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On conservation of the of crystal lattice symmetry in transition at
Curie point in exchange magnets: We show that symmetry of the crystal lattice of exchange magnets (containing
only 3d magneto-active elements) does not change at the Curie point; only the
magnetic symmetry of the crystal is decreasing in the transition point. In the
non-exchange magnets (containing only rare-earth magneto-active elements), on
the contrary, both the magnetic and crystal-chemical symmetry decrease at the
Curie point. There is isotropic magnetic phase in exchange magnets; and their
magnetic symmetry is described by color groups of magnetic symmetry of P-type.
Non-exchange magnets do not have isotropic phase; their symmetry is described
by color groups of magnetic symmetry of Q-type. | cond-mat_str-el |
A fractionalised "$\mathbb{Z}_2$" classical Heisenberg spin liquid: Quantum spin systems are by now known to exhibit a large number of different
classes of spin liquid phases. By contrast, for \textit{classical} Heisenberg
models, only one kind of fractionalised spin liquid phase, the so-called
Coulomb or $U(1)$ spin liquid, has until recently been identified: this
exhibits algebraic spin correlations and impurity moments, `orphan spins',
whose size is a fraction of that of the underlying microscopic degrees of
freedom. Here, we present two Heisenberg models exhibiting fractionalisation in
combination with exponentially decaying correlations. These can be thought of
as a classical continuous spin version of a $\mathbb{Z}_2$ spin liquid. Our
work suggests a systematic search and classification of classical spin liquids
as a worthwhile endeavour. | cond-mat_str-el |
Soft magnon contributions to dielectric constant in spiral magnets with
domain walls: Competing magnetic exchange interactions often result in non-collinear
magnetic states, such as spin spirals, which break the inversion symmetry and
induce ferroelectric polarization. The resulting strong interactions between
magnetic and dielectric degrees of freedom lead to a technologically important
possibility to control magnetic order by electric fields and to electromagnons,
magnetic excitations that can be excited by an electric dipole of the
electromagnetic field. Here we study the effects of chiral domain walls on
magnetoelectric properties of spiral magnets. We use a quasi-1D model
Hamiltonian with competing Heisenberg exchange interactions, leading to a spin
spiral, and Dzyaloshinskii-Moriya interactions, that couple spins and electric
dipoles and mix magnon and phonon excitations. The results suggest that low
frequency dielectric anomalies in spiral magnets, such as TbMnO3 and MnWO4, may
originate from hybrid magnon - polar phonon excitations associated with domain
walls. | cond-mat_str-el |
Basal-Plane Nonlinear Susceptibility: A Direct Probe of the Single-Ion
Physics in URu2Si2: The microscopic nature of the hidden order state in URu2Si2 is dependent on
the low-energy configurations of the uranium ions, and there is currently no
consensus on whether it is predominantly 5f^2 or 5f^3. Here we show that
measurement of the basal-plane nonlinear susceptibility can resolve this issue;
its sign at low-temperatures is a distinguishing factor. We calculate the
linear and nonlinear susceptibilities for specific 5f^2 and 5f^3 crystal-field
schemes that are consistent with current experiment. Because of its dual
magnetic and orbital character, a \Gamma_5 magnetic non-Kramers doublet
ground-state of the U ion can be identified by $\chi_1^c(T) \propto
\chi_3^\perp(T)$ where we have determined the constant of proportionality for
URu2Si2. | cond-mat_str-el |
Effects of Disorder on the Pressure-Induced Mott Transition in
$κ$-BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl: We present a study of the influence of disorder on the Mott metal-insulator
transition for the organic charge-transfer salt
$\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. To this end, disorder was introduced
into the system in a controlled way by exposing the single crystals to x-ray
irradiation. The crystals were then fine-tuned across the Mott transition by
the application of continuously controllable He-gas pressure at low
temperatures. Measurements of the thermal expansion and resistance show that
the first-order character of the Mott transition prevails for low irradiation
doses achieved by irradiation times up to 100 h. For these crystals with a
moderate degree of disorder, we find a first-order transition line which ends
in a second-order critical endpoint, akin to the pristine crystals. Compared to
the latter, however, we observe a significant reduction of both, the critical
pressure $p_c$ and the critical temperature $T_c$. This result is consistent
with the theoretically-predicted formation of a soft Coulomb gap in the
presence of strong correlations and small disorder. Furthermore, we
demonstrate, similar to the observation for the pristine sample, that the Mott
transition after 50 h of irradiation is accompanied by sizable lattice effects,
the critical behavior of which can be well described by mean-field theory. Our
results demonstrate that the character of the Mott transition remains
essentially unchanged at a low disorder level. However, after an irradiation
time of 150 h, no clear signatures of a discontinuous metal-insulator
transition could be revealed anymore. These results suggest that, above a
certain disorder level, the metal-insulator transition becomes a smeared
first-order transition with some residual hysteresis. | cond-mat_str-el |
Lattice dynamics and electronic excitations in a large family of lacunar
spinels with a breathing pyrochlore lattice structure: Reproducing the electronic structure of AM$_4$X$_8$ lacunar spinels with a
breathing pyrochlore lattice is a great theoretical challenge due to the
interplay of various factors. The character of the M$_4$X$_4$ cluster orbitals
is critically influenced by the Jahn-Teller instability, the spin-orbit
interaction, and also by the magnetic state of the clusters. Consequently, to
reproduce the narrow-gap semiconducting nature of these moderately correlated
materials requires advanced approaches, since the strength of the inter-cluster
hopping is strongly affected by the character of the cluster orbitals. In order
to provide a solid experimental basis for theoretical studies, we performed
broadband optical spectroscopy on a large set of lacunar spinels, with
systematically changing ions at the A and M sites as well as the ligand (A=Ga,
Ge, Al; M=V, Mo, Nb, Ta; X=S, Se). Our study covers the range of phonon
excitations and also electronic transitions near the gap edge. In the phonon
excitation spectrum a limited subset of the symmetry allowed modes is observed
in the cubic state, with a few additional modes emerging upon the
symmetry-lowering structural transition. All the infrared active modes are
assigned to vibrations of the ligands and ions at the A sites, with no obvious
contribution from the M-site ions. Concerning the electronic states, we found
that all compounds are narrow-gap semiconductors ($E_\mathrm{g} = 130 -
350\,$meV) already in their room-temperature cubic state and their structural
transitions induce weak, if any, changes in the band gap. The gap value is
decreased when substituting S with Se and also when replacing $3d$ ions by $4d$
or $5d$ ions at the M sites. | cond-mat_str-el |
Ordered spin-ice state in the geometrically frustrated
metallic-ferromagnet Sm2Mo2O7: The recent discovery of Spin-ice is a spectacular example of non-coplanar
spin arrangements that can arise in the pyrochlore A2B2O7 structure. We present
magnetic and thermodynamic studies on the metallic-ferromagnet pyrochlore
Sm2Mo2O7. Our studies, carried out on oriented crystals, suggest that the Sm
spins have an ordered spin-ice ground state below about T* = 15 K. The
temperature- and field-evolution of the ordered spin-ice state are governed by
an antiferromagnetic coupling between the Sm and Mo spins. We propose that as a
consequence of a robust feature of this coupling, the tetrahedra aligned with
the external field adopt a "1-in, 3-out" spin structure as opposed to "3-in,
1-out" in dipolar spin ices, as the field exceeds a critical value. | cond-mat_str-el |
Magnetic field-tuned quantum critical point in CeAuSb_2: Transport, magnetic and thermal properties at high magnetic fields (H) and
low temperatures (T) of the heavy fermion compound CeAuSb_2 are reported. At
H=0 this layered system exhibits antiferromagnetic order below T_N = 6 K.
Applying B along the inter-plane direction, leads to a continuous suppression
of T_N and a quantum critical point at H_c ~ 5.4 T. Although it exhibits Fermi
liquid behavior within the Neel phase, in the paramagnetic state the
fluctuations associated with H_c give rise to unconventional behavior in the
resistivity (sub-linear in T) and to a TlnT dependence in the magnetic
contribution to the specific heat. For H > H_c and low T the electrical
resistivity exhibits an unusual T^3-dependence. | cond-mat_str-el |
Cobalt spin state above the valence and spin-state transition in
(Pr0.7Sm0.3)0.7Ca0.3CoO3: (Pr0.7Sm0.3)0.7Ca0.3CoO3 belongs to a class of cobalt oxides undergoing a
first-order transition (T* close to 90 K) associated to a coupled change in the
valence and spin-state degrees of freedom. The Curie-Weiss regime present
around room temperature (T >> T*) was analysed in detail to address the
controversial issue of the cobalt spin states above the transition. This
magnetic investigation indicates that the Co4+ are in an intermediate
spin-state, while the Co3+ are in a mixed state combining low-spin and
high-spin states. These results are discussed with respect to the literature on
related compounds and recent results of x-ray absorption spectroscopy. | cond-mat_str-el |
The evolution of the Non-Fermi Liquid behavior of BaVS$_3$ under high
pressure: Temperature, pressure, and magnetic field dependencies of the resistivity of
BaVS$_3$ were measured above the critical pressure of $p_{cr}$=2 GPa, which is
associated with the zero temperature insulator-to-metal (MI) transition. The
resistivity exhibits the $T^n$ temperature dependence below $T_g\approx$15 K,
with $n$ of 1.5 at $p_{cr}$, which increases continuously with pressure towards
2. This is interpreted as a crossover from non-Fermi (NFL) to Fermi-liquid (FL)
behavior. Although the spin configuration of the $e_g$ electrons influences the
charge propagation, the NFL behavior is attributed to the pseudogap that
appears in the single particle spectrum of the $d_z^2$ electrons related to
large quasi-one dimensional (Q-1d) 2$k_F$-CDW fluctuations. The non-monotonic
magnetic field dependence of $\Delta$$\rho$/$\rho$ reveals a characteristic
field $B_0\approx$12 T attributed to the full suppression of the pseudogap. | cond-mat_str-el |
Nontrivial ferrimagnetism of the Heisenberg model on the Union Jack
strip lattice: We study the ground-state properties of the S=1/2 antiferromagnetic
Heisenberg model on the Union Jack strip lattice by using the
exact-diagonalization and density matrix renormalization group methods. We
confirm a region of the intermediate-magnetization state between the Neel-like
spin liquid state and the conventional ferrimagnetic state of Lieb-Mattis type.
In the intermediate-state, we find that the spontaneous magnetization changes
gradually with respect to the strength of the inner interaction. In addition,
the local magnetization clearly shows an incommensurate modulation with
long-distance periodicity in the intermediate-magnetization state. These
characteristic behaviors lead to the conclusion that the
intermediate-magnetization state is the non-Lieb-Mattis ferrimagnetic one. We
also discuss the relationship between the ground-state properties of the S=1/2
antiferromagnetic Heisenberg model on the original Union Jack lattice and those
on our strip lattice. | cond-mat_str-el |
Destruction of the Kondo effect in a multi-channel Bose-Fermi Kondo
model: We consider the SU(N) x SU(kappa N) generalization of the spin-isotropic
Bose-Fermi Kondo model in the limit of large N. There are three fixed points
corresponding to a multi-channel non-Fermi liquid phase, a local spin-liquid
phase, and a Kondo-destroying quantum critical point (QCP). We show that the
QCP has strong similarities with its counterpart in the single-channel model,
even though the Kondo phase is very different from the latter. We also discuss
the evolution of the dynamical scaling properties away from the QCP. | cond-mat_str-el |
Susceptibilities of Sr(Cu_(1-x)Zn_x)_2O_3 Studied by Quantum Monte Carlo
Simulation: The effects of non-magnetic impurities randomly doped into a two-leg
Heisenberg spin ladder are investigated. Using the continuous time quantum
Monte Carlo loop algorithm we calculate the uniform and staggered
susceptibilities of such a system. The obtained uniform susceptibility is well
described in terms of an effective model of weakly interacting local moments
induced by non-magnetic impurities for a 1% doping case, but not for higher
concentrations. The staggered susceptibility however is significantly enhanced
over that in the effective model already at 1% doping. Using a mean field
approximation for the interladder coupling, we explain qualitatively the phase
diagram of Sr(Cu_{1-x}Zn_x)_2O_3. | cond-mat_str-el |
Quantum aspects of "hydrodynamic" transport from weak electron-impurity
scattering: Recent experimental observations of apparently hydrodynamic electronic
transport have generated much excitement. However, the understanding of the
observed non-local transport (whirlpool) effects and parabolic
(Poiseuille-like) current profiles has largely been motivated by a
phenomenological analogy to classical fluids. This is due to difficulty in
incorporating strong correlations in quantum mechanical calculation of
transport, which has been the primary angle for interpreting the apparently
hydrodynamic transport. Here we demonstrate that even free fermion systems, in
the presence of (inevitable) disorder, exhibit non-local conductivity effects
such as those observed in experiment because of the fermionic system's
long-range entangled nature. On the basis of explicit calculations of the
conductivity at finite wavevector, $\sigma({\bf q})$, for selected weakly
disordered free fermion systems, we propose experimental strategies for
demonstrating distinctive quantum effects in non-local transport at odds with
the expectations of classical kinetic theory. Our results imply that the
observation of whirlpools or other "hydrodynamic" effects does not guarantee
the dominance of electron-electron scattering over electron-impurity
scattering. | cond-mat_str-el |
Melting of excitonic insulator phase by an intense terahertz pulse in
Ta$_2$NiSe$_5$: In this study, the optical response to a terahertz pulse was investigated in
the transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonic
insulator. First, by irradiating a terahertz pulse with a relatively weak
electric field (0.3 MV/cm), the spectral changes in reflectivity near the
absorption edge due to third-order optical nonlinearity were measured and the
absorption peak characteristic of the excitonic phase just below the interband
transition was identified. Next, by irradiating a strong terahertz pulse with a
strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was
found to be reduced, and a Drude-like response appeared in the mid-infrared
region. These responses can be interpreted as carrier generation by exciton
dissociation induced by the electric field, resulting in the partial melting of
the excitonic phase and metallization. The presence of a distinct threshold
electric field for carrier generation indicates exciton dissociation via
quantum-tunnelling processes. The spectral change due to metallization by the
electric field is significantly different from that due to the strong optical
excitation across the gap, which can be explained by the different melting
mechanisms of the excitonic phase in the two types of excitations. | cond-mat_str-el |
Semiquantitative theory for high-field low-temperature properties of a
distorted diamond spin chain: We consider the antiferromagnetic Heisenberg model on a distorted diamond
chain and use the localized-magnon picture adapted to a distorted geometry to
discuss some of its high-field low-temperature properties. More specifically,
in our study we assume that the partition function for a slightly distorted
geometry has the same form as for ideal geometry, though with slightly
dispersive one-magnon energies. We also discuss the relevance of such a
description to azurite. | cond-mat_str-el |
Magnetic Order in Laser-Irradiated Kagome Antiferromagnets: Dispersionless "zero energy mode'' is one of the hallmarks of frustrated
kagome antiferromagnets (KAFMs). It points to extensive classically degenerate
ground-states. The "zero energy mode'' can be observed experimentally when
lifted to a flat mode at finite energy by a strong intrinsic magnetic
anisotropy. In this letter, we study the effects of irradiation of laser light
on the KAFMs. We adopt the magnon picture without loss of generality. It is
shown that circularly or linearly polarized light lifts the "zero energy
mode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We
find that the circularly polarized light-induced anisotropies have similar
features as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya
interaction in KAFMs. The former stabilizes long-range magnetic order and the
latter induces spin canting out-of-plane with nonzero scalar spin chirality.
The Floquet thermal Hall effect shows that the synthetic magnetic excitation
modes in the case of circularly polarized light are topological, whereas those
of linearly polarized light are not. | cond-mat_str-el |
Disentangling a quantum antiferromagnet with resonant inelastic X-ray
scattering: Low-dimensional copper oxide lattices naturally manifest electronic states
with strong short range quantum entanglement, which are known to lead to
remarkable emergent material properties. However the nanometer scale many-body
wavefunction is challenging to measure or manipulate in a simple way. In this
study, X-ray induced $dd$ electronic transitions are used to suppress spin
entanglement across a single lattice site in the spin-1/2 antiferromagnetic
chain compound SrCuO$_2$, revealing a class of cuprate magnetic excitations
that result from breaking the spin chain. These measurements are the first to
employ two closely spaced X-ray resonances of copper (M$_2$ and M$_3$) as a
form of natural 2-slit interferometer to distinguish between different types of
electronic transition and resolve how they influence the dynamics of nearby
spin-entangled electrons. | cond-mat_str-el |
In situ controllable magnetic phases in doped twisted bilayer
transition-metal dichalcogenides: We study the electronic structure of hole-doped transition metal
dichalcogenides for small twist-angels, where the onsite repulsion is extremely
strong. Using unbiased diagrammatic Monte Carlo simulations, we find evidence
for magnetic correlations which are driven by delocalization and can be
controlled in situ via the dielectric environment. For weak spin-orbit
coupling, we find that the moderately doped system becomes anti-ferromagnetic,
whilst the regime of strong spin-orbit coupling features ferromagnetic
correlations. We show that this behavior is accurately predicted by an
analytical theory based on moment expansion of the Hamiltonian, and analysis of
corresponding particle trajectories. | cond-mat_str-el |
Spin excitations in the kagome-lattice metallic antiferromagnet
Fe$_{0.89}$Co$_{0.11}$Sn: Kagome-lattice materials have attracted tremendous interest due to the broad
prospect for seeking superconductivity, quantum spin liquid states, and
topological electronic structures. Among them, the transition-metal kagome
lattices are high-profile objects for the combination of topological
properties, rich magnetism, and multiple-orbital physics. Here we report an
inelastic neutron scattering study on the spin dynamics of a kagome-lattice
antiferromagnetic metal Fe$_{0.89}$Co$_{0.11}$Sn. Although the magnetic
excitations can be observed up to $\sim$250 meV, well-defined spin waves are
only identified below $\sim$90 meV and can be modeled using Heisenberg exchange
with ferromagnetic in-plane nearest-neighbor coupling $J_1$, in-plane
next-nearest-neighbor coupling $J_2$, and antiferromagnetic (AFM) interlayer
coupling $J_c$ under linear spin-wave theory. Above $\sim$90 meV, the spin
waves enter the itinerant Stoner continuum and become highly damped
particle-hole excitations. At the K point of the Brillouin zone, we reveal a
possible band crossing of the spin wave, which indicates a potential Dirac
magnon. Our results uncover the evolution of the spin excitations from the
planar AFM state to the axial AFM state in Fe$_{0.89}$Co$_{0.11}$Sn, solve the
magnetic Hamiltonian for both states, and confirm the significant influence of
the itinerant magnetism on the spin excitations. | cond-mat_str-el |
Peculiar behavior of the electrical resistivity of MnSi at the
ferromagnetic phase transition: The electrical resistivity of a single crystal of MnSi was measured across
its ferromagnetic phase transition line at ambient and high pressures. Sharp
peaks of the temperature coefficient of resistivity characterize the transition
line. Analysis of these data shows that at pressures to ~0.35 GPa these peaks
have fine structure, revealing a shoulder at ~ 0.5 K above the peak. It is
symptomatic that this structure disappears at pressures higher than ~0.35 GPa,
which was identified earlier as a tricritical point | cond-mat_str-el |
Ground state of the frustrated Hubbard model within DMFT: energetics of
Mott insulator and metal from ePT and QMC: We present a new method, ePT, for extrapolating few known coefficients of a
perturbative expansion. Controlled by comparisons with numerically exact
quantum Monte Carlo (QMC) results, 10th order strong-coupling perturbation
theory (PT) for the Hubbard model on the Bethe lattice is reliably extrapolated
to infinite order. Within dynamical mean-field theory (DMFT), we obtain
continuous estimates of energy E and double occupancy D with unprecedented
precision O(10^{-5}) for the Mott insulator above its stability edge
U_{c1}=4.78 as well as critical exponents. In addition, we derive corresponding
precise estimates for E and D in the metallic ground state from extensive
low-temperature QMC simulations using a fit to weak-coupling PT while enforcing
thermodynamic consistency. | cond-mat_str-el |
La$_2$O$_3$Fe$_2$Se$_2$, a Mott insulator on the brink of
orbital-selective metalization: La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in
sharp contrast with the metallic behavior of FeSe and other parent parent
compounds of iron superconductors. We demonstrate that the key ingredient that
makes La$_2$O$_3$Fe$_2$Se$_2$ a Mott insulator, rather than a correlated metal
dominated by the Hund's coupling is the enhanced crystal-field splitting,
accompanied by a smaller orbital-resolved kinetic energy. The strong deviation
from orbital degeneracy introduced by the crystal-field splitting also pushes
this materials close to an orbital-selective Mott transition. We predict that
either doping or uniaxial external pressure can drive the material into an
orbital-selective Mott state, where only one or few orbitals are metallized
while the others remain insulating. | cond-mat_str-el |
MIEZE Neutron Spin-Echo Spectroscopy of Strongly Correlated Electron
Systems: Recent progress in neutron spin-echo spectroscopy by means of longitudinal
Modulation of IntEnsity with Zero Effort (MIEZE) is reviewed. Key technical
characteristics are summarized which highlight that the parameter range
accessible in momentum and energy, as well as its limitations, are extremely
well understood and controlled. Typical experimental data comprising
quasi-elastic and inelastic scattering are presented, featuring magneto-elastic
coupling and crystal field excitations in Ho2Ti2O7, the skyrmion lattice to
paramagnetic transition under applied magnetic field in MnSi, ferromagnetic
criticality and spin waves in Fe. In addition bench marking studies of the
molecular dynamics in H2O are reported. Taken together, the advantages of MIEZE
spectroscopy in studies at small and intermediate momentum transfers comprise
an exceptionally wide dynamic range of over seven orders of magnitude, the
capability to perform straight forward studies on depolarizing samples or under
depolarizing sample environments, as well as on incoherently scattering
materials. | cond-mat_str-el |
Vibrational edge modes in intrinsically heterogeneous doped transition
metal oxides: By applying an unrestricted Hartree-Fock and a Random Phase approximations to
a multiband Peierls-Hubbard Hamiltonian, we study the phonon mode structure in
models of transition metal oxides in the presence of intrinsic nanoscale
inhomogeneities induced by hole doping. We identify low frequency $local$
vibrational modes pinned to the sharp interfaces between regions of distinct
electronic structure (doped and undoped) and separated in frequency from the
band of extended phonons. A characteristic of these ``edge'' modes is that
their energy is essentially insensitive to the doping level. We discuss the
experimental manifestations of these modes in inelastic neutron scattering, and
also in spin and charge excitation spectra. | cond-mat_str-el |
Electronic structure of the kagome staircase compounds Ni3V2O8 and
Co3V2O8: The electronic structure of the kagome staircase compounds, Ni3V2O8 and
Co3V2O8, has been investigated using soft x-ray absorption, soft x-ray
emission, and resonant inelastic x-ray scattering (RIXS). Comparison between
the two compounds, and with first principles band structure calculations and
crystal-field multiplet models, provide unique insight into the electronic
structure of the two materials. Whereas the location of the narrow (Ni,Co) d
bands is predicted to be close to EF, we experimentally find they lie deeper in
the occupied O 2p and unoccupied V 3d manifolds, and determine their energy via
measured charge-transfer excitations. Additionally, we find evidence for a dd
excitation at 1.5 eV in Ni3V2O8, suggesting the V d states may be weakly
occupied in this compound, contrary to Co3V2O8. Good agreement is found between
the crystal-field dd excitations observed in the experiment and predicted by
atomic multiplet theory. | cond-mat_str-el |
Composite Dirac liquids: parent states for symmetric surface topological
order: We introduce exotic gapless states---`composite Dirac liquids'---that can
appear at a strongly interacting surface of a three-dimensional electronic
topological insulator. Composite Dirac liquids exhibit a gap to all charge
excitations but nevertheless feature a single massless Dirac cone built from
emergent electrically neutral fermions. These states thus comprise electrical
insulators that, interestingly, retain thermal properties similar to those of
the non-interacting topological insulator surface. A variety of novel fully
gapped phases naturally descend from composite Dirac liquids. Most remarkably,
we show that gapping the neutral fermions via Cooper pairing---which crucially
does not violate charge conservation---yields symmetric non-Abelian
topologically ordered surface phases captured in several recent works. Other
(Abelian) topological orders emerge upon alternatively gapping the neutral
Dirac cone with magnetism. We establish a hierarchical relationship between
these descendant phases and expose an appealing connection to paired states of
composite Fermi liquids arising in the half-filled Landau level of
two-dimensional electron gases. To controllably access these states we exploit
a quasi-1D deformation of the original electronic Dirac cone that enables us to
analytically address the fate of the strongly interacting surface. The
algorithm we develop applies quite broadly and further allows the construction
of symmetric surface topological orders for recently introduced bosonic
topological insulators. | cond-mat_str-el |
Optical studies of gap, hopping energies and the Anderson-Hubbard
parameter in the zigzag-chain compound SrCuO2: We have investigated the electronic structure of the zigzag ladder (chain)
compound SrCuO2 combining polarized optical absorption, reflection,
photoreflectance and pseudo-dielectric function measurements with the model
calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at
300 K along (perpendicular) to the Cu-O chains. We have found that the lowest
energy gap, the correlation gap, is temperature independent. The electronic
structure of this oxide is calculated using both the
local-spin-density-approximation with gradient correction method, and the
tight-binding theory for the correlated electrons. The calculated density of
electronic states for non-correlated and correlated electrons shows
quasi-one-dimensional character. The correlation gap values of 1.42 eV
(indirect transition) and 1.88 eV (direct transition) have been calculated with
the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV
(between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV.
We concluded that SrCuO_2 belongs to the correlated-gap insulators. | cond-mat_str-el |
Entanglement in extended Hubbard models and quantum phase transitions: The role of two-point and multipartite entanglement at quantum phase
transitions (QPTs) in correlated electron systems is investigated. We consider
a bond-charge extended Hubbard model exactly solvable in one dimension which
displays various QPTs, with two (qubit) as well as more (qudit) on-site degrees
of freedom involved. The analysis is carried out by means of appropriate
measures of bipartite/multipartite quantum correlations. It is found that all
transitions ascribed to two-point correlations are characterized by an
entanglement range which diverges at the transition points. The exponent
coincides with that of the correlation length at the transitions. We introduce
the correlation ratio, namely, the ratio of quantum mutual information and
single-site entanglement. We show that at T=0, it captures the relative role of
two-point and multipartite quantum correlations at transition points,
generalizing to qudit systems the entanglement ratio. Moreover, a finite value
of quantum mutual information between infinitely distant sites is seen to
quantify the presence of off-diagonal long-range order induced by multipartite
entanglement. | cond-mat_str-el |
Quantum versus thermal fluctuations in the fcc antiferromagnet:
alternative routes to order by disorder: In frustrated magnetic systems with competing interactions fluctuations can
lift the residual accidental degeneracy. We argue that the state selection may
have different outcomes for quantum and thermal order by disorder. As an
example, we consider the semiclassical Heisenberg fcc antiferromagnet with only
the nearest-neighbor interactions. Zero-point oscillations select the type 3
collinear antiferromagnetic state at T=0. Thermal fluctuations favor instead
the type 1 antiferromagnetic structure. The opposite tendencies result in a
finite-temperature transition between the two collinear states. Competition
between effects of quantum and thermal order by disorder is a general
phenomenon and is also realized in the J1-J2 square-lattice antiferromagnet at
the critical point J2 = 0.5 J1. | cond-mat_str-el |
Dynamical signatures of topological order in the driven-dissipative
Kitaev chain: We investigate the effects of dissipation and driving on topological order in
superconducting nanowires. Rather than studying the non-equilibrium steady
state, we propose a method to classify and detect dynamical signatures of
topological order in open quantum systems. Bulk winding numbers for the
Lindblad generator $\hat{\mathcal{L}}$ of the dissipative Kitaev chain are
found to be linked to the presence of Majorana edge master modes -- localized
eigenmodes of $\hat{\mathcal{L}}$. Despite decaying in time, these modes
provide dynamical fingerprints of the topological phases of the closed system,
which are now separated by intermediate regions where winding numbers are
ill-defined and the bulk-boundary correspondence breaks down. Combining these
techniques with the Floquet formalism reveals higher winding numbers and
different types of edge modes under periodic driving. Finally, we link the
presence of edge modes to a steady state current. | cond-mat_str-el |
Incommensurate spin Luttinger liquid phase in a model for the
spin-Peierls materials TiOBr and TiOCl: In the present work we aim to characterize the lattice configurations and the
magnetic behavior in the incommensurate phase of spin-Peierls systems. This
phase emerges when the magnetic exchange interaction is coupled to the
distortions of an underlying triangular lattice and has its experimental
realization in the quasi-one dimensional compound family TiOX (X = Cl, Br).
With a simple model of spin-1/2 chains inserted in a planar triangular geometry
which couples them elastically, we are able to obtain the
uniform-incommensurate and incommensurate-dimerized phase transitions seen in
these compounds. Moreover, we follow the evolution of the wave-vector of the
distortions with temperature inside the incommensurate phase. Finally, we
predict gapless spin excitations for the intermediate phase of TiOX compounds
along with incommensurate spin-spin correlations. This exotic Luttinger
liquid-like behavior could be observed in future experiments. | cond-mat_str-el |
Interplay of electronic structure and unusual development in crystal
structure of YbAuIn and Yb$_{3}$AuGe$_{2}$In$_{3}$: First-principles calculations within the DFT are employed to investigate the
relationship between the electronic structure and the unexpected features of
the hexagonal cell parameters of YbAuIn and Yb$_{3}$AuGe$_{2}$In$_{3}$.
Calculations indicate that YbAuIn is an intermediate valent system with one Yb
4\textit{f} state pinned to the Fermi level, while Yb$_{3}$AuGe$_{2}$In$_{3}$
is closer to integer valency with all Yb 4\textit{f} states occupied.
Structural relaxations performed on LaAuIn and LuAuIn analogs reveal that
expansion of the \textit{c}-parameter in Yb$_{3}$AuGe$_{2}$In$_{3}$ is
attributable to larger size of the divalent Yb compared with intermediate
valent Yb. | cond-mat_str-el |
Weak-field induced nonmagnetic state in a Co-based honeycomb: Layered honeycomb magnets are of interest as potential realizations of the
Kitaev quantum spin liquid (KQSL), a quantum state with long-range spin
entanglement and an exactly solvable Hamiltonian. Conventional magnetically
ordered states are present for all currently known candidate materials,
however, because non-Kitaev terms in the Hamiltonians obscure the Kitaev
physics. Current experimental studies of the KQSL are focused on 4d- or
5d-transition-metal-based honeycombs, in which strong spin-orbit coupling can
be expected, yielding Kitaev interaction that dominate in an applied magnetic
field. In contrast, for 3d-based layered honeycomb magnets, spin orbit coupling
is weak and thus Kitaev-physics should be substantially less accessible. Here
we report our studies on BaCo2(AsO4)2, for which we find that the magnetic
order associated with the non-Kitaev interactions can be fully suppressed by a
relatively low magnetic field, yielding a non-magnetic material and implying
the presence of strong magnetic frustration and weak non-Kitaev interactions. | cond-mat_str-el |
Observation of giant circular dichroism induced by electronic chirality: Chiral phases of matter, characterized by a definite handedness, abound in
nature, ranging from the crystal structure of quartz to spiraling spin states
in helical magnets. In $1T$-TiSe$_2$ a source of chirality has been proposed
that stands apart from these classical examples as it arises from combined
electronic charge and quantum orbital fluctuations. This may allow its
chirality to be accessed and manipulated without imposing either structural or
magnetic handedness. However, direct bulk evidence that broken inversion
symmetry and chirality are intrinsic to TiSe$_2$ remains elusive. Here,
employing resonant elastic scattering of x-rays, we reveal the presence of
giant circular dichroism up to $\sim$ 40$\%$ at forbidden Bragg peaks that
emerge at the charge and orbital ordering transition. The dichroism varies
dramatically with incident energy and azimuthal angle. Comparison to calculated
scattering intensities unambiguously traces its origin to bulk chiral
electronic order in ${\mathrm{TiSe}}_2$ and establishes resonant elastic x-ray
scattering as a sensitive probe to electronic chirality. | cond-mat_str-el |
Quantum Many-Body Scars from Einstein-Podolsky-Rosen States in Bilayer
Systems: Quantum many-body scar states are special eigenstates of nonintegrable models
with distinctive entanglement features that give rise to infinitely long-lived
coherent dynamics under quantum quenches from certain initial states. We
elaborate on a construction of quantum many-body scar states in which they
emerge from Einstein-Podolsky-Rosen (EPR) states in systems with two layers,
wherein the two layers are maximally entangled. We apply this construction to
spin systems as well as systems of itinerant fermions and bosons and
demonstrate how symmetries can be harnessed to enhance its versatility. We show
that several well-known examples of quantum many-body scars, including the
tower of states in the spin-1 XY model and the $\eta$-pairing states in the
Fermi-Hubbard model, can be understood within this formalism. We also
demonstrate how an {\it infinite} tower of many-body scar states can emerge in
bilayer Bose-Hubbard models with charge conservation. | cond-mat_str-el |
Low-energy theory of the Nambu-Goldstone modes of an ultracold $^6Li-$
$^{40}K$ mixture in an optical lattice: A low-energy theory of the Nambu-Goldstone excitation spectrum and the
corresponding speed of sound of an interacting Fermi mixture of Lithium-6 and
Potassium-40 atoms in a two-dimensional optical lattice at finite temperatures
with the Fulde-Ferrell order parameter has been formulated. It is assumed that
the two-species interacting Fermi gas is described by the one-band Hubbard
Hamiltonian with an attractive on-site interaction. The discussion is
restricted to the BCS side of the Feshbach resonance where the Fermi atoms
exhibit superfluidity. The quartic on-site interaction is decoupled via a
Hubbard-Stratonovich transformation by introducing a four-component boson field
which mediates the Hubbard interaction. A functional integral technique and a
Legendre transform are used to give a systematic derivation of the
Schwinger-Dyson equations for the generalized single-particle Green's function
and the Bethe-Salpeter equation for the two-particle Green's function and the
associated collective modes. The numerical solution of the Bethe-Salpeter
equation in the generalized random phase approximation shows that there exist
two distinct sound velocities in the long-wavelength limit. In addition to the
long-wavelength mode (Goldstone mode), the two-species Fermi gas has a
superfluid phase revealed by two rotonlike minima in the asymmetric
collective-mode energy. | cond-mat_str-el |
Low-energy optical properties of the non-magnetic kagome metal
CsV$_3$Sb$_5$: Temperature-dependent reflectivity measurements on the kagome metal
CsV$_3$Sb$_5$ in a broad frequency range of $50-20000$ cm$^{-1}$ down to $T$=10
K are reported. The charge-density wave (CDW) formed below $T_{\rm CDW}$ = 94 K
manifests itself in a prominent spectral-weight transfer from low to higher
energy regions. The CDW gap of 60-75 meV is observed at the lowest temperature
and shows significant deviations from an isotropic BCS-type mean-field
behavior. Absorption peaks appear at frequencies as low as 200 cm$^{-1}$ and
can be identified with interband transitions according to density-functional
calculations. The change in the interband absorption compared to KV$_3$Sb$_5$
reflects the inversion of band saddle points between the K and Cs compounds.
Additionally, a broader and strongly temperature-dependent absorption feature
is observed below 1000 cm$^{-1}$ and assigned to a displaced Drude peak. It
reflects localization effects on charge carriers. | cond-mat_str-el |
Elastic Instabilities within Antiferromagnetically Ordered Phase in the
Orbitally-Frustrated Spinel GeCo$_2$O$_4$: Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$
reveal unusual elastic instabilities due to the phonon-spin coupling within the
antiferromagnetic phase. Shear moduli exhibit anomalies arising from the
coupling to short-range ferromagnetic excitations. Diplike anomalies in the
magnetic-field dependence of elastic moduli reveal magnetic-field-induced
orbital order-order transitions. These results strongly suggest the presence of
geometrical orbital frustration which causes novel orbital phenomena within the
antiferromagnetic phase. | cond-mat_str-el |
Spin diffusion in the low-dimensional molecular quantum Heisenberg
antiferromagnet Cu(pyz)(NO$_{3}$)$_{2}$ detected with implanted muons: We present the results of muon-spin relaxation measurements of spin
excitations in the one-dimensional quantum Heisenberg antiferromagnet
Cu(pyz)(NO$_{3}$)$_{2}$. Using density-functional theory we propose muon sites
and assess the degree of perturbation the muon probe causes on the system. We
identify a site involving the muon forming a hydroxyl-type bond with an oxygen
on the nitrate group that is sensitive to the characteristic spin dynamics of
the system. Our measurements of the spin dynamics show that in the temperature
range $T_{\mathrm{N}}<T<J$ (between the ordering temperature $T_{\mathrm{N}}$
and the exchange energy scale $J$) the field-dependent muon spin relaxation is
characteristic of diffusive transport of spin excitations over a wide range of
applied fields. We also identify a possible crossover at higher applied fields
in the muon probe's response to the fluctuation spectrum, to a regime where the
muon detects early-time transport with a ballistic character. This behavior is
contrasted with that found for $T>J$ and that in the related two-dimensional
system Cu(pyz)$_2$(ClO$_4$)$_{2}$. | cond-mat_str-el |
A Model for the Schottky Anomaly in Metallic $Nd_{2-y}Ce_{y}CuO_{4}$: We present a simple model for the doped compound $Nd_{2-y}Ce_{y}CuO_{4}$, in
order to explain some recent experimental results on the latter. Within a
Hartree-Fock context, we start from an impurity Anderson-like model and
consider the magnetic splitting of the $Nd$-$4f$ ground state Kramers doublet
due to exchange interactions with the ordered $Cu$ moments. Our results are in
very good agreement with the experimental data, yielding a Schottky anomaly
peak for the specific heat that reduces its amplitude, broadens and shifts to
lower temperatures, upon $Ce$ doping. For overdoped compounds at low
temperatures, the specific heat behaves linearly and the magnetic
susceptibility is constant. A smooth transition from this Fermi liquid like
behavior ocurrs as temperature is increased and at high temperatures the
susceptibility exhibits a Curie-like behavior. Finally, we discuss some
improvements our model is amenable to incorporate. | cond-mat_str-el |
Renormalization of f-levels away from the Fermi energy in electron
excitation spectroscopies: Density functional results of
Nd$_{2-x}$Ce$_x$CuO$_4$: Relaxation energies for photoemission, when an occupied electronic state is
excited, and for inverse photoemission, when an empty state is filled, are
calculated within the density functional theory with application to
Nd$_{2-x}$Ce$_x$CuO$_4$. The associated relaxation energies are obtained by
computing differences in total energies between the ground state and an excited
state in which one hole or electron is added into the system. The relaxation
energies of f-electrons are found to be of the order of several eV's,
indicating that f-bands will appear substantially away from the Fermi energy
($E_F$) in their spectroscopic images, even if these bands lie near $E_F$. Our
analysis explains why it would be difficult to observe f electrons at the $E_F$
even in the absence of strong electronic correlations. | cond-mat_str-el |
The J_1-J_2 model revisited : Phenomenology of CuGeO_3: We present a mean field solution of the antiferromagnetic Heisenberg chain
with nearest (J_1) and next to nearest neighbor (J_2) interactions. This
solution provides a way to estimate the effects of frustration. We calculate
the temperature-dependent spin-wave velocity, v_s(T) and discuss the
possibility to determine the magnitude of frustration J_2/J_1 present in quasi
1D compounds from measurements of v_s(T). We compute the thermodynamic
susceptibility at finite temperatures and compare it with the observed
susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to
study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls
transition. | cond-mat_str-el |
Block magnetic excitations in the orbitally selective Mott insulator
BaFe2Se3: Iron pnictides and selenides display a variety of unusual magnetic phases
originating from the interplay between electronic, orbital, and lattice degrees
of freedom. Using powder inelastic neutron scattering on the two-leg ladder
BaFe2Se3, we fully characterize the static and dynamic spin correlations
associated with the Fe4 block state, an exotic magnetic ground state observed
in this low-dimensional magnet and in Rb0.89Fe1.58Se2. All the magnetic
excitations of the Fe4 block state predicted by an effective Heisenberg model
with localized spins are observed below 300 meV and quantitatively reproduced.
However, the data only account for 16 mub^2 per Fe2+, approximatively 2/3 of
the total spectral weight expected for localized S=2 moments. Our results
highlight how orbital degrees of freedom in iron-based magnets can conspire to
stabilize an exotic magnetic state. | cond-mat_str-el |
Scaling approach for the time-dependent Kondo model: We present a new nonperturbative method to deal with the time-dependent
quantum many-body problem, which is an extension of Wegner's flow equations to
time-dependent Hamiltonians. The formalism provides a scaling procedure for the
set of time-dependent interaction constants. We apply these ideas to a Kondo
model with a ferromagnetic exchange coupling switched on over a time scale
$\tau$. We show that the asymptotic expectation value of the impurity spin
interpolates continuously between its quenched and adiabatic value. | cond-mat_str-el |
Spin polarized STM spectra of Dirac Fermions on the surface of a
topological insulator: We provide a theory for the tunneling conductance $G(V)$ of Dirac Fermions on
the surface of a topological insulator as measured by a spin-polarized scanning
tunneling microscope tip for low bias voltages $V$. We show that $G(V)$
exhibits an unconventional dependence on the direction of magnetization of the
tip and can be used to measure the magnitude of the local out-of-plane spin
orientation of the Dirac Fermions on the surface. We also demonstrate that if
the in-plane rotational symmetry on the surface of the topological insulator is
broken by an external field, then $G(V)$ acquires a dependence on the azimuthal
angle of the magnetization of the tip. We explain the role of the Dirac
Fermions in this unconventional behavior and suggest experiments to test our
theory. | cond-mat_str-el |
On the Field-Induced Gap in Cu Benzoate and Other S=1/2 Antiferromagnets: Recent experiments on the S=1/2 antiferromagnetic chain compound, Cu
benzoate, discovered an unexpected gap scaling as approximately the 2/3 power
of an applied magnetic field. A theory of this gap, based on an effective
staggered field, orthogonal to the applied uniform field, resulting from a
staggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading
to a sine-Gordon quantum field theory, has been developed. Here we discuss many
aspects of this subject in considerable detail, including a review of the S=1/2
chain in a uniform field, a spin-wave theory analysis of the uniform plus
staggered field problem, exact amplitudes for the scaling of gap, staggered
susceptibility and staggered magnetization with field or temperature,
intensities of soliton and breather peaks in the structure function and field
and temperature dependence of the total susceptibility. | cond-mat_str-el |
Theory of Ca$_{10}$Cr$_7$O$_{28}$ as a bilayer breathing-kagome magnet:
Classical thermodynamics and semi-classical dynamics: Ca$_{10}$Cr$_7$O$_{28}$ is a novel spin-$1/2$ magnet exhibiting spin liquid
behaviour which sets it apart from any previously studied model or material.
However, understanding Ca$_{10}$Cr$_7$O$_{28}$ presents a significant
challenge, because the low symmetry of the crystal structure leads to very
complex interactions, with up to seven inequivalent coupling parameters in the
unit cell. Here we explore the origin of the spin-liquid behaviour in
Ca$_{10}$Cr$_7$O$_{28}$, starting from the simplest microscopic model
consistent with experiment - a Heisenberg model on a single bilayer of the
breathing-kagome (BBK) lattice. We use a combination of classical Monte Carlo
(MC) simulation and (semi-)classical Molecular Dynamics (MD) simulation to
explore the thermodynamic and dynamic properties of this model, and compare
these with experimental results for Ca$_{10}$Cr$_7$O$_{28}$. We uncover
qualitatively different behaviours on different timescales, and argue that the
ground state of Ca$_{10}$Cr$_7$O$_{28}$ is born out of a slowly-fluctuating
"spiral spin liquid", while faster fluctuations echo the U(1) spin liquid found
in the kagome antiferromagnet. We also identify key differences between
longitudinal and transverse spin excitations in applied magnetic field, and
argue that these are a distinguishing feature of the spin liquid in the BBK
model. | cond-mat_str-el |
Magnetization, thermoelectric, and pressure studies of the magnetic
field-induced metal to insulator transition in tau phase organic conductors: We have investigated the magnetic field-induced metal-insulator transition in
the tau-phase organic conductors, which occurs in fields above 35 T, and below
14 K, by magnetization, thermoelectric, and pressure dependent transport
methods. Our results show that the transition is a bulk thermodynamic process
where a magnetic field-dependent gap opens upon entry into the insulating
state. We argue that the transition involves a magnetic field-induced change in
the electronic structure. | cond-mat_str-el |
Efficient representation of long-range interactions in tensor network
algorithms: We describe a practical and efficient approach to represent physically
realistic long-range interactions in two-dimensional tensor network algorithms
via projected entangled-pair operators (PEPOs). We express the long-range
interaction as a linear combination of correlation functions of an auxiliary
system with only nearest-neighbor interactions. To obtain a smooth and radially
isotropic interaction across all length scales, we map the physical lattice to
an auxiliary lattice of expanded size. Our construction yields a long-range
PEPO as a sum of ancillary PEPOs, each of small, constant bond dimension. This
representation enables efficient numerical simulations with long-range
interactions using projected entangled pair states. | cond-mat_str-el |
Coexisting localized and itinerant gapless excitations in a quantum spin
liquid candidate 1T-TaS$_2$: To reveal the nature of elementary excitations in a quantum spin liquid
(QSL), we measured low temperature thermal conductivity and specific heat of
1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of
spin-1/2. The nonzero temperature linear specific heat coefficient $\gamma$ and
the finite residual linear term of the thermal conductivity in the zero
temperature limit $\kappa_0/T=\kappa/T(T\rightarrow 0)$ are clearly resolved.
This demonstrates the presence of highly mobile gapless excitations, which is
consistent with fractionalized spinon excitations that form a Fermi surface.
Remarkably, an external magnetic field strongly suppresses $\gamma$, whereas it
enhances $\kappa_0/T$. This unusual contrasting behavior in the field
dependence of specific heat and thermal conductivity can be accounted for by
the presence of two types of gapless excitations with itinerant and localized
characters, as recently predicted theoretically (I. Kimchi et al.,
arXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new
insights into the influence of quenched disorder on the QSL. | cond-mat_str-el |
Fermi liquid identities for the Infinite U Anderson Model: We show how the electron gas methods of Luttinger, Ward and Nozi\`eres can be
applied to the infinite U Anderson impurity model within a Schwinger boson
treatment. Working to all orders in a 1/N expansion, we show how the Friedel
Langreth relationship, the Yamada-Yosida-Yoshimori and the Shiba-Korringa
relations can be derived, under the assumption that the spinon and holon fields
are gapped. One of the remarkable features of this treatment, is that the
Landau amplitudes depend on the exchange of low energy virtual spinons and
holons. We end the paper with a discussion on the extension of our approach to
the lattice, where the spinon-holon is expected to close at a quantum critical
point. | cond-mat_str-el |
Spin-Wave Theory for the Scalar Chiral Phase in the Multiple-Spin
Exchange Model on a Triangular Lattice: We study the effects of quantum fluctuations on a non-coplanar tetrahedral
spin structure, which has a scalar chiral order, in the spin-1/2 multiple-spin
exchange model with up to the six-spin exchange interactions on a triangular
lattice. We find that, in the linear spin-wave approximation, the tetrahedral
structure survives the quantum fluctuations because spin waves do not soften in
the whole parameter region of the tetrahedral-structure phase evaluated for the
classical system. In the quantum corrections to the ground-state energy,
sublattice magnetization, and scalar chirality, the effects of the quantum
fluctuations are small for the ferromagnetic nearest-neighbor interactions and
for the strong five-spin interactions. The six-spin interactions have little
effect on the quantum corrections in the tetrahedral-structure phase. This
calculation also corrects an error in the previously reported value of scalar
chirality for the spin-1/2 multiple-spin exchange model with up to the
four-spin exchange interactions. | cond-mat_str-el |
Magnetic hard-axis ordering near ferromagnetic quantum criticality: We investigate the interplay of quantum fluctuations and magnetic
anisotropies in metallic ferromagnets. Our central result is that fluctuations
close to a quantum critical point can drive the moments to point along a
magnetic hard axis. As a proof of concept, we show this behavior explicitly for
a generic two-band model with local Coulomb and Hund's interactions, and a
spin-orbit-induced easy plane anisotropy. The phase diagram is calculated
within the fermionic quantum order-by-disorder approach, which is based on a
self-consistent free energy expansion around a magnetically ordered state with
unspecified orientation. Quantum fluctuations render the transition of the
easy-plane ferromagnet first-order below a tricritical point. At even lower
temperatures, directionally dependent transverse fluctuations dominate the
magnetic anisotropy and the moments flip to lie along the magnetic hard axis.
We discuss our findings in the context of recent experiments that show this
unusual ordering along the magnetic hard direction. | cond-mat_str-el |
LDA+DMFT Spectral Functions and Effective Electron Mass Enhancement in
Superconductor LaFePO: In this Letter we report the first LDA+DMFT results (method combining Local
Density Approximation with Dynamical Mean-Field Theory) for spectral properties
of superconductor LaFePO. Calculated {\bf k}-resolved spectral functions
reproduce recent angle-resolved photoemission spectroscopy (ARPES) data [D. H.
Lu {\it et al}., Nature {\bf 455}, 81 (2008)]. Obtained effective electron mass
enhancement values $m^{*}/m\approx$ 1.9 -- 2.2 are in good agreement with
infrared and optical studies [M. M. Qazilbash {\it et al}., Nature Phys. {\bf
5}, 647 (2009)], de Haas--van Alphen, electrical resistivity, and electronic
specific heat measurements results, that unambiguously evidence for moderate
correlations strength in LaFePO. Similar values of $m^{*}/m$ were found in the
other Fe-based superconductors with substantially different superconducting
transition temperatures. Thus, the dynamical correlation effects are essential
in the Fe-based superconductors, but the strength of electronic correlations
does not determine the value of superconducting transition temperature. | cond-mat_str-el |
Emerging frustration effects in ferromagnetic Ce_2[Pd_{1-x}Ag_x]_2In
alloys: Magnetic and thermal properties of Ferromagnetic (FM)
Ce_{2.15}(Pd_{1-x}Ag_x)_{1.95}In_{0.9} alloys were studied in order to
determine the Quantum Critical Point (QCP) at T_C => 0. The increase of band
electrons produced by Pd/Ag substitution depresses T_C(x) from 4.1K down to
T_C(x=0.5)=1.1K, with a QCP extrapolated to x_{QCP}~ 0.6. Magnetic
susceptibility from T>30K indicates an effective moment slightly decreasing
from \mu_{eff}=2.56\mu_B to 2.4\mu_B at x=0.5. These values and the
paramagnetic temperature \theta_P~ -10K exclude significant Kondo screening
effects. The T_C(x) reduction is accompanied by a weakening of the FM
magnetization and the emergence of a specific heat C_m(T) anomaly at T*~ 1K,
without signs of magnetism detected from AC-susceptibility. The magnetic
entropy collected around 4K (i.e. the T_C of the x=0 sample) practically does
not change with Ag concentration: S_m(4K)~ 0.8 Rln2, suggesting a progressive
transfer of FM degrees of freedom to the non-magnetic (NM) component. No
antecedent was found concerning any NM anomaly emerging from a FM system at
such temperature. The origin of this anomaly is attributed to an 'entropy
bottleneck' originated in the nearly divergent power law dependence for T>T*. | cond-mat_str-el |
Decoupled spin dynamics in the rare-earth orthoferrite YbFeO$_3$:
Evolution of magnetic excitations through the spin-reorientation transition: In this paper we present a comprehensive study of magnetic dynamics in the
rare-earth orthoferrite YbFeO$_3$ at temperatures below and above the
spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6$ K, in magnetic fields
applied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron
scattering, we observed that the spectrum of magnetic excitations consists of
two collective modes well separated in energy: 3D gapped magnons with a
bandwidth of $\sim$60 meV, associated with the antiferromagnetically (AFM)
ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim$1 meV within the
Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe
subsystem changes very little through the SR transition and could be well
described in the frame of semiclassical linear spin-wave theory. On the other
hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$
drastically changes the excitation spectrum of the Yb subsystem, inducing the
transition between two regimes with magnon and spinon-like fluctuations. At $T
< T_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced
ferromagnetic (FM) ground state, and the spectrum consists of a sharp
single-magnon mode, a two-magnon bound state, and a two-magnon continuum,
whereas at $T > T_{\mathrm{SR}}$ only a gapped broad spinon-like continuum
dominates the spectrum. In this work we show that a weak quasi-1D coupling
within the Yb subsystem $J_\text{Yb-Yb}$, mainly neglected in previous studies,
creates unusual quantum spin dynamics on the low energy scales. The results of
our work may stimulate further experimental search for similar compounds with
several magnetic subsystems and energy scales, where low-energy fluctuations
and underlying physics could be "hidden" by a dominating interaction. | cond-mat_str-el |
Orbital order in La0.5Sr1.5MnO4: beyond a common local Jahn-Teller
picture: The standard way to find the orbital occupation of Jahn-Teller (JT) ions is
to use structural data, with the assumption of a one-to-one correspondence
between the orbital occupation and the associated JT distortion, e.g. in O6
octahedron. We show, however, that this approach in principle does not work for
layered systems. Specifically, using the layered manganite La0.5Sr1.5MnO4 as an
example, we found from our x-ray absorption measurements and theoretical
calculations, that the type of orbital ordering strongly contradicts the
standard local distortion approach for the Mn3+O6 octahedra, and that the
generally ignored long-range crystal field effect and anisotropic hopping
integrals are actually crucial to determine the orbital occupation. Our
findings may open a pathway to control of the orbital state in multilayer
systems and thus of their physical properties. | cond-mat_str-el |
Realizing the strongly correlated $d$-Mott state in a fermionic cold
atom optical lattice: We show that a new state of matter, the d-wave Mott-insulator state (d-Mott
state) (introduced recently by [H. Yao, W. F. Tsai, and S. A. Kivelson, Phys.
Rev. B 76, 161104 (2007)]), which is characterized by a non-zero expectation
value of a local plaquette operator embedded in an insulating state, can be
engineered using ultra-cold atomic fermions in two-dimensional double-well
optical lattices. We characterize and analyze the parameter regime where the
$d$-Mott state is stable. We predict the testable signatures of the state in
the time-of-flight measurements. | cond-mat_str-el |
Pressure-Induced Antiferromagnetic Dome in the Heavy-Fermion
$Yb_2Pd_2In_{1-x}Sn_x$ System: In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of
crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida
interactions leads to complex chemical-, pressure-, and magnetic-field phase
diagrams, still to be explored in full detail. By using a series of techniques,
we show that even modest changes of parameters other than temperature are
sufficient to induce multiple quantum-critical transitions in this highly
susceptible heavy-fermion family. In particular, we show that, above $\sim 10$
kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low
temperature, but it likely leads to a reorientation of the Yb magnetic moments
and/or the competition among different antiferromagnetic configurations. | cond-mat_str-el |
Topological Phase Transition in the Hofstadter-Hubbard Model: We study the interplay between topological and conventional long range order
of attractive fermions in a time reversal symmetric Hofstadter lattice using
quantum Monte Carlo simulations, focussing on the case of one-third flux
quantum per plaquette. At half-filling, the system is unstable towards s-wave
pairing and charge-density-wave order at infinitesimally small interactions. At
one-third-filling, the noninteracting system is a topological insulator, and a
nonzero critical interaction strength is needed to drive a transition from the
quantum spin Hall insulator to a superfluid. We probe the topological signature
of the phase transition by threading a magnetic flux through a cylinder and
observe quantized topological charge pumping. | cond-mat_str-el |
Photo-induced states in a Mott insulator: We investigate the properties of the metallic state obtained by photo-doping
carriers into a Mott insulator. In a strongly interacting system, these
carriers have a long life-time, so that they can dissipate their kinetic energy
to a phonon bath. In the relaxed state, the scattering rate saturates at a
non-zero temperature-independent value, and the momentum-resolved spectral
function features broad bands which differ from the well-defined quasi-particle
bands of a chemically doped system. Our results indicate that a photo-doped
Mott insulator behaves as a bad metal, in which strong scattering between
doublons and holes inhibits Fermi-liquid behavior down to low temperature. | cond-mat_str-el |
Magnetic Order in Laser-Irradiated Kagome Antiferromagnets: Dispersionless "zero energy mode'' is one of the hallmarks of frustrated
kagome antiferromagnets (KAFMs). It points to extensive classically degenerate
ground-states. The "zero energy mode'' can be observed experimentally when
lifted to a flat mode at finite energy by a strong intrinsic magnetic
anisotropy. In this letter, we study the effects of irradiation of laser light
on the KAFMs. We adopt the magnon picture without loss of generality. It is
shown that circularly or linearly polarized light lifts the "zero energy
mode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We
find that the circularly polarized light-induced anisotropies have similar
features as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya
interaction in KAFMs. The former stabilizes long-range magnetic order and the
latter induces spin canting out-of-plane with nonzero scalar spin chirality.
The Floquet thermal Hall effect shows that the synthetic magnetic excitation
modes in the case of circularly polarized light are topological, whereas those
of linearly polarized light are not. | cond-mat_str-el |
From Quantum Wires to the Chern-Simons Description of the Fractional
Quantum Hall Effect: We show the explicit connection between two distinct and complementary
approaches to the fractional quantum Hall system (FQHS): the quantum wires
formalism and the topological low-energy effective description given in terms
of an Abelian Chern-Simons theory. The quantum wires approach provides a
description of the FQHS directly in terms of fermions arranged in an array of
one-dimensional coupled wires. In this sense it is usually referred to as a
microscopic description. On the other hand, the effective theory has no
connection with the microscopic modes, involving only the emergent topological
degrees of freedom embodied in an Abelian Chern-Simons gauge field, which
somehow encodes the collective motion of the strongly correlated electrons. The
basic strategy pursued in this work is to bosonize the quantum wires system and
then consider the continuum limit. By examining the algebra of the bosonic
operators of the Hamiltonian, we are able to identify the bosonized microscopic
fields with the components of the field strength (electric and magnetic fields)
of the emergent gauge field. Thus our study provides a bridge between the
microscopic physical degrees of freedom and the emergent topological ones,
without relying on the bulk-edge correspondence. | cond-mat_str-el |
Novel quantum criticality due to emergent topological conservation law
in high-$T_c$ cuprates: We argue that in strongly correlated electron system collective instanton
excitations of the phase field (dual to the charge) arise with a great degree
of stability, governed by gauge flux changes by an integer multiple of $2\pi$.
By unraveling consequences of the non-trivial topology of the charge gauge U(1)
group, we found that the pinning of $\mu$ and the zero-temperature divergence
of charge compressibility $\kappa\sim\partial n_e/\partial\mu$ defines novel
"hidden" quantum criticality on verge of the Mott transition governed by the
protectorate of stable topological numbers rather than Landau paradigm of the
symmetry breaking. | cond-mat_str-el |
Evolution of the 2D antiferromagnetism with temperature and magnetic
field in multiferroic Ba$_2$CoGe$_2$O$_7$: We report on spherical neutron polarimetry and unpolarized neutron
diffraction in zero magnetic field as well as flipping ratio and static
magnetization measurements in high magnetic fields on the multiferroic square
lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$. We found that in zero magnetic
field the magnetic space group is $Cm'm2'$ with sublattice magnetization
parallel to the [100] axis of this orthorhombic setting. The spin canting has
been found to be smaller than $0.2^\circ$ in the ground state. This assignment
is in agreement with the field-induced changes of the magnetic domain structure
below 40 mT as resolved by spherical neutron polarimetry. The magnitude of the
ordered moment has been precisely determined. Above the magnetic ordering
temperature short-range magnetic fluctuations are observed. Based on the
high-field magnetization data, we refined the parameters of the recently
proposed microscopic spin model describing the multiferroic phase of
Ba$_2$CoGe$_2$O$_7$. | cond-mat_str-el |
Flow equation analysis of the anisotropic Kondo model: We use the new method of infinitesimal unitary transformations to calculate
zero temperature correlation functions in the strong-coupling phase of the
anisotropic Kondo model. We find the dynamics on all energy scales including
the crossover behaviour from weak to strong coupling. The integrable structure
of the Hamiltonian is not used in our approach. Our method should also be
useful in other strong-coupling models since few other analytical methods allow
the evaluation of their correlation functions on all energy scales. | cond-mat_str-el |
Pseudoparticle Description of the 1D Hubbard Model Electronic Transport
Properties: We extend the pseudoparticle transport description of the Hubbard chain to
all energy scales. In particular we compute the mean value of the electric
current transported by any Bethe-ansatz state and the transport masses of the
charge carriers. We present numerical results for the optical conductivity of
the model at half-filling for values of U/t=3 and 4. We show that these are in
good agreement with the pseudoparticle description of the finite-energy
transitions involving new pseudoparticle energy bands. | cond-mat_str-el |
Majorana zero modes in a quantum Ising chain with longer-ranged
interactions: A one-dimensional Ising model in a transverse field can be mapped onto a
system of spinless fermions with p-wave superconductivity. In the weak-coupling
BCS regime, it exhibits a zero energy Majorana mode at each end of the chain.
Here, we consider a variation of the model, which represents a superconductor
with longer ranged kinetic energy and pairing amplitudes, as is likely to occur
in more realistic systems. It possesses a richer zero temperature phase diagram
and has several quantum phase transitions. From an exact solution of the model
these phases can be classified according to the number of Majorana zero modes
of an open chain: 0, 1, or 2 at each end. The model posseses a multicritical
point where phases with 0, 1, and 2 Majorana end modes meet. The number of
Majorana modes at each end of the chain is identical to the topological winding
number of the Anderson's pseudospin vector that describes the BCS Hamiltonian.
The topological classification of the phases requires a unitary time-reversal
symmetry to be present. When this symmetry is broken, only the number of
Majorana end modes modulo 2 can be used to distinguish two phases. In one of
the regimes, the wave functions of the two phase shifted Majorana zero modes
decays exponentially in space but but in an oscillatory manner. The wavelength
of oscillation is identical to the asymptotic connected spin-spin correlation
of the XY-model in a transverse field to which our model is dual. | cond-mat_str-el |
Energy scales in 4f1 delafossite magnets: crystal-field splittings
larger than the strength of spin-orbit coupling in KCeO2: Ytterbium-based delafossites with effective S=1/2 moments are investigated
intensively as candidates for quantum spin-liquid ground states. While the
synthesis of related cerium compounds has also been reported,many important
details concerning their crystal, electronic, and magnetic structures are
unclear. Here we analyze the S=1/2 system KCeO2, combining complementary
theoretical methods. The lattice geometry was optimized and the band structure
investigated using density functional theory extended to the level of a GGA+U
calculation in order to reproduce the correct insulating behavior. The Ce 4f1
states were then analyzed in more detail with the help of ab initio
wave-function-based computations. Unusually large effective crystal-field
splittings of up to 320 meV are predicted, which puts KCeO2 in the strong field
coupling regime. Our results reveal a subtle interplay between ligand-cage
electrostatics and the trigonal field generated by the extended crystalline
surroundings, relevant in the context of recent studies on tuning the nature of
the ground-state wave function in 4f triangular-lattice and pyrochlore
compounds. It also makes KCeO2 an interesting model system in relation to the
effect of large crystal-field splittings on the anisotropy of intersite
exchange in spin-orbit coupled quantum magnets. | cond-mat_str-el |
Detection of gapped phases of a 1D spin chain with onsite and spatial
symmetries: We investigate the phase diagram of a quantum spin-1 chain whose Hamiltonian
is invariant under a global onsite $A_4$, translation and lattice inversion
symmetries. We detect different gapped phases characterized by SPT order and
symmetry breaking using matrix product state order parameters. We observe a
rich variety of phases of matter characterized by a combination of symmetry
breaking and symmetry fractionalization and also the interplay between the
onsite and spatial symmetries. Examples of continuous phase transitions
directly between topologically nontrivial SPT phases are also observed. | cond-mat_str-el |
In-plane magnetic field induced double fan spin structure with
$\textit{c}$-axis component in metallic kagome antiferromagnet
YMn$_{6}$Sn$_{6}$: The geometrical frustration nature of the kagome lattice makes it a great
host to flat electronic band, non-trivial topological properties, and novel
magnetisms. Metallic kagome antiferromagnet YMn$_{6}$Sn$_{6}$ exhibits the
topological Hall effect (THE) when an in-plane magnetic field is applied. THE
is typically associated with the nanometer-sized non-coplanar spin structure of
skyrmions in non-centrosymmetric magnets with large Dzyaloshinskii-Moriya
interaction. Here we use single crystal neutron diffraction to determine the
field/temperature dependence of the magnetic structure in YMn$_{6}$Sn$_{6}$. We
find that the observed THE cannot arise from a magnetic skyrmion lattice, but
instead from an in-plane field-induced double fan spin structure with
$\textit{c}$-axis components (DFC). Our work provides the experimental basis
from which a microscopic theory can be established to understand the observed
THE. | cond-mat_str-el |
Electronic structure of Ce2RhIn8 2D heavy Fermion system studied by
angle resolved photoemission spectroscopy: We use angle-resolved photoemission spectroscopy to study heavy fermion
superconductor Ce2RhIn8. The Fermi surface is rather complicated and consists
of several hole and electron pock- ets. We do not observe kz dispersion of
Fermi sheets, which is consistent with 2D character of the electronic
structure. Comparison of the ARPES data and band structure calculations points
to a localized picture of f electrons. Our findings pave the way for
understanding the transport and thermodynamical properties of this material. | cond-mat_str-el |
Superconductivity, pseudogap, and phase separation in topological flat
bands: a quantum Monte Carlo study: We study a two-dimensional model of an isolated narrow topological band at
partial filling with local attractive interactions. Numerically exact quantum
Monte Carlo calculations show that the ground state is a superconductor with a
critical temperature that scales nearly linearly with the interaction strength.
We also find a broad pseudogap regime at temperatures above the superconducting
phase that exhibits strong pairing fluctuations and a tendency towards
electronic phase separation; introducing a small nearest neighbor attraction
suppresses superconductivity entirely and results in phase separation. We
discuss the possible relevance of superconductivity in this unusual regime to
the physics of flat band moir\'{e} materials, and as a route to designing
higher temperature superconductors. | cond-mat_str-el |
Quantum advantage in the charging process of Sachdev-Ye-Kitaev batteries: The exactly-solvable Sachdev-Ye-Kitaev (SYK) model has recently received
considerable attention in both condensed matter and high energy physics because
it describes quantum matter without quasiparticles, while being at the same
time the holographic dual of a quantum black hole. In this Letter, we examine
SYK-based charging protocols of quantum batteries with N quantum cells.
Extensive numerical calculations based on exact diagonalization for N up to 16
strongly suggest that the optimal charging power of our SYK quantum batteries
displays a super-extensive scaling with N that stems from genuine quantum
mechanical effects. While the complexity of the nonequilibrium SYK problem
involved in the charging dynamics prevents us from an analytical proof, we
believe that this Letter offers the first (to the best of our knowledge) strong
numerical evidence of a quantum advantage occurring due to the
maximally-entangling underlying quantum dynamics. | cond-mat_str-el |
Relaxor ferroelectric behavior and intrinsic magnetodielectric behavior
near room temperature in Li2Ni2Mo3O12, a compound with distorted honeycomb
and spin-chains: Keeping current interests to identify materials with intrinsic
magnetodielectric behavior near room temperature and with novel pyroelectric
current anomalies, we report temperature and magnetic-field dependent behavior
of complex dielectric permittivity and pyroelectric current for an oxide,
Li2Ni2Mo3O12, containing magnetic ions with (distorted) honey-comb and chain
arrangement and ordering magnetically below 8 K. The dielectric data reveal the
existence of relaxor ferroelectricity behavior in the range 160-240 K and there
are corresponding Raman mode anomalies as well in that temperature range.
Pyrocurrent behavior is also consistent with this interpretation, with the
pyrocurrent peak-temperature interestingly correlating with the poling
temperature. 7Li NMR offer an evidence for crystallographic disorder intrinsic
to this compound and we therefore conclude that such a disorder is apparently
responsible for the randomness of local electric field leading to relaxor
ferroelectric property. Another observation of emphasis is that there is a
notable decrease in the dielectric constant with the application of magnetic
field to the tune of about -2.4% at 300 K, with the magnitude varying
mariginally with temperature. Small loss factor values validate intrinsic
behavior of the magnetodielectric effect at room temperature. | cond-mat_str-el |
Theoretical constraints for the magnetic-dimer transition in
two-dimensional spin models: From general arguments, that are valid for spin models with sufficiently
short-range interactions, we derive strong constraints on the excitation
spectrum across a continuous phase transition at zero temperature between a
magnetic and a dimerized phase, that breaks the translational symmetry. From
the different symmetries of the two phases, it is possible to predict, at the
quantum critical point, a branch of gapless excitations, not described by
standard semi-classical approaches. By using these arguments, supported by
intensive numerical calculations, we obtain a rather convincing evidence in
favor of a first-order transition from the ferromagnetic to the dimerized phase
in the two-dimensional spin-half model with four-spin ring-exchange
interaction, recently introduced by A.W. Sandvik et al. [Phys. Rev. Lett. 89,
247201 (2002)]. | cond-mat_str-el |
Quantized Berry Phases for a Local Characterization of Spin Liquids in
Frustrated Spin Systems: Recently by using quantized Berry phases, a prescription for a local
characterization of gapped topological insulators is given. One requires the
ground state is gapped and is invariant under some anti-unitary operation. A
spin liquid which is realized as a unique ground state of the Heisenberg spin
system with frustrations is a typical target system, since pairwise exchange
couplings are always time-reversal invariants even with frustrations.
As for a generic Heisenberg model with a finite excitation gap, we locally
modify the Hamiltonian by a continuous SU(2) twist only at a specific link and
define the Berry connection by the derivative. Then the Berry phase evaluated
by the entire many-spin wavefunction is used to define the local topological
order parameter at the link. We numerically apply this scheme for several spin
liquids and show its physical validity. | cond-mat_str-el |
Low frequency Raman response near Ising-nematic quantum critical point:
a memory matrix approach: Recent Raman scattering experiments have revealed a "quasi-elastic peak" in
$\mathrm{FeSe_{1-x}S_x}$ near an Ising-nematic quantum critical point (QCP)
\cite{zhang17}. Notably, the peak occurs at sub-temperature frequencies, and
softens as $T^{\alpha}$ when temperature is decreased toward the QCP, with
$\alpha>1$. In this work, we present a theoretical analysis of the
low-frequency Raman response using a memory matrix approach. We show that such
a quasi-elastic peak is associated with the relaxation of an Ising-nematic
deformation of the Fermi surface. Specifically, we find that the peak frequency
is proportional to $ \tau^{-1}\chi^{-1}$, where $\chi$ is the Ising-nematic
thermodynamic susceptibility, and $\tau^{-1}$ is the decay rate of the nematic
deformation due to an interplay between impurity scattering and
electron-electron scattering mediated by critical Ising-nematic fluctuations.
We argue that the critical fluctuations play a crucial role in determining the
observed temperature dependence of the frequency of the quasi-elastic peak. At
frequencies larger than the temperature, we find that the Raman response is
proportional to $\omega^{1/3}$, consistently with earlier predictions
\cite{klein18a}. | cond-mat_str-el |
Electronic correlations and crystal structure distortions in BaBiO3: BaBiO3 is a material where formally Bi4+ ions with the half-filled 6s-states
form the alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered
insulator. The charge ordering is accompanied by the breathing distortion of
the BiO6 octahedra (extension and contraction of the Bi-O bond lengths).
Standard Density Functional Theory (DFT) calculations fail to obtain the
crystal structure instability caused by the pure breathing distortions.
Combining effects of the breathing distortions and tilting of the BiO6
octahedra allows DFT to reproduce qualitatively experimentally observed
insulator with monoclinic crystal structure but gives strongly underestimate
breathing distortion parameter and energy gap values. In the present work we
reexamine the BaBiO3 problem within the GGA+U method using a Wannier functions
basis set for the Bi 6s-band. Due to high oxidation state of bismuth in this
material the Bi 6s-symmetry Wannier function is predominantly extended
spatially on surrounding oxygen ions and hence differs strongly from a pure
atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with
exclusion of high oxidation state compounds) where the major part a of d-band
Wannier function is concentrated on metal ion and a pure atomic d-orbital can
serve as a good approximation. The GGA+U calculation results agree well with
experimental data, in particular with experimental crystal structure parameters
and energy gap values. Moreover, the GGA+U method allows one to reproduce the
crystal structure instability due to the pure breathing distortions without
octahedra tilting. | cond-mat_str-el |
Quantum critical point in graphene approached in the limit of infinitely
strong Coulomb interaction: Motivated by the physics of graphene, we consider a model of N species of 2+1
dimensional four-component massless Dirac fermions interacting through a 3D
instantaneous Coulomb interaction. We show that in the limit of infinitely
strong Coulomb interaction the system approaches a quantum critical point, at
least for sufficiently large fermion degeneracy. In this regime the system
exhibits invariance under scale transformations in which time and space scale
by different factors. The elementary excitations are fermions with dispersion
relation omega ~ p^z, where the dynamic critical exponent z depends on N. In
the limit of large N we find z=1-4/(\pi^2 N). We argue that due to the
numerically large Coulomb coupling, graphene (freely suspended) in vacuum stays
near the scale-invariant regime in a large momentum window, before eventually
flowing to the trivial fixed point at very low momentum scales. | cond-mat_str-el |
Giant Positive Magnetoresistance and field-induced metal insulator
transition in Cr2NiGa: We report here the magneto-transport properties of the newly synthesized
Heusler compound Cr2NiGa which crystallizes in a disordered cubic B2 structure
belonging to Pm-3m space group. The sample is found to be paramagnetic down to
2 K with metallic character. On application of magnetic field, a significantly
large increase in resistivity is observed which corresponds to
magnetoresistance as high as 112% at 150 kOe of field at the lowest
temperature. Most remarkably, the sample shows negative temperature coefficient
of resistivity below about 50 K under the application of field gretare than or
equal to 80 kOe, signifying a field-induced metal to `insulating' transition.
The observed magnetoresistance follows Kohler's rule below 20 K indicating the
validity of the semiclassical model of electronic transport in metal with a
single relaxation time. A multi-band model for electronic transport, originally
proposed for semimetals, is found to be appropriate to describe the
magneto-transport behavior of the sample. | cond-mat_str-el |
Small-angle interband scattering as the origin of the $T^{3/2}$
resistivity in MnSi: A possible explanation is given for the anomalous $T^{3/2}$ temperature
dependence of the electrical resistivity of MnSi, which is observed in the
high-pressure paramagnetic state. The unusual Fermi surface of MnSi includes
large open sheets that intersect along the faces of the cubic Brillouin zone.
Close to these intersections, long-wavelength interband magnetic spin
fluctuations can scatter electrons from one sheet to the other. The current
relaxation rate due to such interband scattering events is not reduced by
vertex corrections as is that for scattering from intraband ferromagnetic
fluctuations. Consequently, current relaxation proceeds in a manner similar to
that occurring in nearly antiferromagnetic metals, in which low-temperature
$T^{3/2}$ behavior is well known. It is argued that this type of
non-Fermi-liquid behavior can, for a metal with ferromagnetic fluctuations near
Fermi sheet intersections, persist over a much wider temperature range than it
does in nearly antiferromagnetic metals. | cond-mat_str-el |
Exact results for the entanglement in 1D Hubbard models with spatial
constraints: We investigate the entanglement in Hubbard models of hardcore bosons in $1D$,
with an additional hardcore interaction on nearest neighbouring sites. We
derive analytical formulas for the bipartite entanglement entropy for any
number of particles and system size, whose ratio determines the system filling.
At the thermodynamic limit the entropy diverges logarithmically for all
fillings, except for half-filling, with the universal prefactor $1/2$ due to
partial permutational invariance. We show how maximal entanglement can be
achieved by controlling the interaction range between the particles and the
filling which determines the empty space in the system. Our results show how
entangled quantum phases can be created and controlled, by imposing spatial
constraints on states formed in many-body systems of strongly interacting
particles. | cond-mat_str-el |
Lattice and Electronic properties of VO$_2$ with the SCAN(+$U$) approach: Appropriate consideration of the electron correlation is essential to
reproduce the intriguing metal-insulator transition accompanying the
Peierls-type structural transition in VO$_2$. In the density functional
theory-based approach, this depends on the choice of the exchange-correlation
functional. Here, using a newly developed strongly constrained and
appropriately norm (SCAN) functional, we investigate the lattice and electronic
properties of the metallic rutile phase of VO$_2$ ($R$-VO$_2$) from the
first-principles calculations. We also explored the role of the Coulomb
correlation $U$. By adding $U$, we found that the phonon instability properly
describes the Peierls-type distortions. The orbital-decomposed density of
states presents the orbital selective behavior with the SCAN+$U$, which is
susceptible to the one-dimensional Peierls distortion. Our results suggest that
even with the SCAN functional, the explicit inclusion of the Coulomb
interaction is necessary to describe the structural transition of VO$_2$. | cond-mat_str-el |
Double exchange and orbital correlations in electron-doped manganites: A double exchange model for degenerate $e_g$ orbitals with intra- and
inter-orbital interactions has been studied for the electron doped manganites
A$_{1-x}$B$_{x}$MnO$_3$ ($x > 0.5$). We show that such a model reproduces the
observed phase diagram and orbital ordering in the intermediate bandwidth
regime and the Jahn-Teller effect, considered to be crucial for the region
$x<0.5$, does not play a major role in this region. Brink and Khomskii have
already pointed this out and stressed the relevance of the anistropic hopping
across the degenerate $e_g$ orbitals in the infinite Hund's coupling limit.
From a more realistic calculation with finite Hund's coupling, we show that
inclusion of interactions stabilizes the C-phase, the antiferromagnetic
metallic A-phase moves closer to $x=0.5$ while the ferromagnetic phase shrinks.
This is in agreement with the recent observations of Kajimoto et. al. and
Akimoto et. al. | cond-mat_str-el |
Quantum Confinement Transition in a d-wave Superconductor: We study the nature of the zero-temperature phase transition between a d-wave
superconductor and a Mott insulator in two dimensions. In this ``quantum
confinement transition'', spin and charge are confined to form the electron in
the Mott insulator. Within a dual formulation, direct transitions from d-wave
superconductors at half-filling to insulators with spin-Peierls (as well as
other) order emerge naturally. The possibility of striped superconductors is
also discussed within the dual formulation. The transition is described by
nodal fermions and bosonic vortices, interacting via a long-ranged statistical
interaction modeled by two, coupled Chern-Simons gauge fields, and the critical
properties of this model are discussed. | cond-mat_str-el |
One-dimensional conductance through an arbitrary potential: The finite-size Tomonaga-Luttinger Hamiltonian with an arbitrary potential is
mapped onto a non-interacting Fermi gas with renormalized potential. This is
done by means of flow equations for Hamiltonians and is valid for small
electron-electron interaction. This method also yields an alternative
bosonization formula for the transformed field operator which makes no use of
Klein factors. The two-terminal conductance can then be evaluated using the
Landauer formula. We obtain similar results for infinite systems at finite
temperature by identifying the flow parameter with the inverse squared
temperature in the asymptotic regime. We recover the algebraic behavior of the
conductance obtained by Kane and Fisher in the limit of low temperatures and
weak electron-electron interaction, but our results remain valid for arbitrary
external potential. | cond-mat_str-el |
Emergent Bound States and Impurity Pairs in Chemically Doped
Shastry-Sutherland System: The search for novel unconventional superconductors is a central topic of
modern condensed matter physics. Similar to other Mott insulators,
Shastry-Sutherland (SSL) systems are predicted to become superconducting when
chemically doped. This makes SrCu2(BO3)2, an experimental realization of SSL
model, a suitable candidate and understanding of the doping effects in it very
important. Here we report doping-induced emergent states in Mg-doped
SrCu2(BO3)2, which remain stable up to high magnetic fields. Using four
complementary magnetometry techniques and theoretical simulations, a rich
impurity-induced phenomenology at high fields is discovered. The results
demonstrate a rare example in which even a small doping concentration interacts
strongly with both triplets and bound states of triplets, and thus plays a
significant role in the magnetization process even at high magnetic fields.
Moreover, our findings of the emergence of the very stable impurity pairs
provide insights into the anticipated unconventional superconductivity in
SrCu2(BO3)2 and related materials. | cond-mat_str-el |
Electronic Correlation Effects on Stabilizing a Perfect Kagome Lattice
and Ferromagnetic Fluctuation in LaRu$_3$Si$_2$: A perfect Kagome lattice features flat bands that usually lead to strong
electronic correlation effects, but how electronic correlation, in turn,
stabilizes a perfect Kagome lattice has rarely been explored. Here, we study
such effect in a superconducting ($T_c \sim 7.8$ K) Kagome metal LaRu$_3$Si$_2$
with a distorted Kagome plane consisting of pure Ru ions, using density
functional theory plus $U$ and plus dynamical mean-field theory. We find that
increasing electronic correlation can stabilize a perfect Kagome lattice and
induce substantial ferromagnetic fluctuations in LaRu$_3$Si$_2$. By comparing
the calculated magnetic susceptibilities to experimental data, LaRu$_3$Si$_2$
is found to be on the verge of becoming a perfect Kagome lattice. It thus shows
moderate but non-negligible electronic correlations and ferromagnetic
fluctuations, which are crucial to understanding the experimentally observed
non-Fermi-liquid behavior and the pretty high superconducting $T_c$ of
LaRu$_3$Si$_2$. | cond-mat_str-el |
Theoretical investigation of the behavior of CuSe2O5 compound in high
magnetic fields: Based on analytical and numerical approaches, we investigate thermodynamic
properties of CuSe2O5 at high magnetic fields which is a candidate for the
strong intra-chain interaction in quasi one-dimensional (1D) quantum magnets.
Magnetic behavior of the system can be described by the 1D spin-1/2 XXZ model
in the presence of the Dzyaloshinskii-Moriya (DM) interaction. Un- der these
circumstances, there is one quantum critical field in this compound. Below the
quantum critical field the spin chain system is in the gapless Luttinger liquid
(LL) regime, whereas above it one observes a crossover to the gapped saturation
magnetic phase. Indications on the thermodynamic curves confirm the occurrence
of such a phase transition. The main characteristics of the LL phase are
gapless and spin-spin correlation functions decay algebraic. The effects of
zero-temperature quantum phase transition are observed even at rather high
temperatures in comparison with the counterpart compounds. In addition, we
calculate the Wilson ratio in the model. The Wilson ratio at a fixed
temperature remains almost independent of the field in the LL region. In the
vicinity of the quantum critical field, the Wilson ratio increases and exhibits
anomalous enhancement. | cond-mat_str-el |
Control of magnetic interactions between surface adatoms via orbital
repopulation: We propose a reversible mechanism for switching Heisenberg-type exchange
interactions between deposited transition metal adatoms from ferromagnetic to
antiferromagnetic. Using first-principles calculations, we show that this
mechanism can be realized for cobalt atoms on the surface of black phosphorus
by making use of electrically-controlled orbital repopulation, as recently
demonstrated by scanning probe techniques [Nat. Commun. 9, 3904 (2018)]. We
find that field-induced repopulation not only affects the spin state, but also
causes considerable modification of exchange interaction between adatoms,
including its sign. Our model analysis demonstrates that variable
adatom-substrate hybridization is a key factor responsible for this
modification. We perform quantum simulations of inelastic tunneling
characteristics and discuss possible ways to verify the proposed mechanism
experimentally. | cond-mat_str-el |
Neutron Scattering Studies of Spin Fluctuations in High Temperature
Superconductors: Neutron scattering can provide detailed information about the energy and
momentum dependence of the magnetic dynamics of materials provided sufficiently
large single crystals are available. This requirement has limited the number of
rare earth high temperature superconducting materials that have been studied in
any detail. However, improvements in crystal growth in recent years has
resulted in considerable progress in our understanding of the behaviour of the
magnetism of the CuO planes in both the superconducting and normal state. This
review will focus primarily on the spin fluctuations in La_{2-x}Sr_{x}CuO_{4}
and YBa_{2}Cu_{3}O_{7-x} since these are the two systems for which the most
detailed results are available. Although gaps in our understanding remain,
there is now a consistent picture of on the spin fluctuation spectra in both
systems as well as the changes induced by the superconducting transition. For
both La_{2-x}Sr_{x}CuO_{4} and underdoped YBa_{2}Cu_{3}O_{7-x} the normal state
response is characterised by incommensurate magnetic fluctuations. The low
energy excitations are suppressed by the superconducting transition with a
corresponding enhancement in the response at higher energies. For
YBa_{2}Cu_{3}O_{7-x} the superconducting state is accompanied by the rapid
development of a commensurate resonant response whose energy varies with T_{c}.
In underdoped samples this resonance persists above T_{c}. | cond-mat_str-el |
Ce$_{2}$Ir$_{3}$Ga$_{5}$ : a new locally non-centrosymmetric heavy
fermion system: Recently, a new type of unconventional superconductivity with a field-induced
transition between two different superconducting (SC) states was discovered in
the heavy fermion system CeRh$_{2}$As$_{2}$. This unusual SC state was proposed
to be based on specific symmetries of the underlying structure, i.e., a
globally centrosymmetric layered structure, but where the Ce-layers themselves
lack inversion symmetry. This new type of SC state has attracted strong
interest, prompting the search for further heavy fermion systems crystallizing
in structures with appropriate symmetries. We report the discovery and the
study of a new Ce-based heavy fermion system with a globally centrosymmetric
structure but without inversion symmetry on the Ce-site,
Ce$_{2}$Ir$_{3}$Ga$_{5}$. A single crystal X-ray diffraction study revealed an
orthorhombic U$_{2}$Co$_{3}$Si$_{5}$ type structure. Resistivity, specific
heat, and magnetization measurements indicate a moderate-heavy fermion behavior
with a Kondo energy scale of the order of 40 K. Most experimental results
suggest the absence of magnetic order, but a tiny anomaly in the specific heat
opens the possibility for a very weak, itinerant type of ordering. | cond-mat_str-el |
Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the
Kondo system Ce2Rh3Al9: We have used temperature dependent x-ray absorption at the Ce-L3 edge to
investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic
changes of the spectral lineshape with decreasing temperature are analyzed and
found to be related to a change in the $4f$ occupation number, n_f, as the
system undergoes a transition into a Kondo state. The temperature dependence of
$n_f$ indicates a characteristic temperature of 150K, which is clearly related
with the high temperature anomaly observed in the magnetic susceptibility of
the same system. The further anomaly observed in the resistivity of this system
at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo
origin. | cond-mat_str-el |
Heisenberg Necklace Model in Magnetic Field: Motivated by the experimental realizations of nearly one-dimensional spin-1/2
Heisenberg model found in chain cuprates SrCuO$_2$ and Sr$_2$CuO$_3$, which
remain in a quantum-critical Luttinger liquid state down to temperatures that
are much lower than in-chain exchange coupling, we consider the perturbation to
this state caused by interactions with nuclear spins on the same sites. We
study the low-energy sector of the Heisenberg Necklace model and estimate the
effect of the coupling between the nuclear and the electronic spins on the
overall spins dynamics and its dependence on the magnetic field. We find that
the Necklace model has a characteristic energy scale, $\Lambda \sim
J^{1/3}(\gamma I)^{2/3}$, at which the coupling between spins of the necklace
and the spins of the Heisenberg chain becomes strong. In the strong magnetic
field $\mu_B B > \Lambda$ the low energy spectrum is gapless, but two gapless
bosonic modes have different velocities whose ratio at strong fields approaches
a universal number, $\sqrt 2 +1$. In the case of Sr$_2$CuO$_3$ the energy scale
$\Lambda $ is sizable and comparable to the Neel ordering temperature induced
by the inter-chain coupling, and thus could noticeably modify the low
temperature magnon dynamics. We further find that the above energy scale is
insensitive to strong magnetic field, $\mu_B B \gg \Lambda \sim J^{1/3}(\gamma
I)^{2/3}$, and therefore the interaction with nuclear spins cannot lead to
unusually strong magnetic field dependence of the magnon spectrum observed by
ESR in Sr$_2$CuO$_3$, which has been attributed to the magnon interaction with
the Higgs mode. | cond-mat_str-el |
Theory of electronic transport through a triple quantum dot in the
presence of magnetic field: Theory of electronic transport through a triangular triple quantum dot
subject to a perpendicular magnetic field is developed using a tight binding
model. We show that magnetic field allows to engineer degeneracies in the
triple quantum dot energy spectrum. The degeneracies lead to zero electronic
transmission and sharp dips in the current whenever a pair of degenerate states
lies between the chemical potential of the two leads. These dips can occur with
a periodicity of one flux quantum if only two levels contribute to the current
or with half flux quantum if the three levels of the triple dot contribute. The
effect of strong bias voltage and different lead-to-dot connections on
Aharonov-Bohm oscillations in the conductance is also discussed. | cond-mat_str-el |
Supersymmetry on the honeycomb lattice: resonating charge stripes,
superfrustration, and domain walls: We study a model of spinless fermions on the honeycomb lattice with
nearest-neighbor exclusion and extended repulsive interactions that exhibits
`lattice supersymmetry' [P. Fendley, K. Schoutens, and J. de Boer, Phys. Rev.
Lett. 90, 120402 (2003)]. Using a combination of exact diagonalization of large
($N\leq56$ site) systems, mean-field numerics, and symmetry analysis, we
establish a rich phase structure as a function of fermion density, that
includes non-Fermi liquid behavior, resonating charge stripes, domain-wall and
bubble physics, and identify a finite range of fillings with extensive ground
state degeneracy and both gapped and gapless spectra. We comment on the
stability of our results to relaxing the stringent requirements for
supersymmetry, and on their possible broader relevance to systems of
strongly-correlated electrons with extended repulsive interactions. | cond-mat_str-el |
Quantum Entanglement as a Diagnostic of Phase Transitions in Disordered
Fractional Quantum Hall Liquids: We investigate the disorder-driven phase transition from a fractional quantum
Hall state to an Anderson insulator using quantum entanglement methods. We find
that the transition is signaled by a sharp increase in the sensitivity of a
suitably averaged entanglement entropy with respect to disorder -- the
magnitude of its disorder derivative appears to diverge in the thermodynamic
limit. We also study the level statistics of the entanglement spectrum as a
function of disorder. However, unlike the dramatic phase-transition signal in
the entanglement entropy derivative, we find a gradual reduction of level
repulsion only deep in the Anderson insulating phase. | cond-mat_str-el |
Bethe ansatz description of edge-localization in the open-boundary XXZ
spin chain: At large values of the anisotropy \Delta, the open-boundary Heisenberg
spin-1/2 chain has eigenstates displaying localization at the edges. We present
a Bethe ansatz description of this `edge-locking' phenomenon in the entire
\Delta>1 region. We focus on the simplest spin sectors, namely the highly
polarized sectors with only one or two overturned spins, i.e., one-particle and
two-particle sectors. Edge-locking is associated with pure imaginary solutions
of the Bethe equations, which are not commonly encountered in periodic chains.
In the one-particle case, at large anisotropies there are two eigenstates with
imaginary Bethe momenta, related to localization at the two edges. For any
finite chain size, one of the two solutions become real as the anisotropy is
lowered below a certain value. For two particles, a richer scenario is
observed, with eigenstates having the possibility of both particles locked on
the same or different edge, one locked and the other free, and both free either
as single magnons or as bound composites corresponding to `string' solutions.
For finite chains, some of the edge-locked spins get delocalized at certain
values of the anisotropy (`exceptional points'), corresponding to imaginary
solutions becoming real. We characterize these phenomena thoroughly by
providing analytic expansions of the Bethe momenta for large chains, large
anisotropy, and near the exceptional points. In the large-chain limit all the
exceptional points coalesce at the isotropic point (\Delta=1) and edge-locking
becomes stable in the whole \Delta>1 region. | cond-mat_str-el |
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