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Raman response in the nematic phase of FeSe: Raman experiments on bulk FeSe revealed that the low-frequency part of
$B_{1g}$ Raman response $R_{B_{1g}}$, which probes nematic fluctuations,
rapidly decreases below the nematic transition at $T_n \sim 85$K. Such behavior
is usually associated with the gap opening and at a first glance is
inconsistent with the fact that FeSe remains a metal below $T_n$, with sizable
hole and electron pockets. We argue that the drop of $R_{B_{1g}}$ in a nematic
metal comes about because the nematic order drastically changes the orbital
content of the pockets and makes them nearly mono-orbital. In this situation
$B_{1g}$ Raman response gets reduced by the same vertex corrections that
enforce charge conservation. The reduction holds at low frequencies and gives
rise to gap-like behavior of $R_{B_{1g}}$, in full agreement with the
experimental data. | cond-mat_str-el |
Defect-Induced Low-Energy Majorana Excitations in the Kitaev Magnet
$α$-RuCl$_3$: The excitations in the Kitaev spin liquid (KSL) can be described by Majorana
fermions, which have characteristic field dependence of bulk gap and
topological edge modes. In the high-field state of layered honeycomb magnet
$\alpha$-RuCl$_3$, experimental results supporting these Majorana features have
been reported recently. However, there are challenges due to sample dependence
and the impact of inevitable disorder on the KSL is poorly understood. Here we
study how low-energy excitations are modified by introducing point defects in
$\alpha$-RuCl$_3$ using electron irradiation, which induces site vacancies and
exchange randomness. High-resolution measurements of the temperature dependence
of specific heat $C(T)$ under in-plane fields $H$ reveal that while the
field-dependent Majorana gap is almost intact, additional low-energy states
with $C/T=A(H)T$ are induced by introduced defects. At low temperatures, we
obtain the data collapse of $C/T\sim H^{-\gamma}(T/H)$ expected for a
disordered quantum spin system, but with an anomalously large exponent
$\gamma$. This leads us to find a power-law relationship between the
coefficient $A(H)$ and the field-sensitive Majorana gap. These results are
consistent with the picture that the disorder induces low-energy linear
Majorana excitations, which may be considered as a weak localization effect of
Majorana fermions in the KSL. | cond-mat_str-el |
Anisotropy of Kondo-lattice coherence in momentum space for CeCoIn5: We study the electronic and phononic excitations of heavy-fermion metal
CeCoIn$_5$ by polarization-resolved Raman spectroscopy to explore the
Kondo-lattice coherence. Below the coherence temperature T*\,=\,45\,K, the
continuum of electronic excitations in the XY scattering geometry is suppressed
at frequencies below 50\,cm$^{-1}$, whereas the low-frequency continuum in the
X'Y' geometry exhibits no change across T*. We relate the suppression to the
reduced electron-electron scattering rate resulting from the coherence effect.
The presence of suppression in the XY geometry and absence of it in the X'Y'
geometry implies that the $\alpha$ and $\beta$ bands become coherent below T*,
whereas the $\gamma$ band remains largely incoherent down to 10\,K. Moreover,
two optical phonon modes exhibit anomalies in their temperature dependence of
the frequency and linewidth below T*, which results from developing coherent
spectral weight near the Fermi level and reduced electron-phonon scattering
rate. Our results further support the key role of anisotropic hybridization in
CeCoIn$_5$. | cond-mat_str-el |
Excessive Noise as a Test for Many-Body Localization: Recent experimental reports suggested the existence of a finite-temperature
insulator in the vicinity of the superconductor-insulator transition. The rapid
decay of conductivity over a narrow temperature range was theoretically linked
to both a finite-temperature transition to a many-body-localized state, and to
a charge-Berezinskii Kosterlitz Thouless transition. Here we report of
low-frequency noise measurements of such insulators to test for many body
localization. We observed a huge enhancement of the low-temperatures noise when
exceeding a threshold voltage for nonlinear conductivity and discuss our
results in light of the theoretical models. | cond-mat_str-el |
Quantum Disordered Ground States in Frustrated Antiferromagnets with
Multiple Ring Exchange Interactions: We present a certain class of two-dimensional frustrated quantum Heisenberg
spin systems with multiple ring exchange interactions which are rigorously
demonstrated to have quantum disordered ground states without magnetic
long-range order. The systems considered in this paper are s=1/2
antiferromagnets on a honeycomb and square lattices, and an s=1 antiferromagnet
on a triangular lattice. We find that for a particular set of parameter values,
the ground state is a short-range resonating valence bond state or a valence
bond crystal state. It is shown that these systems are closely related to the
quantum dimer model introduced by Rokhsar and Kivelson as an effective
low-energy theory for valence bond states. | cond-mat_str-el |
Spin-phonon coupling effects in transition-metal perovskites:a DFT+$U$
and hybrid-functional study: Spin-phonon coupling effects, as reflected in phonon frequency shifts between
ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic
CaMnO$_3$, SrMnO$_3$, BaMnO$_3$, LaCrO$_3$, LaFeO$_3$ and La$_2$(CrFe)O$_6$,
are investigated using density-functional methods. The calculations are carried
out both with a hybrid-functional (HSE) approach and with a DFT+$U$ approach
using a $U$ that has been fitted to HSE calculations. The phonon frequency
shifts obtained in going from the FM to the AFM spin configuration agree well
with those computed directly from the more accurate HSE approach, but are
obtained with much less computational effort. We find that in the $A$MnO$_3$
materials class with $A$=Ca, Sr, and Ba, this frequency shift decreases as the
A cation radius increases for the $\Gamma$ phonons, while it increases for
R-point phonons. In La$M$O$_3$ with $M$=Cr, Fe, and Cr/Fe, the phonon
frequencies at $\Gamma$ decrease as the spin order changes from AFM to FM for
LaCrO$_3$ and LaFeO$_3$, but they increase for the double perovskite
La$_2$(CrFe)O$_6$. We discuss these results and the prospects for bulk and
superlattice forms of these materials to be useful as multiferroics. | cond-mat_str-el |
Spinon Confinement and a Sharp Longitudinal Mode in Yb$_2$Pt$_2$Pb in
Magnetic Fields: The fundamental excitations in an antiferromagnetic chain of spins-1/2 are
spinons, de-confined fractional quasiparticles that when combined in pairs,
form a triplet excitation continuum. In an Ising-like spin chain the continuum
is gapped and the ground state is N{\'e}el ordered. Here, we report high
resolution neutron scattering experiments, which reveal how a magnetic field
closes this gap and drives the spin chains in \YPP\ to a critical, disordered
Luttinger-liquid state. In \YPP\ the effective spins-1/2 describe the dynamics
of large, Ising-like Yb magnetic moments, ensuring that the measured
excitations are exclusively longitudinal, which we find to be well described by
time-dependent density matrix renormalization group calculations. The
inter-chain coupling leads to the confinement of spinons, a condensed matter
analog of quark confinement in quantum chromodynamics. Insensitive to
transverse fluctuations, our measurements show how a gapless, dispersive
longitudinal mode arises from confinement and evolves with magnetic order. | cond-mat_str-el |
Relationship between single-particle excitation and spin excitation at
the Mott Transition: An intuitive interpretation of the relationship between the dispersion
relation of the single-particle excitation in a metal and that of the spin
excitation in a Mott insulator is presented, based on the results for the one-
and two-dimensional Hubbard models obtained by using the Bethe ansatz,
dynamical density-matrix renormalization group method, and cluster perturbation
theory. The dispersion relation of the spin excitation in the Mott insulator is
naturally constructed from that of the single-particle excitation in the
zero-doping limit in both one- and two-dimensional Hubbard models, which allows
us to interpret the doping-induced states as the states that lose charge
character toward the Mott transition. The characteristic feature of the Mott
transition is contrasted with the feature of a Fermi liquid and that of the
transition between a band insulator and a metal. | cond-mat_str-el |
Anomalous State Sandwiched between Fermi Liquid and Charge Ordered
Mott-Insulating Phases of Ti4O7: The Magneli phase Ti4O7 exhibits two sharp jumps in resistivity with coupled
structural transitions as a function of temperature at Tc1=142 K and Tc2=154 K.
We have studied electronic structure changes across the two transitions using 7
eV laser, soft x-ray and hard x-ray (HX) photoemission spectroscopy (PES). Ti
2p-3d resonant PES and HX-PES show a clear metallic Fermi-edge and mixed
valency above Tc2. The low temperature phase below Tc1 shows a clear insulating
gap of 100 meV. The intermediate phase between Tc1 and Tc2 indicates a
pseudogap coexisting with remnant coherent states. HX-PES and complementary
calculations have confirmed the coherent screening in the strongly correlated
intermediate phase. The results suggest existence of a highly anomalous state
sandwiched between the mixed-valent Fermi liquid and charge ordered
Mott-insulating phase in Ti4O7. | cond-mat_str-el |
Emergence of Jack ground states from two-body pseudopotentials in
fractional quantum Hall systems: The family of "Jack states" related to antisymmetric Jack polynomials are the
exact zero-energy ground states of particular model short-range {\em many-body}
repulsive interactions, defined by a few non-vanishing leading
pseudopotentials. Some Jack states are known or anticipated to accurately
describe many-electron incompressible ground states emergent from the {\em
two-body} Coulomb repulsion in fractional quantum Hall effect. By extensive
numerical diagonalization we demonstrate emergence of Jack states from suitable
pair interactions. We find empirically a simple formula for the optimal
two-body pseudopotentials for the series of most prominent Jack states
generated by {\em contact} many-body repulsion. Furthermore, we seek
realization of arbitrary Jack states in realistic quantum Hall systems with
Coulomb interaction, i.e., in partially filled lowest and excited Landau levels
in quasi-two-dimensional layers of conventional semiconductors like GaAs or in
graphene. | cond-mat_str-el |
Emergent Dipole Gauge Fields and Fractons: We present a realization of fracton-elasticity duality purely formulated in
terms of ordinary gauge fields, encompassing standard elasticity and
incommensurate crystals as those describing twisted bilayer graphene,
quasicrystals or more general moir\'e lattices. Our construction comprises a
description of all types of two-dimensional defects: disclinations,
dislocations, discompressions and point-like defects, and takes into account
body forces and impurities. The original form of the duality in terms of tensor
gauge fields is recovered after partial gauge fixing. We identify the coupling
of each type of defect to the dual gauge fields, and from gauge invariance we
derive generalized continuity equations for the defect currents and the
expected mobility restrictions of elasticity defects. | cond-mat_str-el |
Interactions in Quasicrystals: Although the effects of interactions in solid state systems still remains a
widely open subject, some limiting cases such as the three dimensional Fermi
liquid or the one-dimensional Luttinger liquid are by now well understood when
one is dealing with interacting electrons in {\it periodic} crystalline
structures. This problem is much more fascinating when periodicity is lacking
as it is the case in {\it quasicrystalline} structures. Here, we discuss the
influence of the interactions in quasicrystals and show, on a controlled
one-dimensional model, that they lead to anomalous transport properties,
intermediate between those of an interacting electron gas in a periodic and in
a disordered potential. | cond-mat_str-el |
Holographic Dynamics from Multiscale Entanglement Renormalization Ansatz: The Multiscale Entanglement Renormalization Ansatz (MERA) is a tensor network
based variational ansatz that is capable of capturing many of the key physical
properties of strongly correlated ground states such as criticality and
topological order. MERA also shares many deep relationships with the AdS/CFT
(gauge-gravity) correspondence by realizing a UV complete holographic duality
within the tensor networks framework. Motivated by this, we have re-purposed
the MERA tensor network as an analysis tool to study the real-time evolution of
the 1D transverse Ising model in its low energy excited state sector. We
performed this analysis by allowing the ancilla qubits of the MERA tensor
network to acquire quantum fluctuations, which yields a unitary transform
between the physical (boundary) and ancilla qubit (bulk) Hilbert spaces. This
then defines a reversible quantum circuit which is used as a `holographic
transform' to study excited states and their real-time dynamics from the point
of the bulk ancillae. In the gapped paramagnetic phase of the transverse field
Ising model, we demonstrate the holographic duality between excited states
induced by single spin-flips (Ising `magnons') acting on the ground state and
single ancilla qubit spin-flips. The single ancillae qubit excitation is shown
to be stable in the bulk under real-time evolution and hence defines a stable
holographic quasiparticle which we have named the `hologron'. The `dictionary'
between the bulk and boundary is determined and realizes many features of the
holographic correspondence in a non-CFT limit of the boundary theory. As an
added spin-off, this dictionary together with the extension to multi-hologron
sectors gives us a systematic way to construct quantitatively accurate low
energy effective Hamiltonians. | cond-mat_str-el |
Enhancement of superconductivity by pressure-induced critical
ferromagnetic fluctuations in UCoGe: A $^{59}$Co nuclear quadrupole resonance (NQR) was performed on a
single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The
FM phase vanished at a critical pressure $P_c$, and the NQR spectrum just below
$P_c$ showed phase separation of the FM and paramagnetic (PM) phases below
Curie temperature $T_{\textrm{Curie}}$, suggesting first-order FM quantum phase
transition (QPT). We found that the internal field was absent above $P_c$, but
the superconductivity is almost unchanged. This result suggests the existence
of the nonunitary to unitary transition of the superconductivity around $P_c$.
Nuclear spin-lattice relaxation rate $1/T_1$ showed the FM critical
fluctuations around $P_c$, which persist above $P_c$ and are clearly related to
superconductivity in the PM phase. This FM QPT is understood to be a weak first
order with critical fluctuations. $1/T_1$ sharply decreased in the
superconducting (SC) state above $P_c$ with a single component, in contrast to
the two-component $1/T_1$ in the FM SC state, indicating that the inhomogeneous
SC state is a characteristic feature of the FM SC state in UCoGe. | cond-mat_str-el |
Spin Coulomb Drag: We introduce a distinctive feature of spin-polarized transport, the Spin
Coulomb Drag: there is an intrinsic source of friction for spin currents due to
the Coulomb interaction between spin ``up'' and spin ``down'' electrons. We
calculate the associated ``spin transrestistivity'' in a generalized random
phase approximation and discuss its dependence on temperature, frequency, and
electron density. We show that, in an appropriate range of parameters, such
resistivity is measurable and propose an experiment to measure it. | cond-mat_str-el |
Dynamical Mean Field Theory, Density-Matrix Embedding Theory and
Rotationally Invariant Slave Bosons: a Unified Perspective: We present a unified perspective on Dynamical Mean Field Theory (DMFT),
Density-Matrix Embedding Theory (DMET) and Rotationally Invariant Slave Bosons
(RISB). We show that DMET can be regarded as a simplification of the RISB
method where the quasiparticle weight is set to unity. This relation allows to
easily transpose extensions of a given method to another: for instance, a
temperature-dependent version of RISB can be used to derive a
temperature-dependent free-energy formula for DMET. | cond-mat_str-el |
Interaction-driven Spontaneous Ferromagnetic Insulating States with Odd
Chern Numbers: Motivated by recent experimental work on moir\'e systems in a strong magnetic
field, we compute the compressibility as well as the spin correlations and
Hofstadter spectrum of spinful electrons on a honeycomb lattice with Hubbard
interactions using the determinantal quantum Monte Carlo method. While the
interactions in general preserve quantum and anomalous Hall states, emergent
features arise corresponding to an antiferromagnetic insulator at half-filling
and other incompressible states following the Chern sequence $\pm (2N+1)$.
These odd integer Chern states exhibit strong ferromagnetic correlations and
arise spontaneously without any external mechanism for breaking the
spin-rotation symmetry. Analogs of these magnetic states should be observable
in general interacting quantum Hall systems. In addition, the interacting
Hofstadter spectrum is qualitatively similar to the experimental data at
intermediate values of the on-site interaction. | cond-mat_str-el |
A Quantum Monte Carlo algorithm for out-of-equilibrium Green's functions
at long times: We present a quantum Monte-Carlo algorithm for computing the perturbative
expansion in power of the coupling constant $U$ of the out-of-equilibrium
Green's functions of interacting Hamiltonians of fermions. The algorithm
extends the one presented in Phys. Rev. B 91 245154 (2015), and inherits its
main property: it can reach the infinite time (steady state) limit since the
computational cost to compute order $U^n$ is uniform versus time; the computing
time increases as $2^n$. The algorithm is based on the Schwinger-Keldysh
formalism and can be used for both equilibrium and out-of-equilibrium
calculations. It is stable at both small and long real times including in the
stationary regime, because of its automatic cancellation of the disconnected
Feynman diagrams. We apply this technique to the Anderson quantum impurity
model in the quantum dot geometry to obtain the Green's function and
self-energy expansion up to order $U^{10}$ at very low temperature. We
benchmark our results at weak and intermediate coupling with high precision
Numerical Renormalization Group (NRG) computations as well as analytical
results. | cond-mat_str-el |
Kondo destruction in a quantum paramagnet with magnetic frustration: We report results of isothermal magnetotransport and susceptibility
measurements at elevated magnetic fields B down to very low temperatures T on
high-quality single crystals of the frustrated Kondo-lattice system CePdAl.
They reveal a B*(T) line within the paramagnetic part of the phase diagram.
This line denotes a thermally broadened 'small'-to-'large' Fermi surface
crossover which substantially narrows upon cooling. At B_0* = B*(T=0) = (4.6
+/- 0.1) T, this B*(T) line merges with two other crossover lines, viz. Tp(B)
below and T_FL(B) above B_0*. Tp characterizes a frustration-dominated
spin-liquid state, while T_FL is the Fermi-liquid temperature associated with
the lattice Kondo effect. Non-Fermi-liquid phenomena which are commonly
observed near a 'Kondo destruction' quantum critical point cannot be resolved
in CePdAl. Our observations reveal a rare case where Kondo coupling,
frustration and quantum criticality are closely intertwined. | cond-mat_str-el |
Spin-rotationally symmetric domain flux phases in underdoped cuprates: We propose a new form of inhomogeneous phases consisting of out-of-phase
staggered flux domains separated by diagonal charged domain walls centered on
bonds or on sites. Remarkably, such domain flux phases are spin-rotationally
symmetric and exhibit cone-like quasiparticle dispersion as well as
incommensurate order of orbital currents. Such features are consistent with the
pseudogap behavior and the diagonal stripes observed experimentally in lightly
doped cuprates. A renormalized mean field theory shows that such solutions are
competitive candidates within the $t$--$J$ model. | cond-mat_str-el |
Spectral form factors of clean and random quantum Ising chains: We compute the spectral form factor of two integrable quantum-critical many
body systems in one spatial dimension. The spectral form factor of the quantum
Ising chain is periodic in time in the scaling limit described by a conformal
field theory; we also compute corrections from lattice effects and deviation
from criticality. Criticality in the random Ising chain is described by rare
regions associated with a strong randomness fixed point, and these control the
long time limit of the spectral form factor. | cond-mat_str-el |
Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice
Model : A Qualitative Approach to Oxide Manganites: The transport properties of the ferromagnetic Kondo lattice model in one
dimension are studied via bosonization methods. The antiferromagnetic
fluctuations, which normally appear because of the RKKY interactions, are
explicitly taken into account as a direct exchange between the ``core'' spins.
It is shown that in the paramagnetic regime with the local antiferromagnetic
fluctuations, the resistivity decays exponentially as the temperature increases
while in the ferromagnetic regime the system is an almost perfect conductor. %A
non-perturbative description of localized spin polarons %in the paramagnetic
region is obtained.
The effect of a weak applied field is discussed to be reduced to the case of
the ferromagnetic state leading to band splitting. The qualitative relevance of
the results for the problem of the Oxide Manganites is emphasized. | cond-mat_str-el |
Magnetic Phase Diagram of Spin-1/2 Two-Leg Ladder with Four-Spin Ring
Exchange: We study the spin-1/2 two-leg Heisenberg ladder with four-spin ring exchanges
under a magnetic field. We introduce an exact duality transformation which is
an extension of the spin-chirality duality developed previously and yields a
new self-dual surface in the parameter space. We then determine the magnetic
phase diagram using the numerical approaches of the density-matrix
renormalization-group and exact diagonalization methods. We demonstrate the
appearance of a magnetization plateau and the Tomonaga-Luttinger liquid with
dominant vector-chirality quasi-long-range order for a wide parameter regime of
strong ring exchange. A "nematic" phase, in which magnons form bound pairs and
the magnon-pairing correlation functions dominate, is also identified. | cond-mat_str-el |
Spin-Dependent Correlations of Fermi Liquids at Nonzero Temperatures
within Correlated Density-Matrix Approach: Correlated density matrix theory is generalized to investigate equilibrium
properties of normal Fermi Liquids such as 3He and nuclear matter at nonzero
temperatures. The results also generalize the Fermi-hypernetted-chain technique
that is familiar from studies of the ground state of correlated fermions. By
employing the concept of renormalized bosons and fermions the formal results
are cast in a form that permits the direct evaluation of the statistical
properties of the correlated liquid such as the entropy and the specific heat
at constant volume among other quantities. | cond-mat_str-el |
Symmetric Fracton Matter: Twisted and Enriched: In this paper, we explore the interplay between symmetry and fracton order,
motivated by the analogous close relationship for topologically ordered
systems. Specifically, we consider models with 3D planar subsystem symmetry,
and show that these can realize subsystem symmetry protected topological phases
with gapless boundary modes. Gauging the planar subsystem symmetry leads to a
fracton order in which particles restricted to move along lines exhibit a new
type of statistical interaction that is specific to the lattice geometry. We
show that both the gapless boundary modes of the ungauged theory, and the
statistical interactions after gauging, are naturally captured by a higher-rank
version of Chern-Simons theory. We also show that gauging only part of the
subsystem symmetry can lead to symmetry-enriched fracton orders, with
quasiparticles carrying fractional symmetry charge. | cond-mat_str-el |
Observation of Incompressibility at $ν=4/11$ and $ν=5/13$: The region of filling factors $1/3<\nu<2/5$ is predicted to support new types
of fractional quantum Hall states with topological order different from that of
the Laughlin-Jain or the Moore-Read states. Incompressibility is a necessary
condition for the formation of such novel topological states. We find that at
6.9~mK incompressibility develops only at $\nu=4/11$ and $5/13$, while the
states at $\nu=6/17$ and $3/8$ remain compressible. Our observations at
$\nu=4/11$ and $5/13$ are first steps towards understanding emergent
topological order in these fractional quantum Hall states. | cond-mat_str-el |
Electronic structure of the $Sr_{0.4}Ca_{13.6}Cu_{24}O_{41}$
incommensurate compound: We extracted, from strongly-correlated ab-initio calculations, a complete
model for the chain subsystem of the $Sr_{0.4}Ca_{13.6}Cu_{24}O_{41}$
incommensurate compound. A second neighbor $t-J+V$ model has been determined as
a function of the fourth crystallographic parameter $\tau$, for both low and
room temperature crystallographic structures. The analysis of the obtained
model shows the crucial importance of the structural modulations on the
electronic structure through the on-site energies and the magnetic
interactions. The structural distortions are characterized by their long range
effect on the cited parameters that hinder the reliability of analyses such as
BVS. One of the most striking results is the existence of antiferromagnetic
nearest-neighbor interactions for metal-ligand-metal angles of $90^\circ$. A
detailed analysis of the electron localization and spin arrangement is
presented as a function of the chain to ladder hole transfer and of the
temperature. The obtained spin arrangement is in agreement with
antiferromagnetic correlations in the chain direction at low temperature. | cond-mat_str-el |
TU$^2$FRG -- a scalable approach for truncated unity functional
renormalization group in generic fermionic models: Describing the emergence of phases of condensed matter is one of the central
challenges in physics. For this purpose many numerical and analytical methods
have been developed, each with their own strengths and limitations. The
functional renormalization group is one of these methods bridging between
efficiency and accuracy. In this paper we derive a new truncated unity (TU)
approach unifying real- and momentum space TU, called TU$^2$FRG. This formalism
significantly improves the scaling compared to conventional momentum (TU)FRG
when applied to large unit-cell models and models where the translational
symmetry is broken. | cond-mat_str-el |
Fermionology in the Kondo-Heisenberg model: the case of CeCoIn$_{5}$: Fermi surface of heavy electron systems plays a fundamental role in
understanding their variety of puzzling phenomena, for example, quantum
criticality, strange metal behavior, unconventional superconductivity and even
enigmatic phases with yet unknown order parameters. The spectroscopy
measurement of typical heavy fermion superconductor CeCoIn$_{5}$ has
demonstrated multi-Fermi surface structure, which has not been in detail
studied theoretically in a model system like the Kondo-Heisenberg model. In
this work, we make a step toward such an issue with revisiting the
Kondo-Heisenberg model. It is surprising to find that the usual self-consistent
calculation cannot reproduced the fermionology of the experimental observation
of the system due to the unfounded sign binding between the hopping of the
conduction electrons and the mean-field valence-bond order. To overcome such
inconsistency, we assume that the sign binding should be relaxed and the
mean-field valence-bond order can be considered as a free/fit parameter so as
to meet with real-life experiments. Given the fermionology, the calculated
effective mass enhancement, entropy, superfluid density and Knight shift are
all in qualitative agreement with the experimental results of CeCoIn$_{5}$,
which confirms our assumption. Our result supports a $d_{x^{2}-y^{2}}$-wave
pairing structure in heavy fermion material CeCoIn$_{5}$. In addition, we have
also provided the scanning tunneling microscopy (STM) spectra of the system,
which is able to be tested by the present STM experiments. | cond-mat_str-el |
Interplay of Fractional Chern Insulator and Charge-Density-Wave Phases
in Twisted Bilayer Graphene: We perform an extensive exact diagonalization study of interaction driven
insulators in spin- and valley-polarized moir\'{e} flat bands of twisted
bilayer graphene aligned with its hexagonal boron nitride substrate. In
addition to previously reported fractional Chern insulator phases, we provide
compelling evidence for competing charge-density-wave phases at multiple
fractional fillings of a realistic single-band model. A thorough analysis at
different interlayer hopping parameters, motivated by experimental variability,
and the role of kinetic energy at various Coulomb interaction strengths
highlight the competition between these phases. The interplay of the
single-particle and the interaction induced hole dispersion with the inherent
Berry curvature of the Chern bands is intuitively understood to be the driving
mechanism for the ground-state selection. The resulting phase diagram features
remarkable agreement with experimental findings in a related moir\'{e}
heterostructure and affirms the relevance of our results beyond the scope of
graphene based materials. | cond-mat_str-el |
Anderson localization of spinons in a spin-1/2 antiferromagnetic
Heisenberg chain: Anderson localization is a general phenomenon of wave physics, which stems
from the interference between multiple scattering paths1,2. It was originally
proposed for electrons in a crystal, but later was also observed for light3-5,
microwaves6, ultrasound7,8, and ultracold atoms9-12. Actually, in a crystal,
besides electrons there may exist other quasiparticles such as magnons and
spinons. However the search for Anderson localization of these magnetic
excitations is rare so far. Here we report the first observation of spinon
localization in copper benzoate, an ideal compound of spin-1/2
antiferromagnetic Heisenberg chain, by ultra-low-temperature specific heat and
thermal conductivity measurements. We find that while the spinon specific heat
Cs displays linear temperature dependence down to 50 mK, the spinons thermal
conductivity ks only manifests the linear temperature dependence down to 300
mK. Below 300 mK, ks/T decreases rapidly and vanishes at about 100 mK, which is
a clear evidence for Anderson localization. Our finding opens a new window for
studying such a fundamental phenomenon in condensed matter physics. | cond-mat_str-el |
Structural Transitions in a Classical Two-Dimensional Molecule System: The ground state of a classical two-dimensional (2D) system with finite
number of charged particles, trapped by two positive impurities charges
localized at a distance (zo) from the 2D plane and separated from each other by
a distance xp are obtained. The impurities are allowed to carry more than one
positive charge. This classical system can form a 2D-like classical molecule
that exhibits structural transitions and spontaneous symmetry breaking as a
function of the separation between the positive charges before it transforms
into two independent 2D-like classical atoms. We also observe structural
transitions as a function of the dielectric constant of the substrate which
supports the charged particles, in addition to broken symmetry states and
unbinding of particles. | cond-mat_str-el |
Crystallization in the Fractional Quantum Hall Regime Induced by
Landau-level Mixing: The interplay between strongly correlated liquid and crystal phases for
two-dimensional electrons exposed to a high transverse magnetic field is of
fundamental interest. Through the non-perturbative fixed phase diffusion Monte
Carlo method, we determine the phase diagram of the Wigner crystal in the
$\nu-\kappa$ plane, where $\nu$ is the filling factor and $\kappa$ is the
strength of Landau level mixing. The phase boundary is seen to exhibit a
striking $\nu$ dependence, with the states away from the magic filling factors
$\nu=n/(2pn+1)$ being much more susceptible to crystallization due to Landau
level mixing than those at $\nu=n/(2pn+1)$. Our results explain the qualitative
difference between the experimental behaviors observed in n-doped and p-doped
GaAs quantum wells, and, in particular, the existence of an insulating state
for $\nu<1/3$ and also for $1/3 <\nu< 2/5$ in low density p-doped systems. We
predict that in the vicinity of $\nu=1/5$ and $\nu=2/9$, increasing LL mixing
causes a transition not into an ordinary electron Wigner crystal but rather
into a strongly correlated crystal of composite fermions carrying two vortices. | cond-mat_str-el |
Ab-initio determination of the localized/delocalized f-manifold in
UPd_2Al_3: The electronic structure of UPd_2Al_3 is described using the self-interaction
corrected local-spin-density approximation to density functional theory. The
groundstate is found to be characterized by the coexistence of localized (f^2)
and delocalized U f electrons, in agreement with experimental evidence. We
observe significant difference in electronic structure between UPd_2Al_3 and
the previously studied UPt_3 compound. Even though a trend towards localization
exists in UPt_3, the total energies and the density of states at the Fermi
level favor a groundstate with localized f^1, rather than f^2 U ions. | cond-mat_str-el |
Temperature dependent bilayer ferromagnetism in Sr3Ru2O7: The Ruthenium based perovskites exhibit a wide variety of interesting
collective phenomena related to magnetism originating from the Ru 4d electrons.
Much remains unknown concerning the nature of magnetic fluctuations and
excitations in these systems. We present results of detailed inelastic neutron
scattering measurements of Sr3Ru2O7 as a function of temperature, probing the
ferromagnetic fluctuations of the bilayer structure. A magnetic response is
clearly visible for a range of temperatures, T = 3.8 K up to T = 100 K, and for
energy transfers between 2 and 14 meV. These measurements indicate that the
ferromagnetic fluctuations manifest in the bilayer structure factor persist to
surprisingly large temperatures. This behavior may be related to the proximity
of the system in zero magnetic field to the metamagnetic/ferromagnetic
transition. | cond-mat_str-el |
Search for a quantum phase transition in U(Pt_(1-x)Pd_x)_3: Pd in U(Pt_{1-x}Pd_x)_3 suppresses the superconducting T_c to 0 K at critical
concentration x_c of 0.007 and induces a conventional AFM state for x > x_c.
The resistivity below 1 K shows a deviation from Fermi liquid behavior
described by a power law where the exponent ranges from 2 at x=0 to 1.6 for x =
x_c. This suggests that a quantum phase transition (QPT) may exist near x_c
associated with either the magnetic or superconducting transition temperature =
0 K. Transport for a sample with x = 0.004 < x_c has constant exponent of 1.77
as increasing pressure suppresses T_c to 0 K, suggesting that if a QPT exists
it may be associated with the magnetic transition. | cond-mat_str-el |
Ground States of the Ising Model on the Shastry-Sutherland Lattice and
the Origin of the Fractional Magnetization Plateaus in Rare-Earth
Tetraborides: A complete and exact solution of the ground-state problem for the Ising model
on the Shastry-Sutherland lattice in the applied magnetic field is found. The
magnetization plateau at the one third of the saturation value is shown to be
the only possible fractional plateau in this model. However, stripe magnetic
structures with magnetization 1/2 and $1/n$ ($n > 3$), observed in the
rare-earth tetraborides RB$_4$, occur at the boundaries of the
three-dimensional regions of the ground-state phase diagram. These structures
give rise to new magnetization plateaus if interactions of longer ranges are
taken into account. For instance, an additional third-neighbor interaction is
shown to produce a 1/2 plateau. The results obtained significantly refine the
understanding of the magnetization process in RB$_4$ compounds, especially in
TmB$_4$ and ErB$_4$ which are strong Ising magnets. | cond-mat_str-el |
Nature of the glassy magnetic state in Cu$_{2.84}$Mn$_{0.44}$Al$_{0.72}$
shape memory alloy: The magnetic ground state of the ferromagnetic shape memory alloy of nominal
composition Cu$_{2.84}$Mn$_{0.44}$Al$_{0.72}$ was investigated. The sample
shows reentry of a glassy magnetic phase below the martensitic transition
temperature, which is found to have complex character with two distinct
anomalies in the temperature dependent ac susceptibility data. The sample
retains its glassy phase even below the second transition as evident from the
magnetic memory measurements in different protocols. Existence of two
transitions along with their observed nature suggest that the system can be
described by the mean field Heisenberg model of reentrant spin glass as
proposed by Gabay and Toulous. \cite{rsg-GT1} The sample provides a fascinating
example where a Gabay-Toulous type spin glass state is triggered by a first
order magneto-structural transition. | cond-mat_str-el |
High-Temperature Criticality in Strongly Constrained Quantum Systems: The exotic nature of many strongly correlated materials at reasonably high
temperatures, for instance cuprate superconductors in their normal state, has
lead to the suggestion that such behavior occurs within a quantum critical
region where the physics is controlled by the influence of a phase transition
down at zero temperature. Such a scenario can be thought of as a bottom-up
approach, with the zero temperature mechanisms finding a way to manifest
critical behavior at high temperatures. Here we propose an alternative,
top-down, mechanism by which strong kinematic constraints that can only be
broken at extremely high temperatures are responsible for critical behavior at
intermediate but still high temperatures. This critical behavior may extend all
the way down to zero temperature, but this outcome is not one of necessity, and
the system may order at low temperatures. We provide explicit examples of such
high-temperature criticality when additional strong interactions are introduced
in quantum Heisenberg, transverse field Ising, and some bosonic lattice models. | cond-mat_str-el |
Analytic continuation of the self-energy via Machine Learning techniques: We develop a novel analytic continuation method for self-energies on the
Matsubara domain as computed by quantum Monte Carlo simulations within
dynamical mean field theory (QMC+DMFT). Unlike a maximum entropy (maxEn)
procedure employed for the last thirty years, our approach is based on a
machine learning (ML) technique in combination with the iterative perturbative
theory impurity solver of the dynamical mean field theory self-consistent
process (IPT+DMFT). The input and output training datasets for ML are
simultaneously obtained from IPT+DMFT calculations on Matsubara and real
frequency domains, respectively. The QMC+DMFT self-energy on real frequencies
is determined from the -- usually noisy -- input QMC+DMFT self-energy on the
Matsubara domain and the trained ML kernel. Our approach is free from both,
bias of ML training datasets and from fitting parameters present in the maxEn
method. We demonstrate the efficiency of the method on the testbed frustrated
Hubbard model on the square lattice. | cond-mat_str-el |
Probing light-driven quantum materials with ultrafast resonant inelastic
X-ray scattering: Ultrafast optical pulses are an increasingly important tool for controlling
quantum materials and triggering novel photo-induced phase transitions.
Understanding these dynamic phenomena requires a probe sensitive to spin,
charge, and orbital degrees of freedom. Time-resolved resonant inelastic X-ray
scattering (trRIXS) is an emerging spectroscopic method, which responds to this
need by providing unprecedented access to the finite-momentum fluctuation
spectrum of photoexcited solids. In this Perspective, we briefly review
state-of-the-art trRIXS experiments on condensed matter systems, as well as
recent theoretical advances. We then describe future research opportunities in
the context of light control of quantum matter. | cond-mat_str-el |
Commensurate and Incommensurate Structure of the Neutron Cross Section
in LaSrCuO and YBaCuO: We study the evolution of the d-wave neutron cross-section with variable
frequency \omega and fixed T (below and above Tc) in two different cuprate
families. The evolution from incommensurate to commensurate to incommensurate
peaks is rather generic within an RPA-like scheme. This behavior seems to be in
reasonable accord with experiments, and may help distinguish between this and
the "stripe" scenario. | cond-mat_str-el |
Phase control of magnons in the van der Waals antiferromagnet NiPS$_3$: We demonstrate phase control of magnons in the van der Waals antiferromagnet
NiPS$_3$ using optical excitation by polarized light. The sign of the coherent
precession of spin amplitude changes upon (1) reversing the helicity of a
circularly polarized pump beam, or (2) rotating the polarization of a linearly
polarized pump by $\pi/2$. Because these two excitation pathways have
comparable generation efficiency, the phase of spin precession can be
continuously tuned from 0 to $2\pi$ by controlling the polarization state of
the pump pulse. The ability to excite magnons with a desired phase has
potential applications in the design of a spin-wave phased array and ultrafast
spin information processing. | cond-mat_str-el |
Optimizing configurations for determining the magnetic model based on
ab-initio calculations: In this paper, it is presented a novel strategy to optimize the determination
of magnetic couplings by using ab-initio calculations of the energy. This
approach allows determining efficiently, in terms of a proposed effective
magnetic spin model, an optimal set of magnetic configurations to be simulated
by DFT methods. Moreover, a procedure to estimate the values of the coupling
constants and their error bounds from the estimated energies is proposed. This
method, based on Monte Carlo sampling, takes into account the accuracy of the
ab - initio simulations. A strategy to refine models reusing previously
computed configuration energies is also presented. We apply the method to
determine a magnetic model for the recently synthesized material
Bi$_3$Mn$_4$O$_{12}$(NO$_3$). Finally, an open source software that implements
and automatizes the whole process is presented. | cond-mat_str-el |
Order by disorder in classical kagome antiferromagnets with chiral
interactions: The Heisenberg antiferromagnet on the kagome lattice is an archetypal
instance of how large ground state degeneracies arise, and how they may get
resolved by thermal and quantum fluctuations. Augmenting the Heisenberg model
by chiral spin interactions has proved to be of particular interest in the
discovery of certain chiral quantum spin liquids. Here we consider the
classical variant of this chiral kagome model and find that it exhibits,
similar to the classical Heisenberg antiferromagnet, a remarkably large and
structured ground-state manifold, which combines continuous and discrete
degrees of freedom. This allows for a rich set of order-by-disorder phenomena.
Degeneracy lifting occurs in a highly selective way, choosing already at the
harmonic level specific triaxial states which however retain an emergent $Z_2$
degree of freedom (absent in the conventional Heisenberg model). We also study
the competition of entropic and energetic ground state selection as the model
interpolates between the purely chiral and Heisenberg cases. For this mixed
model, we find a "proximate ordered-by-disorder" finite-temperature regime
where fluctuations overcome the energetic ground state preference of the
perturbation. Finally, a semiclassical route to a spin liquid is provided by
quantum order by disorder in the purely chiral models, where the aforementioned
$Z_2$ degrees of freedom are elevated to the role of an emergent gauge field. | cond-mat_str-el |
The structure of spinful quantum Hall states: a squeezing perspective: We provide a set of rules to define several spinful quantum Hall model
states. The method extends the one known for spin polarized states. It is
achieved by specifying an undressed root partition, a squeezing procedure and
rules to dress the configurations with spin. It applies to both the
excitation-less state and the quasihole states. In particular, we show that the
naive generalization where one preserves the spin information during the
squeezing sequence, may fail. We give numerous examples such as the Halperin
states, the non-abelian spin-singlet states or the spin-charge separated
states. The squeezing procedure for the series (k=2,r) of spinless quantum Hall
states, which vanish as r powers when k+1 particles coincide, is generalized to
the spinful case. As an application of our method, we show that the counting
observed in the particle entanglement spectrum of several spinful states
matches the one obtained through the root partitions and our rules. This
counting also matches the counting of quasihole states of the corresponding
model Hamiltonians, when the latter is available. | cond-mat_str-el |
Electronic correlations in organometallic complexes: We investigate an effective model for organometallic complexes (with
potential uses in optoelectronic devices) via both exact diagonalisation and
the configuration interaction singles (CIS) approximation. This model captures
a number of important features of organometallic complexes, notably the
sensitivity of the radiative decay rate to small chemical changes. We find that
for large parameter ranges the CIS approximation accurately reproduces the low
energy excitations and hence the photophysical properties of the exact
solution. This suggests that electronic correlations do \emph{not} play an
important role in these complexes. This explains why time-dependent density
functional theory works surprisingly well in these complexes. | cond-mat_str-el |
Non-Hermitian Mott Skin Effect: We propose a novel type of skin effects in non-Hermitian quantum many-body
systems which we dub a non-Hermitian Mott skin effect. This phenomenon is
induced by the interplay between strong correlations and the non-Hermitian
point-gap topology. The Mott skin effect induces extreme sensitivity to the
boundary conditions only in the spin degree of freedom (i.e., the charge
distribution is not sensitive to boundary conditions), which is in sharp
contrast to the ordinary non-Hermitian skin effect in non-interacting systems.
Concretely, we elucidate that a bosonic non-Hermitian chain exhibits the Mott
skin effect in the strongly correlated regime by closely examining an effective
Hamiltonian. The emergence of the Mott skin effect is also supported by
numerical diagonalization of the bosonic chain. The difference between the
ordinary non-Hermitian skin effect and the Mott skin effect is also reflected
in the time-evolution of physical quantities; under the time-evolution spin
accumulation is observed while the charge distribution remains spatially
uniform. | cond-mat_str-el |
63,65Cu Nuclear Resonance Study of the Coupled Spin Dimers and Chains
Compound Cu2Fe2Ge4O13: Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of Cu
have been measured in a coupled spin dimers and chains compound Cu2Fe2Ge4O13.
Cu NQR has also been measured in an isostructural material Cu2Sc2Ge4O13
including only spin dimers. Comparison of the temperature dependence of the
63Cu nuclear spin-lattice relaxation rate between the two compounds reveals
that the Fe chains in Cu2Fe2Ge4O13 do not change a spin gap energy of the Cu
dimers from that in Cu2Sc2Ge4O13, contributing additionally to the relaxation
rate at the Cu site. A modestly large internal field of 3.39 T was observed at
the Cu site in the antiferromagnetic state of Cu2Fe2Ge4O13 at 4.2 K, which is
partly because of quantum reduction of the ordered moment of a Cu atom. The
internal field and the ordered moment of Cu are noncollinear due to large
anisotropy of the hyperfine interaction at the Cu site. A model analysis of the
internal field based on the fourfold planar coordination of Cu suggests that a
3d hole of the Cu2+ ion is mainly in the d(x2-y2) orbital state. | cond-mat_str-el |
Arrested Kondo effect and hidden order in URu_2Si_2: Complex electronic matter exhibit subtle forms of self organization which are
almost invisible to the available experimental tools, but which have dramatic
physical consequences. One prominent example is provided by the actinide based
heavy fermion material URu_2Si_2. At high temperature, the U-5f electrons in
URu_2Si_2 carry a very large entropy. This entropy is released at 17.5K via a
second order phase transition to a state which remains shrouded in mystery, and
which was termed a "hidden order" state. Here we develop a first principles
theoretical method to analyze the electronic spectrum of correlated materials
as a function of the position inside the unit cell of the crystal, and use it
to identify the low energy excitations of the URu_2Si_2. We identify the order
parameter of the hidden order state, and show that it is intimately connected
with magnetism. We present first principles results for the temperature
evolution of the electronic states of the material. At temperature below 70K
U-5f electrons undergo a multichannel Kondo effect, which is arrested at low
temperature by the crystal field splitting. At lower temperatures, two broken
symmetry states emerge, characterized by a complex order parameter \psi. A real
$\psi$ describes the hidden order phase, and an imaginary \psi corresponds to
the large moment antiferromagnetic phase, thus providing a unified picture of
the two broken symmetry phases, which are realized in this material. | cond-mat_str-el |
Valence instability across magnetostructural transition in USb$_2$: We have performed pressure dependent X-ray diffraction and resonant X-ray
emission spectroscopy experiments on USb$_2$ to further characterize the AFM-FM
transition occurring near 8 GPa. We have found the magnetic transition
coincides with a tetragonal to orthorhombic transition resulting in a 17%
volume collapse as well as a transient $\textit{f}$-occupation enhancement.
Compared to UAs$_2$ and UAsS, USb$_2$ shows a reduced bulk modulus and
transition pressure and an increased volume collapse at the structural
transition. Except for an enhancement across the transition region, the
$\textit{f}$-occupancy decreases steadily from 1.96 to 1.75. | cond-mat_str-el |
Revisiting the electronic properties of dislocated graphene sheets: The interplay between topological defects, such as dislocations or
disclinations, and the electronic degrees of freedom in graphene has been
extensively studied. In the literature, for the study of this kind of problems,
it is in general used either a gauge theory or a curved spatial Riemannian
geometry approach, where, in the geometric case, the information about the
defects is contained in the metric and the spin-connection. However, these
topological defects can also be associated to a Riemann-Cartan geometry where
curvature and torsion plays an important role. In this article we study the
interplay between a wedge dislocations in a planar graphene sheet and the
properties of its electronic degrees of freedom. Our approach relies in its
relation with elasticity theory through the so called elastic-gauge, where
their typical coefficients, as for example the Poisson's ratio, appear directly
in the metric, and consequently also in the electronic spectrum. | cond-mat_str-el |
Exact SO(8) Symmetry in the Weakly-Interacting Two-Leg Ladder: A perturbative renormalization group analysis of interacting electrons on a
two-leg ladder reveals that at half-filling any weakly repulsive system scales
onto an exactly soluble Gross-Neveu model with a hidden SO(8) symmetry. The
half-filled ground state is a Mott insulator with short-range d-wave pair
correlations. We extract the exact energies, degeneracies, and quantum numbers
of *all* the low energy excited multiplets. One energy (mass) m octets contains
Cooper pair, magnon, and density-wave excitations, two more octets contain
single-particle excitations, and a mass \sqrt{3}m antisymmetric tensor contains
28 "bound states". Exact single-particle and spin gaps are found for the
lightly-doped (d-wave paired one-dimension Bose fluid) system. We also
determine the four other robust phases occuring at half-filling for partially
attractive interactions. All 5 phases have distinct SO(8) symmetries, but share
S.C. Zhang's SO(5) as a common subgroup. | cond-mat_str-el |
Multiple supersonic phase fronts launched at a complex-oxide
hetero-interface: Selective optical excitation of a substrate lattice can drive phase changes
across hetero-interfaces. This phenomenon is a non-equilibrium analogue of
static strain control in heterostructures and may lead to new applications in
optically controlled phase change devices. Here, we make use of time-resolved
non-resonant and resonant x-ray diffraction to clarify the underlying physics,
and to separate different microscopic degrees of freedom in space and time. We
measure the dynamics of the lattice and that of the charge disproportionation
in NdNiO3, when an insulator-metal transition is driven by coherent lattice
distortions in the LaAlO3 substrate. We find that charge redistribution
propagates at supersonic speeds from the interface into the NdNiO3 film,
followed by a sonic lattice wave. When combined with measurements of magnetic
disordering and of the metal-insulator transition, these results establish a
hierarchy of events for ultrafast control at complex oxide hetero-interfaces. | cond-mat_str-el |
Critical charge fluctuations in a pseudogap Anderson model: The Anderson impurity model with a density of states $\rho(\varepsilon)
\propto |\varepsilon|^r$ containing a power-law pseudogap centered on the Fermi
energy ($\varepsilon = 0$) features for $0<r<1$ a Kondo-destruction quantum
critical point (QCP) separating Kondo-screened and local-moment phases. The
observation of mixed valency in quantum critical $\beta$-YbAlB$_4$ has prompted
study of this model away from particle-hole symmetry. The critical spin
response associated with all Kondo destruction QCPs has been shown to be
accompanied, for $r=0.6$ and noninteger occupation of the impurity site, by a
divergence of the local charge susceptibility on both sides of the QCP. In this
work, we use the numerical renormalization-group method to characterize the
Kondo-destruction charge response using five critical exponents, which are
found to assume nontrivial values only for $0.55\lesssim r < 1$. For $0 < r
\lesssim 0.55$, by contrast, the local charge susceptibility shows no
divergence at the QCP, but rather exhibits nonanalytic corrections to a regular
leading behavior. Both the charge critical exponents and the previously
obtained spin critical exponents satisfy a set of scaling relations derived
from an ansatz for the free energy near the QCP. These critical exponents can
all be expressed in terms of just two underlying exponents: the
correlation-length exponent $\nu(r)$ and the gap exponent $\Delta(r)$. The
ansatz predicts a divergent local charge susceptibility for $\nu<2$, which
coincides closely with the observed range $0.55\lesssim r<1$. Many of these
results are argued to generalize to interacting QCPs that have been found in
other quantum impurity models. | cond-mat_str-el |
Phonon spectral function of the one-dimensional Holstein-Hubbard model: We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo
method to calculate the phonon spectral function of the one-dimensional
Holstein-Hubbard model at half-filling. Our results are consistent with a
soft-mode Peierls transition in the adiabatic regime, and the existence of a
central peak related to long-range order in the Peierls phase. We explain a
previously observed feature at small momenta in terms of a hybridization of
charge and phonon excitations. Tuning the system from a Peierls to a metallic
phase with a nonzero Hubbard interaction suppresses the central peak, but a
significant renormalization of the phonon dispersion remains. In contrast, the
dispersion is only weakly modified in the Mott phase. We discuss finite-size
effects, the relation to the dynamic charge structure factor, as well as
additional sum rules and their implications. Finally, we reveal the existence
of a discrete symmetry in a continuum field theory of the Holstein model, which
is spontaneously broken in the Peierls phase. | cond-mat_str-el |
Weak topological insulating phases of hard-core-bosons on the honeycomb
lattice: We study the phases of hard-core-bosons on a two-dimensional periodic
honeycomb lattice in the presence of an on-site potential with alternating sign
along the different y-layers of the lattice. Using quantum Monte Carlo
simulations supported by analytical calculations, we identify a weak
topological insulator, characterized by a zero Chern number but non-zero Berry
phase, which is manifested at either density 1/4 or 3/4, as determined by the
potential pattern. Additionally, a charge-density-wave insulator is observed at
1/2-filling, whereas the phase diagram at intermediate densities is occupied by
a superfluid phase. The weak topological insulator is further shown to be
robust against any amount of nearest-neighbor repulsion, as well as weak
next-nearest-neighbor repulsion. The experimental realization of our model is
feasible in an optical lattice setup. | cond-mat_str-el |
Extending Density Matrix Embedding: A Static Two-Particle Theory: We introduce Extended Density Matrix Embedding Theory (EDMET), a static
quantum embedding theory explicitly self-consistent with respect to local
two-body physics. This overcomes the biggest practical and conceptual
limitation of more traditional one-body embedding methods, namely the lack of
screening and treatment of longer-range interactions. This algebraic
zero-temperature embedding augments a local interacting cluster model with a
minimal number of bosons from a description of the full system correlations via
the random phase approximation, and admits an analytic approach to build a
self-consistent Coulomb-exchange-correlation kernel. For extended Hubbard
models with non-local interactions, this leads to the accurate description of
phase transitions, static quantities and dynamics. We also move towards ab
initio systems via the Parriser--Parr--Pople model of conjugated coronene
derivatives, finding good agreement with experimental optical gaps. | cond-mat_str-el |
The observation of a positive magnetoresistance and close correlation
among lattice, spin and charge around TC in antipervoskite SnCMn3: The temperature dependences of magnetization, electrical transport, and
thermal transport properties of antiperovskite compound SnCMn3 have been
investigated systematically. A positive magnetoresistance (~11%) is observed
around the ferrimagnetic-paramagnetic transition (TC ~ 280 K) in the field of
50 kOe, which can be attributed to the field-induced magnetic phase transition.
The abnormalities of resistivity, Seebeck coefficient, normal Hall effect and
thermal conductivity near TC are suggested to be associated with an abrupt
reconstruction of electronic structure. Further, our results indicate an
essential interaction among lattice, spin and charge degrees of freedom around
TC. Such an interaction among various degrees of freedom associated with sudden
phase transition is suggested to be characteristic of Mn-based antiperovskite
compounds. | cond-mat_str-el |
Phase transitions in topological lattice models via topological symmetry
breaking: We study transitions between phases of matter with topological order. By
studying these transitions in exactly solvable lattice models we show how
universality classes may be identified and critical properties described. As a
familiar example to elucidate our results concretely, we describe in detail a
transition between a fully gapped achiral 2D $p$-wave superconductor ($p+ip$
for pseudospin up/$p-ip$ for pseudospin down) to an $s$-wave superconductor
which we show to be in the 2D transverse field Ising universality class. | cond-mat_str-el |
Dispersionless spin waves in Gadolinium Gallium Garnet: We report the results of neutron scattering on a powder sample of Gd3Ga5O12
at high magnetic fields. We find that in high fields (B>1.8 T) the system is
not fully polarized, but has a small canting of the moments induced by the
dipolar interaction. We show that the degree of canting is accurately predicted
by the standard Hamiltonian which includes the dipolar interaction. The
inelastic scattering is dominated at large momentum transfers by a band of
almost dispersionless excitations. We show that these correspond to the spin
waves localized on ten site rings, expected for a system described by a nearest
neighbor interaction, and that the spectrum at high fields B>1.8 T is
well-described by a spin wave theory. The phase for fields <1.8 T is
characterized by an antiferromagnetic Bragg peak at (210) and an incommensurate
peak. | cond-mat_str-el |
Optimum ground states of generalized Hubbard models with next-nearest
neighbour interaction: We investigate the stability domains of ground states of generalized Hubbard
models with next-nearest neighbour interaction using the optimum groundstate
approach. We focus on the $\eta$-pairing state with momentum P=0 and the fully
polarized ferromagnetic state at half-filling. For these states exact lower
bounds for the regions of stability are obtained in the form of inequalities
between the interaction parameters. For the model with only nearest neighbour
interaction we show that the bounds for the stability regions can be improved
by considering larger clusters. Additional next-nearest neighbour interactions
can lead to larger or smaller stability regions depending on the parameter
values. | cond-mat_str-el |
High-resolution neutron depolarization microscopy of the ferromagnetic
transitions in Ni$_3$Al and HgCr$_2$Se$_4$ under pressure: We performed neutron imaging of ferromagnetic transitions in Ni$_3$Al and
HgCr$_2$Se$_4$ crystals. These neutron depolarization measurements revealed
bulk magnetic inhomogeneities in the ferromagnetic transition temperature with
spatial resolution of about 100~$\mu$m. To obtain such spatial resolution, we
employed a novel neutron microscope equipped with Wolter mirrors as a neutron
image-forming lens and a focusing neutron guide as a neutron condenser lens.
The images of Ni$_3$Al show that the sample does not homogeneously go through
the ferromagnetic transition; the improved resolution allowed us to identify a
distribution of small grains with slightly off-stoichiometric composition.
Additionally, neutron depolarization imaging experiments on the chrome spinel,
HgCr$_2$Se$_4$, under pressures up to 15~kbar highlight the advantages of the
new technique especially for small samples or sample environments with
restricted sample space. The improved spatial resolution enables one to observe
domain formation in the sample while decreasing the acquisition time despite
having a bulky pressure cell in the beam. | cond-mat_str-el |
Microscopic phase separation in triangular-lattice quantum spin magnet
kappa-(BEDT-TTF)2Cu2(CN)3 probed by muon spin relaxation: The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in
which antiferromagnetically-interacting S=1/2 spins are located on a nearly
equilateral triangular lattice attracts considerable interest both from
experimental and theoretical aspects, because a simple antiferromagnetic order
may be inhibited because of the geometrical frustration and hence an exotic
ground state is expected. Furthermore, recent two reports on the ground state
of this system have made it further intriguing by showing completely
controversial results; one indicates the gapless state and the other gapped. By
utilizing microscopic probe of muSR, we have investigated its spin dynamics
below 0.1 K, unveiling its microscopically phase separated ground state at zero
field. | cond-mat_str-el |
Destabilization of the Zhang-Rice singlet at optimal doping: The construction of the Zhang-Rice singlet is revisited in the light of
recent understanding of high-temperature superconductors at optimal doping. A
minimal local model is derived which contains the physical regime found
relevant for ARPES experiments, characterized by significant direct
oxygen-oxygen hopping. For the values of orbital parameters indicated by
experiment, the Zhang-Rice singlet is strongly mixed with a pure oxygen singlet
of the same symmetry. The destabilization of the Zhang-Rice ground state is due
to the oxygen singlet having twice as large a coherence factor with respect to
oxygen-oxygen hopping. An analogous quantum phase transition is identified in
the t-t'-J model. The orbital-antisymmetric copper-oxygen singlet is confirmed
to be irrelevant, as found originally. The usual symmetry analysis is extended
to include dynamical symmetries. | cond-mat_str-el |
Quadriexcitons and excitonic condensate in a symmetric electron-hole
bilayer with valley degeneracy: Using quantum Monte Carlo simulations we have mapped out the zero temperature
phase diagram of a symmetric electron-hole bilayer with twofold valley
degeneracy, as function of the interlayer distance $d$ and in-layer density
$n$. We find that the effect of the valley degeneracy is to shrink the region
of stability of the excitonic condensate, in favor of quadriexcitons at small
$d$ and of the four-component plasma at large $d$, with minor effects on the
value of the excitonic condensate fraction. The enclosure of the condensate in
a density window possibly explains why anomalous tunnelling conductivity,
interpreted as signature of condensation, is observed only between two finite
values of carrier density in graphene bilayers. Our phase diagram may provide
directions to select device parameters for future experiments. | cond-mat_str-el |
Thermoelectric power in one-dimensional Hubbard model: The thermoelectric power S is studied within the one-dimensional Hubbard
model using the linear response theory and the numerical exact-diagonalization
method for small systems. While both the diagonal and off-diagonal dynamical
correlation functions of particle and energy current are singular within the
model even at temperature T>0, S behaves regularly as a function of frequency
$\omega$ and T. Dependence on the electron density n below the half-filling
reveals a change of sign of S at n_0=0.73+/-0.07 due to strong correlations, in
the whole T range considered. Approaching half-filling S is hole-like and can
become large for U>>t although decreasing with T. | cond-mat_str-el |
Remarkably robust and correlated coherence and antiferromagnetism in
(Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$: We present magnetic susceptibility, resistivity, specific heat, and
thermoelectric power measurements on (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ single
crystals (0 $\leq x\leq$ 1). With La substitution, the antiferromagnetic
temperature $T_N$ is suppressed in an almost linear fashion and moves below
0.36 K, the base temperature of our measurements for $x>$ 0.8. Surprisingly, in
addition to robust antiferromagnetism, the system also shows low temperature
coherent scattering below $T_{coh}$ up to $\sim$ 0.9 of La, indicating a small
percolation limit $\sim$ 9$\%$ of Ce that separates a coherent regime from a
single-ion Kondo impurity regime. $T_{coh}$ as a function of magnetic field was
found to have different behavior for $x$< 0.9 and $x$> 0.9. Remarkably,
$(T_{coh})^2$ at $H$ = 0 was found to be linearly proportional to $T_N$. The
jump in the magnetic specific heat $\delta C_{m}$ at $T_N$ as a function of
$T_K/T_N$ for (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ follows the theoretical prediction
based on the molecular field calculation for the $S$ = 1/2 resonant level
model. | cond-mat_str-el |
Induced-Moment Weak Antiferromagnetism and Orbital Order on the
Itinerant-Localized Duality Model with Nested Fermi Surface: A Possible
Origin of Exotic Magnetism in URu${}_{2}$Si$_{2}$: The weak antiferromagnetism of URu${}_{2}$Si${}_{2}$ is discussed on the
basis of a duality model which takes into account salient features of both
itinerant fermions and "localized" component of spin degrees of freedom. The
problem is analyzed in the framework of induced-moment mechanism by taking a
singlet-singlet crystal field scheme together with the nesting property of
partial Fermi surface of itinerant fermions . It is shown that the extremely
small ordered moment $m$ of ${\cal O}$($10^{-2}$$\times$$\mu_{B}$) can be
compatible with the large specific-heat jump at the transition temperature
$T_{N}$. Analysis performed in the presence of external magnetic field shows
that the field dependence of $m$ in the limit T\to 0 and T_{N}$ do not scale
except very near the critical field B which is consistent with a recent
observation by Mentink. It is also shown that the antiferromagnetic magnetic
order gives rise to a tiny amount of antiferromagnetic orbital order of
f-electrons. | cond-mat_str-el |
Spin--orbital interaction for face-sharing octahedra: Realization of a
highly symmetric SU(4) model: Specific features of orbital and spin structure of transition metal compounds
in the case of the face-sharing MO$_6$ octahedra are analyzed. In this
geometry, we consider the form of the spin--orbital Hamiltonian for transition
metal ions with double ($e_g^{\sigma}$) or triple ($t_{2g}$) orbital
degeneracy. Trigonal distortions typical of the structures with face-sharing
octahedra lead to splitting of $t_{2g}$ orbitals into an $a_{1g}$ singlet and
$e_g^{\pi}$ doublet. For both doublets ($e_g^{\sigma}$ and $e_g^{\pi}$), in the
case of one electron or hole per site, we arrive at a symmetric model with the
orbital and spin interaction of the Heisenberg type and the Hamiltonian of
unexpectedly high symmetry: SU(4). Thus, many real materials with this geometry
can serve as a testing ground for checking the prediction of this interesting
theoretical model. We also compare general trends in spin--orbital
("Kugel--Khomskii") exchange interaction for three typical situations: those of
MO$_6$ octahedra with common corner, common edge, and the present case of
common face, which has not been considered yet. | cond-mat_str-el |
A Lattice Model of Intercalation: The thermodynamics of the lattice model of intercalation of ions in crystals
is considered in the mean field approximation. Pseudospin formalism is used for
the description of interaction of electrons with ions and the possibility of
hopping of intercalated ions between different positions is taken into account.
Phase diagrams are built. It is shown that the effective interaction between
intercalated ions can lead to phase separation or to appearance of modulated
phase (it depends on filling of the electron energy band). At high values of
the parameter of ion transfer the ionic subsystem can pass to the
superfluid-like state. | cond-mat_str-el |
Complex Quantum Phenomena in a Bilayered Calcium Ruthenate: Ca$_3$Ru$_2$O$_7$ undergoes an antiferromagnetic transition at
$T_{\text{N}}=56 $K, followed by a Mott-like (MI) transition at
$T_{\text{MI}}=48$ K. This nonmetallic ground state, with a charge gap of 0.1
eV, is suppressed by a highly anisotropic metamagnetic transition that leads to
a fully spin-polarized metallic state. We report the observation of
Shubnikov-de Haas oscillations in the \textit{gapped} state, colossal
magnetoresistance in the inter-plane resistivity with a large anisotropy
different from that observed in the magnetization, and non-Fermi liquid
behavior in the metallic state at high magnetic fields. | cond-mat_str-el |
Collective charge excitations and the metal-insulator transition in the
square lattice Hubbard-Coulomb model: In this article, we discuss the non-trivial collective charge excitations
(plasmons) of the extended square-lattice Hubbard model. Using a fully
non-perturbative approach, we employ the hybrid Monte Carlo algorithm to
simulate the system at half-filling. A modified Backus-Gilbert method is
introduced to obtain the spectral functions via numerical analytic
continuation. We directly compute the single-particle density of states which
demonstrates the formation of Hubbard bands in the strongly-correlated phase.
The momentum-resolved charge susceptibility is also computed on the basis of
the Euclidean charge density-density correlator. In agreement with previous
EDMFT studies, we find that at large strength of the electron-electron
interaction, the plasmon dispersion develops two branches. | cond-mat_str-el |
Fate of entanglement in magnetism under Lindbladian or non-Markovian
dynamics and conditions for their transition to Landau-Lifshitz-Gilbert
classical dynamics: It is commonly assumed in spintronics and magnonics that localized spins
within antiferromagnets are in the N\'{e}el ground state (GS), as well as that
such state evolves, when pushed out of equilibrium by current or external
fields, according to the Landau-Lifshitz-Gilbert (LLG) equation viewing
localized spins as classical vectors of fixed length. On the other hand, the
true GS of antiferromagnets is highly entangled, as confirmed by very recent
neutron scattering experiments witnessing their entanglement. Although GS of
ferromagnets is always unentangled, their magnonic low-energy excitation are
superpositions of many-body spin states and, therefore, entangled. In this
study, we initialize quantum Heisenberg ferro- or antiferromagnetic chains
hosing localized spins $S=1/2$, $S=1$ or $S=5/2$ into unentangled pure state
and then evolve them by quantum master equations (QMEs) of Lindblad or
non-Markovian type, derived by coupling localized spins to a bosonic bath (such
as due to phonons) or by using additional ``reaction coordinate'' in the latter
case. The time evolution is initiated by applying an external magnetic field,
and entanglement of time-evolving {\em mixed} quantum states is monitored by
computing its logarithmic negativity. We find that non-Markovian dynamics
maintains some degree of entanglement, which shrinks the length of the vector
of spin expectation values, thereby making the LLG equation inapplicable.
Conversely, Lindbladian (i.e., Markovian) dynamics ensures that entanglement
goes to zero, thereby enabling quantum-to-classical (i.e., to LLG) transition
in all cases -- $S=1/2$, $S=1$ and $S=5/2$ ferromagnet or $S=5/2$
antiferromagnet -- {\em except} for $S=1/2$ and $S=1$ antiferromagnet. We also
investigate the stability of entangled antiferromagnetic GS upon suddenly
coupling it to the bosonic bath. | cond-mat_str-el |
Spin-Fluctuation Drag Thermopower of Nearly Ferromagnetic Metals: We investigate theoretically the Seebeck effect in materials close to a
ferromagnetic quantum critical point to explain anomalous behaviour at low
temperatures. It is found that the main effect of spin fluctuations is to
enhance the coefficient of the leading $T$-linear term, and a quantum critical
behaviour characterized by a spin-fluctuation temperature appears in the
temperature dependence of correction terms as in the specific heat. | cond-mat_str-el |
Simulation of braiding anyons using Matrix Product States: Anyons exist as point like particles in two dimensions and carry braid
statistics which enable interactions that are independent of the distance
between the particles. Except for a relatively few number of models which are
analytically tractable, much of the physics of anyons remain still unexplored.
In this paper, we show how U(1)-symmetry can be combined with the previously
proposed anyonic Matrix Product States to simulate ground states and dynamics
of anyonic systems on a lattice at any rational particle number density. We
provide proof of principle by studying itinerant anyons on a one dimensional
chain where no natural notion of braiding arises and also on a two-leg ladder
where the anyons hop between sites and possibly braid. We compare the result of
the ground state energies of Fibonacci anyons against hardcore bosons and
spinless fermions. In addition, we report the entanglement entropies of the
ground states of interacting Fibonacci anyons on a fully filled two-leg ladder
at different interaction strength, identifying gapped or gapless points in the
parameter space. As an outlook, our approach can also prove useful in studying
the time dynamics of a finite number of nonabelian anyons on a finite
two-dimensional lattice. | cond-mat_str-el |
Double Exchange in a Magnetically Frustrated System: This work examines the magnetic order and spin dynamics of a double-exchange
model with competing ferromagnetic and antiferromagnetic Heisenberg
interactions between the local moments. The Heisenberg interactions are
periodically arranged in a Villain configuration in two dimensions with
nearest-neighbor, ferromagnetic coupling $J$ and antiferromagnetic coupling
$-\eta J$. This model is solved at zero temperature by performing a
$1/\sqrt{S}$ expansion in the rotated reference frame of each local moment.
When $\eta $ exceeds a critical value, the ground state is a magnetically
frustrated, canted antiferromagnet. With increasing hopping energy $t$ or
magnetic field $B$, the local moments become aligned and the ferromagnetic
phase is stabilized above critical values of $t$ or $B$. In the canted phase, a
charge-density wave forms because the electrons prefer to sit on lines of sites
that are coupled ferromagnetically. Due to a change in the topology of the
Fermi surface from closed to open, phase separation occurs in a narrow range of
parameters in the canted phase. In zero field, the long-wavelength spin waves
are isotropic in the region of phase separation. Whereas the average spin-wave
stiffness in the canted phase increases with $t$ or $\eta $, it exhibits a more
complicated dependence on field. This work strongly suggests that the jump in
the spin-wave stiffness observed in Pr$_{1-x}$Ca$_x$MnO$_3$ with $0.3 \le x \le
0.4$ at a field of 3 T is caused by the delocalization of the electrons rather
than by the alignment of the antiferromagnetic regions. | cond-mat_str-el |
A neutron scattering study of two-magnon states in the quantum magnet
copper nitrate: We report measurements of the two-magnon states in a dimerized
antiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using
inelastic neutron scattering we have measured the one and two magnon excitation
spectra in a large single crystal. The data are in excellent agreement with a
perturbative expansion of the alternating Heisenberg Hamiltonian from the
strongly dimerized limit. The expansion predicts a two-magnon bound state for q
~ (2n+1)pi*d which is consistent with the neutron scattering data. | cond-mat_str-el |
Collective spin excitations in a quantum spin ladder probed by
high-resolution Resonant Inelastic X-ray Scattering: We investigate magnetic excitations in the spin-ladder compound
Sr$_{14}$Cu$_{24}$O$_{41}$ using high-resolution Cu $L_3$-edge Resonant
Inelastic X-ray Scattering (RIXS). Our findings demonstrate that RIXS couples
to collective spin excitations from a quantum spin-liquid ground state. In
contrast to Inelastic Neutron Scattering (INS), the RIXS cross section changes
only moderately over the entire Brillouin Zone (BZ), revealing a high
sensitivity also at small momentum transfers. The two-triplon energy gap is
found to be $100\pm 30$ meV. Our results are supported by calculations within
an effective Hubbard model for a finite-size cluster. | cond-mat_str-el |
Competition between three-sublattice order and superfluidity in the
quantum 3-state Potts model of ultracold bosons and fermions on a square
optical lattice: We study a quantum version of the three-state Potts model that includes as
special cases the effective models of bosons and fermions on the square lattice
in the Mott insulating limit. It can be viewed as a model of quantum
permutations with amplitudes J_parallel and J_perp for identical and different
colors, respectively. For J_parallel=J_perp>0, it is equivalent to the SU(3)
Heisenberg model, which describes the Mott insulating phase of 3-color
fermions, while the parameter range J_perp<min(0,-J_parallel) can be realized
in the Mott insulating phase of 3-color bosonic atoms. Using linear flavor wave
theory, infinite projected entangled-pair states (iPEPS), and continuous-time
quantum Monte-Carlo simulations, we construct the full T=0 phase diagram, and
we explore the T>0 properties for J_perp<0. For dominant antiferromagnetic
J_parallel interactions, a three-sublattice long-range ordered stripe state is
selected out of the ground state manifold of the antiferromagnetic Potts model
by quantum fluctuations. Upon increasing |J_perp|, this state is replaced by a
uniform superfluid for J_perp<0, and by an exotic three-sublattice superfluid
followed by a two-sublattice superfluid for J_perp>0. The transition out of the
uniform superfluid (that can be realized with bosons) is shown to be a peculiar
type of Kosterlitz-Thouless transition with three types of elementary vortices. | cond-mat_str-el |
Pairing Correlations on t-U-J Ladders: Pairing correlations on generalized t-U-J two-leg ladders are reported. We
find that the pairing correlations on the usual t-U Hubbard ladder are
significantly enhanced by the addition of a nearest-neighbor exchange
interaction J. Likewise, these correlations are also enhanced for the t-J model
when the onsite Coulomb interaction is reduced from infinity. Moreover, the
pairing correlations are larger on a t-U-J ladder than on a t-Jeff ladder in
which Jeff has been adjusted so that the two models have the same spin gap at
half-filling. This enhancement of the pairing correlations is associated with
an increase in the pair-binding energy and the pair mobility in the t-U-J model
and point to the importance of the charge transfer nature of the cuprate
systems. | cond-mat_str-el |
Slow scrambling and hidden integrability in a random rotor model: We analyze the out-of-time-order correlation functions of a solvable model of
a large number, $N$, of $M$-component quantum rotors coupled by
Gaussian-distributed random, infinite-range exchange interactions. We focus on
the growth of commutators of operators at a temperature $T$ above the zero
temperature quantum critical point separating the spin-glass and paramagnetic
phases. In the large $N,~M$ limit, the squared commutators of the rotor fields
do not display any exponential growth of commutators, in spite of the absence
of any sharp quasiparticle-like excitations in the disorder-averaged theory. We
show that in this limit, the problem is integrable and point out interesting
connections to random-matrix theory. At leading order in $1/M$, there are no
modifications to the critical behavior but an irrelevant term in the
fixed-point action leads to a small exponential growth of the squared
commutator. We also introduce and comment on a generalized model involving
$p$-pair rotor interactions. | cond-mat_str-el |
Symmetry Enriched U(1) Topological Orders for Dipole-Octupole Doublets
on a Pyrochlore Lattice: Symmetry plays a fundamental role in our understanding of both conventional
symmetry breaking phases and the more exotic quantum and topological phases of
matter. We explore the experimental signatures of symmetry enriched U(1)
quantum spin liquids (QSLs) on the pyrochlore lattice. We point out that the Ce
local moment of the newly discovered pyrochlore QSL candidate
Ce$_2$Sn$_2$O$_7$, is a dipole-octupole doublet. The generic model for these
unusual doublets supports two distinct symmetry enriched U(1) QSL ground states
in the corresponding quantum spin ice regimes. These two U(1) QSLs are dubbed
dipolar U(1) QSL and octupolar U(1) QSL. While the dipolar U(1) QSL has been
discussed in many contexts, the octupolar U(1) QSL is rather unique. Based on
the symmetry properties of the dipole-octupole doublets, we predict the
peculiar physical properties of the octupolar U(1) QSL, elucidating the unique
spectroscopic properties in the externalmagnetic fields. We further predict the
Anderson-Higgs transition from the octupolar U(1) QSL driven by the external
magnetic fields. We identify the experimental relevance with the candidate
material Ce$_2$Sn$_2$O$_7$ and other dipole-octupole doublet systems. | cond-mat_str-el |
Anomalous heavy-fermion and ordered states in the filled skutterudite
PrFe4P12: Specific heat and magnetization measurements have been performed on
high-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to
study the high-field heavy-fermion state (HFS) and low-field ordered state
(ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields
applied along the <100> direction, the Kondo temperature of ~ 9 K and the
existence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear
Schottky contribution, which works as a highly-sensitive on-site probe for the
Pr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03
\mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in
the ODS is nonmagnetic and most probably of quadrupolar origin, combined with
other experimental facts. Significantly suppressed heavy-fermion behavior in
the ODS suggests a possibility that the quadrupolar degrees of freedom is
essential for the heavy quasiparticle band formation in the HFS. Possible
crystalline-electric-field level schemes estimated from the anisotropy in the
magnetization are consistent with this conjecture. | cond-mat_str-el |
Controlling structural distortion in the geometrically frustrated
layered cobaltate YBaCo4O7+δ by Fe substitution and its role on
magnetic correlations: Effects of Fe-substitution on the crystal structure and magnetic correlations
of the geometrically frustrated antiferromagnets YBaCo4-xFexO7+{\delta} (x = 0,
0.2, 0.4, 0.5, 0.6, and 0.8) have been studied by neutron diffraction,
M\"ossbauer spectroscopy, and ac susceptibility. The compounds
YBaCo4-xFexO7+{\delta} have a special layered-type crystal structure with an
alternating Kagom\'e (6c site) and triangular (2a site) layers along the c
axis. Fe3+ ions are found to be substituted at both the crystallographic 2a and
6c sites of Co ions. M\"ossbauer results show a high spin state of Fe3+ ions in
a tetrahedral coordination. A reduction in the distortion of the Kagom\'e
lattice has been observed with the Fe-substitution. The correlation length of
the short-range antiferromagnetic ordering decreases with the Fe-substitution.
The sharpness of the magnetic transition also decreases with the
Fe-substitution. | cond-mat_str-el |
Colossal Positive Magnetoresistance in a Doped Nearly Magnetic
Semiconductor: We report on a positive colossal magnetoresistance (MR) induced by
metallization of FeSb$_{2}$, a nearly magnetic or "Kondo" semiconductor with 3d
ions. We discuss contribution of orbital MR and quantum interference to
enhanced magnetic field response of electrical resistivity. | cond-mat_str-el |
Photoinduced Structural Phase Transitions in Polyacene: There exist two types of structural instability in polyacene: double bonds in
a cis pattern and those in a trans pattern. They are isoenergetic but
spectroscopically distinct. We demonstrate optical characterization and
manipulation of Peierls-distorted polyacene employing both correlated and
uncorrelated Hamiltonians. We clarify the phase boundaries of the cis- and
trans-distorted isomers, elucidate their optical-conductivity spectra, and then
explore their photoresponses. There occurs a photoinduced transformation in the
polyacene structure, but it is one-way switching: The trans configuration is
well convertible into the cis one, whereas the reverse conversion is much less
feasible. Even the weakest light irradiation can cause a transition of
uncorrelated electrons, while correlated electrons have a transition threshold
against light irradiation. | cond-mat_str-el |
Magnetic state dynamics in itinerant paramagnet UM3B2 (M= Co, Ir) probed
by 11B NMR: We have carried out the $^{11}$B NMR measurement on the itinerant
paramagnetic systems U$M_{3}$B$_{2}$ ($M =$ Co, Ir) to investigate the
low-dimensional characteristics of the $5f$-electrons due to the structural
anisotropy. The recent X-ray analysis suggests that UIr$_3$B$_2$ has a
different structure modulated from the ever-known superlattice. The azimuth
angle variation of NMR spectrum within the $ab$-plane clarified that B atoms
occupy the single site, and a certain ligands arrangement surrounding B atom
turns to the same orientation as the another one through the three- or six-fold
rotation around the c-axis. These results have been consistent with the X-ray
proposition. To evaluate the temperature ($T$) development of general
susceptibility ($\chi_{q,\omega}$), Knight shift and nuclear spin-lattice
relaxation rates measurements were performed and the similar variations of
$\chi_{q,\omega}$ were identified in both UCo$_{3}$B$_{2}$ and
UIr$_{3}$B$_{2}$. Above a crossover point defined as $T^{*}\simeq50$ K, the
evolution of $\chi_{q,\omega}$ is dominant at $q=0$, suggesting that
ferromagnetic correlations develop in high-$T$ regimes; meanwhile, below
$T^{*}$, the $q=0$ part in $\chi_{q,\omega}$ shows the saturation tendency, and
a different class of dispersion at finite-$q$ suddenly emerges. This particular
magnetic correlations are interpreted as the antiferromagnetic correlations,
and notable feature of the magnetic state dynamics in low-$T$ regimes is that
the antiferromagnetic correlations arise together with the ferromagnetic
component at the same time. The unique magnetic correlations obtained from NMR
experiment will be discussed by the possible low-dimensionality of
U$M_{3}$B$_{2}$ lattice. | cond-mat_str-el |
$Z_{2}$ fractionalized Chern/topological insulators in an exactly
soluble correlated model: In this paper we propose an exactly soluble model in two-dimensional
honeycomb lattice, from which two phases are found. One is the usual
Chern/topological insulating state and the other is an interesting $Z_2$
fractionalized Chern/topological insulator. While their bulk properties are
similar, the edge-states of physical electrons are quite different. The single
electron excitation of the former shows a free particle-like behavior while the
latter one is gapped, which provides a definite signature to identify the
fractionalized states. The transition between these two phases is found to fall
into the 3D Ising universal class. Significantly, near the quantum transition
point the physical electron in the edge-states shows strong Luttinger liquid
behavior. An extension to the interesting case of the square lattice is also
made. In addition, we also discuss some relationship between our exactly
soluble model and various Hubbard-like models existing in the literature. The
essential difference between the proposed $Z_{2}$ fractionalized Chern
insulator and the hotly pursued fractional Chern insulator is also pointed out.
The present work may be helpful for further study on the fractionalized
insulating phase and related novel correlated quantum phases. | cond-mat_str-el |
Magnetoelectric behavior from cluster multipoles in square cupolas:
Study of Sr(TiO)Cu$_4$(PO$_4$)$_4$ in comparison with Ba and Pb
isostructurals: We report our combined experimental and theoretical study of magnetoelectric
properties of an antiferromagnet Sr(TiO)Cu$_4$(PO$_4$)$_4$, in comparison with
the isostructurals Ba(TiO)Cu$_4$(PO$_4$)$_4$ and Pb(TiO)Cu$_4$(PO$_4$)$_4$. The
family of compounds commonly possesses a low-symmetric magnetic unit called the
square cupola, which is a source of magnetoelectric responses associated with
the magnetic multipoles activated under simultaneous breaking of spatial
inversion and time reversal symmetries. Measuring the full magnetization curves
and the magnetic-field profiles of dielectric constant for
Sr(TiO)Cu$_4$(PO$_4$)$_4$ and comparing them with the theoretical analyses by
the cluster mean-field theory, we find that the effective $S=1/2$ spin model,
which was used for the previous studies for Ba(TiO)Cu$_4$(PO$_4$)$_4$ and
Pb(TiO)Cu$_4$(PO$_4$)$_4$, well explains the experimental results by tuning the
model parameters. Furthermore, elaborating the phase diagram of the model, we
find that the square cupolas could host a variety of magnetic multipoles, i.e.,
monopole, toroidal moment, and quadrupole tensor, depending on the parameters
that could be modulated by deformations of the magnetic square cupolas. Our
results not only provide a microscopic understanding of the series of the
square cupola compounds, but also stimulate further exploration of the
magnetoelectric behavior arising from cluster multipoles harboring in
low-symmetric magnetic units. | cond-mat_str-el |
Observation of orbital order in the Van der Waals material 1T-TiSe2: Besides magnetic and charge order, regular arrangements of orbital occupation
constitute a fundamental order parameter of condensed matter physics. Even
though orbital order is difficult to identify directly in experiments, its
presence was firmly established in a number of strongly correlated,
three-dimensional Mott insulators. Here, reporting resonant X-ray scattering
experiments on the layered Van der Waals compound $1T$-TiSe$_2$, we establish
the emergence of orbital order in a weakly correlated, quasi-two-dimensional
material. Our experimental scattering results are consistent with
first-principles calculations that bring to the fore a generic mechanism of
close interplay between charge redistribution, lattice displacements, and
orbital order. It demonstrates the essential role that orbital degrees of
freedom play in TiSe$_2$, and their importance throughout the family of
correlated Van der Waals materials. | cond-mat_str-el |
Signatures of broken parity and time-reversal symmetry in generalized
string-net models: We study indicators of broken time-reversal and parity symmetries in gapped
topological phases of matter. We focus on phases realized by Levin-Wen
string-net models, and generalize the string-net model to describe phases which
break parity and time-reversal symmetries. We do this by introducing an extra
degree of freedom into the string-net graphical calculus, which takes the form
of a branch cut located at each vertex of the underlying string-net lattice. We
also work with string-net graphs defined on arbitrary (non-trivalent) graphs,
which reveals otherwise hidden information about certain configurations of
anyons in the string-net graph. Most significantly, we show that objects known
as higher Frobenius-Schur indicators can provide several efficient ways to
detect whether or not a given topological phase breaks parity or time-reversal
symmetry. | cond-mat_str-el |
The ubiquitous 1100 charge ordering in organic charge-transfer solids: Charge and spin-orderings in the 1/4-filled organic CT solids are of strong
interest, especially in view of their possible relations to organic
superconductivity. We show that the charge order (CO) in both 1D and 2D CT
solids is of the ...1100... type, in contradiction to mean field prediction of
>...1010... CO. We present detailed computations for metal-insulator and
magnetic insulator-insulator transitions in the theta-ET materials. Complete
agreement with experiments in several theta systems is found. Similar
comparisons between theory and experiments in TCNQ, TMTTF, TMTSF, and ET
materials prove the ubiquity of this phenomenon. | cond-mat_str-el |
Magnetism and metal-insulator transitions in the Rashba-Hubbard model: The nature of metal-insulator and magnetic transitions is still a subject
under intense debate in condensed matter physics. Amongst the many possible
mechanisms, the interplay between electronic correlations and spin-orbit
couplings is an issue of a great deal of interest, in particular when dealing
with quasi-2D compounds. In view of this, here we use a Hartree-Fock approach
to investigate how the Rashba spin-orbit coupling, $V_\text{SO}$, affects the
magnetic ordering provided by a Hubbard interaction, $U$, on a square lattice.
At half-filling, we have found a sequence of transitions for increasing
$V_\text{SO}$: from a Mott insulator to a metallic antiferromagnet, and then to
a paramagnetic Rashba metal. Also, our results indicate that the Rashba
coupling favors magnetic striped phases in the doped regime. By analyzing
spectral properties, we associate the rearrangement of the magnetic ordering
with the emerging chirality created by the spin-orbit coupling. Our findings
provide insights towards clarifying the competition between these tendencies. | cond-mat_str-el |
Physical realization of the four color problem in quantum systems: A multi-component electron model on a lattice is constructed whose ground
state exhibits a spontaneous ordering which follows the rule of map-coloring
used in the solution of the four color problem. The number of components is
determined by the Euler characteristics of a certain surface into which the
lattice is embedded. Combining the concept of chromatic polynomials with the
Heawood-Ringel-Youngs theorem, we derive an index theorem relating the
degeneracy of the ground state with a hidden topology of the lattice. The
system exhibits coloring transition and hidden-topological structure
transition. The coloring phase exhibits a topological order. | cond-mat_str-el |
Fermi arc criterion for surface Majorana modes in superconducting
time-reversal symmetric Weyl semimetals: Many clever routes to Majorana fermions have been discovered by exploiting
the interplay between superconductivity and band topology in metals and
insulators. However, realizations in semimetals remain less explored. We ask,
``under what conditions do superconductor vortices in time-reversal symmetric
Weyl semimetals -- three-dimensional semimetals with only time-reversal
symmetry -- trap Majorana fermions on the surface?'' If each constant-$k_{z}$
plane, where $z$ is the vortex axis, contains equal numbers of Weyl nodes of
each chirality, we predict a generically gapped vortex and derive a topological
invariant $\nu=\pm1$ in terms of the Fermi arc structure that signals the
presence or absence of surface Majorana fermions. In contrast, if certain
constant-$k_{z}$ planes contain a net chirality of Weyl nodes, the vortex is
gapless. We analytically calculate $\nu$ within a perturbative scheme and
provide numerical support with a lattice model. The criteria survive the
presence of other bulk and surface bands and yield phase transitions between
trivial, gapless and topological vortices upon tilting the vortex. We propose
Li(Fe$_{0.91}$Co$_{0.09}$)As and Fe$_{1+y}$Se$_{0.45}$Te$_{0.55}$ with broken
inversion symmetry as candidates for realizing our proposals. | cond-mat_str-el |
An Operational Definition of Topological Order: The unrivaled robustness of topologically ordered states of matter against
perturbations has immediate applications in quantum computing and quantum
metrology, yet their very existence poses a challenge to our understanding of
phase transitions. However, a comprehensive understanding of what actually
constitutes topological order is still lacking. Here we show that one can
interpret topological order as the ability of a system to perform topological
error correction. We find that this operational approach corresponding to a
measurable both lays the conceptual foundations for previous classifications of
topological order and also leads to a successful classification in the hitherto
inaccessible case of topological order in open quantum systems. We demonstrate
the existence of topological order in open systems and their phase transitions
to topologically trivial states. Our results demonstrate the viability of
topological order in nonequilibrium quantum systems and thus substantially
broaden the scope of possible technological applications. | cond-mat_str-el |
Disturbing the dimers: electron- and hole-doping in the intermetallic
insulator FeGa$_3$: Insulating FeGa$_3$ poses peculiar puzzles beyond the occurrence of an
electronic gap in an intermetallic compound. This Fe-based material has a very
distinctive structural characteristic with the Fe atoms occurring in dimers.
The insulating gap can be described comparably well in either the weakly
correlated limit or the strongly correlated limit within density functional
theory viewpoints, where the latter corresponds to singlet formation on the
Fe$_2$ dimers. Though most of the calculated occupied Wannier functions are an
admixture of Fe $3d$ and Ga $4s$ or $4p$ states, there is a single bonding-type
Wannier function per spin centered on each Fe$_2$ dimer. Density functional
theory methods have been applied to follow the evolution of the magnetic
properties and electronic spectrum with doping, where unusual behavior is
observed experimentally. Both electron and hole doping are considered, by Ge
and Zn on the Ga site, and by Co and Mn on the Fe site, the latter introducing
direct disturbance of the Fe$_2$ dimer. Results from weakly and strongly
correlated pictures are compared. Regardless of the method, magnetism including
itinerant phases appears readily with doping. The correlated picture suggests
that in the low doping limit Mn (for Fe) produces an in-gap hole state, while
Co (for Fe) introduces a localized electronic gap state. | cond-mat_str-el |
Magnetic excitations of the field induced states in BaCo2(AsO4)2 probed
by time-domain terahertz spectroscopy: Searching for Kitaev quantum spin liquid (QSL) is a fascinating and
challenging problem. Much effort has been devoted to honeycomb lattice
candidates with strong spin-orbit coupling in 5d-electron iridates and
4delectron RuCl3. Recently, theoretical studies suggested that the 3d7 Co-based
honeycomb materials with high spin state S=3/2 and effective orbital angular
momentum L=1 could also be promising candidates of Kitaev QSL. One of the
candidates, BaCo2(AsO4)2, was revisited recently. The long range magnetic order
in BaCo2(AsO4)2 can be suppressed by very weak in-plane magnetic field,
suggesting its proximity to Kitaev QSL. Here we perform time domain terahertz
spectroscopy measurement to study the magnetic excitations on BaCo2(AsO4)2. We
observe different magnon excitations upon increasing external magnetic field.
In particular, the system is easily driven to a field-polarized paramagnetic
phase, after the long range magnetic order is suppressed by a weak field Hc 2.
The spectra beyond Hc2 are dominated by single magnon and two magnon
excitations without showing signature of QSL. We discuss the similarity and
difference of the excitation spectra between BaCo2(AsO4)2 and the widely
studied Kitaev QSL candidate RuCl3. | cond-mat_str-el |
Phonon-induced topological insulation: We develop an approximate theory of phonon-induced topological insulation in
Dirac materials. In the weak coupling regime, long wavelength phonons may favor
topological phases in Dirac insulators with direct and narrow bandgaps. This
phenomenon originates from electron-phonon matrix elements, which change
qualitatively under a band inversion. A similar mechanism applies to weak
Coulomb interactions and spin-independent disorder; however, the influence of
these on band topology is largely independent of temperature. As applications
of the theory, we evaluate the temperature-dependence of the critical thickness
and the critical stoichiometric ratio for the topological transition in
CdTe/HgTe quantum wells and in BiTl(S$_{1-\delta}$Se$_{\delta})_2$,
respectively. | cond-mat_str-el |
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