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Spin waves in the spiral phase of a doped antiferromagnet: a
strong-coupling approach: We study spin fluctuations in the spiral phase of the two-dimensional Hubbard
model at low doping on the basis of the spin-particle-hole coherent-state path
integral. In the strong correlation limit, we obtain an analytical expression
of the spin-wave excitations over the entire Brillouin zone except in the
vicinity of ${\bf q}=0$. We discuss the validity of the Hartree-Fock and
random-phase approximations in the strong-coupling limit, and compare our
results with previous numerical and analytical calculations. Although the
spiral phase is unstable, as shown by a negative mean-field compressibility and
the presence of imaginary spin-fluctuation modes, we expect the
short-wavelength fluctuation modes (with real energies) to survive in the
actual ground-state of the system. | cond-mat_str-el |
Bosonic Short Range Entangled states Beyond Group Cohomology
classification: We explore and construct a class of bosonic short range entangled (BSRE)
states in all $4k+2$ spatial dimensions, which are higher dimensional
generalizations of the well-known Kitaev's $E_8$ state in 2d. These BSRE states
share the following properties: (1) their bulk is fully gapped and
nondegenerate; (2) their $(4k+1)d$ boundary is described by a "self-dual"
rank-$2k$ antisymmetric tensor gauge field, and it is guaranteed to be gapless
without assuming any symmetry; (3) their $(4k+1)d$ boundary has intrinsic
gravitational anomaly once coupled to the gravitational field; (4) their bulk
is described by an effective Chern-Simons field theory with rank-$(2k+1)$
antisymmetric tensor fields, whose $K^{IJ}$ matrix is identical to that of the
$E_8$ state in $2d$; (5) The existence of these BSRE states lead to various
bosonic symmetry protected topological (BSPT) states as their descendants in
other dimensions; (6) These BSRE states can be constructed by confining
fermionic degrees of freedom from 8 copies of $(4k+2)d$ SRE states with
fermionic $2k-$branes; (7) After compactifying the $(4k+2)d$ BSRE state on a
closed $4k$ dimensional manifold, depending on the topology of the compact $4k$
manifold, the system could reduce to nontrivial $2d$ BSRE states. | cond-mat_str-el |
Topological charge pumping in excitonic insulators: We show that in excitonic insulators with $s$-wave electron-hole pairing, an
applied electric field (either pulsed or static) can induce a $p$-wave
component to the order parameter, and further drive it to rotate in the $s+ip$
plane, realizing a Thouless charge pump. In one dimension, each cycle of
rotation pumps exactly two electrons across the sample. Higher dimensional
systems can be viewed as a stack of one dimensional chains in momentum space in
which each chain crossing the fermi surface contributes a channel of charge
pumping. Physics beyond the adiabatic limit, including in particular
dissipative effects is discussed. | cond-mat_str-el |
Emergent topological spin structures in a centrosymmetric cubic
perovskite: The skyrmion crystal (SkX) characterized by a multiple-q helical spin
modulation has been reported as a unique topological state that competes with
the single-q helimagnetic order in non-centrosymmetric materials. Here we
report the discovery of a rich variety of multiple-q helimagnetic spin
structures in the centrosymmetric cubic perovskite SrFeO3. On the basis of
neutron diffraction measurements, we have identified two types of robust
multiple-q topological spin structures that appear in the absence of external
magnetic fields: an anisotropic double-q spin spiral and an isotropic
quadruple-q spiral hosting a three-dimensional lattice of hedgehog
singularities. The present system not only diversifies the family of SkX host
materials, but furthermore provides an experimental missing link between
centrosymmetric lattices and topological helimagnetic order. It also offers
perspectives for integration of SkXs into oxide electronic devices. | cond-mat_str-el |
Unfrustrating the t-J Model: d-wave BCS Superconductivity in the
$t'$-$J_z$-$V$ Model: The t-J model is believed to be a minimal model that may be capable of
describing the low-energy physics of the cuprate superconductors. However,
although the t-J model is simple in appearance, obtaining a detailed
understanding of its phase diagram has proved to be challenging. We are
therefore motivated to study modifications to the t-J model such that its phase
diagram and mechanism for d-wave superconductivity can be understood
analytically without making uncontrolled approximations. The modified model we
consider is a $t'$-$J_z$-$V$ model on a square lattice, which has a
second-nearest-neighbor hopping $t'$ (instead of a nearest-neighbor hopping
$t$), an Ising (instead of Heisenberg) antiferromagnetic coupling $J_z$, and a
nearest-neighbor repulsion $V$. In a certain strongly interacting limit, the
ground state is an antiferromagnetic superconductor that can be described
exactly by a Hamiltonian where the only interaction is a nearest-neighbor
attraction. BCS theory can then be applied with arbitrary analytical control,
from which nodeless d-wave or s-wave superconductivity can result. | cond-mat_str-el |
On the Problem of the Staggered Field in CuGeO3 Doped with Magnetic
Impurities: The magnitude of the staggered field is calculated from the EPR data for
CuGeO3 doped with Co and Fe magnetic impurities. It is found that this
parameter demonstrate an anomalous temperature and magnetic field dependence
probably due to (i) the specific mechanism of the staggered field generation in
doped CuGeO3 and (ii) a competition between antiferromagnetic interchain
exchange and staggered Zeeman energy. | cond-mat_str-el |
Emergent gravity in graphene: We reconsider monolayer graphene in the presence of elastic deformations. It
is described by the tight - binding model with varying hopping parameters. We
demonstrate, that the fermionic quasiparticles propagate in the emergent 2D
Weitzenbock geometry and in the presence of the emergent U(1) gauge field. Both
emergent geometry and the gauge field are defined by the elastic deformation of
graphene. | cond-mat_str-el |
High magnetic field phase diagram and weak FM breaking in
(Ni0.93Co0.07)3V2O8: We present magnetostriction and thermal expansion measurements on
multiferroic (Ni0.93Co0.07)3V2O8. The high field phase diagrams up to 33 T
along the a, b and c directions are built. For H//a, as the magnetic field
increases, two intermediate phases appear between the incommensurate phase and
the paramagnetic phase at about 7 K, and then a magnetically induced phase
appears above the paramagnetic phase. For H//b,thermal expansion measurement
indicates a mutation in the spin lattice coupling of the high field phases. The
interlaced phase boundary suggests a mixed state in the optical high field
phase. For H//c, an intermediate phase between the commensurate phase and the
incommensurate phase is detected. A nonlinear boundary between the intermediate
phase and the low temperature incommensurate phase, and a clear boundary
between the commensurate phase and the paramagnetic phase are found. These
results indicate that doping Co2+ breaks the weak ferromagnetic moment of the
commensurate phase, which exists in the parent compound Ni3V2O8 and
(Ni0.9Co0.1)3V2O8. This nonlinear influence reflects complicated spin
modulation in Ni3V2O8 by doping Co2+. | cond-mat_str-el |
QMC study of the chiral Heisenberg Gross-Neveu universality class: We investigate a quantum criticality of an antiferromagnetic phase transition
in the Hubbard model on a square lattice with a $d$-wave pairing field by
large-scale auxiliary-field quantum Monte Carlo simulations. Since the $d$-wave
pairing filed induces Dirac cones in the non-interacting single-particle
spectrum, the quantum criticality should correspond to the chiral Heisenberg
universality class in terms of the Gross-Neveu theory, which is the same as
those expected in the Hubbard model on the honeycomb lattice, despite the unit
cells being different (e.g., they contain one and two sites, respectively). We
show that both the two phase transitions, expected to occur on the square and
on the honeycomb lattices, indeed have the same quantum criticality. We also
argue that details of the models, i.e., the way of counting the total number
$N$ of fermion components and the anisotropy of the Dirac cones, do not change
the critical exponents. | cond-mat_str-el |
Ferromagnetic fluctuation and possible triplet superconductivity in
Na_xCoO_2*yH_2O: Fluctuation-exchange study of multi-orbital Hubbard model: Spin and charge fluctuations and superconductivity in a recently discovered
superconductor Na_xCoO_2*yH_2O are studied based on a multi-orbital Hubbard
model. Tight-binding parameters are determined to reproduce the LDA band
dispersions with the Fermi surface, which consist of a large cylindrical one
around the Gamma-point and six hole pockets near the K-points. By applying the
fluctuation-exchange (FLEX) approximation, we show that the Hund's-rule
coupling between the Co t2g orbitals causes ferromagnetic (FM) spin
fluctuation. Triplet f_{y(y^2-3x^2)}-wave and p-wave pairings are favored by
this FM fluctuation on the hole-pocket band. We propose that, in
Na_xCoO_2*yH_2O, the Co t2g orbitals and inter-orbital Hund's-rule coupling
play important roles on the triplet pairing, and this compound can be a first
example of the triplet superconductor in which the orbital degrees of freedom
play substantial roles. | cond-mat_str-el |
Cascade of field-induced magnetic transitions in a frustrated
antiferromagnetic metal: Frustrated magnets can exhibit many novel forms of order when exposed to high
magnetic fields, however, much less is known about materials where frustration
occurs in the presence of itinerant electrons. Here we report thermodynamic and
transport measurements on micron-sized single crystals of the
triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up
to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase
transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and
we combine magnetic torque, specific heat and transport data to construct the
field-temperature phase diagram. The results are discussed in the context of a
frustrated easy-axis Heisenberg model for the localized moments where
intermediate applied magnetic fields are predicted to stabilize a magnetic
supersolid phase. Deviations in the measured phase diagram from this model
predictions are attributed to the role played by the itinerant electrons. | cond-mat_str-el |
Resonant inelastic X-ray scattering response of the Kitaev honeycomb
model: We calculate the resonant inelastic X-ray scattering (RIXS) response of the
Kitaev honeycomb model, an exactly solvable quantum-spin-liquid model with
fractionalized Majorana and flux excitations. We find that the fundamental RIXS
channels, the spin-conserving (SC) and the non-spin-conserving (NSC) ones, do
not interfere and give completely different responses. SC-RIXS picks up
exclusively the Majorana sector with a pronounced momentum dispersion, whereas
NSC-RIXS also creates immobile fluxes, thereby rendering the response only
weakly momentum dependent, as in the spin structure factor measured by
inelastic neutron scattering. RIXS can therefore pick up the fractionalized
excitations of the Kitaev spin liquid separately, making it a sensitive probe
to detect spin-liquid character in potential material incarnations of the
Kitaev honeycomb model. | cond-mat_str-el |
Parasitic small-moment-antiferromagnetism and non-linear coupling of
hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction: We report simultaneous measurements of the distribution of lattice constants
and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and
conventional neutron diffraction, as a function of temperature and pressure up
to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is
purely parasitic and quantitatively originates from the distribution of lattice
constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high
pressure are separated by a line of first-order phase transitions, which ends
in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must
have different symmetry, as expected of the HO being, e.g., incommensurate
orbital currents, helicity order, or multipolar order. | cond-mat_str-el |
Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single
crystals: Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn flux and characterized
by powder and single-crystal x-ray diffraction, respectively, and by
single-crystal electrical resistivity rho, heat capacity Cp, and magnetic
susceptibility chi measurements versus temperature T, and magnetization versus
field M(H) isotherm measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type
structure whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type
structure. The rho(T) data indicate semiconducting behaviors for both compounds
with activation energies of 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2. The
chi(T) and Cp(T) data reveal antiferromagnetic (AFM) ordering at TN = 110 K for
SrMn2Sb2 and 450~K for BaMn2Sb2. The anisotropic chi(T < TN) data also show
that the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane
whereas the ordered moments in BaMn2Sb2 are aligned collinearly along the
tetragonal c axis. The ab-plane M(H) data for SrMn2Sb2 exhibit a continuous
metamagnetic transition at low fields 0 < H < 1 T, whereas BaMn2Sb2 exhibits no
metamagnetic transitions up to 5.5 T. The chi(T) data for both compounds and
the Cp(T) data for SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range
AFM correlations above their respective TN up to at least 900 K within a
local-moment picture, corresponding to quasi-two-dimensional magnetic behavior.
The present results and a survey of the literature for Mn pnictides with the
CaAl2Si2 and ThCr2Si2 crystal structures show that the TN values for the
CaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type
materials. | cond-mat_str-el |
Quantum Criticality of Topological Phase Transitions in 3D Interacting
Electronic Systems: Topological phase transitions in condensed matters accompany emerging
singularities of the electronic wave function, often manifested by gap-closing
points in the momentum space. In conventional topological insulators in three
dimensions (3D), the low energy theory near the gap-closing point can be
described by relativistic Dirac fermions coupled to the long range Coulomb
interaction, hence the quantum critical point of topological phase transitions
provides a promising platform to test the novel predictions of quantum
electrodynamics. Here we show that a new class of quantum critical phenomena
emanates in topological materials breaking either the inversion symmetry or the
time-reversal symmetry. At the quantum critical point, the theory is described
by the emerging low energy fermions, dubbed the anisotropic Weyl fermions,
which show both the relativistic and Newtonian dynamics simultaneously. The
interplay between the anisotropic dispersion and the Coulomb interaction brings
about a new screening phenomena distinct from the conventional Thomas-Fermi
screening in metals and logarithmic screening in Dirac fermions. | cond-mat_str-el |
Thermal DMRG for highly frustrated quantum spin chains: a user
perspective: Thermal DMRG is investigated with emphasis of employability in molecular
magnetism studies. To this end magnetic observables at finite temperature are
evaluated for two one-dimensional quantum spin systems: a Heisenberg chain with
nearest-neighbor antiferromagnetic interaction and a frustrated sawtooth
(delta) chain. It is found that thermal DMRG indeed accurately approximates
magnetic observables for the chain as well as for the sawtooth chain, but in
the latter case only for sufficiently high temperatures. We speculate that the
reason is due to the peculiar structure of the low-energy spectrum of the
sawtooth chain induced by frustration. | cond-mat_str-el |
Thermodynamic and information-theoretic description of the Mott
transition in the two-dimensional Hubbard model: At the Mott transition, electron-electron interaction changes a metal, in
which electrons are itinerant, to an insulator, in which electrons are
localized. This phenomenon is central to quantum materials. Here we contribute
to its understanding by studying the two-dimensional Hubbard model at finite
temperature with plaquette cellular dynamical mean-field theory. We provide an
exhaustive thermodynamic description of the correlation-driven Mott transition
of the half-filled model by calculating pressure, charge compressibility,
entropy, kinetic energy, potential energy and free energy across the
first-order Mott transition and its high-temperature crossover (Widom line).
The entropy is extracted from the Gibbs-Duhem relation and shows complex
behavior near the transition, marked by discontinuous jumps at the first-order
boundary, singular behavior at the Mott endpoint and inflections marking sharp
variations in the supercritical region. The free energy allows us to identify
the thermodynamic phase boundary, to discuss phases stability and
metastability, and to touch upon nucleation and spinodal decomposition
mechanisms for the transition. We complement this thermodynamic description of
the Mott transition by an information-theoretic description. We achieve this by
calculating the local entropy, which is a measure of entanglement, and the
single-site total mutual information, which quantifies quantum and classical
correlations. These information-theoretic measures exhibit characteristic
behaviors that allow us to identify the first-order coexistence regions, the
Mott critical endpoint and the crossovers along the Widom line in the
supercritical region. | cond-mat_str-el |
How Hidden Orders Generate Gaps in 1D Fermionic Systems: We demonstrate that hidden long range order is always present in the gapped
phases of interacting fermionic systems on one dimensional lattices. It is
captured by correlation functions of appropriate nonlocal charge and/or spin
operators, which remain asymptotically finite. The corresponding microscopic
orders are classified. The results are confirmed by DMRG numerical simulation
of the phase diagram of the extended Hubbard model, and of a Haldane insulator
phase. | cond-mat_str-el |
Evidence for Anisotropic Kondo Behavior in Ce0.8La0.2Al3: We have performed an inelastic neutron scattering study of the low energy
spin dynamics of the heavy fermion compound Ce0.8La0.2Al3 as a function of
temperature and external pressure up to 5 kbar. At temperatures below 3 K, the
magnetic response transforms from a quasi-elastic form, common to many heavy
fermion systems, to a single well-defined inelastic peak, which is extremely
sensitive to external pressure. The scaling of the spin dynamics and the
thermodynamic properties are in agreement with the predictions of the
anisotropic Kondo model. | cond-mat_str-el |
Diagrammatic theory for Anderson Impurity Model. Stationary property of
the thermodynamic potential: A diagrammatic theory around atomic limit is proposed for normal state of
Anderson Impurity Model. The new diagram method is based on the ordinary Wick's
theorem for conduction electrons and a generalized Wick's theorem for gtrongly
correlated impurity electrons. This last theorem coincides with the definition
of Kubo cumulants. For the mean value of the evolution operator a linked
cluster theorem is proved and a Dyson's type equations for one-particle
propagators are established. The main element of these equations is the
correlation function which contains the spin, charge and pairing fluctuations
of the system. The thermodynamic potential of the system is expressed through
one-particle renormalized Green's functions and the corelation function. The
stationary property of the thermodynamic potential is established with respect
to the changes of correlation function. | cond-mat_str-el |
Competing Ground States of a Peierls-Hubbard Nanotube: Motivated by iodo platinum complexes assembled within a quadratic-prism
lattice, [Pt(C$_2$H$_8$N$_2$)(C$_{10}$H$_8$N$_2$)I]$_4$(NO$_3$)$_8$, we
investigate the ground-state properties of a Peierls-Hubbard four-legged tube.
Making a group-theoretical analysis, we systematically reveal a variety of
valence arrangements, including half-metallic charge-density-wave states.
Quantum and thermal phase competition is numerically demonstrated with
particular emphasis on doping-induced successive insulator-to-metal transitions
with conductivity increasing stepwise. | cond-mat_str-el |
Phase Thermalization: from Fermi Liquid to Incoherent Metal: When a system consists of a large subsystem (bath) and a small one (probe),
thermalization implies induction of temperature of the bath onto the probe. If
both the bath and the probe are described by same microscopic Hamiltonian,
thermalization further entails that the probe imbibes the phase of the bath. We
refer to this phenomenon as phase thermalization. However, it is not clear
whether this phenomenon is realizable when the probe and the bath are described
by different microscopic Hamiltonians. We show phase thermalization is possible
even when the microscopic Hamiltonians differ significantly. We provide an
explicit example, where the probe is a Fermi liquid realized by a Majorana
chain with $n \gg 1$ fermions per site interacting through random hopping and
the bath is an incoherent metal described by another Majorana chain with $N >
n$ fermions per site interacting through arbitrarily long range random
four-fermion interaction. In deep infrared, the probe turns into an incoherent
metal, with Lyapunov spectrum and diffusion coefficient identical to the bath. | cond-mat_str-el |
Dimensional-Crossover-Driven Mott Insulators in SrVO3 Ultrathin Films: High-quality epitaxial SrVO3 (SVO) thin films of various thicknesses were
grown on (001)-oriented LSAT substrates by pulsed electron-beam deposition
technique. Thick SVO films (~25 nm) exhibited metallic behavior with the
electrical resistivity following the T2 law corresponding to a Fermi liquid
system. We observed a temperature driven metal-insulator transition (MIT) in
SVO ultrathin films with thicknesses below 6.5 nm, the transition temperature
TMIT was found to be at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and
205 K for the 3 nm film. The emergence of the observed MIT can be attributed to
the dimensional crossover from a three-dimensional metal to a two-dimensional
Mott insulator, as the resulting reduction in the effective bandwidth W opens a
band gap at the Fermi level. The magneto-transport study of the SVO ultrathin
films also confirmed the observed MIT is due to the electron-electron
interactions other than localization. | cond-mat_str-el |
Fermi- to non-Fermi-liquid crossover and Kondo transition in
two-dimensional Cu$_{2/3}$V$_{1/3}$V$_2$S$_4$: By means of a specific heat ($C$) and electrical resistivity ($\varrho$)
study, we give evidence of a pronounced Fermi liquid (FL) behavior with sizable
mass renormalization, $m^{\ast}/m = 30$, up to unusually high temperatures
$\sim$70 K in the layered system Cu$_{2/3}$V$_{1/3}$V$_2$S$_4$. At low
temperature, a marked upturn of both $C$ and $\varrho$ is suppressed by
magnetic field, which suggests a picture of Kondo coupling between conduction
electrons in the VS$_2$ layers and impurity spins of the V$^{3+}$ ions located
between layers. This picture opens the possibility of controlling electronic
correlations and the FL to non-FL crossover in simple layered materials. For
instance, we envisage that the coupling between layers provided by the impurity
spins may realize a two-channel Kondo state. | cond-mat_str-el |
An effect of Sm vacancies on the hybridization gap in topological Kondo
insulator candidate SmB$_6$: A necessary element for the predicted topological state in Kondo insulator
SmB$_6$ is the hybridization gap which opens in this compound at low
temperatures. In this work, we present a comparative study of the in-gap
density of states due to Sm vacancies by Raman scattering spectroscopy and heat
capacity for samples where the number of Sm vacancies is equal to or below 1 %.
We demonstrate that hybridization gap is very sensitive to the presence of Sm
vacancies. At the amount of vacancies above 1 % the gap fills in with impurity
states and low temperature heat capacity is enhanced. | cond-mat_str-el |
Exact solution of the topological symplectic Kondo problem: The Kondo effect is an archetypical phenomenon in the physics of strongly
correlated electron systems. Recent attention has focused on the application of
Kondo physics to quantum information science by exploiting overscreened Kondo
impurities with residual anyon-like impurity entropy. While this physics was
proposed in the fine-tuned multi-channel Kondo setup or in the Majorana-based
topological Kondo effect, we here study the Kondo effect with symplectic
symmetry Sp(2k) and present details about the implementation which importantly
only involves conventional s-wave superconductivity coupled to an array of
resonant levels and neither requires perfect channel symmetry nor Majorana
fermions. We carefully discuss the role of perturbations and show that a global
Zeeman drives the system to a 2-channel SU(k) fixed point. Exact results for
the residual entropy, specific heat, and magnetization are derived using the
thermodynamic Bethe Ansatz for Sp(2k). This solution not only proves the
existence of a quantum critical ground state with anyon-like Hilbert space
dimension but also a particularly weak non-Fermi liquid behavior at
criticality. We interpret the weakness of non-analyticities as a manifestation
of suppressed density of states at the impurity causing only a very weak
connection of putative anyons and conduction electrons. Given this weak
connection, the simplicity of the design, and the stability of the effect, we
conjecture that the symplectic Kondo effect may be particularly suitable for
quantum information applications. | cond-mat_str-el |
Manifestation of topological behaviors in interacting Weyl systems:
one-body verse two-body correlations: Understanding correlation effects in topological phases of matter is at the
forefront of current research in condensed matter physics. Here we try to
clarify some subtleties in studying topological behaviors of interacting Weyl
semimetals. It is well-known that there exist two topological invariants
defined to identify their topological character. One is the many-body Chern
number, which can be directly linked to the Hall conductivity and thus to the
two-particle correlations. The other is the topological index constructed from
the single-particle Green's functions. Because the information of Green's
functions is easier to be achieved than the many-body wavefunctions, usually
only the latter is employed in the literature. However, the approach based on
the single-particle Green's function can break down in the strongly correlated
phase. For illustration, an exactly solvable two-orbital model with
momentum-local two-body interactions is discussed, in which both topological
invariants can be calculated analytically. We find that the topological index
calculated from the Green's function formalism can be nonzero even for a
non-topological strongly correlated phase with vanishing many-body Chern
number. In addition, we stress that the physical surface states implied by
nonzero many-body Chern numbers should be the edge modes of particle-hole
collective excitations, rather than those of quasiparticle nature derived from
the Green's function formalism. Our observations thus demonstrate the
limitation of the validity of Green's function formalism in the investigations
of interacting topological materials. | cond-mat_str-el |
Revised crystal structure and electronic properties of high dielectric
Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ ceramics: Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_3$ (BFN) ceramics are considered to be promising
for technological applications owing to their high dielectric constant over a
wide range of temperatures. However, there exists considerable discrepancy over
the structural details. We address this discrepancy through a combined x-ray
diffraction at room temperature and neutron powder diffraction measurements in
the range from 5K up to room temperature, supplemented by a comparative
analysis of the earlier reported structures. Our study reveals a cubic
structure with space group Pm$\bar{3}$m at all measured temperatures. Further,
the x-ray near edge structure and the extended x-ray absorption fine structure
studies on the local environment of the Fe ions is consistent with the cubic
symmetry. An appropriate value of $U$ for DFT+$U$ calculations is obtained by
comparison with x-ray absorption spectroscopy, which agrees well with the
earlier reported electronic properties. | cond-mat_str-el |
Capturing long range correlations in two-dimensional quantum lattice
systems using correlator product states: We study the suitability of correlator product states for describing
ground-state properties of two-dimensional spin models. Our ansatz for the
many-body wave function takes the form of either plaquette or bond correlator
product states and the energy is optimized by varying the correlators using
Monte Carlo minimization. For the Ising model we find that plaquette
correlators are best for estimating the energy while bond correlators capture
the expected long-range correlations and critical behavior of the system more
faithfully. For the antiferromagnetic Heisenberg model, however, plaquettes
outperform bond correlators at describing both local and long-range
correlations because of the substantially larger number of local parameters
they contain. These observations have quantitative implications for the
application of correlator product states to other more complex systems, and
give important heuristic insights: in particular the necessity of carefully
tailoring the choice of correlators to the system considered, its interactions
and symmetries. | cond-mat_str-el |
Resistivity and Thermal Conductivity of an Organic Insulator
beta'-EtMe3Sb[Pd(dmit)2]2: A finite residual linear term in the thermal conductivity at zero temperature
in insulating magnets indicates the presence of gapless excitations of
itinerant quasiparticles, which has been observed in some candidate materials
of quantum spin liquids (QSLs). In the organic triangular insulator
beta'-EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature
thermal conductivity depends on the cooling process and the finite residual
term is observed only in samples with large thermal conductivity. Moreover, the
cooling rate dependence is largely sample dependent. Here we find that, while
the low-temperature thermal conductivity significantly depends on the cooling
rate, the high-temperature resistivity is almost perfectly independent of the
cooling rate. These results indicate that in the samples with the finite
residual term, the mean free path of the quasiparticles that carry the heat at
low temperatures is governed by disorders, whose characteristic length scale of
the distribution is much longer than the electron mean free path that
determines the high-temperature resistivity. This explains why recent X-ray
diffraction and nuclear magnetic resonance measurements show no cooling rate
dependence. Naturally, these measurements are unsuitable for detecting
disorders of the length scale relevant for the thermal conductivity, just as
they cannot determine the residual resistivity of metals. Present results
indicate that very careful experiments are needed when discussing itinerant
spin excitations in beta'-EtMe3Sb[Pd(dmit)2]2. | cond-mat_str-el |
Restricted Boltzmann Machines for Quantum States with Nonabelian or
Anyonic Symmetries: Although artificial neural networks have recently been proven to provide a
promising new framework for constructing quantum many-body wave functions, the
parameterization of a quantum wavefunction with nonabelian symmetries in terms
of a Boltzmann machine inherently leads to biased results due to the basis
dependence. We demonstrate that this problem can be overcome by sampling in the
basis of irreducible representations instead of spins, for which the
corresponding ansatz respects the nonabelian symmetries of the system. We apply
our methodology to find the ground states of the one-dimensional
antiferromagnetic Heisenberg (AFH) model with spin-half and spin-1 degrees of
freedom, and obtain a substantially higher accuracy than when using the
$s_z$-basis as input to the neural network. The proposed ansatz can target
excited states, which is illustrated by calculating the energy gap of the AFH
model. We also generalize the framework to the case of anyonic spin chains. | cond-mat_str-el |
Kondo effect and STM spectra through ferromagnetic nanoclusters: Motivated by recent scanning tunneling microscope (STM) experiments on cobalt
clusters adsorbed on single wall metallic nanotubes [Odom {\em et al.}, Science
{\bf 290}, 1549 (2000)], we study theoretically the size dependence of STM
spectra and spin-flip scattering of electrons from finite size ferromagnetic
clusters adsorbed on metallic surfaces. We study two models of nanometer size
ferromagnets: (i) An itinerant model with delocalized s, p and d electrons and
(ii) a local moment model with both localized d-level spins and delocalized
cluster electrons. The effective exchange coupling between the spin of the
cluster and the conduction electrons of the metallic substrate depends on the
specific details of the single particle density of states on the cluster. The
calculated Kondo coupling is inversely proportional to the total spin of the
ferromagnetic cluster in both models and thus the Kondo temperature is rapidly
suppressed as the size of the cluster increases. Mesoscopic fluctuations in the
charging energies and magnetization of nanoclusters can lead to large
fluctuations in the Kondo temperatures and a very asymmetric voltage dependence
of the STM spectra. We compare our results to the experiments. | cond-mat_str-el |
Resonant Nernst effect in the metallic and field-induced spin density
wave states of (TMTSF)2ClO4: We examine an unusual phenomenon where, in tilted magnetic fields near magic
angles parallel to crystallographic planes, a "giant" resonant Nernst signal
has been observed by Wu et al.[Phys. Rev. Lett. 91 56601(2003)] in the metallic
state of an organic conducting Bechgaard salt. We show that this effect appears
to be a general feature of these materials, and is also present in the field
induced spin density wave phase with even larger amplitude. Our results place
new restrictions on models that treat the metallic state as an unconventional
density wave or as a state with finite Cooper pairing. | cond-mat_str-el |
Simple mechanisms that impede the Berry phase identification from
magneto-oscillations: The phase of quantum magneto-oscillations is often associated with the Berry
phase and is widely used to argue in favor of topological nontriviality of the
system (Berry phase $2\pi n+\pi$). Nevertheless, the experimentally determined
value may deviate from $2\pi n+\pi$ arbitrarily, therefore more care should be
made analyzing the phase of magneto-oscillations to distinguish trivial systems
from nontrivial. In this paper we suggest two simple mechanisms dramatically
affecting the experimentally observed value of the phase in three-dimensional
topological insulators: (i) magnetic field dependence of the chemical
potential, and (ii) possible nonuniformity of the system. These mechanisms are
not limited to topological insulators and can be extended to other
topologically trivial and non-trivial systems. | cond-mat_str-el |
Investigation of Ultrafast Demagnetization and Gilbert Damping and their
Correlation in Different Ferromagnetic Thin Films Grown Under Identical
Conditions: Following the demonstration of laser-induced ultrafast demagnetization in
ferromagnetic nickel, several theoretical and phenomenological propositions
have sought to uncover its underlying physics. In this work we revisit the
three temperature model (3TM) and the microscopic three temperature model
(M3TM) to perform a comparative analysis of ultrafast demagnetization in
20-nm-thick cobalt, nickel and permalloy thin films measured using an
all-optical pump-probe technique. In addition to the ultrafast dynamics at the
femtosecond timescales, the nanosecond magnetization precession and damping are
recorded at various pump excitation fluences revealing a fluence-dependent
enhancement in both the demagnetization times and the damping factors. We
confirm that the Curie temperature to magnetic moment ratio of a given system
acts as a figure of merit for the demagnetization time, while the
demagnetization times and damping factors show an apparent sensitivity to the
density of states at the Fermi level for a given system. Further, from
numerical simulations of the ultrafast demagnetization based on both the 3TM
and the M3TM, we extract the reservoir coupling parameters that best reproduce
the experimental data and estimate the value of the spin flip scattering
probability for each system. We discuss how the fluence-dependence of
inter-reservoir coupling parameters so extracted may reflect a role played by
nonthermal electrons in the magnetization dynamics at low laser fluences. | cond-mat_str-el |
High-density two-dimensional small polaron gas in a delta-doped Mott
insulator: Heterointerfaces in complex oxide systems open new arenas in which to test
models of strongly correlated material, explore the role of dimensionality in
metal-insulator-transitions (MITs) and small polaron formation. Close to the
quantum critical point Mott MITs depend on band filling controlled by random
disordered substitutional doping. Delta-doped Mott insulators are potentially
free of random disorder and introduce a new arena in which to explore the
effect of electron correlations and dimensionality. Epitaxial films of the
prototypical Mott insulator GdTiO3 are delta-doped by substituting a single
(GdO)+1 plane with a monolayer of charge neutral SrO to produce a
two-dimensional system with high planar doping density. Unlike metallic SrTiO3
quantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally
activated DC and optical conductivity that agree in a quantitative manner with
predictions of small polaron transport but with an extremely high
two-dimensional density of polarons, ~ 7E14 cm-2 | cond-mat_str-el |
Dissipative Majorana quantum wires: In this paper, we formulate and quantitatively examine the effect of
dissipation on topological systems. We use a specific model of Kitaev quantum
wire with an onsite Ohmic dissipation, and perform a numerically exact quantum
Monte Carlo simulation to investigate this interacting open quantum system with
a strong system-bath (SB) coupling beyond the scope of Born-Markovian
approximation. We concentrate on the effect of dissipation on the topological
features of the system (e.g. the Majorana edge mode) at zero temperature, and
find that even though the topological phase is robust against weak SB couplings
as it is supposed to be, it will eventually be destroyed by sufficiently strong
dissipations via either a continuous quantum phase transition or a crossover
depending on the symmetry of the system. The dissipation-driven quantum
criticality is also discussed. In addition, using the framework of Abelian
bosonization, we provide an analytical description of the interplay between
pairing, dissipation and interaction in our model. | cond-mat_str-el |
Spin model for the Honeycomb $\rm NiPS_3$: In the Van der Waal material $\rm NiPS_3$, Ni atoms have spin S=1 and realize
a honeycomb lattice. Six sulfur atoms surround each Ni and split their d
manifold into three filled and two unfilled bands. Aimed to determine the spin
Hamiltonian of $\rm NiPS_3$, we study its exchange mechanisms using a two-band
half-filled Hubbard model. Hopping between d orbitals is mediated by p orbitals
of sulfur and gives rise to bilinear and biquadratic spin couplings in the
limit of strong electronic correlations. The microscopic model exposed a
ferromagnetic biquadratic spin interaction $\rm K_1$ allowing the completion of
a minimal $\rm J_1-J_3-K_1$ spin Hamiltonian for $\rm NiPS_3$. In bulk, a
ferromagnetic first nearest neighbor $\rm J_1$ and a more significant
antiferromagnetic third nearest neighbor spin coupling $\rm J_3$ agreed with
the literature, while in monolayer $\rm J_1$ is positive and very small in
comparison. Using a variational scheme we found that a zig-zag
antiferromagnetic order is the ground state of bulk samples. The zig-zag
pattern is adjacent to commensurate and incommensurate spin spirals, which
could hint at the puzzling results reported in $\rm NiPS_3$ monolayers. | cond-mat_str-el |
Determination of Fermi surface by charge density correlations: The Fermi surface topology in the two-dimensional Hubbard model is
particularly relevant for the high-temperature superconductors, whereas its
theoretical research encounters with the difficulty of the analytical
continuation problem. To this end, we proposed the concept of the
momentum-dependent compressibility, defined as the variation of the momentum
distribution function with respect to the chemical potential. The surface
determined by the maximum of the momentum-dependent compressibility is nearly
identical to the Fermi surface in the weakly and intermediate coupling regions
according to our numerical results. In the correlated region, this surface also
exhibits pocket and arc features, just like the Fermi surface in
high-temperature superconductors. Therefore, for theoretical studies, this
surface can be used as an alternative to determine the underlying Fermi
surface. Considering that the momentum-dependent compressibility is closely
related to the charge density correlations, our work also shows a connection
between the Fermi surface topology and the charge density fluctuations. | cond-mat_str-el |
Duality and ground-state phase diagram for the quantum XYZ model with
arbitrary spin $s$ in one spatial dimension: Five duality transformations are unveiled for the quantum XYZ model with
arbitrary spin $s$ in one spatial dimension. The presence of these duality
transformations drastically reduces the entire ground-state phase diagram to
two {\it finite} regimes - the principal regimes, with all the other ten
regimes dual to them. Combining with the determination of critical points from
the conventional order parameter approach and/or the fidelity approach to
quantum phase transitions, we are able to map out the ground-state phase
diagram for the quantum XYZ model with arbitrary spin $s$. This is explicitly
demonstrated for $s=1/2,1,3/2$ and 2. As it turns out, all the critical points,
with central charge $c=1$, are self-dual under a respective duality
transformation for half-integer as well as integer spin $s$. However, in the
latter case, the presence of the so-called symmetry protected topological
phase, i.e., the Haldane phase, results in extra lines of critical points with
central charge $c=1/2$, which is not self-dual under any duality
transformation. | cond-mat_str-el |
High-pressure versus isoelectronic doping effect on the honeycomb
iridate Na$_2$IrO$_3$: We study the effect of isoelectronic doping and external pressure in tuning
the ground state of the honeycomb iridate Na$_2$IrO$_3$ by combining optical
spectroscopy with synchrotron x-ray diffraction measurements on single
crystals. The obtained optical conductivity of Na$_2$IrO$_3$ is discussed in
terms of a Mott insulating picture versus the formation of quasimolecular
orbitals and in terms of Kitaev-interactions. With increasing Li content $x$,
(Na$_{1-x}$Li$_x$)$_2$IrO$_3$ moves deeper into the Mott insulating regime and
there are indications that up to a doping level of 24\% the compound comes
closer to the Kitaev-limit. The optical conductivity spectrum of single
crystalline $\alpha$-Li$_2$IrO$_3$ does not follow the trends observed for the
series up to $x=0.24$. There are strong indications that $\alpha$-Li$_2$IrO$_3$
is less close to the Kitaev-limit compared to Na$_2$IrO$_3$ and closer to the
quasimolecular orbital picture. Except for the pressure-induced hardening of
the phonon modes, the optical properties of Na$_2$IrO$_3$ seem to be robust
against external pressure. Possible explanations of the unexpected evolution of
the optical conductivity with isolectronic doping and the drastic change
between $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes
of lattice parameters and the optical conductivity with the corresponding
changes induced by doping. | cond-mat_str-el |
Quantum correlations in the spin-1/2 Heisenberg XXZ chain with modulated
Dzyaloshinskii-Moriya interaction: We study a one-dimensional spin-1/2 XXZ Heisenberg model with alternating
Dzyaloshinskii- Moriya interaction, using the numerical Lanczos method.
Recently, the ground state (GS) phase diagram of this model has been
established using the bosonization approach and extensive density matrix
renormalization group computations. Four quantum phases - saturated
ferromagnetic (FM), Luttinger liquid (LL), and two (C1 and C2) gapped phases
with composite structure of GS order, characterized by the coexistence of
long-range alternating dimer, chirality and antiferromagnetic order have been
identified. Here we reexamine the same problem using the exact diagonalization
Lanczos method for chains up to N = 26 sites and explicitly detect positions of
quantum critical points (QCP) by investigating the quantum correlations as the
entanglement and the quantum discord (QD). It is shown that the entanglement
quantified by concurrence and the first derivative of the QD are able to reveal
besides the standard FM QCP also the Berezinskii-Kosterlitz-Thouless (BKT)
phase transition point between the LL and the gapped C1 phase and the Ising
type critical point separating the C1 and C2 phases. | cond-mat_str-el |
Quantum magnetism on the Cairo pentagonal lattice: We present an extensive analytical and numerical study of the
antiferromagnetic Heisenberg model on the Cairo pentagonal lattice, the dual of
the Shastry-Sutherland lattice with a close realization in the S=5/2 compound
Bi2Fe4O9. We consider a model with two exchange couplings suggested by the
symmetry of the lattice, and investigate the nature of the ground state as a
function of their ratio x and the spin S. After establishing the classical
phase diagram we switch on quantum mechanics in a gradual way that highlights
the different role of quantum fluctuations on the two inequivalent sites of the
lattice. The most important findings for S=1/2 include: (i) a surprising
interplay between a collinear and a four-sublattice orthogonal phase due to an
underlying order-by-disorder mechanism at small x (related to an emergent J1-J2
effective model with J2 >> J1), and (ii) a non-magnetic and possibly
spin-nematic phase with d-wave symmetry at intermediate x. | cond-mat_str-el |
Gaussian state approximation of quantum many-body scars: Quantum many-body scars are atypical, highly nonthermal eigenstates of
kinetically constrained systems embedded in a sea of thermal eigenstates. These
special eigenstates are characterized, for example, by a bipartite entanglement
entropy that scales as most logarithmically with subsystem size. We use
numerical optimization techniques to investigate if quantum many-body scars of
the experimentally relevant PXP model are well approximated by Gaussian states.
These states are described by a number of parameters that scales quadratically
with system size, thereby having a much lower complexity than generic quantum
many-body states. We find that this is a good description for the quantum
many-body scars away from the center of the spectrum. | cond-mat_str-el |
A phason disordered two dimensional quantum antiferromagnet: We examine a novel type of disorder in quantum antiferromagnets. Our model
consists of localized spins with antiferromagnetic exchanges on a bipartite
quasiperiodic structure, which is geometrically disordered in such a way that
no frustration is introduced. In the limit of zero disorder, the structure is
the perfect Penrose rhombus tiling. This tiling is progressively disordered by
augmenting the number of random "phason flips" or local tile-reshuffling
operations. The ground state remains N\'eel ordered, and we have studied its
properties as a function of increasing disorder using linear spin wave theory
and quantum Monte Carlo. We find that the ground state energy decreases,
indicating enhanced quantum fluctuations with increasing disorder. The magnon
spectrum is progressively smoothed, and the effective spin wave velocity of low
energy magnons increases with disorder. For large disorder, the ground state
energy as well as the average staggered magnetization tend towards limiting
values characteristic of this type of randomized tilings. | cond-mat_str-el |
Patterning of two-dimensional electron systems in SrTiO3 based
heterostructures using a CeO2 template: Two-dimensional electron systems found at the interface of SrTiO3-based oxide
heterostructures often display anisotropic electric transport whose origin is
currently under debate. To characterize transport along specific
crystallographic directions, we developed a hard-mask patterning routine based
on an amorphous CeO2 template layer. The technique allows preparing
well-defined microbridges by conventional ultraviolet photolithography which,
in comparison to standard techniques such as ion- or wet-chemical etching, does
not induce any degradation of interfacial conductance. The patterning scheme is
described in details and the successful production of microbridges based on
amorphous Al2O3-SrTiO3 heterostructures is demonstrated. Significant
anisotropic transport is observed for T < 30 K which is mainly related to
impurity/defect scattering of charge carriers in these heterostructures. | cond-mat_str-el |
Odd-Parity Triplet Pair Induced by Hund's Rule Coupling: We discuss microscopic aspects of odd-parity triplet pair in orbital
degenerate systems. From the concept of off-diagonal long-range order, a pair
state is unambiguously defined as the eigenstate with the maximum eigenvalue of
pair correlation function. Performing this scheme by a numerical technique, we
clarify that the odd-parity triplet pair occurs as an out-of-phase combination
of local triplets induced by Hund's rule coupling for the lattice including two
sites in the unit cell. | cond-mat_str-el |
Intrinsic Structural Disorder and the Magnetic Ground State in Bulk
EuTiO3: The magnetic properties of single-crystal EuTiO3 are suggestive of nanoscale
disorder below its cubic-tetragonal phase transition. We demonstrate that
electric field cooling acts to restore monocrystallinity, thus confirming that
emergent structural disorder is an intrinsic low-temperature property of this
material. Using torque magnetometry, we deduce that tetragonal EuTiO3 enters an
easy-axis antiferromagnetic phase at 5.6 K, with a first-order transition to an
easy-plane ground state below 3 K. Our data is reproduced by a 3D anisotropic
Heisenberg spin model. | cond-mat_str-el |
Superfluid-Insulator transition of quantum Hall domain walls in bilayer
graphene: We consider the zero-filled quantum-Hall ferromagnetic state of bilayer
graphene subject to a kink-like perpendicular electric field, which generates
domain walls in the electronic state and low-energy collective modes confined
to move along them. In particular, it is shown that two pairs of collective
helical modes are formed at opposite sides of the kink, each pair consisting of
modes with identical helicities. We derive an effective field-theoretical model
of these modes in terms of two weakly coupled anisotropic quantum spin-ladders,
with parameters tunable through control of the electric and magnetic fields.
This yields a rich phase diagram, where due to the helical nature of the modes,
distinct phases possess very different charge conduction properties. Most
notably, this system can potentially exhibit a transition from a superfluid to
an insulating phase. | cond-mat_str-el |
Probabilistic Simulation of Quantum Circuits with the Transformer: The fundamental question of how to best simulate quantum systems using
conventional computational resources lies at the forefront of condensed matter
and quantum computation. It impacts both our understanding of quantum materials
and our ability to emulate quantum circuits. Here we present an exact
formulation of quantum dynamics via factorized generalized measurements which
maps quantum states to probability distributions with the advantage that local
unitary dynamics and quantum channels map to local quasi-stochastic matrices.
This representation provides a general framework for using state-of-the-art
probabilistic models in machine learning for the simulation of quantum
many-body dynamics. Using this framework, we have developed a practical
algorithm to simulate quantum circuits with the Transformer, a powerful ansatz
responsible for the most recent breakthroughs in natural language processing.
We demonstrate our approach by simulating circuits which build GHZ and linear
graph states of up to 60 qubits, as well as a variational quantum eigensolver
circuit for preparing the ground state of the transverse field Ising model on
six qubits. Our methodology constitutes a modern machine learning approach to
the simulation of quantum physics with applicability both to quantum circuits
as well as other quantum many-body systems. | cond-mat_str-el |
Tripartite entangled plaquette state in a cluster magnet: Using large-scale quantum Monte Carlo simulations we show that a spin-$1/2$
XXZ model on a two-dimensional anisotropic Kagome lattice exhibits a tripartite
entangled plaquette state that preserves all of the Hamiltonian symmetries. It
is connected via phase boundaries to a ferromagnet and a valence-bond solid
that break U(1) and lattice translation symmetries, respectively. We study the
phase diagram of the model in detail, in particular the transitions to the
tripartite entangled plaquette state, which are consistent with conventional
order-disorder transitions. Our results can be interpreted as a description of
the charge sector dynamics of a Hubbard model applied to the description of the
spin liquid candidate ${\mathrm{LiZn}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$,
as well as a model of strongly correlated bosonic atoms loaded onto highly
tunable {\it trimerized} optical Kagome lattices. | cond-mat_str-el |
Magnetic behavior, Griffiths phase and magneto-transport study in 3$d$
based nano-crystalline double perovskite Pr$_2$CoMnO$_6$: Double perovskite (DP) oxide material receive extensive research interest due
to exciting physical properties with potential technological application. 3$d$
based DP oxides are promising for exciting physics like magnetodielectric,
ferroelectric, Griffith phase etc., specially Co/Mn DPs are gaining much
research interest. In this paper we present the study of magnetic phase and
transport properties in nano-crystalline Pr$_2$CoMnO$_6$ a 3$d$ based double
perovskite compound. This material shows a paramagnetic (PM) to ferromagnetic
(FM) phase transition below 173 K marked by a rapid increase in magnetic moment
due to spin ordering. We found divergence in inverse magnetic susceptibility
($\chi$$^{-1}$) from Curie weiss behavior around 206 K which indicates the
evolution of Griffiths phase before actual PM-FM transition. We found that the
Griffiths phase suppressed with increasing applied magnetic filed. For the
understanding of charge transport in this material we have measured temperature
dependent electrical resistivity. Pr$_2$CoMnO$_6$ is a strong insulator where
resistivity increase abruptly below magnetic phase transition. To understand
the effect of magnetic field on transport behavior we have also measured the
magnetoresistance (MR) at different temperatures. Sample shows the negative MR
with maximum value $\sim$22 $\%$ under applied magnetic field of 50 kOe at 125
K. MR follows quadratic field dependency above $T_c$ however below $T_c$ the MR
shows deviation from this field dependency at low field. | cond-mat_str-el |
Electrical and thermal transport in van der Waals magnets
2H-M$_x$TaS$_2$ (M = Mn, Co): We report a detailed study of electrical and thermal transport properties in
2H-M$_x$TaS$_2$ (M = Mn, Co) magnets where M atoms are intercalated in the van
der Waals gap. The intercalation induces ferromagentism with an easy-plane
anisotropy in 2H-Mn$_x$TaS$_2$, but ferromagnetism with a strong uniaxial
anisotropy in 2H-Co$_{0.22}$TaS$_2$, which finally evolves into a
three-dimensional antiferromagnetism in 2H-Co$_{0.34}$TaS$_2$.
Temperature-dependent electrical resistivity shows metallic behavior for all
samples. Thermopower is negative in the whole temperature range for
2H-Co$_x$TaS$_2$, whereas the sign changes from negative to positive with
increasing Mn for 2H-Mn$_x$TaS$_2$. The diffusive thermoelectric response
dominates in both high- and low-temperature ranges for all samples. A clear
kink in electrical resistivity, a weak anomaly in thermal conductivity, as well
as a slope change in thermopower were observed at the magnetic transitions for
2H-Mn$_{0.28}$TaS$_2$ ($T_\textrm{c}$ $\approx$ 82 K) and 2H-Co$_{0.34}$TaS$_2$
($T_\textrm{N}$ $\approx$ 36 K), respectively, albeit weaker for lower $x$
crystals. Co-intercalation promoted ferromagnetic to antiferromagnetic
transition is further confirmed by the Hall resistivity; the sign change of the
ordinary Hall coefficient indicates a multi-band behavior in 2H-Co$_x$TaS$_2$. | cond-mat_str-el |
Matrix Product State applications for the ALPS project: The density-matrix renormalization group method has become a standard
computational approach to the low-energy physics as well as dynamics of
low-dimensional quantum systems. In this paper, we present a new set of
applications, available as part of the ALPS package, that provide an efficient
and flexible implementation of these methods based on a matrix-product state
(MPS) representation. Our applications implement, within the same framework,
algorithms to variationally find the ground state and low-lying excited states
as well as simulate the time evolution of arbitrary one-dimensional and
two-dimensional models. Implementing the conservation of quantum numbers for
generic Abelian symmetries, we achieve performance competitive with the best
codes in the community. Example results are provided for (i) a model of
itinerant fermions in one dimension and (ii) a model of quantum magnetism. | cond-mat_str-el |
Doping Effects on the two-dimensional Spin Dimer Compound
$SrCu_2(BO_3)_2$: A series of compounds M$_{0.1}$Sr$_{0.9}$Cu$_2$(BO$_3$)$_2$ with Sr
substituted by M=Al, La, Na and Y were prepared by solid state reaction. XRD
analysis showed that these doping compounds are isostructural to
SrCu$_2$(BO$_3$)$_2$. The magnetic susceptibility from 1.9K to 300K in an
applied magnetic field of 1.0T and the specific heat from 1.9K to 25K in
applied fields up to 14T were measured. The spin gap is deduced from the low
temperature susceptibility as well as the specific heat. It is found that the
spin gap is strongly suppressed by magnetic fields. No superconductivity is
observed in all four samples. | cond-mat_str-el |
Magnetism in Kitaev Quantum Spin Liquid Candidate RuBr$_3$: The present studies show that long-range magnetic order takes place in
RuBr$_3$ at $\approx$ 34 K. The observations of clear oscillations in the muon
time spectra demonstrate the presence of well-defined internal fields at the
muon sites. The magnetic ordering appears to be very robust and static
suggesting a more conventional nature of magnetic ordering in the RuBr$_3$
system at zero field. Present investigations prove that in RuBr$_3$ the Kitaev
interactions are likely to be weakened at zero field in comparison to the
$\alpha$-RuCl$_3$ system. This proves that it is possible to tune the Kitaev
interactions by replacing Cl with heavier halogen elements such as Br. | cond-mat_str-el |
Combining complex and radial slave boson fields within the
Kotliar-Ruckenstein representation of correlated impurities: The gauge symmetry group of any slave boson representation allows to gauge
away the phase of bosonic fields. One benefit of this radial field formulation
is the elimination of spurious Bose condensations when saddle-point
approximation is performed. Within the Kotliar-Ruckenstein representation,
three of the four bosonic fields can be radial while the last one has to remain
complex. In this work, we present the procedure to carry out the functional
integration involving constrained fermionic fields, complex bosonic fields, and
radial bosonic fields. The correctness of the representation is verified by
exactly evaluating the partition function and the Green's function of the
Hubbard model in the atomic limit. | cond-mat_str-el |
Microscopic model realization of $\boldsymbol{d}$-wave pseudospin
current order in Sr$_{\boldsymbol{2}}$IrO$_{\boldsymbol{4}}$: The $d$-wave pseudospin current order ($d$PSCO) with staggered circulating
pseudospin current has been proposed as the hidden electronic order to describe
the unexpected breaking of spatial symmetries in stoichiometric
Sr$_{2}$IrO$_{4}$ and the unconventional pseudogap phenomena in electron doped
Sr$_{2}$IrO$_{4}$. However, a microscopic model for the emergence of $d$PSCO is
still lacking. The nearest neighbor Coulomb repulsion $V$, which is expected to
be significant in Sr$_{2}$IrO$_{4}$ due to the large spatial extension of the
Ir $5d$ orbitals, is capable of driving $d$PSCO on the mean-field level, albeit
the latter is energetically degenerate to the staggered flux phase with
circulating charge current. We find the in-plane anisotropy $\Gamma_2$ in the
effective superexchange interaction between $J_\text{eff}={1\over 2}$
pseudospins, originating from the cooperative interplay between Hund's rule
coupling and spin-orbit coupling of Ir $5d$ electrons, is able to lift the
degeneracy and stabilize the pseudospin currents. The effective single-orbital
model of $J_\text{eff}={1\over 2}$ electrons, including onsite Coulomb
repulsion $U$, nearest neighbor Coulomb repulsion $V$, and the in-plane
anisotropy $\Gamma_2$, is then studied. We obtain the mean-field ground states,
analyze their properties, and determine the phase diagram of stoichiometric
Sr$_{2}$IrO$_{4}$ in the plane spanned by $U$ and $V$ at a fixed $\Gamma_2$. We
demonstrate the realization of $d$PSCO, and its competition and coexistence
with antiferromagnetism. Remarkably, we find the coexistence of $d$PSCO and
antiferromagnetism naturally leads to spin bond nematicity, with the spin
directions of these three orders forming nontrivial chirality. Furthermore, we
show that the emergence of the coexistent state and its chirality can be tuned
by carrier doping. | cond-mat_str-el |
Valley dependent many-body effects in 2D semiconductors: We calculate the valley degeneracy ($g_v$) dependence of the many-body
renormalization of quasiparticle properties in multivalley 2D semiconductor
structures due to the Coulomb interaction between the carriers. Quite
unexpectedly, the $g_v$ dependence of many-body effects is nontrivial and
non-generic, and depends qualitatively on the specific Fermi liquid property
under consideration. While the interacting 2D compressibility manifests
monotonically increasing many-body renormalization with increasing $g_v$, the
2D spin susceptibility exhibits an interesting non-monotonic $g_v$ dependence
with the susceptibility increasing (decreasing) with $g_v$ for smaller (larger)
values of $g_v$ with the renormalization effect peaking around $g_v\sim 1-2$.
Our theoretical results provide a clear conceptual understanding of recent
valley-dependent 2D susceptibility measurements in AlAs quantum wells. | cond-mat_str-el |
Destruction of long-range order in non-collinear two-dimensional
antiferromagnets by random-bond disorder: We consider frustrated Heisenberg antiferromagnets, whose clean-limit ground
state is characterized by non-collinear long-range order with non-zero vector
chirality, and study the effects of quenched bond disorder, i.e., random
exchange couplings. A single bond defect is known to induce a dipolar texture
in the spin background independent of microscopic details. Using general
analytical arguments as well as large-scale simulations for the classical
triangular-lattice Heisenberg model, we show that any finite concentration of
such defects destroys long-range order for spatial dimension $d\leq 2$, in
favor of a glassy state whose correlation length in $d=2$ is exponentially
large for small randomness. Our results are relevant for a wide range of
layered frustrated magnets. | cond-mat_str-el |
On the dangers of partial diagrammatic summations: Benchmarks for the
two-dimensional Hubbard model in the weak-coupling regime: We study the two-dimensional Hubbard model in the weak-coupling regime and
compare the self-energy obtained from various approximate diagrammatic schemes
to the result of diagrammatic Monte Carlo simulations, which sum up all
weak-coupling diagrams up to a given order. While dynamical mean-field theory
provides a good approximation for the local part of the self-energy, including
its frequency dependence, the partial summation of bubble and/or ladder
diagrams typically yields worse results than second order perturbation theory.
Even widely used self-consistent schemes such as GW or the fluctuation-exchange
approximation (FLEX) are found to be unreliable. Combining the dynamical
mean-field self-energy with the nonlocal component of GW in GW+DMFT yields
improved results for the local self-energy and nonlocal self-energies of the
correct order of magnitude, but here, too, a more reliable scheme is obtained
by restricting the nonlocal contribution to the second order diagram. FLEX+DMFT
is found to give accurate results in the low-density regime, but even worse
results than FLEX near half-filling. | cond-mat_str-el |
Hall resistance in quantum Hall metals due to Pancharatnam phase
retardation and energy level spacing: We derive the trial Hall resistance formula for the quantum Hall metals to
address both the integer and fractional quantum Hall effects. Within the
degenerate Landau levels, Zeeman splitting and level crossings in the presence
of changing magnetic-field strength determine the Pancharatnam phase
retardation, including the phase acceleration or deceleration, which are
related to the changes in the phase and group momenta of a wavefunction. We
discuss the relevant physical postulates with respect to Pancharatnam phase
retardation to qualitatively reproduce the measured Hall resistance's zigzag
curve for both the integer and the fractional filling factors. Along the way,
we give out some hints to falsify our postulates with experiments. | cond-mat_str-el |
Kondo effect due to a hydrogen impurity in graphene: a multichannel
Kondo problem with diverging hybridization: We consider the Kondo effect arising from a hydrogen impurity in graphene. As
a first approximation, the strong covalent bond to a carbon atom removes that
carbon atom without breaking the $C_{3}$ rotation symmetry, and we only retain
the Hubbard interaction on the three nearest neighbors of the removed carbon
atom which then behave as magnetic impurities. These three impurity spins are
coupled to three conduction channels with definite helicity, two of which
support a diverging local density of states (LDOS) $\propto 1/ [ | \omega \ |
\ln ^{2}( \Lambda /| \omega \ | \ ) \ ] $ near the Dirac point $\omega
\rightarrow 0$ even though the bulk density of states vanishes linearly. We
study the resulting 3-impurity multi-channel Kondo model using the numerical
renormalization group method. For weak potential scattering, the ground state
of the Kondo model is a particle-hole symmetric spin-$1/2$ doublet, with
ferromagnetic coupling between the three impurity spins; for moderate potential
scattering, the ground state becomes a particle-hole asymmetric spin singlet,
with antiferromagnetic coupling between the three impurity spins. This behavior
is inherited by the Anderson model containing the hydrogen impurity and all
four carbon atoms in its vicinity. | cond-mat_str-el |
Spin Correlations in Quantum Wires: We consider theoretically spin correlations in an 1D quantum wire with
Rashba-Dresselhaus spin-orbit interaction (RDI). The correlations of
non-interacting electrons display electron-spin resonance at a frequency
proportional to the RDI coupling. Interacting electrons on varying the
direction of external magnetic field transit from the state of Luttinger liquid
(LL) to the spin density wave (SDW) state. We show that the two-time total spin
correlations of these states are significantly different. In the LL the
projection of total spin to the direction of the RDI induced field is conserved
and the corresponding correlator is equal to zero. The correlators of two
components perpendicular to the RDI field display a sharp ESR driven by RDI
induced intrinsic field. In contrast, in the SDW state the longitudinal
projection of spin dominates, whereas the transverse components are suppressed.
This prediction indicates a simple way for experimental diagnostic of the SDW
in a quantum wire. | cond-mat_str-el |
Coulomb correlation effects in LaOFeAs: LDA+DMFT(QMC) study: Effects of Coulomb correlation on LaOFeAs electronic structure have been
investigated by LDA+DMFT(QMC) method. The calculation results show that LaOFeAs
is in the regime of intermediate correlation strength with significant part of
the spectral density moved from the Fermi energy to Hubbard bands. However the
system is not on the edge of metal insulator-transition because increase of the
Coulomb interaction parameter value from $U$=4.0 eV to $U$=5.0 eV did not
result in insulator state. Correlations affect different d-orbitals not in the
same way. $t_{2g}$ states ($xz,yz$ and $x^2-y^2$ orbitals) have higher energy
due to crystal filed splitting and are nearly half-filled. Their spectral
functions have pseudogap with Fermi energy position on the higher sub-band
slope. Lower energy $e_g$ set of d-orbitals ($3z^2-r^2$ and $xy$) have
significantly larger occupancy values with typically metallic spectral
functions. | cond-mat_str-el |
Spin-frame field theory of a three-sublattice antiferromagnet: We present a nonlinear field theory of a three-sublattice hexagonal
antiferromagnet. The order parameter is the spin frame, an orthogonal triplet
of vectors related to sublattice magnetizations and spin chirality. The
exchange energy, quadratic in spin-frame gradients, has three coupling
constants, only two of which manifest themselves in the bulk. As a result, the
three spin-wave velocities satisfy a universal relation. Vortices generally
have an elliptical shape with the eccentricity determined by the Lam\'e
parameters. | cond-mat_str-el |
Transport properties of Metallic Ruthenates: a DFT+DMFT investigation: We present a systematical theoretical study on the transport properties of an
archetypal family of Hund's metals, Sr$_2$RuO$_4$,Sr$_3$Ru$_2$O$_7$, SrRuO$_3$
and CaRuO$_3$, within the combination of first principles density functional
theory and dynamical mean field theory. The agreement between theory and
experiments for optical conductivity and resistivity is good, which indicates
that electron-electron scattering dominates the transport of ruthenates. We
demonstrate that in the single-site dynamical mean field approach the transport
properties of Hund's metals fall into the scenario of "resilient
quasiparticles". We explains why the single layered compound Sr$_2$RuO$_4$ has
a relative weak correlation with respect to its siblings, which corroborates
its good metallicity. | cond-mat_str-el |
Fermi liquid state and enhanced electron correlations in the new iron
pnictide CaFe$_4$As$_3$: The newly discovered CaFe$_4$As$_3$ system displays low-temperature Fermi
liquid behavior, with enhanced electron-electron correlations. At high
temperatures, the magnetic susceptibility shows Curie-Weiss behavior, with a
large temperature-independent contribution. Antiferromagnetic ordering is
observed below T$_N$ = (88.0 $\pm$ 1.0) K, possibly via a spin density wave
(SDW) transition. A remarkably sharp drop in resistivity occurs below T$_2$ =
(26.4 $\pm$ 1.0) K, correlated with a similarly abrupt increase in the
susceptibility, but no visible feature in the specific heat. The electronic
specific heat coefficient $\gamma$ at low temperatures is close to 0.02 J
mol$^{-1}_{Fe}$ K$^{-2}$, but a higher value for $\gamma$ ($\sim$0.08 J
mol$^{-1}_{Fe}$ K$^{-2}$ can be inferred from a linear C$ / $T \textit{vs.}
T$^2$ just above T$_2$. The Kadowaki-Woods ratio A$/\gamma^2$ = 55$*10^{-5}$
$\mu \Omega$cm mol$^2$ K$^2 $mJ$^{-2}$ is nearly two orders of magnitude larger
than that of heavy fermions. | cond-mat_str-el |
A Criterion for Strange Metallicity in the Lorenz Ratio: The Wiedemann-Franz (WF) law, stating that the Lorenz ratio $L =
\kappa/(T\sigma)$ between the thermal and electrical conductivities in a metal
approaches a universal constant $L_0=\pi^2 k_B^2/ (3 e^2)$ at low temperatures,
is often interpreted as a signature of fermionic Landau quasi-particles. In
contrast, we show that various models of weakly disordered non-Fermi liquids
also obey the WF law at $T \to 0$. Instead, we propose using the leading
low-temperature correction to the WF law, $L(T)-L_0$ (proportional to the
inelastic scattering rate), to distinguish different types of strange metals.
As an example, we demonstrate that in a solvable model of a marginal Fermi
liquid, $L(T)-L_0\propto -T$. Using the quantum Boltzmann equation (QBE)
approach, we find analogous behavior in a class of marginal- and non-Fermi
liquids with a weakly momentum-dependent inelastic scattering. In contrast, in
a Fermi liquid, $L(T)-L_0$ is proportional to $-T^2$. This holds even when the
resistivity grows linearly with $T$, due to $T-$linear quasi-elastic scattering
(as in the case of electron-phonon scattering at temperatures above the Debye
frequency). Finally, by exploiting the QBE approach, we demonstrate that the
transverse Lorenz ratio, $L_{xy} = \kappa_{xy}/(T\sigma_{xy})$, exhibits the
same behavior. | cond-mat_str-el |
Spin-filtering by field dependent resonant tunneling: We consider theoretically transport in a spinfull one-channel interacting
quantum wire placed in an external magnetic field. For the case of two
point-like impurities embedded in the wire, under a small voltage bias the
spin-polarized current occurs at special points in the parameter space, tunable
by a single parameter. At sufficiently low temperatures complete
spin-polarization may be achieved, provided repulsive interaction between
electrons is not too strong. | cond-mat_str-el |
Signatures of a liquid-crystal transition in spin-wave excitations of
skyrmions: Understanding the spin-wave excitations of chiral magnetic order, such as the
skyrmion crystal (SkX), is of fundamental interest to confirm such exotic
magnetic order. The SkX is realized by competing Dzyaloshinskii-Moriya and
ferromagnetic-exchange interactions with a magnetic field or anisotropy. Here
we compute the dynamical spin structure factor, using Monte Carlo and spin
dynamics simulations, extracting the spin-wave spectrum in the SkX, in the
vicinity of the paramagnet to SkX transition. Inside the SkX, we find six
spin-wave modes, which are supplemented by another mode originating from the
ferromagnetic background. Above the critical temperature $T_s$ for the skyrmion
crystallization, we find a diffusive regime, reminiscent of the
liquid-to-crystal transition, revealing that topological spin texture of
skyrmionic character starts to develop above $T_s$ as the precursor of the SkX.
We discuss the opportunities for the detection of the spin waves of the SkX
using inelastic-neutron-scattering experiments in manganite-iridate
heterostructures. | cond-mat_str-el |
Candidate local parent Hamiltonian for 3/7 fractional quantum Hall
effect: While a parent Hamiltonian for Laughlin $1/3$ wave function has been long
known in terms of the Haldane pseudopotentials, no parent Hamiltonians are
known for the lowest-Landau-level projected wave functions of the composite
fermion theory at $n/(2n+1)$ with $n\geq2$. If one takes the two lowest Landau
levels to be degenerate, the Trugman-Kivelson interaction produces the
unprojected 2/5 wave function as the unique zero energy solution. If the lowest
three Landau levels are assumed to be degenerate, the Trugman-Kivelson
interaction produces a large number of zero energy states at $\nu=3/7$. We
propose that adding an appropriately constructed three-body interaction yields
the unprojected $3/7$ wave function as the unique zero energy solution, and
report extensive exact diagonalization studies that provide strong support to
this proposal. | cond-mat_str-el |
On the effective reconstruction of expectation values from ab initio
quantum embedding: Quantum embedding is an appealing route to fragment a large interacting
quantum system into several smaller auxiliary `cluster' problems to exploit the
locality of the correlated physics. In this work we critically review
approaches to recombine these fragmented solutions in order to compute
non-local expectation values, including the total energy. Starting from the
democratic partitioning of expectation values used in density matrix embedding
theory, we motivate and develop a number of alternative approaches, numerically
demonstrating their efficiency and improved accuracy as a function of
increasing cluster size for both energetics and non-local two-body observables
in molecular and solid state systems. These approaches consider the
$N$-representability of the resulting expectation values via an implicit global
wave~function across the clusters, as well as the importance of including
contributions to expectation values spanning multiple fragments simultaneously,
thereby alleviating the fundamental locality approximation of the embedding. We
clearly demonstrate the value of these introduced functionals for reliable
extraction of observables and robust and systematic convergence as the cluster
size increases, allowing for significantly smaller clusters to be used for a
desired accuracy compared to traditional approaches in ab initio wave~function
quantum embedding. | cond-mat_str-el |
On the origin of the quantum-critical transition in the bilayer
Heisenberg model: The bilayer Heisenberg antiferromagnet is known to exhibit a quantum-critical
transition at a particular value of the inter-layer coupling. Using a new type
of coherent state, appropriate to the special order parameter structure of the
bilayer, we map the problem onto the quantum non-linear sigma model. It is
found that the bare coupling constant diverges at the classical transition of
Chubukov and Morr, so that in any finite dimension the actual transition occurs
inside the ordered phase of the classical theory. | cond-mat_str-el |
Interacting Topological Superconductors and possible Origin of $16n$
Chiral Fermions in the Standard Model: Motivated by the observation that the Standard Model of particle physics
(plus a right-handed neutrino) has precisely 16 Weyl fermions per generation,
we search for $(3+1)$-dimensional chiral fermionic theories and chiral gauge
theories that can be regularized on a 3 dimensional spatial lattice when and
only when the number of flavors is an integral multiple of 16. All these
results are based on the observation that local interactions reduce the
classification of certain $(4+1)$-dimensional topological superconductors from
$\mathbb{Z}$ to $\mathbb{Z}_{8}$, which means that one of their
$(3+1)$-dimensional boundaries can be gapped out by interactions without
breaking any symmetry when and only when the number of boundary chiral fermions
is an integral multiple of $16$. | cond-mat_str-el |
Short-range antiferromagnetic correlations in the superconducting state
of filled skutterudite alloys Pr$_{1-x}$Eu$_x$Pt$_4$Ge$_{12}$: Motivated by current research efforts towards exploring the interplay between
magnetism and superconductivity in multiband electronic systems, we have
investigated the effects of Eu substitution through thermodynamic measurements
on the superconducting filled skutterudite alloys
Pr$_{1-x}$Eu$_x$Pt$_4$Ge$_{12}$. An increase in Eu concentration leads to a
suppression of the superconducting transition temperature consistent with an
increase of magnetic entropy due to Eu local moments. While the low-temperature
heat capacity anomaly is present over the whole doping range, we find that in
alloys with $x\leq0.5$ the Schottky peaks in the heat capacity in the
superconducting state appear to be due to Zeeman splitting by an internal
magnetic field. Our theoretical modeling suggests that this field is a result
of the short-range antiferromagnetic correlations between the europium ions.
For the samples with $x > 0.5$, the peaks in the heat capacity signal the onset
of antiferromagnetic (AFM) ordering of the Eu moments. | cond-mat_str-el |
Temperature-driven hidden 5f itinerant-localized crossover in
heavy-fermion compound PuIn3: The temperature-dependent evolution pattern of 5f electrons helps to
elucidate the long-standing itinerant-localized dual nature in plutonium-based
compounds. In this work, we investigate the correlated electronic states of
PuIn3 dependence on temperature by using a combination of the density
functional theory and the dynamical mean-field theory. Not only the
experimental photoemission spectroscopy is correctly reproduced, but also a
possible hidden 5f itinerant-localized crossover is identified. Moreover, it is
found that the quasiparticle multiplets from the many-body transitions
gradually enhance with decreasing temperature, accompanied by the
hybridizations with 5f electrons and conduction bands. The temperature-induced
variation of Fermi surface topology suggests a possible electronic Lifshitz
transition and the onset of magnetic order at low temperature. Finally, the
ubiquitous existence orbital selective 5f electron correlation is also
discovered in PuIn3. These illuminating results shall enrich the understanding
on Pu-based compounds and serve as critical predictions for ongoing
experimental research. | cond-mat_str-el |
Flat-band ferromagnetism in a correlated topological insulator on a
honeycomb lattice: We study the flat-band ferromagnetic phase of a spinfull and time-reversal
symmetric Haldane-Hubbard model on a honeycomb lattice within a bosonization
formalism for flat-band Z$_2$ topological insulators. Such a study extend our
previous one [L. S. G. Leite and R. L. Doretto, Phys. Rev. B {\bf 104}, 155129
(2021)] concerning the flat-band ferromagnetic phase of a correlated Chern
insulator described by a Haldane-Hubbard model. We consider the topological
Hubbard model at $1/4$ filling of its corresponding noninteracting limit and in
the nearly flat band limit of its lower free-electronic bands. We show that it
is possible to define boson operators associated with two distinct spin-flip
excitations, one that changes (mixed-lattice excitations) and a second one that
preserves (same-lattice excitations) the index related with the two triangular
sublattices. Within the bosonization scheme, the fermionic model is mapped into
an effective interacting boson model, whose quadratic term is considered at the
harmonic approximation in order to determine the spin-wave excitation spectrum.
For both mixed and same-lattice excitations, we find that the spin-wave
spectrum is gapped and has two branches, with an energy gap between the lower
and the upper bands at the $K$ and $K'$ points of the first Brillouin zone.
Such a behavior is distinct from the one of the corresponding correlated Chern
insulator, whose spin-wave spectrum has a Goldstone mode at the center of the
first Brillouin zone and Dirac points at $K$ and $K'$ points. We also find some
evidences that the spin-wave bands for the same-lattice excitations might be
topologically nontrivial even in the completely flat band limit. | cond-mat_str-el |
Theory of Half-metallic Ferrimagnetism in Double Perovskites: We present a comprehensive theory of the temperature- and disorder-dependence
of half-metallic ferrimagnetism in the double perovskite Sr$_2$FeMoO$_6$ (SFMO)
with $T_c$ above room temperature. We show that the magnetization $M(T)$ and
conduction electron polarization $P(T)$ are both proportional to the
magnetization $M_S(T)$ of localized Fe spins. We derive and validate an
effective spin Hamiltonian, amenable to large-scale three-dimensional
simulations. We show how $M(T)$ and $T_c$ are affected by disorder, ubiquitous
in these materials. We suggest a way to enhance $T_c$ in SFMO without
sacrificing polarization. | cond-mat_str-el |
Exact band structures for 1D superlattices beyond the tight-binding
approximation: The band structures describing non-interacting particles in one-dimensional
superlattices of arbitrary periodicity are obtained by an analytical
diagonalization of the Hamiltonian without adopting the popular tight-binding
approximation. The results are compared with those of the tight-binding
approximation. In this way, a quantitative prediction of the validity and
failure of the tight-binding approximation becomes possible. In particular, it
is demonstrated that in contrast to the prediction of the tight-binding
approximation the central energy bands do not touch for periodicities $\tau$ of
the lattice where $\tau=4n$ and $n$ is an integer. | cond-mat_str-el |
Spin Dynamics at Very Low Temperature in Spin Ice Dy$_2$Ti$_2$O$_7$: We have performed AC susceptibility and DC magnetic relaxation measurements
on the spin ice system Dy$_2$Ti$_2$O$_7$ down to 0.08 K. The relaxation time of
the magnetization has been estimated below 2 K down to 0.08 K. The spin
dynamics of Dy$_2$Ti$_2$O$_7$ is well described by using two relaxation times
($\tau_{\rm S}$ (short time) and $\tau_{\rm L}$ (long time)). Both $\tau_{\rm
S}$ and $\tau_{\rm L}$ increase on cooling. Assuming the Arrhenius law in the
temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K,
both $\tau_{\rm S}$ and $\tau_{\rm L}$ show a clear deviation from the thermal
activated dynamics toward temperature independent relaxation, suggesting a
quantum dynamics. | cond-mat_str-el |
Exact solution of electronic transport in semiconductors dominated by
scattering on polaronic impurities: The scattering of electrons on impurities with internal degrees of freedom is
bound to produce the signatures of the scatterer's own dynamics and results in
nontrivial electronic transport properties. Previous studies of polaronic
impurities in low-dimensional structures, like molecular junctions and
one-dimensional nanowire models, have shown that perturbative treatments cannot
account for a complex energy dependence of the scattering cross section in such
systems. Here we derive the exact solution of polaronic impurities shaping the
electronic transport in bulk (3D) systems. In the model with a short-ranged
electron-phonon interaction, we solve for and sum over all elastic and
inelastic partial cross sections, abundant in resonant features. The
temperature dependence of the charge mobility shows the power-law dependence,
$\mu(T)\propto T^{-\nu}$, with $\nu$ being highly sensitive to impurity
parameters. The latter may explain nonuniversal power-law exponents observed
experimentally, e.g. in high-quality organic molecular semiconductors. | cond-mat_str-el |
Electronic structure and parity effects in correlated nanosystems: We discuss the spectral, transport and magnetic properties of quantum
nanowires composed of N\leq 13 atoms and containing either even or odd numbers
of valence electrons. In our approach we combine Exact Diagonalization and Ab
Initio calculations (EDABI method). The analysis is performed as a function of
the interatomic distance. The momentum distribution differs drastically for
those obtained for even N with those for odd N, whereas the Drude weight
evolves smoothly. A role of boundary conditions is stressed. | cond-mat_str-el |
Colloquium: Hidden Order, Superconductivity, and Magnetism -- The
Unsolved Case of URu2Si2: This Colloquium reviews the 25 year quest for understanding the continuous
(second-order) mean-field-like phase transition occurring at 17.5 K in URu2Si2.
About ten years ago, the term hidden order (HO) was coined and has since been
utilized to describe the unknown ordered state, whose origin cannot be
disclosed by conventional solid-state probes, such as x rays, neutrons, or
muons. HO is able to support superconductivity at lower temperatures (Tc ~ 1.5
K), and when magnetism is developed with increasing pressure both the HO and
the superconductivity are destroyed. Other ways of probing the HO are via
Rh-doping and very large magnetic fields. During the last few years a variety
of advanced techniques have been tested to probe the HO state and their
attempts will be summarized. A digest of recent theoretical developments is
also included. It is the objective of this Colloquium to shed additional light
on the HO state and its associated phases in other materials. | cond-mat_str-el |
Universal Duality in Luttinger Liquid Coupled to Generic Environment: We study a Luttinger Liquid (LL) coupled to a generic environment consisting
of bosonic modes with arbitrary density-density and current-current
interactions. The LL can be either in the conducting phase and perturbed by a
weak scatterer or in the insulating phase and perturbed by a weak link. The
environment modes can also be scattered by the imperfection in the system with
arbitrary transmission and reflection amplitudes. We present a general method
of calculating correlation functions under the presence of the environment and
prove the duality of exponents describing the scaling of the weak scatterer and
of the weak link. This duality holds true for a broad class of models and is
sensitive to neither interaction nor environmental modes details, thus it shows
up as the universal property. It ensures that the environment cannot generate
new stable fixed points of the RG flow. Thus, the LL always flows toward either
conducting or insulating phase. Phases are separated by a sharp boundary which
is shifted by the influence of the environment. Our results are relevant, for
example, for low-energy transport in (i) an interacting quantum wire or a
carbon nanotube where the electrons are coupled to the acoustic phonons
scattered by the lattice defect; (ii) a mixture of interacting fermionic and
bosonic cold atoms where the bosonic modes are scattered due to an abrupt local
change of the interaction, (iii) mesoscopic electric circuits. | cond-mat_str-el |
Effective electron-electron interaction in a two dimensional
paramagnetic system: We analyze the effective electron-electron interaction in a two dimensional
polarized paramagnetic system. The spin degree of freedom, s, is manifestly
present in the expressions of spin dependent local field factors that describe
the short range exchange (x) and correlation (c) effects. Starting from the
exact asymptotic values of the local field correction functions for large and
small momentum at zero frequency we obtain self-consistent expressions across
the whole spectrum of momenta. Then, the effective interaction between two
electrons with spins s and s' is calculated. We find that the four effective
interactions, up-up, up-down, down-up and down-down, are different. We also
obtain their qualitative dependence on the electronic density and polarization
and note that these results are independent of the approximation used for the
local field correction functions at intermediate momenta. | cond-mat_str-el |
Electron-phonon coupling and spin-charge separation in one-dimensional
Mott insulators: We examine the single-particle excitation spectrum in the one-dimensional
Hubbard-Holstein model at half-filling by performing the dynamical density
matrix renormalization group (DDMRG) calculation. The DDMRG results are
interpreted as superposition of spectra for a spinless carrier dressed with
phonons. The superposition is a consequence of robustness of the spin-charge
separation against electron-phonon coupling. The separation is in contrast to
the coupling between phonon and spin degrees of freedom in two-dimensional
systems. We discuss implication of the results of the recent angle-resolved
photoemission spectroscopy measurements on SrCuO${}_{2}$. | cond-mat_str-el |
Anomalous Hall Effect in Graphite: We report on the experimental observation of an anomalous Hall effect (AHE)
in highly oriented pyrolytic graphite samples. The overall data indicate that
the AHE in graphite can be self-consistently understood within the frameworks
of the magnetic-field-driven excitonic pairing models. | cond-mat_str-el |
Gapless edge states of BF field theory and translation-symmetric Z2 spin
liquids: We study possible gapless edge states of translation-symmetric Z2 spin
liquids. The gapless edge states emerge from dangling Majorana fermions at the
boundary. We construct a series of mean-field Hamiltonians of Z2 spin liquids
on the square lattice; these models can be obtained by generalization of Wen's
exactly solvable plaquette model. We also study the details of the edge theory
of these Z2 spin liquids and find their effective BF theory descriptions. The
effective BF theories are shown to describe the crystal momenta of the ground
states and their degeneracies and to predict the edge theories of these Z2 spin
liquids. As a byproduct, we obtained a way to classify the BF theories
reflecting the lattice symmetries. We discuss in closing three-dimensional Z2
spin liquids with gapless surface states on the cubic lattice. | cond-mat_str-el |
Quantum phase transitions in d-wave superconductors: Motivated by the strong, low temperature damping of nodal quasiparticles
observed in some cuprate superconductors, we study quantum phase transitions in
d_{x^2-y^2} superconductors with a spin-singlet, zero momentum, fermion
bilinear order parameter. We present a complete, group-theoretic classification
of such transitions into 7 distinct cases (including cases with nematic order)
and analyze fluctuations by the renormalization group. We find that only 2, the
transitions to d_{x^2-y^2}+is and d_{x^2-y^2} + i d_{xy} pairing, possess
stable fixed points with universal damping of nodal quasiparticles; the latter
leaves the gapped quasiparticles along (1,0), (0,1) essentially undamped. | cond-mat_str-el |
Renormalized SO(5) symmetry in ladders with next-nearest-neighbor
hopping: We study the occurrence of SO(5) symmetry in the low-energy sector of
two-chain Hubbard-like systems by analyzing the flow of the running couplings
($g$-ology) under renormalization group in the weak-interaction limit. It is
shown that SO(5) is asymptotically restored for low energies for rather general
parameters of the bare Hamiltonian. This holds also with inclusion of a
next-nearest-neighbor hopping which explicitly breaks particle-hole symmetry
provided one accounts for a different single-particle weight for the
quasiparticles of the two bands of the system. The physical significance of
this renormalized SO(5) symmetry is discussed. | cond-mat_str-el |
Fine structures in the spectrum of the open-boundary Heisenberg chain at
large anisotropies: At large anisotropies, the spectrum of the Heisenberg XXZ spin chain
separates into `bands' with energies largely determined by the number of domain
walls. The band structure is richer with open boundary conditions: there are
more bands and the bands develop intricate fine structures. We characterize and
explain these structures and substructures in the open-boundary chain. The fine
structures are explained using degenerate perturbation theory. We also present
some dynamical consequences of these sub-band structures, through explicit time
evolution of the wavefunction from initial states motivated by the fine
structure analysis. | cond-mat_str-el |
Smeared quantum phase transition in the dissipative random quantum Ising
model: We investigate the quantum phase transition in the random transverse-field
Ising model under the influence of Ohmic dissipation. To this end, we
numerically implement a strong-disorder renormalization-group scheme. We find
that Ohmic dissipation destroys the quantum critical point and the associated
quantum Griffiths phase by smearing. Our results quantitatively confirm a
recent theory [Phys. Rev. Lett. {\bf 100}, 240601 (2008)] of smeared quantum
phase transitions. | cond-mat_str-el |
Two Anderson impurities in a 2D host with Rashba spin-orbit interaction: We have studied the two-dimensional two-impurity Anderson model with
additional Rashba spin-orbit interaction by means of the modified perturbation
theory. The impurity Green's functions we have constructed exactly reproduce
the first four spectral moments. We discuss the height and the width of the
even/odd Kondo peaks as functions of the inter-impurity distance and the Rashba
energy $E_R$ (the strength of the Rashba spin-orbit interaction). For small
impurity separations the Kondo temperature shows a non-monotonic dependence on
$E_R$ being different in the even and the odd channel. We predict that the
Kondo temperature has only almost linear dependence on $E_R$ and not an
exponential increase with $E_R$ | cond-mat_str-el |
Origin of infrared peaks in the optical conductivity of ytterbium
compounds: We have calculated optical conductivity [$\sigma(\omega)$] spectra of
ytterbium compounds (YbAl$_3$, YbAl$_2$, YbCu$_2$Si$_2$, YbNi$_2$Ge$_2$,
YbInCu$_4$, YbRh$_2$Si$_2$, YbIr$_2$Si$_2$, and YbB$_{12}$) based on the direct
interband transition derived from first-principle band calculation and compared
the results with the experimentally obtained $\sigma(\omega)$ spectra. The
spectral feature of a peak in the middle-infrared region (mid-IR peak) and a
shoulder structure in the far-infrared region (far-IR shoulder) in the
experimental $\sigma(\omega)$ spectra can be described by the band calculation
with a common renormalization factor. This result indicates that the infrared
spectra of Yb compounds originate from the interband transition from the Yb
$4f$ state but that the Yb $4f$ state shifts to the Fermi level with strong
electron correlation. | cond-mat_str-el |
Spin functional renormalization group for dimerized quantum spin systems: We investigate dimerized quantum spin systems using the spin functional
renormalization group approach proposed by Krieg and Kopietz [Phys. Rev. B 99,
060403(R) (2019)] which directly focuses on the physical spin correlation
functions and avoids the representation of the spins in terms of fermionic or
bosonic auxiliary operators. Starting from decoupled dimers as initial
condition for the renormalization group flow equations, we obtain the spectrum
of the triplet excitations as well as the magnetization in the quantum
paramagnetic, ferromagnetic, and thermally disordered phases at all
temperatures. Moreover, we compute the full phase diagram of a weakly coupled
dimerized spin system in three dimensions, including the correct mean field
critical exponents at the two quantum critical points. | cond-mat_str-el |
Novel Orbital Ordering induced by Anisotropic Stress in a Manganite Thin
Film: We performed resonant and nonresonant x-ray diffraction studies of a
Nd0.5Sr0.5MnO3 thin film that exhibits a clear first-order transition. Lattice
parameters vary drastically at the metal-insulator transition at 170K (=T_MI),
and superlattice reflections appear below 140K (=T_CO). The electronic
structure between T_MI and T_CO is identified as A-type antiferromagnetic with
the d_{x2-y2} ferroorbital ordering. Below T_CO, a new type of antiferroorbital
ordering emerges. The accommodation of the large lattice distortion at the
first-order phase transition and the appearance of the novel orbital ordering
are brought about by the anisotropy in the substrate, a new parameter for the
phase control. | cond-mat_str-el |
Anisotropic Magnetic Response in Kondo Lattice with Antiferromagnetic
Order: Magnetic properties are investigated for the Kondo lattice by using the
continuous time quantum Monte Colro (CT-QMC) and the dynamical mean field
theory (DMFT). The DMFT+CT-QMC approach is extended so as to derive the
anisotropic magnetic response in the antiferromagnetic phase. The longitudinal
and transverse magnetic susceptibilities are numerically derived in the
antiferromagnetic phase. For the RKKY regime with a small Kondo coupling, the
transverse susceptibility does not decrease below the transition temperature
while the longitudinal susceptibility decreases as expected from the mean field
picture. In the competing region between the RKKY interaction and the Kondo
effect, however, both longitudinal and transverse susceptibilities decrease
below the transition temperature. The results obtained naturally explain the
temperature dependence of the magnetic susceptibility observed in
CeT$_2$Al$_{10}$ ($T$=Ru,Os,Fe) family. | cond-mat_str-el |
Model for the Fractional Quantum Hall Effect problem: A simple one-dimensional model is proposed, in which N spinless repulsively
interacting fermions occupy M>N degenerate states. It is argued that the energy
spectrum and the wavefunctions of this system strongly resemble the spectrum
and wavefunctions of 2D electrons in the lowest Landau level (the problem of
the Fractional Quantum Hall Effect). In particular, Laughlin-type wavefunctions
describe ground states at filling factors v = N/M = 1(2m+1). Within this model
the complimentary wavefunction for v = 1-1/(2m + 1) is found explicitly and
extremely simple ground state wavefunctions for arbitrary odd-denominator
filling factors are proposed. | cond-mat_str-el |
Proximity-induced superconductivity in a 2D Kondo lattice of an
f-electron-based surface alloy: Realizing hybrids of low-dimensional Kondo lattices and superconducting
substrates leads to fascinating platforms for studying the exciting physics of
strongly correlated electron systems with induced superconducting pairing.
Here, we report a scanning tunneling microscopy and spectroscopy study of a new
type of two-dimensional (2D) La-Ce alloy grown epitaxially on a superconducting
Re(0001) substrate. We observe the characteristic spectroscopic signature of a
hybridization gap evidencing the coherent spin screening in the 2D Kondo
lattice realized by the ultrathin La-Ce alloy film on normal conducting
Re(0001). Upon lowering the temperature below the critical temperature of
rhenium, a superconducting gap is induced with an in-gap Shiba band arising
from the interaction of residual unscreened magnetic moments with the
superconducting substrate. A positive correlation between the Kondo
hybridization gap and the binding energy of the subgap Shiba band maximum is
found. Our results open up a promising route toward the design of artificial
superconducting Kondo and heavy fermion systems. | cond-mat_str-el |
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