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Disordered and interacting parabolic semimetals in two and three
dimensions: A clean noninteracting parabolic semimetal is characterized by quadratic band
touching between the conduction and the valence bands at isolated diabolic
points in the Brillouin zone and describes a fermionic quantum critical system
with dynamic exponent z=2. We consider the stability of such a semimetal
against electronic interaction and quenched disorder using a perturbative
renormalization group analysis for two and three spatial dimensions. For the
noninteracting problem infinitesimally weak disorder leads to an Anderson
insulator and a diffusive metal respectively in two and three dimensions. On
the other hand, the long range Coulomb interaction causes an excitonic
instability for the clean interacting problem towards a broken symmetry ground
state in both dimensions. Our weak coupling analysis of the combined effects of
disorder and interaction suggests the competition between a broken symmetry and
a disorder controlled metallic or insulating states, but is inadequate for
describing the quantum phase transitions among them. We discuss the relevance
of our results for bilayer graphene and some 227 iridate compounds, and
identify these materials as promising candidates for exploring novel disorder
and interaction controlled quantum critical phenomena. | cond-mat_str-el |
Thickness-dependent magnetic properties and strain-induced orbital
magnetic moment in SrRuO3 thin films: Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy
magnetization axis depending on the epitaxial strain and undergo a
metal-to-insulator transition with decreasing film thickness. We have
investigated the magnetic properties of SRO thin films with varying thicknesses
fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism
(XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film
thickness, the film changes from ferromagnetic to non-magnetic around
3monolayer thickness, consistent with previous magnetization and
magneto-optical Kerr effect measurements. The orbital magnetic moment
perpendicular to the film was found to be ~ 0.1{\mu}B/Ru atom, and remained
nearly unchanged with decreasing film thickness while the spin magnetic moment
decreases. Mechanism for the formation of the orbital magnetic moment is
discussed based on the electronic structure of the compressively strained SRO
film. | cond-mat_str-el |
Expansion of the tetragonal magnetic phase with pressure in the
iron-arsenide superconductor Ba{1-x}KxFe2As2: In the temperature-concentration phase diagram of most iron-based
superconductors, antiferromagnetic order is gradually suppressed to zero at a
critical point, and a dome of superconductivity forms around that point. The
nature of the magnetic phase and its fluctuations is of fundamental importance
for elucidating the pairing mechanism. In Ba{1-x}KxFe2As2 and Ba{1-x}NaxFe2As2,
it has recently become clear that the usual stripe-like magnetic phase, of
orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal
symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we
report measurements of the electrical resistivity of Ba{1-x}KxFe2As2 under
applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We
track the onset of the tetragonal magnetic phase using the sharp anomaly it
produces in the resistivity. In the temperature-concentration phase diagram of
Ba{1-x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic
phase, while the stripe-like phase shrinks. This raises the interesting
possibility that the fluctuations of the former phase might be involved in the
pairing mechanism responsible for the superconductivity. | cond-mat_str-el |
Field-Tuned Quantum Effects in a Triangular-Lattice Ising Magnet: We report thermodynamic and neutron scattering measurements of the
triangular-lattice quantum Ising magnet TmMgGaO 4 in longitudinal magnetic
fields. Our experiments reveal a quasi-plateau state induced by quantum
fluctuations. This state exhibits an unconventional non-monotonic field and
temperature dependence of the magnetic order and excitation gap. In the high
field regime where the quantum fluctuations are largely suppressed, we observed
a disordered state with coherent magnon-like excitations despite the
suppression of the spin excitation intensity. Through detailed semi-classical
calculations, we are able to understand these behaviors quantitatively from the
subtle competition between quantum fluctuations and frustrated Ising
interactions. | cond-mat_str-el |
Antiferromagnetic and d-wave pairing correlations in the strongly
interacting two-dimensional Hubbard model from the functional renormalization
group: Using the dynamical mean-field theory (DMFT) as a `booster-rocket', the
functional renormalization group (fRG) can be upgraded from a weak-coupling
method to a powerful computation tool for strongly interacting fermion systems.
The strong local correlations are treated non-perturbatively by the DMFT, while
the fRG flow can be formulated such that it is driven exclusively by non-local
correlations, which are more amenable to approximations. We show that the full
frequency dependence of the two-particle vertex needs to be taken into account
in this approach, and demonstrate that this is actually possible -- in spite of
the singular frequency dependence of the vertex at strong coupling. We are thus
able to present the first results obtained from the DMFT-boosted fRG for the
two-dimensional Hubbard model in the strongly interacting regime. We find
strong antiferromagnetic correlations from half-filling to 18 percent
hole-doping, and, at the lowest temperature we can access, a sizable $d$-wave
pairing interaction driven by magnetic correlations at the edge of the
antiferromagnetic regime. | cond-mat_str-el |
Spin Precession and Real Time Dynamics in the Kondo Model: A
Time-Dependent Numerical Renormalization-Group Study: A detailed derivation of the recently proposed time-dependent numerical
renormalization-group (TD-NRG) approach to nonequilibrium dynamics in quantum
impurity systems is presented. We demonstrate that the method is suitable for
fermionic as well as bosonic baths. A comparison with exact analytical results
for the charge relaxation in the resonant-level model and for dephasing in the
spin-boson model establishes the accuracy of the method. The real-time dynamics
of a single spin coupled to both types of baths is investigated. We use the
TD-NRG to calculate the spin relaxation and spin precession of a single Kondo
impurity. The short- and long-time dynamics is studied as a function of
temperature in the ferromagnetic and antiferromagnetic regimes. The short-time
dynamics agrees very well with analytical results obtained at second order in
the exchange coupling $J$. In the ferromagnetic regime, the long-time spin
decay is described by the scaling variable $x = 2\rho_F J(T) T t$. In the
antiferromagnetic regime it is governed for $T < T_K$ by the Kondo time scale
$1/T_K$. Here $\rho_F$ is the conduction-electron density of states and $T_K$
is the Kondo temperature. Results for spin precession are obtained by rotating
the external magnetic field from the x axis to the z axis. | cond-mat_str-el |
Pinball liquid phase from Hund's coupling in frustrated transition metal
oxides: The interplay of non-local Coulomb repulsion and Hund's coupling in the
d-orbital manifold in frustrated triangular lattices is analyzed by a mutliband
extended Hubbard model. We find a rich phase diagram with several competing
phases, including a robust pinball liquid phase, which is an unconventional
metal characterized by threefold charge order, bad metallic behavior and the
emergence of high spin local moments. Our results naturally explain the
anomalous charge-ordered metallic state observed in the triangular layered
compound AgNiO2. The potential relevance to other triangular transition metal
oxides is discussed. | cond-mat_str-el |
Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte
Carlo study: We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy. | cond-mat_str-el |
Anderson impurity in the one-dimensional Hubbard model on finite size
systems: An Anderson impurity in a Hubbard model on chains with finite length is
studied using the density-matrix renormalization group (DMRG) technique. In the
first place, we analyzed how the reduction of electron density from
half-filling to quarter-filling affects the Kondo resonance in the limit of
Hubbard repulsion U=0. In general, a weak dependence with the electron density
was found for the local density of states (LDOS) at the impurity except when
the impurity, at half-filling, is close to a mixed valence regime. Next, in the
central part of this paper, we studied the effects of finite Hubbard
interaction on the chain at quarter-filling. Our main result is that this
interaction drives the impurity into a more defined Kondo regime although
accompanied in most cases by a reduction of the spectral weight of the impurity
LDOS. Again, for the impurity in the mixed valence regime, we observed an
interesting nonmonotonic behavior. We also concluded that the conductance,
computed for a small finite bias applied to the leads, follows the behavior of
the impurity LDOS, as in the case of non-interacting chains. Finally, we
analyzed how the Hubbard interaction and the finite chain length affect the
spin compensation cloud both at zero and at finite temperature, in this case
using quantum Monte Carlo techniques. | cond-mat_str-el |
Modified Curie-Weiss Law for $j_{\rm eff}$ Magnets: In spin-orbit-coupled magnetic materials, the usually applied Curie-Weiss law
can break down. This is due to potentially sharp temperature-dependence of the
local magnetic moments. We therefore propose a modified Curie-Weiss formula
suitable for analysis of experimental susceptibility. We show for octahedrally
coordinated materials of $d^5$ filling that the Weiss constant obtained from
the improved formula is in excellent agreement with the calculated Weiss
constant from microscopic exchange interactions. Reanalyzing the measured
susceptibility of several Kitaev candidate materials with the modified formula
resolves apparent discrepancies between various experiments regarding the
magnitude and anisotropies of the underlying magnetic couplings. | cond-mat_str-el |
Low Curie-temperature ferromagnetic phase in SmPt2Cd20 possibly
accompanied by strong quantum fluctuations: Electrical resistivity, magnetization and specific heat have been measured
for single crystals of SmPt$_{2}$Cd$_{20}$. It has been found that
SmPt$_{2}$Cd$_{20}$ exhibits a ferromagnetic (FM) transition at $T_{\rm C} =
0.64$ K, the lowest among cubic compounds. Specific heat divided by temperature
increases with decreasing temperature even below $T_{\rm C}$ and attains 4.5
J/mol K$^{2}$ at 0.26 K, implying substantial magnetic quantum fluctuations. An
analysis of the magnetic entropy suggests the crystalline-electric-field
splitting of the Sm $J = 5/2$ multiplet with a $\Gamma_{7}$ doublet ground
state and a $\Gamma_8$ quartet excited state (the excitation energy of $\sim30$
K). The electrical resistivity shows a power-law behavior with $T^{0.74}$ below
2 K without showing any noticeable anomaly at $T_{\rm C}$. SmPt$_{2}$Cd$_{20}$
is regarded as a rare cubic system that is located in the vicinity of a FM
quantum critical point. | cond-mat_str-el |
Spin polarization induced tenfold magneto-resistivity of highly metallic
2D holes in a narrow GaAs quantum well: We observe that an in-plane magnetic field ($B_{||}$) can induce an order of
magnitude enhancement in the low temperature ($T$) resistivity ($\rho$) of
metallic 2D holes in a narrow (10nm) GaAs quantum well. Moreover, we show the
first observation of saturating behavior of $\rho(B_{||})$ at high $B_{||}$ in
GaAs system, which suggests our large positive $\rho(B_{||})$ is due to the
spin polarization effect alone. We find that this tenfold increase in
$\rho(B_{||})$ even persists deeply into the 2D metallic state with the high
$B_{||}$ saturating values of $\rho$ lower than 0.1$\times$h/e$^2$. The
dramatic effect of $B_{||}$ we observe on the highly conductive 2D holes (with
$B$=0 conductivity as high as 75e$^2$/h) sets strong constraint on models for
the spin dependent transport in dilute metallic 2D systems. | cond-mat_str-el |
X-Ray Resonant Scattering as a Direct Probe of Orbital Ordering in
Transition-Metal Oxides: X-ray resonant scattering at the K-edge of transition metal oxides is shown
to measure the orbital order parameter, supposed to accompany magnetic ordering
in some cases. Virtual transitions to the 3d-orbitals are quadrupolar in
general. In cases with no inversion symmetry, such as V$_2$O$_3$, treated in
detail here, a dipole component enhances the resonance. Hence, we argue that
the detailed structure of orbital order in V$_2$O$_3$ is experimentally
accessible. | cond-mat_str-el |
Far Infrared absorption of non center of mass modes and optical sum rule
in a few electron quantum dot with Rashba spin-orbit coupling: Spin-orbit interaction in a quantum dot couples far infrared radiation to non
center of mass excitation modes, even for parabolic confinement and dipole
approximation. The intensities of the absorption peaks satisfy the optical sum
rule, giving direct information on the total number of electrons inside the
dot. In the case of a circularly polarized radiation the sum rule is
insensitive to the strength of a Rashba spin-orbit coupling due to an electric
field orthogonal to the dot plane, but not to other sources of spin-orbit
interaction, thus allowing to discriminate between the two. | cond-mat_str-el |
Unconventional electron states in $δ$-doped SmTiO$_3$: The Mott-insulating distorted perovskite SmTiO$_3$, doped with a single SrO
layer in a quantum-well architecture is studied by the combination of density
functional theory with dynamical mean-field theory. A rich correlated
electronic structure in line with recent experimental investigations is
revealed by the given realistic many-body approach to a large-unit-cell oxide
heterostructure. Coexistence of conducting and Mott-insulating TiO$_2$ layers
prone to magnetic order gives rise to multi-orbital electronic transport beyond
standard Fermi-liquid theory. First hints towards a pseudogap opening due to
electron-electron scattering within a background of ferromagnetic and
antiferromagnetic fluctuations are detected. | cond-mat_str-el |
Flavor Degeneracy and Effects of Disorder in Ultracold Atom Systems: Cold atoms in optical lattices offer an exciting new laboratory where quantum
many-body phenomena can be realized in a highly controlled way. They can even
serve as quantum simulators for notoriously difficult problems like
high-temperature superconductivity. This review is focussed on recent
developments and new results in multi-component systems. Fermionic atoms with
SU(N) symmetry have exotic superfluid and flavor-ordered ground states. We
discuss symmetry breaking, collective modes and detection issues. Bosonic
multi-flavor ensembles allow for engineering of spin Hamiltonians which are
interesting from a quantum computation point of view. Finally, we will address
the competition of disorder and interaction in optical lattices. We present a
complete phase diagram obtained within dynamical mean-field theory and discuss
experimental observability of the Mott and Anderson phases. | cond-mat_str-el |
Reduced fidelity in topological quantum phase transitions: We study the reduced fidelity between local states of lattice systems
exhibiting topological order. By exploiting mappings to spin models with
classical order, we are able to analytically extract the scaling behavior of
the reduced fidelity at the corresponding quantum phase transitions out of the
topologically ordered phases. Our results suggest that the reduced fidelity,
albeit being a local measure, generically serves as a faithful marker of a
topological quantum phase transition. | cond-mat_str-el |
Development of spin fluctuations under the presence of $d$-wave bond
order in cuprate superconductors: In cuprate superconductors, superconductivity appears below the CDW
transition temperature $T_{CDW}$. However, many-body electronic states under
the CDW order are still far from understood. Here, we study the development of
the spin fluctuations under the presence of $d$-wave bond order (BO) with
wavevector $q=(\pi/2,0),(0,\pi/2)$, which is derived from the paramagnon
interference mechanism in recent theoretical studies. Based on the $4 \times 1$
and $4 \times 4$ cluster Hubbard models, the feedback effects between spin
susceptibility and self-energy are calculated self-consistently by using the
fluctuation-exchange (FLEX) approximation. It is found that the $d$-wave BO
leads to a sizable suppression of the nuclear magnetic relaxation rate $1/T_1$.
In contrast, the reduction in $T_c$ is small, since the static susceptibility
$\chi^s(Q_s)$ is affected by the BO just slightly. It is verified that the
$d$-wave BO scenario is consistent with the experimental electronic properties
below $T_{CDW}$. | cond-mat_str-el |
A Proposal to Use Neutron Scattering to Measure Scalar Spin Chirality
Fluctuations in Kagome Lattices: In the theory of quantum spin liquids, gauge fluctuations are emergent
excitations at low energy. The gauge magnetic field is proportional to the
scalar spin chirality, S1.(S2xS3). It is therefore highly desirable to measure
the fluctuation spectrum of the scalar spin chirality. We show that in the
Kagome lattice with a Dzyaloshinskii-Moriya term, the fluctuation in Sz which
is readily measured by neutron scattering contains a piece which is
proportional to the chirality fluctuation. | cond-mat_str-el |
Emergent Potts order in the kagomé $J_1-J_3$ Heisenberg model: Motivated by the physical properties of Vesignieite
BaCu$_3$V$_2$O$_8$(OH)$_2$, we study the $J_1-J_3$ Heisenberg model on the
kagom\'e lattice, that is proposed to describe this compound for $J_1<0$ and
$J_3\gg|J_1|$. The nature of the classical ground state and the possible phase
transitions are investigated through analytical calculations and parallel
tempering Monte Carlo simulations. For $J_1<0$ and
$J_3>\frac{1+\sqrt{5}}4|J_1|$, the ground states are not all related by an
Hamiltonian symmetry. Order appears at low temperature via the order by
disorder mechanism, favoring colinear configurations and leading to an emergent
$q=4$ Potts parameter. This gives rise to a finite temperature phase
transition. Effect of quantum fluctuations are studied through linear spin wave
approximation and high temperature expansions of the $S=1/2$ model. For $J_3$
between $\frac14|J_1|$ and $\frac{1+\sqrt{5}}4|J_1|$, the ground state goes
through a succession of semi-spiral states, possibly giving rise to multiple
phase transitions at low temperatures. | cond-mat_str-el |
Theory of inelastic light scattering in spin-1 systems: resonant regimes
and detection of quadrupolar order: Motivated by the lack of an obvious spectroscopic probe to investigate
non-conventional order such as quadrupolar orders in spin S>1/2 systems, we
present a theoretical approach to inelastic light scattering for spin-1 quantum
magnets in the context of a two-band Hubbard model. In contrast to the S=1/2
case, where the only type of local excited state is a doubly occupied state of
energy $U$, several local excited states with occupation up to 4 electrons are
present. As a consequence, we show that two distinct resonating scattering
regimes can be accessed depending on the incident photon energy. For
$\hbar\omega_{in}\lesssim U$, the standard Loudon-Fleury operator remains the
leading term of the expansion as in the spin-1/2 case. For
$\hbar\omega_{in}\lesssim4U$, a second resonant regime is found with a leading
term that takes the form of a biquadratic coupling $\sim({\bf S}_{i}\cdot{\bf
S}_{j)^{2}$. Consequences for the Raman spectra of S=1 magnets with magnetic or
quadrupolar order are discussed. Raman scattering appears to be a powerful
probe of quadrupolar order. | cond-mat_str-el |
lattice-symmetries: A package for working with quantum many-body bases: Exact diagonalization (ED) is one of the most reliable and established
numerical methods of quantum many-body theory. The main limiting factor of the
method is the exponential scaling of Hilbert space dimension with system size.
Fortunately, by symmetry considerations the effective dimension can be reduced
by multiple orders of magnitude. Here, we present lattice-symmetries, a package
for working with such symmetry-adapted quantum many-body bases and operators.
It supports bases for spin-1/2 particles with arbitrary user-defined symmetries
and generic 1-, 2-, 3-, and 4-point operators. As an example application we
discuss SpinED program which allows to easily diagonalize clusters of at least
42 sites on a single node thus making large-scale ED easily accessible to
people with no background in numerical methods and computational physics. | cond-mat_str-el |
Anyonic braiding via quench dynamics in fractional quantum Hall liquids: In a Laughlin fractional quantum Hall state, one- and two-quasihole states
can be obtained by diagonalizing the many-body Hamiltonian with a trapping
potential or, for larger systems, from the linear combination of the edge Jack
polynomials. The quasihole states live entirely in the subspace of the
lowest-energy branch in the energy spectrum with a fixed number of orbits, or a
hard-wall confinement. The reduction in the Hilbert space dimension facilitates
the study of time evolution of the quasihole states after, say, the removal of
the trapping potential. We explore the quench dynamics under a harmonic
external potential, which rotates the quasiholes in the droplet, and discuss
the effect of long-range interaction and more realistic confinement. Accurate
evaluation of the mutual statistics phase of anyons for a wide range of anyon
separation can be achieved from the Berry-phase calculation. | cond-mat_str-el |
Metallic mean-field stripes, incommensurability and chemical potential
in cuprates: We perform a systematic slave-boson mean-field analysis of the three-band
model for cuprates with first-principle parameters. Contrary to widespread
believe based on earlier mean-field computations low doping stripes have a
linear density close to 1/2 added hole per lattice constant. We find a
dimensional crossover from 1D to 2D at doping $\sim 0.1$ followed by a breaking
of particle-hole symmetry around doping 1/8 as doping increases. Our results
explain in a simple way the behavior of the chemical potential, the magnetic
incommensurability, and transport experiments as a function of doping. Bond
centered and site-centered stripes become degenerate for small overdoping. | cond-mat_str-el |
Localized excitation in the hybridization gap in YbAl3: The intermediate valence compound YbAl3 exhibits a broad magnetic excitation
with characteristic energy E1 ~ 50meV, of order of the Kondo energy (TK ~
600-700K). In the low temperature (T < Tcoh ~ 40K) Fermi liquid state, however,
a new magnetic excitation arises at E2 ~ 33meV, which lies in the hybridization
gap that exists in this compound. We show, using inelastic neutron scattering
on a single-crystal sample, that while the scattering at energies near E1 has
the momentum (Q-) dependence expected for interband scattering across the
indirect gap, the scattering near E2 is independent of Q. This suggests that it
arises from a spatially-localized excitation in the hybridization gap. | cond-mat_str-el |
Theory of Low-Temperature Hall Effect in Stripe--Ordered Cuprates: We investigate the effect of static anti-phase stripe order on the weak-field
Hall effect of electrons on a two-dimensional square lattice with electron
dispersion appropriate to the high T$_c$ cuprates. We first consider the cases
where the magnitudes of the spin and charge stripe potentials are smaller than
or of the same order as the bandwidth of the two-dimensional electrons, so that
the electronic properties are not too strongly one-dimensional. In a model with
only spin stripe potential, and at carrier concentrations appropriate to
hole-doped cuprates, increasing the stripe scattering potential from zero leads
to an increase in $R_H$, followed by a sign change. If the scattering amplitude
is yet further increased, a second sign change occurs. The results are in
semiquantitative agreement with data. In a charge-stripe-potential-only model,
$R_H$ increases as the charge stripe scattering strength increases, with no
sign change occurring. In a model with both spin and charge stripe potentials,
$R_H$ may be enhanced or may change sign, depending on the strengths of the two
scattering potentials. We also consider the case in which the magnitudes of the
stripe potentials are much larger than the bandwidth, where analytical results
can be obtained. In this limit, the system is quasi-one-dimensional, while
$R_H$ remains finite and its sign is determined by the carrier density and the
electron band parameters. | cond-mat_str-el |
Magnetic phase diagram of a spin S=1/2 antiferromagnetic two-leg ladder
in the presence of modulated along legs Dzyaloshinskii-Moriya interaction: We study the ground-state magnetic phase diagram of a spin S=1/2
antiferromagnetic two-leg ladder in the presence of period two lattice units
modulated, Dzyaloshinskii-Moriya (DM) interaction along the legs. We consider
the case of collinear DM vectors and strong rung exchange and magnetic field.
In this limit we map the initial ladder model onto the effective spin
$\sigma=1/2$ XXZ chain and study the latter using the continuum-limit
bosonization approach. We identified four quantum phase transitions and
corresponding critical magnetic fields, which mark transitions from the spin
gapped regimes into the gapless quantum spin-liquid regimes. In the gapped
phases the magnetization curve of the system shows plateaus at magnetisation
M=0 and to its saturation value per rung M=1. We have shown that the very
presence of alternating DM interaction leads to opening of a gap in the
excitation spectrum at magnetization M=0.5. The width of the magnetization
plateau at M=0.5, is determined by the associated with the dynamical generation
of a gap in the spectrum is calculated and is shown that its length scales as
$(D_{0}D_{1}/J^{2})^{\alpha}$ where $D_{0},D_{1}$ are uniform and staggered
components of the DM term, $J$ is the intraleg exchange and $\alpha \leq 3/4$
and weakly depends on the DM couplings. | cond-mat_str-el |
Low-dimensional quantum magnetism in Cu(NCS)$_2$: A molecular framework
material: Low-dimensional magnetic materials with spin-$\frac{1}{2}$ moments can host a
range of exotic magnetic phenomena due to the intrinsic importance of quantum
fluctuations to their behavior. Here, we report the structure, magnetic
structure and magnetic properties of copper(II) thiocyanate, Cu(NCS)$_2$, a
one-dimensional coordination polymer which displays low-dimensional quantum
magnetism. Magnetic susceptibility, electron paramagnetic resonance (EPR)
spectroscopy, $^{13}$C magic-angle spinning nuclear magnetic resonance (MASNMR)
spectroscopy, and density functional theory (DFT) investigations indicate that
Cu(NCS)$_2$ behaves as a two-dimensional array of weakly coupled
antiferromagnetic spin chains ($J_2 = 133(1)$ K, $\alpha = J_1/J_2 = 0.08$).
Powder neutron-diffraction measurements confirm that Cu(NCS)$_2$ orders as a
commensurate antiferromagnet below $T_\mathrm{N} = 12$ K, with a strongly
reduced ordered moment (0.3 $\mu_\mathrm{B}$) due to quantum fluctuations. | cond-mat_str-el |
Comment on ``Exact bosonization for an interacting Fermi gas in
arbitrary dimensions'': This a comment on arXiv:0907.3243v2. We demonstrate that the method proposed
by Efetov {\it et. al.} is just a reformulation of the Blankenbeckler,
Scalapino, and Sugar approach and thus it contains exactly the same sign
problem, including the dependence of the sign on the smoothness of the paths. | cond-mat_str-el |
Analysis of the Knight shift data on Li and Zn substituted YBCO: The Knight shift data on Li and Zn substituted YBa$_2$Cu$_3$O$_{6+x}$ are
analysed using an itinerant model with short-range antiferromagnetic
correlations. The model parameters, which are determined by fitting the
experimental data on the transverse nuclear relaxation rate $T_2^{-1}$ of pure
YBa$_2$Cu$_3$O$_{6+x}$, are used to calculate the Knight shifts for various
nuclei around a nonmagnetic impurity located in the CuO$_2$ planes. The
calculations are carried out for Li and Zn impurities substituted into
optimally doped and underdoped YBa$_2$Cu$_3$O$_{6+x}$. The results are compared
with the $^7$Li and $^{89}$Y Knight shift measurements on these materials. | cond-mat_str-el |
Spin and charge density waves in the Lieb lattice: We study the mean-field phase diagram of the two-dimensional (2D) Hubbard
model in the Lieb lattice allowing for spin and charge density waves. Previous
studies of this diagram have shown that the mean-field magnetization
surprisingly deviates from the value predicted by Lieb's theorem
\cite{Lieb1989} as the on-site repulsive Coulomb interaction ($U$) becomes
smaller \cite{Gouveia2015}. Here, we show that in order for Lieb's theorem to
be satisfied, a more complex mean-field approach should be followed in the case
of bipartite lattices or other lattices whose unit cells contain more than two
types of atoms. In the case of the Lieb lattice, we show that, by allowing the
system to modulate the magnetization and charge density between sublattices,
the difference in the absolute values of the magnetization of the sublattices,
$m_{\text{Lieb}}$, at half-filling, saturates at the exact value $1/2$ for any
value of $U$, as predicted by Lieb. Additionally, Lieb's relation,
$m_{\text{Lieb}}=1/2$, is verified approximately for large $U$, in the $n \in
[2/3,4/3]$ range. This range includes not only the ferromagnetic region of the
phase diagram of the Lieb lattice (see Ref.~\onlinecite{Gouveia2015}), but also
the adjacent spiral regions. In fact, in this lattice, below or at
half-filling, $m_{\text{Lieb}}$ is simply the filling of the quasi-flat bands
in the mean-field energy dispersion both for large and small $U$. | cond-mat_str-el |
Quantum criticality and non-Fermi-liquid behavior in a two-level
two-lead quantum dot: Analytical and continuous-time quantum Monte Carlo methods are used to
investigate the possibility of occupation switching and quantum criticality in
a model of two quantum impurities coupled to two leads. A general discussion of
potential occupancy-switching related quantum critical points is given, and a
detailed analysis is made of a specific model which has been recently
discussed. For spinless electrons, no phase transition is found. For electrons
with spin, a critical value of the interaction strength separates a weak
coupling regime in which all properties vary smoothly with parameters from a
strong coupling phase in which occupation numbers vary discontinuously as level
energies are changed. The discontinuity point is characterized by
non-Fermi-liquid behavior. Results for self-energies and correlation functions
are given. Phase diagrams are presented. | cond-mat_str-el |
Spin Drag and Spin-Charge Separation in Cold Fermi Gases: Low-energy spin and charge excitations of one-dimensional interacting
fermions are completely decoupled and propagate with different velocities.
These modes however can decay due to several possible mechanisms. In this paper
we expose a new facet of spin-charge separation: not only the speeds but also
the damping rates of spin and charge excitations are different. While the
propagation of long-wavelength charge excitations is essentially ballistic,
spin propagation is intrinsically damped and diffusive. We suggest that cold
Fermi gases trapped inside a tight atomic waveguide offer the opportunity to
measure the spin-drag relaxation rate that controls the broadening of a spin
packet. | cond-mat_str-el |
Relationship between ferroelectricity and Dzyaloshinskii-Moriya
interaction in multiferroics and the effect of bond-bending: We studied the microscopic mechanism of multiferroics, in particular with the
"spin current" model (Hosho Katsura, Naoto Nagaosa and Aleander V. Balatsky,
Phys. Rev. Lett. 95, 057205 (2005)). Starting from a system with helical spin
configuration, we solved for the forms of the electron wave functions and
analyzed their characteristics. The relation between ferroelectricity and
Dzyaloshinskii-Moriya interaction (I. Dzyaloshinskii, J. Phys. Chem. Solids 4,
241 (1958) and T. Moriya, Phys. Rev. 120, 91 (1960)) is clearly established.
There is also a simple relation between the electric polarization and the wave
vector of magnetic orders. Finally, we show that the bond-bending exists in
transition metal oxides can enhance ferroelectricity. | cond-mat_str-el |
Quasiparticle lifetime behaviour in a simplified self-consistent
T-matrix treatment of the attractive Hubbard model in 2D: The attractive Hubbard model on a 2-D square lattice is studied at low
electronic densities using the ladder approximation for the pair
susceptibility. This model includes (i) the short coherence lengths known to
exist experimentally in the cuprate superconductors, and (ii) two-particle
bound states that correspond to electron pairs. We study the quasiparticle
lifetimes in both non self-consistent and self-consistent theories, the latter
including interactions between the pairs. We find that if we include the
interactions between pairs the quasiparticle lifetimes vary approximately
linearly with the inverse temperature, consistent with experiment. | cond-mat_str-el |
Spin-Wave and Electromagnon Dispersions in Multiferroic MnWO4 as
Observed by Neutron Spectroscopy: Isotropic Heisenberg Exchange versus
Anisotropic Dzyaloshinskii-Moriya Interaction: High resolution inelastic neutron scattering reveals that the elementary
magnetic excitations in multiferroic MnWO4 consist of low energy dispersive
electromagnons in addition to the well-known spin-wave excitations. The latter
can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling
extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1)
meV. Two electromagnon branches appear at lower energies of 0.07(1) meV and
0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric
coupling and persist in both, the collinear magnetic and paraelectric AF1
phase, and the spin spiral ferroelectric AF2 phase. These excitations are
associated with the Dzyaloshinskii-Moriya exchange interaction, which is
significant due to the rather large spin-orbit coupling. | cond-mat_str-el |
Polaronic signatures in the optical properties of
Nd$_{2-x}$Ce$_x$CuO$_4$: We investigate the temperature and doping dependence of the optical
conductivity $\sigma(\omega)$ of Nd$_{2-x}$Ce$_x$CuO$_4$ in terms of
magnetic/lattice polaron formation. We employ dynamical mean-field theory in
the context of the Holstein-t-J model where an exact analytical solution is
available in the limit of infinite connectivity. We show that the pseudogap
features in the optical conductivity of this compound can be associated to the
formation of lattice polarons assisted by the magnetic interaction. | cond-mat_str-el |
Symmetry-protected topological phases of alkaline-earth cold fermionic
atoms in one dimension: We investigate the existence of symmetry-protected topological phases in
one-dimensional alkaline-earth cold fermionic atoms with general half-integer
nuclear spin I at half filling. In this respect, some orbital degrees of
freedom are required. They can be introduced by considering either the
metastable excited state of alkaline-earth atoms or the p-band of the optical
lattice. Using complementary techniques, we show that SU(2) Haldane topological
phases are stabilised from these orbital degrees of freedom. On top of these
phases, we find the emergence of topological phases with enlarged SU(2I+1)
symmetry which depend only on the nuclear spin degrees of freedom. The main
physical properties of the latter phases are further studied using a
matrix-product state approach. On the one hand, we find that these phases are
symmetry-protected topological phases, with respect to inversion symmetry, when
I=1/2,5/2,9/2,..., which is directly relevant to ytterbium and strontium cold
fermions. On the other hand, for the other values of I(=half-odd integer),
these topological phases are stabilised only in the presence of exact
SU(2I+1)-symmetry. | cond-mat_str-el |
On the stability of topological order in tensor network states: We construct a tensor network representation of the 3d toric code ground
state that is stable to a generating set of uniform local tensor perturbations,
including those that do not map to local operators on the physical Hilbert
space. The stability is established by mapping the phase diagram of the
perturbed tensor network to that of the 3d Ising gauge theory, which has a
non-zero finite temperature transition. More generally, we find that the
stability of a topological tensor network state is determined by the form of
its virtual symmetries and the topological excitations created by virtual
operators that break those symmetries. In particular, a dual representation of
the 3d toric code ground state, as well as representations of the X-cube and
cubic code ground states, for which point-like excitations are created by such
operators, are found to be unstable. | cond-mat_str-el |
Extended DFT+U+V method with on-site and inter-site electronic
interactions: In this article we introduce a generalization of the popular DFT+U method
based on the extended Hubbard model that includes on-site and inter-site
electronic interactions. The novel corrective Hamiltonian is designed to study
systems for which electrons are not completely localized on atomic states
(according to the general scheme of Mott localization) and hybridization
between orbitals from different sites plays an important role. The application
of the extended functional to archetypal Mott - charge-transfer (NiO) and
covalently bonded insulators (Si and GaAs) demonstrates its accuracy and
versatility and the possibility to obtain a unifying and equally accurate
description for a broad range of very diverse systems. | cond-mat_str-el |
Possible interaction driven topological phases in (111) bilayers of
LaNiO3: We use the variational mean-field approach to systematically study the phase
diagram of a bilayer heterostructure of the correlated transition metal oxide
LaNiO3, grown along the (111) direction. The Ni 3+ ions with d7 (or eg1)
configuration form a buckled honeycomb lattice. We show that as a function of
the strength of the on-site interactions, various topological phases emerge. In
the presence of a reasonable size of the Hund's coupling, as the correlation is
tuned from intermediate to strong, the following sequence of phases is found:
(1) a Dirac half-semimetal phase, (2) a quantum anomalous hall insulator (QAHI)
phase with Chern number one, and (3) a ferromagnetic nematic phase breaking the
lattice point group symmetry. The spin-orbit couplings and magnetism are both
dynamically generated in the QAHI phase. | cond-mat_str-el |
Nine classes of integrable boundary conditions for the eight-state
supersymmetric fermion model: Nine classes of integrable boundary conditions for the eight-state
supersymmetric model of strongly correlated fermions are presented. The
boundary systems are solved by using the coordinate Bethe ansatz method and the
Bethe ansatz equations for all nine cases are given. | cond-mat_str-el |
Extension of the spin-1/2 frustrated square lattice model: the case of
layered vanadium phosphates: We study the influence of the spin lattice distortion on the properties of
frustrated magnetic systems and consider the applicability of the spin-1/2
frustrated square lattice model to materials lacking tetragonal symmetry. We
focus on the case of layered vanadium phosphates AA'VO(PO4)2 (AA' = Pb2, SrZn,
BaZn, and BaCd). To provide a proper microscopic description of these
compounds, we use extensive band structure calculations for real materials and
model structures and supplement this analysis with simulations of thermodynamic
properties, thus facilitating a direct comparison with the experimental data.
Due to the reduced symmetry, the realistic spin model of layered vanadium
phosphates AA'VO(PO4)2 includes four inequivalent exchange couplings: J1 and
J1' between nearest-neighbors and J2 and J2' between next-nearest-neighbors.
The estimates of individual exchange couplings suggest different regimes, from
J1'/J1 and J2'/J2 close to 1 in BaCdVO(PO4)2, a nearly regular frustrated
square lattice, to J1'/J1 ~ 0.7 and J2'/J2 ~ 0.4 in SrZnVO(PO4)2, a frustrated
square lattice with sizable distortion. The underlying structural differences
are analyzed, and the key factors causing the distortion of the spin lattice in
layered vanadium compounds are discussed. We propose possible routes for
finding new frustrated square lattice materials among complex vanadium oxides.
Full diagonalization simulations of thermodynamic properties indicate the
similarity of the extended model to the regular one with averaged couplings. In
case of moderate frustration and moderate distortion, valid for all the
AA'VO(PO4)2 compounds reported so far, the distorted spin lattice can be
considered as a regular square lattice with the couplings (J1+J1')/2 between
nearest-neighbors and (J2+J2')/2 between next-nearest-neighbors. | cond-mat_str-el |
Effects of quantum impurity spins on the magnetic properties of zigzag
and linear spin chains: We investigated the magnetic ground state and low-energy excitations of the
spin chains compounds SrCuO$_{2}$ (zigzag chains) and Sr$_{2}$CuO$_{3}$ (linear
chains) in the presence of quantum impurities induced by lightly doping ($\leq
1 \%$) with Zn$^{2+}$ ($S = 0$), Co$^{2+}$ ($S =1/2$) and Ni$^{2+}$ ($S = 1$)
impurities at the Cu$^{2+}$ site. We show that the ground states and the nature
of low-lying excitations (i.e., gapped or gapless) depend on the spin state and
symmetry of the defects. For Ni doped chains a spin gap is observed but for Zn
and Co doping the excitations remain gapless. Co-doped chains exhibit magnetic
order with critical temperatures significantly enhanced compared to those of
the pristine compounds. In the specific case of 1 \% Co impurities, the linear
chains exhibit long-range order below 11 K, while the zigzag chain is
characterized by a quasi-long range ordered phase below 6 K with correlation
lengths of about 12\textit{a} and 40\textit{c} units along the crystal axes
\textit{a} and \textit{c}, respectively. The different magnetic behaviours of
these two compounds with comparable intra- and interchain couplings underpin
the role of spin frustration in the zigzag chains. | cond-mat_str-el |
Unraveling Orbital Correlations via Magnetic Resonant Inelastic X-ray
Scattering: Although orbital degrees of freedom are a factor of fundamental importance in
strongly correlated transition metal compounds, orbital correlations and
dynamics remain very difficult to access, in particular by neutron scattering.
Via a direct calculation of scattering amplitudes we show that instead magnetic
resonant inelastic x-ray scattering (RIXS) does reveal orbital correlations. In
contrast to neutron scattering, the intensity of the magnetic excitations in
RIXS depends very sensitively on both the symmetry of the orbitals that spins
occupy, and on photon polarizations. We show in detail how this effect allows
magnetic RIXS to distinguish between alternating orbital ordered and
ferro-orbital (or orbital liquid) states. | cond-mat_str-el |
Emergent SU(3) symmetry in random spin-1 chains: We show that generic SU(2)-invariant random spin-1 chains have phases with an
emergent SU(3) symmetry. We map out the full zero-temperature phase diagram and
identify two different phases: (i) a conventional random singlet phase (RSP) of
strongly bound spin pairs (SU(3) "mesons") and (ii) an unconventional RSP of
bound SU(3) "baryons", which are formed, in the great majority, by spin trios
located at random positions. The emergent SU(3) symmetry dictates that
susceptibilities and correlation functions of both dipolar and quadrupolar spin
operators have the same asymptotic behavior. | cond-mat_str-el |
Comment on "Critical spin dynamics of the 2D quantum Heisenberg
antiferromagnets: Sr2CuO2Cl2 and Sr2Cu3O4Cl2": We compare the neutron measurements of Kim et al. (cond-mat/0012239) on
two-dimensional, S=1/2 antiferromagnets with the continuum quasiclassical
theory of S. Sachdev and O.A. Starykh (cond-mat/9904354). The damping of the
lowest energy spin excitations is characterized by a dimensionless number whose
temperature dependence was predicted to be determined entirely by that of the
uniform spin susceptibility. Theory and experiment are consistent with each
other. | cond-mat_str-el |
Energy band of graphene ribbons under the tensile force: According to the tight-binding approximation, we investigate the electronic
structures of graphene ribbons with zigzag shaped edges (ZGRs) and armchair
shaped edges (AGRs) drawn by the tensile force, and obtain the analytic
relations between the energy bands of pi-electrons in ZGR, AGR and the tensile
force based on only considering the nearest-neighbor interaction and the
hydrogen-like atomic wave function is considered as pi-electron wave function.
Importantly, we find the tensile force can open an energy gap at the K point
for ZGR and AGR, and the force perpendicular to the zigzag edges can open
energy gap more easily besides the gap values of ZGR and AGR at the K point
both increase as the tensile force increases. | cond-mat_str-el |
Dimensionality Control of d-orbital Occupation in Oxide Superlattices: Manipulating the orbital state in a strongly correlated electron system is of
fundamental and technological importance for exploring and developing novel
electronic phases. Here, we report an unambiguous demonstration of orbital
occupancy control between t2g and eg multiplets in quasi-twodimensional
transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3
and a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its
fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs
changed from a Mott insulator, in which both t2g and eg orbitals are partially
filled, to a band insulator by completely filling (emptying) the t2g (eg)
orbitals. We found the reduction of dimensionality has a profound effect on the
electronic structure evolution, which is, whereas, insensitive to the epitaxial
strain. The remarkable orbital controllability shown here offers a promising
pathway for novel applications such as catalysis and photovoltaics, where the
energy of d level is an essential parameter. | cond-mat_str-el |
Pseudofermion ferromagnetism in the Kondo lattices: a mean-field
approach: Ground state ferromagnetism of the Kondo lattices is investigated within
slave fermion approach by Coleman and Andrei within a mean-field approximation
in the effective hybridization model. Conditions for formation of both
saturated (half-metallic) and non-saturated magnetic state are obtained for
various lattices. A description in terms of universal functions which depend
only on bare electron density of states (DOS) is presented. A crucial role of
the energy dependence of the bare DOS (especially, of DOS peaks) for the
small-moment ferromagnetism formation is demonstrated. | cond-mat_str-el |
Ferromagnetism and Fermi-surface transition in the periodic Anderson
model: Second-order phase transition without symmetry breaking: We study ferromagnetism in the periodic Anderson model with and without a
magnetic field by the Gutzwiller theory. We find three ferromagnetic phases: a
weak ferromagnetic phase (FM0), a half-metallic phase without Fermi surface for
the majority spin (FM1), and a ferromagnetic phase with almost completely
polarized f-electrons (FM2). The Fermi surface changes from the large
Fermi-surface in the paramagnetic state to the small Fermi-surface in FM2. We
also find that the transitions between the ferromagnetic phases can be
second-order phase transitions in spite of the absence of symmetry breaking.
While we cannot define an order parameter for such transitions in an ordinary
way, the topology of the Fermi surface characterizes the transitions, i.e.,
they are Lifshitz transitions. | cond-mat_str-el |
Steady-state superconductivity in electronic materials with repulsive
interactions: We study the effect of laser driving on a minimal model for a hexagonal
two-dimensional material with broken inversion symmetry. Through the
application of circularly polarised light and coupling to a thermal free
electron bath, the system is driven into a nonequilibrium steady state with
asymmetric, nonthermal carrier populations in the two valleys. We show that, in
this steady state, interband superconducting correlations between electrons can
develop independent of the sign of the electron-electron interactions. We
discuss how our results apply, for example, to transition metal
dichalcogenides. This work opens the door to technological applications of
superconductivity in a range of materials that were hitherto precluded from it. | cond-mat_str-el |
Correlation-driven electronic nematicity in the Dirac semimetal BaNiS2: In BaNiS2 a Dirac nodal-line band structure exists within a two-dimensional
Ni square lattice system, in which significant electronic correlation effects
are anticipated. Using scanning tunneling microscopy, we discover signs of
correlated-electron behavior, namely electronic nematicity appearing as a pair
of C2-symmetry striped patterns in the local density-of-states at ~60 meV above
the Fermi energy. In observations of quasiparticle interference, as well as
identifying scattering between Dirac cones, we find that the striped patterns
in real space stem from a lifting of degeneracy among electron pockets at the
Brillouin zone boundary. We infer a momentum-dependent energy shift with d-form
factor, which we model numerically within a density wave equation framework
that considers spin-fluctuation-driven nematicity. This suggests an unusual
mechanism driving the nematic instability, stemming from only a small
perturbation to the Fermi surface, in a system with very low density of states
at the Fermi energy. The Dirac points lie at nodes of the d-form factor, and
are almost unaffected by it. These results highlight BaNiS2 as a unique
material in which Dirac electrons and symmetry-breaking electronic correlations
coexist. | cond-mat_str-el |
Quantum chaos on a critical Fermi surface: We compute parameters characterizing many-body quantum chaos for a critical
Fermi surface without quasiparticle excitations. We examine a theory of $N$
species of fermions at non-zero density coupled to a $U(1)$ gauge field in two
spatial dimensions, and determine the Lyapunov rate and the butterfly velocity
in an extended random-phase approximation. The thermal diffusivity is found to
be universally related to these chaos parameters i.e. the relationship is
independent of $N$, the gauge coupling constant, the Fermi velocity, the Fermi
surface curvature, and high energy details. | cond-mat_str-el |
Magnetic critical properties and basal-plane anisotropy of Sr$_2$IrO$_4$: The anisotropic magnetic properties of Sr$_2$IrO$_4$ are investigated, using
longitudinal and torque magnetometry. The critical scaling across $T_c$ of the
longitudinal magnetization is the one expected for the 2D XY universality
class. Modeling the torque for a magnetic field in the basal-plane, and taking
into account all in-plane and out-of-plane magnetic couplings, we derive the
effective 4-fold anisotropy $K_4 \approx$ 1 10$^5$ erg mole$^{-1}$. Although
larger than for the cuprates, it is found too small to account for a
significant departure from the isotropic 2D XY model. The in-plane torque also
allows us to put an upper bound for the anisotropy of a field-induced shift of
the antiferromagnetic ordering temperature. | cond-mat_str-el |
Angular dependence of the Hall effect of lsmo films: We find that the Hall effect resistivity ($\rho_{xy}$) of thin films of
\lsmo\ varies as a function of the angle $\theta$ between the applied magnetic
field and the film normal as $\rho_{xy}=a\cos \theta + b\cos 3\theta$, where
$|b|$ increases with increasing temperature and decreases with increasing
magnetic field. We find that the angular dependence of the longitudinal
resistivity and the magnetization cannot fully explain the surprising term $b$,
suggesting it is a manifestation of an intrinsic transport property. | cond-mat_str-el |
Spin Density and Non-Collinear Magnetization in Frustrated Pyrochlore
\tbti from Polarized Neutron Scattering: We used a local susceptibility approach in extensive polarized neutron
diffraction studies of the spin liquid \tbti. For a magnetic field applied
along the [110] and [111] directions, we found that, at high temperature, all
Tb moments are collinear and parallel to the field. With decreasing
temperature, the Tb moments reorient from the field direction to their local
anisotropy axes. For the [110] field direction, the field induced magnetic
structure at 10 K is spin ice-like, but with two types of Tb moments of very
different magnitudes. For a field along [111], the magnetic structure resembles
the so-called "one in-three out" found in spin ices, with the difference that
all Tb moments have an additional component along the [111] direction due to
the magnetic field. The temperature evolution of the local susceptibilities
clearly demonstrates a progressive change from Heisenberg to Ising behavior of
the Tb moments when lowering the temperature, which appears to be a crystal
field effect. | cond-mat_str-el |
Microscopic characterization of the magnetic properties of the itinerant
antiferromagnet La2Ni7 by 139La NMR/NQR measurements: 139La nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)
measurements have been performed to investigate the magnetic properties of the
itinerant magnet La2Ni7 which shows a series of antiferromagnetic (AFM) phase
transitions at $T_{N1}$=61 K, $T_{N2}$=56 K, and $T_{N3}$=42 K under zero
magnetic field. Two distinct La NMR signals were observed due to the two
crystallographically inequivalent La sites in La2Ni7 (La1 and La2 in the La2Ni4
and the LaNi5 sub-units of the La2Ni7 unit cell, respectively). From the 139La
NQR spectrum in the AFM state below $T_{N3}$, the AFM state was revealed to be
a commensurate state where Ni ordered moments align along the crystalline c
axis. Owing to the two different La sites, we were able to estimate the average
values of the Ni ordered moments ($\sim$0.09-0.10 $\mu_{B}$/Ni and
$\sim$0.17$\mu_{B}$/Ni around La1 and La2, respectively) from 139La NMR
spectrum measurements in the AFM state below $T_{N3}$, suggesting a non-uniform
distribution of the Ni-ordered moments in the AFM state. In contrast, a more
uniform distribution of the Ni-ordered moments in the saturated paramagnetic
state induced by the application of high magnetic fields is observed. The
temperature dependence of the sublattice magnetization measured by the internal
field at the La2 site in the AFM state was reproduced by a local moment model
better than the self-consistent renormalization (SCR) theory for weak itinerant
antiferromagnets. Given the small Ni-ordered moments in the magnetically
ordered state, our results suggest that La2Ni7 has characteristics of both
itinerant and localized natures in its magnetism. With this in mind, it is
noteworthy that the temperature dependence of nuclear spin-relaxation rates in
the paramagnetic state above $T_{N1}$ measured at zero magnetic field can be
explained qualitatively by both the SCR theory and the local-moment model. | cond-mat_str-el |
Metamagnetic transition in the two $f$ orbitals Kondo lattice model: In this work, we study the effects of a transverse magnetic field in a Kondo
lattice model with two $f$ orbitals interacting with the conduction electrons.
The $f$ electrons that are present on the same site interact through Hund's
coupling, while on neighboring sites they interact through intersite exchange.
We consider here that part of $f$ electrons are localized (orbital 1) while
another part (orbital 2) are delocalized, as it is frequent in uranium systems.
Then, only electrons in the localized orbital 1 interact through exchange
interaction with the neighboring ones, while electrons in orbital 2 are coupled
with conduction electrons through a Kondo interaction. We obtain a solution
where ferromagnetism and Kondo effect coexist for small values of an applied
transverse magnetic field for $T\rightarrow0$. Increasing the transverse field,
two situations can be obtained when Kondo coupling vanishes: first, a
metamagnetic transition occurs just before or at the same time of the fully
polarized state, and second, a metamagnetic transition occurs when the spins
are already pointing out along the magnetic field. | cond-mat_str-el |
Renormalization Group Potential for Quasi-One-Dimensional Correlated
Systems: We studied the correlated quasi-one-dimensional systems by one-loop
renormalization group techniques in weak coupling. In contrast to conventional
g-ology approach, we formulate the theory in terms of bilinear currents and
obtain all possible interaction vertices. Furthermore, the one-loop
renormalization group equations are derived by operator product expansions of
these currents at short length scale. It is rather remarkable that these
coupled non-linear equations, after appropriate rescaling, can be casted into
potential flows. The existence of what we nicknamed "RG potential" provides a
natural explanation of the emergent symmetry enhancement in ladder systems.
Further implications arisen from the RG potential are also discussed at the
end. | cond-mat_str-el |
Zero frequency divergence and gauge phase factor in the optical response
theory: The static current-current correlation leads to the definitional zero
frequency divergence (ZFD) in the optical susceptibilities. Previous
computations have shown nonequivalent results between two gauges (${\bf p\cdot
A}$ and ${\bf E \cdot r}$) under the exact same unperturbed wave functions. We
reveal that those problems are caused by the improper treatment of the
time-dependent gauge phase factor in the optical response theory. The gauge
phase factor, which is conventionally ignored by the theory, is important in
solving ZFD and obtaining the equivalent results between these two gauges. The
Hamiltonians with these two gauges are not necessary equivalent unless the
gauge phase factor is properly considered in the wavefunctions. Both
Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models of
trans-polyacetylene serve as our illustrative examples to study the linear
susceptibility $\chi^{(1)}$ through both current-current and dipole-dipole
correlations. Previous improper results of the $\chi^{(1)}$ calculations and
distribution functions with both gauges are discussed. The importance of gauge
phase factor to solve the ZFD problem is emphasized based on SSH and TLM
models. As a conclusion, the reason why dipole-dipole correlation favors over
current-current correlation in the practical computations is explained. | cond-mat_str-el |
Majorana Edge States for Z2 Topological Orders of the Wen-plaquette
Model and the Toric-code Model: In this paper we study the symmetry protected Majorana edge states for the Z2
topological order of the Wen-plaquette model and the toric-code model and
calculate the dispersion of the Majorana edge states. For the system with
translational symmetry, the Majorana edge states are gapless and have the nodal
points at k=0 and k=pi. For the edge states of the toric-code model without
translational symmetry, the edge modes become gapped. | cond-mat_str-el |
Reply to 'Comment on "Dynamic correlations of the spinless Coulomb
Luttinger liquid [Phys. Rev. B 65, 125109 (2002)]"': We show that the criticism of our paper [Phys. Rev. B 65, 125109 (2002)] by
Wang, Millis, and Das Sarma [cond-mat/0206203] is based on a trivial
mathematical mistake they have committed. | cond-mat_str-el |
Symmetry projected Jastrow mean field wavefunction in variational Monte
Carlo: We extend our low-scaling variational Monte Carlo (VMC) algorithm to optimize
the symmetry projected Jastrow mean field (SJMF) wavefunctions. These
wavefunctions consist of a symmetry-projected product of a Jastrow and a
general broken-symmetry mean field reference. Examples include Jastrow
antisymmetrized geminal power (JAGP), Jastrow-Pfaffians, and resonating valence
bond (RVB) states among others, all of which can be treated with our algorithm.
We will demonstrate using benchmark systems including the nitrogen molecule, a
chain of hydrogen atoms, and the 2-D Hubbard model that a significant amount of
correlation can be obtained by optimizing the energy of the SJMF wavefunction.
This can be achieved at a relatively small cost when multiple symmetries
including spin, particle number, and complex conjugation are simultaneously
broken and projected. We also show that reduced density matrices can be
calculated using the optimized wavefunctions, which allows us to calculate
other observables such as correlation functions and will enable us to embed the
VMC algorithm in a complete active space self-consistent field (CASSCF)
calculation. | cond-mat_str-el |
Matrix-product-state method with a dynamical local basis optimization
for bosonic systems out of equilibrium: We present a method for simulating the time evolution of one-dimensional
correlated electron-phonon systems which combines the time-evolving block
decimation algorithm with a dynamical optimization of the local basis. This
approach can reduce the computational cost by orders of magnitude when boson
fluctuations are large. The method is demonstrated on the nonequilibrium
Holstein polaron by comparison with exact simulations in a limited functional
space and on the scattering of an electronic wave packet by local phonon modes.
Our study of the scattering problem reveals a rich physics including transient
self-trapping and dissipation. | cond-mat_str-el |
Floquet second-order topological insulators in non-Hermitian systems: Second-order topological insulator (SOTI) is featured with the presence of
$(d-2)$-dimensional boundary states in $d$-dimension systems. The
non-Hermiticity induced breakdown of bulk-boundary correspondence (BBC) and the
periodic driving on systems generally obscure the description of non-Hermitian
SOTI. To prompt the applications of SOTIs, we explore the role of periodic
driving in controllably creating exotic non-Hermitian SOTIs both for 2D and 3D
systems. A scheme to retrieve the BBC and a complete description to SOTIs via
the bulk topology of such nonequilibrium systems are proposed. It is found that
rich exotic non-Hermitian SOTIs with a widely tunable number of 2D corner
states and 3D hinge states and a coexistence of the first- and second-order
topological insulators are induced by the periodic driving. Enriching the
family of topological phases, our result may inspire the exploration to apply
SOTIs via tuning the number of corner/hinge states by the periodic driving. | cond-mat_str-el |
Stability of skyrmions in perturbed ferromagnetic chiral magnets: Magnetic skyrmions, topological spin textures observed in chiral magnets,
have attracted huge interest due to their applications in the field of
spintronics. In this work we study the stability of circular isolated skyrmions
in ferromagnetic chiral magnets under the influence of different perturbations
and external fields. To this end we develop a general systematic procedure
based in a harmonic expansion series of the skyrmion boundary which allows the
identifycation of the breakdown of the skyrmion circular shape on each
instability channel independently. We apply our approach to a few
representative spin models with actual interest in order to obtain the zero
temperature phase diagram, where isolated skyrmions emerge as metaestable
states. The results presented in this paper are in agreement with properties of
isolated skyrmions observed in recent experiments opening the possibility of
extending the analysis to more complex situations. | cond-mat_str-el |
Floquet multi-Weyl points in crossing-nodal-line semimetals: Weyl points with monopole charge $\pm 1$ have been extensively studied,
however, real materials of multi-Weyl points, whose monopole charges are higher
than $1$, have yet to be found. In this Rapid Communication, we show that
nodal-line semimetals with nontrivial line connectivity provide natural
platforms for realizing Floquet multi-Weyl points. In particular, we show that
driving crossing nodal lines by circularly polarized light generates
double-Weyl points. Furthermore, we show that monopole combination and
annihilation can be observed in crossing-nodal-line semimetals and nodal-chain
semimetals. These proposals can be experimentally verified in pump-probe
angle-resolved photoemission spectroscopy. | cond-mat_str-el |
The possibility of measuring intrinsic electronic correlations in
graphene using a d-wave contact Josephson junction: While not widely recognized, electronic correlations might play an important
role in graphene. Indeed, Pauling's resonance valence bond (RVB) theory for the
pp-bonded planar organic molecules, of which graphene is the infinite
extension, already established the importance of the nearest neighbor
spin-singlet bond (SB) state in these materials. However, despite the recent
growth of interest in graphene, there is still no quantitative estimate of the
effects of Coulomb repulsion in either undoped or doped graphene. Here we use a
tight-binding Bogoliubov-de Gennes (TB BdG) formalism to show that in
unconventional d-wave contact graphene Josephson junctions the intrinsic SB
correlations are strongly enhanced. We show on a striking effect of the SB
correlations in both proximity effect and Josephson current as well as
establishing a 1/(T-T_c) functional dependence for the superconducting decay
length. Here T_c is the superconducting transition temperature for the
intrinsic SB correlations, which depends on both the effects of Coulomb
repulsion and the doping level. We therefore propose that d-wave contact
graphene Josephson junctions will provide a promising experimental system for
the measurement of the effective strength of intrinsic SB correlations in
graphene. | cond-mat_str-el |
Bending and Breaking of Stripes in a Charge-Ordered Manganite: In complex electronic materials, coupling between electrons and the atomic
lattice gives rise to remarkable phenomena, including colossal
magnetoresistance and metal-insulator transitions. Charge-ordered phases are a
prototypical manifestation of charge-lattice coupling, in which the atomic
lattice undergoes periodic lattice displacements (PLDs). Here we directly map
the picometer scale PLDs at individual atomic columns in the room temperature
charge-ordered manganite Bi$_{0.35}$Sr$_{0.18}$Ca$_{0.47}$MnO$_3$ using
aberration corrected scanning transmission electron microscopy (STEM). We
measure transverse, displacive lattice modulations of the cations, distinct
from existing manganite charge-order models. We reveal locally unidirectional
striped PLD domains as small as $\sim$5 nm, despite apparent bidirectionality
over larger length scales. Further, we observe a direct link between disorder
in one lattice modulation, in the form of dislocations and shear deformations,
and nascent order in the perpendicular modulation. By examining the defects and
symmetries of PLDs near the charge-ordering phase transition, we directly
visualize the local competition underpinning spatial heterogeneity in a complex
oxide. | cond-mat_str-el |
Valence Bond Phases in $S=1/2$ Kane-Mele-Heisenberg Model: The phase diagram of Kane-Mele-Heisenberg (KMH) model in classical
limit~\cite{zare}, contains disordered regions in the coupling space, as the
result of to competition among different terms in the Hamiltonian, leading to
frustration in finding a unique ground state. In this work we explore the
nature of these phase in the quantum limit, for a $S=1/2$. Employing exact
diagonalization (ED) in $S_z$ and nearest neighbor valence bond (NNVB) bases,
bond and plaquette valence bond mean field theories, We show that the
disordered regions are divided into ordered quantum states in the form of
plaquette valence bond crystal(PVBC) and staggered dimerized (SD) phases. | cond-mat_str-el |
Correlation effects in partially ionized mass asymmetric electron-hole
plasmas: The effects of strong Coulomb correlations in dense three-dimensional
electron-hole plasmas are studied by means of unbiased direct path integral
Monte Carlo simulations. The formation and dissociation of bound states, such
as excitons and bi-excitons is analyzed and the density-temperature region of
their appearance is identified. At high density, the Mott transition to the
fully ionized metallic state (electron-hole liquid) is detected. Particular
attention is paid to the influence of the hole to electron mass ratio $M$ on
the properties of the plasma. Above a critical value of about M=80 formation of
a hole Coulomb crystal was recently verified [Phys. Rev. Lett. {\bf 95}, 235006
(2005)] which is supported by additional results. Results are related to the
excitonic phase diagram of intermediate valent Tm[Se,Te], where large values of
$M$ have been observed experimentally. | cond-mat_str-el |
The ALPS project release 1.3: open source software for strongly
correlated systems: We present release 1.3 of the ALPS (Algorithms and Libraries for Physics
Simulations) project, an international open source software project to develop
libraries and application programs for the simulation of strongly correlated
quantum lattice models such as quantum magnets, lattice bosons, and strongly
correlated fermion systems. Development is centered on common XML and binary
data formats, on libraries to simplify and speed up code development, and on
full-featured simulation programs. The programs enable non-experts to start
carrying out numerical simulations by providing basic implementations of the
important algorithms for quantum lattice models: classical and quantum Monte
Carlo (QMC) using non-local updates, extended ensemble simulations, exact and
full diagonalization (ED), as well as the density matrix renormalization group
(DMRG). Changes in the new release include a DMRG program for interacting
models, support for translation symmetries in the diagonalization programs, the
ability to define custom measurement operators, and support for inhomogeneous
systems, such as lattice models with traps. The software is available from our
web server at http://alps.comp-phys.org/ . | cond-mat_str-el |
Magnetic excitations in hole-doped Sr2IrO4: A comparison with
electron-doped cuprates: We have studied the evolution of magnetic and orbital excitations as a
function of hole-doping in single crystal samples of Sr2Ir(1-x)Rh(x)O4 (0.07 <
x < 0.42) using high resolution Ir L3-edge resonant inelastic x-ray scattering
(RIXS). Within the antiferromagnetically ordered region of the phase diagram (x
< 0.17) we observe highly dispersive magnon and spin-orbit exciton modes.
Interestingly, both the magnon gap energy and the magnon bandwidth appear to
increase as a function of doping, resulting in a hardening of the magnon mode
with increasing hole doping. As a result, the observed spin dynamics of
hole-doped iridates more closely resemble those of the electron-doped, rather
than hole-doped, cuprates. Within the paramagnetic region of the phase diagram
(0.17 < x < 0.42) the low-lying magnon mode disappears, and we find no evidence
of spin fluctuations in this regime. In addition, we observe that the orbital
excitations become essentially dispersionless in the paramagnetic phase,
indicating that magnetic order plays a crucial role in the propagation of the
spin-orbit exciton. | cond-mat_str-el |
The Thermoelectric Effect and Its Natural Heavy Fermion Explanation in
Twisted Bilayer and Trilayer Graphene: We study the interacting transport properties of twisted bilayer graphene
(TBG) using the topological heavy-fermion (THF) model. In the THF model, TBG
comprises localized, correlated $f$-electrons and itinerant, dispersive
$c$-electrons. We focus on the Seebeck coefficient, which quantifies the
voltage difference arising from a temperature gradient. We find that the TBG's
Seebeck coefficient shows unconventional (strongly-interacting) traits:
negative values with sawtooth oscillations at positive fillings, contrasting
typical band-theory expectations. This behavior is naturally attributed to the
presence of heavy (correlated, short-lived $f$-electrons) and light
(dispersive, long-lived $c$-electrons) electronic bands. Their longer lifetime
and stronger dispersion lead to a dominant transport contribution from the
$c$-electrons. At positive integer fillings, the correlated TBG insulators
feature $c$- ($f$-)electron bands on the electron (hole) doping side, leading
to an overall negative Seebeck coefficient. Additionally, sawtooth oscillations
occur around each integer filling due to gap openings. Our results highlight
the essential importance of electron correlations in understanding the
transport properties of TBG and, in particular, of the lifetime asymmetry
between the two fermionic species (naturally captured by the THF model). Our
findings are corroborated by new experiments in both twisted bilayer and
trilayer graphene, and show the natural presence of strongly-correlated heavy
and light carriers in the system. | cond-mat_str-el |
Emergent soft-gap Anderson models at quantum criticality in a lattice
Hamiltonian within dynamical mean field theory: Local quantum criticality in itinerant fermion systems has been extensively
investigated through the soft-gap Anderson impurity model, wherein a localized,
correlated impurity, hybridizes with a broad conduction band with a singular,
$|\omega|^r$, density of states. However, lattice models hosting quantum
critical points (QCPs), do not appear to have such a spectrum emerging at the
QCP. In this work, we report the emergence of such a singular form of the
density of states in a three-orbital lattice model, within dynamical mean field
theory, precisely at a quantum critical point, separating a gapless, Fermi
liquid, metallic phase from a gapped, Mott insulating phase. A
temperature-dependent exponent, $\alpha$, defined using the corresponding
Matsubara self-energy, is found to vary from $+1$ deep in the FL regime, to
$-1$ in the Mott insulator regime. Interestingly, we find that $\alpha$ becomes
temperature independent, and hence isosbestic, precisely at the QCP. The
isosbestic exponent is shown to lead to an emergent soft-gap spectrum,
$|\omega|^r$ at the QCP, where $r = |\alpha_{\rm iso}|$. We discuss the
implications of our findings for non-Fermi liquid behaviour in the quantum
critical region of the phase diagram. | cond-mat_str-el |
Calculating ground state properties of correlated fermionic systems with
BCS trial wave functions in Slater determinant path-integral approaches: We introduce an efficient and numerically stable technique to make use of a
BCS trial wave function in the computation of correlation functions of strongly
correlated quantum fermion systems. The technique is applicable to any
projection approach involving paths of independent-fermion propagators, for
example in mean-field or auxiliary-field quantum Monte Carlo (AFQMC)
calculations. Within AFQMC, in the absence of the sign problem, the methodology
allows the use of a BCS reference state which can greatly reduce the required
imaginary time of projection, and improves Monte Carlo sampling efficiency and
statistical accuracy for systems where pairing correlations are important. When
the sign problem is present, the approach provides a powerful generalization of
the constrained-path AFQMC technique which usually uses Slater determinant
trial wave functions. As a demonstration of the capability of the methodology,
we present benchmark results for the attractive Hubbard model, both
spin-balanced (no sign problem) and with a finite spin polarization (with sign
problem). | cond-mat_str-el |
Demonstration of a robust pseudogap in a three-dimensional correlated
electronic system: We outline a partial-fractions decomposition method for determining the
one-particle spectral function and single-particle density of states of a
correlated electronic system on a finite lattice in the non self-consistent
T-matrix approximation to arbitrary numerical accuracy, and demonstrate the
application of these ideas to the attractive Hubbard model. We then demonstrate
the effectiveness of a finite-size scaling ansatz which allows for the
extraction of quantities of interest in the thermodynamic limit from this
method. In this approximation, in one or two dimensions, for any finite lattice
or in the thermodynamic limit, a pseudogap is present and its energy diverges
as Tc is approached from above; this is an unphysical manifestation of using an
approximation that predicts a spurious phase transition in one or two
dimensions. However, in three dimensions one expects the transition predicted
by this approximation to represent a true continuous phase transition, and in
the thermodynamic limit any pseudogap predicted by this formulation will remain
finite. We have applied our method to the attractive Hubbard model on a
three-dimensional simple cubic lattice, and find that for intermediate coupling
a prominent pseudogap is found in the single-particle density of states, and
this gap persists over a large temperature range. In addition, we also show
that for weak coupling a pseudogap is also present. The pseudogap energy at the
transition temperature is almost a factor of three larger than the T=0 BCS gap
for intermediate coupling, whereas for weak coupling the pseudogap and BCS gap
energies are essentially equal. | cond-mat_str-el |
Fermi surface reconstruction by a charge-density-wave with finite
correlation length: Even a small amplitude charge-density-wave (CDW) can reconstruct a Fermi
surface, giving rise to new quantum oscillation frequencies. Here, we
investigate quantum oscillations when the CDW has a finite correlation length
$\xi$ -- a case relevant to the hole-doped cuprates. By considering the Berry
phase induced by a spatially varying CDW phase, we derive an effective Dingle
factor that depends exponentially on the ratio of the cyclotron orbit radius,
$R_c$, to $\xi$. In the context of YBCO, we conclude that the values of $\xi$
reported to date for bidirectional CDW order are, prima facie, too short to
account for the observed Fermi surface reconstruction; on the other hand, the
values of $\xi$ for the unidirectional CDW are just long enough. | cond-mat_str-el |
A look at the crossover region between BCS superconductivity and Bose
Einstein Condensation: Pair fluctuation theory has been used to study the crossover from the weak
coupling BCS theory to the strong coupling Bose Einstein Condensation. The
effect of fluctuations has been studied over the whole crossover regime. It has
been shown that the pair fluctuations are enhanced considerably in both two and
three dimensions and hence mean field theory is inadequate to study the
physical properties in this regime. A self consistent scheme for calculating
the pair susceptibility is given. | cond-mat_str-el |
Magnetic field dependence of the many-electron states in a magnetic
quantum dot: The ferromagnetic-antiferromagnetic transition: The electron-electron correlations in a many-electron (Ne = 1, 2,..., 5)
quantum dot confined by a parabolic potential is investigated in the presence
of a single magnetic ion and a perpendicular magnetic field. We obtained the
energy spectrum and calculated the addition energy which exhibits cusps as
function of the magnetic field. The vortex properties of the many-particle wave
function of the ground state are studied and for large magnetic fields are
related to composite fermions. The position of the impurity influences strongly
the spin pair correlation function when the external field is large. In small
applied magnetic field, the spin exchange energy together with the Zeeman terms
leads to a ferromagnetic-antiferromagnetic(FM-AFM) transition. When the
magnetic ion is shifted away from the center of the quantum dot a remarkable
re-entrant AFM-FM-AFM transition is found as function of the strength of the
Coulomb interaction. Thermodynamic quantities as the heat capacity, the
magnetization, and the susceptibility are also studied. Cusps in the energy
levels show up as peaks in the heat capacity and the susceptibility. | cond-mat_str-el |
Thermodynamic investigations in the precursor region of FeGe: High-resolution DC magnetization and AC-specific heat data of the cubic
helimagnet FeGe have been measured as function of temperature and magnetic
field. The magnetization data as well as the isothermal susceptibility data
confirm the complexity of the magnetic phase diagram in the vicinity of the
onset of long-rang magnetic order (Tc = 278.5 K) and the existence of a
segmented A-phase region. Moreover, these data revealed independent and clear
indications of phase boundaries and crossovers within the A-phase region.
Together with the anomalies in the specific-heat data around Tc and at small
magnetic fields (H < 600 Oe) a complex magnetic phase diagram of FeGe is
obtained. | cond-mat_str-el |
Formation of energy gap in higher dimensional spin-orbital liquids: A Schwinger boson mean field theory is developed for spin liquids in a
symmetric spin-orbital model in higher dimensions. Spin, orbital and coupled
spin-orbital operators are treated equally. We evaluate the dynamic correlation
functions and collective excitations spectra. As the collective excitations
have a finite energy gap, we conclude that the ground state is a spin-orbital
liquid with a two-fold degeneracy, which breaks the discrete spin-orbital
symmetry. Possible relevence of this spin liquid state to several realistic
systems, such as CaV$_4$V$_9$ and Na$_2$Sb$_2$Ti$_2$O, are discussed. | cond-mat_str-el |
Emergent Anisotropic Non-Fermi Liquid at a Topological Phase Transition
in Three Dimensions: Understanding correlation effects in topological phases and their transitions
is a cutting-edge area of research in recent condensed matter physics. We study
topological quantum phase transitions (TQPTs) between double-Weyl semimetals
(DWSMs) and insulators, and argue that a novel class of quantum criticality
appears at the TQPT characterized by emergent anisotropic non-Fermi liquid
behaviors, in which the interplay between the Coulomb interaction and
electronic critical modes induces not only anisotropic renormalization of the
Coulomb interaction but also strongly correlated electronic excitation in three
spatial dimensions. Using the standard renormalization group methods, large
$N_f$ theory and the $\epsilon= 4-d$ method with fermion flavor number $N_f$
and spatial dimension $d$, we obtain the anomalous dimensions of electrons
($\eta_f=0.366/N_f $) in large $N_f$ theory and the associated anisotropic
scaling relations of various physical observables. Our results may be observed
in candidate materials for DWSMs such as HgCr$_2$Se$_4$ or SrSi$_2$ when the
system undergoes a TQPT. | cond-mat_str-el |
Dynamical Effects from Anomaly: Modified Electrodynamics in Weyl
Semimetal: We discuss the modified quantum electrodynamics from a time-reversal-breaking
Weyl semimetal coupled with a $U(1)$ gauge (electromagnetic) field. A key role
is played by the soft dispersion of the photons in a particular direction, say
$\hat{z}$, due to the Hall conductivity of the Weyl semimetal. Due to the soft
photon, the fermion velocity in $\hat{z}$ is logarithmically reduced under
renormalization group flow, together with the fine structure constant.
Meanwhile, fermions acquire a finite lifetime from spontaneous emission of the
soft photon, namely the Cherenkov radiation. At low energy $E$, the inverse of
the fermion lifetime scales as $\tau^{-1}\sim E/{\rm PolyLog}(E)$. Therefore,
even though fermion quasiparticles are eventually well-defined at very low
energy, over a wide intermediate energy window the Weyl semimetal behaves like
a marginal Fermi liquid. Phenomenologically, our results are more relevant for
emergent Weyl semimetals, where the fermions and photons all emerge from
strongly correlated lattice systems. Possible experimental implications are
discussed. | cond-mat_str-el |
Model for the Magnetic Order and Pairing Channels in Fe Pnictide
Superconductors: A two-orbital model for Fe-pnictide superconductors is investigated using
computational techniques on two-dimensional square clusters. The hopping
amplitudes are derived from orbital overlap integrals, or by band structure
fits, and the spin frustrating effect of the plaquette-diagonal Fe-Fe hopping
is remarked. A spin 'striped' state is stable in a broad range of couplings in
the undoped regime, in agreement with neutron scattering. Adding two electrons
to the undoped ground state of a small cluster, the dominant pairing operators
are found. Depending on parameters, two pairing operators were identified: they
involve inter-xz-yz orbital combinations forming spin singlets or triplets,
transforming according to the B_2g and A_2g representations of the D_4h group,
respectively. | cond-mat_str-el |
Light-induced magnetization driven by interorbital charge motion in a
spin-orbit assisted Mott insulator alpha-RuCl3: In a honeycomb-lattice spin-orbit assisted Mott insulator {\alpha}-RuCl3, an
ultrafast magnetization is induced by circularly polarized excitation below the
Mott gap. Photo-carriers play an important role, which are generated by turning
down the synergy of the on-site Coulomb interaction and the spin-orbit
interaction realizing the insulator state. An ultrafast 6- fs measurement of
photo-carrier dynamics and a quantum mechanical analysis clarify the mechanism,
according to which the magnetization emerges from a coherent charge motion
between different t2g orbitals (dyz-dxz-dxy) of Ru3+ ions. This ultrafast
magnetization is weakened in the antiferromagnetic (AF) phase, which is
opposite to the general tendency that the inverse Faraday effect is larger in
AF compounds than in paramagnetic ones. This temperature dependence indicates
that the interorbital charge motion is affected by pseudo-spin rotational
symmetry breaking in the AF phase. | cond-mat_str-el |
Wigner-molecule supercrystal in transition-metal dichalcogenide moiré
superlattices: Lessons from the bottom-up approach: The few-body problem for $N=4$ fermionic charge carriers in a double-well
moir\'{e} quantum dot (MQD), representing the first step in a bottom-up
strategy to investigate formation of molecular supercrystals in transition
metal dichalcogenide (TMD) moir\'e superlattices with integral fillings, $\nu >
1$, is solved exactly by employing large-scale exact-diagonalization via full
configuration interaction (FCI) computations. A comparative analysis with the
mean-field solutions of the often used spin-and-space unrestricted Hartree Fock
(sS-UHF) demonstrates the limitations of the UHF method (by itself) to provide
a proper description of the influence of the interdot Coulomb interaction. In
particular, it is explicitly shown for $\nu=2$ that the exact charge densities
(CDs) within each MQD retain the ring-like shape characteristic (for a wide
range of relevant parameters) of a fully isolated MQD, as was found for sliding
Wigner molecules (WMs). This deeply quantum-mechanical behavior contrasts
sharply with the UHF CDs that portray solely orientationally pinned and well
localized dumbbell dimers. An improved CD, which agrees with the FCI-calculated
one, derived from the restoration of the sS-UHF broken parity symmetries is
further introduced, suggesting a beyond-mean-field methodological roadmap for
correcting the sS-UHF results. It is conjectured that the conclusions for the
$\nu=2$ moir\'e TMD superlattice case extend to all cases with integral
fillings that are associated with sliding WMs in isolated MQDs. The case of
$\nu=3$, associated with a pinned WM in isolated MQDs, is an exception. | cond-mat_str-el |
Excitation Spectra and Thermodynamic Response of Segmented Heisenberg
Spin Chains: The spectral and thermodynamic response of segmented quantum spin chains is
analyzed using a combination of numerical techniques and finite-size scaling
arguments. Various distributions of segment lengths are considered, including
the two extreme cases of quenched and annealed averages. As the impurity
concentration is increased, it is found that (i) the integrated spectral weight
is rapidly reduced, (ii) a pseudo-gap feature opens up at small frequencies,
and (iii) at larger frequencies a discrete peak structure emerges, dominated by
the contributions of the smallest cluster segments. The corresponding
low-temperature thermodynamic response has a divergent contribution due to the
odd-site clusters and a sub-dominant exponentially activated component due to
the even-site segments whose finite-size gap is responsible for the spectral
weight suppression at small frequencies. Based on simple scaling arguments,
approximate low-temperature expressions are derived for the uniform
susceptibility and the heat capacity. These are shown to be in good agreement
with numerical solutions of the Bethe ansatz equations for ensembles of
open-end chains. | cond-mat_str-el |
Itinerant Quantum Critical Point with Fermion Pockets and Hot Spots: Metallic quantum criticality is among the central theme in the understanding
of correlated electronic systems, and converging results between analytical and
numerical approaches are still under calling. In this work, we develop
state-of-art large scale quantum Monte Carlo simulation technique and
systematically investigate the itinerant quantum critical point on a 2D square
lattice with antiferromagnetic spin fluctuations at wavevector
$\mathbf{Q}=(\pi,\pi)$ -- a problem that resembles the Fermi surface setup and
low-energy antiferromagnetic fluctuations in high-Tc cuprates and other
critical metals, which might be relevant to their non-Fermi-liquid behaviors.
System sizes of $60\times 60 \times 320$ ($L \times L \times L_\tau$) are
comfortably accessed, and the quantum critical scaling behaviors are revealed
with unprecedingly high precision. We found that the antiferromagnetic spin
fluctuations introduce effective interactions among fermions and the fermions
in return render the bare bosonic critical point into a new universality,
different from both the bare Ising universality class and the
Hertz-Mills-Moriya RPA prediction. At the quantum critical point, a finite
anomalous dimension $\eta\sim 0.125$ is observed in the bosonic propagator, and
fermions at hot spots evolve into a non-Fermi-liquid. In the
antiferromagnetically ordered metallic phase, fermion pockets are observed as
energy gap opens up at the hot spots. These results bridge the recent
theoretical and numerical developments in metallic quantum criticality and can
be served as the stepping stone towards final understanding of the 2D
correlated fermions interacting with gapless critical excitations. | cond-mat_str-el |
First-order melting of a weak spin-orbit Mott insulator into a
correlated metal: The electronic phase diagram of the weak spin-orbit Mott insulator
(Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon
doping electrons via La substitution, an immediate collapse in resistivity
occurs along with a narrow regime of nanoscale phase separation comprised of
antiferromagnetic, insulating regions and paramagnetic, metallic puddles
persisting until x~0.04. Continued electron doping results in an abrupt,
first-order phase boundary where the Neel state is suppressed and a homogenous,
correlated, metallic state appears with an enhanced spin susceptibility and
local moments. As the metallic state is stabilized, a weak structural
distortion develops and suggests a competing instability with the parent
spin-orbit Mott state. | cond-mat_str-el |
Local Potential Functional Embedding Theory: A Self-Consistent Flavor of
Density Functional Theory for Lattices without Density Functionals: The recently proposed Householder transformed density-matrix functional
embedding theory (Ht-DMFET) [Sekaran et al., Phys. Rev. B 104, 035121 (2021)],
which is equivalent to (but formally simpler than) density matrix embedding
theory (DMET) in the non-interacting case, is revisited from the perspective of
density-functional theory (DFT). An in-principle-exact density-functional
version of Ht-DMFET is derived for the one-dimensional Hubbard lattice with a
single embedded impurity. On the basis of well-identified density-functional
approximations, a local potential functional embedding theory (LPFET) is
formulated and implemented. Even though LPFET performs better than Ht-DMFET in
the low-density regime, in particular when electron correlation is strong, both
methods are unable to describe the density-driven Mott-Hubbard transition, as
expected. These results combined with our formally exact density-functional
embedding theory reveal that a single statically embedded impurity can in
principle describe the gap opening, provided that the complementary correlation
potential (that describes the interaction of the embedding cluster with its
environment, which is simply neglected in both Ht-DMFET and LPFET) exhibits a
derivative discontinuity (DD) at half filling. The extension of LPFET to
multiple impurities (which would enable to circumvent the modeling of DDs) and
its generalization to quantum chemical Hamiltonians are left for future work. | cond-mat_str-el |
Detection of long-range entanglement in gapped quantum spin liquids by
local measurements: Topological order, reflected in long range patterns of entanglement, is
quantified by the topological entanglement entropy (TEE) $\gamma$. We show that
for gapped quantum spin liquids (QSL) it is possible to extract $\gamma$ using
two-spin local correlators. We demonstrate our method for the gapped
$\mathbb{Z}_2$ Kitaev spin liquid on a honeycomb lattice with anisotropic
interactions. We show that the $\gamma = \log 2$ for $\mathbb{Z}_2$ topological
order can be simply extracted from local two-spin correlators across two
different bonds, with an accuracy comparable or higher than the Kitaev-Preskill
construction. This implies that the different superselection sectors of
$\mathbb{Z}_2$ gauge theory determined by global Wilson loop operators can be
fully reflected locally in the matter majorana sector. | cond-mat_str-el |
Ring Exchange Mechanism for Triplet Superconductivity in a Two-Chain
Hubbard Model: Possible Relevance to Bechgaard Salts: The density-matrix renormalization group method is used to study the ground
state of the two-chain zigzag-bond Hubbard model at quarter filling. We show
that, with a proper choice of the signs of hopping integrals, the ring exchange
mechanism yields ferromagnetic spin correlations between interchain neighboring
sites, and produces the attractive interaction between electrons as well as the
long-range pair correlations in the spin-triplet channel, thereby leading the
system to triplet superconductivity. We argue that this novel mechanism may
have possible relevance to observed superconductivity in Bechgaard salts. | cond-mat_str-el |
Geometrical quadrupolar frustration in DyB$_4$: Physical properties of DyB$_4$ have been studied by magnetization, specific
heat, and ultrasonic measurements. The magnetic entropy change and the
ultrasonic properties in the intermediate phase II indicate that the degeneracy
of internal degrees of freedom is not fully lifted in spite of the formation of
magnetic order. The ultrasonic attenuation and the huge softening of $C_{44}$
in phase II suggests existence of electric-quadrupolar (orbital) fluctuations
of the 4$f$-electron. These unusual properties originate from the geometrical
quadrupolar frustration. | cond-mat_str-el |
Signatures of a topological Weyl loop in Co$_3$Sn$_2$S$_2$: The search for novel topological phases of matter in quantum magnets has
emerged as a frontier of condensed matter physics. Here we use state-of-the-art
angle-resolved photoemission spectroscopy (ARPES) to investigate single
crystals of Co$_3$Sn$_2$S$_2$ in its ferromagnetic phase. We report for the
first time signatures of a topological Weyl loop. From fundamental symmetry
considerations, this magnetic Weyl loop is expected to be gapless if spin-orbit
coupling (SOC) is strictly zero but gapped, with possible Weyl points, under
finite SOC. We point out that high-resolution ARPES results to date cannot
unambiguously resolve the SOC gap anywhere along the Weyl loop, leaving open
the possibility that Co$_3$Sn$_2$S$_2$ hosts zero Weyl points or some non-zero
number of Weyl points. On the surface of our samples, we further observe a
possible Fermi arc, but we are unable to clearly verify its topological nature
using the established counting criteria. As a result, we argue that from the
point of view of photoemission spectroscopy the presence of Weyl points and
Fermi arcs in Co$_3$Sn$_2$S$_2$ remains ambiguous. Our results have
implications for ongoing investigations of Co$_3$Sn$_2$S$_2$ and other
topological magnets. | cond-mat_str-el |
Asymptotic Freedom and Large Spin Antiferromagnetic Chains: Building on the mapping of large-$S$ spin chains onto the O($3$) nonlinear
$\sigma$ model with coupling constant $2/S$, and on general properties of that
model (asymptotic freedom, implying that perturbation theory is valid at high
energy, and Elitzur's conjecture that rotationally invariant quantities are
infrared finite in perturbation theory), we use the Holstein-Primakoff
representation to derive analytic expressions for the equal-time and dynamical
spin-spin correlations valid at distances smaller than $S^{-1} \exp(\pi S)$ or
at energies larger than $J S^2 \exp(-\pi S)$, where $J$ is the Heisenberg
exchange coupling. This is supported by comparing the static correlations with
quantum Monte Carlo simulations for $S = 5/2$. | cond-mat_str-el |
Imaginary part of Hall conductivity in tilted doped Weyl semimetal with
both broken time reversal and inversion symmetry: We consider a Weyl semimetal (WSM) with finite doping and tilt within a
continuum model Hamiltonian with both broken time reversal and inversion
symmetry. We calculate the absorptive part of the anomalous AC Hall
conductivity as a function of photon energy ($\Omega$) for both type I and type
II Weyl semimetal. For a given Weyl node, changing the sign of its chirality or
of its tilt changes the sign of its contribution to the absorptive Hall
conductivity with no change in magnitude. For a noncentrosymmetric system we
find that there are ranges of photon energies for which only the positive or
only the negative chirality node contributes to the imaginary (absorptive) part
of the Hall conductivity. There are also other photon energies where both
chirality contribute and there can be other ranges of $\Omega$ where there is
no absorption associated with the AC Hall conductivity in type I and regions
where it is instead constant for type II. We comment on implications for the
absorption of circular polarized light. | cond-mat_str-el |
Orbital Polarization in Strained LaNiO$_{3}$: Structural Distortions and
Correlation Effects: Transition-metal heterostructures offer the fascinating possibility of
controlling orbital degrees of freedom via strain. Here, we investigate
theoretically the degree of orbital polarization that can be induced by
epitaxial strain in LaNiO$_3$ films. Using combined electronic structure and
dynamical mean-field theory methods we take into account both structural
distortions and electron correlations and discuss their relative influence. We
confirm that Hund's rule coupling tends to decrease the polarization and point
out that this applies to both the $d^8\underline{L}$ and $d^7$ local
configurations of the Ni ions. Our calculations are in good agreement with
recent experiments, which revealed sizable orbital polarization under tensile
strain. We discuss why full orbital polarization is hard to achieve in this
specific system and emphasize the general limitations that must be overcome to
achieve this goal. | cond-mat_str-el |
Comment on "Origin of Giant Optical Nonlinearity in
Charge-Transfer--Mott Insulators: A New Paradigm for Nonlinear Optics": Comment on Phys. Rev. Lett. 86, 2086 (2001) | cond-mat_str-el |
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