text
stringlengths 89
2.49k
| category
stringclasses 19
values |
|---|---|
Crossover to Fermi-liquid behavior for weakly-coupled Luttinger liquids
in the anisotropic large-dimension limit: We study the problem of the crossover from one- to higher-dimensional metals
by considering an array of Luttinger liquids (one-dimensional chains) coupled
by a weak interchain hopping {\tp.} We evaluate the exact asymptotic low-energy
behavior of the self-energy in the anisotropic infinite-dimension limit. This
limit extends the dinamical mean field concept to the case of a chain embedded
in a self-consistent medium. The system flows to a Fermi-liquid fixed point for
energies below the dimensional crossover temperature, and the anomalous
exponent $\al$ renormalizes to zero, in the case of equal spin and charge
velocities. In particular, the single-particle spectral function shows sharp
quasiparticle peaks with nonvanishing weight along the whole Fermi surface, in
contrast to the lowest-order result. Our result is obtained by carring out a
resummation of all diagrams of the expansion in \tp contributing to the
anisotropic $D\to\infty$ limit. This is done by solving, in an almost
completely analytic way, an asymptotically exact recursive equation for the
renormalized vertices, within a skeleton expansion. Our outcome shows that
perturbation expansions in \tp restricted to lowest orders are unreliable below
the crossover temperature. The extension to finite dimensions is discussed.
This work extends our recent Letter [Phys. Rev. Lett. {\bf 83}, 128 (1999)],
and includes all mathematical details. | cond-mat_str-el |
Incommensurate magnetic order in Ag$_{2}$NiO$_{2}$: The nature of the magnetic transition of the half-filled triangular
antiferromagnet Ag$_{2}$NiO$_2$ with $T_{\rm N}$=56K was studied with positive
muon-spin-rotation and relaxation ($\mu^+$SR) spectroscopy. Zero field
$\mu^+$SR measurements indicate the existence of a static internal magnetic
field at temperatures below $T_{\rm N}$. Two components with slightly different
precession frequencies and wide internal-field distributions suggest the
formation of an incommensurate antiferromagnetic order below 56 K. This implies
that the antifrerromagnetic interaction is predominant in the NiO$_2$ plane in
contrast to the case of the related compound NaNiO$_2$. An additional
transition was found at $\sim$22 K by both $\mu^+$SR and susceptibility
measurements. It was also clarified that the transition at $\sim$260 K observed
in the susceptibility of Ag$_{2}$NiO$_{2}$ is induced by a purely structural
transition. | cond-mat_str-el |
Ferromagnetic diagonal stripe states in the two-dimensional Hubbard
model with $U\lesssim\infty$: We have performed a variational Monte Carlo simulation to study the ground
state of a two-dimensional Hubbard model on a square lattice in the strong
coupling region. The energy gain of possible inhomogeneous electron states are
computed as a function of $U$ when the hole density $\epsilon=1/8$ and next
nearest-neighbor hopping $t'/t=-0.30$. The bond-centered ferromagnetic diagonal
stripe state is stabilized in the strong coupling region ($U/t\geq$16), which
is due to the gain of both kinetic energy and on-site Coulomb interaction
energy due to the holon moving over the ferromagnetic domain and the gain of
kinetic-exchange-interaction energy at the antiferromagnetic domain wall. | cond-mat_str-el |
The Parent of Misfit-Layered Cobalt Oxides: [Sr2O2]qCoO2: Misfit-layered (ML) cobalt oxides of the general formula of [MmA2Om+2]qCoO2
have been proven to be efficient thermoelectric materials as the structure is
capable in accommodating the two seemingly contradictory characteristics of
high electrical conductivity and large thermo-electric power. They are also
potential hosts for other oxymoron-like functions. The known phases all contain
one or two square-planar MO (M = Co, Bi, Pb, Tl, etc.) layers sandwiched
together with AO (A = Ca, Sr, Ba, etc.) planes of square symmetry and CoO2
layers of hexagonal symmetry. Here we report realization of the simplest (m =
0) ML phase forming in the Sr-Co-O system with the cation ratio, Sr/Co = 1.
Atomic-resolution TEM imaging confirms for the new phase the parent three-layer
crystal structure, SrO-SrO-CoO2, which is compatible with the formula of
[Sr2O2]qCoO2. Electron diffraction reveals that the phase is rather
commensurate, i.e. the "misfit parameter" q is 0.5. Nevertheless, in terms of
the transport-property characteristics the new ML parent is comparable to its
earlier-established and more complex derivatives. | cond-mat_str-el |
Model Calculation of Electron-Phonon Couplings in a Dimer with a
Non-Degenerate Orbital: We evaluate all the electron-phonon couplings derived from the one-body
electronic interactions, in both the adiabatic and extreme non-adiabatic limit,
for a dimer with a non-degenerate orbital built from atomic wave functions of
Gaussian shape. We find largely different values of the coupling parameters in
the two cases, as well as different expressions of the corresponding terms in
the Hamiltonian. | cond-mat_str-el |
Domain switching and exchange bias control by electric field in
multiferroic conical magnet Mn$_2$GeO$_4$: The electric field effect on magnetism was examined in the multiferroic
conical magnet Mn$_2$GeO$_4$, which shows a strong coupling between
ferromagnetic and ferroelectric order parameters. The systematic evaluation of
the electric polarization in the multiferroic phase below 5.5 K under various
field cooling conditions reveals that small magnetic fields of 0.1 T
significantly reduce the required electric fields needed to reach saturation.
By applying electric fields during magnetic field dependent hysteresis
measurements of magnetization M and polarization P an electrically controllable
exchange bias was observed, a phenomenon exceedingly rare in single phase
multiferroics. Furthermore, non-reversible electric switching of P and M
domains was achieved under specific magnetic field conditions. | cond-mat_str-el |
Valence Bond Phases in $S=1/2$ Kane-Mele-Heisenberg Model: The phase diagram of Kane-Mele-Heisenberg (KMH) model in classical
limit~\cite{zare}, contains disordered regions in the coupling space, as the
result of to competition among different terms in the Hamiltonian, leading to
frustration in finding a unique ground state. In this work we explore the
nature of these phase in the quantum limit, for a $S=1/2$. Employing exact
diagonalization (ED) in $S_z$ and nearest neighbor valence bond (NNVB) bases,
bond and plaquette valence bond mean field theories, We show that the
disordered regions are divided into ordered quantum states in the form of
plaquette valence bond crystal(PVBC) and staggered dimerized (SD) phases. | cond-mat_str-el |
Correlation-driven insulator-metal transition in near-ideal vanadium
dioxide films: We use polarization- and temperature-dependent x-ray absorption spectroscopy,
in combination with photoelectron microscopy, x-ray diffraction and electronic
transport measurements, to study the driving force behind the insulator-metal
transition in VO2. We show that both the collapse of the insulating gap and the
concomitant change in crystal symmetry in homogeneously strained
single-crystalline VO2 films are preceded by the purely-electronic softening of
Coulomb correlations within V-V singlet dimers. This process starts 7 K (+/-
0.3 K) below the transition temperature, as conventionally defined by
electronic transport and x-ray diffraction measurements, and sets the energy
scale for driving the near-room-temperature insulator-metal transition in this
technologically-promising material. | cond-mat_str-el |
Single-molecule-mediated heat current between an electronic and a
bosonic bath: In molecular devices electronic degrees of freedom are coupled to vibrational
modes of the molecule, offering an opportunity to study fundamental aspects of
this coupling between at the nanoscale. To this end we consider the
nonequilibrium heat exchange between a conduction band and a bosonic bath
mediated by a single molecule. For molecules large enough so that on-site
interactions can be dropped we carry out an asymptotically exact calculation of
the heat current, governed by the smallness of the electron-phonon coupling,
and obtain the steady state heat current driven by a finite temperature drop.
At low temperatures the heat current is found to have a power-law behavior with
respect to the temperature difference with the power depending on the nature of
the bosonic bath. At high temperatures, on the other hand, the current is
linear in the temperature difference for all types of bosonic baths. The
crossover between these behaviors is described. Some of the results are given a
physical explanation by comparing to a perturbative Master equation calculation
(whose limitation we examine). | cond-mat_str-el |
Strong correlation effects in theoretical STM studies of magnetic
adatoms: We present a theoretical study for the scanning tunneling microscopy (STM)
spectra of surface-supported magnetic nanostructures, incorporating strong
correlation effects. As concrete examples, we study Co and Mn adatoms on the
Cu(111) surface, which are expected to represent the opposite limits of the
Kondo physics and local moment behavior, using a combination of density
functional theory and both quantum Monte Carlo and exact diagonalization
impurity solvers. We examine in detail the effects of temperature $T$,
correlation strength $U$, and the impurity $d$ electron occupancy $N_d$ on the
local density of states. We also study the effective coherence energy scale,
i.e., the Kondo temperature $T_K$, which can be extracted from the STM spectra.
Theoretical STM spectra are computed as a function of the STM tip position
relative to each adatom. Because of the multi-orbital nature of the adatoms,
the STM spectra are shown to consist of a complicated superposition of orbital
contributions, with different orbital symmetries, self-energies and Kondo
temperatures. For the Mn adatom, which is close to half-filling, the STM
spectra are featureless near the Fermi level. On the other hand, the
quasiparticle peak for Co adatom gives rise to strongly position-dependent Fano
line-shapes. | cond-mat_str-el |
Strong electron correlations in the normal state of FeSe0.42Te0.58: We investigate the normal state of the '11' iron-based superconductor
FeSe0.42Te0.58 by angle resolved photoemission. Our data reveal a highly
renormalized quasiparticle dispersion characteristic of a strongly correlated
metal. We find sheet dependent effective carrier masses between ~ 3 - 16 m_e
corresponding to a mass enhancement over band structure values of m*/m_band ~ 6
- 20. This is nearly an order of magnitude higher than the renormalization
reported previously for iron-arsenide superconductors of the '1111' and '122'
families but fully consistent with the bulk specific heat. | cond-mat_str-el |
Local Pairing at U-impurities in BCS Superconductors: We analyse here the role electrons on Anderson-U impurities play in
superconductivity in a metal alloy. We find that phonon coupling at impurities
counteracts the traditional effects which dominate T_c suppression in the
non-magnetic limit. In some cases, we findt hat non-magnetic impurities can
enhance T_c. In the Kondo limit, a Fermi liquid analysis reveals that it is the
enhancement in the density of states arising from the Kondo resonancethat
counteracts pair-weakening. | cond-mat_str-el |
Nonmagnetic-magnetic transition and magnetically ordered structure in
SmS: SmS, a prototypical intermediate valence compound, has been studied by
performing high-pressure nuclear magnetic resonance measurements on a
$^{33}$S-enriched sample. The observation of an additional signal below 15-20 K
above a nonmagnetic-magnetic transition pressure $P_{\rm c2} \approx 2$ GPa
gives evidence of a magnetic transition. The absence of a Curie-term in the
Knight shift near $P_{\rm c2}$ indicates that the localized character of $4f$
electrons is entirely screened and the mechanism of the magnetic ordering is
not described within a simple localized model. Simultaneously, the line shape
in the magnetically ordered state is incompatible with a spin density wave
order. These suggest that the magnetic order in SmS may require an
understanding beyond the conventional framework for heavy fermions. The fact
that hyperfine fields from the ordered moments cancel out at the S site leads
us to a conclusion that the ordered phase has a type II antiferromagnetic
structure. | cond-mat_str-el |
Selection of factorizable ground state in a frustrated spin tube: Order
by disorder and hidden ferromagnetism: The interplay between frustration and quantum fluctuation in magnetic systems
is known to be the origin of many exotic states in condensed matter physics. In
this paper, we consider a frustrated four-leg spin tube under a magnetic field.
This system is a prototype to study the emergence of a nonmagnetic ground state
factorizable into local states and the associated order parameter without
quantum fluctuation, that appears in a wide variety of frustrated systems. The
one-dimensional nature of the system allows us to apply various techniques: a
path-integral formulation based on the notion of order by disorder,
strong-coupling analysis where magnetic excitations are gapped, and
density-matrix renormalization group. All methods point toward an interesting
property of the ground state in the magnetization plateaus, namely, a quantized
value of relative magnetizations between different sublattices (spin imbalance)
and an almost perfect factorization of the ground state. | cond-mat_str-el |
Wave Function and Strange Correlator of Short Range Entangled states: We demonstrate the following conclusion: If $|\Psi\rangle$ is a $1d$ or $2d$
nontrivial short range entangled state, and $|\Omega \rangle$ is a trivial
disordered state defined on the same Hilbert space, then the following quantity
(so called strange correlator) $C(r, r^\prime) = \frac{\langle \Omega|\phi(r)
\phi(r^\prime) | \Psi\rangle}{\langle \Omega| \Psi\rangle}$ either saturates to
a constant or decays as a power-law in the limit $|r - r^\prime| \rightarrow
+\infty$, even though both $| \Omega\rangle$ and $| \Psi\rangle$ are quantum
disordered states with short-range correlation. $\phi(r)$ is some local
operator in the Hilbert space. This result is obtained based on both field
theory analysis, and also an explicit computation of $C(r, r^\prime)$ for four
different examples: $1d$ Haldane phase of spin-1 chain, $2d$ quantum spin Hall
insulator with a strong Rashba spin-orbit coupling, $2d$ spin-2 AKLT state on
the square lattice, and the $2d$ bosonic symmetry protected topological phase
with $Z_2$ symmetry. This result can be used as a diagnosis for short range
entangled states in $1d$ and $2d$. A possible diagnosis for $3d$ short range
entangled states is also proposed. | cond-mat_str-el |
Direct determination of the spin structure of Nd$_2$Ir$_2$O$_7$ by means
of neutron diffraction: We report on the spin structure of the pyrochlore iridate Nd$_2$Ir$_2$O$_7$
that could be directly determined by means of powder neutron diffraction. Our
magnetic structure refinement unravels a so-called all-in/all-out magnetic
structure that appears in both, the Nd and the Ir sublattice. The ordered
magnetic moments at 1.8 K amount to 0.34(1) $\mu_\mathrm{B}$/Ir$^{4+}$ and
1.27(1) $\mu_\mathrm{B}$/Nd$^{3+}$. The Nd$^{3+}$ moment size at 1.8 K is
smaller than that expected for the Nd$^{3+}$ ground state doublet. On the other
hand, the size of the ordered moments of the Ir$^{4+}$ ions at 1.8 K agrees
very well with the value expected for a $J_\mathrm{eff}$ = 1/2 state based on
the presence of strong spin-orbit coupling in this system. Finally, our
measurements reveal a parallel alignment of the Nd$^{3+}$ moments with the net
moment of its six nearest neighboring Ir$^{4+}$ ions. | cond-mat_str-el |
Interesting magnetic properties of Fe$_{1-x}$Co$_x$Si alloys: Solid solution between nonmagnetic narrow gap semiconductor FeSi and
diamagnetic semi-metal CoSi gives rise to interesting metallic alloys with
long-range helical magnetic ordering, for a wide range of intermediate
concentration. We report various interesting magnetic properties of these
alloys, including low temperature re-entrant spin-glass like behaviour and a
novel inverted magnetic hysteresis loop. Role of Dzyaloshinski-Moriya
interaction in the magnetic response of these non-centrosymmetric alloys is
discussed. | cond-mat_str-el |
Incoherent transport in a classical spin liquid: We study the energy and spin transport of the classical spin liquid hosted by
the pyrochlore Heisenberg antiferromagnet in the large $S$ limit. Molecular
dynamics calculation suggests that both the energy and spin diffusion constants
approach finite limits as the temperature tends to zero. We explain our results
in terms of an effective disorder model, where the energy/spin-carrying normal
modes propagate in a quasi-static disordered spin background. The finite zero
temperature limits of the diffusion constants are then naturally understood as
a result of the finite mean free path of the normal modes due to the effective
disorder. | cond-mat_str-el |
Experimentally Realized Correlated Electron Materials: From
Superconductors to Topological Insulators: Recent discoveries, as well as open questions, in experimentally realized
correlated electron materials are reviewed. In particular, high temperature
superconductivity in the cuprates and in the recently discovered iron
pnictides, possible chiral p-wave superconductivity in strontium ruthenate, the
search for quantum spin liquid behavior in real materials, and new experimental
discoveries in topological insulators are discussed. | cond-mat_str-el |
Spectral functions in a magnetic field as a probe of spin-charge
separation in a Luttinger liquid: We show that the single-particle spectral functions in a magnetic field can
be used to probe spin-charge separation of a Luttinger liquid. Away from the
Fermi momentum, the magnetic field splits both the spinon peak and holon peak;
here the spin-charge separation nature is reflected in the different magnitude
of the two splittings. At the Fermi momentum, the magnetic field splits the
zero-field peak into {\it four} peaks. The feasibility of experimentally
studying this effect is discussed. | cond-mat_str-el |
The electronic structure of epitaxially stabilized 5d perovskite
Ca_{1-x}Sr_xIrO_3 (x = 0, 0.5, and 1) thin films: the role of strong
spin-orbit coupling: We have investigated the electronic structure of meta-stable perovskite
Ca1-xSrxIrO3 (x = 0, 0.5, and 1) thin films using transport measurements,
optical spectroscopy, and first-principles calculations. We artificially
fabricated the perovskite phase of Ca1-xSrxIrO3, which has a hexagonal or post
perovskite crystal structure in bulk form, by growing epitaxial thin films on
perovskite GdScO3 substrates using epi-stabilization technique. The transport
properties of the perovskite Ca1-xSrxIrO3 films systematically changed from
nearly insulating (or semi-metallic) for x = 0 to bad metallic for x = 1. Due
to the extended wavefunctions, 5d electrons are usually delocalized. However,
the strong spin-orbit coupling in Ca1-xSrxIrO3 results in the formation of
effective total angular momentum Jeff = 1/2 and 3/2 states, which puts
Ca1-xSrxIrO3 in the vicinity of a metal-insulator phase boundary. As a result,
the electrical properties of the Ca1-xSrxIrO3 films are found to be sensitive
to x and strain. | cond-mat_str-el |
A novel continuous time quantum Monte Carlo solver for dynamical mean
field theory in the compact Legendre representation: Dynamical mean-field theory (DMFT) is one of the most widely-used methods to
treat accurately electron correlation effects in ab-initio real material
calculations. Many modern large-scale implementations of DMFT in electronic
structure codes involve solving a quantum impurity model with a Continuous-Time
Quantum Monte Carlo (CT-QMC) solver. The main advantage of CT-QMC is that,
unlike standard quantum Monte Carlo approaches, it is able to generate the
local Green's functions of the correlated system on an arbitrarily fine
imaginary time grid, and is free of any systematic errors. In this work, we
extend a hybrid QMC solver proposed by Khatami et al. and Rost et al. to a
multi-orbital context. This has the advantage of enabling impurity solver QMC
calculations to scale linearly with inverse temperature and permit its
application to d and f band materials. In addition, we present a novel Green's
function processing scheme which generates accurate quasi-continuous imaginary
time solutions of the impurity problem which overcome errors inherent to
standard QMC approaches. This solver and processing scheme are incorporated
into a full DFT+DMFT calculation using the CASTEP DFT code. Benchmark
calculations for strontium vanadate properties are presented. | cond-mat_str-el |
Detecting Quantum Anomalies in Open Systems: Symmetries and quantum anomalies serve as powerful tools for constraining
complicated quantum many-body systems, offering valuable insights into
low-energy characteristics based on their ultraviolet structure. Nevertheless,
their applicability has traditionally been confined to closed quantum systems,
rendering them largely unexplored for open quantum systems described by density
matrices. In this work, we introduce a novel and experimentally feasible
approach to detect quantum anomalies in open systems. Specifically, we claim
that, when coupled with an external environment, the mixed 't Hooft anomaly
between spin rotation symmetry and lattice translation symmetry gives
distinctive characteristics for half-integer and integer spin chains in
measurements of $\exp(\rm{i}\theta S^z_{\rm tot})$ as a function of $\theta$.
Notably, the half-integer spin chain manifests a topological phenomenon akin to
the ``level crossing" observed in closed systems. To substantiate our
assertion, we develop a lattice-level spacetime rotation to analyze the
aforementioned measurements. Based on the matrix product density operator and
transfer matrix formalism, we analytically establish and numerically
demonstrate the unavoidable singular behavior of $\exp(\rm{i}\theta S^z_{\rm
tot})$ for half-integer spin chains. Conceptually, our work demonstrates a way
to discuss notions like ``spectral flow'' and ``flux threading'' in open
systems not necessarily with a Hamiltonian. | cond-mat_str-el |
Generating function for projected entangled-pair states: Diagrammatic summation is a common bottleneck in modern applications of
projected entangled-pair states, especially in computing low-energy excitations
of a two-dimensional quantum many-body system. To solve this problem, here we
extend the generating function approach for tensor network diagrammatic
summation, a scheme previously proposed in the context of matrix product
states. Taking the form of a one-particle excitation, we show that the excited
state can be computed efficiently in the generating function formalism, which
can further be used in evaluating the dynamical structure factor of the system.
Our benchmark results for the spin-$1/2$ transverse-field Ising model and
Heisenberg model on the square lattice provide a desirable accuracy, showing
good agreement with known results. We then study the spin-$1/2$ $J_1$-$J_2$
model on the same lattice and investigate the dynamical properties of the
putative gapless spin liquid phase. We conclude with a discussion on
generalizations to multi-particle excitations. | cond-mat_str-el |
Non-equilibrium conductivity at quantum critical points: Quantum criticality provides an important route to revealing universal
non-equilibrium behaviour. A canonical example of a quantum critical point is
the Bose-Hubbard model, which we study under the application of an electric
field. A Boltzmann transport formalism and $\epsilon$-expansion are used to
obtain the non-equilibrium conductivity and current noise. This approach allows
us to explicitly identify how a universal non-equilibrium steady state is
maintained, by identifying the rate-limiting step in balancing Joule heating
and dissipation to a heat bath. It also reveals that the non-equilibrium
distribution function is very far from a thermal distribution. | cond-mat_str-el |
Charge and orbital order due to cooperative Jahn-Teller effect in
manganite chains: We derive an effective Hamiltonian that takes into account the quantum nature
of phonons and models cooperative Jahn-Teller effect in the adiabatic regime
and at strong electron-phonon coupling in one dimension. Our approach involves
mapping a strong-coupling problem to a weak-coupling one by using a duality
transformation. Subsequently, a sixth-order perturbation theory is employed in
the polaronic frame of reference where the small parameter is inversely
(directly) proportional to the coupling (adiabaticity). We study charge and
orbital order in ferromagnetic manganite chains and address the pronounced
electron-hole asymmetry in the observed phase diagram. In particular, at strong
coupling, we offer an explanation for the observed density dependence of the
wavevector of charge modulation, i.e., wavevector is proportional to
(independent of) electron density on the electron-doped (hole-doped) side of
the phase diagram of manganites. We also provide a picture for the charge and
orbital order at special fillings $ \frac{1}{2}$, $\frac{1}{3}$, $\frac{1}{4}$,
and $ \frac{1}{5}$; while focusing on the ordering controversy at fillings
$\frac{1}{3}$ and $\frac{1}{4}$, we find that Wigner-crystal arrangement is
preferred over bi-stripe order. | cond-mat_str-el |
Thermally activated exchange narrowing of the Gd3+ ESR fine structure in
a single crystal of Ce1-xGdxFe4P12 (x = 0.001) skutterudite: We report electron spin resonance (ESR) measurements in the Gd3+ doped
semiconducting filled skutterudite compound Ce1-xGdxFe4P12 (x = 0.001). As the
temperature T varies from T = 150 K to T = 165 K, the Gd3+ ESR fine and
hyperfine structures coalesce into a broad inhomogeneous single resonance. At T
= 200 K the line narrows and as T increases further, the resonance becomes
homogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest
that the origin of these features may be associated to a subtle interdependence
of thermally activated mechanisms that combine: i) an increase with T of the
density of activated conduction-carriers across the T-dependent semiconducting
pseudogap; ii) the Gd3+ Korringa relaxation process due to an exchange
interaction, J_{fd}S.s, between the Gd3+ localized magnetic moments and the
thermally activated conduction-carriers and; iii) a relatively weak confining
potential of the rare-earth ions inside the oversized (Fe2P3)4 cage, which
allows the rare-earths to become rattler Einstein oscillators above T = 148 K.
We argue that the rattling of the Gd3+ ions, via a motional narrowing
mechanism, also contributes to the coalescence of the ESR fine and hyperfine
structure. | cond-mat_str-el |
Magnetic ordering tendencies in hexagonal boron nitride-bilayer graphene
moiré structures: When hexagonal boron nitride (hBN) and graphene are aligned at zero or small
twist angle, a moir\'e structure is formed due to the small lattice constant
mismatch between the two structures. In this work, we analyze magnetic ordering
tendencies, driven by onsite Coulomb interactions, of encapsulated bilayer
graphene (BG) forming a moir\'e structure with one (hBN-BG) or both hBN layers
(hBN-BG-hBN), using the random phase approximation. The calculations are
performed in a fully atomistic Hubbard model that takes into account all
$\pi$-electrons of the carbon atoms in one moir\'e unit cell. We analyze the
charge neutral case and find that the dominant magnetic ordering instability is
uniformly antiferromagnetic. Furthermore, at low temperatures, the critical
Hubbard interaction $U_c$ required to induce magnetic order is slightly larger
in those systems where the moir\'e structure has caused a band gap opening in
the non-interacting picture, although the difference is less than 6%.
Mean-field calculations are employed to estimate how such an
interaction-induced magnetic order may change the observable single-particle
gap sizes. | cond-mat_str-el |
Orbital Degree of Freedom and Phase Separation in Ferromagnetic
Manganites at Finite Temperatures: The spin and orbital phase diagram for perovskite manganites are investigated
as a function of temperature and hole concentration. The superexchange and
double exchange interactions dominate the ferromagnetic phases in the low and
high concentration regions of doped holes, respectively.The two interactions
favor different orbital states each other. Between the phases, two interactions
compete with each other and the phase separation appears in the wide range of
temperature and hole concentration. The anisotropy of the orbital space causes
discontinuous changes of the orbital state and promotes the phase separation.
The relation between the phase separation and the stripe- and sheet-type charge
segregation is discussed. | cond-mat_str-el |
Investigation of the commensurate magnetic structure in heavy fermion
CePt2In7 using magnetic resonant X-ray diffraction: We investigated the magnetic structure of the heavy fermion compound
CePt$_2$In$_7$ below $T_N~=5.34(2)$ K using magnetic resonant X-ray diffraction
at ambient pressure. The magnetic order is characterized by a commensurate
propagation vector ${k}_{1/2}~=~\left( \frac{1}{2} , \frac{1}{2},
\frac{1}{2}\right)$ with spins lying in the basal plane. Our measurements did
not reveal the presence of an incommensurate order propagating along the high
symmetry directions in reciprocal space but cannot exclude other incommensurate
modulations or weak scattering intensities. The observed commensurate order can
be described equivalently by either a single-${k}$ structure or by a
multi-${k}$ structure. Furthermore we explain how a commensurate-only ordering
may explain the broad distribution of internal fields observed in nuclear
quadrupolar resonance experiments (Sakai et al. 2011, Phys. Rev. B 83 140408)
that was previously attributed to an incommensurate order. We also report
powder X-ray diffraction showing that the crystallographic structure of
CePt$_2$In$_7$ changes monotonically with pressure up to $P~=~7.3$ GPa at room
temperature. The determined bulk modulus $B_0~=~81.1(3)$ GPa is similar to the
ones of the Ce-115 family. Broad diffraction peaks confirm the presence of
pronounced strain in polycrystalline samples of CePt$_2$In$_7$. We discuss how
strain effects can lead to different electronic and magnetic properties between
polycrystalline and single crystal samples. | cond-mat_str-el |
Enhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic
transition in LaFeAs(O_{1-x}F_x) probed by ^{75}As and ^{139}La NMR: $^{75}$As and $^{139}$La NMR results of LaFeAs(O$_{1-x}$F$_x$) ($x$=0, 0.025,
and 0.04) were reported. Upon F-doping, the tetragonal-to-orthorhombic
structural phase transition temperature $T_S$, antiferromagnetic transition
temperature $T_N$ and internal magnetic field $\mu_0H_{\rm int}$ are gradually
reduced for $x<0.04$. However, at $x=0.04$, $T_N$ is abruptly suppressed to be
30 K along with a tiny $\mu_0H_{\rm int}$, which is distinct from the
continuous disappearance of the ordered phases in the Ba122 systems of
Ba(Fe,Co)$_2$As$_2$ and BaFe$_2$(As,P)$_2$. The anisotropy of the spin-lattice
relaxation rate $T_1^{-1}$, $(T_1)^{-1}_{H\parallel ab}/(T_1)^{-1}_{H\parallel
c}$, in the paramagnetic phase of $x = 0$ and 0.025 is constant ($\sim 1.5$),
but increases abruptly below $T_S$ due to the enhancement of
$(T_1)^{-1}_{H\parallel ab}$ by the slowing down of magnetic fluctuations. This
indicates that the tetragonal-to-orthorhombic structural distortion enhances
the anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit
interaction. | cond-mat_str-el |
Gutzwiller variational theory for the Hubbard model with attractive
interaction: We investigate the electronic and superconducting properties of a negative-U
Hubbard model. For this purpose we evaluate a recently introduced variational
theory based on Gutzwiller-correlated BCS wave functions. We find significant
differences between our approach and standard BCS theory, especially for the
superconducting gap. For small values of $|U|$, we derive analytical
expressions for the order parameter and the superconducting gap which we
compare to exact results from perturbation theory. | cond-mat_str-el |
Persistent half-metallic ferromagnetism in a (111)-oriented manganite
superlattice: Heterostructures of mixed-valence manganites are still under intense
scrutiny, due to the occurrence of exotic quantum phenomena linked to
electronic correlation and interfacial composition. For instance, if two
anti-ferromagnetic insulators as LaMnO$_3$ and SrMnO$_3$ are grown in a
(001)-oriented superlattice, a half-metallic ferromagnet may form, provided
that the thickness is sufficiently small to allow tunneling across interfaces.
In this article, we employ electronic structure calculations to show that all
the layers of a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice retain a
half-metallic ferromagnetic character for a much larger thickness than in its
(001) counterpart. This behavior is shown to be linked to the charge transfer
across the interface, favored by the octahedral connectivity between the
layers. This also results in a symmetry-induced quenching of the Jahn-Teller
distortions, which are replaced by breathing modes. The latter are coupled to
charge and spin oscillations, whose components have a pure e g character. Most
interestingly, the magnetization reaches its maximum value inside the LaMnO$_3$
region and not at the interface, which is fundamentally different from what
observed for the (001) orientation. The analysis of the inter-atomic exchange
coupling shows that the magnetic order arises from the double-exchange
mechanism, despite competing interactions inside the SrMnO$_3$ region. Finally,
the van Vleck distortions and the spin oscillations are found to be crucially
affected by the variation of Hund's exchange and charge doping, which allows us
to speculate that our system behaves as a Hund's metal, creating an interesting
connection between manganites and nickelates. | cond-mat_str-el |
Band structure approach to the resonant x-ray scattering: We study the resonance behaviour of the forbidden 600 and 222 x-ray Bragg
peaks in Ge using LDA band structure methods. These Bragg peaks remain
forbidden in the resonant dipole scattering approximation even taking into
account the non local nature of the band states. However they become allowed at
resonance if the eigenstates of the unoccupied conduction band involve a
hybridization of p like and d like atomic states. We show that the energy
dependence of the resonant behaviour, including the phase of the scattering, is
a direct measure of this p-d hybridization.and obtain quantitative agreement
with experiment. A simple physical picture involving a product of dipole and
quadrupolar transition matrix elements explains this behaviour and shows that
it should be generally true for cases where the resonating atom is not at an
inversion center. This has strong implications for the description of the
resonance behavior of x-ray scattering in materials where the resonant atom is
not at an inversion center such as V2O3 and in ferro and antiferro electric and
piezo electric materials in general. | cond-mat_str-el |
Theory of magnetostriction for multipolar quantum spin ice in pyrochlore
materials: Multipolar magnetism is an emerging field of quantum materials research. The
building blocks of multipolar phenomena are magnetic ions with a non-Kramers
doublet, where the orbital and spin degrees of freedom are inextricably
intertwined, leading to unusual spin-orbital entangled states. The detection of
such subtle forms of matter has, however, been difficult due to a limited
number of appropriate experimental tools. In this work, motivated by a recent
magnetostriction experiment on Pr$_2$Zr$_2$O$_7$, we theoretically investigate
how multipolar quantum spin ice, an elusive three dimensional quantum spin
liquid, and other multipolar ordered phases in the pyrochlore materials can be
detected using magnetostriction. We provide theoretical results based on
classical and/or quantum studies of non-Kramers and Kramers magnetic ions, and
contrast the behaviors of distinct phases in both systems. Our work paves an
important avenue for future identification of exotic ground states in
multipolar systems. | cond-mat_str-el |
Density Matrix Spectra and Order Parameters in the 1D Extended Hubbard
Model: Without any knowledge of the symmetry existing in the system, we derive the
exact forms of the order parameters which show long-range correlation in the
ground state of the one-dimensional extended Hubbard model using a quantum
information approach. Our work demonstrates that the quantum information
approach can help us to find the explicit form of the order parameter, which
cannot be derived systematically via traditional methods in the condensed
matter theory. | cond-mat_str-el |
Magnetic-field induced band-structure change in CeBiPt: We report on a field-induced change of the electronic band structure of
CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic
fields. Above ~25 T, the charge-carrier concentration increases nearly 30% with
a concomitant disappearance of the Shubnikov-de Haas signal. These features are
intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt
the Fermi surface remains unaffected. Electronic band-structure calculations
point to a 4f-polarization-induced change of the Fermi-surface topology. | cond-mat_str-el |
Double-Pulse Deexcitations in a One-Dimensional Strongly Correlated
System: We investigate the ultrafast optical response of the one-dimensional
half-filled extended Hubbard model exposed to two successive laser pulses. By
using the time-dependent Lanczos method, we find that following the first
pulse, the excitation and deexcitation process between the ground state and
excitonic states can be precisely controlled by the relative temporal
displacement of the pulses. The underlying physics can be understood in terms
of a modified Rabi model. Our simulations clearly demonstrate the
controllability of ultrafast transition between excited and deexcited phases in
strongly correlated electron systems. | cond-mat_str-el |
Multiple Diffusion-Freezing Mechanisms in Molecular Hydrogen Films: Molecular hydrogen is a fascinating candidate for quantum fluid showing
bosonic and fermionic superfluidity. We have studied diffusion dynamics of thin
films of H$_2$, HD and D$_2$ adsorbed on a glass substrate by measurements of
elasticity. The elasticity shows multiple anomalies well below bulk triple
point. They are attributed to three different diffusion mechanisms of
admolecules and their "freezing" into localized state: classical thermal
diffusion of vacancies, quantum tunneling of vacancies, and diffusion of
molecules in the uppermost surface. The surface diffusion is active down to 1
K, below which the molecules become localized. This suggests that the surface
layer of hydrogen films is on the verge of quantum phase transition to
superfluid state. | cond-mat_str-el |
Phase diagram of a one-dimensional Ising model with an anomalous Z_2
symmetry: Anomalous global symmetries, which can be realized on the boundary of
symmetry-protected topological phases, brings new phases and phase transitions
to condensed matter physics. In this work, we study a one dimensional model
with an anomalous Z2 symmetry, using the density-matrix renormalization group
method. Besides a symmetry-breaking ferromagnetic phase, we find a gapless
phase described by the SU(2)_1 conformal field theory, despite the existence of
only discrete Z_2 symmetry in the Hamiltonian. The phase transition between the
ferromagnetic phase and the gapless phase is continuous and has the same
critical scaling as in the gapless phase. Our numerical finding is compatible
of theoretical constraints on possible phases resulting from the symmetry
anomaly. | cond-mat_str-el |
Compensation of Coulomb blocking and energy transfer in the current
voltage characteristic of molecular conduction junctions: We have studied the influence of both exciton effects and Coulomb repulsion
on current in molecular nanojunctions. We show that dipolar energy-transfer
interactions between the sites in the wire can at high voltage compensate
Coulomb blocking for particular relationships between their values. Tuning this
relationship may be achieved by using the effect of plasmonic nanostructure on
dipolar energy transfer interactions. | cond-mat_str-el |
Relationship between the ground-state wave function of a magnet and its
static structure factor: We state and prove two theorems about the ground state of magnetic systems
described by very general Heisenberg-type models and discuss their implications
for magnetic neutron scattering. The first theorem states that two models
cannot have the same correlator without sharing the corresponding ground
states. According to the second theorem, an $N$-qubit wave function cannot
reproduce the correlators of a given system unless it represents a true ground
state of that system. We discuss the implications for neutron scattering
inverse problems. We argue that the first theorem provides a framework to
understand neutron-based Hamiltonian learning. Furthermore, we propose a
variational approach to quantum magnets based on the second theorem where a
representation of the wave function (held, for instance, in a neural network or
in the qubit register of a quantum processor) is optimised to fit experimental
neutron-scattering data directly, without the involvement of a model
Hamiltonian. | cond-mat_str-el |
Tuning the bond order wave (BOW) phase of half-filled extended Hubbard
models: Theoretical and computational studies of the quantum phase diagram of the
one-dimensional half-filled extended Hubbard model (EHM) indicate a narrow bond
order wave (BOW) phase with finite magnetic gap $E_m$ for on-site repulsion $U
< U^*$, the critical point, and nearest neighbor interaction $V_c \approx U/2$
near the boundary of the charge density wave (CDW) phase. Potentials with more
extended interactions that retain the EHM symmetry are shown to have a less
cooperative CDW transition with higher $U^*$ and wider BOW phase. Density
matrix renormalization group (DMRG) is used to obtain $E_m$ directly as the
singlet-triplet gap, with finite $E_m$ marking the BOW boundary $V_s(U)$. The
BOW/CDW boundary $V_c(U)$ is obtained from exact finite-size calculations that
are consistent with previous EHM determinations. The kinetic energy or bond
order provides a convenient new estimate of $U^*$ based on a metallic point at
$V_c(U)$ for $U < U^*$. Tuning the BOW phase of half-filled Hubbard models with
different intersite potentials indicates a ground state with large charge
fluctuations and magnetic frustration. The possibility of physical realizations
of a BOW phase is raised for Coulomb interactions. | cond-mat_str-el |
Entanglement switching via the Kondo effect in triple quantum dots: We consider a triple quantum dot system in a triangular geometry with one of
the dots connected to metallic leads. Using Wilson's numerical renormalization
group method, we investigate quantum entanglement and its relation to the
thermodynamic and transport properties, in the regime where each of the dots is
singly occupied on average, but with non-negligible charge fluctuations. It is
shown that even in the regime of significant charge fluctuations the formation
of the Kondo singlets induces switching between separable and perfectly
entangled states. The quantum phase transition between unentangled and
entangled states is analyzed quantitatively and the corresponding phase diagram
is explained by exactly solvable spin model. | cond-mat_str-el |
Observation of re-entrant correlated insulators and interaction driven
Fermi surface reconstructions at one magnetic flux quantum per moiré unit
cell in magic-angle twisted bilayer graphene: The discovery of flat bands with non-trivial band topology in magic angle
twisted bi-layer graphene (MATBG) has provided a unique platform to study
strongly correlated phe-nomena including superconductivity, correlated
insulators, Chern insulators and magnetism. A fundamental feature of the MATBG,
so far unexplored, is its high magnetic field Hof-stadter spectrum. Here we
report on a detailed magneto-transport study of a MATBG de-vice in external
magnetic fields of up to B = 31 T, corresponding to one magnetic flux quan-tum
per moir\'e unit cell {\Phi}0. At {\Phi}0, we observe a re-entrant correlated
insulator at a flat band filling factor of {\nu} = +2, and interaction-driven
Fermi surface reconstructions at other fillings, which are identified by new
sets of Landau levels originating from these. These ex-perimental observations
are supplemented by theoretical work that predicts a new set of 8 well-isolated
flat bands at {\Phi}0 , of comparable band width but with different topology
than in zero field. Overall, our magneto-transport data reveals a qualitatively
new Hofstadter spec-trum in MATBG, which arises due to the strong electronic
correlations in the re-entrant flat bands. | cond-mat_str-el |
Unified Fock space representation of fractional quantum Hall states: Many bosonic (fermionic) fractional quantum Hall states, such as Laughlin,
Moore-Read and Read-Rezayi wavefunctions, belong to a special class of
orthogonal polynomials: the Jack polynomials (times a Vandermonde determinant).
This fundamental observation allows to point out two different recurrence
relations for the coefficients of the permanent (Slater) decomposition of the
bosonic (fermionic) states. Here we provide an explicit Fock space
representation for these wavefunctions by introducing a two-body squeezing
operator which represents them as a Jastrow operator applied to reference
states, which are in general simple periodic one dimensional patterns.
Remarkably, this operator representation is the same for bosons and fermions,
and the different nature of the two recurrence relations is an outcome of
particle statistics. | cond-mat_str-el |
Tensor renormalization group approach to classical dimer models: We analyze classical dimer models on the square and triangular lattice using
a tensor network representation of the dimers. The correlation functions are
numerically calculated using the recently developed "Tensor renormalization
group" (TRG) technique. The partition function for the dimer problem can be
calculated exactly by the Pfaffian method which is used here as a platform for
comparing the numerical results. TRG turns out to be a powerful tool for
describing gapped systems with exponentially decaying correlations very
efficiently due to its fast convergence. This is the case for the dimer model
on the triangular lattice. However, the convergence becomes very slow and
unstable in case of the square lattice where the model has algebraically
decaying correlations. We highlight these aspects with numerical simulations
and critically appraise the robustness of TRG approach by contrasting the
results for small and large system sizes against the exact calculations.
Furthermore, we benchmark our TRG results with classical Monte Carlo (MC)
method. | cond-mat_str-el |
The Effect of Intrinsic Quantum Fluctuations on the Phase Diagram of
Anisotropic Dipolar Magnets: The rare-earth material $\mathrm{LiHoF_4}$ is believed to be an experimental
realization of the celebrated (dipolar) Ising model, and upon the inclusion of
a transverse field $B_x$, an archetypal quantum Ising model. Moreover, by
substituting the magnetic Ho ions by non-magnetic Y ions, disorder can be
introduced into the system giving rise to a dipolar disordered magnet and at
high disorders to a spin-glass. Indeed, this material has been scrutinized
experimentally, numerically and theoretically over many decades with the aim of
understanding various collective magnetic phenomena. One of the to-date open
questions is the discrepancy between the experimental and theoretical $B_x -T$
phase diagram at low-fields and high temperatures. Here we propose a mechanism,
backed by numerical results, that highlights the importance of quantum
fluctuations induced by the off-diagonal dipolar terms, in determining the
critical temperature of anisotropic dipolar magnets in the presence and in the
absence of a transverse field. We thus show that the description as a simple
Ising system is insufficient to quantitatively describe the full phase diagram
of $\mathrm{LiHoF_4}$, for the pure as well as for the dilute system. | cond-mat_str-el |
Cumulant-based calculations of the correlation energy in a molecule: The problem of constructing a guaranteed convergent sequence of corrections
to the Hartree--Fock ground state energy of a molecule without storing the
many-electron wave function is considered. Several methods based on cumulants
are considered and it is shown that such a sequence is obtained by Lanczos
tridiagonalization, in which the elements of the tridiagonal matrix are
calculated through cumulants. | cond-mat_str-el |
Numerical Study of the Pairing Correlation of the t-J Type Models: We reported that the pair-pair correlation function of the two-dimensional
t-J model does not have long-range d-wave superconducting correlations in the
interesting parameter range of $J/t \leq 0.5$. The power-Lanczos method is used
under the assumption of monotonic behavior. This assumption has been well
checked in the two-dimensional t-J and attractive Hubbard model. Here we
re-examine this criterion of monotonic behavior of the pairing correlation
function for the one-dimensional and two-leg t-J ladder where other accurate
numerical results are available. The method seems to be working well. | cond-mat_str-el |
Road to zero-field antiferromagnetic skyrmions in a frustrated AFM/FM
heterostructure: We demonstrate a mechanism of significant reduction, including complete
elimination, of the external magnetic field required for the stabilization of a
skyrmion lattice (SkX) phase in a frustrated triangular Heisenberg
antiferromagnet (AFM) with the Dzyaloshinskii-Moriya interaction. It is
achieved by coupling of such a AFM plane to a reference ferromagnetic (FM)
layer, which generates an effective field cooperating with the external
magnetic field. If the FM layer shows some axial single-ion anisotropy then the
effective field can also be generated in zero external field due to a
spontaneous FM long-range ordering. Then a sufficiently large interlayer
coupling can fully substitute the external magnetic field and the SkX phase in
the AFM layer can be stabilized even in zero external field. | cond-mat_str-el |
Exact treatment of exciton-polaron formation by Diagrammatic Monte Carlo: We develop an approximation-free Diagrammatic Monte Carlo technique to study
fermionic particles interacting with each other simultaneously through both an
attractive Coulomb potential and bosonic excitations of the underlying medium.
Exemplarily we apply the method to the long-standing exciton-polaron problem
and present numerically exact results for the wave function, ground-state
energy, binding energy and effective mass of this quasiparticle. Focusing on
the electron-hole pair bound-state formation, we discuss various limiting cases
of a generic exciton-polaron model. The frequently used instantaneous
approximation to the retarded interaction due to the phonon exchange is found
to be of very limited applicability. For the case of a light electron and heavy
hole the system is well approximated by a particle in the field of a static
attractive impurity. | cond-mat_str-el |
Magnon crystals and magnetic phases in a Kagomé-stripe antiferromagnet: In this work we analyze the magnetization properties of an antiferromagnetic
Kagom\'e stripe lattice, motivated by the recent synthesis of materials
exhibiting this structure. By employing a variety of techniques that include
numerical methods as Density Matrix Renormalization Group and Monte Carlo
simulations, as well as analytical techniques, as perturbative low energy
effective models and exact solutions, we characterize the magnetization process
and magnetic phase diagram of a Kagom\'e stripe lattice. The model captures a
variety of behaviors present in the two dimensional Kagom\'e lattice, which are
described here by analytical models and numerically corroborated. In addition
to the characterization of semiclassical intermediate plateaus, it is worth
noting the determination of an exact magnon crystal phase which breaks the
underlying symmetry of the lattice. This magnon crystal phase generalizes
previous findings and according to our knowledge is reported here for the first
time. | cond-mat_str-el |
Helical magnetic ordering in Sr(Co1-xNix)2As2: SrCo2As2 is a peculiar itinerant magnetic system that does not order
magnetically, but inelastic neutron scattering experiments observe the same
stripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based
superconductors along with evidence of magnetic frustration. Here we present
results from neutron diffraction measurements on single crystals of
Sr(Co1-xNix)2As2 that show the development of long-range AF order with
Ni-doping. However, the AF order is not stripe-type. Rather, the magnetic
structure consists of ferromagnetically-aligned (FM) layers (with moments
laying in the layer) that are AF arranged along c with an incommensurate
propagation vector of (0 0 tau), i.e. a helix. Using high-energy x-ray
diffraction, we find no evidence for a temperature-induced structural phase
transition that would indicate a collinear AF order. This finding supports a
picture of competing FM and AF interactions within the square transition-metal
layers due to flat-band magnetic instabilities. However, the composition
dependence of the propagation vector suggests that far more subtle Fermi
surface and orbital effects control the interlayer magnetic correlations. | cond-mat_str-el |
t2g-orbital model on a honeycomb lattice: application to antiferromagnet
SrRu2O6: Motivated by the recent discovery of high temperature antiferromagnet
SrRu$_2$O$_6$ and its potential to be the parent of a new superconductor, we
construct a minimal $t_{2g}$-orbital model on a honeycomb lattice to simulate
its low energy band structure. Local Coulomb interaction is taken into account
through both random phase approximation and mean field theory. Experimentally
observed Antiferromagnetic order is obtained in both approximations. In
addition, our theory predicts that the magnetic moments on three
$t_{2g}$-orbitals are non-collinear as a result of the strong spin-orbit
coupling of Ru atoms. | cond-mat_str-el |
Concentration dependence in kinetic arrest of first order magnetic
transition in Ta doped $HfFe_2$: Magnetic behavior of the pseudo-binary alloy $Hf_{1-x}Ta_xFe_2$ has been
studied, for which the zero field ferromagnetic (FM) to antiferromagnetic (AFM)
transition temperature is tuned near to T=0 K. These alloys show anomalous
thermomagnetic irreversibility at low temperature due to kinetic arrest of the
first order AFM-FM transition. All the three studied compositions show
re-entrant transition in zero field cooled warming curve and anomalous
non-monotonic variation of upper critical field in isothermal magnetization.
The region in H-T space, where these features of kinetic arrest manifest
themselves, increases with increasing Ta concentration. | cond-mat_str-el |
Quantum criticality and confinement in weak Mott insulators: Electrons undergoing a Mott transition may shed their charge but persist as
neutral excitations of a quantum spin liquid (QSL). We introduce concrete
two-dimensional models exhibiting this exotic behavior as they transition from
superconducting or topological phases into fully charge-localized insulators.
We study these Mott transitions and the confinement of neutral fermions at a
second transition into a symmetry-broken phase. In the process, we also derive
coupled-wire parent Hamiltonians for a non-Abelian QSL and a $\mathbb{Z}_4$
QSL. | cond-mat_str-el |
Dynamical Properties of small Polarons: On the basis of the two-site polaron problem, which we solve by exact
diagonalization, we analyse the spectral properties of polaronic systems in
view of discerning localized from itinerant polarons and bound polaron pairs
from an ensemble of single polarons. The corresponding experimental techniques
for that concern photoemission and inverse photoemission spectroscopy. The
evolution of the density of states as a function of concentration of charge
carriers and strength of the electron-phonon interaction clearly shows the
opening up of a gap between single polaronic and bi-polaronic states, in
analogy to the Hubbard problem for strongly correlated electron systems. The
crossover regime between adiabatic and anti-adiabatic small polarons is
triggered by two characteristic time scales: the renormalized electron hopping
rate and the renormalized vibrational frequency becoming equal. This crossover
regime is then characterized by temporarily alternating self- localization and
delocalization of the charge carriers which is accompanied by phase slips in
the charge and molecular deformation oscillations and ultimately leads to a
dephasing between these two dynamical components of the polaron problem. We
visualize these features by a study of the temporal evolution of the charge
redistribution and the change in molecular deformations. The spectral and
dynamical properties of polarons discussed here are beyond the applicability of
the standard Lang Firsov approach to the polaron problem. | cond-mat_str-el |
Elementary Building Blocks for Cluster Mott Insulators: Mott insulators, in which strong Coulomb interactions fully localize
electrons on single atomic sites, play host to an incredibly rich and exciting
array of strongly correlated physics. One can naturally extend this concept to
cluster Mott insulators, wherein electrons localize not on single atoms but
across clusters of atoms, forming ``molecules in solids''. The resulting
localized degrees of freedom incorporate the full spectrum of electronic
degrees of freedom, spin, orbital, and charge. These serve as the building
blocks for cluster Mott insulators, and understanding them is an important
first step toward understanding the many-body physics that emerges in candidate
cluster Mott insulators. Here, we focus on elementary building blocks,
neglecting some of the complexity present in real materials which can often
obfuscate the underlying principles at play. Through an extensive set of exact
theoretical calculations on clusters of varying geometry, number of orbitals,
and number of electrons, we uncover some of the basic organizing principles of
cluster Mott phases, particularly when interactions dominate and negate a
simple single-particle picture. These include, for example, the identification
of an additional ``cluster Hund's rule'', of cluster ground states that are
best understood from a purely interacting perspective, and of several localized
degrees of freedom which are protected by an unusual combination of discrete
spatial or orbital symmetries. Finally, we discuss the impact of adding
additional terms, relevant to material candidates, on the phase diagrams
presented throughout, as well as the potential next steps in the journey to
building a more complete picture of cluster Mott insulators. | cond-mat_str-el |
Phase diagram of the one-dimensional extended attractive Hubbard model
for large nearest-neighbor repulsion: We consider the extended Hubbard model with attractive on-site interaction U
and nearest-neighbor repulsions V. We construct an effective Hamiltonian
H_{eff} for hopping t<<V and arbitrary U<0. Retaining the most important terms,
H_{eff} can be mapped onto two XXZ models, solved by the Bethe ansatz. The
quantum phase diagram shows two Luttinger liquid phases and a region of phase
separation between them. For density n<0.422 and U<-4, singlet superconducting
correlations dominate at large distances. For some parameters, the results are
in qualitative agreement with experiments in BaKBiO. | cond-mat_str-el |
Hidden antiferro-nematic order in Fe-based superconductor BaFe$_2$As$_2$
and NaFeAs above $T_S$: In several Fe-based superconductors, slight $C_4$ symmetry breaking occurs at
$T^*$, which is tens of Kelvin higher than the structural transition
temperature $T_S$. In this "hidden" nematic state at $T_S<T<T^*$, the
orthorhombicity is tiny [$\phi=(a-b)/(a+b) \ll 0.1$%], but clear evidences of
bulk phase transition have been accumulated. To explain this long-standing
mystery, we propose the emergence of antiferro-bond (AFB) order with the
antiferro wavevector ${\bf q}=(0,\pi)$ at $T=T^*$, by which the characteristic
phenomena below $T^*$ are satisfactorily explained. This AFB order originates
from the inter-orbital nesting between the $d_{xy}$-orbital hole-pocket and the
electron-pocket, and this inter-orbital bond order naturally explains the
pseudogap, band-folding, and tiny nematicity that is linear in $T^*-T$. The
hidden AFB order explains key experiments in both BaFe$_2$As$_2$ and NaFeAs,
but it is not expected to occur in FeSe because of the absence of the
$d_{xy}$-orbital hole-pocket. | cond-mat_str-el |
Spin-valley skyrmions in graphene at filling factor $ν=-1$: We model quantum Hall skyrmions in graphene monolayer at quarter filling by a
theory of CP3 fields and study the energy minimizing skyrmions in presence of
valley pseudospin anisotropy and Zeeman coupling. We present a diagram of all
types of skyrmions in a wide range of the anisotropy parameters. For each type
of skyrmion, we visualize it on three Bloch spheres, and present the profiles
of its texture on the graphene honeycomb lattice, thus providing references for
the STM/STS imaging of spin-pseudospin textures in graphene monolayer in
quantum Hall regime. Besides the spin and pseudospin skyrmions for the
corresponding degrees of freedom of an electron in the N=0 Landau level, we
discuss two unusual types -- the "entanglement skyrmion" whose texture lies in
the space of the entanglement of spin and pseudospin, as well as the "deflated
pseudospin skyrmion" with partial entanglement. For all skyrmion types, we
study the dependence of the energy and the size of a skyrmion on the anisotropy
parameters and perpendicular magnetic field. We also propose three ways to
modify the anisotropy energy, namely the sample tilting, the substrate
anisotropy and the valley pseudospin analogue of Zeeman coupling. | cond-mat_str-el |
Probing the stability of the spin liquid phases in the Kitaev-Heisenberg
model using tensor network algorithms: We study the extent of the spin liquid phases in the Kitaev-Heisenberg model
using infinite Projected Entangled-Pair States tensor network ansatz wave
functions directly in the thermodynamic limit. To assess the accuracy of the
ansatz wave functions we perform benchmarks against exact results for the
Kitaev model and find very good agreement for various observables. In the case
of the Kitaev-Heisenberg model we confirm the existence of 6 different phases:
N\'eel, stripy, ferromagnetic, zigzag and two spin liquid phases. We find
finite extents for both spin liquid phases and discontinuous phase transitions
connecting them to symmetry-broken phases. | cond-mat_str-el |
Existence of Fine Structure inside Spin Gap in CeRu2Al10: We investigate the magnetic field effect on the spin gap state in CeRu2Al10
by measuring the magnetization and electrical resistivity. We found that the
magnetization curve for the magnetic field H//c shows a metamagnetic-like
anomaly at H*~4 T below T_0=27 K, but no anomaly for H//a and H//b. A shoulder
of the electrical resistivity at Ts~5 K for I//c is suppressed by applying a
longitudinal magnetic field above 5 T. Many anomalies are also found in the
magnetoresistance for Hkc below ~5 K. The obtained magnetic phase diagram
consists of at least two or three phases below T_0. These results strongly
indicate the existence of a fine structure at a low energy side in a spin gap
state with the excitation energy of 8 meV recently observed in the inelastic
neutron scattering experiments. | cond-mat_str-el |
Magnetic frustrations in the face centered cubic antiferromagnet NiS2: Neutron scattering experiment on NiS2 single crystal revealed a honeycomb
pattern of the intensity distribution in reciprocal lattice space
(continuous-line structure along the fcc zone boundary) providing the first
direct evidence for nearly frustrated antiferromagnetism (AF) on the face
centered cubic (fcc) lattice. A small but finite lattice distortion below 30.9
K lifts the degeneracy of the magnetic ground state due to the frustration and
eventually result in the coexistence of the type I and the type II AF long
range orderings, which are mutually incompatible in the fcc symmetry at higher
temperatures. | cond-mat_str-el |
High-pressure crystal growth and investigation of the metal-to-metal
transition of Ruddlesden-Popper trilayer nickelates La$_4$Ni$_3$O$_{10}$: Single crystals of Ruddlesden-Popper nickelates La$_4$Ni$_3$O$_{10}$ were
grown by means of the floating-zone technique at oxygen pressure of 20~bar. Our
results reveal the effects of the annealing process under pressure on the
crystal structure. We present the requirements for crystal growth and show how
a reported ferromagnetic impurity phase can be avoided. The different growth
and post-annealing processes result in two distinct phases $P2_1/a$ and {\it
Bmab} in which the metal-to-metal transitions occur at 152~K and 136~K,
respectively. | cond-mat_str-el |
Quantum critical phenomena in a spin-1/2 frustrated square lattice with
spatial anisotropy: We present a model compound with a spin-1/2 spatially anisotropic frustrated
square lattice, in which three antiferromagnetic interactions and one
ferromagnetic interaction are competing. We observe an unconventional gradual
increase in the low-temperature magnetization curve reminiscent of the quantum
critical behavior between gapped and gapless phases. In addition, the specific
heat and electron spin resonance signals indicate one-dimensional
characteristics. These results demonstrate quantum critical behavior associated
with one dimensionalization caused by frustrated interactions in the spin-1/2
spatially anisotropic square lattice. | cond-mat_str-el |
Topological Majorana Two-Channel Kondo Effect: A one-dimensional time-reversal-invariant topological superconductor hosts a
Majorana Kramers pair at each end, where time-reversal symmetry acts as a
supersymmetry that flips local fermion parity. We examine the transport anomaly
of such a superconductor, floating and tunnel-coupled to normal leads at its
two ends. We demonstrate the realization of a topologically-protected,
channel-symmetric, two-channel Kondo effect without fine-tuning. Whereas the
nonlocal teleportation vanishes, a lead present at one end telecontrols the
universal transport through the other end. | cond-mat_str-el |
Constructing Landau framework for topological order: Quantum chains and
ladders: We studied quantum phase transitions in the antiferromagnetic dimerized
spin-1/2 XY chain andvtwo-leg ladders. From analysis of several spin models we
present our main result: the framework to deal with topological orders and
hidden symmetries within the Landau paradigm. After mapping of the spin
Hamiltonians onto the tight-binding models with Dirac or Majorana fermions and,
when necessary, the mean-field approximation, the analysis can be done
analytically. By utilizing duality transformations the calculation of nonlocal
string order parameters is mapped onto the local order problem in some dual
representation and done without further approximations. Calculated phase
diagrams, phase boundaries, order parameters and their symmetries for each of
the phases provide a comprehensive quantitative Landau description of the
quantum critical properties of the models considered. Complementarily, the
phases with hidden orders can also be distinguished by the Pontryagin (winding)
numbers which we have calculated as well. This unified framework can be
straightforwardly applied for various spin chains and ladders, topological
insulators and superconductors. Applications to other systems are under way. | cond-mat_str-el |
Investigating the electronic structure of MSi (M = Cr, Mn, Fe $\&$ Co)
and calculating $\textit{$U_{eff}$}$ $\&$ $\textit{J}$ by using cDFT: We have investigated electronic energy band structures and partial density of
states of intermetallic compounds $\textit{viz.}$ CrSi, MnSi, FeSi and CoSi, by
using density functional theory (DFT). CrSi \& MnSi, FeSi and CoSi have
metallic, indirect band gap semiconducting (band gap $ \sim$ 90 meV) and
semi-metallic ground state, respectively. On studying the band structures while
going across the series Cr-Co, the occupied bands around the Fermi level are
getting narrower while the unoccupied bands are getting wider. Similarly, band
edge in partial density of states is shifting away from the Fermi level due to
increased hybridizations. The effective mass of holes for FeSi is found to be
much larger than that of electrons, giving rise to positive Seebeck coefficient
and negative Hall coefficient, which is consistent with experimental results.
For different ionic states of 3\textit{d}-metal, the values of
$\textit{$U_{eff}$}$ and $\textit{J}$ are evaluated by using constrained DFT
method. $\textit{$U_{eff}$}$ ($\textit{J}$) for $2^{+} $ ionic state across the
series are $\sim $ 3.3 eV ($\sim $0.65 eV), $\sim $ 3.7 eV ($\sim $0.72 eV),
$\sim $ 4.4 eV ($\sim $ 0.82 eV) and $\sim $ 4.5 eV ($\sim $ 0.87 eV).
$\lambda$ and $\textit{J}$ are also calculated by considering Yukawa form of
Coulomb interaction. $\lambda $ values for $2^{+} $ ionic state along the
series are $\sim $ 1.97 a.$u^{-1}$, $\sim $ 2.07 a.$u^{-1}$, $\sim $ 2.07
a.$u^{-1}$ and $\sim $ 2.34 a.$u^{-1}$. 4$\textit{s}$ electrons are found to be
contributing more in screening the 3$\textit{d}$ electrons as compared to
4$\textit{p}$ electrons of 3$\textit{d}$ metals. | cond-mat_str-el |
Quadrupolar Superexchange Interactions, Multipolar Order and Magnetic
Phase Transition in UO$_2$: The origin of non-collinear magnetic order in UO$_{2}$ is studied by an ab
initio dynamical-mean-field-theory framework in conjunction with a
linear-response approach for evaluating inter-site superexchange interactions
between U 5$f^{2}$ shells. The calculated quadrupole-quadruple superexchange
interactions are found to unambiguously resolve the frustration of
face-centered-cubic U sublattice toward stabilization of the experimentally
observed non-collinear 3k-magnetic order. Therefore, the exotic 3k
antiferromagnetic order in UO$_{2}$ can be accounted for by a purely electronic
exchange mechanism acting in the undistorted cubic lattice structure. The
quadrupolar short-range order above magnetic ordering temperature $T_N$ is
found to qualitatively differ from the long-range order below $T_N$. | cond-mat_str-el |
Electronic structure of CrN: A comparison between different exchange
correlation potentials: We report a series of electronic structure calculations for CrN using
different exchange correlation potentials: PBE, LDA+$U$, the Tran-Blaha
modified Becke-Johnson, and hybrid functionals. In every case, our calculations
show that the onset of magnetism in CrN should be accompanied by a gap opening.
The experimentally found antiferromagnetic order always leads to an insulating
behavior. Our results give further evidence that the Tran-Blaha functional is
very useful for treating the electronic structure of correlated semiconductors
allowing a parameter free description of the system. Hybrid functionals are
also well capable of describing the electronic structure of CrN. The analysis
of the system is complemented with our calculations of the thermopower that are
in agreement with the experimental data. | cond-mat_str-el |
Bridging the Gap between Deep Learning and Frustrated Quantum Spin
System for Extreme-scale Simulations on New Generation of Sunway
Supercomputer: Efficient numerical methods are promising tools for delivering unique
insights into the fascinating properties of physics, such as the highly
frustrated quantum many-body systems. However, the computational complexity of
obtaining the wave functions for accurately describing the quantum states
increases exponentially with respect to particle number. Here we present a
novel convolutional neural network (CNN) for simulating the two-dimensional
highly frustrated spin-$1/2$ $J_1-J_2$ Heisenberg model, meanwhile the
simulation is performed at an extreme scale system with low cost and high
scalability. By ingenious employment of transfer learning and CNN's
translational invariance, we successfully investigate the quantum system with
the lattice size up to $24\times24$, within 30 million cores of the new
generation of sunway supercomputer. The final achievement demonstrates the
effectiveness of CNN-based representation of quantum-state and brings the
state-of-the-art record up to a brand-new level from both aspects of remarkable
accuracy and unprecedented scales. | cond-mat_str-el |
Kagome Materials I: SG 191, ScV$_6$Sn$_6$. Flat Phonon Soft Modes and
Unconventional CDW Formation: Microscopic and Effective Theory: Kagome Materials with flat bands exhibit wildly different physical properties
depending on symmetry group, and electron number. For the case of ScV$_6$Sn$_6$
in space group 191, we investigate the existence of a charge density wave (CDW)
at vector $\bar{K}=(\frac{1}{3},\frac{1}{3},\frac{1}{3})$ and its relationship
with the phonon behavior. The experimental findings reveal a $\sim$95K CDW
without nesting/peaks in the electron susceptibility at $\bar{K}$. Notably,
ScV$_6$Sn$_6$ exhibits a collapsed phonon mode at
$H=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ and an imaginary flat phonon band in
the vicinity of $H$. The soft phonon is attributed to triangular Sn ($Sn^T$)
mirror-even vibrations along the $z$-direction. We develop a simple force
constant model to describe the entire soft phonon dispersion. By employing a
new (Gaussian) approximation of the hopping parameter, we demonstrate the
renormalization of the phonon frequency and the consequent collapse of the $H$
phonon. Additionally, we propose an effective model with two order parameters
(OPs) to explain the appearance of the CDW at $\bar{K}$, which competes with
the collapsed phonon at $H$. Through comparisons with experimental data, we
show that the $H$ OP undergoes a second-order phase transition while exhibiting
substantial fluctuations, ultimately inducing the first-order transition of the
$\bar{K}$ OP. Furthermore, we extend our analysis to the similar compound
YV$_6$Sn$_6$, which lacks a CDW phase, attributing this difference to the
participation of the heavier Y atom in the out-of-plane phonon behavior. Our
comprehensive study not only elucidates the CDW in ScV$_6$Sn$_6$ but also
presents a significant advancement in modeling complex electronic systems,
fostering collaborations between ab-initio simulations and analytical
approaches. | cond-mat_str-el |
Variational Monte Carlo Method in the Presence of Spin-Orbit Interaction
and Its Application to the Kitaev and Kitaev-Heisenberg Model: We propose an accurate variational Monte Carlo method applicable in the
presence of the strong spin-orbit interaction. Our variational wave functions
consist of generalized Pfaffian-Slater wave functions that involve mixtures of
singlet and triplet Cooper pairs, Jastrow-Gutzwiller-type pro- jections, and
quantum number projections. The generalized wave functions allow any symmetry
breaking states, ranging from magnetic and/or charge ordered states to
superconducting states and their fluctuations, on equal footing without any ad
hoc ansatz for variational wave functions. We de- tail our optimization scheme
for the generalized Pfaffian-Slater wave functions with complex-number
variational parameters. Generalized quantum number projections are also
introduced, which im- poses the conservation of not only spin and momentum
quantum numbers but also Wilson loops. As a demonstration of the capability of
the present variational Monte Carlo method, the accuracy and efficiency of the
present method is tested for the Kitaev and Kitaev-Heisenberg models. The
Kitaev model serves as a critical benchmark of the present method: The exact
ground state of the model is a typical gapless quantum spin liquid far beyond
the applicability range of simple mean-field wave functions. The newly
introduced quantum number projections precisely reproduce the ground state
degeneracy of the Kitaev spin liquids, in addition to their ground state
energy. Our framework offers accurate solutions for the systems where strong
electron correlation and spin-orbit interaction coexist. | cond-mat_str-el |
Nonlinear sigma Model Treatment of Quantum Antiferromagnets in a
Magnetic Field: We present a theoretical analysis of the properties of low-dimensional
quantum antiferromagnets in applied magnetic fields. In a nonlinear sigma model
description, we use a spin stiffness analysis, a 1/N expansion, and a
renormalization group approach to describe the broken-symmetry regimes of
finite magnetization, and, in cases of most interest, a low-field regime where
symmetry is restored by quantum fluctuations. We compute the magnetization,
critical fields, spin correlation functions, and decay exponents accessible by
nuclear magnetic resonance experiments. The model is relevant to many systems
exhibiting Haldane physics, and provides good agreement with data for the
two-chain spin ladder compound CuHpCl. | cond-mat_str-el |
Effect of strong correlations on the high energy anomaly in hole- and
electron-doped high-Tc superconductors: Recently, angle-resolved photoemission spectroscopy (ARPES) has been used to
highlight an anomalously large band renormalization at high binding energies in
cuprate superconductors: the high energy 'waterfall' or high energy anomaly
(HEA). This paper demonstrates, using a combination of new ARPES measurements
and quantum Monte Carlo simulations, that the HEA is not simply the by-product
of matrix element effects, but rather represents a cross-over from a
quasiparticle band at low binding energies near the Fermi level to valence
bands at higher binding energy, assumed to be of strong oxygen character, in
both hole- and electron-doped cuprates. While photoemission matrix elements
clearly play a role in changing the aesthetic appearance of the band
dispersion, i.e. the 'waterfall'-like behavior, they provide an inadequate
description for the physics that underlies the strong band renormalization
giving rise to the HEA. Model calculations of the single-band Hubbard
Hamiltonian showcase the role played by correlations in the formation of the
HEA and uncover significant differences in the HEA energy scale for hole- and
electron-doped cuprates. In addition, this approach properly captures the
transfer of spectral weight accompanying both hole and electron doping in a
correlated material and provides a unifying description of the HEA across both
sides of the cuprate phase diagram. | cond-mat_str-el |
Optimal grouping of arbitrary diagrammatic expansions via analytic pole
structure: We present a general method to optimize the evaluation of Feynman
diagrammatic expansions, which requires the automated symbolic assignment of
momentum/energy conserving variables to each diagram. With this symbolic
representation, we utilize the pole structure of each diagram to automatically
sort the Feynman diagrams into groups that are likely to contain nearly equal
or nearly cancelling diagrams, and we show that for some systems this
cancellation is exact. This allows for a potentially massive cancellation
during the numerical integration of internal momenta variables, leading to an
optimal suppression of the `sign problem' and hence reducing the computational
cost. Although we define these groups using a frequency space representation,
the equality or cancellation of diagrams within the group remains valid in
other representations such as imaginary time used in standard diagrammatic
Monte Carlo. As an application of the approach we apply this method, combined
with algorithmic Matsubara integration (AMI) [Phys. Rev. B 99, 035120 (2019)]
and Monte Carlo methods, to the Hubbard model self-energy expansion on a 2D
square lattice up to sixth order which we evaluate and compare with existing
benchmarks. | cond-mat_str-el |
Topological classification of non-Hermitian Hamiltonians with frequency
dependence: We develop a topological classification of non-Hermitian effective
Hamiltonians that depend on momentum and frequency. Such effective Hamiltonians
are in one-to-one correspondence to single-particle Green's functions of
systems that satisfy translational invariance in space and time but may be
interacting or open. We employ K-theory, which for the special case of
noninteracting systems leads to the well-known tenfold-way topological
classification of insulators and fully gapped superconductors. Relevant
theorems for K-groups are reformulated and proven in the more transparent
language of Hamiltonians instead of vector bundles. We obtain 54 symmetry
classes for frequency-dependent non-Hermitian Hamiltonians satisfying
anti-unitary symmetries. Employing dimensional reduction, the group structure
for all these classes is calculated. This classification leads to a group
structure with one component from the momentum dependence, which corresponds to
the non-Hermitian generalization of topological insulators and superconductors,
and two additional parts resulting from the frequency dependence. These parts
describe winding of the effective Hamiltonian in the frequency direction and in
combined momentum-frequency space. | cond-mat_str-el |
Superfluid and supersolid phases of lattice bosons with ring-exchange
interaction: We examine the superfluid phase of a hard-core boson model with
nearest-neighbor exchange J and four-particle ring-exchange K at half-filling
on the square lattice. At zero temperature we find that the superfluid in the
pure-J model is quickly destroyed by the inclusion of negative-K ring-exchange
interactions, favoring a state with a (pi,pi) ordering wavevector. Minimization
of the mean-field energy suggests that a supersolid state with coexisting
superfluidity, charge-density wave, and valence-bond-like order is formed. We
also study the behavior of the finite-T Kosterlitz-Thouless phase transition in
the superfluid phase, by forcing the Nelson-Kosterlitz universal jump condition
on the finite-T spin wave superfluid density. Away from the pure J point,
T_{KT} decreases rapidly for negative K, while for positive K, T_{KT} reaches a
maximum at some K \neq 0 in agreement with recent quantum Monte Carlo
simulations. | cond-mat_str-el |
Analytic Relations between Localizable Entanglement and String
Correlations in Spin Systems: We study the relation between the recently defined localizable entanglement
and generalized correlations in quantum spin systems. Differently from the
current belief, the localizable entanglement is always given by the average of
a generalized string. Using symmetry arguments we show that in most spin 1/2
and spin 1 systems the localizable entanglement reduces to the spin-spin or
string correlations, respectively. We prove that a general class of spin 1
systems, which includes the Heisenberg model, can be used as perfect quantum
channel. These conclusions are obtained in analytic form and confirm some
results found previously on numerical grounds. | cond-mat_str-el |
Charge and spin correlations of a one dimensional electron gas on the
continuum: We present a variational Monte Carlo study of a model one dimensional
electron gas on the continuum, with long-range interaction (1/r decay). At low
density the reduced dimensionality brings about pseudonodes of the many-body
wavefunction, yielding non-ergodic behavior of naive Monte Carlo sampling,
which affects the evaluation of pair correlations and the related structure
factors. The problem is however easily solved and we are able to carefully
analyze the structure factors obtained from an optimal trial function, finding
good agreement with the exact predictions for a Luttinger-like hamiltonian with
an interaction similar to the one used in the present study. | cond-mat_str-el |
Bond-bond correlations, gap relations and thermodynamics of spin-$1/2$
chains with spin-Peierls transitions and bond-order-wave phases: The spin-$1/2$ chain with antiferromagnetic exchange $J_1$ and $J_2 = \alpha
J_1$ between first and second neighbors, respectively, has both gapless and
gapped ($\Delta(\alpha) > 0$) quantum phases at frustration $0 \le \alpha \le
3/4$. The ground state instability of regular ($\delta = 0$) chains to
dimerization ($\delta > 0$) drives a spin-Peierls transition at
$T_{SP}(\alpha)$ that varies with $\alpha$ in these strongly correlated
systems. The thermodynamic limit of correlated states is obtained by exact
treatment of short chains followed by density matrix renormalization
calculations of progressively longer chains. The doubly degenerate ground
states of the gapped regular phase are bond order waves (BOWs) with long-range
bond-bond correlations and electronic dimerization $\delta_e(\alpha)$. The $T$
dependence of $\delta_e(T,\alpha)$ is found using four-spin correlation
functions and contrasted to structural dimerization $\delta(T,\alpha)$ at $T
\le T_{SP}(\alpha)$. The relation between $T_{SP}(\alpha)$ and the $T = 0$ gap
$\Delta(\delta(0),\alpha)$ varies with frustration in both gapless and gapped
phases. The magnetic susceptibility $\chi(T,\alpha)$ at $T > T_{SP}$ can be
used to identify physical realizations of spin-Peierls systems. The $\alpha =
1/2$ chain illustrates the characteristic BOW features of a regular chain with
a large singlet-triplet gap and electronic dimerization. | cond-mat_str-el |
Transport across junctions of a Weyl and a multi-Weyl semimetal: We study transport across junctions of a Weyl and a multi-Weyl semimetal (WSM
and a MSM) separated by a region of thickness $d$ which has a barrier potential
$U_0$. We show that in the thin barrier limit ($U_0 \to \infty$ and $d \to 0$
with $\chi=U_0 d/(\hbar v_F)$ kept finite, where $v_F$ is velocity of
low-energy electrons and $\hbar$ is Planck's constant), the tunneling
conductance $G$ across such a junction becomes independent of $\chi$. We
demonstrate that such a barrier independence is a consequence of the change in
the topological winding number of the Weyl nodes across the junction and point
out that it has no analogue in tunneling conductance of either junctions of
two-dimensional topological materials (such as graphene or topological
insulators) or those made out of WSMs or MSMs with same topological winding
numbers. We study this phenomenon both for normal-barrier-normal (NBN) and
normal-barrier-superconductor (NBS) junctions involving WSMs and MSMs with
arbitrary winding numbers and discuss experiments which can test our theory. | cond-mat_str-el |
Suppression of electronic susceptibility in metal-Mott insulator
alternating material, (Me-3,5-DIP)[Ni(dmit)2]2: Frequency shifts and nuclear relaxations of 13C NMR of the metal-insulator
alternating material, (Me-3,5-DIP)[Ni(dmit)2]2, are presented. The NMR
absorption lines originating from metallic and insulating layers are well
resolved, which evidences the coexistence of localized spins (\pi_loc) and
conduction \pi-electrons. The insulating layer is newly found to undergo
antiferromagnetic long range order at about 2.5 K, suggesting emergence of
S=1/2 Mott insulator. In the metallic layer, we found significant suppressions
of static and dynamical susceptibilities of conduction electrons below 35 K,
where antiferromagnetic correlation in the insulating layer evolves. We propose
a dynamical effect through strong \pi-\pi_loc coupling between the metallic and
insulating layers as an origin of the reduction of the density of states. | cond-mat_str-el |
Neutron scattering and muon-spin spectroscopy studies of the magnetic
triangular-lattice compounds $A_2$La$_2$NiW$_2$O$_{12}$ ($A$ = Sr, Ba): We report on the geometrically frustrated two-dimensional triangular-lattice
magnets $A_2$La$_2$NiW$_2$O$_{12}$ ($A$ = Sr, Ba) studied mostly by means of
neutron powder diffraction (NPD) and muon-spin rotation and relaxation
($\mu$SR) techniques. The chemical pressure induced by the Ba-for-Sr
substitution suppresses the ferromagnetic (FM) transition from 6.3 K in the
Ba-compound to 4.8 K in the Sr-compound. We find that the $R\bar{3}$ space
group reproduces the NPD patterns better than the previously reported
$R\bar{3}m$ space group. Both compounds adopt the same magnetic structure with
a propagation vector $\boldsymbol{k} = (0, 0, 0)$, in which the Ni$^{2+}$
magnetic moments are aligned ferromagnetically along the $c$-axis. The
zero-field {\textmu}SR results reveal two distinct internal fields (0.31 and
0.10 T), caused by the long-range ferromagnetic order. The small transverse
muon-spin relaxation rates reflect the homogeneous internal field distribution
in the ordered phase and, thus, further support the simple FM arrangement of
the Ni$^{2+}$ moments. The small longitudinal muon-spin relaxation rates, in
both the ferromagnetic- and paramagnetic states of A$_2$La$_2$NiW$_2$O$_{12}$,
indicate that spin fluctuations are rather weak. Our results demonstrate that
chemical pressure indeed changes the superexchange interactions in
$A_2$La$_2$NiW$_2$O$_{12}$ compounds, with the FM interactions being dominant. | cond-mat_str-el |
A continuous transition between fractional quantum Hall and superfluid
states: We develop a theory of a direct, continuous quantum phase transition between
a bosonic Laughlin fractional quantum Hall (FQH) state and a superfluid,
generalizing the Mott insulator to superfluid phase diagram of bosons to allow
for the breaking of time-reversal symmetry. The direct transition can be
protected by a spatial symmetry, and the critical theory is a pair of Dirac
fermion fields coupled to an emergent Chern-Simons gauge field. The transition
may be achieved in optical traps of ultracold atoms by starting with a $\nu =
1/2$ bosonic Laughlin state and tuning an appropriate periodic potential to
change the topology of the composite fermion band structure. | cond-mat_str-el |
Loop currents in quantum matter: In many quantum materials, strong electron correlations lead to the emergence
of new states of matter. In particular, the study in the last decades of the
complex phase diagram of high temperature superconducting cuprates highlighted
intra-unit-cell electronic instabilities breaking discrete Ising-like
symmetries, while preserving the lattice translation invariance. Polarized
neutron diffraction experiments have provided compelling evidences supporting a
new form of intra-unit-cell magnetism, emerging concomitantly with the
so-called pseudogap state of these materials. This observation is currently
interpreted as the magnetic hallmark of an intra-unit-cell loop current order,
breaking both parity and time-reversal symmetries. More generally, this
magneto-electric state is likely to exist in a wider class of quantum materials
beyond superconducting cuprates. For instance, it has been already observed in
hole-doped Mott insulating iridates or in the spin liquid state of hole-doped
2-leg ladder cuprates. | cond-mat_str-el |
Coexistence of antiferromagnetism and dimerization in a disordered
spin-Peierls model: exact results: A model of disordered spin-Peierls system is considered, where domain walls
are randomly distributed as a telegraph noise. For this realization of the
disorder in an XX spin chain, we calculate exactly the density of states as
well as several thermodynamic quantities. The resulting physical behavior
should be qualitatively unchanged even for an XXZ chain, up to the isotropic
XXX point. For weak disorder, besides a high energy regime where the behavior
of a pure spin-Peierls system is recovered, there is a cross-over to a low
energy regime with singular thermodynamic properties and enhanced
antiferromagnetic fluctuations. These regimes are analyzed with the help of
exact results, and the relevant energy scales determined. We discuss the
possible relevance of such a disorder realization to the doped inorganic
spin-Peierls compound CuGeO$_3$. | cond-mat_str-el |
The Kondo effect in the quantum $XX$ spin chain: We investigate the boundary phenomena that arise in a finite-size $XX$ spin
chain interacting through an $XX$ interaction with a spin$-\frac{1}{2}$
impurity located at its edge. Upon Jordan-Wigner transformation, the model is
described by a quadratic Fermionic Hamiltonian. Our work displays, within this
ostensibly simple model, the emergence of the Kondo effect, a quintessential
hallmark of strongly correlated physics. We also show how the Kondo cloud
shrinks and turns into a single particle bound state as the impurity coupling
increases beyond a critical value. Using both \textit{Bethe Ansatz} and
\textit{exact diagonalization} techniques, we show that the local moment of the
impurity is screened by different mechanisms depending on the ratio of the
boundary and bulk coupling. When the ratio falls below the critical value
$\sqrt{2}$, the impurity is screened via the multiparticle Kondo effect.
However, when the ratio between the coupling exceeds the critical value , a
bound mode is formed at the impurity site which screens the spin of the
impurity. We show that the boundary phase transition is reflected in local
ground state properties by calculating the spinon density of states, the
magnetization at the impurity site in the presence of a global magnetic field,
and the finite temperature susceptibility. We find that the spinon density of
states in the Kondo phase has the characteristic Lorentzian peak that moves
from the Fermi level to the maximum energy of the spinon as the impurity
coupling is increased and becomes a localized bound mode in the bound mode
phase. Moreover, the impurity magnetization and the finite temperature impurity
susceptibility behave differently in the two phases. When the boundary coupling
$J_{\mathrm{imp}}$ exceeds the critical value $\sqrt{2}J$, the model is no
longer boundary conformal invariant as a massive bound mode appears at the
impurity site. | cond-mat_str-el |
One-particle spectral properties of the t-J-$V$ model on the triangular
lattice near charge order: We study the t-J-$V$ model beyond mean field level at finite doping on the
triangular lattice. The Coulomb repulsion $V$ between nearest neighbors brings
the system to a charge ordered state for $V$ larger than a critical value
$V_c$. One-particle spectral properties as self-energy, spectral functions and
the quasiparticle weight are studied near and far from the charge ordered
phase. When the system approaches the charge ordered state, charge fluctuations
become soft and they strongly influence the system leading to incoherent
one-particle excitations. Possible implications for cobaltates are given. | cond-mat_str-el |
Low rank compression in the numerical solution of the nonequilibrium
Dyson equation: We propose a method to improve the computational and memory efficiency of
numerical solvers for the nonequilibrium Dyson equation in the Keldysh
formalism. It is based on the empirical observation that the nonequilibrium
Green's functions and self energies arising in many problems of physical
interest, discretized as matrices, have low rank off-diagonal blocks, and can
therefore be compressed using a hierarchical low rank data structure. We
describe an efficient algorithm to build this compressed representation on the
fly during the course of time stepping, and use the representation to reduce
the cost of computing history integrals, which is the main computational
bottleneck. For systems with the hierarchical low rank property, our method
reduces the computational complexity of solving the nonequilibrium Dyson
equation from cubic to near quadratic, and the memory complexity from quadratic
to near linear. We demonstrate the full solver for the Falicov-Kimball model
exposed to a rapid ramp and Floquet driving of system parameters, and are able
to increase feasible propagation times substantially. We present examples with
262144 time steps, which would require approximately five months of computing
time and 2.2 TB of memory using the direct time stepping method, but can be
completed in just over a day on a laptop with less than 4 GB of memory using
our method. We also confirm the hierarchical low rank property for the driven
Hubbard model in the weak coupling regime within the GW approximation, and in
the strong coupling regime within dynamical mean-field theory. | cond-mat_str-el |
Real time cumulant approach for charge transfer satellites in x-ray
photoemission spectra: X-ray photoemission spectra generally exhibit satellite features in addition
to the quasi-particle peaks due to many-body excitations, which have been of
considerable theoretical and experimental interest. However, the satellites
attributed to charge-transfer (CT) excitations in correlated materials have
proved difficult to calculate from first principles. Here we report a
real-time, real-space approach for such calculations based on a cumulant
representation of the core-hole Green's function and time-dependent density
functional theory. This approach also yields an interpretation of CT satellites
in terms of a complex oscillatory, transient response to a suddenly created
core hole. Illustrative results for TiO$_2$ and NiO are in good agreement with
experiment. | cond-mat_str-el |
Charge and spin stripe in La$_{2-x}$Sr$_{x}$NiO$_{4}$ (x=1/3,1/2): Electronic structure of stripe ordered La$_{2-x}$Sr$_{x}$NiO$_{4}$ is
investigated. The system with x=1/3 is insulator, in LSDA+U calculations, and
shows charge and spin stripe, consistent with the experimental results. Highly
correlated system of x=1/2 is studied by using exact diagonalization of
multi-orbital many body Hamiltonian derived from LDA calculations and including
on-site and inter-site Coulomb interactions. The fluctuation of the residual
spin on Ni$^{3+}$ (hole) site couples with the charge fluctuation between
Ni$^{3+}$ and Ni$^{2+}$ states and this correlation lowers the total energy.
The resultant ground state is insulator with charge and spin stripe of the
energy gap 0.9eV, consistent with observed one. The on-site Coulomb interaction
stabilizes integral valency of each Ni ion (Ni$^{3+}$ and Ni$^{2+}$), but does
not induce the charge order. Two quantities, inter-site Coulomb interaction and
anisotropy of hopping integrals, play an important role to form the charge and
spin stripe order in a system of x=1/2. | cond-mat_str-el |
The boundaries of 2+1D fermionic topological orders: $2+1$D bosonic topological orders can be characterized by the $S,T$ matrices
that encode the statistics of topological excitations. In particular, the $S,T$
matrices can be used to systematically obtain the gapped boundaries of bosonic
topological orders. Such an approach, however, does not naively apply to
fermionic topological orders (FTOs). In this work, we propose a systematic
approach to obtain the gapped boundaries of $2+1$D abelian FTOs. The main trick
is to construct a bosonic extension in which the fermionic excitation is
"condensed" to form the associated FTOs. Here we choose the parent bosonic
topological order to be the $\mathbb{Z}_2$ topological order, which indeed has
a fermionic excitation. Such a construction allows us to find an explicit
correspondence between abelian FTOs (described by odd $K$-matrix $K_F$) and the
"fermion-" condensed $\mathbb{Z}_2$ topological orders (described by even
$K$-matrix $K_B$). This provides a systematic algorithm to obtain the modular
covariant boundary partition functions as well as the boundary topological
excitations of abelian FTOs. For example, the $\nu=1-\frac{1}{m}$ Laughlin's
states have exactly one type of gapped boundary when $m$ is a square, whose
boundary excitations form a $\mathbb{Z}_{2}\times\mathbb{Z}_{\sqrt{m}}$ fusion
ring. Our approach can be easily generalized to obtain gapped and gapless
boundaries of non-abelian fermionic topological orders. | cond-mat_str-el |
Anomalies in a slightly doped insulator with strong particle-hole
asymmetry and narrow gap---the case for SmB$_6$?: SmB$_6$, known to be a Kondo insulator, has received intense scrutiny in
recent years due to its paradoxical experimental signatures: while some
quantities show an insulating behavior, others point to a metallic state. This
has led to the conjecture that SmB$_6$ hosts nontrivial excitations within its
bulk gap, and has spawned several theories to that effect. In principle, there
exists an alternative possibility: the system is a metal but unusually with
both metal- and insulator-like properties. Inspired by this possibility, I
consider a minimal model of a Kondo insulator---a flat band hybridized with a
parabolic band---that is slightly electron doped, i.e., the chemical potential
is in the conduction band but close to the band edge. By calculating the dc
conductivity, ac conductivity, specific heat, and quantum oscillations at the
phenomenological level, I show that these quantities exhibit unusual behaviors
that are, surprisingly, qualitatively consistent with those observed
experimentally in SmB$_6$. The rapid change of band curvature around the
chemical potential arising from the strong particle-hole asymmetry and the
narrow gap in the model, a feature not usually encountered in the textbook
cases of metals or insulators, is at the heart of the unusual behaviors. | cond-mat_str-el |
Short Range Interaction Effects on the Density of States of Disordered
Two Dimensional Crystals with a half--filled band: The Density of electronic States (DoS) of a two--dimensional square lattice
with substitutional impurities is calculated in the presence of short--range
electron--electron interactions. In the middle of the energy band, the Bragg
reflections off the Brillouin zone boundary are shown to lead to additional
quantum corrections to the DoS, the sign of which is opposite to the sign of
the Altshuler--Aronov's logarithmic correction. The resulting quantum
correction to the DoS at half--filling is positive, i.e. the DoS increases
logarithmically as the Fermi energy is approached. However, far from the
commensurate points where the Bragg reflections are suppressed, the negative
logarithmic corrections to the DoS survive. | cond-mat_str-el |
Interconnected Renormalization of Hubbard Bands and Green's Function
Zeros in Mott Insulators Induced by Strong Magnetic Fluctuations: We analyze the role of spatial electronic correlations and, in particular, of
the magnetic fluctuations in Mott insulators. A half-filled Hubbard model is
solved at large strength of the repulsion U on a two-dimensional square lattice
using an advanced diagrammatic non-perturbative approach capable of going
beyond Hartree-Fock and single-site dynamical mean-field theories. We show that
at high temperatures the magnetic fluctuations are weak, and the electronic
self-energy of the system is mainly local and is well reproduced by the atomic
(Hubbard-I) approximation. Lowering the temperature toward the low-temperature
magnetically ordered phase, the non-locality of the self-energy becomes crucial
in determining the momentum-dispersion of the Hubbard bands and the Green's
function zeros. We therefore establish a precise link between Luttinger
surface, non-local correlations and spectral properties of the Hubbard bands. | cond-mat_str-el |
Electron correlation effects in superconducting nanowires in and out of
equilibrium: One-dimensional nanowires with strong spin-orbit coupling and
proximity-induced superconductivity are predicted to exhibit topological
superconductivity with condensed-matter analogues to Majorana fermions. Here,
the nonequilibrium Green's function approach with the generalized Kadanoff-Baym
ansatz is employed to study the electron-correlation effects and their role in
the topological superconducting phase in and out of equilibrium.
Electron-correlation effects are found to affect the transient signatures
regarding the zero-energy Majorana states, when the superconducting nanowire is
subjected to external perturbations such as magnetic-field quenching,
laser-pulse excitation, and coupling to biased normal-metal leads. | cond-mat_str-el |
New gapped quantum phases for S=1 spin chain with D2h symmetry: We study different quantum phases in integer spin systems with on-site
D2h=D2xZ2 and translation symmetry. We find four distinct non-trivial phases in
S=1 spin chains despite they all have the same symmetry. All the four phases
have gapped bulk excitations, doubly-degenerate end states and the
doubly-degenerate entanglement spectrum. These non-trivial phases are examples
of symmetry protected topological (SPT) phases introduced by Gu and Wen. One of
the SPT phase correspond to the Haldane phase and the other three are new.
These four SPT phases can be distinguished experimentally by their different
response of the end states to weak external magnetic fields. According to
Chen-Gu-Wen classification, the D2h symmetric spin chain can have totally 64
SPT phases that do not break any symmetry. Here we constructed seven nontrivial
phases from the seven classes of nontrivial projective representations of D2h
group. Four of these are found in S=1 spin chains and studied in this paper,
and the other three may be realized in S=1 spin ladders or S=2 models. | cond-mat_str-el |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.