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The Effect of $f$-$d$ Magnetic Coupling in Multiferroic $R$MnO$_3$
Crystals: We have established detailed magnetoelectric phase diagrams of
(Eu$_{0.595}$Y$_{0.405}$)$_{1-x}$Tb$_x$MnO$_3$ ($0 \le x \le 1$) and
(Eu,Y)$_{1-x}$Gd$_x$MnO$_3$ ($0 \le x \le 0.69$), whose average ionic radii of
$R$-site ($R$: rare earth) cations are equal to that of Tb$^{3+}$, in order to
reveal the effect of rare earth 4$f$ magnetic moments on the magnetoelectric
properties. In spite of the same $R$-site ionic radii, the magnetoelectric
properties of the two systems are remarkably different from each other. A small
amount of Tb substitution on $R$ sites ($x \sim 0.2$) totally destroys
ferroelectric polarization along the a axis ($P_a$), and an increase in Tb
concentration stabilizes the $P_c$ phase. On the other hand, Gd substitution
($x \sim 0.2$) extinguishes the $P_c$ phase, and slightly suppresses the $P_a$
phase. These results demonstrate that the magnetoelectric properties of
$R$MnO$_3$ strongly depend on the characteristics of the rare earth 4$f$
moments. | cond-mat_str-el |
Competition between Interactions and Randomness in Photoinduced
Synchronization of Charge Oscillations on a Dimer Lattice: The synchronization of charge oscillations after photoexcitation that has
been realized through the emergence of an electronic breathing mode on dimer
lattices is studied here from the viewpoint of the competition between
interactions and randomness. We employ an extended Hubbard model at
three-quarter filling on a simple dimer lattice and add random numbers to all
transfer integrals between nearest-neighbor sites. Photoinduced dynamics are
calculated using the time-dependent Schr\"odinger equation by the exact
diagonalization method. Although the randomness tends to unsynchronize charge
oscillations on different bonds during and after photoexcitation, sufficiently
strong on-site repulsion $U$ overcomes this effect and synchronizes these
charge oscillations some time after strong photoexcitation. The degree of
synchronization is evaluated using an order parameter that is derived from the
time profiles of the current densities on all bonds. As to the nearest-neighbor
interaction $V$, if $V$ is weakly attractive, it increases the order parameter
by facilitating the charge oscillations. The relevance of these findings to
previously reported experimental and theoretical results for the organic
conductor $\kappa$-(bis[ethylenedithio]tetrathiafulvalene)$_2$Cu[N(CN)$_2$]Br
is discussed. | cond-mat_str-el |
New gap equation for a marginal Fermi liquid: Assuming a phenomenological self-energy $Im \Sigma(\omega) \sim
|\omega|^{\beta\}, (\beta=1 $), which becomes gapped below $T_c$, we derived a
new gap equation. The new gap equation contains the effect of the kinetic
energy gain upon developing a superconducting order parameter. However, this
new kinetic energy gain mechanism works only for a repulsive pairing potential
leading to a s-wave state. In this case, compared to the usual potential energy
gain in the superconducting state as in the BCS gap equation, the kinetic
energy gain is more effective to easily achieve a high critical temperature
$T_c$, since it is naturally Fermi energy scale. In view of the experimental
evidences of the d-wave pairing state in the hole-doped copper-oxide high-$T_c$
superconductors, we discuss the implications of our results. | cond-mat_str-el |
New Paired-Wavefunction for the Frustrated Antiferromagnetic Spin-Half
Chain: I propose a new paired-wavefunction with a parameter that continuously
interpolates from the 1D Jastrow-product to the Majumdar-Ghosh
dimer-wavefunction appropriate for the frustrated Heisenberg $S = 1/2$
antiferromagnet. This spin paired-state constructed in $S_z$ basis is an
alternative to the well-known resonating-valence-bond basis state for
describing the $S = 0$ ground-state with no apparent long-range spin order.
Some numerical evidences are presented. | cond-mat_str-el |
Fragile Mott Insulators: We prove that there exists a class of crystalline insulators, which we call
"fragile Mott insulators" which are not adiabatically connected to any sort of
band insulator provided time-reversal and certain point-group symmetries are
respected, but which are otherwise unspectacular in that they exhibit no
topological order nor any form of fractionalized quasiparticles. Different
fragile Mott insulators are characterized by different nontrivial
one-dimensional representations of the crystal point group. We illustrate this
new type of insulators with two examples: the d-Mott insulator discovered in
the checkerboard Hubbard model at half-filling and the
Affleck-Kennedy-Lieb-Tasaki insulator on the square lattice. | cond-mat_str-el |
Unusual behaviors in the transport properties of REFe$_{4}$P$_{12}$ (RE:
La, Ce, Pr, and Nd): We have investigated the resistivity ($\rho$), thermoelectric power (TEP) and
Hall coefficient ($R_{H}$) on high quality single crystals of
REFe$_{4}$P$_{12}$. TEP in CeFe$_{4}$P$_{12}$ is extremely large ($\sim$
0.5mV/K at 290K) with a peak of $\sim$ 0.75mV/K at around 65K. The Hall
mobility also shows a peak at $\sim$ 65K, suggesting carriers with heavy masses
developed at lower temperatures related with the f-hybridized band. Both Pr-
and Nd- systems exhibit an apparent increase of $\rho$ with decreasing
temperature far above their magnetic transition temperatures. In the same
temperature ranges, TEP exhibits unusually large absolute values of -50$\mu$V/K
for PrFe$_{4}$P$_{12}$ and -15$\mu$V/K for NdFe$_{4}$P$_{12}$, respectively.
For PrFe$_{4}$P$_{12}$, such anomalous transport properties suggest an unusual
ground state, possibly related with the Quadrupolar Kondo effect. | cond-mat_str-el |
Antiferromagnetic chiral spin density wave and strain-induced Chern
insulator in the square lattice Hubbard model with frustration: We employ the Hartree-Fock approximation to identify the magnetic ground
state of the Hubbard model on a frustrated square lattice. We investigate the
phase diagram as a function of the Coulomb repulsion's strength $U$, and the
ratio $t'/t$ between the nearest and next nearest neighbor hoppings $t$ and
$t'$. At half-filling and for a sufficiently large $U$, an antiferromagnetic
chiral spin density wave order with nonzero spin chirality emerges as the
ground state in a wide regime of the phase diagram near $t'/t=1/\sqrt{2}$,
where the Fermi surface is well-nested for both $(\pi,\pi)$ and
$(\pi,0)/(0,\pi)$ wave vectors. This triple-${\bf Q}$ chiral phase is
sandwiched by a single-${\bf Q}$ N\'{e}el phase and a double-${\bf Q}$ coplanar
spin-vortex crystal phase, at smaller and larger $t'/t$, respectively. The
energy spectrum in the chiral spin density wave phase consists of four pairs of
degenerate bands. These give rise to two pairs of Dirac cones with the same
chirality at the point $({\pi \over 2},{\pi\over 2})$ of the Brillouin zone. We
demonstrate that the application of a diagonal strain induces a $d_{xy}$-wave
next nearest neighbor hopping which, in turn, opens gaps in the two Dirac cones
with opposite masses. As a result, four pairs of well-separated
topologically-nontrivial bands emerge, and each pair of those contributes with
a Chern number $\pm1$. At half-filling, this leads to a zero total Chern number
and renders the topologically-notrivial properties observable only in the ac
response regime. Instead, we show that at $3/4$ filling, the triple-${\bf Q}$
chiral phase yields a Chern insulator exhibiting the quantum anomalous Hall
effect. | cond-mat_str-el |
Localized moments and the stability of antiferromagnetic order in Yb3Pt4: We present here the results of electrical resistivity {\rho}, magnetization
M, ac susceptibility \c{hi}ac', and specific heat CM measurements that have
been carried out on single crystals of Yb3Pt4 over a wide range of fields and
temperatures. The 2.4-K N\'eel temperature that is found in zero field
collapses under field to a first-order transition TN=0 at BCEP=1.85 T. In the
absence of antiferromagnetic order, the specific heat CM(T,B), the
magnetization M(T,B), and even the resistivity {\rho}(T,B) all display B/T
scaling, indicating that they are dominated by strong paramagnetic
fluctuations, where the only characteristic energy scale results from the
Zeeman splitting of an energetically isolated, Yb doublet ground state. This
paramagnetic scattering disappears with the onset of antiferromagnetic order,
revealing Fermi liquid behavior {\Delta}{\rho}=AT2 that persists up to the
antiferromagnetic phase line TN(B), but not beyond. The first-order character
of TN=0 and the ubiquity of the paramagnetic fluctuations imply that
non-Fermi-liquid behaviors are absent in Yb3Pt4. In contrast to heavy fermions
such as YbRh2Si2, Yb3Pt4 represents an extremely simple regime of f-electron
behavior where the Yb moments and conduction electrons are almost decoupled,
and where Kondo physics plays little role. | cond-mat_str-el |
An effect of the uniaxial strain on the temperature of Bose-Einstein
condensation of the intersite bipolarons: We have studied an effect of uniaxial strain to the temperature of
Bose-Einstein condensation of intersite bipolarons within the framework of
Extended Holstein-Hubbard model. Uniaxial lattice strains are taken into an
account by introducing a generalized density-displacement type force for
electron-lattice interaction. Associating the superconducting critical
temperature $T_c$ with the temperature of Bose-Einstein condensation $T_{BEC}$
of intersite bipolarons we have calculated strain derivatives of $T_{BEC}$ and
satisfactorily explained the results of the experiments on La-based high-$T_c$
films. | cond-mat_str-el |
Staggered Flux State in Two-Dimensional Hubbard Models: The stability and other properties of a staggered flux (SF) state or a
correlated d-density wave state are studied for the Hubbard (t-t'-U) model on
extended square lattices, as a low-lying state that competes with the
d(x2-y2)-wave superconductivity (d-SC) and possibly causes the pseudogap
phenomena in underdoped high-Tc cuprates and organic kappa-BEDT-TTF salts. In
calculations, a variational Monte Carlo method is used. In the trial wave
function, a configuration-dependent phase factor, which is vital to treat a
current-carrying state for a large U/t, is introduced in addition to ordinary
correlation factors. Varying U/t, t'/t, and the doping rate (delta)
systematically, we show that the SF state becomes more stable than the normal
state (projected Fermi sea) for a strongly correlated (U/t\gtrsim 5) and
underdoped (delta\lesssim 0.16) area. The decrease in energy is sizable,
particularly in the area where Mott physics prevails and the circular current
(order parameter) is strongly suppressed. These features are consistent with
those for the t-J model. The effect of the frustration t'/t plays a crucial
role in preserving charge homogeneity and appropriately describing the behavior
of hole- and electron-doped cuprates and kappa-BEDT-TTF salts. We argue that
the SF state does not coexist with d-SC and is not a `normal state' from which
d-SC arises. We also show that a spin current (flux or nematic) state is never
stabilized in the same regime. | cond-mat_str-el |
Nuclear magnetic resonance signature of the spin-nematic phase in
LiCuVO$_{4}$ at high magnetic fields: We report a 51V nuclear magnetic resonance investigation of the frustrated
spin-1/2 chain compound LiCuVO4, performed in pulsed magnetic fields and
focused on high-field phases up to 55 T. For the crystal orientations H // c
and H // b we find a narrow field region just below the magnetic saturation
where the local magnetization remains uniform and homogeneous, while its value
is field dependent. This behavior is the first microscopic signature of the
spin-nematic state, breaking spin-rotation symmetry without generating any
transverse dipolar order, and is consistent with theoretical predictions for
the LiCuVO4 compound. | cond-mat_str-el |
Stability of the doped antiferromagnetic state of the t-t'-Hubbard model: The next-nearest-neighbour hopping term t' is shown to stabilize the AF state
of the doped Hubbard model with respect to transverse perturbations in the
order- parameter by strongly suppressing the intraband particle-hole processes.
For a fixed sign of t', this stabilization is found to be significantly
different for electron and hole doping, which qualitatively explains the
observed difference in the degree of robustness of the AF state in the
electron-doped (Nd_{2-x}Ce_{x}CuO_{4}) and hole-doped (La_{2-x}Sr_{x}CuO_{4})
cuprates. The t'-U phase diagram is obtained for both signs of the t' term,
showing the different regions of stability and instability of the doped
antiferromagnet. Doping is shown to suppress the t'-induced frustration due to
the competing interaction J'. A study of transverse spin fluctuations in the
metallic AF state reveals that the decay of magnons into particle-hole
excitations yields an interesting low-energy result \Gamma \sim \omega for
magnon damping. | cond-mat_str-el |
First order transition from ferromagnetism to antiferromagnetism in
Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$: a magnetotransport study: The magnetotransport behaviour is investigated in detail across the first
order magnetic phase transition from ferromagnetic to antiferromagnetic state
in polycrystalline Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ sample. The study clearly
brings out various generic features associated with a first order transition,
viz., hysteresis, phase coexistence, supercooling and superheating, presence
and limits of the metastable regimes. These results of magnetotransport study
exhibit and support all the interesting thermomagnetic history effects that
were observed in our earlier dc-magnetisation study on the same sample. Most
notable here is the initial (or virgin) resistivity vs. field curve lying
outside the hysteretic "butterfly shaped" magnetoresistivity loops obtained on
cyclying the magnetic field between high enough positive and negative
strengths. These findings, bearing one-to-one similarity with the data obtained
in their magnetic counterpart (i.e., dc-magnetisation), have been ascribed an
origin due to the arresting of this first order transition kinetics at low
temperature and high magnetic field. | cond-mat_str-el |
$\require{mhchem}$Quantum paramagnetism in the decorated square-kagome
antiferromagnet $\ce{Na6Cu7BiO4(PO4)4Cl3}$: $\require{mhchem}$The square-kagome lattice Heisenberg antiferromagnet is a
highly frustrated Hamiltonian whose material realizations have been scarce. We
theoretically investigate the recently synthesized $\ce{Na6Cu7BiO4(PO4)4Cl3}$
where a Cu$^{2+}$ spin-$1/2$ square-kagome lattice (with six site unit cell) is
decorated by a seventh magnetic site alternatingly above and below the layers.
The material does not show any sign of long-range magnetic order down to 50 mK
despite a Curie-Weiss temperature of $-212$ K indicating a quantum paramagnetic
phase. Our DFT energy mapping elicits a purely antiferromagnetic Hamiltonian
that features longer range exchange interactions beyond the pure square-kagome
model and, importantly, we find the seventh site to be strongly coupled to the
plane. We combine two variational Monte Carlo approaches,
pseudo-fermion/Majorana functional renormalization group and Schwinger-Boson
mean field calculations to show that the complex Hamiltonian of
$\ce{Na6Cu7BiO4(PO4)4Cl3}$ still features a nonmagnetic ground state. We
explain how the seventh Cu$^{2+}$ site actually aids the stabilization of the
disordered state. We predict static and dynamic spin structure factors to guide
future neutron scattering experiments. | cond-mat_str-el |
Symmetry Protected Topological Order by Folding a One-Dimensional
Spin-$1/2$ Chain: We present a toy model with a Hamiltonian $H^{(2)}_T$ on a folded
one-dimensional spin chain. The non-trivial ground states of $H^{(2)}_T$ are
separated by a gap from the excited states. By analyzing the symmetries in the
model, we find that the topological order is protected by a $\mathbb{Z}_2$
global symmetry. However, by using perturbation series and excluding thermal
effects, we show that the $\mathbb{Z}_2$ symmetry is stable in comparison to a
standard nearest-neighbor Ising model with a Hamiltonian $H_I$. We find that
$H^{(2)}_T$ is a member of a family of Hamiltonians that are adiabatically
connected to $H_I$. Furthermore, the generalizations of this class of
Hamiltonians, their adiabatic connection to $H_I$, and the relation to quantum
error-correcting codes are discussed. Finally, we show the correspondence
between the two ground states of $H^{(2)}_T$ and the unpaired Majorana modes,
and provide numerical examples. | cond-mat_str-el |
Polaron with Quadratic Electron-phonon Interaction: We present the first numerically exact study of a polaron with quadratic
coupling to the oscillator displacement, using two alternative methodological
developments. Our results cover both anti-adiabatic and adiabatic regimes and
the entire range of electron-phonon coupling $g_2$, from the system's stability
threshold at attractive $g_2=-1$ to arbitrary strong repulsion at $g_2 \gg 1$.
Key properties of quadratic polarons prove dramatically different from their
linear counterparts. They (i) are insensitive even to large quadratic coupling
except in the anti-adiabatic limit near the threshold of instability at
attraction; (ii) depend only on the adiabatic ratio but are insensitive to the
electron dispersion and dimension of space; (iii) feature weak lattice
deformations even at the instability point. Our results are of direct relevance
to properties of electrons at low densities in polar materials, including
recent proposals for their superconducting states. | cond-mat_str-el |
Optical manipulation of electronic dimensionality in a quantum material: Exotic phenomenon can be achieved in quantum materials by confining
electronic states into two dimensions. For example, relativistic fermions are
realised in a single layer of carbon atoms, the quantized Hall effect can
result from two-dimensional (2D) systems, and the superconducting transition
temperature can be enhanced significantly in a one-atomic-layer material.
Ordinarily, 2D electronic system can be obtained by exfoliating the layered
materials, growing monolayer materials on substrates, or establishing
interfaces between different materials. Herein, we use femtosecond infrared
laser pulses to invert the periodic lattice distortion sectionally in a
three-dimensional (3D) charge density wave material, creating macroscopic
domain walls of transient 2D ordered electronic states with exotic properties.
The corresponding ultrafast electronic and lattice dynamics are captured by
time- and angle-resolved photoemission spectroscopy and MeV ultrafast electron
diffraction. Surprisingly, a novel energy gap state, which might be a signature
of light-induced superconductivity, is identified in the photoinduced 2D domain
wall near the surface. Such optical modulation of atomic motion is a new path
to realise 2D electronic states and will be a new platform for creating novel
phases in quantum materials. | cond-mat_str-el |
Mott Transition of MnO under Pressure: Comparison of Correlated Band
Theories: The electronic structure, magnetic moment, and volume collapse of MnO under
pressure are obtained from four different correlated band theory methods; local
density approximation + Hubbard U (LDA+U), pseudopotential self-interaction
correction (pseudo-SIC), the hybrid functional (combined local exchange plus
Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each
method treats correlation among the five Mn 3d orbitals (per spin), including
their hybridization with three O $2p$ orbitals in the valence bands and their
changes with pressure. The focus is on comparison of the methods for rocksalt
MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa
range). Each method predicts a first-order volume collapse, but with variation
in the predicted volume and critical pressure. Accompanying the volume collapse
is a moment collapse, which for all methods is from high-spin to low-spin (5/2
to 1/2), not to nonmagnetic as the simplest scenario would have. The specific
manner in which the transition occurs varies considerably among the methods:
pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives
insulator-to-insulator and the hybrid method gives an insulator-to-semimetal
transition. Projected densities of states above and below the transition are
presented for each of the methods and used to analyze the character of each
transition. In some cases the rhombohedral symmetry of the
antiferromagnetically ordered phase clearly influences the character of the
transition. | cond-mat_str-el |
Conductance and Kondo effect of a controlled single atom contact: The tip of a low-temperature scanning tunneling microscope is brought into
contact with individual Kondo impurities (cobalt atoms) adsorbed on a Cu(100)
surface. A smooth transition from the tunneling regime to a point contact with
a conductance of $G\approx\text{G}_0$ occurs. Spectroscopy in the contact
regime, {\it i. e.}, at currents in a $\mu\text{A}$ range was achieved. A
modified line shape is observed indicating a significant change of the Kondo
temperature $T_{\text{K}}$ at contact. Model calculations indicate that the
proximity of the tip shifts the cobalt $d$-band and thus affects
$T_{\text{K}}$. | cond-mat_str-el |
Probing Critical Surfaces in Momentum Space Using Real-Space
Entanglement Entropy: Bose versus Fermi: A co-dimension one critical surface in the momentum space can be either a
familiar Fermi surface, which separates occupied states from empty ones in the
non-interacting fermion case, or a novel Bose surface, where gapless bosonic
excitations are anchored. Their presence gives rise to logarithmic violation of
entanglement entropy area law. When they are convex, we show that the shape of
these critical surfaces can be determined by inspecting the leading logarithmic
term of real space entanglement entropy. The fundamental difference between a
Fermi surface and a Bose surface is revealed by the fact that the logarithmic
terms in entanglement entropies differ by a factor of two: $S^{Bose}_{log} = 2
S^{Fermi}_{log}$, even when they have identical geometry. Our method has
remarkable similarity with determining Fermi surface shape using quantum
oscillation. We also discuss possible probes of concave critical surfaces in
momentum space. | cond-mat_str-el |
Spin-orbit coupling and ESR theory for carbon nanotubes: A theoretical description of ESR in 1D interacting metals is given, with
primary emphasis on carbon nanotubes. The spin-orbit coupling is derived, and
the resulting ESR spectrum is analyzed by field theory and exact
diagonalization. Drastic differences in the ESR spectra of single-wall and
multi-wall nanotubes are found. For single-wall tubes, the predicted double
peak spectrum could reveal spin-charge separation. | cond-mat_str-el |
Quantum phase transition in the one-dimensional compass model: We introduce a one-dimensional model which interpolates between the Ising
model and the quantum compass model with frustrated pseudospin interactions
$\sigma_i^z\sigma_{i+1}^z$ and $\sigma_i^x\sigma_{i+1}^x$, alternating between
even/odd bonds, and present its exact solution by mapping to quantum Ising
models. We show that the nearest neighbor pseudospin correlations change
discontinuosly and indicate divergent correlation length at the first order
quantum phase transition. At this transition one finds the disordered ground
state of the compass model with high degeneracy $2\times 2^{N/2}$ in the limit
of $N\to\infty$. | cond-mat_str-el |
Hidden Integrability of a Kondo Impurity in an Unconventional Host: We study a spin-1/2 Kondo impurity coupled to an unconventional host in which
the density of band states vanishes either precisely at (``gapless'' systems)
or on some interval around the Fermi level (``gapped''systems). Despite an
essentially nonlinear band dispersion, the system is proven to exhibit hidden
integrability and is diagonalized exactly by the Bethe ansatz. | cond-mat_str-el |
A High Pressure Neutron Study of Colossal Magnetoresistant
NdMnAsO0.95F0.05: A high pressure neutron diffraction study of the oxypnictide NdMnAsO0.95F0.05
has been performed at temperatures of 290 K - 383 K and pressures up to 8.59
GPa. The results demonstrate that the antiferromagnetic order of the Mn spins
is robust to pressures of up to 8.59 GPa. TN is enhanced from 360 K to 383 K
upon applying an external pressure of 4.97 GPa, a rate of 4.63 K/GPa.
NdMnAsO0.95F0.05 is shown to violate Bloch's rule which would suggest that
NdMnAsO0.95F0.05 is on the verge of a localised to itinerant transition. There
is no evidence of a structural transition but applied pressure tends to result
in more regular As-Mn-As and Nd-O-Nd tetrahedra. The unit cell is significantly
more compressible along the c-axis than the a-axis, as the inter-layer coupling
is weaker than the intrinsic bonds contained within NdO and MnAs slabs. | cond-mat_str-el |
Spin Configuration in the 1/3 Magnetization Plateau of Azurite
Determined by NMR: High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local
spin polarization in the 1/3 magnetization plateau of azurite,
Cu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond
antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the
Cu2 (dimer) sites is found to be field independent, negative and strongly
anisotropic, corresponding to $\approx$10 % of fully polarized spin in a
$d$-orbital. This is close to the expected configuration of the "quantum"
plateau, where a singlet state is stabilized on the dimer. However, the
observed non-zero spin polarization points to some triplet admixture, induced
by strong asymmetry of the diamond bonds $J_1$ and $J_3$. | cond-mat_str-el |
Exploring the spin-orbital ground state of Ba3CuSb2O9: Motivated by the absence of both spin freezing and a cooperative Jahn-Teller
effect at the lowest measured temperatures, we study the ground state of
Ba3CuSb2O9. We solve a general spin-orbital model on both the honeycomb and the
decorated honeycomb lattice, revealing rich phase diagrams. The spin-orbital
model on the honeycomb lattice contains an SU(4) point, where previous studies
have shown the existence of a spin-orbital liquid with algebraically decaying
correlations. For realistic parameters on the decorated honeycomb lattice, we
find a phase that consists of clusters of nearest-neighbour spin singlets,
which can be understood in terms of dimer coverings of an emergent square
lattice. While the experimental situation is complicated by structural
disorder, we show qualitative agreement between our theory and a range of
experiments. | cond-mat_str-el |
The one dimensional Kondo lattice model at partial band filling: The Kondo lattice model introduced in 1977 describes a lattice of localized
magnetic moments interacting with a sea of conduction electrons. It is one of
the most important canonical models in the study of a class of rare earth
compounds, called heavy fermion systems, and as such has been studied
intensively by a wide variety of techniques for more than a quarter of a
century. This review focuses on the one dimensional case at partial band
filling, in which the number of conduction electrons is less than the number of
localized moments. The theoretical understanding, based on the bosonized
solution, of the conventional Kondo lattice model is presented in great detail.
This review divides naturally into two parts, the first relating to the
description of the formalism, and the second to its application. After an
all-inclusive description of the bosonization technique, the bosonized form of
the Kondo lattice hamiltonian is constructed in detail. Next the
double-exchange ordering, Kondo singlet formation, the RKKY interaction and
spin polaron formation are described comprehensively. An in-depth analysis of
the phase diagram follows, with special emphasis on the destruction of the
ferromagnetic phase by spin-flip disorder scattering, and of recent numerical
results. The results are shown to hold for both antiferromagnetic and
ferromagnetic Kondo lattice. The general exposition is pedagogic in tone. | cond-mat_str-el |
LaMnO$_3$ is a Mott Insulator: an precise definition and an evaluation
of the local interaction strength: We compare the interaction parameters measured on LaMnO$_3$ to single site
dynamical mean field estimates of the critical correlation strength needed to
drive a Mott transition, finding that the total correlation strength
(electron-electron plus electron-lattice) is very close to but slightly larger
than the critical value, while if the electron lattice interaction is neglected
the model is metallic. Our results emphasize the importance of additional
physics including the buckling of the Mn-O-Mn bonds. | cond-mat_str-el |
"Self-Dual" Quantum Critical Point on the surface of $3d$ Topological
Insulator: In the last few years a lot of exotic and anomalous topological phases were
constructed by proliferating the vortex like topological defects on the surface
of the $3d$ topological insulator (TI). In this work, rather than considering
topological phases at the boundary, we will study quantum critical points
driven by vortex like topological defects. In general we will discuss a
$(2+1)d$ quantum phase transition described by the following field theory:
$\mathcal{L} = \bar{\psi}\gamma_\mu (\partial_\mu - i a_\mu) \psi +
|(\partial_\mu - i k a_\mu)\phi|^2 + r |\phi|^2 + g |\phi|^4$, with tuning
parameter $r$, arbitrary integer $k$, Dirac fermion $\psi$ and complex scalar
bosonic field $\phi$ which both couple to the same $(2+1)d$ dynamical
noncompact U(1) gauge field $a_\mu$. The physical meaning of these
quantities/fields will be explained in the text. We demonstrate that this
quantum critical point has a quasi self-dual nature. And at this quantum
critical point, various universal quantities such as the electrical
conductivity, and scaling dimension of gauge invariant operators can be
calculated systematically through a $1/k^2$ expansion, based on the observation
that the limit $k \rightarrow + \infty$ corresponds to an ordinary $3d$ XY
transition. | cond-mat_str-el |
Hydrostatic pressure effect on Co-based honeycomb magnet BaCo2(AsO4)2: The honeycomb antiferromagnet BaCo2(AsO4)2, in which small in-plane magnetic
fields (H1 = 0.26 T and H2 = 0.52 T at T = 1.8 K < TN = 5.4 K) induce two
magnetic phase transitions, has attracted attention as a possible candidate
material for the realization of Kitaev physics based on the 3d element Co2+.
Here, we report on the change of the transition temperature TN and the critical
fields H1 and H2 of BaCo2(AsO4)2 with hydrostatic pressure up to ~ 20 kbar, as
determined from magnetization and specific heat measurements. Within this
pressure range, a marginal increase of the magnetic ordering temperature is
observed. At the same time, the critical fields are changed significantly (up
to ~ 25-35 %). Specifically, we find that H1 is increased with hydrostatic
pressure, i.e., the antiferromagnetic state is stabilized with hydrostatic
pressure, whereas H2, which was previously associated with a transition into a
proposed Kitaev spin liquid state, decreases with increasing pressure. These
results put constraints on the magnetic models that are used to describe the
low-temperature magnetic properties of BaCo2(AsO4)2. | cond-mat_str-el |
An exactly size consistent geminal power via Jastrow factor networks in
a local one particle basis: The accurate but expensive product of geminals ansatz may be approximated by
a geminal power, but this approach sacrifices size consistency. Here we show
both analytically and numerically that a size consistent form very similar to
the product of geminals can be recovered using a network of location specific
Jastrow factors. Upon variational energy minimization, the network creates
particle number projections that remove the charge fluctuations responsible for
size inconsistency. This polynomial cost approach captures strong many-electron
correlations, giving a maximum error of just 1.8 kcal/mol during the
double-bond dissociation of H2O in an STO-3G atomic orbital basis. | cond-mat_str-el |
Deconfined quantum criticality driven by Dirac fermions in SU(2)
antiferromagnets: Quantum electrodynamics in 2+1 dimensions is an effective gauge theory for
the so called algebraic quantum liquids. A new type of such a liquid, the
algebraic charge liquid, has been proposed recently in the context of
deconfined quantum critical points [R. K. Kaul {\it et al.}, Nature Physics
{\bf 4}, 28 (2008)]. In this context, we show by using the renormalization
group in $d=4-\epsilon$ spacetime dimensions, that a deconfined quantum
critical point occurs in a SU(2) system provided the number of Dirac fermion
species $N_f\geq 4$. The calculations are done in a representation where the
Dirac fermions are given by four-component spinors. The critical exponents are
calculated for several values of $N_f$. In particular, for $N_f=4$ and
$\epsilon=1$ ($d=2+1$) the anomalous dimension of the N\'eel field is given by
$\eta_N=1/3$, with a correlation length exponent $\nu=1/2$. These values change
considerably for $N_f>4$. For instance, for $N_f=6$ we find $\eta_N\approx
0.75191$ and $\nu\approx 0.66009$. We also investigate the effect of chiral
symmetry breaking and analyze the scaling behavior of the chiral holon
susceptibility, $G_\chi(x)\equiv<\bar \psi(x)\psi(x)\bar \psi(0)\psi(0)>$. | cond-mat_str-el |
Flat bands and $Z_2$ topological phases in a non-Abelian kagome lattice: We introduce a non-Abelian kagome lattice model that has both time-reversal
and inversion symmetries and study the flat band physics and topological phases
of this model. Due to the coexistence of both time-reversal and inversion
symmetries, the energy bands consist of three doubly degenerate bands whose
energy and conditions for the presence of flat bands could be obtained
analytically, allowing us to tune the flat band with respect to the other two
dispersive bands from the top to the middle and then to the bottom of the three
bands. We further study the gapped phases of the model and show that they
belong to the same phase as the band gaps only close at discrete points of the
parameter space, making any two gapped phases adiabatically connected to each
other without closing the band gap. Using the Pfaffian approach based on the
time-reversal symmetry and parity characterization from the inversion symmetry,
we calculate the bulk topological invariants and demonstrate that the unique
gapped phases belong to the $Z_2$ quantum spin Hall phase, which is further
confirmed by the edge state calculations. | cond-mat_str-el |
Defect-induced edge ferromagnetism and fractional spin excitations of
the SU(4) $π$-flux Hubbard model on honeycomb lattice: Recently, a SU(4) $\pi$-flux Hubbard model on the honeycomb lattice has been
proposed to study the spin-orbit excitations of $\alpha$-ZrCl$_3$
[Phys.~Rev.~Lett. 121.097201~(2017)]. Based on this model with a zigzag edge,
we show the edge defects can induce edge flat bands that result in a SU(4) edge
ferromagnetism. We develop an effective one-dimensional interaction Hamiltonian
to study the corresponding SU(4) spin excitations. Remarkably, SU(4) spin
excitations of the edge ferromagnet appear as a continuum covering the entire
energy region rather than usual magnons. Through further entanglement entropy
analysis, we suggest that the continuum consists of fractionalized spin
excitations from the disappeared magnons, except for that from the
particle-hole Stoner excitations. Moreover, in ribbon systems with finite
widths, the disappeared magnons can be restored in the gap formed by the
finite-size effect and the optical branch of the restored magnons are found to
be topological nontrivial. | cond-mat_str-el |
Phase-Space Berry Phases in Chiral Magnets: Dzyaloshinskii-Moriya
Interaction and the Charge of Skyrmions: The semiclassical motion of electrons in phase space, x=(R, k), is influenced
by Berry phases described by a 6-component vector potential, A=(A^R, A^k). In
chiral magnets Dzyaloshinskii-Moriya (DM) interactions induce slowly varying
magnetic textures (helices and skyrmion lattices) for which all components of A
are important inducing effectively a curvature in mixed position and momentum
space. We show that for smooth textures and weak spin-orbit coupling phase
space Berry curvatures determine the DM interactions and give important
contributions to the charge. Using ab initio methods we calculate the strength
of DM interactions in MnSi in good agreement with experiment and estimate the
charge of skyrmions. | cond-mat_str-el |
Visualizing Strange Metallic Correlations in the 2D Fermi-Hubbard Model
with AI: Strongly correlated phases of matter are often described in terms of
straightforward electronic patterns. This has so far been the basis for
studying the Fermi-Hubbard model realized with ultracold atoms. Here, we show
that artificial intelligence (AI) can provide an unbiased alternative to this
paradigm for phases with subtle, or even unknown, patterns. Long- and
short-range spin correlations spontaneously emerge in filters of a
convolutional neural network trained on snapshots of single atomic species. In
the less well-understood strange metallic phase of the model, we find that a
more complex network trained on snapshots of local moments produces an
effective order parameter for the non-Fermi-liquid behavior. Our technique can
be employed to characterize correlations unique to other phases with no obvious
order parameters or signatures in projective measurements, and has implications
for science discovery through AI beyond strongly correlated systems. | cond-mat_str-el |
Exciton doublet in the Mott-Hubbard LiCuVO$_4$ insulator identified by
spectral ellipsometry: Spectroscopic ellipsometry was used to study the dielectric function of
LiCuVO$_{4}$, a compound comprised of chains of edge-sharing CuO$_4$
plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K.
For photon polarization along the chains, the data reveal a weak but
well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral
weight is strongly enhanced upon cooling near the magnetic ordering
temperature. We identify these features as an exciton doublet in the
Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction
between electrons on nearest and next-nearest neighbor sites along the chains.
Our results and methodology can be used to address the role of the long-range
Coulomb repulsion for compounds with doped copper-oxide chains and planes. | cond-mat_str-el |
Persistence of Ising-like easy-axis spin correlations in the
paramagnetic state of the spin-1 chain compound NiTe$_2$O$_5$: A $^{125}$Te nuclear magnetic resonance (NMR) study was carried out in the
paramagnetic state of the recently discovered quasi-one-dimensional spin-1
chain compound NiTe$_2$O$_5$. We observed that the $^{125}$Te NMR spectrum
splits into two in a magnetic field applied along the $c$ axis. Based on the
strong temperature variation of the relative intensity ratio of the split
lines, we infer that the line splitting arises from the two sublattice
susceptibilities induced in opposite directions along the chains. In great
support of this interpretation, a quantitative analysis of the spin-lattice
relaxation rate $T_1^{-1}$ and the Knight shift data unravels dominant
transverse spin fluctuations. We conclude that Ising-like uniaxial spin
correlations persist up to surprisingly high temperatures compared to the
exchange energy scales. Spin-charge coupling mechanism via a self-doping effect
may be important. | cond-mat_str-el |
Thermodynamic behavior of the XXZ Heisenberg s=1/2 chain around the
factorizing magnetic field: We have investigated the zero and finite temperature behaviors of the
anisotropic antiferromagnetic Heisenberg XXZ spin-1/2 chain in the presence of
a transverse magnetic field (h). The attention is concentrated on an interval
of magnetic field between the factorizing field (h_f) and the critical one
(h_c). The model presents a spin-flop phase for 0<h<h_f with an energy scale
which is defined by the long range antiferromagnetic order while it undergoes
an entanglement phase transition at h=h_f. The entanglement estimators clearly
show that the entanglement is lost exactly at h=h_f which justifies different
quantum correlations on both sides of the factorizing field. As a consequence
of zero entanglement (at h=h_f) the ground state is known exactly as a product
of single particle states which is the starting point for initiating a spin
wave theory. The linear spin wave theory is implemented to obtain the specific
heat and thermal entanglement of the model in the interested region. A double
peak structure is found in the specific heat around h=h_f which manifests the
existence of two energy scales in the system as a result of two competing
orders before the critical point. These results are confirmed by the low
temperature Lanczos data which we have computed. | cond-mat_str-el |
Lattice and orbital fluctuations in TiPO4: In the s = 1/2 antiferromagnetic spin chain material TiPO4 the formation of a
spin gap takes place in a two step process with two characteristic
temperatures, T*=111 K and TSP=74 K. We observe an unusual lattice dynamics
over a large temperature regime as well as evidence for an orbital instability
preceding the spin-Peierls transition. We relate different intrachain exchange
interactions of the high temperature compared to the spin-Peierls phase to a
modification of the orbital ordering pattern. In particular, our observation of
a high energy excitation of mixed electronic and lattice origin suggests an
exotic dimerization process different from other spin-Peierls materials. | cond-mat_str-el |
Fractional Quantum Hall Effect of Hard-Core Bosons in Topological Flat
Bands: Recent proposals of topological flat band (TFB) models have provided a new
route to realize the fractional quantum Hall effect (FQHE) without Landau
levels. We study hard-core bosons with short-range interactions in two
representative TFB models, one of which is the well known Haldane model (but
with different parameters). We demonstrate that FQHE states emerge with
signatures of even number of quasi-degenerate ground states on a torus and a
robust spectrum gap separating these states from higher energy spectrum. We
also establish quantum phase diagrams for the filling factor 1/2 and illustrate
quantum phase transitions to other competing symmetry-breaking phases. | cond-mat_str-el |
A time and spatially resolved quench of the fermionic Hubbard model
showing restricted equilibration: We investigate the quench of half-filled 1D and 2D fermionic Hubbard models
to models without Coulomb interaction. Since the time propagation is gaussian
we can use a variety of time-dependent quantum Monte Carlo methods to tackle
this problem without generating a dynamical sign problem. Using a continuous
time quantum Monte Carlo method (CTQMC) we achieve a system size of 128 sites
in 1D, and using a Blankenbecler-Scalapino-Sugar (BSS) type algorithm we were
able to simulate 20 x 20 square lattices. Applying these methods to study the
dynamics after the quench, we observe that the final state of the system can be
reasonably well described by a thermal single-particle density matrix that
takes the initial single particle conservation laws into account. The
characteristic decay towards this limit is found to be oscillatory with an
additional power law decay that depends on the dimensionality. This numerically
exact result is shown to compare favorable to mean-field approximations as well
as to perturbation theory. Furthermore we observe the information propagation
in the 1D-case in the charge charge and spin spin correlations and find that it
is linear with a velocity of roughly v = 4 in units of the hopping amplitude. | cond-mat_str-el |
Ground State of the Easy-Axis Rare-Earth Kagomé Langasite
Pr$_3$Ga$_5$SiO$_{14}$: We report muon spin relaxation ($\mu$SR) and $^{69,71}$Ga nuclear quadrupolar
resonance (NQR) local-probe investigations of the kagom\'e compound
Pr$_3$Ga$_5$SiO$_{14}$. Small quasi-static random internal fields develop below
40 K and persist down to our base temperature of 21 mK. They originate from
hyperfine-enhanced $^{141}$Pr nuclear magnetism which requires a non-magnetic
Pr$^{3+}$ crystal-field (CF) ground state. Besides, we observe a broad maximum
of the relaxation rate at $\simeq 10$ K which we attribute to the population of
the first excited magnetic CF level. Our results yield a Van-Vleck paramagnet
picture, at variance with the formerly proposed spin-liquid ground state. | cond-mat_str-el |
Supersolid phases of hardcore bosons on the square lattice: Correlated
hopping, next-nearest neighbor hopping and frustration: We discuss the appearance of supersolid phases for interacting hardcore
bosons on the square lattice when, in addition to the standard nearest neighbor
hopping and repulsion, correlated or next-nearest neighbor hopping is present.
Having in mind dimer-based quantum magnets in a field described by effective
bosonic models of this kind, we put special emphasis on a comparison between
the different cases of relative signs of the kinetic processes, which
correspond to unfrustrated or frustrated magnetic models. In the unfrustrated
case, we compare Quantum Monte Carlo simulations with a mean-field (classical)
approach, which is shown to give qualitatively correct results. Using this
classical approach for the frustrated case, we find that the phase diagram is
generically richer than in the unfrustrated case. We also investigate in detail
the differences between standard next-nearest neighbour and correlated hopping
over the diagonal, with the conclusion that both cases are similar if
checkerboard order is present at half-filling, while a supersolid phase can be
stabilized without any adjacent solid phase only in the case of correlated
hopping. | cond-mat_str-el |
Multipolar ordering from dynamical mean field theory with application to
CeB6: Magnetic and multipolar ordering in f electron systems takes place at low
temperatures of order 1-10 Kelvin. Combinations of first-principles with
many-body calculations for such low-energy properties of correlated materials
are challenging problems. We address multipolar ordering in f electron systems
based on the dynamical mean-field theory (DMFT) combined with density
functional theory. We derive the momentum-dependent multipolar susceptibilities
and interactions by solving the Bethe-Salpeter (BS) equation of the
two-particle Green's function. We apply the formalism to the prototypical
example of multipolar ordering CeB6, and demonstrate that the experimental
quadrupole transition is correctly reproduced. This first-principles formalism
based on DMFT and BS equation has applications which are beyond the reach of
the traditional RKKY formula. In particular, more itinerant electron systems
including 5f, 4d and 5d electrons can be addressed. | cond-mat_str-el |
Magnetic properties of the Anderson model: a local moment approach: We develop a local moment approach to static properties of the symmetric
Anderson model in the presence of a magnetic field, focussing in particular on
the strong coupling Kondo regime. The approach is innately simple and
physically transparent; but is found to give good agreement, for essentially
all field strengths, with exact results for the Wilson ratio, impurity
magnetization, spin susceptibility and related properties. | cond-mat_str-el |
Unusual spin pseudogap behavior in the spin web lattice Cu$_3$TeO$_6$
probed by $^{125}$Te nuclear magnetic resonance: We present a $^{125}$Te nuclear magnetic resonance (NMR) study in the
three-dimensional spin web lattice Cu$_3$TeO$_6$, which harbors topological
magnons. The $^{125}$Te NMR spectra and the Knight shift $\mathcal{K}$ as a
function of temperature show a drastic change at $T_\text{S}\sim 40$ K much
lower than the N\'eel ordering temperature $T_\text{N}\sim 61$ K, providing
evidence for the first-order structural phase transition within the
magnetically ordered state. Most remarkably, the temperature dependence of the
spin-lattice relaxation rate $T_1^{-1}$ unravels spin-gap-like magnetic
excitations, which sharply sets in at $T^*\sim 75$ K, the temperature well
above $T_\text{N}$. The spin gap behavior may be understood by weakly
dispersive optical magnon branches of high-energy spin excitations originating
from the unique corner-sharing Cu hexagon spin-1/2 network with low
coordination number. | cond-mat_str-el |
Excitation Spectra and Hard-core Thermodynamics of Bosonic Atoms In
Double Well Optical Lattices: A general coupled representation is proposed for bosonic atoms in double well
optical lattices. Based on such a representation, we investigate the excitation
spectra and thermodynamics of bosonic atoms in the optical lattices. We find
excitation spectra of the double well system with filling factor equal to one
can be described by simultaneous excitations composed of pseudo particles
corresponding to doubly occupied and empty double wells. It is demonstrated
that hard-core bosonic statistics must be taken into account in order to
properly describe the equilibrium properties at finite temperatures. For this
we calculate the temperature dependence curves of particle numbers and heat
capacity, in which the hard-core features are clearly shown. At last, the cases
with filling factors unequal to one are also briefly discussed. | cond-mat_str-el |
Organizing symmetry-protected topological phases by layering and
symmetry reduction: a minimalist perspective: It is demonstrated that fermionic/bosonic symmetry-protected topological
(SPT) phases across different dimensions and symmetry classes can be organized
using geometric constructions that increase dimensions and symmetry-forgetting
maps that change symmetry groups. Specifically, it is shown that the
interacting classifications of SPT phases with and without glide symmetry fit
into a short exact sequence, so that the classification with glide is
constrained to be a direct sum of cyclic groups of order 2 or 4. Applied to
fermionic SPT phases in the Wigner-Dyson class AII, this implies that the
complete interacting classification in the presence of glide is ${\mathbb
Z}_4{\oplus}{\mathbb Z}_2{\oplus}{\mathbb Z}_2$ in 3 dimensions. In particular,
the hourglass-fermion phase recently realized in the band insulator KHgSb must
be robust to interactions. Generalizations to spatiotemporal glide symmetries
are discussed. | cond-mat_str-el |
Magnetic interactions in strongly correlated systems: spin and orbital
contributions: We present a technique to map an electronic model with local interactions (a
generalized multi-orbital Hubbard model) onto an effective model of interacting
classical spins, by requiring that the thermodynamic potentials associated to
spin rotations in the two systems are equivalent up to second order in the
rotation angles. This allows to determine the parameters of relativistic and
non-relativistic magnetic interactions in the effective spin model in terms of
equilibrium Green's functions of the electronic model. The Hamiltonian of the
electronic system includes, in addition to the non-relativistic part,
relativistic single-particle terms such as the Zeeman coupling to an external
magnetic fields, spin-orbit coupling, and arbitrary magnetic anisotropies; the
orbital degrees of freedom of the electrons are explicitly taken into account.
We determine the complete relativistic exchange tensors, accounting for
anisotropic exchange, Dzyaloshinskii-Moriya interactions, as well as additional
non-diagonal symmetric terms (which may include dipole-dipole interaction). Our
procedure provides the complete exchange tensors in a unified framework,
including previously disregarded features such as the vertices of two-particle
Green's functions and non-local self-energies. We do not assume any smallness
in spin-orbit coupling, so our treatment is in this sense exact. Finally, we
show how to distinguish and address separately the spin, orbital and
spin-orbital contributions to magnetism. | cond-mat_str-el |
Loop braiding statistics in exactly soluble 3D lattice models: We construct two exactly soluble lattice spin models that demonstrate the
importance of three-loop braiding statistics for the classification of 3D
gapped quantum phases. The two models are superficially similar: both are
gapped and both support particle-like and loop-like excitations similar to that
of charges and vortex lines in a $\mathbb{Z}_2 \times \mathbb{Z}_2$ gauge
theory. Furthermore, in both models the particle excitations are bosons, and in
both models the particle and loop excitations have the same mutual braiding
statistics. The difference between the two models is only apparent when one
considers the recently proposed three-loop braiding process in which one loop
is braided around another while both are linked to a third loop. We find that
the statistical phase associated with this process is different in the two
models, thus proving that they belong to two distinct phases. An important
feature of this work is that we derive our results using a concrete approach:
we construct string and membrane operators that create and move the particle
and loop excitations and then we extract the braiding statistics from the
commutation algebra of these operators. | cond-mat_str-el |
Effect of antiferromagnetic spin correlations on lattice distortion and
charge ordering in Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$: We use neutron scattering to study the lattice and magnetic structure of the
layered half-doped manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$. On cooling from high
temperature, the system first becomes charge- and orbital- ordered (CO/OO) near
$T_{CO}=300$ K and then develops checkerboard-like antiferromagnetic (AF) order
below $T_{N}=130$ K. At temperatures above $T_{N}$ but below $T_{CO}$
($T_N<T<T_{CO}$), the appearance of short-range AF spin correlations suppresses
the CO/OO induced orthorhombic strain, contrasting with other half-doped
manganites, where AF order has no observable effect on the lattice distortion.
These results suggest that a strong spin-lattice coupling and the competition
between AF exchange and CO/OO ordering ultimately determines the
low-temperature properties of the system. | cond-mat_str-el |
Anisotropic magnetization studies of $R_2 Co Ga_8$ (R = Gd, Tb, Dy, Ho,
Er, Tm, Y and Lu) single crystals: Single crystals of R$_2$CoGa$_8$ series of compounds were grown, for the
first time, by high temperature solution growth (flux) method. These compounds
crystallize in a tetragonal crystal structure with the space group $P4/mmm$. It
has been found that R$_2$CoGa$_8$ phase forms only with the heavier rare
earths, starting from Gd with a relatively large $c/a$ ratio of $\approx$ 2.6.
The resultant anisotropic magnetic properties of the compounds were
investigated along the two principal crystallographic directions of the crystal
viz., along [100] and [001]. The nonmagnetic compounds Y$_2$CoGa$_8$ and
Lu$_2$CoGa$_8$ show diamagnetic behavior down to the lowest temperature (1.8 K)
pointing out the non-magnetic nature of Co in these compounds and a relatively
low density of electronic states at the Fermi level. Compounds with the
magnetic rare earths order antiferromagnetically at temperatures lower than 30
K. The easy axis of magnetization for R$_2$CoGa$_8$ (R = Tb, Dy and Ho) is
found to be along the [001] direction and it changes to [100] direction for
Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$. The magnetization behavior is analyzed on
the basis of crystalline electric field (CEF) model. The estimated crystal
field parameters explains the magnetocrystalline anisotropy in this series of
compounds. | cond-mat_str-el |
Quantum effects for the 2D soliton in isotropic ferromagnets: We evaluate a zero-point quantum correction to a Belavin-Polyakov soliton in
an isotropic 2D ferromagnet. By revising the scattering problem of
quasi-particles by a soliton we show that it leads to the Aharonov-Bohm type of
scattering, hence the scattering data can not be obtained by the Born
approximation. We proof that the soliton energy with account of quantum
corrections does not have a minimum as a function of its radius, which is
usually interpreted as a soliton instability. On the other hand, we show that
long lifetime solitons can exist in ferromagnets due to an additional integral
of motion, which is absent for the sigma-model. | cond-mat_str-el |
Variational Ansatz for an Abelian to non-Abelian Topological Phase
Transition in $ν= 1/2 + 1/2$ Bilayers: We propose a one-parameter variational ansatz to describe the
tunneling-driven Abelian to non-Abelian transition in bosonic $\nu=1/2+1/2$
fractional quantum Hall bilayers. This ansatz, based on exact matrix product
states, captures the low-energy physics all along the transition and allows to
probe its characteristic features. The transition is continuous, characterized
by the decoupling of antisymmetric degrees of freedom. We futhermore determine
the tunneling strength above which non-Abelian statistics should be observed
experimentally. Finally, we propose to engineer the inter-layer tunneling to
create an interface trapping a neutral chiral Majorana. We microscopically
characterize such an interface using a slightly modified model wavefunction. | cond-mat_str-el |
Microscopic theory for nematic fractional quantum Hall effect: We analyse various microscopic properties of the nematic fractional quantum
Hall effect (FQHN) in the thermodynamic limit, and present necessary conditions
required of the microscopic Hamiltonians for the nematic FQHE to be robust.
Analytical expressions for the degenerate ground state manifold, ground state
energies, and gapless nematic modes are given in compact forms with the input
interaction and the corresponding ground state structure factors. We relate the
long wavelength limit of the neutral excitations to the guiding center metric
deformation, and show explicitly the family of trial wavefunctions for the
nematic modes with spatially varying nematic order near the quantum critical
point. For short range interactions, the dynamics of the FQHN is completely
determined by the long wavelength part of the ground state structure factor.
The special case of the FQHN at $\nu=1/3$ is discussed with new theoretical
insights from the Haffnian parent Hamiltonian, leading to a number of rigorous
statements and experimental implications. | cond-mat_str-el |
Effects of exchange distortions in the magnetic Kagome lattice: This study examines the effect of distorted triangular magnetic interactions
in the Kagome lattice. Using a Holstein-Primakoff expansion, we determine the
analytical solutions for classical energies and the spin-wave modes for various
magnetic configurations. By understanding the magnetic phase diagram, we
characterize the changes in the spin waves and examine the spin distortions of
the ferromagnetic (FM), Antiferrimagnetic (AfM), and 120$^{\circ}$ phases that
are produced by variable exchange interactions and lead to various
non-collinear phases, which provides a deeper understanding of the magnetic
fingerprints of these configurations for experimental characterization and
identification. | cond-mat_str-el |
Exciton dissociation mediated by phonons in organic photovoltaics: It is well known that phonons can overscreen the bare Coulomb
electron-electron repulsion, turning it into the effective attraction that
binds the Cooper pairs responsible for BCS superconductivity. Here, we use a
simple lattice model to prove that the counterpart of this is also possible,
whereby phonons overscreen the bare electron-hole attraction and may turn it
repulsive at short distances, driving exciton dissociation in certain regions
of the parameter space. We argue that this phonon-mediated short-range
screening plays an important role in the physics of organic solar cell
materials (and other materials with strong electron-phonon coupling) and could
point the way to new strategies for optimizing their efficiencies. | cond-mat_str-el |
Few-body nature of Kondo correlated ground states: The quenching of degenerate impurity states in metals generally induces a
long-range correlated quantum state known as the Kondo screening cloud. While a
macroscopic number of particles clearly take part in forming this extended
structure, assessing the number of truly entangled degrees of freedom requires
a careful analysis of the relevant many-body wavefunction. For this purpose, we
examine the natural single-particle orbitals that are eigenstates of the
single-particle density (correlation) matrix for the ground state of two
quantum impurity problems: the interacting resonant level model (IRLM) and the
single impurity Anderson model (SIAM). As a simple and general probe for
few-body versus many-body character we consider the rate of exponential decay
of the correlation matrix eigenvalues towards inactive (fully empty or filled)
orbitals. We find that this rate remains large in the physically most relevant
region of parameter space, implying a few-body character. Genuine many-body
correlations emerge only when the Kondo temperature becomes exponentially
small, for instance near a quantum critical point. In addition, we demonstrate
that a simple numerical diagonalization of the few-body problem restricted to
the Fock space of the most correlated orbitals converges exponentially fast
with respect to the number of orbitals, to the true ground state of the IRLM.
We also show that finite size effects drastically affect the correlation
spectrum, shedding light on an apparent paradox arising from previous studies
on short chains. | cond-mat_str-el |
Pseudoparticle approach to 1D integrable quantum models: Over the last three decades a large number of experimental studies on several
quasi one-dimensional (1D) metals and quasi1D Mott-Hubbard insulators have
produced evidence for distinct spectral features identified with charge-only
and spin-only fractionalized particles. They can be also observed in ultra-cold
atomic 1D optical lattices a nd quantum wires. 1D exactly solvable models
provide nontrivial tests of the approaches for these systems relying on field
theories. Different schemes such as the pseudofermion dynamical theory (PDT)
and the mobile quantum impurity model (MQIM) have revealed that the 1D
correlated models high-energy physics is qualitatively different from that of a
low-energy Tomonaga-Luttinger liquid (TLL). This includes the momentum
dependence of the exponents that control the one- and two-particle dynamical
correlation functions near their spectra edges and in the vicinity of
one-particle singular spectral features.
On the one hand, the low-energy charge-only and spin-only fractionalized
particles are usually identified with holons and spinons, respectively. On the
other hand, `particle-like' representations in terms of {\it pseudoparticles},
related PDT {\it pseudofermions}, and MQIM particles are suitable for the
description of both the low-energy TLL physics and high-energy spectral and
dynamical properties of 1D correlated systems. The main goal of this review is
to revisit the usefulness of pseudoparticle and PDT pseudofermion
representations for the study of both static and high-energy spectral and
dynamical properties of the 1D Lieb-Liniger Bose gas, spin-$1/2$ isotropic
Heisenberg chain, and 1D Hubbard model. Moreover, the relation between the PDT
and the MQIM is clarified. | cond-mat_str-el |
Magnetic field induced quantum phase transitions in the two-impurity
Anderson model: In the two-impurity Anderson model, the inter-impurity spin exchange
interaction favors a spin singlet state between two impurities leading to the
breakdown of the Kondo effect. We show that a local uniform magnetic field can
delocalize the quasiparticles to restore the Kondo resonance. This transition
is found to be continuous, accompanied by not only the divergence of the
staggered (antiferromagnetic) susceptibility, but also the divergence of the
uniform spin susceptibility. This may imply that the magnetic field induced
quantum phase transitions in Kondo systems are in favor of the local critical
type. | cond-mat_str-el |
Zigzag and Checkerboard Magnetic Patterns in Orbitally Directional
Double-Exchange Systems: We analyze a $t_{2g}$ double-exchange system where the orbital directionality
of the itinerant degrees of freedom is a key dynamical feature that
self-adjusts in response to doping and leads to a phase diagram dominated by
two classes of ground-states with zigzag and checkerboard patterns. The
prevalence of distinct orderings is tied to the formation of orbital molecules
that in one-dimensional paths make insulating zigzag states kinetically more
favorable than metallic stripes, thus allowing for a novel doping-induced
metal-to-insulator transition. We find that the basic mechanism that controls
the magnetic competition is the breaking of orbital directionality through
structural distortions and highlight the consequences of the interorbital
Coulomb interaction. | cond-mat_str-el |
The magnetic structure of Ce$_3$TiBi$_5$ and its relation to
current-induced magnetization: The control of magnetization using electric fields has been extensively
studied in magnetoelectric multiferroic insulator materials. Changes in
magnetization in bulk metals caused by electric currents have attracted less
attention. The recently discovered metallic magnet Ce$_3$TiBi$_5$ has been
reported to exhibit current-induced magnetization. Here we determined the
magnetic structure of Ce$_3$TiBi$_5$ using neutron diffraction, aiming to
understand the microscopic origin of this magnetoelectric phenomenon in a
metal. We established that the antiferromagnetic order emerging below $T_N=5$ K
is a cycloid order described by $P6_3/mcm.1'(0,0,g)00sss$ with small moment
sizes of $0.50(2)~\mu_B$ and propagation vector ${\bf k}=(0,0,0.386)$.
Surprisingly, the symmetry of this magnetic structure is inconsistent with the
presence of current-induced magnetization and potential origins of this
inconsistency with previous results are discussed. Additionally, our results
suggest that moments order along their hard magnetic direction in
Ce$_3$TiBi$_5$, a phenomenon which has been observed in other Kondo systems. | cond-mat_str-el |
Gd pyrochlore under a staggered molecular field in Gd$_2$Ir$_2$O$_7$: The influence of a staggered molecular field in frustrated rare-earth
pyrochlores, produced via the magnetic iridium occupying the transition metal
site, can generate exotic ground states, such as the fragmentation of the
magnetization in the Ho compound. At variance with the Ising Ho$^{3+}$ moment,
we focus on the behavior of the quasi isotropic magnetic moment of the
Gd$^{3+}$ ion at the rare-earth site. By means of macroscopic measurements and
neutron scattering, we find a complex situation where different components of
the magnetic moment contribute to two antiferromagnetic non-collinear
arrangements: a high temperature all in - all out order induced by the Ir
molecular field, and Palmer and Chalker correlations that tend to order at much
lower temperatures. This is enabled by the anisotropic nature of the Gd-Gd
interactions and requires a weak easy-plane anisotropy of the Gd$^{3+}$ moment
due to the mixing of the ground state with multiplets of higher spectral terms. | cond-mat_str-el |
Phase diagram of the Kondo necklace: a mean-field renormalization group
approach: In this paper we investigate the magnetic properties of heavy fermions in the
antiferromagnetic and dense Kondo phases in the framework of the Kondo necklace
model. We use a mean field renormalization group approach to obtain a
temperature versus Kondo coupling $(T-J)$ phase diagram for this model in
qualitative agreement with Doniach's diagram, proposed on physical grounds. We
further analyze the magnetically disordered phase using a two-sites approach.
We calculate the correlation functions and the magnetic susceptibility that
allow to identify the crossover between the spin-liquid and the local moment
regimes, which occurs at a {\em coherence} temperature. | cond-mat_str-el |
Unifying static and dynamic properties in 3D quantum antiferromagnets: Quantum Monte Carlo simulations offer an unbiased means to study the static
and dynamic properties of quantum critical systems, while quantum field theory
provides direct analytical results. We study three dimensional, critical
quantum antiferromagnets by performing a combined analysis using both quantum
field theory calculations and quantum Monte Carlo data. Explicitly, we analyze
the order parameter (staggered magnetization), N\'eel temperature,
quasiparticle gaps, and the susceptibilities in the scalar and vector channels.
We connect the two approaches by deriving descriptions of the quantum Monte
Carlo observables in terms of the quasiparticle excitations of the field
theory. The remarkable agreement not only unifies the description of the static
and dynamic properties of the system, but also constitutes a thorough test of
perturbative O(3) quantum field theory and opens new avenues for the analytical
guidance of detailed numerical studies. | cond-mat_str-el |
Charge and spin dynamics of the Hubbard chains: We calculate the local correlation functions of charge and spin for the
one-chain and two-chain Hubbard model using the density matrix renormalization
group method and the recursion technique. Keeping only finite number of states
we get good accuracy for the low energy excitations. We study the charge and
spin gaps, bandwidths and weights of the spectra for various values of the
on-site Coulomb interaction U and the electron filling. In the low energy part,
the local correlation functions are different for the charge and spin. The
bandwidths are proportional to t for the charge and J for the spin,
respectively. | cond-mat_str-el |
Entanglement and logarithmic spirals in a quantum spin-1 many-body
system with competing dimer and trimer interactions: Spontaneous symmetry breaking (SSB) with type-B Goldstone modes is
investigated in the macroscopically degenerate phase for a quantum spin-1
many-body system with competing dimer and trimer interactions. The SSB involves
three distinct patterns. The first occurs at the dimer point, with the pattern
from staggered ${\rm SU}(3)$ to ${\rm U}(1)\times{\rm U}(1)$. The second occurs
at the trimer point, with the pattern from uniform ${\rm SU}(3)$ to ${\rm
U}(1)\times{\rm U}(1)$. The third occurs in the dimer-trimer regime, with the
pattern from uniform ${\rm SU}(2)$ to ${\rm U}(1)$. The number of type-B
Goldstone modes is thus two, two and one for the three patterns, respectively.
The ground state degeneracies arising from the three patterns are exponential
with the system size, which may be recognized as sequences of integers relevant
to self-similar logarithmic spirals. This in turn is attributed to the presence
of an emergent symmetry operation tailored to a specific degenerate ground
state. As a consequence, the residual entropy is non-zero, which measures the
disorder present in a unit cell of highly degenerate ground state generated
from a generalized highest weight state. An exact Schmidt decomposition exists
for the highly degenerate ground states, thus exposing the self-similarities
underlying an abstract fractal, described by the fractal dimension. The latter
is extracted from performing a universal finite system-size scaling analysis of
the entanglement entropy, which is identical to the number of type-B Goldstone
modes. The model under investigation thus accommodates an exotic scale
invariant quantum state of matter. | cond-mat_str-el |
Melting of magnetization plateaus for kagome and square-kagome lattice
antiferromagnets: Unconventional features of the magnetization curve at zero temperature such
as plateaus or jumps are a hallmark of frustrated spin systems. Very little is
known about their behavior at non-zero temperatures. Here we investigate the
temperature dependence of the magnetization curve of the kagome lattice
antiferromagnet in particular at 1/3 of the saturation magnetization for large
lattice sizes of up to N=48 spins. We discuss the phenomenon of asymmetric
melting and trace it back to a combined effect of unbalanced magnetization
steps on either side of the investigated plateau as well as on the behavior of
the density of states across the plateau. We compare our findings to the
square-kagome lattice that behaves similarly at low temperatures at zero field,
but as we will demonstrate differently at 1/3 of the saturation magnetization.
Both systems possess a flat one-magnon band and therefore share with the class
of flat-band systems the general property that the plateau that precedes the
jump to saturation melts asymmetrically but now with a minimal susceptibility
that bends towards lower fields with increasing temperature. | cond-mat_str-el |
Stacking faults in $α$-RuCl$_3$ revealed by local electric
polarization: We present out-of-plane dielectric and magnetodielectric measurements of
single crystallines $\alpha$-RuCl$_3$ with various degrees of stack faults. A
frequency dependent, but field independent, dielectric anomaly appears at
$T_{A}\:(f=100\:\mathrm{kHz})\sim$ 4 K once both magnetic transitions at
$T_{N1}\sim$ 7 K and $T_{N2}\sim$ 14 K set in. The observed dielectric anomaly
is attributed to the emergency of possible local electric polarizations whose
inversion symmetry is broken by inhomogeneously distributed stacking faults. A
field-induced intermediate phase is only observed when a magnetic field is
applied perpendicular to the Ru-Ru bonds for samples with minimal stacking
faults. Less pronounced in-plane anisotropy is found in samples with sizable
contribution from stacking imperfections. Our findings suggest that dielectric
measurement is a sensitive probe in detecting the structural and magnetic
properties, which may be a promising tool especially in studying
$\alpha$-RuCl$_3$ thin film devices. Moreover, the stacking details of RuCl$_3$
layers strongly affect the ground state both in the magnetic and electric
channels. Such a fragile ground state against stacking faults needs to be
overcome for realistic applications utilizing the magnetic and/or electric
properties of Kitaev based physics in $\alpha$-RuCl$_3$. | cond-mat_str-el |
Tomonaga-Luttinger parameters for doped Mott insulators: The Tomonaga--Luttinger parameter $K_{\rho}$ determines the critical behavior
in quasi one-dimensional correlated electron systems, e.g., the exponent
$\alpha$ for the density of states near the Fermi energy. We use the numerical
density-matrix renormalization group method to calculate $K_{\rho}$ from the
slope of the density-density correlation function in momentum space at zero
wave vector. We check the accuracy of our new approach against exact results
for the Hubbard and XXZ Heisenberg models. We determine $K_{\rho}$ in the phase
diagram of the extended Hubbard model at quarter filling, $n_{\rm c}=1/2$, and
confirm the bosonization results $K_{\rho}=n_{\rm c}^2=1/4$ on the critical
line and $K_{\rho}^{\rm CDW}=n_{\rm c}^2/2=1/8$ at infinitesimal doping of the
charge-density-wave (CDW) insulator for all interaction strengths. The doped
CDW insulator exhibits exponents $\alpha>1$ only for small doping and strong
correlations. | cond-mat_str-el |
Emergence of nontrivial magnetic excitations in a spin liquid state of
kagome volborthite: When quantum fluctuations destroy underlying long-range ordered states, novel
quantum states emerge. Spin-liquid (SL) states of frustrated quantum
antiferromagnets, in which highly-correlated spins keep to fluctuate down to
very low temperatures, are prominent examples of such quantum states. SL states
often exhibit exotic physical properties, but the precise nature of the
elementary excitations behind such phenomena remains entirely elusive. Here we
utilize thermal Hall measurements that can capture the unexplored property of
the elementary excitations in SL states, and report on the observation of
anomalous excitations that may unveil the unique features of the SL state. Our
principal finding is a negative thermal Hall conductivity (k_xy) which the
charge-neutral spin excitations in a gapless SL state of the two-dimensional
kagome insulator volborthite Cu_3V_2O_7(OH)_2 \cdot 2H_2O exhibit, in much the
same way in which charged electrons give rise to the conventional electric Hall
effect. We find that k_xy is absent in the high-temperature paramagnetic state
and develops upon entering the SL state in accordance with the growth of the
short-range spin correlations, demonstrating that k_xy is a key signature of
the elementary excitation formed in the SL state. These results suggest the
emergence of nontrivial elementary excitations in the gapless SL state which
feel the presence of fictitious magnetic flux, whose effective Lorentz force is
found to be less than 1/100 of that experienced by free electrons. | cond-mat_str-el |
Low-energy excitations of the Hubbard model on the Kagomé lattice: The Hubbard model on the Kagom\'e lattice is investigated in a metallic phase
at half-filling. By introducing anisotropic electron hopping on the lattice, we
control geometrical frustration and clarify how the lattice geometry affects
physical properties. By means of the fluctuation exchange (FLEX) approximation,
we calculate the spin and charge susceptibilities, the one-particle spectral
function, the quasi-particle renormalization factor, and the Fermi velocity. It
is found that geometrical frustration of the Kagom\'e lattice suppresses the
instability to various ordered states through the strong reduction of the
wavevector dependence of susceptibilities, thereby stabilizing the formation of
quasi-particles due to the almost isotropic spin fluctuations in the Brillouin
zone. These characteristic properties are discussed in connection with the
effects of geometrical frustration in the strong coupling regime. | cond-mat_str-el |
Local and nonlocal order parameters in the Kitaev chain: We have calculated order parameters for the phases of the Kitaev chain with
interaction and dimerization at a special symmetric point applying the
Jordan-Wigner and other duality transformations. We use string order parameters
(SOPs) defined via the correlation functions of the Majorana string operators.
The SOPs are mapped onto the local order parameters of some dual Hamiltonians
and easily calculated. We have shown that the phase diagram of the interacting
dimerized chain comprises the phases with the conventional local order as well
as the phases with nonlocal SOPs. From the results for the critical indices we
infer the 2D Ising universality class of criticality at the particular symmetry
point where the model is exactly solvable. | cond-mat_str-el |
Quantum criticality with a twist - interplay of correlations and Kohn
anomalies in three dimensions: A general understanding of quantum phase transitions in strongly correlated
materials is still lacking. By exploiting a cutting-edge quantum many-body
approach, the dynamical vertex approximation, we make an important progress,
determining the quantum critical properties of the antiferromagnetic transition
in the fundamental model for correlated electrons, the Hubbard model in three
dimensions. In particular, we demonstrate that -in contradiction to the
conventional Hertz-Millis-Moriya theory- its quantum critical behavior is
driven by the Kohn anomalies of the Fermi surface, even when electronic
correlations become strong. | cond-mat_str-el |
Doped Mott phase and charge correlations in monolayer 1T-NbSe$_2$: The doped Hubbard model is one of the paradigmatic platforms to engineer
exotic quantum many-body states, including charge-ordered states, strange
metals and unconventional superconductors. While undoped and doped correlated
phases have been experimentally realized in a variety twisted van der Waals
materials, experiments in monolayer materials, and in particular 1T transition
metal dichalcogenides, have solely reached the conventional insulating undoped
regime. Correlated phases in monolayer two-dimensional materials have much
higher associated energy scales than their twisted counterparts, making doped
correlated monolayers an attractive platform for high temperature correlated
quantum matter. Here, we demonstrate the realization of a doped Mott phase in a
van der Waals dichalcogenide 1T-NbSe$_2$ monolayer. The system is electron
doped due to electron transfer to a monolayer van der Waals substrate via
proximity, leading to a correlated triangular lattice with both half-filled and
fully-filled sites. We analyze the distribution of the half-filled and filled
sites and show the arrangement is unlikely to be controlled by disorder alone,
and we show that the presence of competing non-local many-body correlations
would account for the charge correlations found experimentally. Our results
establish 1T-NbSe$_2$ as a potential monolayer platform to explore correlated
doped Mott physics in a frustrated lattice. | cond-mat_str-el |
Hydrodynamic spin fluctuations in the antiferromagnetic Heisenberg chain: We study the finite temperature, low energy, long wave-length spectrum of the
dynamic structure factor of the spin-$1/2$ antiferromagnetic Heisenberg chain
in the presence of exchange anisotropy and external magnetic fields. Using
imaginary-time quantum Monte-Carlo we extract parameters, relevant to
characterize a {\it renormalized} Luttinger liquid. For small momentum our
results are consistent with a change from propagating spinon density waves to
spin diffusion, described by a finite-frequency spin-current relaxation rate.
Results for this relaxation rate as well as other Luttinger liquid parameters
are presented versus temperature, momentum, magnetic field, and anisotropy,
including finite-size analysis, and checks for anomalous diffusion. Our results
are consistent with exact diagonalization and Bethe Ansatz, where available,
and with corroborate findings of other previous studies using bosonization,
transfer matrix renormalization group, and quantum Monte-Carlo. | cond-mat_str-el |
Lock-in of a Chiral Soliton Lattice by Itinerant Electrons: Chiral magnets often show intriguing magnetic and transport properties
associated with their peculiar spin textures. A typical example is a chiral
soliton lattice, which is found in monoaxial chiral magnets, such as
CrNb$_3$S$_6$ and Yb(Ni$_{1-x}$Cu$_x$)$_3$Al$_9$ in an external magnetic field
perpendicular to the chiral axis. Here, we theoretically investigate the
electronic and magnetic properties in the chiral soliton lattice by a minimal
itinerant electron model. Using variational calculations, we find that the
period of the chiral soliton lattice can be locked at particular values
dictated by the Fermi wave number, in stark contrast to spin-only models. We
discuss this behavior caused by the spin-charge coupling as a possible
mechanism for the lock-in discovered in Yb(Ni$_{1-x}$Cu$_x$)$_3$Al$_9$. We also
show that the same mechanism leads to the spontaneous formation of the chiral
soliton lattice even in the absence of the magnetic field. | cond-mat_str-el |
Recent Progress of Point Contact Spectroscopy as a Probe of Correlated
Electron States: We review recent progress in point contact spectroscopy (PCS) to extract
spectroscopic information out of correlated electron materials, with the
emphasis on non-superconducting states. PCS has been used to detect bosonic
excitations in normal metals, where signatures (e.g. phonons) are usually less
than 1$\%$ of the measured conductance. In the superconducting state, point
contact Andreev reflection (PCAR) has been widely used to study properties of
the superconducting gap in various superconductors. In the last decade, there
have been more and more experimental results suggesting that the point contact
conductance could reveal new features associated with the unusual single
electron dynamics in non-superconducting states, shedding a new light on
exploring the nature of the competing phases in correlated materials. We will
summarize the theories for point contact spectroscopy developed from different
approaches and highlight these conceptual differences distinguishing point
contact spectroscopy from tunneling-based probes. Moreover, we will show how
the Schwinger-Kadanoff-Baym-Keldysh (SKBK) formalism together with the
appropriate modeling of the nano-scale point contacts randomly distributed
across the junction leads to the conclusion that the point contact conductance
is proportional to the {\it effective density of states}, a physical quantity
that can be computed if the electron self energy is known. The experimental
data on iron based superconductors and heavy fermion compounds will be analyzed
in this framework. These recent developments have extended the applicability of
point contact spectroscopy to correlated materials, which will help us achieve
a deeper understanding of the single electron dynamics in strongly correlated
systems. | cond-mat_str-el |
Magnetic Structure and Spin Fluctuations in Colossal Magnetoresistance
Ferrimagnet Mn3Si2Te6: The ferrimagnetic insulator Mn3Si2Te6, which features a Curie temperature Tc
at 78 K and a delicate yet consequential magnetic frustration, exhibits
colossal magnetoresistance (CMR) when the magnetic field is applied along the
magnetic hard axis, surprisingly inconsistent with existing precedents [Y. Ni,
H. Zhao, Y. Zhang et al. Phys. Rev. B 103, L161105 (2021)]. This discovery
motivates a thorough single-crystal neutron diffraction study in order to gain
insights into the magnetic structure and its hidden correlation with the new
type of CMR. Here we report a noncollinear magnetic structure below the Tc
where the moments lie predominantly within the basal plane but tilt toward the
c axis by ~10o at ambient conditions. A substantial magnetic diffuse scattering
decays slowly and persists well above the Tc. The evolution of the spin
correlation lengths agrees well with the electrical resistivity, underscoring
the role of spin fluctuation contributing to the magnetoresistivity near the
transition. Application of magnetic field along the c axis, renders a swift
occurrence of CMR but only a slow tilting of the magnetic moments toward the c
axis. The unparalleled changes indicate a non-consequential role of magnetic
polarization. | cond-mat_str-el |
Kondo and anti-Kondo coupling to local moments in EuB$_6$: With a treatment of the 4$f$ states of EuB$_6$ based on LDA+U method, the
mixing of Eu $f$ states with B $p$ states around the X point of the Brillouin
zone is shown to have unexpected consequences for the effective exchange
interactions. We analyze in detail the orbital character of electronic states
close to the Fermi level and discuss the effective exchange between the
itinerant electrons and the local $4f$ moments. The analysis suggests that the
ordered phase may provide the first example of a {\it half metallic semimetal},
and that the physics of EuB$_6$ should be described in terms of a two band
Kondo lattice model with parallel (ferromagnetic) coupling of the conduction
electrons and antiparallel (antiferromagnetic) coupling of the valence
electrons to the local $4f$ moments. | cond-mat_str-el |
Optical studies of Cr$^{3+}$-Cr$^{2+}$ pair center in KZnF$_{3}$ crystal: Optical absorption spectra of Cr$^{3+}$-Cr$^{2+}$ pair center in
KZnF$_{3}$:Cr$^{3+}$,Cr$^{2+}$ crystal were investigated in wide temperature
range. Broad band at 30800 cm$^{-1}$ is attributed to cation-cation
(e_g)-electron transfer transition. Narrow lines with maxima at 16720 cm$^{-1}$
and 19880 cm$^{-1}$ have been assigned to purely electronic exchange-induced
electric-dipole transitions from the ground
(Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^5E_g)) state to excited
(Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^a)) and
(Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^b)) states, respectively. It's vibronic
satellites corresponding to (a_{1g}) local mode of Cr$^{3+}$ fluorine
octahedron of the pair are also observed. Energy of the local mode for the
ground and mentioned excited states are 580, 540 and 530 cm$^{-1}$. Instead of
expected double exchange for mixed valence pair ferromagnetic superexchange for
Cr$^{3+}$-Cr$^{2+}$ pair in KZnF$_{3}$ crystal is realized. Exchange integral
(J=-14.9\pm0.4) cm$^{-1}$ and Jahn-Teller splitting (\Delta_{JT}=340\pm40)
cm$^{-1}$ for the ground state of the pair were obtained by analysis of the
temperature dependence of absorption lines. Important features of the crossover
double exchange - ferromagnetic superexchange are discussed. | cond-mat_str-el |
Quantum Mott Transition and Multi-Furcating Criticality: Phenomenological theory of the Mott transition is presented. When the
critical temperature of the Mott transition is much higher than the quantum
degeneracy temperature, the transition is essentially described by the Ising
universality class. Below the critical temperature, phase separation or
first-order transition occurs. However, if the critical point is involved in
the Fermi degeneracy region, a marginal quantum critical point appears at zero
temperature. The originally single Mott critical point generates subsequent
many unstable fixed points through various Fermi surface instabilities induced
by the Mott criticality characterized by the diverging charge susceptibility or
doublon susceptibility. This occurs in marginal quantum-critical region.
Charge, magnetic and superconducting instabilitites compete severely under
these critical charge fluctuations. The quantum Mott transition triggers
multi-furcating criticality, which goes beyond the conventional concept of
multicriticality in quantum phase transitions. Near the quantum Mott
transition, the criticality generically drives growth of inhomogeneous
structure in the momentum space with singular points of flat dispersion on the
Fermi surface. The singular points determine the quantum dynamics of the Mott
transition by the dynamical exponent $z=4$. We argue that many of
filling-control Mott transitions are classified to this category. Recent
numerical results as well as experimental results on strongly correlated
systems including transition metal oxides, organic materials and $^3$He layer
adsorbed on a substrate are consistently analyzed especially in two-dimensional
systems. | cond-mat_str-el |
Spatio-temporal dynamics of quantum-well excitons: We investigate the lateral transport of excitons in ZnSe quantum wells by
using time-resolved micro-photoluminescence enhanced by the introduction of a
solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps,
respectively. Strong deviation from classical diffusion is observed up to 400
ps. This feature is attributed to the hot-exciton effects, consistent with
previous experiments under cw excitation. The coupled transport-relaxation
process of hot excitons is modelled by Monte Carlo simulation. We prove that
two basic assumptions typically accepted in photoluminescence investigations on
excitonic transport, namely (i) the classical diffusion model as well as (ii)
the equivalence between the temporal and spatial evolution of the exciton
population and of the measured photoluminescence, are not valid for
low-temperature experiments. | cond-mat_str-el |
Low-temperature spin Coulomb drag in a two-dimensional electron gas: The phenomenon of low-temperature spin Coulomb drag in a two-dimensional
electron gas is investigated. The spin transresistivity coefficient is
essentially enhanced in the diffusive regime, as compared to conventional
predictions. The origin of this enhancement is the quantum coherence of spin-up
and spin-down electrons propagating in the same random impurity potential and
coupled via the Coulomb interaction. A comprehensive analysis of spin and
interlayer Coulomb drag effects is presented. | cond-mat_str-el |
Energy Scale Deformation on Regular Polyhedra: A variant of energy scale deformation is considered for the S = 1/2
antiferromagnetic Heisenberg model on polyhedra. The deformation is induced by
the perturbations to the uniform Hamiltonian, whose coefficients are determined
by the bond coordinates. On the tetrahedral, octahedral, and cubic clusters,
the perturbative terms do not affect the ground state of the uniform
Hamiltonian when they are sufficiently small. On the other hand, for the
icosahedral and dodecahedral clusters, it is numerically confirmed that the
ground state of the uniform Hamiltonian is almost insensitive to the
perturbations unless they lead to a discontinuous change in the ground state.
The obtained results suggest the existence of a generalization of sine-square
deformation in higher dimensions. | cond-mat_str-el |
A generalization of the injectivity condition for Projected Entangled
Pair States: We introduce a family of tensor network states that we term semi-injective
Projected Entangled-Pair States (PEPS). They extend the class of injective PEPS
and include other states, like the ground states of the AKLT and the CZX models
in square lattices. We construct parent Hamiltonians for which semi-injective
PEPS are unique ground states. We also determine the necessary and sufficient
conditions for two tensors to generate the same family of such states in two
spatial dimensions. Using this result, we show that the third cohomology
labeling of Symmetry Protected Topological phases extends to semi-injective
PEPS. | cond-mat_str-el |
Quantitative functional renormalization for three-dimensional quantum
Heisenberg models: We employ a recently developed variant of the functional renormalization
group method for spin systems, the so-called pseudo Majorana functional
renormalization group, to investigate three-dimensional spin-1/2 Heisenberg
models at finite temperatures. We study unfrustrated and frustrated Heisenberg
systems on the simple cubic and pyrochlore lattices. Comparing our results with
other quantum many-body techniques, we demonstrate a high quantitative accuracy
of our method. Particularly, for the unfrustrated simple cubic lattice
antiferromagnet ordering temperatures obtained from finite-size scaling of
one-loop data deviate from error controlled quantum Monte Carlo results by
$\sim5\%$ and we further confirm the established values for the critical
exponent $\nu$ and the anomalous dimension $\eta$. As the PMFRG yields results
in good agreement with QMC, but remains applicable when the system is
frustrated, we next treat the pyrochlore Heisenberg antiferromagnet as a
paradigmatic magnetically disordered system and find nearly perfect agreement
of our two-loop static homogeneous susceptibility with other methods. We
further investigate the broadening of pinch points in the spin structure factor
as a result of quantum and thermal fluctuations and confirm a finite width in
the extrapolated limit $T\rightarrow0$. While extensions towards higher loop
orders $\ell$ seem to systematically improve our approach for magnetically
disordered systems we also discuss subtleties when increasing $\ell$ in the
presence of magnetic order. Overall, the pseudo Majorana functional
renormalization group is established as a powerful many-body technique in
quantum magnetism with a wealth of possible future applications. | cond-mat_str-el |
Quantum critical point in the spin glass-antiferromagnetism competition
in Kondo-lattice systems: A theory is proposed to describe the competition among antiferromagnetism
(AF), spin glass (SG) and Kondo effect. The model describes two Kondo
sublattices with an intrasite Kondo interaction strength $J_{K}$ and an
interlattice quantum Ising interaction in the presence of a transverse field
$\Gamma$. The interlattice coupling is a random Gaussian distributed variable
(with average $-2J_0/N$ and variance $32 J^{2}/N$) while the $\Gamma$ field is
introduced as a quantum mechanism to produce spin flipping. The path integral
formalism is used to study this fermionic problem where the spin operators are
represented by bilinear combinations of Grassmann fields. The disorder is
treated within the framework of the replica trick. The free energy and the
order parameters of the problem are obtained by using the static ansatz and by
choosing both $J_0/J$ and $\Gamma/J \approx (J_k/J)^2$ to allow, as previously,
a better comparison with the experimental findings.
The results indicate the presence of a SG solution at low $J_K/J$ and for
temperature $T<T_{f}$ ($T_{f}$ is the freezing temperature). When $J_K/J$ is
increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo
state is obtained for high values of $J_{K}/J$. Moreover, the behaviors of the
freezing and Neel temperatures are also affected by the relationship between
$J_{K}$ and the transverse field $\Gamma$. The first one presents a slight
decrease while the second one decreases towards a Quantum Critical Point (QCP).
The obtained phase diagram has the same sequence as the experimental one for
$Ce_{2}Au_{1-x}Co_{x}Si_{3}$, if $J_{K}$ is assumed to increase with $x$, and
in addition, it also shows a qualitative agreement concerning the behavior of
the freezing and the Neel temperatures. | cond-mat_str-el |
Giant Magnetoresistance in Hubbard Chains: We use numerically unbiased methods to show that the one-dimensional Hubbard
model with periodically distributed on-site interactions already contains the
minimal ingredients to display the phenomenon of magnetoresistance; i.e., by
applying an external magnetic field, a dramatic enhancement on the charge
transport is achieved. We reach this conclusion based on the computation of the
Drude weight and of the single-particle density of states, applying twisted
boundary condition averaging to reduce finite-size effects. The known picture
that describes the giant magnetoresistance, by interpreting the scattering
amplitudes of parallel or antiparallel polarized currents with local
magnetizations, is obtained without having to resort to different entities;
itinerant and localized charges are indistinguishable. | cond-mat_str-el |
A simple metal-insulator criterion for the doped Mott-Hubbard materials: We derived a simple metal-insulator criterion in analytical form for the
doped Mott-Hubbard materials. Its readings closely related to the orbital and
spin nature of the ground states of the unit cell. The available criterion
readings (metal or insulator) in the paramagnetic phase points to the
possibility of the insulator state of doped materials with the forbidden first
removal electron states. According to its physical meaning the result is
similar to Wilsons criterion in the itinerant electron systems. An application
of the criterion to the high-Tc cuprates discussed. | cond-mat_str-el |
A Primer on Weyl Semimetals: Down the Discovery of Topological Phases: Recently discovered Weyl semimetals (WSM) have found special place in
topological condensed matter studies for they represent first example of
massless Weyl fermions found in condensed matter systems. A WSM shows gapless
bulk energy spectra with Dirac-like point degeneracies, famously called Weyl
nodes, which carry with themselves well defined chiralities and topologically
protected chiral charges. One finds the Berry curvature of the Bloch bands to
become singular, like in a magnetic monopole, at these Weyl nodes. Moreover,
these systems feature topological surface states in the form of open Fermi
arcs. In this review, we undergo a concise journey from graphene based Dirac
physics to Weyl semimetals: the underlying Hamiltonians, their basic features
and their unique response to external electric and magnetic fields in order to
provide a basic walk-through of how the Weyl physics unfolded with time
starting from the discovery of Graphene. | cond-mat_str-el |
Photo-Induced Dynamics in Charge-Frustrated Systems: Photo-excited charge dynamics of interacting charge-frustrated systems are
studied using a spinless fermion model on an anisotropic triangular lattice.
Real-time evolution of the system after irradiating a pump-photon pulse is
analyzed by the exact diagonalization method. We focus on photo-excited states
in the two canonical charge-ordered (CO) ground states, i.e., horizontal
stripe-type and vertical stripe-type COs, which compete with each other owing
to the charge frustration. We find that the photo-induced excited states from
the two types of COs are distinct. From the horizontal stripe-type CO, a
transition to another CO state called the three-fold CO phase occurs. In sharp
contrast, the vertical stripe-type CO phase is only weakened by
photo-irradiation. Our observations are attributable to the charge frustration
effects occurring in the photo-excited states. | cond-mat_str-el |
Effect of screening of the electron-phonon interaction on mass
renormalization and optical conductivity of the Extended Holstein model
polarons: An interacting electron-phonon system is considered within the Extended
Holstein model at strong coupling regime and nonadiabatic approximation. It is
assumed that screening of an electron-phonon interaction is due to the excess
electrons in a lattice. An influence of the screening on the mass and optical
conductivity of a lattice polarons is studied. A more general form Yukawa-type
electron-phonon interaction potential potential is accepted and corresponding
forces are derived in a lattice. It is emphasized that the screening effect is
more pronounced at the values of screening radius comparable with a lattice
constant. It is shown that the mass of a lattice polaron obtained using
Yukawa-type electron-phonon interaction potential is less renormalized than
those of the early studied works at the same screening regime. Optical
conductivity of lattice polarons is calculated at different screening regimes.
The screening lowers the value of energy that corresponds to the peak of the
optical conductivity curve. The shift (lowering) is more pronounced at small
values of screening radius too. The factors that give rise to this shift is
briefly discussed. | cond-mat_str-el |
Terahertz-Light Driven Coupling of Antiferromagnetic Spins to Lattice: Understanding spin-lattice coupling represents a key challenge in modern
condensed matter physics, with crucial importance and implications for
ultrafast and 2D-magnetism. The efficiency of angular momentum and energy
transfer between spins and the lattice imposes fundamental speed limits on the
ability to control spins in spintronics, magnonics and magnetic data storage.
We report on an efficient nonlinear mechanism of spin-lattice coupling driven
by THz light pulses. A nearly single-cycle THz pulse resonantly interacts with
a coherent magnonic state in the antiferromagnet CoF2 and excites the
Raman-active THz phonon. The results reveal the unique functionality of
antiferromagnets allowing ultrafast spin-lattice coupling using light. | cond-mat_str-el |
Anderson localization of spinons in a spin-1/2 antiferromagnetic
Heisenberg chain: Anderson localization is a general phenomenon of wave physics, which stems
from the interference between multiple scattering paths1,2. It was originally
proposed for electrons in a crystal, but later was also observed for light3-5,
microwaves6, ultrasound7,8, and ultracold atoms9-12. Actually, in a crystal,
besides electrons there may exist other quasiparticles such as magnons and
spinons. However the search for Anderson localization of these magnetic
excitations is rare so far. Here we report the first observation of spinon
localization in copper benzoate, an ideal compound of spin-1/2
antiferromagnetic Heisenberg chain, by ultra-low-temperature specific heat and
thermal conductivity measurements. We find that while the spinon specific heat
Cs displays linear temperature dependence down to 50 mK, the spinons thermal
conductivity ks only manifests the linear temperature dependence down to 300
mK. Below 300 mK, ks/T decreases rapidly and vanishes at about 100 mK, which is
a clear evidence for Anderson localization. Our finding opens a new window for
studying such a fundamental phenomenon in condensed matter physics. | cond-mat_str-el |
Antiferromagnetic structure of alkali metal superoxide CsO$_2$: We have performed a powder neutron diffraction study on CsO$_2$, where the
unpaired electron with $s=1/2$ in the $\pi^*$ orbital of the O$_2^-$ ion is
responsible for the magnetism. The magnetic reflections 0 $\frac{1}{2}$ 0 and 0
$\frac{1}{2}$ 1 were observed below the N\'{e}el temperature of about 10 K. An
antiferromagnetic structure with a propagation vector of (0 ,$\frac{1}{2}$, 0)
and magnetic moments parallel to the $a$-axis is the most plausible. The
magnitude of the ordered moment is about 0.2 $\mu_B$, which is considered to be
strongly suppressed due to the one-dimensionality of the system. We propose a
possible $\pi^*$ orbital order that can explain the obtained magnetic
structure, and discuss its relation to the one-dimensionality. | cond-mat_str-el |
Hund's coupling driven interorbital entanglement in orbital-selective
Mott phase: We examine the orbital-selective Mott transition in the non-hybridized
two-band Hubbard model using the dynamical mean-field theory. We find that the
orbital-selective Mott transition could be depicted by the local quantum state
fidelity. Additionally, within the orbital-selective Mott phase, the combined
characteristics of the two orbitals lead to the presence of interorbital
entanglement, which is characterized by the non-semi-integer values of local
quantum state fidelity. It is demonstrated that this entanglement is driven by
transverse Hund's coupling, and the mechanisms underlying the orbital-selective
Mott transition show prominent variations depending on the presence or absence
of Hund's coupling and its transverse terms. | cond-mat_str-el |
Wannier Permanent Wave Functions and Featureless Bosonic Mott Insulators
on the 1/3 filled Kagome Lattice: We study Bose-Hubbard models on tight-binding, non-Bravais lattices, with a
filling of one boson per unit cell -- and thus fractional site filling. At
integer filling of a unit cell neither symmetry breaking nor topological order
is required, and in principle a trivial and featureless (i.e.,
symmetry-unbroken) insulator is allowed. We demonstrate by explicit
construction of a family of wavefunctions that such a featureless Mott
insulating state exists at 1/3 filling on the kagome lattice, and construct
Hamiltonians for which these wavefunctions are exact ground states. We briefly
comment on the experimental relevance of our results to cold atoms in optical
lattices. Such wavefunctions also yield 1/3 magnetization plateau states for
spin models in an applied field. The featureless Mott states we discuss can be
generalized to any lattice for which symmetric exponentially localized Wannier
orbitals can be found at the requisite filling, and their wavefunction is given
by the permanent over all Wannier orbitals. | cond-mat_str-el |
Quantum Hall Charge Kondo Criticality: The long-thought charge Kondo effects have recently been experimentally
realized in the quantum Hall regime. This experiment, supported by numerics,
exemplifies the realization of two-channel Kondo state, a non-Fermi Liquid, and
its crossover to the one-channel counterpart, a Fermi liquid. Scaling up such a
platform, we find a hierarchy of non-Fermi Liquids and their tunable crossovers
based on a renormalization group analysis. Utilizing results from a conformal
field theory, we further examine the universal conductances of this strongly
correlated system and their finite temperature scaling, which elucidate the
sharp distinctions between charge and spin Kondo physics. | cond-mat_str-el |
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