mapping to a trimerized SU(3) spin chain. We establish that
the correlation functions of ferroquadrupolar operators can be used to reveal
the emergent symmetry. | cond-mat_str-el |
Diagnosing a strong topological insulator by quantum oscillations: We show how quantum oscillation measurements of surface states in an
insulator may allow to diagnose a strong topological insulator and distinguish
it from its weak or topologically trivial counterpart. The criterion is defined
by the parity of the number of fundamental frequencies in the surface-state
quantum oscillation spectrum: an even number of frequencies implies a weak or a
topologically trivial insulator, whereas an odd number points to a strong
topological insulator. We also discuss various aspects and issues related to
applying this criterion in practice. | cond-mat_str-el |
Spin-phonon interaction increased by compressive strain in
antiferromagnetic MnO thin films: MnO thin films with various thicknesses and strains were grown on MgO
substrates by pulsed laser deposition, then characterized using x-ray
diffraction and infrared reflectance spectroscopy. Films grown on
(001)-oriented MgO substrates exhibit homogenous biaxial compressive strain
which increases as the film thickness is reduced. For that reason, the
frequency of doubly-degenerate phonon increases with the strain, and splits
below N\'eel temperature TN due to the magnetic-exchange interaction. Films
grown on (110)-oriented MgO substrates exhibit a huge phonon splitting already
at room temperature due to the anisotropic in-plane compressive strain. Below
TN, additional phonon is activated in the IR spectra; this trend is evidence
for a spin-order-induced structural phase transition from tetragonal to
monoclinic phase. Total phonon splitting is 55 cm-1 in (110)-oriented MnO film,
which is more than twice the value in bulk MnO. This result is evidence that
the nearest neighbor exchange interaction, which is responsible for the
magnetically driven phonon splitting, is greatly increased in compressively
strained films. | cond-mat_str-el |
A note on GMP algebra, dipole symmetry, and Hohenberg-Mermin-Wagner
theorem in the lowest Landau level: After projection to the lowest Landau level translational invariance and
particle conservation combine into dipole symmetry. We show that the new
symmetry forbids spontaneous $U(1)$ symmetry breaking at zero temperature. In
the case of the spatially inhomogeneous magnetic field, where the translational
invariance is absent, we show that the dipole symmetry disappears and the
constraint on the symmetry breaking is lifted. We pay special attention to the
fate of the Girvin-Macdonald-Platzman algebra in the inhomogeneous magnetic
field and show that a natural generalization of it is still present even though
the dipole symmetry is not. | cond-mat_str-el |
Single-hole dynamics in the t-J model: The quasi-particle weight of a single hole in an antiferromagnetic background
is studied in the semiclassical approximation. We start from the t-J model,
generalize it to arbitrary spin S by employing an appropriate coherent state
representation for the hole, and derive an effective action for the dynamics in
the long-wavelength low-energy limit. In the same limit, we find an expression
for the single-hole Green's function which we evaluate in an 1/S expansion. Our
approach has the advantage of being applicable in one and in two dimensions. We
find two qualitatively different results in these two cases: while in one
dimension our results are compatible with a vanishing quasi-particle weight,
this weight is found to be finite in two dimensions, indicating normal
quasi-particle behavior of the hole in this last case. | cond-mat_str-el |
Collective modes for helical edge state interacting with quantum light: We investigate the light-matter interaction between the edge state of a 2D
topological insulator and quantum electromagnetic field. The interaction
originates from the Zeeman term between the spin of the edge electrons and the
magnetic field, and also through the Peierls substition. The continuous U(1)
symmetry of the system in the absence of the vector potential reduces into
discrete time reversal symmetry in the presence of the vector potential. Due to
light-matter interaction, a superradiant ground state emerges with
spontaneously broken time reversal symmetry, accompanied by a net photocurrent
along the edge, generated by the vector potential of the quantum light. The
spectral function of the photon field reveals polariton continuum excitations
above a threshold energy, corresponding to a Higgs mode and another low energy
collective mode due to the phase fluctuations of the ground state. This
collective mode is a zero energy Goldstone mode that arises from the broken
continuous U(1) symmetry in the absence of the vector potential, and acquires
finite a gap in the presence of the vector potential. The optical conductivity
of the edge electrons is calculated using the random phase approximation by
taking the fluctuation of the order parameter into account. It contains the
collective modes as a Drude peak with renormalized effective mass, which moves
to finite frequencies as the symmetry of the system is lowered by the inclusion
of the vector potential. | cond-mat_str-el |
Uniform electron gas at finite temperatures: We calculate the free energy of the quantum uniform electron gas for
temperatures from near zero to 100 times the Fermi energy, approaching the
classical limit. An extension of the Vashista-Singwi theory to finite
temperatures and self-consistent compressibility sum rule is presented.
Comparisons are made to other local field correction methods, as well as recent
quantum Monte Carlo simulation and classical map based results. Accurate fits
to the exchange-correlation free energy from both theory and simulation are
given for future practical applications. | cond-mat_str-el |
The transport-structural correspondence across the nematic phase
transition probed by elasto-x-ray diffraction: Electronic nematicity in iron pnictide materials is coupled to both the
lattice and the conducting electrons, which allows both structural and
transport observables to probe nematic fluctuations and the order parameter.
Here we combine simultaneous transport and x-ray diffraction measurements with
in-situ tunable strain (elasto-XRD) to measure the temperature dependence of
the shear modulus and elastoresistivity above the nematic transition and the
spontaneous orthorhombicity and resistivity anisotropy below the nematic
transition, all within a single sample of $Ba(Fe_{0.96}Co_{0.04})_{2} As_{2}$.
The ratio of transport to structural quantities is nearly
temperature-independent over a 74 K range and agrees between the ordered and
disordered phases. These results show that elasto-XRD is a powerful technique
to probe the nemato-elastic and nemato-transport couplings, which have
important implications to the nearby superconductivity. It also enables the
measurement in the large strain limit, where the breakdown of mean field
description reveals the intertwined nature of nematicity. | cond-mat_str-el |
Novel aspects and strong correlation in the electronic structure of
Sr$_2$FeMoO$_6$: We investigate the electronic structure of Sr$_2$FeMoO$_6$ combining
photoemission spectroscopy with a wide range of photon energies and electronic
structure calculations based on first-principle as well as model Hamiltonian
approaches to reveal several interesting aspects. We find evidence for
unusually strong Coulomb correlation effects both in the Fe 3$d$ and O 2$p$
states, with an enhanced manifestation in the majority spin channel.
Additionally, O 2$p$ states exhibit a spin-splitting of nonmagnetic origin,
which nevertheless is likely to have subtle influence on the stability of novel
ferromagnetism of this compound. | cond-mat_str-el |
Doping-dependent magnetization plateaux in p-merized Hubbard chains: We study zero-temperature Hubbard chains with periodically modulated hopping
at arbitrary filling n and magnetization m. We show that the magnetization
curves have plateaux at certain values of m which depend on the periodicity p
and the filling. At commensurate filling n a charge gap opens and then
magnetization plateaux correspond to fully gapped situations. However, plateaux
also arise in the magnetization curves at fixed n between the commensurate
values and then the plateau-value of of m depends continuously on n and can
thus also become irrational. In particular for the case of dimerized hopping
(p=2) and fixed doping we find that a plateau appears at m=1-n. In this case,
there is still a gapless mode on the plateau leading to thermodynamic behavior
which is different from a completely gapped situation. | cond-mat_str-el |
Photoemission Study of Rare-Earth Ditelluride Compounds (ReTe_2 : Re =
La, Pr, Sm, and Gd): We studied the electronic structure of rare-earth ditelluride (ReTe_2 : Re =
La, Pr, Sm, and Gd) using photoemission spectroscopy. From the x-ray
photoelectron spectroscopy (XPS) study of the 3d core levels of rare-earth
elements, we found that all the rare earth elements are trivalent. We have also
made theoretical calculations using the Gunnarsson and Schoenhammer
approximation and multiplet calculations for the rare earth elements to find
that the La and Gd~3d peaks are well explained using our calculations. There is
no considerable change in the line-shape of the Te~3d peaks depending on
different rare earth elements. On ther other hand, valence band spectra studied
with the ultraviolet photoelectron spectroscopy (UPS) show a small change in
the Te p band depending on rare-earth elements. According to the UPS data,
LaTe_2 has very low carrier density at the Fermi level while SmTe_2 and PrTe_2
show strongly metallic band structure effects near the Fermi level. | cond-mat_str-el |
Matrix product state recursion methods for strongly correlated quantum
systems: We present a method for extrapolation of real-time dynamical correlation
functions which can improve the capability of matrix product state methods to
compute spectral functions. Unlike the widely used linear prediction method,
which ignores the origin of the data being extrapolated, our recursion methods
utilize a representation of the wavefunction in terms of an expansion of the
same wavefunction and its translations at earlier times. This recursion method
is exact for a noninteracting Fermi system. Surprisingly, the recursion method
is also more robust than linear prediction at large interaction strength. We
test this method on the Hubbard two-leg ladder and present more accurate
results for the spectral function than previous studies. | cond-mat_str-el |
Ground-State Phase Diagram of the Bond-Alternating $S=2$ Quantum Spin
Chain with the $XXZ$ and On-Site Anisotropies -- Symmetry Protected
Topological Phase versus Trivial Phase: We investigate the ground-state phase diagram of the bond-alternating $S=2$
quantum spin chain with the $XXZ$ and on-site anisotropies. For the on-site
anisotropies, in addition to the popular $D_2 \sum_j (S_j^z)^2$ term, we
consider the $D_4 \sum_j (S_j^z)^4$ term. Mainly we use the exact
diagonalization and the level spectroscopy analysis. We show that the Haldane
state, large-$D$ state and the Dimer2 state belong to the same trivial phase,
by finding the existence of adiabatic paths directly connecting these states
without the quantum phase transition. Similarly, we show that the
intermediate-$D$ state and the Dimer1 state belong to the same symmetry
protected topological phase. | cond-mat_str-el |
Equation of motion approach to the solution of Anderson model: Based on an equation of motion approach the single impurity Anderson
model(SIAM) is reexamined. Using the cluster expansions the equations of motion
of Green functions are transformed into the corresponding equations of motion
of connected Green functions, which provides a natural and uniform truncation
scheme. A factor of two missing in the Lacroix's approximation for the Kondo
temperature is gained in the next higher order truncation beyond Lacroix's. A
quantitative improvement in the density of states at the Fermi level is also
obtained. | cond-mat_str-el |
Origin of room-temperature ferromagnetism in Mn doped semiconducting
CdGeP2: CdGeP2 chalcopyrites doped with Mn have been recently found to exhibit room
temperature ferromagnetism. Isovalent substitution of the Cd site is expected,
however, to create antiferromagnetism, in analogy with the well-known CdTe:Mn
(d^5) case. However, chalcopyrite semiconductors exhibit low-energy intrinsic
defects. We show theoretically how ferromagnetism results from the interaction
of Mn with hole-producing intrinsic defects. | cond-mat_str-el |
Dynamic electronic correlation effects in NbO$_2$ as compared to VO$_2$: In this work we present a comparative investigation of the electronic
structures of NbO$_2$ and VO$_2$ obtained within the combination of density
functional theory and cluster-dynamical mean field theory calculations. We
investigate the role of dynamic electronic correlations on the electronic
structure of the metallic and insulating phases of NbO$_2$ and VO$_2$, with
focus on the mechanism responsible for the gap opening in the insulating
phases. For the rutile metallic phases of both oxides, we obtain that
electronic correlations lead to strong renormalization of the $t_{2g}$
subbands, as well as the emergence of incoherent Hubbard subbands, signaling
that electronic correlations are also important in the metallic phase of
NbO$_2$. Interestingly, we find that nonlocal dynamic correlations do play a
role in the gap formation of the (bct) insulating phase of NbO$_2$, by a
similar physical mechanism as that recently proposed by us in the case of the
(M$_1$) dimerized phase of VO$_2$ (\textit{Phys. Rev. Lett. 117, 056402
(2016)}). Although the effect of nonlocal dynamic correlations in the gap
opening of bct phase is less important than in the (M$_1$ and M$_2$) monoclinic
phases of VO$_2$, their presence indicates that the former is not a purely
Peierls-type insulator, as it was recently proposed. | cond-mat_str-el |
A quantum theory of the nearly frozen charge glass: We study long-range interacting electrons on the triangular lattice using
mixed quantum/classical simulations going beyond the usual classical
descriptions of the lattice Coulomb fluid. Our results in the strong
interaction limit indicate that the emergence and proliferation of quantum
defects governs the low-temperature dynamics of this strongly frustrated
system, in a way that crucially depends on the degree of anisotropy of the
electronic structure. The present theoretical findings explain the
phenomenology observed in the $\theta$-ET$_2$X charge ordering materials as
they fall out of equilibrium. The approach devised here can be easily
generalized to address other systems where charge frustration is lifted by
quantum fluctuations. | cond-mat_str-el |
Optical study of phase transitions in single-crystalline RuP: RuP single crystals of MnP-type orthorhombic structure were synthesized by
the Sn flux method. Temperature-dependent x-ray diffraction measurements reveal
that the compound experiences two structural phase transitions, which are
further confirmed by enormous anomalies shown in temperature-dependent
resistivity and magnetic susceptibility. Particularly, the resistivity drops
monotonically upon temperature cooling below the second transition, indicating
that the material shows metallic behavior, in sharp contrast with the
insulating ground state of polycrystalline samples. Optical conductivity
measurements were also performed in order to unravel the mechanism of these two
transitions. The measurement revealed a sudden reconstruction of band structure
over a broad energy scale and a significant removal of conducting carriers
below the first phase transition, while a charge-density-wave-like energy gap
opens below the second phase transition. | cond-mat_str-el |
Spectral Properties of the Attractive Hubbard Model: Deviations from Fermi liquid behavior are well documented in the normal state
of the cuprate superconductors, and some of these differences are possibly
related to pre-formed pairs appearing at temperatures above T_c. In order to
test these ideas we have investigated the attractive Hubbard model within a
self-consistent, conserving ladder approximation. In this version of the
theory, no feature is present which can be related to the pseudo gap found in
the high-T_c materials. Further, the interactions between two-particle bound
states change the physics of the superconducting instability in a profound
fashion, and lead to a completely different phenomenology that one predicts
based on the non-self-consistent version of the same theory. | cond-mat_str-el |
Expansion potentials for exact far-from-equilibrium spreading of
particles and energy: The rates at which energy and particle densities move to equalize arbitrarily
large temperature and chemical potential differences in an isolated quantum
system have an emergent thermodynamical description whenever energy or particle
current commutes with the Hamiltonian. Concrete examples include the energy
current in the 1D spinless fermion model with nearest-neighbor interactions
(XXZ spin chain), energy current in Lorentz-invariant theories or particle
current in interacting Bose gases in arbitrary dimension. Even far from
equilibrium, these rates are controlled by state functions, which we call
``expansion potentials'', expressed as integrals of equilibrium Drude weights.
This relation between nonequilibrium quantities and linear response implies
non-equilibrium Maxwell relations for the Drude weights. We verify our results
via DMRG calculations for the XXZ chain. | cond-mat_str-el |
Kinetic theory of spin-polarized systems in electric and magnetic fields
with spin-orbit coupling: I. Kinetic equation and anomalous Hall and
spin-Hall effects: The coupled kinetic equation for density and spin Wigner functions are
derived including spin-orbit coupling, electric and magnetic field as well as
selfconsistent Hartree meanfields suited for SU(2) transport. The interactions
are assumed to be with scalar and magnetic impurities as well as scalar and
spin-flip potentials among the particles. The spin-orbit interaction is used in
a form suitable to solid state physics with Rashba or Dresselhaus coupling,
graphene, extrinsic spin-orbit coupling as well as effective nuclear matter
coupling. The deficiencies of the two-fluid model are worked out consisting in
the appearance of an effective in-medium spin-precession. The stationary
solution of all these systems shows a band splitting controlled by an effective
medium-dependent Zeeman field. The selfconsistent precession direction is
discussed and a cancellation of linear spin-orbit coupling at zero temperature
is reported. The precession of spin around this effective direction caused by
spin-orbit coupling leads to anomalous charge and spin currents in an electric
field. Anomalous Hall conductivity is shown to consists of the known results
obtained from Kubo formula or Berry phases and a new symmetric part interpreted
as inverse Hall effect. Analogously the spin-Hall and inverse spin-Hall effect
of spin currents are discussed which are present even without magnetic fields
showing a spin accumulation triggered by currents. The analytical dynamical
expressions for zero temperature are derived and discussed in dependence on the
magnetic field and effective magnetizations. The anomalous Hall and spin-Hall
effect changes sign at higher than a critical frequency dependent on the
relaxation time. | cond-mat_str-el |
Momentum-dependent magnon lifetime in the metallic non-collinear
triangular antiferromagnet CrB2: Non-collinear magnetic order arises for various reasons in several magnetic
systems and exhibits interesting spin dynamics. Despite its ubiquitous
presence, little is known of how magnons, otherwise stable quasiparticles,
decay in these systems, particularly in metallic magnets. Using inelastic
neutron scattering, we examine the magnetic excitation spectra in a metallic
non-collinear antiferromagnet CrB$_{2}$, in which Cr atoms form a triangular
lattice and display incommensurate magnetic order. Our data show intrinsic
magnon damping and continuum-like excitations that cannot be explained by
linear spin wave theory. The intrinsic magnon linewidth $\Gamma(q,E_{q})$ shows
very unusual momentum dependence, which our analysis shows to originate from
the combination of two-magnon decay and the Stoner continuum. By comparing the
theoretical predictions with the experiments, we identify where in the momentum
and energy space one of the two factors becomes more dominant. Our work
constitutes a rare comprehensive study of the spin dynamics in metallic
non-collinear antiferromagnets. It reveals, for the first time, definite
experimental evidence of the higher-order effects in metallic antiferromagnets. | cond-mat_str-el |
Application of polynomial-expansion Monte Carlo method to a spin-ice
Kondo lattice model: We present the results of Monte Carlo simulation for a Kondo lattice model in
which itinerant electrons interact with Ising spins with spin-ice type
easy-axis anisotropy on a pyrochlore lattice. We demonstrate the efficiency of
the truncated polynomial expansion algorithm, which enables a large scale
simulation, in comparison with a conventional algorithm using the exact
diagonalization. Computing the sublattice magnetization, we show the
convergence of the data with increasing the number of polynomials and
truncation distance. | cond-mat_str-el |
Nonlocal Effect of Local Nonmagnetic Impurity in High-Tc
Superconductors: Induced Local Moment and Huge Residual Resistivity: We study a Hubbard model with a strong onsite impurity potential based on an
improved fluctuation-exchange (FLEX) approximation, which we call the GVI-FLEX
method. We find that (i) both local and staggered susceptibilities are strongly
enhanced around the impurity. By this reason, (ii) the quasiparticle lifetime
as well as the local density of states (DOS) are strongly suppressed in a wide
area around the impurity (like a Swiss cheese hole), which causes the ``huge
residual resistivity'' beyond the s-wave unitary scattering value. These
results by the GVI method naturally explains the various impurity effects in
HTSC's in a unified way, which had been a long-standing theoretical problem. | cond-mat_str-el |
Data-driven estimation of transfer integrals in undoped cuprates: Undoped cuprates are an abundant class of magnetic insulators, in which the
synergy of rich chemistry and sizable quantum fluctuations leads to a variety
of magnetic behaviors. Understanding the magnetism of these materials is
impossible without the knowledge of the underlying spin model. The typically
dominant antiferromagnetic superexchanges can be accurately estimated from the
respective electronic transfer integrals. Density functional theory
calculations mapped onto an effective one-orbital model in the Wannier basis
are an accurate, albeit computationally cumbersome method to estimate such
transfer integrals in cuprates. We demonstrate that instead an Artificial
Neural Network (ANN), trained on the results of high-throughput calculations,
can predict the transfer integrals using the crystal structure as the only
input. Descriptors of the ANN model encode the spatial configuration and the
chemical composition of the local crystalline environment. A virtual toolbox
employing our model can be readily employed to determine leading superexchange
paths as well as for rapidly assessing the relevant spin model in yet unknown
cuprates. | cond-mat_str-el |
Toward a systematic 1/d expansion: Two particle properties: We present a procedure to calculate 1/d corrections to the two-particle
properties around the infinite dimensional dynamical mean field limit. Our
method is based on a modified version of the scheme of Ref.
onlinecite{SchillerIngersent}}. To test our method we study the Hubbard model
at half filling within the fluctuation exchange approximation (FLEX), a
selfconsistent generalization of iterative perturbation theory. Apart from the
inherent unstabilities of FLEX, our method is stable and results in causal
solutions. We find that 1/d corrections to the local approximation are
relatively small in the Hubbard model. | cond-mat_str-el |
Two-fluid coexistence in a spinless fermions chain with pair hopping: We show that a simple one-dimensional model of spinless fermions with pair
hopping displays a phase in which a Luttinger liquid of paired fermions
coexists with a Luttinger liquid of unpaired fermions. Our results are based on
extensive numerical density-matrix renormalisation group calculations and are
supported by a two-fluid model that captures the essence of the coexistence
region. | cond-mat_str-el |
Gate-tunable heavy fermion quantum criticality in a moiré Kondo
lattice: We propose a realization of Kondo-lattice physics in moir\'e superlattices at
the interface between a WX$_2$ homobilayer and MoX$_2$ monolayer (where
X=S,Se). Under appropriate gating conditions, the interface-WX$_2$-layer forms
a triangular lattice of local moments that couple to itinerant electrons in the
other WX$_2$-layer via a gate-tunable Kondo exchange interaction. Using a
parton mean-field approach we identify a range of twist-angles which support a
gate-tuned quantum phase transition between a heavy-fermion liquid with large
anomalous Hall conductance and a fractionalized chiral spin-liquid coexisting
with a light Fermi liquid, and describe experimental signatures to distinguish
among competing theoretical scenarios. | cond-mat_str-el |
Equilibration in a chiral Luttinger liquid: We explore the weak-strong-coupling Bose-Fermi duality in a model of a
single-channel integer or fractional quantum Hall edge state with a
finite-range interaction. The system is described by a chiral Luttinger liquid
with non-linear dispersion of bosonic and fermonic excitations. We use the
bosonization, a unitary transformation, and a refermionization to map the
system onto that of weakly interacting fermions at low temperature $T$ or
weakly interacting bosons at high $T$. We calculate the equilibration rate
which is found to scale with temperature as $T^5$ and $T^{14}$ in the
high-temperature ("bosonic") and the low-temperature ("fermonic") regimes,
respectively. The relaxation rate of a hot particle with the momentum $k$ in
the fermonic regime scales as $k^7T^7$. | cond-mat_str-el |
Disordered quantum spin state in the stripe lattice system consisting of
triangular and square tilings: Quantum fluctuations originating phase competition or geometrical frustration
of spins lead to novel states such as a quantum critical point and a quantum
spin liquid where the strong quantum fluctuations suppress any ordered states
even at 0 K. Utilizing site-selective NMR for a quasi-two dimensional organic
conductor $\lambda$-(STF)$_2$GaCl$_4$, we investigate the non-magnetic
insulating phase of the stripe lattice system consisting of triangular and
square tilings. We found development of AF spin fluctuations with decreasing
temperature. Regardless of large enhancement of spin-lattice relaxation rate
$1/T_1$ owing to critical slowing down below 10 K, no long-range magnetic
ordering was observed down to 1.63 K two orders of magnitude less than the
exchange interaction $J/k_{\rm B} \simeq$ 194 K. Moreover, $1/T_1$ saturated
below 3.5 K. These results are in stark contrast to observed behaviors so far
in other non-magnetic ground states discussed in terms of spin liquids,
demonstrating realization of an exotic quantum state accompanying quantum
criticality. | cond-mat_str-el |
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