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Topological Nature of Anomalous Hall Effect in Ferromagnet: The anomalous Hall effect in two-dimensional ferromagnets is discussed to be
the physical realization of the parity anomaly in (2+1)D, and the band crossing
points behave as the topological singularity in the Brillouin zone. This
appears as the sharp peaks and the sign changes of the transverse conductance
$\sigma_{xy}$ as a function of the Fermi energy and/or the magnetization. The
relevance to the experiments including the three dimensional systems is also
discussed. | cond-mat_str-el |
CsMn$_4$As$_3$: A layered tetragonal transition-metal pnictide compound
with antiferromagnetic ground state: We report the synthesis and properties of a new layered tetragonal ternary
compound CsMn$_4$As$_3$ (structure: KCu$_4$S$_3$-type, space group: $P4/mmm$,
No. 123 and $Z = 2$). The material is a small band-gap semiconductor and
exhibits an antiferromagnetic ground state associated with Mn spins. The
compound exhibits a signature of a distinct magnetic moment canting event at
150(5)~K with a canting angle of $\approx 0.3^{\circ}$. Although, some features
of the magnetic characteristics of this new compound are qualitatively similar
to those of the related BaMn$_2$As$_2$, the underlying Mn sublattices of the
two materials are quite different. While the Mn square-lattice layers in
BaMn$_2$As$_2$ are equally spaced along the $c$-direction with the interlayer
distance $d_{\rm L\,Ba} = 6.7341(4)$ Ang., the Mn sublattice forms bilayers in
CsMn$_4$As$_3$ with the interlayer distance within a bilayer $d_{\rm L\,Cs} =
3.1661(6)$ Ang. and the distance between the two adjacent bilayers $d_{\rm B} =
7.290(6)$ Ang. This difference in the Mn sublattice is bound to significantly
alter the energy balance between the $J_{1}$, $J_{2}$ and $J_{c}$ exchange
interactions within the J1-J2-Jc model compared to that in BaMn$_2$As$_2$ and
the other related 122 compounds including the well-known iron-arsenide
superconductor parent compound BaFe$_2$As$_2$. Owing to the novelty of its
transition metal sublattice, this new addition to the family of tetragonal
materials related to the iron-based superconductors brings prospects for doping
and pressure studies in the search of new superconducting phases as well as
other exciting correlated-electron properties. | cond-mat_str-el |
The phase diagram and the structure of CDW state in high magnetic field
in quasi-1D materials: mean-field approach: We develop the mean-field theory of a charge-density wave (CDW) state in
magnetic field and study the properties of this state below the transition
temperature. We show that the CDW state with shifted wave vector in high
magnetic field (CDW$_x$ phase) has at least double harmonic modulation on the
most part of the phase diagram. In the perfect nesting case the single harmonic
CDW state with shifted wave vector exists only in a very narrow region near the
tricritical point where the fluctuations are very strong. We show that the
transition from CDW$_0$ to CDW$_x$ state below the critical temperature is
accompanied by a jump of the CDW order parameter and of the wave vector rather
than by their continuous increase. This implies a first order transition
between these CDW states and explains the strong hysteresis accompanying this
transition in many experiments. We examine how the phase diagram changes in the
case of imperfect nesting. | cond-mat_str-el |
Chern and $Z_{2}$ topological insulating phases in perovskite-derived
$4d$ and $5d$ oxide buckled honeycomb lattices: Based on density functional theory calculations including a Coulomb repulsion
parameter $U$, we explore the topological properties of
(La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$(111) with $X=$ $4d$ and $5d$ cations. The
metastable ferromagnetic phases of LaTcO$_3$ and LaPtO$_3$ preserve P321
symmetry and emerge as Chern insulators (CI) with $C$=2 and 1 and band gaps of
41 and 38 meV at the lateral lattice constant of LaAlO$_3$, respectively. Berry
curvatures, spin textures as well as edge states provide additional insight
into the nature of the CI states. While for $X$=Tc the CI phase is further
stabilized under tensile strain, for $X$=Pd and Pt a site disproportionation
takes place when increasing the lateral lattice constant from $a_{\rm LAO}$ to
$a_{\rm LNO}$. The CI phase of $X$=Pt shows a strong dependence on the Hubbard
$U$ parameter with sign reversal for higher values associated with the change
of band gap opening mechanism. Parallels to the previously studied
($X_2$O$_3$)$_1$/(Al$_2$O$_3$)$_5$(0001) honeycomb corundum layers are
discussed. Additionally, non-magnetic systems with $X$=Mo and W are identified
as potential candidates for $Z_2$ topological insulators at $a_{\rm LAO}$ with
band gaps of 26 and 60 meV, respectively. The computed edge states and $Z_{2}$
invariants underpin the non-trivial topological properties. | cond-mat_str-el |
Constraint Effective Potential of the Magnetization in the Quantum XY
Model: Using an improved estimator in the loop-cluster algorithm, we investigate the
constraint effective potential of the magnetization in the spin $\tfrac{1}{2}$
quantum XY model. The numerical results are in excellent agreement with the
predictions of the corresponding low-energy effective field theory. After its
low-energy parameters have been determined with better than permille precision,
the effective theory makes accurate predictions for the constraint effective
potential which are in excellent agreement with the Monte Carlo data. This
shows that the effective theory indeed describes the physics in the low-energy
regime quantitatively correctly. | cond-mat_str-el |
Phonon anomalies due to stripe collective modes in high T_c cuprates: Phonon anomalies observed in various high $T_c$ cuprates by neutron
experiments are analyzed theoretically in terms of the stripe concept. The
phonon self-energy correction is evaluated by taking into account the charge
collective modes of stripes, giving rise to dispersion gap, or kink and shadow
phonon modes at twice the wave number of spin stripe. These features coincide
precisely with observations. The gapped branches of the phonon are found to be
in-phase and out-of-phase oscillations relative to the charge collective mode. | cond-mat_str-el |
Hole-pair hopping in arrangements of hole-rich/hole-poor domains in a
quantum antiferromagnet: We study the motion of holes in a doped quantum antiferromagnet in the
presence of arrangements of hole-rich and hole-poor domains such as the
stripe-phase in high-$T_C$ cuprates. When these structures form, it becomes
energetically favorable for single holes, pairs of holes or small bound-hole
clusters to hop from one hole-rich domain to another due to quantum
fluctuations. However, we find that at temperature of approximately 100 K, the
probability for bound hole-pair exchange between neighboring hole-rich regions
in the stripe phase, is one or two orders of magnitude larger than single-hole
or multi-hole droplet exchange. As a result holes in a given hole-rich domain
penetrate further into the antiferromagnetically aligned domains when they do
it in pairs. At temperature of about 100 K and below bound pairs of holes hop
from one hole-rich domain to another with high probability. Therefore our main
finding is that the presence of the antiferromagnetic hole-poor domains act as
a filter which selects, from the hole-rich domains (where holes form a
self-bound liquid), hole pairs which can be exchanged throughout the system.
This fluid of bound hole pairs can undergo a superfluid phase ordering at the
above mentioned temperature scale. | cond-mat_str-el |
Analysis of the magnetic response of the edge-sharing chain cuprate
Li$_2$CuO$_2$ within TMRG: It is widely accepted that the low-energy physics in edge-sharing cuprate
materials has one-dimensional (1D) character. The relevant model to study such
systems is believed to be the 1D extended Heisenberg model with ferromagnetic
nearest-neighbor (NN) interaction and antiferromagnetic next-nearest-neighbor
one. Thus far, however, theoretical studies of such materials have been
confined to the case of isotropic interactions. In the present work, we compare
the spin susceptibility of the 1D extended Heisenberg model with anisotropy in
the NN channel, obtained by means of the Transfer Matrix Renormalization Group
method, with that of the edge-sharing chain cuprate Li$_2$CuO$_2$. | cond-mat_str-el |
Functional renormalization group for frustrated magnets with nondiagonal
spin interactions: In the field of quantum magnetism, the advent of numerous spin-orbit assisted
Mott insulating compounds, such as the family of Kitaev materials, has led to a
growing interest in studying general spin models with non-diagonal interactions
that do not retain the SU(2) invariance of the underlying spin degrees of
freedom. However, the exchange frustration arising from these non-diagonal and
often bond-directional interactions for two- and three-dimensional lattice
geometries poses a serious challenge for numerical many-body simulation
techniques. In this paper, we present an extended formulation of the
pseudo-fermion functional renormalization group that is capable of capturing
the physics of frustrated quantum magnets with generic (diagonal and
off-diagonal) two-spin interaction terms. Based on a careful symmetry analysis
of the underlying flow equations, we reveal that the computational complexity
grows only moderately, as compared to models with only diagonal interaction
terms. We apply the formalism to a kagome antiferromagnet which is augmented by
general in-plane and out-of-plane Dzyaloshinskii-Moriya (DM) interactions, as
argued to be present in the spin liquid candidate material herbertsmithite. We
calculate the complete ground state phase diagram in the strength of in-plane
and out-of-plane DM couplings, and discuss the extended stability of the spin
liquid of the unperturbed kagome antiferromagnet in the presence of these
couplings. | cond-mat_str-el |
The stability of 3D skyrmions under mechanical stress studied via Monte
Carlo calculations: Using Monte Carlo (MC) simulations, we study the skyrmion
stability/instability as a response to uniaxial mechanical stresses. Skyrmions
emerge in chiral magnetic materials as a stable spin configuration under
external magnetic field $\vec{B}$ with the competition of ferromagnetic
interaction and Dzyaloshinskii-Moriya interaction (DMI) at low temperature $T$.
Skyrmion configurations are also known to be stable (unstable) under a
compressive stress applied parallel (perpendicular) to $\vec{B}$. To understand
the origin of such experimentally confirmed stability/instability, we use the
Finsler geometry modeling technique with a new degree of freedom for strains,
which plays an essential role in DMI being anisotropic. We find from MC data
that the area of the skyrmion state on the $B$-$T$ phase diagram increases
(decreases) depending on the direction of applied stresses, in agreement with
reported experimental results. This change in the area of the skyrmion state
indicates that skyrmions become more (less) stable if the tensile strain
direction is parallel (perpendicular) to $\vec{B}$. From the numerical data in
this paper, we find that the so-called magneto-elastic effect is suitably
implemented in the effective DMI theory with the strain degree of freedom
without complex magneto-elastic coupling terms for chiral magnetic materials.
This result confirms that experimentally-observed skyrmion stability and
instability are caused by DMI anisotropy. | cond-mat_str-el |
Luttinger liquid coupled to Bose-Einstein condensation reservoirs: We investigate the transport properties for a Luttinger liquid coupled to two
identical Bose-Einstein condensation reservoirs. Using the approach of equation
of motion for the Green function of the system, we find that the distance
between the two resonant transmission probability peaks of the system is
determined by the bosonic interaction strengths, and the sharpness of these
resonant peaks is mainly determined by the Rabi frequency and phase of the
Bose-Einstein condensation reservoir. These results for the proposed system
involving a Luttinger liquid may build a bridge between the controling
transport properties of cold atom in atom physics and the interacting boson
transport in low-dimensional condensed matter physics. | cond-mat_str-el |
Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6: We investigate both the longitudinal thermal conductivity ($\kappa_{xx}$) and
the planar thermal Hall conductivity ($\kappa_{xy}$) in the Kitaev spin liquid
candidate of Co-based honeycomb antiferromagnet Na$_2$Co$_2$TeO$_6$ in a
magnetic field ($B$) applied along the $a$ and $a^*$ axes. A finite
$\kappa_{xy}$ is resolved for both field directions in the antiferromagnetic
(AFM) phase below the N\'eel temperature of 27 K. The temperature dependence of
$\kappa_{xy}/T$ shows the emergence of topological bosonic excitations. In
addition, the field dependence of $\kappa_{xy}$ shows sign reversals at the
critical fields in the AFM phase, suggesting the changes in the Chern number
distribution of the topological magnons. Remarkably, a finite $\kappa_{xy}$ is
observed in $B \parallel a^*$ between the first-order transition field in the
AFM phase and the saturation field, which is prohibited in a disordered state
by the two-fold rotation symmetry around the $a^*$ axis of the honeycomb
lattice, showing the presence of a magnetically ordered state that breaks the
two-fold rotation symmetry. Our results demonstrate the presence of topological
magnons in this compound in the whole field range below the saturation field. | cond-mat_str-el |
Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions
from Charge-Order-Insulator to Metal in Quasi-Two-Dimensional Organic
Conductors: To elucidate different photoinduced melting dynamics of charge orders
observed in quasi-two-dimensional organic conductors $ \theta
$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ and $ \alpha $-(BEDT-TTF)$_2$I$_3$
[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], we theoretically study
photoinduced time evolution of charge and spin correlation functions on the
basis of exact many-electron wave functions coupled with classical phonons in
extended Peierls-Hubbard models on anisotropic triangular lattices. In both
salts, the so-called horizontal-stripe charge order is stabilized by
nearest-neighbor repulsive interactions and by electron-lattice interactions.
In $ \theta $-(BEDT-TTF)$_2$RbZn(SCN)$_4$ (abbreviated as $ \theta $-RbZn), the
stabilization energy due to lattice distortion is larger, so that larger
quantity of energy needs to be absorbed for the melting of the charge and
lattice orders. The photoinduced charge dynamics shows a complex behavior owing
to a substantial number of nearly degenerate eigenstates involved. This is
related to the high structural symmetry when the lattice is undistorted. In $
\alpha $-(BEDT-TTF)$_2$I$_3$ (abbreviated as $ \alpha $-I$_3$), the lattice
stabilization energy is smaller, and smaller quantity of energy is sufficient
to melt the charge and lattice orders leading to a metallic phase. The
photoinduced charge dynamics shows a sinusoidal oscillation. In $ \alpha
$-I$_3$, the low structural symmetry ensures nearly spin-singlet bonds between
hole-rich sites, where the spin correlation survives even after
photoexcitation. | cond-mat_str-el |
Spin-glass like dynamics of ferromagnetic clusters in
La$_{0.75}$Ba$_{0.25}$CoO$_3$: We report the magnetization study of the compound
La$_{0.75}$Ba$_{0.25}$CoO$_3$ where Ba$^{2+}$ doping is just above the critical
limit for percolation of ferromagnetic clusters. The field cooled (FC) and zero
field cooled (ZFC) magnetization exhibit a thermomagnetic irreversibility and
the ac susceptibility show a frequency dependent peak at the ferromagnetic
ordering temperature (T$_C$$\approx$203~K) of the clusters. These features
indicate about the presence of a non-equilibrium state below T$_C$. In the
non-equilibrium state, the dynamic scaling of the imaginary part of ac
susceptibility and the static scaling of the nonlinear susceptibility clearly
establish a spin-glass like cooperative freezing of ferromagnetic clusters at
200.9(2)~K. The existence of spin-glass like freezing of ferromagnetic clusters
is further substantiated by the ZFC aging and memory experiments. We also
observe certain dynamical features which are not present in a typical
spin-glass, such as, initial magnetization after ZFC aging first increases and
then decreases with the wait time and an imperfect recovery of relaxation in
negative temperature cycling experiments. This imperfect recovery transforms to
perfect recovery on concurrent field cycling. Our analysis suggests that these
additional dynamical features have their origin in inter-cluster exchange
interaction and cluster size distribution. The inter-cluster exchange
interaction above the magnetic percolation gives a superferromagnetic state in
some granular thin films but our results show the absence of typical
superferromagnetic like state in La$_{0.75}$Ba$_{0.25}$CoO$_3$. | cond-mat_str-el |
Theory of the field-revealed Kitaev spin liquid: Elementary excitations in highly entangled states such as quantum spin
liquids may exhibit exotic statistics, different from those obeyed by
fundamental bosons and fermions. Excitations called non-Abelian anyons are
predicted to exist in a Kitaev spin liquid - the ground state of an exactly
solvable model proposed by Kitaev almost a decade ago. A smoking-gun signature
of such non-Abelian anyons, namely a half-integer quantized thermal Hall
conductivity, was recently reported in $\alpha$-RuCl$_3$. While fascinating, a
microscopic theory for this phenomenon in $\alpha$-RuCl$_3$ remains elusive
because the pure Kitaev phase cannot capture these anyons appearing in an
intermediate magnetic field. Here we present a microscopic theory of the Kitaev
spin liquid emerging between the low- and high-field states. Essential to this
result is an antiferromagnetic off-diagonal symmetric interaction that permits
the Kitaev spin liquid to protrude from the pure ferromagnetic Kitaev limit
under a magnetic field. This generic model captures a field-revealed Kitaev
spin liquid, and displays strong anisotropy of field effects. A wide regime of
non-Abelian anyon Kitaev spin liquid is predicted when the magnetic field is
perpendicular to the honeycomb plane. | cond-mat_str-el |
Unconventional spin transport in strongly correlated kagome systems: Recent progress in material design enables the study of correlated,
low-temperature phases and associated anomalous transport in two-dimensional
kagome systems. Here, we show that unconventional spin transport can arise in
such systems even at elevated temperatures due to emergent dynamical
constraints. To demonstrate this effect, we consider a strong-coupling limit of
an extended Hubbard model on the kagome lattice with density of $2/3$. We
numerically investigate the charge and spin transport by a cellular automaton
circuit, allowing us to perform simulations on large systems to long times
while preserving the essential conservation laws. The charge dynamics reflects
the constraints and can be understood by a Gaussian field theory of a scalar
height field. Moreover, the system exhibits a hidden spin conservation law with
a dynamic sublattice structure, which enables additional slow relaxation
pathways for spin excitations. These features can be directly tested by
measuring the dynamic spin structure factor with neutron scattering. | cond-mat_str-el |
Dynamical Conductivity of Dirac Materials: For graphene (a Dirac material) it has been theoretically predicted and
experimentally observed that DC resistivity is proportional to $ T^4$ when the
temperature is much less than Bloch- Gr\"{u}neisen ($\Theta_{BG}$) temperature
and T linear in opposite case ($T>>\Theta_{BG}$). Going beyond the DC case, we
investigate the dynamical conductivity in graphene using the powerful method of
memory function formalism. In the DC (zero frequency regime) limit, we obtained
the above mention behavior which was previously obtained using the
Bloch-Boltzmann kinetic equation. In the finite frequency regime, we obtained
several new results: (1) the generalized Drude scattering rate, in the zero
temperature limit, shows $\omega^4 $ behavior at low frequencies ($\omega <<
k_B \Theta_{BG}/ \hbar$) and saturates at higher frequencies. We also observed
the Holstein Mechanism, however, with different power laws from that in the
case of metals; (2) At higher frequencies, $\omega>>k_B \Theta_{BG}/ \hbar$,
and higher temperatures $T>>\Theta_{BG}$, we observed that the generalized
Drude scattering rate is linear in temperature. In addition, several other
results are also obtained. With the experimental advancement of this field,
these results should be experimentally tested. | cond-mat_str-el |
Metal-Mott insulator interfaces: Motivated by the direct observation of electronic phase separation in
first-order Mott transitions, we model the interface between the
thermodynamically coexisting metal and Mott insulator. We show how to model the
required slab geometry and extract the electronic spectra. We construct an
effective Landau free energy and compute the variation of its parameters across
the phase diagram. Finally, using a linear mixture of the density and
double-occupancy, we identify a natural Ising order parameter which unifies the
treatment of the bandwidth and filling controlled Mott transitions. | cond-mat_str-el |
Magnetic Field Induced Exotic Phases in Isotropic Frustrated Spin-1/2
chain: The frustrated isotropic $J_1-J_2$ model with ferromagnetic $J_1$ and
anti-ferromagnetic $J_2$ interactions in presence of an axial magnetic field
shows many exotic phases, such as vector chiral and multipolar phases. The
existing studies of the phase boundaries of these systems are based on the
indirect evidences such as correlation functions {\it etc}. In this paper, the
phase boundaries of these exotic phases are calculated based on order
parameters and jumps in the magnetization. In the strong magnetic field, $Z_2$
symmetry is broken, therefore, order parameter of the vector chiral phase is
calculated using the broken symmetry states. Our results obtained using the
modified density matrix renormalization group and exact diagonalization
methods, suggest that the vector chiral phase exist only in narrow range of
parameter space $J_2/J_1$. | cond-mat_str-el |
Pomeranchuk instability in doped graphene: The density of states of graphene has Van Hove singularities that can be
reached by chemical doping and have already been explored in photoemission
experiments. We show that in the presence of Coulomb interactions the system at
the Van Hove filling is likely to undergo a Pomeranchuk instability breaking
the lattice point group symmetry. In the presence of an on--site Hubbard
interaction the system is also unstable towards ferromagnetism. We explore the
competition of the two instabilities and build the phase diagram. We also
suggest that, for doping levels where the trigonal warping is noticeable, the
Fermi liquid state in graphene can be stable up to zero temperature avoiding
the Kohn--Luttinger mechanism and providing an example of two dimensional Fermi
liquid at zero temperature. | cond-mat_str-el |
Accessing the spectral function in a current-carrying device: The presence of an electrical transport current in a material is one of the
simplest and most important realisations of non-equilibrium physics. The
current density breaks the crystalline symmetry and can give rise to dramatic
phenomena, such as sliding charge density waves [1], insulator-to-metal
transitions [2,3] or gap openings in topologically protected states [4]. Almost
nothing is known about how a current influences the electron spectral function,
which characterizes most of the solid's electronic, optical and chemical
properties. Here we show that angle-resolved photoemission spectroscopy with a
nano-scale light spot (nanoARPES) provides not only a wealth of information on
local equilibrium properties, but also opens the possibility to access the
local non-equilibrium spectral function in the presence of a transport current.
Unifying spectroscopic and transport measurements in this way allows
non-invasive local measurements of the composition, structure, many-body
effects and carrier mobility in the presence of high current densities. | cond-mat_str-el |
Parametrization of LSDA+$U$ for noncollinear magnetic configurations:
Multipolar magnetism in UO$_2$: To explore the formation of noncollinear magnetic configurations in materials
with strongly correlated electrons, we derive a noncollinear LSDA+$U$ model
involving only one parameter $U$, as opposed to the difference between the
Hubbard and Stoner parameters $U-J$. Computing $U$ in the constrained random
phase approximation, we investigate noncollinear magnetism of uranium dioxide
UO$_2$ and find that the spin-orbit coupling (SOC) stabilizes the 3$\textbf{k}$
ordered magnetic ground state. The estimated SOC strength in UO$_2$ is as large
as 0.73 eV per uranium atom, making spin and orbital degrees of freedom
virtually inseparable. Using a multipolar pseudospin Hamiltonian, we show how
octupolar and dipole-dipole exchange coupling help establish the 3$\textbf{k}$
magnetic ground state with canted ordering of uranium $f$-orbitals. The
cooperative Jahn-Teller effect does not appear to play a significant part in
stabilizing the noncollinear 3$\textbf{k}$ state, which has the lowest energy
even in an undistorted lattice. The choice of parameter $U$ in the LSDA+$U$
model has a notable quantitative effect on the predicted properties of UO$_2$,
in particular on the magnetic exchange interaction and, perhaps trivially, on
the band gap: The value of $U=3.46$ eV computed fully $ab$ $initio$ delivers
the band gap of 2.11~eV in good agreement with experiment, and a balanced
account of other pertinent energy scales. | cond-mat_str-el |
Characterizing The Many-Body Localization via Studying State Sensitivity
to Boundary Conditions: We introduce novel characterizations for many-body phase transitions between
delocalized and localized phases based on the system's sensitivity to boundary
conditions. In particular, we change boundary conditions from periodic to
antiperiodic and calculate shift in the system's energy and shifts in the
single-particle density matrix eigenvalues in the corresponding energy window.
We employ the typical model for studying MBL, a one-dimensional disordered
system of fermions with nearest-neighbor repulsive interaction where disorder
is introduced as randomness on on-site energies. By calculating numerically the
shifts in the system's energy and eigenvalues of the single-particle density
matrix, we observe that in the localized regime, both shifts are vanishing;
while in the extended regime, both shifts are on the order of the corresponding
level spacing. We also applied these characterizations of the phase transition
to the case of having next-nearest-neighbor interactions in addition to the
nearest-neighbor interactions, and studied its effect on the transition. | cond-mat_str-el |
Ab initio downfolding for electron-phonon coupled systems: constrained
density-functional perturbation theory (cDFPT): We formulate an ab initio downfolding scheme for electron-phonon coupled
systems. In this scheme, we calculate partially renormalized phonon frequencies
and electron-phonon coupling, which include the screening effects of
high-energy electrons, to construct a realistic Hamiltonian consisting of
low-energy electron and phonon degrees of freedom. We show that our scheme,
which we call constrained density-functional perturbation theory (cDFPT), can
be implemented by slightly modifying the conventional DFPT, which is one of the
standard methods to calculate phonon properties from first principles. Our
scheme can be applied to various phonon-related problems, such as
superconductivity, electron and thermal transport, thermoelectricity,
piezoelectricity, dielectricity and multiferroicity. We believe that the cDFPT
provides a firm basis for the understanding of the role of phonons in strongly
correlated materials. Here, we apply the scheme to the fullerene
superconductors and discuss how the realistic low-energy Hamiltonian is
constructed. | cond-mat_str-el |
Gutzwiller-correlated wave functions for degenerate bands: exact results
in infinite dimensions: We introduce Gutzwiller-correlated wave functions for the variational
investigation of general multi-band Hubbard models. We set up a diagrammatic
formalism which allows us to evaluate analytically ground-state properties in
the limit of infinite spatial dimensions. In this limit recent results obtained
within the Gutzwiller approximation are seen to become exact for these wave
functions. We further show that the Slave Boson mean-field theory for
degenerate bands becomes variationally controlled at zero temperature in
infinite dimensions. Lastly, we briefly comment on the variational approach to
the Anderson transition in strongly correlated electron systems. | cond-mat_str-el |
Field-induced intermediate phase in $α$-RuCl$_3$: Non-coplanar
order, phase diagram, and proximate spin liquid: Frustrated magnets with strong spin-orbit coupling promise to host
topological states of matter, with fractionalized excitations and emergent
gauge fields. Kitaev's proposal for a honeycomb-lattice Majorana spin liquid
has triggered an intense search for experimental realizations, with
bond-dependent Ising interaction being the essential building block. A prime
candidate is $\alpha$-RuCl$_3$ whose phase diagram in a magnetic field is,
however, not understood to date. Here we present conclusive experimental
evidence for a novel field-induced ordered phase in $\alpha$-RuCl$_3$,
sandwiched between the zigzag and quantum disordered phases at low and high
fields, respectively. We provide a detailed theoretical study of the relevant
effective spin model which we show to display a field-induced intermediate
phase as well. We fully characterize the intermediate phase within this model,
including its complex spin structure, and pinpoint the parameters relevant to
$\alpha$-RuCl$_3$ based on the experimentally observed critical fields. Most
importantly, our study connects the physics of $\alpha$-RuCl$_3$ to that of the
Kitaev-$\Gamma$ model, which displays a quantum spin liquid phase in zero
field, and hence reveals the spin liquid whose signatures have been detected in
a variety of dynamical probes of $\alpha$-RuCl$_3$. | cond-mat_str-el |
Entanglement entropy of the $ν=1/2$ composite fermion non-Fermi liquid
state: The so-called ``non-Fermi liquid'' behavior is very common in strongly
correlated systems. However, its operational definition in terms of ``what it
is not'' is a major obstacle against theoretical understanding of this
fascinating correlated state. Recently there has been much interest in
entanglement entropy as a theoretical tool to study non-Fermi liquids. So far
explicit calculations have been limited to models without direct experimental
realizations. Here we focus on a two dimensional electron fluid under magnetic
field and filling fraction $\nu=1/2$, which is believed to be a non-Fermi
liquid state. Using the composite fermion (CF) wave-function which captures the
$\nu=1/2$ state very accurately, we compute the second R\'enyi entropy using
variational Monte-Carlo technique and an efficient parallel algorithm. We find
the entanglement entropy scales as $L\log L$ with the length of the boundary
$L$ as it does for free fermions, albeit with a pre-factor twice that of the
free fermion. We contrast the results against theoretical conjectures and
discuss the implications of the results. | 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 |
Evolution of magnetism in Pd-substituted Ce$_2$RhIn$_8$ single crystals: The evolution of magnetism and superconductivity in
Ce$_2$Rh$_{1-x}$Pd$_x$In$_8$ solid solutions has been studied within the entire
concentration range by means of thermodynamic and magnetic measurements at
ambient pressure and at temperatures between 0.35 K and room temperature. For
this purpose, single crystals with Pd concentrations x = 0, 0.10, 0.15, 0.30,
0.45, 0.55, 0.85 and 1 have been grown from In self-flux and characterized by
x-ray diffraction and microprobe analysis. Starting from the antiferromagnet
Ce$_2$RhIn$_8$, the N\'eel temperature gradually decreases with increasing Pd
concentration and the antiferromagnetism has disappeared for $x \ge 0.45$.
Superconductivity has been observed only for Ce$_2$PdIn$_8$. | cond-mat_str-el |
Interpretation of high-pressure experiments on FeAs superconductors: In two recent articles (cond-mat/0606177 and arXiv:0804.1615), we have
suggested a unified theory of superconductivity based on the real-space
spin-parallel electron pairing and superconducting mechanism and have shown
that the stable hexagonal and tetragonal vortex lattices (the optimal doping
phases) can be expected in the newly discovered LaO{1-x}F{x}FeAs
(x0=1/7=0.1428) and SmO{1-x}F{x}FeAs (x0=1/6=0.1667), respectively. In this
paper, we present a theoretical study of the effects of hydrostatic and
anisotropic pressure on the superconducting transition temperature Tc of the
Fe-based layered superconductors based on the above mentioned theory. Our
results indicate a strong doping-dependent pressure effects on the Tc of this
compound system. Under high hydrostatic pressure, we find that dTc/dP is
negative when x>x0 (the so-called overdoped region) and is positive when x<x0
(the so-called underdoped region). Qualitatively, our finding is in good
agreement with the existing experimental data in LaO{1-x}F{x}FeAs (x=0.11<1/7)
(arXiv:0803.4266) and SmO{1-x}F{x}FeAs (x=0.13<1/6 and x=0.3>1/6)
(arXiv:0804.1582). Furthermore, Tc of both overdoped and underdoped samples
shows an increase with uniaxial pressure in the charge stripe direction and a
decrease with pressure in the direction perpendicular to the stripes. We
suggest that the mechanism responsible for the pressure effect is not specific
to the iron-based family and it may also be applicable to other superconducting
materials. | cond-mat_str-el |
Integrable Extended Hubbard Hamiltonians from Symmetric Group Solutions: We consider the most general form of extended Hubbard Hamiltonian conserving
the total spin and number of electrons, and find all the 1-dimensional
completely integrable models which can be derived from first degree polynomial
solution of the Yang-Baxter equation. It is shown that such models are 96. They
are identified with the 16-dimensional representation of a new class of
solutions of symmetric group relations, acting as generalized permutators. As
particular examples, the EKS and some other known models are obtained. | cond-mat_str-el |
Topological Magnons: A Review: At sufficiently low temperatures magnetic materials often enter a correlated
phase hosting collective, coherent magnetic excitations such as magnons or
triplons. Drawing on the enormous progress on topological materials of the last
few years, recent research has led to new insights into the geometry and
topology of these magnetic excitations. Berry phases associated to magnetic
dynamics can lead to observable consequences in heat and spin transport while
analogues of topological insulators and semimetals can arise within magnon band
structures from natural magnetic couplings. Magnetic excitations offer a
platform to explore the interplay of magnetic symmetries and topology, to drive
topological transitions using magnetic fields. examine the effects of
interactions on topological bands and to generate topologically protected spin
currents at interfaces. In this review, we survey progress on all these topics,
highlighting aspects of topological matter that are unique to magnon systems
and the avenues yet to be fully investigated. | cond-mat_str-el |
Pair Superfluid and Supersolid of Correlated Hard-Core Bosons on a
Triangular Lattice: We have systematically studied the hard-core Bose-Hubbard model with
correlated hopping on a triangular lattice using density-matrix renormalization
group method. A rich ground state phase diagram is determined. In this phase
diagram there is a supersolid phase and a pair superfluid phase due to the
interplay between the ordinary frustrated boson hopping and an unusual
correlated hopping. In particular, we find that the quantum phase transition
between the supersolid phase and the pair superfluid phase is continuous. | cond-mat_str-el |
Coulomb matrix elements in multi-orbital Hubbard models: Coulomb matrix elements are needed in all studies in solid-state theory that
are based on Hubbard-type multi-orbital models. Due to symmetries, the matrix
elements are not independent. We determine a set of independent Coulomb
parameters for a $d$-shell and a $f$-shell and all point groups with up to $16$
elements ($O_h$, $O$, $T_d$, $T_h$, $D_{6h}$, and $D_{4h}$). Furthermore, we
express all other matrix elements as a function of the independent Coulomb
parameters. Apart from the solution of the general point-group problem we
investigate in detail the spherical approximation and first-order corrections
to the spherical approximation. | cond-mat_str-el |
The electronic structure of La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$
probed by high- and low-energy angle-resolved photoelectron spectroscopy:
evolution with probing depth: We present angle-resolved photoelectron spectroscopy data probing the
electronic structure of the Nd-substituted high-$T_c$ cuprate
La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ (Nd-LSCO). Data have been acquired at
low and high photon energies, $h\nu$ = 55 and 500 eV, respectively. Earlier
comparable low-energy studies of La$_{1.4-x}$Nd$_{0.6}$Sr$_{x}$CuO$_4$ ($x =
0.10, 0.12, 0.15$) have shown strongly suppressed photoemission intensity, or
absence thereof, in large parts of the Brillouin zone. Contrary to these
findings we observe spectral weight at all points along the entire Fermi
surface contour at low and high photon energies. No signs of strong charge
modulations are found. At high photon energy, the Fermi surface shows obvious
differences in shape as compared to the low-energy results presented here and
in similar studies. The observed difference in shape and the high
bulk-sensitivity at this photon energy suggest intrinsic electronic structure
differences between the surface and bulk regions. | cond-mat_str-el |
Terahertz Electrodynamics of Mixed-Valent YbAl$_3$ and LuAl$_3$ Thin
Films: We present THz measurements of thin films of mixed-valent YbAl$_3$ and its
structural analogue LuAl$_3$. Combined with traditional Fourier transform
infrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study
the low-frequency transport of these materials. We find that LuAl$_3$
demonstrates conventional Drude transport whereas at low temperatures YbAl$_3$
demonstrates a sharply renormalized Drude peak and a mid-infrared (MIR) peak in
the conductivity, indicative of the formation of a heavy Fermi liquid. In
YbAl$_3$ the extended Drude framework shows a consistency of the scattering
rate with Fermi-liquid behavior below $T < 40$ K and a moderate mass
enhancement. While a $\omega^2$ Fermi liquid-like frequency dependence is not
clearly exhibited, the temperature dependence of the Drude scattering rate and
effective mass is consistent with the formation of a low-temperature moderately
heavy Fermi liquid, albeit one with a smaller mass than observed in single
crystals. The extended Drude analysis also supports a slow crossover between
the Fermi liquid state and the normal state in YbAl$_3$. | cond-mat_str-el |
Magnetic bubble crystal in tetragonal magnets: A magnetic bubble crystal is a two-dimensional soliton lattice consisting of
multiple spin density waves similar to a magnetic skyrmion crystal.
Nevertheless, the emergence of the bubble crystal with a collinear spin texture
is rare compared to that of the skyrmion crystal with a noncoplanar spin
texture. Here we theoretically report the stabilization mechanisms of the
bubble crystal in tetragonal magnets. By performing numerical calculations
based on an efficient steepest descent method for an effective spin model with
magnetic anisotropy and multiple spin interactions in momentum space on a
two-dimensional square lattice, we construct magnetic field-temperature phase
diagrams for various sets of model parameters. We find that the bubble crystal
is stabilized at finite temperatures near the skyrmion crystal by an easy-axis
anisotropic two-spin interaction. Through a detailed analysis, we also show
that the high-harmonic wave-vector interaction and the biquadratic interaction
play important roles in the stability of the bubble crystal. Our results
indicate a close relationship between the bubble crystal and the skyrmion
crystal in terms of the stabilization mechanisms, which suggests the
possibility of the bubble crystal in the skyrmion-hosting materials by
controlling the easy-axis magnetic anisotropy through external and/or chemical
pressure. | cond-mat_str-el |
The Square-Lattice Heisenberg Antiferromagnet at Very Large Correlation
Lengths: The correlation length of the square-lattice spin-1/2 Heisenberg
antiferromagnet is studied in the low-temperature (asymptotic-scaling) regime.
Our novel approach combines a very efficient loop cluster algorithm --
operating directly in the Euclidean time continuum -- with finite-size scaling.
This enables us to probe correlation lengths up to $\xi \approx 350,000$
lattice spacings -- more than three orders of magnitude larger than any
previous study. We resolve a conundrum concerning the applicability of
asymptotic-scaling formulae to experimentally- and numerically-determined
correlation lengths, and arrive at a very precise determination of the
low-energy observables. Our results have direct implications for the
zero-temperature behavior of spin-1/2 ladders. | cond-mat_str-el |
Singlet Ground State and Magnetization Plateaus in Ba$_3$Mn$_2$O$_8$: Magnetic susceptibility and the magnetization process have been measured in
\green polycrystal. In this compound, the magnetic manganese ion exists as
Mn$^{5+}$ in a tetrahedral environment, and thus the magnetic interaction can
be described by an S=1 Heisenberg model. The ground state was found to be a
spin singlet with an excitation gap $\Delta/k_{\rm B}=11.2$ K. Magnetization
plateaus were observed at zero and at half of the saturation magnetization.
These results indicate that the present system can be represented by a coupled
antiferromagnetic dimer model. | cond-mat_str-el |
Two-stage spin-flop transitions in S = 1/2 antiferromagnetic spin chain
BaCu_2Si_2O_7: Two-stage spin-flop transitions are observed the in quasi-one-dimensional
antiferromagnet, BaCu${}_2$Si${}_2$O${}_7$. A magnetic field applied along the
easy axis induces a spin-flop transition at 2.0 T followed by a second
transition at 4.9 T. The magnetic susceptibility indicates the presence of
Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain
neighboring spins. We discuss a possible mechanism whereby the geometrical
competition between DM and interchain interactions, as discussed for the
two-dimensional antiferromagnet La${}_2$CuO${}_4$, causes the two-stage
spin-flop transitions. | cond-mat_str-el |
Curvature induced drift and deformation of magnetic skyrmions:
comparison of ferro- and antiferromagnetic cases: The influence of the geometrical curvature of chiral magnetic films on the
static and dynamic properties of hosted skyrmions are studied theoretically. We
predict the effects of the curvature-induced drift of skyrmions under the
action of the curvature gradients without any external stimuli. The strength of
the curvature-induced driving force essentially depends on the skyrmion type,
Neel or Bloch, while the trajectory of motion is determined by the type of
magnetic ordering: ferro- or antiferromagnetic. When moving on the surface,
skyrmions undergo deformations that depend on the type of skyrmion. In the
small-curvature limit, using the collective-variable approach we show, that the
driving force acting on a Neel skyrmion is linear in the gradient of the mean
curvature. The driving acting on a Bloch skyrmion is much smaller: it is
proportional to the product of the mean curvature and its gradient. In contrast
to the fast Neel skyrmions, the dynamics of the slow Bloch skyrmions is
essentially affected by the skyrmion profile deformation. For the sake of
simplicity, we restrict ourselves to the case of zero Gaussian curvature and
consider cylindrical surfaces of general type. Equations of motion for
ferromagnetic and antiferromagnetic skyrmions in curved magnetic films are
obtained in terms of collective variables. All analytical predictions are
confirmed by numerical simulations. | cond-mat_str-el |
An Ising model on a 3D honeycomb zigzag-ladder lattice: a solution to
the ground-state problem and application to the SrRE$_2$O$_4$ and
BaRE$_2$O$_4$ magnets: An exact solution (incomplete) of the ground-state problem for an Ising model
in an external field on a 3D honeycomb zigzag-ladder lattice with two types of
sites is found. It is shown that the geometrical frustration due to the
presence of triangle elements leads to the emergence of a variety of magnetic
phases. The majority of these are partially disordered (highly degenerate). The
theoretical results are used to explain the sequence of experimentally observed
phase transitions in the honeycomb zigzag-ladder magnets and to predict the
appearance of new phases. | cond-mat_str-el |
Photoemission spectra of LaMnO3 controlled by orbital excitations: We investigate the spectral function of a hole moving in the orbital-ordered
ferromagnetic planes of LaMnO$_3$, and show that it depends critically on the
type of orbital ordering. While the hole does not couple to the spin
excitations, it interacts strongly with the excitations of $e_g$ orbitals
(orbitons), leading to new type of quasiparticles with a dispersion on the
orbiton energy scale and with strongly enhanced mass and reduced weight.
Therefore we predict a large redistribution of spectral weight with respect to
the bands found in local density approximation (LDA) or in LDA+U. | cond-mat_str-el |
Finite-temperature dynamic structure factor of the spin-1 XXZ chain with
single-ion anisotropy: Improving matrix-product state techniques based on the purification of the
density matrix, we are able to accurately calculate the finite-temperature
dynamic response of the infinite spin-1 XXZ chain with single-ion anisotropy in
the Haldane, large-$D$ and antiferromagnetic phases. Distinct thermally
activated scattering processes make a significant contribution to the spectral
weight in all cases. In the Haldane phase intraband magnon scattering is
prominent, and the onsite anisotropy causes the magnon to split into singlet
and doublet branches. In the large-$D$ phase response, the intraband signal is
separated from an exciton-antiexciton continuum. In the antiferromagnetic
phase, holons are the lowest-lying excitations, with a gap that closes at the
transition to the Haldane state. At finite temperatures, scattering between
domain-wall excitations becomes especially important and strongly enhances the
spectral weight for momentum transfer $\pi$. | cond-mat_str-el |
Local atomic and magnetic structure of multiferroic (Sr,Ba)(Mn,Ti)O$_3$: We present a detailed study of the local atomic and magnetic structure of the
type-I multiferroic perovskite system (Sr,Ba)(Mn,Ti)O$_3$ using x-ray and
neutron pair distribution function (PDF) analysis, polarized neutron
scattering, and muon spin relaxation ($\mu$SR) techniques. The atomic PDF
analysis reveals widespread nanoscale tetragonal distortions of the crystal
structure even in the paraelectric phase with average cubic symmetry,
corresponding to incipient ferroelectricity in the local structure. Magnetic
PDF analysis, polarized neutron scattering, and $\mu$SR likewise confirm the
presence of short-range antiferromagnetic correlations in the paramagnetic
state, which grow in magnitude as the temperature approaches the magnetic
transition. We show that these short-range magnetic correlations coincide with
a reduction of the tetragonal (i.e. ferroelectric) distortion in the average
structure, suggesting that short-range magnetism can play an important role in
magnetoelectric and/or magnetostructural phenomena even without genuine
long-range magnetic order. The reduction of the tetragonal distortion scales
linearly with the local magnetic order parameter, pointing to spontaneous
linear magnetoelectric coupling in this system. These findings provide greater
insight into the multiferroic properties of (Sr,Ba)(Mn,Ti)O$_3$ and demonstrate
the importance of investigating the local atomic and magnetic structure to gain
a deeper understanding of the intertwined degrees of freedom in multiferroics. | cond-mat_str-el |
A robust but disordered collapsed-volume phase in a cerium alloy under
the application of pulsed magnetic fields: We report synchrotron x-ray powder diffraction measurements of
Ce0.8La0.1Th0.1 subject to pulsed magnetic fields as high as 28 Tesla. This
alloy is known to exhibit a continuous volume collapse on cooling at ambient
pressure, which is a modification of the gamma -> alpha transition in elemental
cerium. Recently, it has been suggested on the basis of field-cooled
resistivity and pulsed field magnetization measurements that the volume
collapse in this alloy can be suppressed by the application of magnetic fields.
Conversely, our direct diffraction measurements show a robust collapsed phase,
which persists in magnetic fields as high as 28 Tesla. We also observe
nanoscale disorder in the collapsed phase, which increasingly contaminates the
high temperature phase on thermal cycling. | cond-mat_str-el |
The Effect of Randomness on the Mott State: We reinvestigate the competition between the Mott and the Anderson insulator
state in a one-dimensional disordered fermionic system by a combination of
instanton and renormalization group methods. Tracing back both the
compressibility and the ac-conductivity to a vanishing kink energy of the
electronic displacement field we do not find any indication for the existence
of an intermediate (Mott glass) phase. | cond-mat_str-el |
Magnetic effects at the interface between nonmagnetic oxides: The electronic reconstruction at the interface between two insulating oxides
can give rise to a highly-conductive interface. In analogy to this remarkable
interface-induced conductivity we show how, additionally, magnetism can be
induced at the interface between the otherwise nonmagnetic insulating
perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the
interface is found, together with a logarithmic temperature dependence of the
sheet resistance. At low temperatures, the sheet resistance reveals magnetic
hysteresis. Magnetic ordering is a key issue in solid-state science and its
underlying mechanisms are still the subject of intense research. In particular,
the interplay between localized magnetic moments and the spin of itinerant
conduction electrons in a solid gives rise to intriguing many-body effects such
as Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, the Kondo effect, and
carrier-induced ferromagnetism in diluted magnetic semiconductors. The
conducting oxide interface now provides a versatile system to induce and
manipulate magnetic moments in otherwise nonmagnetic materials. | cond-mat_str-el |
Global phase diagram of the spin-1 antiferromagnet with uniaxial
anisotropy on the kagome lattice: The phase diagram of the XXZ spin-1 quantum magnet on the kagome lattice is
studied for all cases where the $J_z$ coupling is antiferromagnetic. In the
zero magnetic field case, the six previously introduced phases, found using
various methods, are: the nondegenerate gapped photon phase which breaks no
space symmetry or spin symmetry; the six-fold degenerate phase with plaquette
order, which breaks both time reversal symmetry and translational symmetry; the
"superfluid" (ferromagnetic) phase with an in-plane global U(1) symmetry
broken, when $J_{xy} < 0$; the $\sqrt{3}\times\sqrt{3}$ order when $J_{xy} >
0$; the nematic phase when $D < 0$ and large; and a phase with resonating
dimers on each hexagon. We obtain all of these phases and partial information
about their quantum phase transitions in a single framework by studying
condensation of defects in the six-fold plaquette phases. The transition
between nematic phase and the six-fold degenerate plaquette phase is
potentially an unconventional second-order critical point. In the case of a
nonzero magnetic field along $\hat{z}$, another ordered phase with translation
symmetry broken is opened up in the nematic phase. Due to the breaking of
time-reversal symmetry by the field, a supersolid phase emerges between the
six-fold plaquette order and the superfluid phase. This phase diagram might be
accessible in nickel compounds, BF$_4$ salts, or optical lattices of atoms with
three degenerate states on every site. | cond-mat_str-el |
Strong electronic correlations in superconducting organic charge
transfer salts: We review the role of strong electronic correlations in
quasi--two-dimensional organic charge transfer salts such as (BEDT-TTF)$_2X$,
(BETS)$_2Y$ and $\beta'$-[Pd(dmit)$_2$]$_2Z$. We begin by defining minimal
models for these materials. It is necessary to identify two classes of
material: the first class is strongly dimerised and is described by a
half-filled Hubbard model; the second class is not strongly dimerised and is
described by a quarter filled extended Hubbard model. We argue that these
models capture the essential physics of these materials. We explore the phase
diagram of the half-filled quasi--two-dimensional organic charge transfer
salts, focusing on the metallic and superconducting phases. We review work
showing that the metallic phase, which has both Fermi liquid and `bad metal'
regimes, is described both quantitatively and qualitatively by dynamical mean
field theory (DMFT). The phenomenology of the superconducting state is still a
matter of contention. We critically review the experimental situation, focusing
on the key experimental results that may distinguish between rival theories of
superconductivity, particularly probes of the pairing symmetry and measurements
of the superfluid stiffness. We then discuss some strongly correlated theories
of superconductivity, in particular, the resonating valence bond (RVB) theory
of superconductivity. We conclude by discussing some of the major challenges
currently facing the field. | cond-mat_str-el |
Critical Metal Phase at the Anderson Metal-Insulator Transition with
Kondo Impurities: It is well-known that magnetic impurities can change the symmetry class of
disordered metallic systems by breaking spin and time-reversal symmetry. At low
temperature these symmetries can be restored by Kondo screening. It is also
known that at the Anderson metal-insulator transition, wave functions develop
multifractal fluctuations with power law correlations. Here, we consider the
interplay of these two effects. We show that multifractal correlations open
local pseudogaps at the Fermi energy at some random positions in space. When
dilute magnetic impurities are at these locations, Kondo screening is strongly
suppressed. We find that when the exchange coupling J is smaller than a certain
value J*, the metal-insulator transition point extends to a critical region in
the disorder strength parameter and to a band of critical states. The width of
this critical region increases with a power of the concentration of magnetic
impurities. | cond-mat_str-el |
Hidden Charge Order in an Iron Oxide Square-Lattice Compound: Since the discovery of charge disproportionation in the FeO$_2$
square-lattice compound Sr$_3$Fe$_2$O$_7$ by M\"ossbauer spectroscopy more than
fifty years ago, the spatial ordering pattern of the disproportionated charges
has remained "hidden" to conventional diffraction probes, despite numerous
x-ray and neutron scattering studies. We have used neutron Larmor diffraction
and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge
order in the FeO$_2$ planes that vanishes at a sharp second-order phase
transition upon heating above 332 K. Stacking disorder of the checkerboard
pattern due to frustrated interlayer interactions broadens the corresponding
superstructure reflections and greatly reduces their amplitude, thus explaining
the difficulty to detect them by conventional probes. We discuss implications
of these findings for research on "hidden order" in other materials. | cond-mat_str-el |
Ground state of S=1 zigzag spin-orbital chain: We investigate ground-state properties of a $t_{\rm 2g}$-orbital Hubbard
model on a zigzag chain relevant for CaV$_{2}$O$_{4}$, by exploiting numerical
techniques such as Lanczos diagonalization and density-matrix renormalization
group. Assuming a V$^{3+}$ ion, a local spin $S$=$1$ state is formed by two
electrons in the $t_{\rm 2g}$ orbitals. That is, the system is a Haldane system
with active $t_{\rm 2g}$-orbital degrees of freedom. We observe orbital-state
transitions, yielding a distinct spin system under the orbital-ordered
background. We also discuss the orbital structure induced by open edges,
originating in the spatial anisotropy of the $t_{\rm 2g}$ orbitals. | cond-mat_str-el |
Evidence for a fractional quantum Hall state with anisotropic
longitudinal transport: At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau
level (LL), a clean two-dimensional electron system (2DES) exhibits numerous
incompressible liquid phases which display the fractional quantized Hall effect
(FQHE) (Das Sarma and Pinczuk, 1997). These liquid phases do not break
rotational symmetry, exhibiting resistivities which are isotropic in the plane.
In contrast, at lower fields, when the Fermi level lies in the $N\ge2$ third
and several higher LLs, the 2DES displays a distinctly different class of
collective states. In particular, near half filling of these high LLs the 2DES
exhibits a strongly anisotropic longitudinal resistance at low temperatures
(Lilly et al., 1999; Du et al., 1999). These "stripe" phases, which do not
exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing
broken rotational symmetry and orientational order (Koulakov et al., 1996;
Fogler et al., 1996; Moessner and Chalker, 1996; Fradkin and Kivelson, 1999;
Fradkin et al, 2010). Here we report a surprising new observation: An
electronic configuration in the N=1 second LL whose resistivity tensor
simultaneously displays a robust fractionally quantized Hall plateau and a
strongly anisotropic longitudinal resistance resembling that of the stripe
phases. | cond-mat_str-el |
How do we interrogate the electrons without roughing them up?: Electrons are indistinguishable, but the energy of each electron is different
in different materials and if we can exploit this energy, then we can
systematically study the changes of electronic properties in non free-electron
metals. | cond-mat_str-el |
Entanglement and Topology in Su-Schrieffer-Heeger Cavity Quantum
Electrodynamics: Cavity materials are a frontier to investigate the role of light-matter
interactions on the properties of electronic phases of matter. In this work, we
raise a fundamental question: can non-local interactions mediated by cavity
photons destabilize a topological electronic phase? We investigate this
question by characterizing entanglement, energy spectrum and correlation
functions of the topological Su-Schrieffer-Heeger (SSH) chain interacting with
an optical cavity mode. Employing density-matrix renormalization group (DMRG)
and exact diagonalization (ED), we demonstrate the stability of the edge state
and establish an area law scaling for the ground state entanglement entropy,
despite long-range correlations induced by light-matter interactions. These
features are linked to gauge invariance and the scaling of virtual photon
excitations entangled with matter, effectively computed in a low-dimensional
Krylov subspace of the full Hilbert space. This work provides a framework for
characterizing novel equilibrium phenomena in topological cavity materials. | cond-mat_str-el |
From order to randomness: Onset and evolution of the random-singlet
state in bond-disordered BaCu$_2$(Si$_{1-x}$Ge$_x$)$_2$O$_7$ spin-chain
compounds: Heisenberg-type spin-chain materials have been extensively studied over the
years, yet not much is known about their behavior in the presence of disorder.
Starting from BaCu$_2$Si$_2$O$_7$, a typical spin-1/2 chain system, we
investigate a series of compounds with different degrees of bond disorder,
where the systematic replacement of Si with Ge results in a re-modulation of
the Cu$^{2+}$ exchange interactions. By combining magnetometry measurements
with nuclear magnetic resonance studies we follow the evolution of the
disorder-related properties from the well-ordered BaCu$_2$Si$_2$O$_7$ to the
maximally disordered BaCu$_2$SiGeO$_7$. Our data indicate that already a weak
degree of disorder of only 5% Ge, apart from reducing the 3D magnetic ordering
temperature $T_\mathrm{N}$ quite effectively, induces a qualitatively different
state in the paramagnetic regime. At maximum disorder our data indicate that
this state may be identified with the theoretically predicted random singlet
(RS) state. With decreasing disorder the extension of the RS regime at
temperatures above $T_\mathrm{N}$ is reduced, yet its influence is clearly
manifest, particularly in the features of NMR relaxation data. | cond-mat_str-el |
Quantum-critical transport of marginal Fermi-liquids: We present exact results for the electrical and thermal conductivity and
Seebeck coefficient at low temperatures and frequencies in the quantum-critical
region for fermions on a lattice scattering with the collective fluctuations of
the quantum xy model. This is done by the asymptotically exact solution of the
vertex equation in the Kubo formula for these transport properties. The model
is applicable to the fluctuations of the loop-current order in cuprates as well
as to a class of quasi-two dimensional heavy-fermion and other metallic
antiferromagnets, and proposed recently also for the possible loop-current
order in Moir\'{e} twisted bi-layer graphene and bi-layer WSe$_2$. All these
metals have a linear in temperature electrical resistivity in the
quantum-critical region of their phase diagrams, often termed "Planckian"
resistivity. The solution of the integral equation for the vertex in the Kubo
equation for transport shows that all vertex renormalizations except due to
Aslamazov-Larkin processes are absent. The latter appear as an Umklapp
scattering matrix, which is shown to give only a temperature independent
multiplicative factor for electrical resistivity which is non-zero in the pure
limit only if the Fermi-surface is large enough, but do not affect thermal
conductivity. We also show that the mass renormalization which gives a
logarithmic enhancement of the marginal Fermi-liquid specific heat does not
appear in the electrical resistivity as well as in the thermal conductivity. On
the other hand the mass renormalization appears in the Seebeck coefficient. The
results for transport properties are derived for any Fermi-surface on any
lattice. As an example, the linear in $T$ electrical resistivity is explicitly
calculated for large enough circular Fermi-surfaces on a square lattice. We
also discuss in detail the conservation laws that play a crucial role in all
transport properties. | cond-mat_str-el |
The chemical bond as an emergent phenomenon: We first argue that the covalent bond and the various closed-shell
interactions can be thought of as symmetry broken versions of one and the same
interaction, viz., the multi-center bond. We use specially chosen molecular
units to show that the symmetry breaking is controlled by density and
electronegativity variation. We show that the bond order changes with bond
deformation but in a step-like fashion, regions of near constancy separated by
electronic localization transitions. These will often cause displacive
transitions as well so that the bond strength, order, and length are
established self-consistently. We further argue on the inherent relation of the
covalent, closed-shell, and multi-center interactions with ionic and metallic
bonding. All of these interactions can be viewed as distinct sectors on a phase
diagram with density and electronegativity variation as control variables; the
ionic and covalent/secondary sectors are associated with on-site and bond-order
charge density wave respectively, the metallic sectorwith an electronic fluid.
While displaying a contiguity at low densities, the metallic and ionic
interactions represent distinct phases separated by discontinuous transitions
at sufficiently high densities. Multi-center interactions emerge as a hybrid of
the metallic and ionic bond that results from spatial coexistence of
delocalized and localized electrons. In the present description, the issue of
the stability of a compound is that of mutual miscibility of electronic fluids
with distinct degrees of electron localization, supra-atomic ordering in
complex inorganic compounds comes about naturally. The notions of electronic
localization advanced hereby suggest a high throughput, automated procedure for
screening candidate compounds and structures with regard to stability, without
the need for computationally costly geometric optimization. | cond-mat_str-el |
First-Order Reversal Curves of the Magnetostructural Phase Transition in
FeTe: We apply the first-order reversal curve (FORC) method, borrowed from studies
of ferromagnetic materials, to the magneto-structural phase transition of FeTe.
FORC measurements reveal two features in the hysteretic phase transition, even
in samples where traditional temperature measurements display only a single
transition. For Fe1.13Te, the influence of magnetic field suggests that the
main feature is primarily structural while a smaller, slightly
higher-temperature transition is magnetic in origin. By contrast Fe1.03Te has a
single transition which shows a uniform response to magnetic field, indicating
a stronger coupling of the magnetic and structural phase transitions. We also
introduce uniaxial stress, which spreads the distribution width without
changing the underlying energy barrier of the transformation. The work shows
how FORC can help disentangle the roles of the magnetic and structural phase
transitions in FeTe. | cond-mat_str-el |
Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8): It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase
transition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic
insulator (FI) phase. The magnetization shows a sharp jump at the transition
field accompanying with a remarkable increase of magnetoresistance. Striction
measurements clarified that this transition is associated with the structural
change from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic
phase. These results evidently show that the FI phase with a pseudo cubic
symmetry is more stable in high fields than the FM phase due to the double
exchange interaction. The driving force of this transition is explained by the
enhancement of the ferromagnetic superexchange interaction induced by an
antiferromagnetic type orbital ordering in the pseudo cubic phase, which was
recently found in the anomalous X-ray scattering experiments. | cond-mat_str-el |
Doping evolution of the electron-hole asymmetric s-wave pseudogap in
underdoped high-Tc cuprate superconductors: We study the doping evolution of the electronic structure in the pseudogap
state of high-Tc cuprate superconductors, by means of a cluster extension of
the dynamical mean-field theory applied to the two-dimensional Hubbard model.
The calculated single-particle excitation spectra in the strongly underdoped
regime show a marked electron-hole asymmetry and reveal a "s-wave" pseudogap,
which display a finite amplitude in all the directions in the momentum space
but not always at the Fermi level: The energy location of the gap strongly
depends on momentum, and in particular in the nodal region, it is above the
Fermi level. With increasing hole doping, the pseudogap disappears everywhere
in the momentum space. We show that the origin and the "s-wave" structure of
the pseudogap can be ascribed to the emergence of a strong-scattering surface,
which appears in the energy-momentum space close to the Mott insulator. | cond-mat_str-el |
The spatial range of the Kondo effect: a numerical analysis: The spatial length of the Kondo screening is still a controversial issue
related to Kondo physics. While renormalization group and Bethe Anzats
solutions have provided detailed information about the thermodynamics of
magnetic impurities, they are insufficient to study the effect on the
surrounding electrons, i.e., the spatial range of the correlations created by
the Kondo effect between the localized magnetic moment and the conduction
electrons. The objective of this work is to present a quantitative way of
measuring the extension of these correlations by studying their effect directly
on the local density of states (LDOS) at arbitrary distances from the impurity.
The numerical techniques used, the Embedded Cluster Approximation, the Finite U
Slave Bosons, and Numerical Renormalization Group, calculate the Green
functions in real space. With this information, one can calculate how the local
density of states away from the impurity is modified by its presence, below and
above the Kondo temperature, and then estimate the range of the disturbances in
the non-interacting Fermi sea due to the Kondo effect, and how it changes with
the Kondo temperature $T_{\rm K}$. The results obtained agree with results
obtained through spin-spin correlations, showing that the LDOS captures the
phenomenology of the Kondo cloud as well. To the best of our knowledge, it is
the first time that the LDOS is used to estimate the extension of the Kondo
cloud. | cond-mat_str-el |
On the existence of the excitonic insulator phase in the extended
Falicov-Kimball model: an SO(2)-invariant slave-boson approach: We re-examine the three-dimensional spinless Falicov-Kimball model with
dispersive $f$ electrons at half-filling, addressing the dispute about the
formation of an excitonic condensate, which is closely related to the problem
of electronic ferroelectricity. To this end, we work out a slave-boson
functional integral representation of the suchlike extended Falicov-Kimball
model that preserves the $SO(2)\otimes U(1)^{\otimes 2}$ invariance of the
action. We find a spontaneous pairing of $c$ electrons with $f$ holes, building
an excitonic insulator state at low temperatures, also for the case of
initially non-degenerate orbitals. This is in contrast to recent predictions of
scalar slave-boson mean-field theory but corroborates previous Hartree-Fock and
RPA results. Our more precise treatment of correlation effects, however, leads
to a substantial reduction of the critical temperature. The different behavior
of the partial densities of states in the weak and strong inter-orbital Coulomb
interaction regimes supports a BCS-BEC transition scenario. | cond-mat_str-el |
Thermal Conductivity due to Spinons in the One-Dimensional Quantum Spin
System Sr2V3O9: We have measured the thermal conductivity along different directions of the S
= 1/2 one-dimensional (1D) spin system Sr2V3O9 in magnetic fields up to 14 T.
It has been found that the thermal conductivity along the [10-1] direction,
\k{appa}[10-1], is large and markedly suppressed by the application of magnetic
field, indicating that there is a large contribution of spinons to
\k{appa}[10-1] and that the spin chains run along the [10-1] direction. The
maximum value of the thermal conductivity due to spinons is ~14 W/Km along the
[10-1] direction, supporting the empirical law that the magnitude of the
thermal conductivity due to spinons is roughly proportional to the
antiferromagnetic interaction between the nearest neighboring spins. | cond-mat_str-el |
Effects of electron coupling to intra- and inter-molecular vibrational
modes on the transport properties of single crystal organic semiconductors: Electron coupling to intra- and inter-molecular vibrational modes is
investigated in models appropriate to single crystal organic semiconductors,
such as oligoacenes. Focus is on spectral and transport properties of these
systems beyond perturbative approaches. The interplay between different
couplings strongly affects the temperature band renormalization that is the
result of a subtle equilibrium between opposite tendencies: band narrowing due
to interaction with local modes, band widening due to electron coupling to non
local modes. The model provides an accurate description of the mobility as
function of temperature: indeed, it has the correct order of magnitude, at low
temperatures, it scales as a power-law $T^{-\delta}$ with the exponent $\delta$
larger than unity, and, at high temperatures, shows an hopping behavior with a
small activation energy. | cond-mat_str-el |
Effects of Dissipation on Solitons in the Hydrodynamic Regime of
Graphene: We use hydrodynamic techniques to analyze the one-dimensional propagation of
solitons in gated graphene on an arbitrary uniform background current. Results
are derived for both the Fermi liquid and Dirac fluid regimes. We find that
these solutions satisfy the Korteweg-de Vries-Burgers equation. Viscous
dissipation and ohmic heating are included, causing the solitons to decay.
Experiments are proposed to measure this decay and thereby quantify the shear
viscosity in graphene. | cond-mat_str-el |
Conserving quasiparticle calculations for small metal clusters: A novel approach for GW-based calculations of quasiparticle properties for
finite systems is presented, in which the screened interaction is obtained
directly from a linear response calculation of the density-density correlation
function. The conserving nature of our results is shown by explicit evaluation
of the $f$-sum rule. As an application, energy renormalizations and level
broadenings are calculated for the closed-shell Na$_9^+$ and Na$_{21}^+$
clusters, as well as for Na$_4$. Pronounced improvements of conserving
approximations to RPA-level results are obtained. | cond-mat_str-el |
Laser-excited ultrahigh-resolution photoemission spectroscopy of
NaxCoO2.yH2O:Evidence for pseudogap formation: We have studied the temperature-dependent electronic structure near the Fermi
level (EF) of the layered cobaltate superconductor, Na0.35CoO2.1.3H2O, and
related materials, using laser-excited ultrahigh-resolution photoemission
spectroscopy. We observe the formation of a pseudogap with an energy scale of ~
20 meV in Na0.35CoO2.1.3H2O and Na0.35CoO2.0.7H2O, which is clearly absent in
Na0.7CoO2. The energy scale of the pseudogap is larger than the expected value
for the superconducting gap, suggesting an additional competing order parameter
at low temperatures. We discuss implications of the pseudogap in relation to
available transport and magnetic susceptibility results. | cond-mat_str-el |
Current response of nonequilibrium steady states in Landau-Zener
problem: Nonequilibrium Green's function approach: The carrier generation in insulators subjected to strong electric fields is
characterized by the Landau-Zener formula for the tunneling probability with a
nonperturbative exponent. Despite its long history with diverse applications
and extensions, study of nonequilibrium steady states and associated current
response in the presence of the generated carriers has been mainly limited to
numerical simulations so far. Here, we develop a framework to calculate the
nonequilibrium Green's function of generic insulating systems under a DC
electric field, in the presence of a fermionic reservoir. Using asymptotic
expansion techniques, we derive a semi-quantitative formula for the Green's
function with nonperturbative contribution. This formalism enables us to
calculate dissipative current response of the nonequilibrium steady state,
which turns out to be not simply characterized by the intraband current
proportional to the tunneling probability. We also apply the present formalism
to noncentrosymmetric insulators, and propose nonreciprocal charge and spin
transport peculiar to tunneling electrons. | cond-mat_str-el |
Anisotropic optical properties of detwinned BaFe$_{2}$As$_{2}$: The optical properties of a large, detwinned single crystal of BaFe$_2$As$_2$
have been examined over a wide frequency range above and below the structural
and magnetic transition at $T_{\rm N}\simeq 138$ K. Above $T_{\rm N}$ the real
part of the optical conductivity and the two infrared-active lattice modes are
almost completely isotropic; the lattice modes show a weak polarization
dependence just above $T_{\rm N}$. For $T<T_{\rm N}$, the optical conductivity
due to the free-carrier response is anisotropic, being larger along the $a$
axis than the $b$ axis below $\simeq 30$ meV; above this energy the optical
conductivity is dominated by the interband contributions, which appear to be
isotropic. The splitting of the low-energy infrared-active mode below $T_{\rm
N}$ is clearly observed, and the polarization modulation of the new modes may
be used to estimate that the crystal is $\simeq 70$% detwinned. The
high-frequency mode, with a threefold increase in strength of the lower branch
below $T_{\rm N}$ and nearly silent upper branch, remains enigmatic. | cond-mat_str-el |
Some exact results for the zero-bandwidth extended Hubbard model with
intersite charge and magnetic interactions: The extended Hubbard model in the zero-bandwidth limit is studied. The
effective Hamiltonian consists of (i) on-site $U$ interaction and intersite
(ii) density-density interaction $W$ and (iii) Ising-like magnetic exchange
interaction $J$ (between the nearest-neighbors). We present rigorous (and
analytical) results obtained within the transfer-matrix method for 1D-chain in
two particular cases: (a) $W=0$ and $n=1$; (b) $U\rightarrow+\infty$ and
$n=1/2$ ($W\neq 0$, $J\neq 0$). We obtain the exact formulas for the partition
functions which enables to calculate thermodynamic properties such as entropy,
specific heat ($c$), and double occupancy per site. In both cases the system
exhibits an interesting temperature dependence of $c$ involving a
characteristic two-peak structure. There are no phase transitions at finite
temperatures and the only transitions occur in the ground state. | cond-mat_str-el |
Strongly-correlated crystal-field approach to 3d oxides - the orbital
magnetism in 3d-ion compounds: We have developed the crystal-field approach with strong electron
correlations, extended to the Quantum Atomistic Solid-State theory (QUASST), as
a physically relevant theoretical model for the description of electronic and
magnetic properties of 3d-atom compounds. Its applicability has been
illustrated for LaCoO3, FeBr2 and Na2V3O7. According to the QUASST theory in
compounds containing open 3d-/4f-/5f-shell atoms the discrete atomic-like
low-energy electronic structure survives also when the 3d atom becomes the full
part of a solid matter. This low-energy atomic-like electronic structure, being
determined by local crystal-field interactions and the intra-atomic spin-orbit
coupling, predominantly determines electronic and magnetic properties of the
whole compound.
We understand our theoretical research as a continuation of the Van Vleck's
studies on the localized magnetism. We point out, however, the importance of
the orbital magnetism and the intra-atomic spin-orbit coupling for the
physically adequate description of real 3d-ion compounds and 3d magnetism. Our
studies clearly indicate that it is the highest time to ''unquench'' the
orbital moment in solid-state physics in description of 3d-atom containing
compounds.
PACS No: 75.10.D; 71.70.E
Keywords: magnetism, transition-metal compounds, 3d magnetism, crystal field,
spin-orbit coupling, orbital magnetism | cond-mat_str-el |
The Temperature Evolution of the Out-of-Plane Correlation Lengths of
Charge-Stripe Ordered La(1.725)Sr(0.275)NiO(4): The temperature dependence of the magnetic order of stripe-ordered
La(1.725)Sr(0.275)NiO(4) is investigated by neutron diffraction. Upon cooling,
the widths if the magnetic Bragg peaks are observed to broaden. The degree of
broadening is found to be very different for l = odd-integer and l =
even-integer magnetic peaks. We argue that the observed behaviour is a result
of competition between magnetic and charge order. | cond-mat_str-el |
Magnetic properties of pressurized CsV$_{3}$Sb$_{5}$ calculated by using
a hybrid functional: Based on the hybrid functional, we find that at 0 GPa, the pristine
CsV$_{3}$Sb$_{5}$ has local magnetic moment of 0.85 $\mu_B$ /unit cell, which
is suppressed at pressure of 2.5 GPa resulting in a spin-crossover. Since the
ground sate of CsV$_{3}$Sb$_{5}$ with charge density wave (CDW) distortion is
non-magnetic state, the local magnetic moment of pristine CsV$_{3}$Sb$_{5}$
will be suppressed by temperature-induced CDW transition at 94 K. The schematic
evolution of magnetic moments as functions of pressure and temperature are
presented. At low temperature, CsV$_{3}$Sb$_{5}$ is a rare example of materials
hosting pressureinduced local magnetic moment, and we suggeste that the effects
of local magnetic moments should be considered for understanding its
properties. | cond-mat_str-el |
Stroboscopic Symmetry-Protected Topological Phases: Symmetry-protected topological (SPT) phases of matter have been the focus of
many recent theoretical investigations, but controlled mechanisms for
engineering them have so far been elusive. In this work, we demonstrate that by
driving interacting spin systems periodically in time and tuning the available
parameters, one can realize lattice models for bosonic SPT phases in the limit
where the driving frequency is large. We provide concrete examples of this
construction in one and two dimensions, and discuss signatures of these phases
in stroboscopic measurements of local observables. | cond-mat_str-el |
Spin Liquid State in the 3D Frustrated Antiferromagnet PbCuTe2O6: NMR
and muSR Studies: PbCuTe2O6 is a rare example of a spin liquid candidate featuring a three
dimensional magnetic lattice. Strong geometric frustration arises from the
dominant antiferromagnetic interaction which generates a hyperkagome network of
Cu2+ ions although additional interactions enhance the magnetic lattice
connectivity. Through a combination of magnetization measurements and local
probe investigation by NMR and muSR down to 20 mK, we provide a robust evidence
for the absence of magnetic freezing in the ground state. The local spin
susceptibility probed by the NMR shift hardly deviates from the macroscopic one
down to 1 K pointing to a homogeneous magnetic system with a low defect
concentration. The saturation of the NMR shift and the sublinear power law
temperature (T) evolution of the 1/T1 NMR relaxation rate at low T point to a
non-singlet ground state favoring a gapless fermionic description of the
magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics
is witnessed, which may signal a reconstruction of spinon Fermi surface.
Nonetheless, the compound remains in a fluctuating spin liquid state down to
the lowest temperature of the present investigation. | cond-mat_str-el |
Simple parametrization for the ground-state energy of the infinite
Hubbard chain incorporating Mott physics, spin-dependent phenomena and
spatial inhomogeneity: Simple analytical parametrizations for the ground-state energy of the
one-dimensional repulsive Hubbard model are developed. The charge-dependence of
the energy is parametrized using exact results extracted from the Bethe-Ansatz.
The resulting parametrization is shown to be in better agreement with highly
precise data obtained from fully numerical solution of the Bethe-Ansatz
equations than previous expressions [Lima et al., Phys. Rev. Lett. 90, 146402
(2003)]. Unlike these earlier proposals, the present parametrization correctly
predicts a positive Mott gap at half filling for any U>0. The construction is
extended to spin-dependent phenomena by parametrizing the
magnetization-dependence of the ground-state energy using further exact results
and numerical benchmarking. Lastly, the parametrizations developed for the
spatially uniform model are extended by means of a simple local-density-type
approximation to spatially inhomogeneous models, e.g., in the presence of
impurities, external fields or trapping potentials. Results are shown to be in
excellent agreement with independent many-body calculations, at a fraction of
the computational cost. | cond-mat_str-el |
Metal-insulator transition and local-moment collapse in negative
charge-transfer CaFeO$_3$ under pressure: We compute the electronic structure, spin and charge state of Fe ions, and
structural phase stability of paramagnetic CaFeO$_3$ under pressure using a
fully self-consistent in charge density DFT+dynamical mean-field theory method.
We show that at ambient pressure CaFeO$_3$ is a negative charge-transfer
insulator characterized by strong localization of the Fe $3d$ electrons. It
crystallizes in the monoclinic $P2_1/n$ crystal structure with a cooperative
breathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations
and local moments are consistent with robust charge disproportionation of Fe
ions in the insulating $P2_1/n$ phase, the physical charge density difference
around the structurally distinct Fe A and Fe B ions with the ``contracted'' and
``expanded'' oxygen octahedra, respectively, is rather weak, $\sim$0.04. This
implies the importance of the Fe $3d$ and O $2p$ negative charge transfer and
supports the formation of a bond-disproportionated state characterized by the
Fe A $3d^{5-\delta}\underline{L}^{2-\delta}$ and Fe B $3d^5$ valence
configurations with $\delta \ll 1$, in agreement with strong hybridization
between the Fe $3d$ and O $2p$ states. Upon compression above $\sim$41 GPa
CaFeO$_3$ undergoes the insulator-to-metal phase transition (IMT) which is
accompanied by a structural transformation into the orthorhombic $Pbnm$ phase.
The phase transition is accompanied by suppression of the cooperative breathing
mode distortion of the lattice and, hence, results in the melting of bond
disproportionation of the Fe ions. Our analysis suggests that the IMT
transition is associated with orbital-dependent delocalization of the Fe $3d$
electrons and leads to a remarkable collapse of the local magnetic moments. Our
results imply the crucial importance of the interplay of electronic
correlations and structural effects to explain the properties of CaFeO$_3$. | cond-mat_str-el |
Dynamical Signature of Fractionalization at the Deconfined Quantum
Critical Point: Deconfined quantum critical points govern continuous quantum phase
transitions at which fractionalized (deconfined) degrees of freedom emerge.
Here we study dynamical signatures of the fractionalized excitations in a
quantum magnet (the easy-plane J-Q model) that realize a deconfined quantum
critical point with emergent O(4) symmetry. By means of large-scale quantum
Monte Carlo simulations and stochastic analytic continuation of imaginary-time
correlation functions, we obtain the dynamic spin structure factors in the
$S^{x}$ and $S^{z}$ channels. In both channels, we observe broad continua that
originate from the deconfined excitations. We further identify several distinct
spectral features of the deconfined quantum critical point, including the lower
edge of the continuum and its form factor on moving through the Brillouin Zone.
We provide field-theoretical and lattice model calculations that explain the
overall shapes of the computed spectra, which highlight the importance of
interactions and gauge fluctuations to explaining the spectral-weight
distribution. We make further comparisons with the conventional Landau O(2)
transition in a different quantum magnet, at which no signatures of
fractionalization are observed. The distinctive spectral signatures of the
deconfined quantum critical point suggest the feasibility of its experimental
detection in neutron scattering and nuclear magnetic resonance experiments. | cond-mat_str-el |
From Kosterlitz-Thouless to Pokrovsky-Talapov transitions in spinless
fermions and spin chains with next-nearest neighbour interactions: We investigate the nature of the quantum phase transition out of
charge-density-wave phase in the spinless fermion model with nearest- and
next-nearest-neighbor interaction at one-third filling. Using an extensive
Density Matrix Renormalization Group (DMRG) simulations we show that the
transition changes it nature. We show that for weak next-nearest-neighbor
coupling the transition is of Kosterlitz-Thouless type in agreement with
bosonisation predictions. We also provide strong numerical evidences that for
large next-nearest-neighbor repulsion the transition belongs to the
Pokrovsky-Talapov univerality class describing a non-conformal
commensurate-incommensurate transition. Finally, we argue that the change of
the nature of the transition is a result of incommensurability induced by
frustration and realized even at zero doping. The implications in the context
of XXZ chain with next-nearest-neighbor Ising interaction is briefly discussed. | cond-mat_str-el |
A finite-frequency functional RG approach to the single impurity
Anderson model: We use the Matsubara functional renormalization group (FRG) to describe
electronic correlations within the single impurity Anderson model. In contrast
to standard FRG calculations, we account for the frequency-dependence of the
two-particle vertex in order to address finite-energy properties (e.g, spectral
functions). By comparing with data obtained from the numerical renormalization
group (NRG) framework, the FRG approximation is shown to work well for
arbitrary parameters (particularly finite temperatures) provided that the
electron-electron interaction U is not too large. We demonstrate that aspects
of (large U) Kondo physics which are described well by a simpler
frequency-independent truncation scheme are no longer captured by the
'higher-order' frequency-dependent approximation. In contrast, at small to
intermediate U the results obtained by the more elaborate scheme agree better
with NRG data. We suggest to parametrize the two-particle vertex not by three
independent energy variables but by introducing three functions each of a
single frequency. This considerably reduces the numerical effort to integrate
the FRG flow equations. | cond-mat_str-el |
Graded Projected Entangled-Pair State Representations and An Algorithm
for Translationally Invariant Strongly Correlated Electronic Systems on
Infinite-Size Lattices in Two Spatial Dimensions: An algorithm to find a graded Projected Entangled-Pair State representation
of the ground state wave functions is developed for translationally invariant
strongly correlated electronic systems on infinite-size lattices in two spatial
dimensions. It is tested for the two-dimensional t-J model at and away from
half filling, with truncation dimensions up to 6. We are able to locate a line
of phase separation, which qualitatively agrees with the results based on the
high-temperature expansions. We find that the model exhibits an extended s-wave
superconductivity for J=0.4t at quarter filling. However, we emphasize that the
currently accessible truncation dimensions are not large enough, so it is
necessary to incorporate the symmetry of the system into the algorithm, in
order to achieve results with higher precision. | cond-mat_str-el |
Spin and orbital ordering in double-layered manganites: We study theoretically the phase diagram of the double-layered perovskite
manganites taking into account the orbital degeneracy, the strong Coulombic
repulsion, and the coupling with the lattice deformation. Observed spin
structural changes as the increased doping are explained in terms of the
orbital ordering and the bond-length dependence of the hopping integral along
$c$-axis. Temperature dependence of the neutron diffraction peak corresponding
to the canting structure is also explained. Comparison with the 3D cubic system
is made. | cond-mat_str-el |
Electric field-induced Skyrmion distortion and giant lattice rotation in
the magnetoelectric insulator Cu2OSeO3: Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected
magnetic spin vortex-like objects, display a magnetoelectric coupling and can
be manipulated by externally applied electric (E) fields. Here, we explore the
E-field coupling to the magnetoelectric Skyrmion lattice phase, and study the
response using neutron scattering. Giant E-field induced rotations of the
Skyrmion lattice are achieved that span a range of $\sim$25$^{\circ}$.
Supporting calculations show that an E-field-induced Skyrmion distortion lies
behind the lattice rotation. Overall, we present a new approach to Skyrmion
control that makes no use of spin-transfer torques due to currents of either
electrons or magnons. | cond-mat_str-el |
Effects of Backflow Correlation in the Three-Dimensional Electron Gas:
Quantum Monte Carlo Study: The correlation energy of the homogeneous three-dimensional interacting
electron gas is calculated using the variational and fixed-node diffusion Monte
Carlo methods, with trial functions that include backflow and three-body
correlations. In the high density regime the effects of backflow dominate over
those due to three-body correlations, but the relative importance of the latter
increases as the density decreases. Since the backflow correlations vary the
nodes of the trial function, this leads to improved energies in the fixed-node
diffusion Monte Carlo calculations. The effects are comparable to those found
for the two-dimensional electron gas, leading to much improved variational
energies and fixed-node diffusion energies equal to the release-node energies
of Ceperley and Alder within statistical and systematic errors. | cond-mat_str-el |
Tuning the Magnetic Ground State by Charge Transfer Energy in SrCoO2.5
via Strain Engineering: SrCoO2.5 (SCO) is a charge transfer insulator with 3d6 ground state
configuration leading to antiferromagnetic nature. It is observed that
substrate induced strain engineering modifies the ground state of SCO thin film
with 3d7L (L:O-2p hole) configuration causing negative charge transfer
energy.The consequent strong hybridization between O-2p and Co-3d bands causes
a hole in O-2p band leading to hole mediated unconventional ferromagnetic
ordering in SrCoO2.5 thin film. This opens up a new avenue to tune the
electronic structure vis a vis magnetic property via strain engineering. | cond-mat_str-el |
Role of surface states in STM spectroscopy of (111) metal surfaces with
Kondo adsorbates: A nearly-free-electron (NFE) model to describe STM spectroscopy of (111)
metal surfaces with Kondo impurities is presented. Surface states are found to
play an important role giving a larger contribution to the conductance in the
case of Cu(111) and Au(111) than Ag(111) surfaces. This difference arises from
the farther extension of the Ag(111) surface state into the substrate. The
different line shapes observed when Co is adsorbed on different substrates can
be explained from the position of the surface band onset relative to the Fermi
energy. The lateral dependence of the line shape amplitude is found to be
bulk-like for R|| < 4 Amstrongs and surface-like at larger distances, in
agreement with experimental data. | cond-mat_str-el |
Interacting surface states of three-dimensional topological insulators: We numerically investigate the surface states of a strong topological
insulator in the presence of strong electron-electron interactions. We choose a
spherical topological insulator geometry to make the surface amenable to a
finite size analysis. The single-particle problem maps to that of Landau
orbitals on the sphere with a magnetic monopole at the center that has unit
strength and opposite sign for electrons with opposite spin. Assuming
density-density contact interactions, we find superconducting and anomalous
(quantum) Hall phases for attractive and repulsive interactions, respectively,
as well as chiral fermion and chiral Majorana fermion boundary modes between
different phases. Our setup is preeminently adapted to the search for
topologically ordered surface terminations that could be microscopically
stabilized by tailored surface interaction profiles. | cond-mat_str-el |
Spectroscopic signatures and origin of a hidden order in Ba$_2$MgReO$_6$: Clarifying the underlying mechanisms that govern ordering transitions in
condensed matter systems is crucial for comprehending emergent properties and
phenomena. While transitions are often classified as electronically driven or
lattice-driven, we present a departure from this conventional paradigm in the
case of the double perovskite Ba$_2$MgReO$_6$. Leveraging resonant and
non-resonant elastic x-ray scattering techniques, we unveil the simultaneous
ordering of structural distortions and charge quadrupoles at a critical
temperature of $T_\mathrm{q}$$\sim$33 K. Using a variety of complementary
first-principles-based computational techniques, we demonstrate that while
electronic interactions drive the ordering at $T_\mathrm{q}$, it is ultimately
the lattice that dictates the specific ground state that emerges. Our findings
highlight the crucial interplay between electronic and lattice degrees of
freedom, providing a unified framework to understand and predict unconventional
emergent phenomena in quantum materials. | cond-mat_str-el |
Dynamical vertex approximation for the two-dimensional Hubbard model: Recently, diagrammatic extensions of dynamical mean field theory (DMFT) have
been proposed for including short- and long-range correlations beyond DMFT on
an equal footing. We employ one of these, the dynamical vertex approximation
(D$\Gamma$A), and study the two-dimensional Hubbard model on a square lattice.
We define two transition lines in the phase diagram which correspond,
respectively, to the opening of the gap in the nodal direction and throughout
the Fermi surface. Our self-energy data show that the evolution between the two
regimes occurs in a gradual way (crossover) and also that at low enough
temperatures the whole Fermi surface is always gapped. Furthermore, we present
a comparison of our D$\Gamma$A calculations at a parameter set where data
obtained by other techniques are available. | cond-mat_str-el |
Effect of Li doping on magnetic and transport properties of CoV2O4 and
FeV2O4: The structural, magnetic and transport properties have been studied of Li
doped CoV2O4 and FeV2O4. Li doping increases the ferri-magnetic ordering
temperature of both the samples but decreases the spin-glass transition
temperature of CoV2O4. The Li-doping decreases the V-V distance which in effect
increases the A-V coupling. Thus the increased A-V coupling dominate over the
decrease in A-V coupling due to doping of non-magnetic Li. | cond-mat_str-el |
Quasiparticle evolution and pseudogap formation in V2O3: An infrared
spectroscopy study: The infrared conductivity of V2O3 is measured in the whole phase diagram.
Quasiparticles appear above the Neel temperature TN and eventually disappear
further enhancing the temperature, leading to a pseudogap in the optical
spectrum above 425 K. Our calculations demonstrate that this loss of coherence
can be explained only if the temperature dependence of lattice parameters is
considered. V2O3 is therefore effectively driven from the metallic to the
insulating side of the Mott transition as the temperature is increased. | cond-mat_str-el |
Hubbard nanoclusters far from equilibrium: The Hubbard model is a prototype for strongly correlated many-particle
systems, including electrons in condensed matter and molecules, as well as for
fermions or bosons in optical lattices. While the equilibrium properties of
these systems have been studied in detail, the nonequilibrium dynamics
following a strong non-perturbative excitation only recently came into the
focus of experiments and theory. It is of particular interest how the dynamics
depend on the coupling strength and on the particle number and whether there
exist universal features in the time evolution. Here, we present results for
the dynamics of finite Hubbard clusters based on a selfconsistent
nonequilibrium Green functions (NEGF) approach invoking the generalized
Kadanoff--Baym ansatz (GKBA). We discuss the conserving properties of the GKBA
with Hartree--Fock propagators in detail and present a generalized form of the
energy conservation criterion of Baym and Kadanoff for NEGF. Furthermore, we
demonstrate that the HF-GKBA cures some artifacts of prior two-time NEGF
simulations. Besides, this approach substantially speeds up the numerical
calculations and thus presents the capability to study comparatively large
systems and to extend the analysis to long times allowing for an accurate
computation of the excitation spectrum via time propagation. Our data obtained
within the second Born approximation compares favorably with exact
diagonalization results (available for up to 13 particles) and are expected to
have predictive capability for substantially larger systems in the weak
coupling limit. | cond-mat_str-el |
Partition Functions of Strongly Correlated Electron Systems as
"Fermionants": We introduce a new mathematical object, the "fermionant"
${\mathrm{Ferm}}_N(G)$, of type $N$ of an $n \times n$ matrix $G$. It
represents certain $n$-point functions involving $N$ species of free fermions.
When N=1, the fermionant reduces to the determinant. The partition function of
the repulsive Hubbard model, of geometrically frustrated quantum
antiferromagnets, and of Kondo lattice models can be expressed as fermionants
of type N=2, which naturally incorporates infinite on-site repulsion. A
computation of the fermionant in polynomial time would solve many interesting
fermion sign problems. | cond-mat_str-el |
Superfluid--Insulator Transition in Strongly Disordered One-dimensional
Systems: We present an asymptotically exact renormalization-group theory of the
superfluid--insulator transition in one-dimensional disordered systems, with
emphasis on an accurate description of the interplay between the
Giamarchi--Schulz (instanton--anti-instanton) and weak-link (scratched-XY)
criticalities. Combining the theory with extensive quantum Monte Carlo
simulations allows us to shed new light on the ground-state phase diagram of
the one-dimensional disordered Bose-Hubbard model at unit filling. | cond-mat_str-el |
Repulsive Fermi gases in a two-dimensional lattice with non-Abelian
gauge fields: Motivated by the recent experiment realizing bidirectional spin-orbit-coupled
Bose-Einstein condensates (BEC), we theoretically explore the properties of
repulsive fermions in the two-dimensional (2D) optical lattice with such
non-Abelian gauge fields. Within the mean-field level, we find a novel phase of
topological antiferromagnetic (TAFM) order which incorporates both the
non-trivial topology due to spin-flip hopping and spontaneous symmetry breaking
(SSB) for the in-plane spin order. We argue that the appearance of such a phase
is generic for repulsive fermions in Chern bands achieved through spin-orbit
coupling. Our work paves the way for further studies of fermionic
generalization of 2D non-Abelian spin-orbit-coupled quantum gases. | cond-mat_str-el |
Topological Equivalence between the Fibonacci Quasicrystal and the
Harper Model: One-dimensional quasiperiodic systems, such as the Harper model and the
Fibonacci quasicrystal, have long been the focus of extensive theoretical and
experimental research. Recently, the Harper model was found to be topologically
nontrivial. Here, we derive a general model that embodies a continuous
deformation between these seemingly unrelated models. We show that this
deformation does not close any bulk gaps, and thus prove that these models are
in fact topologically equivalent. Remarkably, they are equivalent regardless of
whether the quasiperiodicity appears as an on-site or hopping modulation. This
proves that these different models share the same boundary phenomena and
explains past measurements. We generalize this equivalence to any
Fibonacci-like quasicrystal, i.e., a cut and project in any irrational angle. | cond-mat_str-el |
Two-particle self-consistent approach for broken symmetry phases: Spontaneous symmetry breaking of interacting fermion systems constitutes a
major challenge for many-body theory due to the proliferation of new
independent scattering channels once absent or degenerate in the symmetric
phase. One example is given by the ferro/antiferromagnetic broken symmetry
phase (BSP) of the Hubbard model, where vertices in the spin-transverse and
spin-longitudinal channels become independent with a consequent increase in the
computational power for their calculation. Here we generalize the formalism of
the non-perturbative Two-Particle-Self-Consistent method (TPSC) to treat broken
SU(2) magnetic phases of the Hubbard model, providing with a efficient yet
reliable method. We show that in the BSP, the sum-rule enforcement of
susceptibilities must be accompanied by a modified gap equation resulting in a
renormalisation of the order parameter, vertex corrections and the preservation
of the gap-less feature of the Goldstone modes. We then apply the theory to the
antiferromagnetic phase of the Hubbard model in the cubic lattice at
half-filling. We compare our results of double occupancies and staggered
magnetisation to the ones obtained using Diagrammatic Monte Carlo showing
excellent quantitative agreement. We demonstrate how vertex corrections play a
central role in lowering the Higgs resonance with respect to the quasi-particle
excitation gap in the spin-longitudinal susceptibility, yielding a well visible
Higgs-mode. | cond-mat_str-el |
Quantum phase transitions in antiferromagnets and superfluids: We present a general introduction to the non-zero temperature dynamic and
transport properties of low-dimensional systems near a quantum phase
transition. Basic results are reviewed in the context of experiments on the
spin-ladder compounds, insulating two-dimensional antiferromagnets, and
double-layer quantum Hall systems. Recent large N computations on an extended
t-J model (cond-mat/9906104) motivate a global scenario of the quantum phases
and transitions in the high temperature superconductors, and connections are
made to numerous experiments. | cond-mat_str-el |
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