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Phonon-Dressed Two-Dimensional Carriers on the ZnO Surface: Two-dimensional (2D) metallic states formed on the ZnO(10$\bar{1}$0) surface
by hydrogen adsorption have been investigated using angle-resolved
photoelectron spectroscopy (ARPES). The observed metallic state is
characterized by a peak-dip-hump structure at just below the Fermi level and a
long tail structure extending up to 600 meV in binding energy. The peak and
hump positions are separated by about 70 meV, a value close to the excitation
energy of longitudinal optical (LO) phonons. Spectral functions formulated on
the basis of the 2D electron-phonon coupling well reproduce the ARPES intensity
distribution of the metallic states. This spectral analysis suggests that the
2D electrons accumulated on the ZnO surface couple to the LO phonons and that
this coupling is the origin of the anomalous long tail. Our results indicate
that the 2D electrons at the ZnO surface are described as the electron liquid
model. | cond-mat_str-el |
Semiclassical description of anisotropic magnets for spin S=1: In this paper, nonlinear equations describing one-dimensional non-Heisenberg
ferromagnetic model are studied by use of generalized coherent states in a real
parameterization. Also dissipative spin wave equation for dipole and quadruple
branches is obtained if there is a small linear excitation from the ground
state. | cond-mat_str-el |
Hall coefficient diagnostics of surface state in pressurized SmB6: In this study, we report the first results of the high-pressure Hall
coefficient (RH) measurements in the putative topological Kondo insulator SmB6
up to 37 GPa. Below 10 GPa, our data reveal that RH(T) exhibits a prominent
peak upon cooling below 20 K. Remarkably, the temperature at which surface
conduction dominates coincides with the temperature of the peak in RH(T). The
temperature dependent resistance and Hall coefficient can be well fitted by a
two-channel model with contributions from the metallic surface and the
thermally activated bulk states. When the bulk of SmB6 becomes metallic and
magnetic at ~ 10 GPa, both the RH(T) peak and the resistance plateau disappear
simultaneously. Our results indicate that the RH(T) peak is a fingerprint to
diagnose the presence of a metallic surface state in SmB6. The high-pressure
magnetic state of SmB6 is robust to 180 GPa, and no evidence of
superconductivity is observed in the metallic phase. | cond-mat_str-el |
Gutzwiller Density Functional Theory: a formal derivation and
application to ferromagnetic nickel: We present a detailed derivation of the Gutzwiller Density Functional Theory
that covers all conceivable cases of symmetries and Gutzwiller wave functions.
The method is used in a study of ferromagnetic nickel where we calculate ground
state properties (lattice constant, bulk modulus, spin magnetic moment) and the
quasi-particle band structure. Our method resolves most shortcomings of an
ordinary Density Functional calculation on nickel. However, the quality of the
results strongly depends on the particular choice of the double-counting
correction. This constitutes a serious problem for all methods that attempt to
merge Density Functional Theory with correlated-electron approaches based on
Hubbard-type local interactions. | cond-mat_str-el |
Quantum phase transitions and superconductivity in the pressurized
heavy-fermion compound CeCuP2: The tilted balance among competing interactions can yield a rich variety of
ground states of quantum matter. In most Ce-based heavy fermion systems, this
can often be qualitatively described by the famous Doniach phase diagram, owing
to the competition between the Kondo screening and the
Ruderman-Kittel-Kasuya-Yoshida exchange interaction. Here, we report an unusual
pressure-temperature phase diagram beyond the Doniach one in CeCuP2. At ambient
pressure, CeCuP2 displays typical heavy-fermion behavior, albeit with a very
low carrier density. With lowering temperature, it shows a crossover from a non
Fermi liquid to a Fermi liquid at around 2.4 K. But surprisingly, the Kondo
coherence temperature decreases with increasing pressure, opposite to that in
most Ce-based heavy fermion compounds. Upon further compression, two
superconducting phases are revealed. At 48.0 GPa, the transition temperature
reaches 6.1 K, the highest among all Ce-based heavy fermion superconductors. We
argue for possible roles of valence tuning and fluctuations associated with its
special crystal structure in addition to the hybridization effect. These
unusual phase diagrams suggest that CeCuP2 is a novel platform for studying the
rich heavy fermions physics beyond the conventional Doniach paradigm. | cond-mat_str-el |
Strong Correlation, Bloch Bundle Topology and Spinless Haldane-Hubbard
Model: Different realizations of the Hubbard operators in different Hilbert spaces
give rise to various microscopic lattice electron models driven by strong
correlations. In terms of the Gutzwiller projected operators, the most familiar
examples are the t-J and the BCS-Hubbard models at strong coupling. We focus on
the spin-dopon representation of the Hubbard operators. In this case the no
double occupancy constraint (NDO) can be reexpressed as a Kondo interaction. As
an explicit example, the effective low energy action is derived in terms of
itinerant spineless fermions (dopons) strongly interacting with localized
lattice spins.The spontaneous breaking of time reversal symmetry describes a
spinless version of the Haldane-Hubbard topological theory. Our consideration
suggests that the topologically non-trivial U(1) Bloch bundle associated with
this model can be realized dynamically due to the presence of strong
correlations even in the absence of any external flux. | cond-mat_str-el |
Spin-gap opening accompanied by a strong magnetoelastic response in the
S=1 magnetic dimer system Ba3BiRu2O9: Neutron diffraction, magnetization, resistivity, and heat capacity
measurements on the 6H-perovskite Ba3BiRu2O9 reveal simultaneous magnetic and
structural dimerization driven by strong magnetoelastic coupling. An
isostructural but strongly displacive first-order transition on cooling through
T*=176 K is associated with a change in the nature of direct Ru-Ru bonds within
Ru2O9 face-sharing octahedra. Above T*, Ba3BiRu2O9 is an S=1 magnetic dimer
system with intradimer exchange interactions J0/kB=320 K and interdimer
exchange interactions J'/kB=-160 K. Below T*, a spin-gapped state emerges with
\Delta\approx220 K. Ab initio calculations confirm antiferromagnetic exchange
within dimers, but the transition is not accompanied by long range-magnetic
order. | cond-mat_str-el |
Non-local scaling operators with entanglement renormalization: The multi-scale entanglement renormalization ansatz (MERA) can be used, in
its scale invariant version, to describe the ground state of a lattice system
at a quantum critical point. From the scale invariant MERA one can determine
the local scaling operators of the model. Here we show that, in the presence of
a global symmetry $\mathcal{G}$, it is also possible to determine a class of
non-local scaling operators. Each operator consist, for a given group element
$g\in\mathcal{G}$, of a semi-infinite string $\tGamma_g$ with a local operator
$\phi$ attached to its open end. In the case of the quantum Ising model,
$\mathcal{G}= \mathbb{Z}_2$, they correspond to the disorder operator $\mu$,
the fermionic operators $\psi$ and $\bar{\psi}$, and all their descendants.
Together with the local scaling operators identity $\mathbb{I}$, spin $\sigma$
and energy $\epsilon$, the fermionic and disorder scaling operators $\psi$,
$\bar{\psi}$ and $\mu$ are the complete list of primary fields of the Ising
CFT. Thefore the scale invariant MERA allows us to characterize all the
conformal towers of this CFT. | cond-mat_str-el |
Current orderings of interacting electrons in bilayer graphene: By taking into account the possibility of all the intralayer as well as the
interlayer current orderings, we derive an eight-band model for interacting
electrons in bilayer graphene. With the numerical solution to the model, we
show that only the current orderings between the same sublattice sites can
exist within the range of the physical interacting strength. This result
confirms our previous model of spin-polarized-current phase for the
ground-state of interacting electrons in bilayer graphene that resolves a
number of experimental puzzles. | cond-mat_str-el |
Evaluation of High Order Terms for the Hubbard Model in the
Strong-coupling Limit: The ground-state energy of the Hubbard model on a Bethe lattice with infinite
connectivity at half filling is calculated for the insulating phase. Using
Kohn's transformation to derive an effective Hamiltonian for the
strong-coupling limit, the resulting class of diagrams is determined. We
develop an algorithm for an algebraic evaluation of the contributions of
high-order terms and check it by applying it to the Falicov-Kimball model that
is exactly solvable. For the Hubbard model, the ground-state energy is exactly
calculated up to order t^12/U^11. The results of the strong-coupling expansion
deviate from numerical calculations as quantum Monte Carlo (or density-matrix
renormalization-group) by less than 0.13% (0.32% respectively) for U>4.76. | cond-mat_str-el |
Emergence of the XY-like phase in the deformed spin-3/2 AKLT systems: Affleck, Kennedy, Lieb and Taski (AKLT) constructed an exemplary spin-3/2
valence-bond solid (VBS) state on the hexagonal lattice, which is the ground
state of an isotropic quantum antiferromagnet and possesses no spontaneous
magnetization but finite correlation length. This is distinct from the N\'eel
ordered state of the spin-3/2 Heisenberg model on the same lattice. Niggemann,
Kl\"umper and Zittartz then generalized the AKLT Hamiltonian to one family
invariant under spin rotation about the z-axis. The ground states of this
family can be parameterized by a single parameter that deforms the AKLT state,
and this system exhibits a quantum phase transition between the VBS and N\'eel
phases, as the parameter increases from the AKLT point to large anisotropy. We
investigate the opposite regime when the parameter decreases from the AKLT
point and find that there appears to be a Berezinskii-Kosterlitz-Thouless-like
transition from the VBS phase to an XY phase. Such a transition also occurs in
the deformation of other types of AKLT states with triplet-bond constructions
on the same lattice. However, we do not find such an XY-like phase in the
deformed AKLT models on other trivalent lattices, such as square-octagon, cross
and star lattices. On the star lattice, the deformed family of AKLT states
remain in the same phase as the isotropic AKLT state throughout the whole
region of the parameter. However, for two triplet-bond generalizations, the
triplet VBS phase is sandwiched between two ferromagnetic phases (for large and
small deformation parameters, respectively), which are characterized by
spontaneous magnetizations along different axes. Along the way, we also discuss
how various deformed AKLT states can be used for the purpose of universal
quantum computation. | cond-mat_str-el |
Crucial role of Internal Collective Modes in Underdoped Cuprates: The enigmatic cuprate superconductors have attracted resurgent interest with
several recent reports and discussions of competing orders in the underdoped
side. Motivated by this, here we address the natural question of fragility of
the d-wave superconducting state in underdoped cuprates. Using a combination of
theoretical approaches we study t-J like models, and discover an - as yet
unexplored - instability that is brought about by an "internal" (anti-symmetric
mode) fluctuation of the d-wave state. This new theoretical result is in good
agreement with recent STM and ARPES studies of cuprates. We also suggest
experimental directions to uncover this physics. | cond-mat_str-el |
Kondo resonances and Fano antiresonances in transport through quantum
dots: The transmission of electrons through a non-interacting tight-binding chain
with an interacting side quantum dot (QD) is analized. When the Kondo effect
develops at the dot the conductance presents a wide minimum, reaching zero at
the unitary limit. This result is compared to the opposite behaviour found in
an embedded QD. Application of a magnetic field destroys the Kondo effect and
the conductance shows pairs of dips separated by the charging energy U. The
results are discussed in terms of Fano antiresonances and explain qualitatively
recent experimental results. | cond-mat_str-el |
Theory of Large Intrinsic Spin Hall Effect in Iridate Semimetals: We theoretically investigate the mechanism to generate large intrinsic spin
Hall effect in iridates or more broadly in 5d transition metal oxides with
strong spin-orbit coupling. We demonstrate such a possibility by taking the
example of orthorhombic perovskite iridate with nonsymmorphic lattice symmetry,
SrIrO$_3$, which is a three-dimensional semimetal with nodal line spectrum. It
is shown that large intrinsic spin Hall effect arises in this system via the
spin-Berry curvature originating from the nearly degenerate electronic spectra
surrounding the nodal line. This effect exists even when the nodal line is
gently gapped out, due to the persistent nearly degenerate electronic
structure, suggesting a distinct robustness. The magnitude of the spin Hall
conductivity is shown to be comparable to the best known example such as doped
topological insulators and the biggest in any transition metal oxides. To gain
further insight, we compute the intrinsic spin Hall conductivity in both of the
bulk and thin film systems. We find that the geometric confinement in thin
films leads to significant modifications of the electronic states, leading to
even bigger spin Hall conductivity in certain cases. We compare our findings
with the recent experimental report on the discovery of large spin Hall effect
in SrIrO$_3$ thin films. | cond-mat_str-el |
A non-perturbative study of bulk photovoltaic effect enhanced by an
optically induced phase transition: Solid systems with strong correlations and interactions under light
illumination have the potential for exhibiting interesting bulk photovoltaic
behavior in the non-perturbative regime, which has remained largely unexplored
in the past theoretical studies. We investigate the bulk photovoltaic response
of a perovskite manganite with strongly coupled electron-spin-lattice dynamics,
using real-time simulations performed with a tight-binding model. The transient
changes in the band structure and the photoinduced phase transitions, emerging
from spin and phonon dynamics, result in a nonlinear current versus intensity
behavior beyond the perturbative limit. The current rises sharply across a
photoinduced magnetic phase transition, which later saturates at higher light
intensities due to excited phonon and spin modes. The predicted peak
photoresponsivity is orders of magnitude higher than other known ferroelectric
oxides such as BiFeO$_3$. We disentangle phonon-and spin-assisted components to
the ballistic photocurrent, showing that they are comparable in magnitude. Our
results illustrate a promising alternative way for controlling and optimizing
the bulk photovoltaic response through the photoinduced phase transitions in
strongly-correlated systems. | cond-mat_str-el |
Competeing orders in spin-1 and spin-3/2 XXZ Kagome antiferromagnets: A
series expansion study: We study the competition between $\sqrt{3} \times \sqrt{3}$ (RT3) and $q=0$
(Q0) magnetic orders in spin-one and spin-$3/2$ Kagome-lattice XXZ
antiferromagnets with varying XY anisotropy parameter $\Delta$, using series
expansion methods. The Hamiltonian is split into two parts: an $H_0$ which
favors the classical order in the desired pattern and an $H_1$, which is
treated in perturbation theory by a series expansion. We find that the ground
state energy series for the RT3 and Q0 phases are identical up to sixth order
in the expansion, but ultimately a selection occurs, which depends on spin and
the anisotropy $\Delta$. Results for ground state energy and the magnetization
are presented. These results are compared with recent spin-wave theory and
coupled-cluster calculations. The series results for the phase diagram are
close to the predictions of spin-wave theory. For the spin-one model at the
Heisenberg point ($\Delta=1$), our results are consistent with a vanishing
order parameter, that is an absence of a magnetically ordered phase. We also
develop series expansions for the ground state energy of the spin-one
Heisenberg model in the trimerized phase. We find that the ground state energy
in this phase is lower than those of magnetically ordered ones, supporting the
existence of a spontaneously trimerized phase in this model. | cond-mat_str-el |
Diagnostics for plasmon satellites and Hubbard bands in transition metal
oxides: Coulomb correlations between the electrons imprint characteristic signatures
to the spectral properties of materials. Among others, they are at the origin
of a rich phenomenology of satellite features, either stemming from atomic-like
multiplets or from interactions with particle-hole excitations or plasmons.
While in many cases the latter lie at considerably higher energies than the
former, suggesting clear distinction criteria, this picture has recently become
blurred by indications that satellites of different types can coexist in the
same energy range. It is now generally accepted that the identification of the
nature of spectral features is a highly non-trivial task. In this article we
propose a general procedure for tracing the origin of satellites of different
types within modern ab initio calculations. As an illustration, we analyze the
ternary transition metal oxides SrVO$_3$ and SrMoO$_3$, which are drosophila
compounds for the coexistence of Hubbard and plasmonic satellites, reconciling
previous seemingly contradictory findings in an unexpected manner. | cond-mat_str-el |
Quantum oscillations in the anomalous phase in Sr3Ru2O7: We report measurements of quantum oscillations detected in the putative
nematic phase of Sr3Ru2O7. Significant improvements in sample purity enabled
the resolution of small amplitude dHvA oscillations between two first order
metamagnetic transitions delimiting the phase. Two distinct frequencies were
observed, and their amplitudes follow the normal Lifshitz-Kosevich profile. The
Fermi surface sheets seem to correspond to a subset of those detected outside
the phase. Variations of the dHvA frequencies are explained in terms of a
chemical potential shift produced by reaching a peak in the density of states,
and an anomalous field dependence of the oscillatory amplitude provides
information on domains. | cond-mat_str-el |
Multipole Ordering and Fluctuations in f-Electron Systems: We investigate effects of multipole moments in f-electron systems both from
phenomenological and microscopic viewpoints. First, we discuss significant
effects of octupole moment on the magnetic susceptibility in a paramagnetic
phase. It is found that even within mean-field approximation, the magnetic
susceptibility deviates from the Curie-Weiss law due to interactions between
dipole and octupole moments. Next, we proceed to a microscopic theory for
multipole ordering on the basis of a j-j coupling scheme. After brief
explanation of a method to derive multipole interactions from the $f$-electron
model, we discuss several multipole ordered phases depending on lattice
structure. Finally, we show our new development of the microscopic approach to
the evaluation of multipole response functions. We apply fluctuation exchange
approximation to the f-electron model, and evaluate multipole response
functions. | cond-mat_str-el |
Thermal conductivity of anisotropic and frustrated spin-1/2 chains: We analyze the thermal conductivity of anisotropic and frustrated spin-1/2
chains using analytical and numerical techniques. This includes mean-field
theory based on the Jordan-Wigner transformation, bosonization, and exact
diagonalization of systems with N<=18 sites. We present results for the
temperature dependence of the zero-frequency weight of the conductivity for
several values of the anisotropy \Delta. In the gapless regime, we show that
the mean-field theory compares well to known results and that the
low-temperature limit is correctly described by bosonization. In the
antiferromagnetic and ferromagnetic gapped regime, we analyze the temperature
dependence of the thermal conductivity numerically. The convergence of the
finite-size data is remarkably good in the ferromagnetic case. Finally, we
apply our numerical method and mean-field theory to the frustrated chain where
we find a good agreement of these two approaches on finite systems. Our
numerical data do not yield evidence for a diverging thermal conductivity in
the thermodynamic limit in case of the antiferromagnetic gapped regime of the
frustrated chain. | cond-mat_str-el |
Orbital Localization and Delocalization Effects in the U 5f^2
Configuration: Impurity Problem: Anderson models, based on quantum chemical studies of the molecule of
U(C_8H_8)_2, are applied to investigate the problem of an U impurity in a
metal. The special point here is that the U 5f-orbitals are divided into two
subsets: an almost completely localized set and a considerably delocalized one.
Due to the crystal field, both localized and delocalized U 5f-orbitals affect
the low-energy physics. A numerical renormalization group study shows that
every fixed point is characterized by a residual local spin and a phase shift.
The latter changes between 0 and \pi/2, which indicates the competition between
two different fixed points. Such a competition between the different local
spins at the fixed points reflects itself in the impurity magnetic
susceptibility at high temperatures. These different features cannot be
obtained if the special characters of U 5f-orbitals are neglected. | cond-mat_str-el |
Kinetic theory of the non-local electrodynamic response in anisotropic
metals: skin effect in 2D systems: The electrodynamic response of ultra-pure materials at low temperatures
becomes spatially non-local. This non-locality gives rise to phenomena such as
hydrodynamic flow in transport and the anomalous skin effect in optics. In
systems characterized by an anisotropic electronic dispersion, the non-local
dynamics becomes dependent on the relative orientation of the sample with
respect to the applied field, in ways that go beyond the usual, homogeneous
response. Such orientational dependence should manifest itself not only in
transport experiments, as recently observed, but also in optical spectroscopy.
In this paper we develop a kinetic theory for the distribution function and the
transverse conductivity of two- and three-dimensional Fermi systems with
anisotropic electronic dispersion. By expanding the collision integral into the
eigenbasis of a collision operator, we include momentum-relaxing scattering as
well as momentum-conserving collisions. We examine the isotropic 2D case as a
reference, as well as anisotropic hexagonal and square Fermi-surface shapes. We
apply our theory to the quantitative calculation of the skin depth and the
surface impedance, in all regimes of skin effect. We find qualitative
differences between the frequency dependence of the impedance in isotropic and
anisotropic systems. Such differences are shown to persist even for more
complex 2D Fermi surfaces, including the ''supercircle'' geometry and an
experimental parametrization for PdCoO$_2$, which deviate from an ideal
polygonal shape. We study the orientational dependence of skin effect due to
Fermi-surface anisotropy, thus providing guidance for the experimental study of
non-local optical effects. | cond-mat_str-el |
Low rank Green's function representations applied to dynamical
mean-field theory: Several recent works have introduced highly compact representations of
single-particle Green's functions in the imaginary time and Matsubara frequency
domains, as well as efficient interpolation grids used to recover the
representations. In particular, the intermediate representation with sparse
sampling and the discrete Lehmann representation (DLR) make use of low-rank
compression techniques to obtain optimal approximations with controllable
accuracy. We consider the use of the DLR in dynamical mean-field theory (DMFT)
calculations, and in particular, show that the standard full Matsubara
frequency grid can be replaced by the compact grid of DLR Matsubara frequency
nodes. We test the performance of the method for a DMFT calculation of
Sr$_2$RuO$_4$ at temperature $50$K using a continuous-time quantum Monte Carlo
impurity solver, and demonstrate that Matsubara frequency quantities can be
represented on a grid of only $36$ nodes with no reduction in accuracy, or
increase in the number of self-consistent iterations, despite the presence of
significant Monte Carlo noise. | cond-mat_str-el |
Are multiphase competition & order-by-disorder the keys to understanding
Yb2Ti2O7?: If magnetic frustration is most commonly known for undermining long-range
order, as famously illustrated by spin liquids, the ability of matter to
develop new collective mechanisms in order to fight frustration is no less
fascinating, providing an avenue for the exploration and discovery of
unconventional properties of matter. Here we study an ideal minimal model of
such mechanisms which, incidentally, pertains to the perplexing quantum spin
ice candidate Yb2Ti2O7. Specifically, we explain how thermal and quantum
fluctuations, optimized by order-by-disorder selection, conspire to expand the
stability region of an accidentally degenerate continuous symmetry U(1)
manifold against the classical splayed ferromagnetic ground state that is
displayed by the sister compound Yb2Sn2O7. The resulting competition gives rise
to multiple phase transitions, in striking similitude with recent experiments
on Yb2Ti2O7 [Lhotel et al., Phys. Rev. B 89 224419 (2014)]. Considering the
effective Hamiltonian determined for Yb2Ti2O7, we provide, by combining a gamut
of numerical techniques, compelling evidence that such multiphase competition
is the long-sought missing key to understanding the intrinsic properties of
this material. As a corollary, our work offers a pertinent illustration of the
influence of chemical pressure in rare-earth pyrochlores. | cond-mat_str-el |
From itinerant to local-moment antiferromagnetism in Kondo lattices:
Adiabatic continuity vs. quantum phase transitions: Motivated by both experimental and theoretical activities, we discuss the
fate of Kondo screening and possible quantum phase transitions in
antiferromagnetically ordered phases of Kondo lattices. While transitions with
topological changes of the Fermi surface may occur, we demonstrate that an
entirely continuous evolution from itinerant to local-moment antiferromagnetism
(i.e. from strong to negligible Kondo screening) is possible as well. This
situation is in contrast to that in a non-symmetry-broken situation where a
quantum phase transition towards an exotic metallic spin-liquid state
necessarily accompanies the disappearance of Kondo screening. We discuss
criteria for the existence of topological transitions in the antiferromagnetic
phase, as well as implications for theoretical scenarios and for current
experiments. | cond-mat_str-el |
Hund electronic correlation in La$_3$Ni$_2$O$_7$ under high pressure: By means of density functional theory plus dynamical mean-field theory
(DFT+DMFT), we investigate the correlated electronic structures of
La$_3$Ni$_2$O$_7$ under high pressure. Our calculations show that
La$_3$Ni$_2$O$_7$ is a multi-orbital Hund metal. Both the 3$d_{z^2}$ and
3$d_{x^2 - y^2}$ orbitals of Ni are close to be half filled and contribute the
bands across the Fermi level. Band renormalization and orbital selective
electronic correlation are observed. Through imaginary-time correlation
functions, the discovery of high-spin configuration, spin-frozen phase, and
spin-orbital separation shows that the system is in a frozen moment phase at
high temperatures above 290 K and is a Fermi liquid at low temperatures, which
is further comfirmed by the calculated spin, orbital, and charge
susceptibilities under high temperatures. Our study uncovers Hundness in
La$_3$Ni$_2$O$_7$ under high pressure. | cond-mat_str-el |
Screened hybrid functional applied to 3d^0-->3d^8 transition-metal
perovskites LaMO3 (M=Sc-Cu): influence of the exchange mixing parameter on
the structural, electronic and magnetic properties: We assess the performance of the Heyd-Scuseria-Ernzerhof (HSE) screened
hybrid density functional scheme applied to the perovskite family LaMO3
(M=Sc-Cu) and discuss the role of the mixing parameter alpha (which determines
the fraction of exact Hartree-Fock exchange included in the density functional
theory (DFT) exchange-correlation functional) on the structural, electronic,
and magnetic properties. The physical complexity of this class of compounds,
manifested by the largely varying electronic characters
(band/Mott-Hubbard/charge-transfer insulators and metals), magnetic orderings,
structural distortions (cooperative Jahn-Teller like instabilities), as well as
by the strong competition between localization/delocalization effects
associated with the gradual filling of the t_2g and e_g orbitals, symbolize a
critical and challenging case for theory. Our results indicates that HSE is
able to provide a consistent picture of the complex physical scenario
encountered across the LaMO3 series and significantly improve the standard DFT
description. The only exceptions are the correlated paramagnetic metals LaNiO3
and LaCuO3, which are found to be treated better within DFT. By fitting the
ground state properties with respect to alpha we have constructed a set of
'optimum' values of alpha from LaScO3 to LaCuO3: it is found that the 'optimum'
mixing parameter decreases with increasing filling of the d manifold (LaScO3:
0.25; LaTiO3 & LaVO3: 0.10-0.15; LaCrO3, LaMnO3, and LaFeO3: 0.15; LaCoO3:
0.05; LaNiO3 & LaCuO3: 0). This trend can be nicely correlated with the
modulation of the screening and dielectric properties across the LaMO3 series,
thus providing a physical justification to the empirical fitting procedure. | cond-mat_str-el |
Angular dependence of Hall effect and magnetoresistance in
SrRuO$_3$-SrIrO$_3$ heterostructures: Perovskite SrRuO$_3$ is a prototypical itinerant ferromagnet which allows
interface engineering of its electronic and magnetic properties. We report
synthesis and investigation of atomically flat artificial multilayers of
SrRuO$_3$ with the spin-orbit semimetal SrIrO$_3$ in combination with
band-structure calculations with a Hubbard $U$ term and topological analysis.
They reveal an electronic reconstruction and emergence of flat Ru-4d$_{xz}$
bands near the interface, ferromagnetic interlayer coupling and negative
Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall
effect and magnetoresistance measurements as a function of the field angle from
out of plane towards in-plane orientation (either parallel or perpendicular to
the current direction) by a two-channel model. The magnetic easy direction is
tilted by about $20^\circ$ from the sample normal for low magnetic fields,
rotating towards the out-of-plane direction by increasing fields. Fully
strained epitaxial growth enables a strong anisotropy of magnetoresistance. An
additional Hall effect contribution, not accounted for by the two-channel model
is compatible with stable skyrmions only up to a critical angle of roughly
$45^\circ$ from the sample normal. Within about $20^\circ$ from the thin film
plane an additional peak-like contribution to the Hall effect suggests the
formation of a non-trivial spin structure. | cond-mat_str-el |
Anisotropic magnetoresistance and piezoelectric effect in GaAs Hall
samples: In this work, we argue that an anisotropic interaction potential may
stabilize anisotropic liquid phases of electrons even in a strong magnetic
field regime where normally one expects to see only isotropic quantum Hall or
isotropic Fermi liquid states. We use this approach to support a theoretical
framework that envisions the possibility of an anisotropic liquid crystalline
state of electrons in the lowest Landau level. In particular, we argue that an
anisotropic liquid state of electrons may stabilize in the lowest Landau level
close to the liquid-solid transition region at filling factor $\nu=1/6$ for a
given anisotropic Coulomb interaction potential. Quantum Monte Carlo
simulations for a liquid crystalline state with broken rotational symmetry
indicate stability of liquid crystalline order consistent with the existence of
an anisotropic liquid state of electrons stabilized by anisotropy at filling
factor $\nu=1/6$ of the lowest Landau level. | cond-mat_str-el |
Magnetic Order in Laser-Irradiated Kagome Antiferromagnets: Dispersionless "zero energy mode'' is one of the hallmarks of frustrated
kagome antiferromagnets (KAFMs). It points to extensive classically degenerate
ground-states. The "zero energy mode'' can be observed experimentally when
lifted to a flat mode at finite energy by a strong intrinsic magnetic
anisotropy. In this letter, we study the effects of irradiation of laser light
on the KAFMs. We adopt the magnon picture without loss of generality. It is
shown that circularly or linearly polarized light lifts the "zero energy
mode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We
find that the circularly polarized light-induced anisotropies have similar
features as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya
interaction in KAFMs. The former stabilizes long-range magnetic order and the
latter induces spin canting out-of-plane with nonzero scalar spin chirality.
The Floquet thermal Hall effect shows that the synthetic magnetic excitation
modes in the case of circularly polarized light are topological, whereas those
of linearly polarized light are not. | cond-mat_str-el |
Inducing topological order in a honeycomb lattice: We explore the possibility of inducing a topological insulator phase in a
honeycomb lattice lacking spin-orbit interaction using a metallic (or Fermi
gas) environment. The lattice and the metallic environment interact through a
density-density interaction without particle tunneling, and integrating out the
metallic environment produces a honeycomb sheet with in-plane oscillating
long-ranged interactions. We find the ground state of the interacting system in
a variational mean-field method and show that the Fermi wave vector, kF, of the
metal determines which phase occurs in the honeycomb lattice sheet. This is
analogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism in which the
metal's kF determines the interaction profile as a function of the distance.
Tuning kF and the interaction strength may lead to a variety of ordered phases,
including a topological insulator and anomalous quantum-hall states with
complex next-nearest-neighbor hopping, as in the Haldane and the Kane-Mele
model. We estimate the required range of parameters needed for the topological
state and find that the Fermi vector of the metallic gate should be of the
order of 3Pi/8a (with a being the graphene lattice constant). The net coupling
between the layers, which includes screening in the metal, should be of the
order of the honeycomb lattice bandwidth. This configuration should be most
easily realized in a cold-atoms setting with two interacting Fermionic species. | cond-mat_str-el |
Spectral signatures of the Luttinger liquid to charge-density-wave
transition: Electron- and phonon spectral functions of the one-dimensional,
spinless-fermion Holstein model at half filling are calculated in the four
distinct regimes of the phase diagram, corresponding to an attractive or
repulsive Luttinger liquid at weak electron-phonon coupling, and a band- or
polaronic insulator at strong coupling. The results obtained by means of kernel
polynomial and systematic cluster approaches reveal substantially different
physics in these regimes and further indicate that the size of the phonon
frequency significantly affects the nature of the quantum Peierls phase
transition. | cond-mat_str-el |
Ground-State Phase Diagram of (1/2,1/2,1) Mixed Diamond Chains: The ground-state phases of mixed diamond chains with ($S, \tau^{(1)},
\tau^{(2)})=(1/2,1/2,1)$, where $S$ is the magnitude of vertex spins, and
$\tau^{(1)}$ and $\tau^{(2)}$ are those of apical spins, are investigated. The
two apical spins in each unit cell are coupled by an exchange coupling
$\lambda$. The vertex spins are coupled with the top and bottom apical spins by
exchange couplings $1+\delta$ and $1-\delta$, respectively. Although this model
has an infinite number of local conservation laws for $\delta=0$, they are lost
for finite $\delta$. The ground-state phase diagram is determined using the
numerical exact diagonalization and DMRG method in addition to the analytical
approximations in various limiting cases. The phase diagram consists of a
nonmagnetic phase and several kinds of ferrimagnetic phases. We find two
different ferrimagnetic phases without spontaneous translational symmetry
breakdown. It is also found that the quantized ferrimagnetic phases with large
spatial periodicities present for $\delta=0$ are easily destroyed by small
$\delta$ and replaced by a partial ferrimagnetic phase. The nonmagnetic phase
is considered to be a gapless Tomonaga-Luttinger liquid phase based on the
recently extended Lieb-Schultz-Mattis theorem to the site-reflection invariant
spin chains and numerical diagonalization results. | cond-mat_str-el |
The free energy of anisotropic quantum spin systems: Functional integral
representation: In this work, we propose a method for calculating the free energy of
anisotropic quantum spin systems. We use the Hubbard-Stratonovich
transformation to express the partition function of a generic bilinear
super-exchange Hamiltonian in terms of a functional integral over classical
time-dependent fields. In the general case the result is presented as an
infinite series. The series may be summed up in the case of Ising-type models.
For any ordered state we derive a compact expression for the contribution of
Gaussian spin fluctuations to the free energy. | cond-mat_str-el |
The electronic structure of the high-symmetry perovskite iridate Ba2IrO4: We report angle-resolved photoemission (ARPES) measurements, density
functional and model tight-binding calculations on Ba$_2$IrO$_4$ (Ba-214), an
antiferromagnetic ($T_N=230$ K) insulator. Ba-214 does not exhibit the
rotational distortion of the IrO$_6$ octahedra that is present in its sister
compound Sr$_2$IrO$_4$ (Sr-214), and is therefore an attractive reference
material to study the electronic structure of layered iridates. We find that
the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at
the predominant role of the spin-orbit interaction in these materials.
Temperature-dependent ARPES data show that the energy gap persists well above
$T_N$, and favour a Mott over a Slater scenario for this compound. | cond-mat_str-el |
Field induced tricritical behavior in the S=1/2 quasi one-dimensional
frustrated Ising antiferromagnet: The results of extensive histogram cluster heat-bath Monte Carlo simulations
on the critical behavior of the quasi-one dimensional Ising antiferromagnet on
a stacked triangular lattice are presented. A small applied field is shown to
induce a crossover from XY universality to mean-field tricritical behavior.
Experimental estimates of critical exponents suggest that these two types of
phase transitions are observed in S=1 CsNiCl$_3$ and $S=1/2$ CsCoBr$_3$,
respectively. The present results demonstrate that this difference can be
explained by an unusual staggered magnetic field arising from quantum exchange
mixing previously proposed to account for spin excitations in $S=1/2$
quasi-one-dimensional Ising antiferromagnets. | cond-mat_str-el |
Diagrammatic quantum Monte Carlo study of an acoustic lattice polaron: We present the first approximation free diagrammatic Monte Carlo study of a
lattice polaron interacting with an acoustic phonon branch through the
deformation potential. Weak and strong coupling regimes are separated by a
self-trapping region where quantum resonance between various possible lattice
deformations is seen in the ground state properties, spectral function, and
optical conductivity. The unique feature of such polaron is the interplay
between long- and short wavelength acoustic vibrations creating a composite
phonon cloud and leading to persistent self-trapping due to the existence of
multiple quasi-stable states. This results in a spectral response whose
structure is much more complex than in any of the previously considered polaron
models. | cond-mat_str-el |
Quantum Skyrmion Lattices in Heisenberg Ferromagnets: Skyrmions are topological magnetic textures that can arise in
non-centrosymmetric ferromagnetic materials. In most systems experimentally
investigated to date, skyrmions emerge as classical objects. However, the
discovery of skyrmions with nanometer length scales has sparked interest in
their quantum properties. Here, we simulate the ground states of
two-dimensional spin-$1/2$ Heisenberg lattices with Dzyaloshinskii-Moriya
interactions and discover a broad region in the zero-temperature phase diagram
which hosts quantum skyrmion lattices. We argue that the quantum skyrmion
lattice phase can be detected experimentally in the magnetization profile via
local magnetic polarization measurements as well as in the spin structure
factor measurable via neutron scattering experiments. Finally, we explore the
resulting quantum skyrmion state, analyze its real-space polarization profile
and show that it is a non-classical state featuring entanglement between
quasiparticle and environment mainly localized near the boundary spins of the
skyrmion. | cond-mat_str-el |
LDA+DMFT approach to resonant inelastic x-ray scattering in correlated
materials: We present a computational study of $L$-edge resonant inelastic x-ray
scattering (RIXS) in correlated 3$d$ transition-metal oxides using an $ab$
$initio$ method based on local density approximation + dynamical mean-field
theory (DMFT). The present method, building on Anderson impurity model with an
optimized continuum bath within DMFT, is an extension of the cluster model to
include unbound electron-hole pair excitations as well as material-specific
charge-transfer excitations with less empirical parameters. We find a good
agreement with available experimental data. The relationship between correlated
bands and fluorescence-like feature in the RIXS spectra is discussed. | cond-mat_str-el |
Magnetic model for A2CuP2O7 (A = Na, Li) revisited: 1D versus 2D
behavior: We report magnetization measurements, full-potential band structure
calculations, and microscopic modeling for the spin-1/2 Heisenberg magnets
A2CuP2O7 (A = Na, Li). Based on a quantitative evaluation of the leading
exchange integrals and the subsequent quantum Monte-Carlo simulations, we
propose a quasi-one-dimensional magnetic model for both compounds, in contrast
to earlier studies that conjectured on the two-dimensional scenario. The
one-dimensional nature of A2CuP2O7 is unambiguously verified by magnetization
isotherms measured in fields up to 50 T. The saturation fields of about 40 T
for both Li and Na compounds are in excellent agreement with the intrachain
exchange J1 ~ 27 K extracted from the magnetic susceptibility data. The
proposed magnetic structure entails spin chains with the dominating
antiferromagnetic nearest-neighbor interaction J1 and two inequivalent,
nonfrustrated antiferromagnetic interchain couplings of about 0.01*J1 each. A
possible long-range magnetic ordering is discussed in comparison with the
available experimental information. | cond-mat_str-el |
Topological Blocking in Quantum Quench Dynamics: We study the non-equilibrium dynamics of quenching through a quantum critical
point in topological systems, focusing on one of their defining features:
ground state degeneracies and associated topological sectors. We present the
notion of 'topological blocking', experienced by the dynamics due to a mismatch
in degeneracies between two phases and we argue that the dynamic evolution of
the quench depends strongly on the topological sector being probed. We
demonstrate this interplay between quench and topology in models stemming from
two extensively studied systems, the transverse Ising chain and the Kitaev
honeycomb model. Through non-local maps of each of these systems, we
effectively study spinless fermionic $p$-wave paired superconductors. Confining
the systems to ring and toroidal geometries, respectively, enables us to
cleanly address degeneracies, subtle issues of fermion occupation and parity,
and mismatches between topological sectors. We show that various features of
the quench, which are related to Kibble-Zurek physics, are sensitive to the
topological sector being probed, in particular, the overlap between the
time-evolved initial ground state and an appropriate low-energy state of the
final Hamiltonian. While most of our study is confined to translationally
invariant systems, where momentum is a convenient quantum number, we briefly
consider the effect of disorder and illustrate how this can influence the
quench in a qualitatively different way depending on the topological sector
considered. | cond-mat_str-el |
Orbital polarons in the metal-insulator transition of manganites: The metal-insulator transition in manganites is strongly influenced by the
concentration of holes present in the system. Based upon an orbitally
degenerate Mott-Hubbard model we analyze two possible localization scenarios to
account for this doping dependence: First, we rule out that the transition is
initiated by a disorder-order crossover in the orbital sector, showing that its
effect on charge itineracy is only small. Second, we introduce the idea of
orbital polarons originating from a strong polarization of orbitals in the
vicinity of holes. Considering this direct coupling between charge and orbital
degree of freedom in addition to lattice effects we are able to explain well
the phase diagram of manganites for low and intermediate hole concentrations. | cond-mat_str-el |
Quantized gravitational responses and the sign problem: It is believed that not all quantum systems can be simulated efficiently
using classical computational resources. This notion is supported by the fact
that in quantum Monte Carlo (QMC) simulations for a large number of important
problems it is not known how to express the partition function in a sign-free
manner. The answer to the question --- whether there is an fundamental
obstruction to such a sign-free representation in generic quantum systems ---
remains unclear. Here, focussing on systems with bosonic degrees of freedom, we
show that quantized gravitational responses appear as obstructions to local
sign-free QMC. In condensed matter physics settings these responses, such as
thermal Hall conductance, are associated with fractional quantum Hall effects.
We show that similar arguments hold also in the case of spontaneously broken
time-reversal (TR) symmetry such as in the chiral phase of a perturbed quantum
Kagome antiferromagnet. The connection between quantized gravitational
responses and the sign problem is also clearly manifested in certain vertex
models, where TR symmetry is preserved. | cond-mat_str-el |
Unconventional many-body phase transitions in a non-Hermitian Ising
chain: We study many-body phase transitions in a one-dimensional ferromagnetic
transversed field Ising model with an imaginary field and show that the system
exhibits three phase transitions: one second-order phase transition and two
$\mathcal{PT}$ phase transitions. The second-order phase transition occurring
in the ground state is investigated via biorthogonal and self-normal
entanglement entropy, for which we develop an approach to perform finite-size
scaling theory to extract the central charge for small systems. Compared with
the second-order phase transition, the first $\mathcal{PT}$ transition is
characterized by the appearance of an exceptional point in the full energy
spectrum, while the second $\mathcal{PT}$ transition only occurs in specific
excited states. Furthermore, we interestingly show that both of exceptional
points are second-order in terms of scalings of imaginary parts of the energy.
This work provides an exact solution for unconventional many-body phase
transitions in non-Hermitian systems. | cond-mat_str-el |
Matrix product states approaches to operator spreading in ergodic
quantum systems: We review different tensor network approaches to study the spreading of
operators in generic nonintegrable quantum systems. As a common ground to all
methods, we quantify this spreading by means of the Frobenius norm of the
commutator of a spreading operator with a local operator, which is usually
referred to as the out of time order correlation (OTOC) function. We compare
two approaches based on matrix-product states in the Schr\"odinger picture: the
time dependent block decimation (TEBD) and the time dependent variational
principle (TDVP), as well as TEBD based on matrix-product operators directly in
the Heisenberg picture. The results of all methods are compared to numerically
exact results using Krylov space exact time evolution. We find that for the
Schr\"odinger picture the TDVP algorithm performs better than the TEBD
algorithm. Moreover the tails of the OTOC are accurately obtained both by TDVP
MPS and TEBD MPO. They are in very good agreement with exact results at short
times, and appear to be converged in bond dimension even at longer times.
However the growth and saturation regimes are not well captured by both
methods. | cond-mat_str-el |
Charge collective modes in strongly correlated electron systems with
long range interactions: Elucidating the impact of strong electronic correlations on the collective
modes of metallic systems has been of longstanding interest, mainly due to the
inadequacy of the random phase approximation (RPA) in the strongly correlated
regime. In his regard, we analyze the charge excitation spectrum of a Hubbard
model on the face centered cubic lattice, extended with long range
interactions, in different coupling regimes ranging from uncorrelated to the
metal-to-insulator transition at half filling. We argue that the slave boson
representation introduced by Kotliar and Ruckenstein, when formulated in radial
gauge, constitutes a suitable framework to carry out this endeavor, and we
compare its results to conventional RPA as a benchmark. We focus on the
influence of the local and long range couplings on the particle-hole excitation
continuum and the quantum collective phenomena generically comprised in our
spectra, and find numerous qualitative and quantitative discrepancies between
our method and standard RPA in the intermediate-to-strong coupling regime. At
the onset of the Mott transition, the plasmon gap is found to vanish,
supporting a quasiparticle description of the mode. | cond-mat_str-el |
Relativistic and thermal effects on the magnon spectrum of a
ferromagnetic monolayer: A spin model including magnetic anisotropy terms and Dzyaloshinsky-Moriya
interactions is studied for the case of a ferromagnetic monolayer with C2v
symmetry like Fe/W(110). Using the quasiclassical stochastic
Landau-Lifshitz-Gilbert equations, the magnon spectrum of the system is derived
using linear response theory. The Dzyaloshinsky-Moriya interaction leads to
asymmetry in the spectrum, while the anisotropy terms induce a gap. It is shown
that in the presence of lattice defects, both the Dzyaloshinsky-Moriya
interactions and the two-site anisotropy lead to a softening of the magnon
energies. Two methods are developed to investigate the magnon spectrum at
finite temperatures. The theoretical results are compared to atomistic spin
dynamics simulations and a good agreement is found between them. | cond-mat_str-el |
Metal-Insulator-Like Behavior in Semimetallic Bismuth and Graphite: When high quality bismuth or graphite crystals are placed in a magnetic field
directed along the c-axis (trigonal axis for bismuth) and the temperature is
lowered, the resistance increases as it does in an insulator but then
saturates. We show that the combination of unusual features specific to
semimetals, i.e., low carrier density, small effective mass, high purity, and
an equal number of electrons and holes (compensation), gives rise to a unique
ordering and spacing of three characteristic energy scales, which not only is
specific to semimetals but which concomitantly provides a wide window for the
observation of apparent field induced metal-insulator behavior. Using
magnetotransport and Hall measurements, the details of this unusual behavior
are captured with a conventional multi-band model, thus confirming the
occupation by semimetals of a unique niche between conventional metals and
semiconductors. | cond-mat_str-el |
Real Space Coulomb Interaction: A Pairing Glue for FeAs Superconductors: In this paper we present a real space pairing glue for the iron-based layered
superconductors. It is shown that two static electrons embedded symmetrically
into two adjacent Fe plaquettes of the superconductor can be bounded due to the
Coulombic interaction. The pairing mechanism favors the existence of the
pseudogap in the underdoped FeAs superconductors. A criterion is introduced to
distinguish whether or not the pseudogap can open in a material. | cond-mat_str-el |
Anomalies in bosonic SPT edge theories: connection to F-symbols and a
method for calculation: We describe a systematic procedure for determining the identity of a 2D
bosonic symmetry protected topological (SPT) phase from the properties of its
edge excitations. Our approach applies to general bosonic SPT phases with
either unitary or antiunitary symmetries, and with either continuous or
discrete symmetry groups, with the only restriction being that the symmetries
must be on-site. Concretely, our procedure takes a bosonic SPT edge theory as
input, and produces an element $\omega$ of the cohomology group $H^3(G,
U_T(1))$. This element $\omega \in H^3(G, U_T(1))$ can be interpreted as either
a label for the bulk 2D SPT phase or a label for the anomaly carried by the SPT
edge theory. The basic idea behind our approach is to compute the $F$-symbol
associated with domain walls in a symmetry broken edge theory; this domain wall
$F$-symbol is precisely the anomaly we wish to compute. We demonstrate our
approach with several SPT edge theories including both lattice models and
continuum field theories. | cond-mat_str-el |
From the double-exchange Hamiltonian to the $t-J$ model: Classical spins: From the double-exchange Hamiltonian with classical localized spins in the
limit of large but finit Hund exchange coupling we obtain the $t-J$ model (with
classical localized spins). | cond-mat_str-el |
Theory of Twist Liquids: Gauging an Anyonic Symmetry: Topological phases in (2+1)-dimensions are frequently equipped with global
symmetries, like conjugation, bilayer or electric-magnetic duality, that
relabel anyons without affecting the topological structures. Twist defects are
static point-like objects that permute the labels of orbiting anyons. Gauging
these symmetries by quantizing defects into dynamical excitations leads to a
wide class of more exotic topological phases referred as twist liquids, which
are generically non-Abelian. We formulate a general gauging framework,
characterize the anyon structure of twist liquids and provide solvable lattice
models that capture the gauging phase transitions. We explicitly demonstrate
the gauging of the $\mathbb{Z}_2$-symmetric toric code, $SO(2N)_1$ and
$SU(3)_1$ state as well as the $S_3$-symmetric $SO(8)_1$ state and a
non-Abelian chiral state we call the "4-Potts" state. | cond-mat_str-el |
Thermal drag in spin ladders coupled to phonons: We study the spin-phonon drag effect in the magnetothermal transport of
spin-1/2 two-leg ladders coupled to lattice degrees of freedom. Using a bond
operator description for the triplon excitations of the spin ladder and
magnetoelastic coupling to acoustic phonons, we employ the time convolutionless
projection operator method to derive expressions for the diagonal and
off-diagonal thermal conductivities of the coupled two-component triplon-phonon
system. We find that for magnetoelastic coupling strengths and diagonal
scattering rates relevant to copper-oxide spin-ladders the drag heat
conductivity can be of similar magnitude as the diagonal triplon heat
conductivity. Moreover, we show that the drag and diagonal conductivities
display very similar overall temperature dependences. Finally, the drag
conductivity is shown to be rather susceptible to external magnetic fields. | cond-mat_str-el |
Twofold van Hove singularity and origin of charge order in topological
kagome superconductor CsV3Sb5: The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently
discovered family of topological kagome metals with a rich phenomenology of
strongly correlated electronic phases including charge order and
superconductivity. Understanding how the singularities inherent to the kagome
electronic structure are linked to the observed many-body phases is a topic of
great interest and relevance. Here, we combine angle-resolved photoemission
spectroscopy and density functional theory to reveal multiple kagome-derived
van Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and
analyze their contribution to electronic symmetry breaking. Intriguingly, the
vHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate
from their pure and mixed sublattice characters, respectively. This twofold vHs
is unique property of the kagome lattice, and its flavor critically determines
the pairing symmetry and ground states emerging in AV3Sb5 series. We establish
that, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome
band and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to
set the stage for electronic symmetry breaking. The former band exhibits
pronounced Fermi surface nesting, while the latter contributes via higher-order
vHs. Our work reveals the essential role of kagome-derived vHs for the
collective phenomena realized in the AV3Sb5 family, paving the way to a deeper
understanding of strongly correlated topological kagome systems. | cond-mat_str-el |
A Gate-tunable Polarized Phase of Two-Dimensional Electrons at the
LaAlO3/SrTiO3 Interface: Controlling the coupling between localized spins and itinerant electrons can
lead to exotic magnetic states. A novel system featuring local magnetic moments
and extended 2D electrons is the interface between LaAlO3 and SrTiO3. The
magnetism of the interface, however, was observed to be insensitive to the
presence of these electrons and is believed to arise solely from extrinsic
sources like oxygen vacancies and strain. Here we show the existence of
unconventional electronic phases in the LaAlO3/SrTiO3 system pointing to an
underlying tunable coupling between itinerant electrons and localized moments.
Using anisotropic magnetoresistance and anomalous Hall effect measurements in a
unique in-plane configuration, we identify two distinct phases in the space of
carrier density and magnetic field. At high densities and fields, the
electronic system is strongly polarized and shows a response, which is highly
anisotropic along the crystalline directions. Surprisingly, below a
density-dependent critical field, the polarization and anisotropy vanish
whereas the resistivity sharply rises. The unprecedented vanishing of the easy
axes below a critical field is in sharp contrast with other coupled magnetic
systems and indicates strong coupling with the moments that depends on the
symmetry of the itinerant electrons. The observed interplay between the two
phases indicates the nature of magnetism at the LaAlO3/SrTiO3 interface as both
having an intrinsic origin and being tunable. | cond-mat_str-el |
Strong enhancement of magnetic order from bulk to stretched monolayer
FeSe as Hund's metals: Despite of the importance of magnetism in possible relation to other key
properties in iron-based superconductors, its understanding is still far from
complete especially for FeSe systems. On one hand, the origin of the absence of
magnetic orders in bulk FeSe is yet to be clarified. On the other hand, it is
still not clear how close monolayer FeSe on SrTiO$_3$, with the highest
transition temperature among iron-based superconductors, is to a magnetic
instability. Here we investigate magnetic properties of bulk and monolayer FeSe
using dynamical mean-field theory combined with density-functional theory. We
find that suppressed magnetic order in bulk FeSe is associated with the
reduction of inter-orbital charge fluctuations, an effect of Hund's coupling,
enhanced by a larger crystal field splitting. Meanwhile, spatial isolation of
Fe atoms in expanded monolayer FeSe leads into a strong magnetic order, which
is completely destroyed by a small electron doping. Our work provides a
comprehensive understanding of the magnetic order in iron-based superconductors
and other general multi-orbital correlated systems as Hund's metals. | cond-mat_str-el |
How does a quadratic term in the energy dispersion modify the
single-particle Green's function of the Tomonaga-Luttinger model?: We calculate the effect of a quadratic term in the energy dispersion on the
low-energy behavior of the Green's function of the spinless Tomonaga-Luttinger
model (TLM). Assuming that for small wave-vectors q = k - k_F the fermionic
excitation energy relative to the Fermi energy is v_F q + q^2 / (2m), we
explicitly calculate the single-particle Green's function for finite but small
values of lambda = q_c /(2k_F). Here k_F is the Fermi wave-vector, q_c is the
maximal momentum transfered by the interaction, and v_F = k_F / m is the Fermi
velocity. Assuming equal forward scattering couplings g_2 = g_4, we find that
the dominant effect of the quadratic term in the energy dispersion is a
renormalization of the anomalous dimension. In particular, at weak coupling the
anomalous dimension is tilde{gamma} = gamma (1 - 2 lambda^2 gamma), where gamma
is the anomalous dimension of the TLM. We also show how to treat the change of
the chemical potential due to the interactions within the functional
bosonization approach in arbitrary dimensions. | cond-mat_str-el |
Dynamical Mean-Field Theory for Doped Antiferromagnets: We have generalized the dynamical mean-field theory to study the doping
dependence of the crossover from antiferromagnetic to short-range order
modelled by an incommensurate spin density wave in the Hubbard model. The local
selfenergy which includes spin fluctuations gives quasiparticle weights and
spectral properties in good agreement with quantum Monte Carlo and exact
diagonalization data in two dimensions. The spectra at finite doping are
characterized by a Mott-Hubbard `gap' accompanied by a pseudogap induced by the
local spin order. | cond-mat_str-el |
Doped carrier formulation of the t-J model: the projection constraint
and the effective Kondo-Heisenberg lattice representation: We show that the recently proposed doped carrier Hamiltonian formulation of
the t-J model should be complemented with the constraint that projects out the
unphysical states. With this new important ingredient, the previously used and
seemingly different spin-fermion representations of the t-J model are shown to
be gauge related to each other. This new constraint can be treated in a
controlled way close to half-filling suggesting that the doped carrier
representation provides an appropriate theoretical framework to address the t-J
model in this region. This constraint also suggests that the t-J model can be
mapped onto a Kondo-Heisenberg lattice model. Such a mapping highlights
important physical similarities between the quasi two-dimensional heavy
fermions and the high-T$_c$ superconductors. Finally we discuss the physical
implications of our model representation relating in particular the small
versus large Fermi surface crossover to the closure of the lattice spin gap. | cond-mat_str-el |
Orbital-selective Mott transitions in two-band Hubbard models: The anisotropic two-orbital Hubbard model is investigated at low temperatures
using high-precision quantum Monte Carlo (QMC) simulations within dynamical
mean-field theory (DMFT). We demonstrate that two distinct orbital-selective
Mott transitions (OSMTs) occur for a bandwidth ratio of 2 even without
spin-flip contributions to the Hund exchange, and we quantify numerical errors
in earlier QMC data which had obscured the second transition. The limit of
small inter-orbital coupling is introduced via a new generalized Hamiltonian
and studied using QMC and Potthoff's self-energy functional method, yielding
insight into the nature of the OSMTs and the non-Fermi-liquid OSM phase and
opening the possibility for a new quantum-critical point. | cond-mat_str-el |
Topological and trivial magnetic oscillations in nodal loop semimetals: Nodal loop semimetals are close descendants of Weyl semimetals and possess a
topologically dressed band structure. We argue by combining the conventional
theory of magnetic oscillation with topological arguments that nodal loop
semimetals host coexisting topological and trivial magnetic oscillations. These
originate from mapping the topological properties of the extremal Fermi surface
cross sections onto the physics of two dimensional semi Dirac systems, stemming
from merging two massless Dirac cones. By tuning the chemical potential and the
direction of magnetic field, a sharp transition is identified separating purely
trivial oscillations, arising from the Landau levels of a normal two
dimensional (2D) electron gas, to a phase where oscillations of topological and
trivial origin coexist, originating from 2D massless Dirac and semi Dirac
points, respectively. These could in principle be directly identified in
current experiments. | cond-mat_str-el |
Quantum adiabatic theorem for chemical reactions and systems with
time-dependent orthogonalization: A general quantum adiabatic theorem with and without the time-dependent
orthogonalization is proven, which can be applied to understand the origin of
activation energies in chemical reactions. Further proofs are also developed
for the oscillating Schwinger Hamiltonian to establish the relationship between
the internal (due to time-dependent eigenfunctions) and external (due to
time-dependent Hamiltonian) time scales. We prove that this relationship needs
to be taken as an independent quantum adiabatic approximation criterion. We
give four examples, including logical expositions based on the spin-1/2
two-level system to address the gapped and gapless (due to energy level
crossings) systems, as well as to understand how does this theorem allows one
to study dynamical systems such as chemical reactions. | cond-mat_str-el |
The Falicov-Kimball model in external magnetic field: orbital effects: We study thermodynamic properties of the two-dimensional (2D) Falicov-Kimball
model in the presence of external magnetic field perpendicular to the lattice.
The field is taken into account by the Peierls substitution in the hopping
term. In the non-interacting case the field dependent energy spectrum forms the
famous Hofstadter butterfly. Our results indicate that for arbitrary nonzero
interaction strength and arbitrary magnetic field there is a gap in the energy
spectrum at sufficiently low temperature. The gap vanishes with increase of
temperature for weak coupling, however, it persists at high temperatures if the
coupling is strong enough. Numerical results have been obtained with the help
of Monte Carlo technique based on a modified Metropolis algorithm. | cond-mat_str-el |
Excitations and relaxation dynamics in multiferroic GeV4S8 studied by
THz and dielectric spectroscopy: We report on THz time-domain spectroscopy on multiferroic GeV4S8, which
undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits
antiferromagnetic order below 14.6 K. The THz experiments are complemented by
dielectric experiments at audio and radio frequencies. We identify a low-lying
excitation close to 15 cm-1, which is only weakly temperature dependent and
probably corresponds to a molecular excitation within the electronic level
scheme of the V4 clusters. In addition, we detect complex temperature-dependent
behavior of a low-lying phononic excitation, closely linked to the onset of
orbitally-driven ferroelectricity. In the high-temperature cubic phase, which
is paramagnetic and orbitally disordered, this excitation is of relaxational
character, becomes an overdamped Lorentzian mode in the orbitally ordered phase
below the Jahn-Teller transition, and finally appears as well-defined phonon
excitation in the antiferromagnetic state. Abrupt changes in real and imaginary
parts of the complex dielectric permittivity show that orbital ordering appears
via a structural phase transition with strong first-order character and that
the onset of antiferromagnetic order is accompanied by significant structural
changes, which are of first-order character, too. Dielectric spectroscopy
documents that, at low frequencies, significant dipolar relaxations are present
in the orbitally ordered, paramagnetic phase only. In contrast to the closely
related GaV4S8, this relaxation dynamics that most likely mirrors coupled
orbital and polar fluctuations does not seem to be related to the dynamic
processes detected in the THz regime. | cond-mat_str-el |
Electronic Liquid Crystalline Phases in a Spin-Orbit Coupled
Two-Dimensional Electron Gas: We argue that the ground state of a two-dimensional electron gas with Rashba
spin-orbit coupling realizes one of several possible liquid crystalline or
Wigner crystalline phases in the low-density limit, even for short-range
repulsive electron-electron interactions (which decay with distance with a
power larger than 2). Depending on specifics of the interactions, preferred
ground-states include an anisotropic Wigner crystal with an increasingly
anisotropic unit cell as the density decreases, a striped or electron smectic
phase, and a ferromagnetic phase which strongly breaks the lattice point-group
symmetry, i.e. exhibits nematic order. Melting of the anisotropic Wigner
crystal or the smectic phase by thermal or quantum fluctuations can gives rise
to a non-magnetic nematic phase which preserves time-reversal symmetry. | cond-mat_str-el |
Spin-polarization coupling in multiferroic transition-metal oxides: A systematic microscopic theory of magnetically induced ferroelectricity and
lattice modulation is presented for all electron configurations of
Mott-insulating transition-metal oxides. Various mechanisms of polarization are
identified in terms of a strong-coupling perturbation theory. Especially, the
spin-orbit interaction acting on the ligand p orbitals is shown to give the
ferroelectric polarization of the spin-current form, which plays a crucial role
particularly in eg systems. Semiquantitative agreements with the multiferroic
TbMnO3 are obtained. Predictions for X-ray and neutron scattering experiments
are proposed to clarify the microscopic mechanism of the spin-polarization
coupling in different materials. | cond-mat_str-el |
Determination of intrinsic ferroelectric polarization in lossy improper
ferroelectric systems: We measured the intrinsic hysteretic polarization in lossy improper and
nanoferroelectric systems where the nonhysteretic polarization and leakage are
large and the relaxation takes place over a broader time scale. We used
different measurement protocols such as standard single triangular voltage
pulse, a pulse train of PUND (Positive Up Negative Down), and an even more
complicated pulse train of fourteen voltage pulses and compared the results
obtained. We show that a protocol which sends a train of fourteen pulses is
more appropriate for extracting relaxed (i.e., time scale independent) and
intrinsic remanent polarization for these samples. We also point out that it is
possible to select and design an appropriate measurement protocol depending on
the magnitude of polarization and leakage of the system. | cond-mat_str-el |
Spin Waves in Antiferromagnetic Spin Chains with Long Range Interactions: We study antiferromagnetic spin chains with unfrustrated long-range
interactions that decays as a power law with exponent $\beta$, using the spin
wave approximation. We find for sufficiently large spin $S$, the Neel order is
stable at T=0 for $\beta < 3$, and survive up to a finite Neel temperature for
$\beta < 2$, validating the spin-wave approach in these regimes. We estimate
the critical values of $S$ and $T$ for the Neel order to be stable. The spin
wave spectra are found to be gapless but have non-linear momentum dependence at
long wave length, which is responsible for the suppression of quantum and
thermal fluctuations and stabilizing the Neel state. We also show that for
$\beta\le 1$ and for a large but finite-size system size $L$, the excitation
gap of the system approaches zero slower than $L^{-1}$, a behavior that is in
contrast to the Lieb-Schulz-Mattis theorem. | cond-mat_str-el |
Spin-orbit physics of j=1/2 Mott insulators on the triangular lattice: The Heisenberg-Kitaev (HK) model on the triangular lattice is conceptually
interesting for its interplay of geometric and exchange frustration. HK models
are also thought to capture the essential physics of the spin-orbital
entanglement in effective $j=1/2$ Mott insulators studied in the context of
various 5d transition metal oxides. Here we argue that the recently synthesized
Ba$_3$IrTi$_2$O$_9$ is a prime candidate for a microscopic realization of the
triangular HK model. We establish that an infinitesimal Kitaev exchange
destabilizes the 120$^\circ$ order of the quantum Heisenberg model and results
in the formation of an extended $\mathbb{Z}_2$-vortex crystal phase in the
parameter regime most likely relevant to the real material. Using a combination
of analytical and numerical techniques we map out the entire phase diagram of
the model, which further includes various ordered phases as well as an extended
nematic phase around the antiferromagnetic Kitaev point. | cond-mat_str-el |
Details of Sample Dependence and Transport Properties of URu2Si2: Resistivity and specific heat measurements were performed in the low carrier
unconventional superconductor URu2Si2 on various samples with very different
qualities. The superconducting transition temperature (TSC) and the hidden
order transition temperature (THO) of these crystals were evaluated as a
function of the residual resistivity ratio (RRR). In high quality single
crystals the resistivity does not seem to follow a T2 dependence above TSC,
indicating that the Fermi liquid regime is restricted to low temperatures.
However, an analysis of the isothermal longitudinal magnetoresistivity points
out that the T2 dependence may be "spoiled" by residual inhomogeneous
superconducting contribution. We discuss a possible scenario concerning the
distribution of TSC related with the fact that the hidden order phase is very
sensitive to the pressure inhomogeneity. | cond-mat_str-el |
Interaction Correction of Conductivity Near a Ferromagnetic Quantum
Critical Point: We calculate the temperature dependence of conductivity due to interaction
correction for a disordered itinerant electron system close to a ferromagnetic
quantum critical point which occurs due to a spin density wave instability. In
the quantum critical regime, the crossover between diffusive and ballistic
transport occurs at a temperature $T^{\ast}=1/[\tau \gamma (E_{F}\tau)^{2}]$,
where $\gamma$ is the parameter associated with the Landau damping of the spin
fluctuations, $\tau$ is the impurity scattering time, and $E_{F}$ is the Fermi
energy. For a generic choice of parameters, $T^{\ast}$ is few orders of
magnitude smaller than the usual crossover scale $1/\tau$. In the ballistic
quantum critical regime, the conductivity has a $T^{(d-1)/3}$ temperature
dependence, where $d$ is the dimensionality of the system. In the diffusive
quantum critical regime we get $T^{1/4}$ dependence in three dimensions, and
$\ln^2 T$ dependence in two dimensions. Away from the quantum critical regime
we recover the standard results for a good metal. | cond-mat_str-el |
Quantum Wire Hybridized with a Single-Level Impurity: We have studied low-temperature properties of interacting electrons in a
one-dimensional quantum wire (Luttinger liquid) side-hybridized with a
single-level impurity. The hybridization induces a back-scattering of electrons
in the wire which strongly affects its low energy properties. Using a one-loop
renormalization group approach valid for a weak electron-electron interaction,
we have calculated a transmission coefficient through the wire,
$\mathcal{T}(\varepsilon)$, and a local density of states, $\nu(\varepsilon)$
at low energies $\varepsilon $. In particular, we have found that the
antiresonance in $\mathcal{T}(\varepsilon)$ has a generalized Breit-Wigner
shape with the effective width $\Gamma(\varepsilon)$ which diverges at the
Fermi level. | cond-mat_str-el |
Kondo Effect and Persistent Currents in a Mesoscopic Ring: Numerically
Exact Results: We study the persistent current circulating along a mesoscopic ring with a
dot side-coupled to it when threaded by a magnetic field. A cluster including
the dot and its vicinity is diagonalized and embedded into the rest of the
system. The result is numerically exact. We show that a ring of any size can be
in the Kondo regime, although for small sizes it depends upon the magnetic
flux. In the Kondo regime, the current can be a smooth or a strongly dependent
function of the gate potential according to the structure of occupation of the
highest energetic electrons of the system. | cond-mat_str-el |
Nonlinear spectroscopy of collective modes in excitonic insulator: The nonlinear optical response of an excitonic insulator coupled to lattice
degrees of freedom is shown to depend in strong and characteristic ways on
whether the insulating behavior originates primarily from electron-electron or
electron-lattice interactions. Linear response optical signatures of the
massive phase mode and the amplitude (Higgs) mode are identified. Upon
nonlinear excitation resonant to the phase mode, a new in-gap mode at twice the
phase mode frequency is induced, leading to a huge second harmonic response.
Excitation of in-gap phonon modes leads to different and much smaller effects.
A Landau-Ginzburg theory analysis explain these different behavior and reveals
that a parametric resonance of the strongly excited phase mode is the origin of
the photo-induced mode in the electron-dominant case. The difference in the
nonlinear optical response serve as a measure of the dominant mechanism of the
ordered phase. | cond-mat_str-el |
The Chern-Simons Invariant in the Berry Phase of a Two by Two
Hamiltonian: The positive (negaive)-energy eigen vectors of the two by two Hamiltonian
$H=\v{r}\cdot\vec{\s}$ where $\vec{\s}$ are the Pauli matrices and $\v{r}$ is a
3-vector, form a U(1) fiber bundle when $\v{r}$ sweeps over a manifold $\cM$ in
the three dimensional parameter space of $\v{r}$ . For appropriately chosen
base space $\cM$ the resulting fiber bundle can have non-trivial topology. For
example when $\cM=S^2\equiv\{\v{r}; |\v{r}|=1\}$ the corresponding bundle has a
non-zero Chern number, which is the indicator that it is topologically
non-trivial. In this paper we construct a two by two Hamiltonian whose eigen
bundle shows a more subtle topological non-triviality over
$\cM=R^3\bigcup\{\infty\}$, the stereographic projection of $S^3$. This
non-triviality is characterized by a non-zero Chern-Simons invariant. | cond-mat_str-el |
Energetics of Domain Walls in the 2D t-J model: Using the density matrix renormalization group, we calculate the energy of a
domain wall in the 2D t-J model as a function of the linear hole density
\rho_\ell, as well as the interaction energy between walls, for J/t=0.35. Based
on these results, we conclude that the ground state always has domain walls for
dopings 0 < x < 0.3. For x < 0.125, the system has (1,0) domain walls with
\rho_\ell ~ 0.5, while for 0.125 < x < 0.17, the system has a possibly
phase-separated mixture of walls with \rho_\ell ~ 0.5 and \rho_\ell =1. For x >
0.17, there are only walls with \rho_\ell =1. For \rho_\ell = 1, diagonal (1,1)
domain walls have very nearly the same energy as (1,0) domain walls. | cond-mat_str-el |
A photo-induced strange metal with electron and hole quasi-particles: Photo-doping of Mott insulators or correlated metals can create an unusual
metallic state which simultaneously hosts hole-like and electron-like
particles. We study the dynamics of this state up to long times, as it passes
its kinetic energy to the environment. When the system cools down, it crosses
over from a bad metal into a resilient quasiparticle regime, in which
quasiparticle bands are formed with separate Fermi levels for electrons and
holes, but quasiparticles do not yet satisfy the Fermi liquid paradigm.
Subsequently, the transfer of energy to the environment slows down
significantly, and the system does not reach the Fermi liquid state even on the
timescale of picoseconds. The transient photo-doped strange metal exhibits
unusual properties of relevance for ultrafast charge and heat transport: In
particular, there can be an asymmetry in the properties of electrons and holes,
and strong correlations between electrons and holes, as seen in the spectral
properties. | cond-mat_str-el |
Spin excitations in metallic kagome lattice FeSn and CoSn: In two-dimensional (2D) metallic kagome lattice materials, destructive
interference of electronic hopping pathways around the kagome bracket can
produce nearly localized electrons, and thus electronic bands that are flat in
momentum space. When ferromagnetic order breaks the degeneracy of the
electronic bands and splits them into the spin-up majority and spin-down
minority electronic bands, quasiparticle excitations between the spin-up and
spin-down flat bands should form a narrow localized spin-excitation Stoner
continuum coexisting with well-defined spin waves in the long wavelengths. Here
we report inelastic neutron scattering studies of spin excitations in 2D
metallic Kagome lattice antiferromagnetic FeSn and paramagnetic CoSn, where
angle resolved photoemission spectroscopy experiments found spin-polarized and
nonpolarized flat bands, respectively, below the Fermi level. Although our
initial measurements on FeSn indeed reveal well-defined spin waves extending
well above 140 meV coexisting with a flat excitation at 170 meV, subsequent
experiments on CoSn indicate that the flat mode actually arises mostly from
hydrocarbon scattering of the CYTOP-M commonly used to glue the samples to
aluminum holder. Therefore, our results established the evolution of spin
excitations in FeSn and CoSn, and identified an anomalous flat mode that has
been overlooked by the neutron scattering community for the past 20 years. | cond-mat_str-el |
The emergence of charged collective modes from a large N extrapolation
of the Hubbard model: We consider a symplectic extrapolation of the Hubbard model of N fold
replicated electrons and solve this model exactly in two special cases, at
N=infinity in the bosonic sector and for any N on a dimer of two points. At
N=infinity we find a multiplet of collective modes that contains neutral spin
fluctuations and charged pair fluctuations that are degenerate with each other
at zero doping. Our solution of the symplectic model on a dimer of two points
for any N interpolates smoothly between N=1 and N=infinity without any visible
discontinuity. These results suggest that the inclusion of charged pairing
modes in weakly doped antiferromagnets is essential and that an expansion about
the N=infinity limit is appropriate in this context. | cond-mat_str-el |
Heavy holes: precursor to superconductivity in antiferromagnetic CeIn3: Numerous phenomenological parallels have been drawn between f- and d-
electron systems in an attempt to understand their display of unconventional
superconductivity. The microscopics of how electrons evolve from participation
in large moment antiferromagnetism to superconductivity in these systems,
however, remains a mystery. Knowing the origin of Cooper paired electrons in
momentum space is a crucial prerequisite for understanding the pairing
mechanism. Of especial interest are pressure-induced superconductors CeIn3 and
CeRhIn5 in which disparate magnetic and superconducting orders apparently
coexist - arising from within the same f-electron degrees of freedom. Here we
present ambient pressure quantum oscillation measurements on CeIn3 that
crucially identify the electronic structure - potentially similar to high
temperature superconductors. Heavy pockets of f-character are revealed in
CeIn3, undergoing an unexpected effective mass divergence well before the
antiferromagnetic critical field. We thus uncover the softening of a branch of
quasiparticle excitations located away from the traditional spin-fluctuation
dominated antiferromagnetic quantum critical point. The observed Fermi surface
of dispersive f-electrons in CeIn3 could potentially explain the emergence of
Cooper pairs from within a strong moment antiferromagnet. | cond-mat_str-el |
On planar fermions with quartic interaction at finite temperature and
density: We study the breaking of parity symmetry in the 2+1 Gross-Neveu model at
finite temperature with chemical potential $\mu$, in the presence of an
external magnetic field. We find that the requirement of gauge invariance,
which is considered mandatory in the presence of gauge fields, breaks parity at
any finite temperature and provides for dynamical mass generation, preventing
symmetry restoration for any non-vanishing $\mu$. The dynamical mass becomes
negligibly small as temperature is raised. We comment on the relevance of our
observation for the gap generation of nodal quasi-particles in the pseudo-gap
phase of high $T_c$ superconductors. | cond-mat_str-el |
Composition and field tuned magnetism and superconductivity in
Nd1-xCexCoIn5: The Nd1-xCexCoIn5 alloys evolve from local moment magnetism (x = 0) to heavy
fermion superconductivity (x =1). Magnetic order is observed over a broad range
of x. For a substantial range of x (0.83 <= x <= 0.95) in the temperature -
composition phase diagram we find that superconductivity may coexist with spin
- density wave magnetic order at the Fermi surface. We show that a delicate
balance betwen superconducting and magnetic instabilities can be reversibly
tuned by both the Ce/Nd ratio and magnetic field, offering a new and unique
model electronic system. | cond-mat_str-el |
Boundary effects in the critical scaling of entanglement entropy in 1D
systems: We present exact diagonalization and density matrix renormalization group
results for the entanglement entropy of critical spin-1/2 XXZ chains. We find
that open boundary conditions induce an alternating term in both the energy
density and the entanglement entropy which are approximately proportional,
decaying away from the boundary with a power-law. The power varies with
anisotropy along the XXZ critical line and is corrected by a logarithmic
factor, which we calculate analytically, at the isotropic point. A heuristic
resonating valence bond explanation is suggested. | cond-mat_str-el |
The effects of k-dependent self-energy in the electronic structure of
correlated materials: It is known from self-energy calculations in the electron gas and sp
materials based on the GW approximation that a typical quasiparticle
renormalization factor (Z factor) is approximately 0.7-0.8. Band narrowing in
electron gas at rs = 4 due to correlation effects, however, is only
approximately 10%, significantly smaller than the Z factor would suggest. The
band narrowing is determined by the frequency-dependent self-energy, giving the
Z factor, and the momentum-dependent or nonlocal self-energy. The results for
the electron gas point to a strong cancellation between the effects of
frequency- and momentum-dependent self-energy. It is often assumed that for
systems with a nar- row band the self-energy is local. In this work we show
that even for narrow-band materials, such as SrVO3, the nonlocal self-energy is
important. | cond-mat_str-el |
Absence of static stripes in the two-dimensional $t{-}J$ model by an
accurate and systematic quantum Monte Carlo approach: We examine the two-dimensional $t{-}J$ model by using variational approach
combined with well established quantum Monte Carlo techniques [S. Sorella {\it
et al.}, \prl {\bf 88}, 117002 (2002)] that are used to improve systematically
the accuracy of the variational ansatz. Contrary to recent density-matrix
renormalization group and projected entangled-pair state calculations [P.
Corboz {\it et al.}, \prb {\bf 84}, 041108(R) (2011)], a uniform phase is found
for $J/t=0.4$, even when the calculation is biased with an ansatz that
explicitly contains stripe order. Moreover, in the small hole doping regime,
i.e., $\delta \lesssim 0.1$, our results support the coexistence of
antiferromagnetism and superconductivity. | cond-mat_str-el |
Magnetic Field Effect on Crossover Temperature from Non-Fermi Liquid to
Fermi Liquid Behavior in f^2-Impurity Systems with Crystalline-Electric-Field
Singlet State Competing with Kondo-Yosida Singlet State: We investigate the magnetic field dependence of the physical properties of
f^2-configuration systems with a crystalline-electric field (CEF) singlet
ground state, which gives rise to a non- Fermi liquid (NFL) fixed point due to
the competition between the Kondo-Yosida singlet and CEF singlet states. On the
basis of the numerical renormalization group method, we find that the magnetic
field breaks this NFL fixed point via two mechanisms: one causing the
polarization of f-electrons and the other giving the "channel" anisotropy.
These two mechanisms induce a difference in the magnetic field dependence of
the characteristic temperature T_F^{*}(H), the crossover temperature from NFL
to Fermi-liquid behavior. While the polarization of f-electrons gives
T_F^{*}(H) \propto H^x (x\sim2.0), the "channel" anisotropy gives the
H-independent T_F^{*}(H). These two mechanisms cross over continuously at
approximately the crossover magnetic field H_c, where an anomalous H-dependence
of T_F^{*}(H) appears. Such T_F^{*}(H) well reproduces the NFL behaviors
observed in Th_{1-x}U_xRu_2Si_2. We also find that the H-dependence of the
resistivity and the magnetic susceptibility are in good agreement with the
experimental results of this material. These results suggest that the NFL
behaviors observed in Th_{1-x}U_xRu_2Si_2 can be understood if this material is
located in the CEF singlet side near the critical phase boundary between the
two singlet states. | cond-mat_str-el |
Reply to "Comment on `Orbital-selective Mott transitions in the
anisotropic two-band Hubbard model at finite temperatures'": In a Comment [cond-mat/0506138] on our recent e-print [cond-mat/0505106]
Liebsch claimed "excellent correspondence" between our high-precision quantum
Monte-Carlo (QMC) data for the anisotropic two-band Hubbard model with Ising
type exchange couplings and his earlier QMC results. Liebsch also claimed that
the sequence of two orbital-selective Mott transitions, identified by us in
this model, had already been reported in his earlier work. Here we demonstrate
that both claims are incorrect. We establish that Liebsch's previous QMC
estimates for the quasiparticle weight Z have relative errors exceeding 100%
near transitions and cannot be used to infer the existence of a second Mott
transition (for U_{c2}~2.5). We further show that Liebsch's attribution of our
findings to his own earlier work is disproved by the published record.
Consequently, the Comment is unwarranted; all results and formulations of our
e-print remain valid. | cond-mat_str-el |
Crystalline Solutions of Kohn-Sham Equations in the Fractional Quantum
Hall Regime: A Kohn-Sham density functional approach has recently been developed for the
fractional quantum Hall effect, which maps the strongly interacting electrons
into a system of weakly interacting composite fermions subject to an exchange
correlation potential as well as a density dependent gauge field that mimics
the "flux quanta" bound to composite fermions. To get a feel for the role of
various terms, we study the behavior of the self-consistent solution as a
function of the strength of the exchange correlation potential, which is varied
through an {\it ad hoc} multiplicative factor. We find that a crystal phase is
stabilized when the exchange correlation interaction is sufficiently strong
relative to the composite-fermion cyclotron energy. Various properties of this
crystal are examined. | cond-mat_str-el |
The spin-1 ladder : A bosonization study: We construct a field-theoretic description of two coupled spin-1 Heisenberg
chains, starting with the known representation of a single spin-1 chain in
terms of Majorana fermions (or Ising models). After reexamining the
bosonization rules for two Ising models, taking particular care of order and
disorder operators, we obtain a bosonic description of the spin-1 ladder. From
renormalization-group and mean-field arguments, we conclude that, for a small
interchain coupling, the spin-1 ladder is approximately described by three
decoupled, two-frequency sine-Gordon models. We then predict that, starting
with decoupled chains, the spin gap decreases linearly with interchain
coupling, both in the ferromagnetic and antiferromagnetic directions. Finally,
we discuss the possibility of an incommensurate phase in the spin-1 zigzag
chain. | cond-mat_str-el |
Effect of Interdots Electronic Repulsion in the Majorana Signature for a
Double Dot Interferometer: We investigate theoretically the features of the Majorana hallmark in the
presence of Coulomb repulsion between two quantum dots describing a spinless
Aharonov-Bohm-like interferometer, where one of the dots is strongly coupled to
a Kitaev wire within the topological phase. Such a system has been originally
proposed without Coulomb interaction in J. of Appl. Phys. 116, 173701 (2014).
Our findings reveal that for dots in resonance, the ratio between the strength
of Coulomb repulsion and the dot-wire coupling changes the width of the
Majorana zero-bias peak for both Fano regimes studied, indicating thus that the
electronic interdots correlation influences the Majorana state lifetime in the
dot hybridized with the wire. Moreover, for the off-resonance case, the swap
between the energy levels of the dots also modifies the width of the Majorana
peak, which does not happen for the noninteracting case. The results obtained
here can guide experimentalists that pursuit a way of revealing Majorana
signatures. | cond-mat_str-el |
Reply to Millis et al. on "A Tale of Two Theories: Quantum Griffiths
Effects in Metallic Systems": In a recent paper (cond-mat/0411197) we showed the equivalence of two
seemingly contradictory theories on Griffiths-McCoy singularities (GMS) in
metallic antiferromagnets close to a quantum critical point (QCP). In a recent
comment, Millis {\it et al.} (cond-mat/0411738) argue that in heavy-fermion
materials the electronic damping is large leading to the freezing of locally
magnetically ordered droplets at high temperatures. In this reply we show that
this erroneous conclusion is based on a treatment of the problem of disorder
close to a QCP which is not self-consistent. We argue that a self-consistent
treatment of the ordered droplets must lead to weak damping and to a large
region of GMS behavior, in agreement with the our ealier results. | cond-mat_str-el |
Transport in a classical model of an one-dimensional Mott insulator:
Influence of conservation laws: We study numerically how conservation laws affect the optical conductivity
sigma(w) of a slightly doped one-dimensional Mott insulator. We investigate a
regime where the average distance between charge excitations is large compared
to their thermal de Broglie wave length and a classical description is
possible. Due to conservation laws, the dc-conductivity is infinite and the
Drude weight D is finite even at finite temperatures. Our numerical results
test and confirm exact theoretical predictions for D both for integrable and
non-integrable models. Small deviations from integrability induce slowly
decaying modes and, consequently, low-frequency peaks in sigma(w) which can be
described by a memory matrix approach. | cond-mat_str-el |
Magnetism and berry phase manipulation in an emergent structure of
perovskite ruthenate by (111) strain engineering: The interplay among symmetry of lattices, electronic correlations, and Berry
phase of the Bloch states in solids has led to fascinating quantum phases of
matter. A prototypical system is the magnetic Weyl candidate SrRuO3, where
designing and creating electronic and topological properties on artificial
lattice geometry is highly demanded yet remains elusive. Here, we establish an
emergent trigonal structure of SrRuO3 by means of heteroepitaxial strain
engineering along the [111] crystallographic axis. Distinctive from bulk, the
trigonal SrRuO3 exhibits a peculiar XY-type ferromagnetic ground state, with
the coexistence of high-mobility holes likely from linear Weyl bands and
low-mobility electrons from normal quadratic bands as carriers. The presence of
Weyl nodes are further corroborated by capturing intrinsic anomalous Hall
effect, acting as momentum-space sources of Berry curvatures. The experimental
observations are consistent with our first-principles calculations, shedding
light on the detailed band topology of trigonal SrRuO3 with multiple pairs of
Weyl nodes near the Fermi level. Our findings signify the essence of magnetism
and Berry phase manipulation via lattice design and pave the way towards
unveiling nontrivial correlated topological phenomena. | cond-mat_str-el |
Superconducting properties of Pr-based Filled skutterudite
PrRu$_4$As$_{12}$: We report on systematic study of superconducting characteristics and Pr
crystalline-electric-field (CEF) levels of filled-skutterudite \pra ($T_{\rm
c}$ = 2.33 K). The temperature dependences of the upper critical field $H_{\rm
c2}$ and the Ginzburg-Landau (Maki) parameter $\kappa_2$ suggest an s-wave
clean-limit superconductivity. The electronic specific heat coefficient $\gamma
\sim 95$ mJ/K$^2$mol, being $\sim 1.5$ times larger than that for \lra,
indicates $4f$-originating quasiparticle mass enhancement. Magnetic
susceptibility $\chi(T)$ indicates that the CEF ground state is a $\Gamma_1$
singlet and a $\Gamma_4^{(1)}$ triplet first excited state lies at $\Delta_{\rm
CEF}\sim 30$ K above. Systematic comparison among \pos, \prs, \pra and La-based
reference compounds suggests that inelastic exchange- and
aspherical-charge-scatterings of conduction electrons from CEF-split $4f$
levels play an essential role for the quasiparticle mass enhancement and the
value of $T_{\rm c}$ in the Pr-based filled skutterudites. | cond-mat_str-el |
Transfer of spectral weight across the gap of Sr2IrO4 induced by La
doping: We study with Angle Resolved PhotoElectron Spectroscopy (ARPES) the evolution
of the electronic structure of Sr2IrO4, when holes or electrons are introduced,
through Rh or La substitutions. At low dopings, the added carriers occupy the
first available states, at bottom or top of the gap, revealing an anisotropic
gap of 0.7eV in good agreement with STM measurements. At further doping, we
observe a reduction of the gap and a transfer of spectral weight across the
gap, although the quasiparticle weight remains very small. We discuss the
origin of the in-gap spectral weight as a local distribution of gap values. | cond-mat_str-el |
Conversion of glassy antiferromagnetic-insulating phase to equilibrium
ferromagnetic-metallic phase by devitrification and recrystallization in Al
substituted Pr${_{0.5}}$Ca$_{0.5}$MnO${_3}$: We show that Pr${_{0.5}}$Ca$_{0.5}$MnO${_3}$ with 2.5% Al substitution and
La${_{0.5}}$Ca$_{0.5}$MnO${_3}$ (LCMO) exhibit qualitatively similar and
visibly anomalous M-H curves at low temperature. Magnetic field causes a broad
first-order but irreversible antiferromagnetic (AF)-insulating (I) to
ferromagnetic (FM)-metallic (M) transition in both and gives rise to soft FM
state. However, the low temperature equilibrium state of
Pr$_{0.5}$Ca$_{0.5}$Mn$_{0.975}$Al$_{0.025}$O$_3$ (PCMAO) is FM-M whereas that
of LCMO is AF-I. In both the systems the respective equilibrium phase coexists
with the other phase with contrasting order, which is not in equilibrium, and
the cooling field can tune the fractions of the coexisting phases. It is shown
earlier that the coexisting FM-M phase behaves like `magnetic glass' in LCMO.
Here we show from specially designed measurement protocols that the AF-I phase
of PCMAO has all the characteristics of magnetic glassy states. It devitrifies
on heating and also recrystallizes to equilibrium FM-M phase after annealing.
This glass-like AF-I phase also shows similar intriguing feature observed in
FM-M magnetic glassy state of LCMO that when the starting coexisting fraction
of glass is larger, successive annealing results in larger fraction of
equilibrium phase. This similarity between two manganite systems with
contrasting magnetic orders of respective glassy and equilibrium phases points
toward a possible universality. | cond-mat_str-el |
Comment on arXiv:0811.1575 entitled "Quantum phase transitions in the
Hubbard model on triangular lattice" by T. Yoshioka, A. Koga and N. Kawakami: We show that the phase boundary between the paramagnetic metal and the
nonmagnetic Mott insulator for the Hubbard model on a triangular lattice
obtained by Yoshioka et al. in arXiv:0811.1575 does not correctly represent
that of the thermodynamic limit but is an artifact of the 6 by 6 lattice they
rely on. After the system size extrapolation, the phase boundary is located at
U/t=5.2 as proposed by Morita et al., J. Phys. Soc. Jpn. 71 (2008) 2109 and in
contrast to Yoshioka et al. Here, U is the onsite Coulomb repulsion and t is
the nearest-neighbor transfer. | cond-mat_str-el |
Magnetic order in Tb$_2$Sn$_2$O$_7$ under high pressure: from ordered
spin ice to spin liquid and antiferromagnetic order: We have studied the Tb$_2$Sn$_2$O$_7$ frustrated magnet by neutron
diffraction under isotropic pressure of 4.6 GPa, combined with uniaxial
pressure of 0.3 GPa, in the temperature range 0.06 K$<$T$<$100 K. Magnetic
order persists under pressure but the ordered spin ice structure stabilized at
ambient pressure below 1.3 K partly transforms into an antiferromagnetic one.
The long range ordered moment at 0.06 K is reduced under pressure, which is
interpreted by a pressure induced enhancement of the spin liquid fluctuations.
Above the ordering transition, short range spin correlations are affected by
pressure, and ferromagnetic correlations are suppressed. The influence of
pressure on the ground state is discussed considering both isotropic and stress
effects. | cond-mat_str-el |
Abrupt disappearance and reemergence of the SU(2) and SU(4) Kondo
effects due to population inversion: The interplay of almost degenerate levels in quantum dots and molecular
junctions with possibly different couplings to the reservoirs has lead to many
observable phenomena, such as the Fano effect, transmission phase slips and the
SU(4) Kondo effect. Here we predict a dramatic repeated disappearance and
reemergence of the SU(4) and anomalous SU(2) Kondo effects with increasing gate
voltage. This phenomenon is attributed to the level occupation switching which
has been previously invoked to explain the universal transmission phase slips
in the conductance through a quantum dot. We use analytical arguments and
numerical renormalization group calculations to explain the observations and
discuss their experimental relevance and dependence on the physical parameters. | cond-mat_str-el |
Emergence of a 2d macro-spin liquid in a highly frustrated 3d quantum
magnet: The classical Ising model on the frustrated 3d swedenborgite lattice has
disordered spin liquid ground states for all ratios of inter- and intra-planar
couplings. Quantum fluctuations due to a transverse field give rise to several
exotic quantum phenomena. In the limit of weakly coupled Kagom\'e layers we
find a 3d version of disorder by disorder. For large out-of-plane couplings 1d
macro-spins are formed which realize a disordered macro-spin liquid on an
emerging triangular lattice. Signatures of this dimensional reduction are also
found in critical exponents of the quantum phase transition out of the fully
polarized phase into the macro-spin liquid displaying quantum criticality
typical for 2d quantum systems. | cond-mat_str-el |
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