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Ineffectiveness of the Dzyaloshinskii-Moriya interaction in the
dynamical quantum phase transition in the ITF model: Quantum phase transition occurs at a quantum critical value of a control
parameter such as the magnetic field in the Ising model in a transverse
magnetic field (ITF). Recently, it is shown that ramping across the quantum
critical point generates non-analytic behaviors in the time evolution of a
closed quantum system in the thermodynamic limit at zero temperature. The
mentioned phenomenon is called the dynamical quantum phase transition (DQPT).
Here, we consider the one-dimensional (1D) ITF model with added the
Dzyaloshinskii-Moriya interaction (DMI). Using the fermionization technique,
the Hamiltonian is exactly diagonalized. Although the DM interaction induces
chiral phase in the ground state phase diagram of the model, the study of the
rate function of the return probability has proven that the DMI does not affect
in the DQPT. We conclude accordingly that the ramping across the quantum
critical point is not a necessary and sufficient condition for DQPT. | cond-mat_str-el |
Strong correlation induced charge localization in antiferromagnets: The fate of an injected hole in a Mott antiferromagnet is an outstanding
issue of strongly correlated physics. It provides important insights into doped
Mott insulators closely related to high-temperature superconductivity in
cuprates. Here, we report a systematic numerical study based on the density
matrix renormalization group (DMRG). It reveals a remarkable novelty and
surprise for the single hole's motion in otherwise well-understood Mott
insulators. Specifically, we find that the charge of the hole is self-localized
by a novel quantum interference mechanism purely of strong correlation origin,
in contrast to Anderson localization due to disorders. The common belief of
quasiparticle picture is invalidated by the charge localization concomitant
with spin-charge separation: the spin of the doped hole is found to remain a
mobile object. Our findings unveil a new paradigm for doped Mott insulators
that emerges already in the simplest single hole case. | cond-mat_str-el |
Hartree-Fock study of the moiré Hubbard model for twisted bilayer
transition metal dichalcogenides: Twisted bilayer transition metal dichalcogenides have emerged as important
model systems for the investigation of correlated electron physics because
their interaction strength, carrier concentration, band structure, and
inversion symmetry breaking are controllable by device fabrication, twist
angle, and most importantly, gate voltage, which can be varied in situ. The low
energy physics of some of these materials has been shown to be described by a
"moir\'e Hubbard model" generalized from the usual Hubbard model by the
addition of strong, tunable spin orbit coupling and inversion symmetry
breaking. In this work, we use a Hartree-Fock approximation to reach a
comprehensive understanding of the moir\'e Hubbard model on the mean field
level. We determine the magnetic and metal-insulator phase diagrams, and assess
the effects of spin orbit coupling, inversion symmetry breaking, and the
tunable van Hove singularity. We also consider the spin and orbital effects of
applied magnetic fields. This work provides guidance for experiments and sets
the stage for beyond mean-field calculations. | cond-mat_str-el |
Vortex creation and control in the Kitaev spin liquid by local bond
modulations: The Kitaev model realizes a quantum spin liquid where the spin excitations
are fractionalized into itinerant Majorana fermions and localized
$\mathbb{Z}_2$ vortices. Quantum entanglement between the fractional
excitations can be utilized for decoherence-free topological quantum
computation. Of particular interest is the anyonic statistics realized by
braiding the vortex excitations under a magnetic field. Despite the promising
potential, the practical methodology for creation and control of the vortex
excitations remains elusive thus far. Here we theoretically propose how one can
create and move the vortices in the Kitaev spin liquid. We find that the
vortices are induced by a local modulation of the exchange interaction;
especially, the local Dzyaloshinskii-Moriya (symmetric off-diagonal)
interaction can create vortices most efficiently in the (anti)ferromagnetic
Kitaev model, as it effectively flips the sign of the Kitaev interaction. We
test this idea by performing the {\it ab initio} calculation for a candidate
material $\alpha$-RuCl$_3$ through the manipulation of the ligand positions
that breaks the inversion symmetry and induces the local Dzyaloshinskii-Moriya
interaction. We also demonstrate a braiding of vortices by adiabatically and
successively changing the local bond modulations. | cond-mat_str-el |
Reply to Comment on "Magnetotransport signatures of a single nodal
electron pocket constructed from Fermi arcs": In a recent manuscript, we showed how an electron pocket in the shape of a
diamond with concave sides could potentially explain changes in sign of the
Hall coefficient R_H in the underdoped high-Tc cuprates as a function of
magnetic field and temperature. For simplicity, this Fermi surface is assumed
to be constructed from arcs of a circle connected at vertices which is an idea
borrowed from Banik and Overhauser. Such a diamond-shaped pocket is proposed to
be the product of biaxial charge-density wave order, which was subsequently
confirmed in x-ray scattering experiments. Since those x-ray scattering
experiments were performed, the biaxial Fermi surface reconstruction scheme has
garnered widespread support in the scientific literature. It has been shown to
accurately account for the cross-section of the Fermi surface pocket observed
in quantum oscillation measurements, the sign and behavior of the Hall
coefficient, the size of the high magnetic field electronic contribution to the
heat capacity and more recently the form of the angle-dependent
magnetoresistance.In their comment, Chakravarty and Wang raise several
important questions relating to the validity of the Hall coefficient we
calculated for such a diamond-shaped Fermi surface pocket. These questions
concern specifically (1) whether a change in sign of the Hall coefficient R_H
with magnetic field and temperature is dependent on a `special' form for the
rounding of the vertices, (2) whether a pocket of such a geometry can produce
quantum oscillations in R_H in the absence of other Fermi surface sections and
(3) whether a reconstructed Fermi surface consisting of a single pocket is less
`natural' than one consisting of multiple pockets. Below we consider each of
these in turn. | cond-mat_str-el |
Supersymmetric Approach to Heavy-Fermion Systems: We present a new supersymmetric approach to the Kondo lattice model in order
to describe simultaneously the quasiparticle excitations and the low-energy
magnetic fluctuations in heavy-Fermion systems. This approach mixes the
fermionic and the bosonic representation of the spin following the standard
rules of superalgebra. Our results show the formation of a bosonic band within
the hybridization gap reflecting the spin collective modes. The density of
states at the Fermi level is strongly renormalized while the Fermi surface sum
rule includes $n_{c}+1$ states. The dynamical susceptibility is made of a Fermi
liquid superimposed on a localized magnetism contribution. | cond-mat_str-el |
Electrically controllable magnetic order in the bilayer Hubbard model on
honeycomb lattice --- a determinant quantum Monte Carlo study: Layered antiferromagnetic spin density wave (LAF) state is one of the
plausible ground states of charge neutral Bernal stacked bilayer graphene. In
this paper, we use determinant quantum Monte Carlo method to study the effect
of the electric field on the magnetic order in bilayer Hubbard model on a
honeycomb lattice. Our results qualitatively support the LAF ground state found
in the mean field theory. The obtained magnetic moments, however, are much
smaller than what are estimated in the mean field theory. As electric field
increases, the magnetic order parameter rapidly decreases. | cond-mat_str-el |
Thermal stability and irreversibility of skyrmion-lattice phases in
Cu$_2$OSeO$_3$: Small angle neutron scattering measurements have been performed to study the
thermodynamic stability of skyrmion-lattice phases in Cu$_2$OSeO$_3$. We found
that the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be
stabilized through different thermal histories; by cooling from the
paramagnetic phase under finite magnetic field, the SkX(2) phase is selected.
On the other hand, the 30$^{\circ}$-rotated SkX(1) phase becomes dominant by
heating the sample from the ordered conical phase under finite field. This
difference in stabilization is surprisingly similar to the irreversibility
observed in spin glasses. The zero-field cooling results in the co-existence of
the two phases. It is further found that once one of the skyrmion-lattice
phases is formed, it is hardly destabilized. This indicates unusual thermal
stability of the two skyrmion-lattice phases originating from an unexpectedly
large energy barrier between them. | cond-mat_str-el |
Disorder effects in spiral spin liquids: Long-range spin textures,
Friedel-like oscillations, and spiral spin glasses: Spiral spin liquids are correlated states of matter in which a frustrated
magnetic system evades order by fluctuating between a set of (nearly)
degenerate spin spirals. Here, we investigate the response of spiral spin
liquids to quenched disorder in a $J_1$-$J_2$ honeycomb-lattice Heisenberg
model. At the single-impurity level, we identify different
order-by-quenched-disorder phenomena and analyze the ensuing spin textures. In
particular, we show that the latter generally display Friedel-like
oscillations, which encode direct information about the spiral contour, i.e.,
the classical ground-state manifold. At finite defect concentrations, we
perform extensive numerical simulations and characterize the resulting phases
at zero temperature. As a result, we find that the competition between
incompatible order-by-quenched-disorder mechanisms can lead to spiral spin
glass states already at low to moderate disorder. Finally, we discuss
extensions of our conclusions to nonzero temperatures and higher-dimensional
systems, as well as their applications to experiments. | cond-mat_str-el |
The Composite Particle-Hole Spinor of the Lowest Landau Level: We propose to form a two-component effective field theory from L = (L_ce +
L_ch)/2, where L_ce is the Lagrangian of composite electrons with a
Chern-Simons term, and L_ch is the particle-hole conjugate of L_ce - the
Lagrangian of composite holes. In the theory, the two-component fermion field
phi is a composite particle-hole spinor coupled to an emergent effective gauge
field in the presence of a background electromagnetic field. The Chern-Simons
terms for both the composite electrons and composite holes are exactly
cancelled out, and a 1/2 pseudospin degree of freedom, which responses to the
emergent gauge field the same way as the real spin to the electromagnetic
field, emerges automatically. Furthermore, the composite particle-hole spinor
theory has exactly the same form as the non-relativistic limit of the massless
Dirac composite fermion theory after expanded to the four-component form and
with a mass term added. | cond-mat_str-el |
Phase transitions in chiral magnets from Monte Carlo simulations: Motivated by the unusual temperature dependence of the specific heat in MnSi,
comprising a combination of a sharp first-order feature accompanied by a broad
hump, we study the extended Heisenberg model with competing exchange $J$ and
anisotropic Dzyaloshinskii-Moriya $D$ interactions in a broad range of ratio
$D/J$. Utilizing classical Monte Carlo simulations we find an evolution of the
temperature dependence of the specific heat and magnetic susceptibility with
variation of $D/J$. Combined with an analysis of the Bragg intensity patterns,
we clearly demonstrate that the observed puzzling hump in the specific heat of
MnSi originates from smearing out of the virtual ferromagnetic second order
phase transition by helical fluctuations, which manifest themselves in the
transient multiple spiral state. These fluctuations finally condense into the
helical ordered phase via a first order phase transition as is indicated by the
specific heat peak. Thus the model demonstrates a crossover from a second-order
to a first-order transition with increasing $D/J$. Upon further increasing
$D/J$ another crossover from a first-order to a second-order transition takes
place in the system. Moreover, the results of the calculations clearly indicate
that these competing interactions are the primary factor responsible for the
appearance of first order phase transitions in helical magnets with the
Dzyaloshinskii-Moriya (DM) interaction. | cond-mat_str-el |
Magnetic structure determination of Ca$_3$LiOsO$_6$ using neutron and
x-ray scattering: We present a neutron and x-ray scattering investigation of Ca$_3$LiOsO$_6$, a
material predicted to host magnetic ordering solely through an extended
superexchange pathway involving two anions, an interaction mechanism that has
undergone relatively little investigation. This contrasts with the ubiquitous
superexchange interaction mechanism involving a single anion that has well
defined and long standing rules. Despite the apparent 1D nature and triangular
units of magnetic osmium ions the onset of magnetic correlations has been
observed at a high temperature of 117 K in bulk measurements. We experimentally
determine the magnetically ordered structure and show it to be long range and
three dimensional. Our results support the model of extended superexchange
interaction. | cond-mat_str-el |
Spin polarons in the t-J model in an unconstrained representation: The report discusses the slave-fermion representations of the t-J model and
describes another representation, in which fermions and bosons are completely
commuting and in which the properties of fermions are directly related to the
properties of physical holes. For a study of the system in the new
representation at half-filling, interaction of fermions with two magnons is
treated in mean-field theory. The obtained effective model, in comparison to
that of the usual slave-fermion representation, has an additional bare hole
dispersion due to the hole moving by using quantum spin fluctuations present in
the undoped antiferromagnetic ground state. The single-hole Green's function at
half-filling is then found numerically using the self-consistent Born
approximation. For all studied quantities good or excellent agreement with
numerical data is observed in the entire parameter range, noticeably better
than in the studies with the slave-fermion representation. Using the same
effective model, the two-hole problem is also studied by solving numerically
the Bethe-Salpeter equation with noncrossing diagrams. | cond-mat_str-el |
DMRG Approach to Optimizing Two-Dimensional Tensor Networks: Tensor network algorithms have been remarkably successful solving a variety
of problems in quantum many-body physics. However, algorithms to optimize
two-dimensional tensor networks known as PEPS lack many of the aspects that
make the seminal density matrix renormalization group (DMRG) algorithm so
powerful for optimizing one-dimensional tensor networks known as matrix product
states. We implement a framework for optimizing two-dimensional PEPS tensor
networks which includes all of steps that make DMRG so successful for
optimizing one-dimension tensor networks. We present results for several 2D
spin models and discuss possible extensions and applications. | cond-mat_str-el |
Results on the symmetries of integrable fermionic models on chains: We investigate integrable fermionic models within the scheme of the graded
Quantum Inverse Scattering Method, and prove that any symmetry imposed on the
solution of the Yang-Baxter Equation reflects on the constants of motion of the
model; generalizations with respect to known results are discussed. This
theorem is shown to be very effective when combined with the Polynomial
$\Rc$-matrix Technique (PRT): we apply both of them to the study of the
extended Hubbard models, for which we find all the subcases enjoying several
kinds of (super)symmetries. In particular, we derive a geometrical construction
expressing any $gl(2,1)$-invariant model as a linear combination of EKS and
U-supersymmetric models. Furtherly, we use the PRT to obtain 32 integrable
$so(4)$-invariant models. By joint use of the Sutherland's Species technique
and $\eta$-pairs construction we propose a general method to derive their
physical features, and we provide some explicit results. | cond-mat_str-el |
Multiple supersonic phase fronts launched at a complex-oxide
hetero-interface: Selective optical excitation of a substrate lattice can drive phase changes
across hetero-interfaces. This phenomenon is a non-equilibrium analogue of
static strain control in heterostructures and may lead to new applications in
optically controlled phase change devices. Here, we make use of time-resolved
non-resonant and resonant x-ray diffraction to clarify the underlying physics,
and to separate different microscopic degrees of freedom in space and time. We
measure the dynamics of the lattice and that of the charge disproportionation
in NdNiO3, when an insulator-metal transition is driven by coherent lattice
distortions in the LaAlO3 substrate. We find that charge redistribution
propagates at supersonic speeds from the interface into the NdNiO3 film,
followed by a sonic lattice wave. When combined with measurements of magnetic
disordering and of the metal-insulator transition, these results establish a
hierarchy of events for ultrafast control at complex oxide hetero-interfaces. | cond-mat_str-el |
Monte Carlo modeling the phase diagram of magnets with the
Dzyaloshinskii - Moriya interaction: We use classical Monte Carlo calculations to model the high-pressure behavior
of the phase transition in the helical magnets. We vary values of the exchange
interaction constant J and the Dzyaloshinskii-Moriya interaction constant D,
which is equivalent to changing spin-spin distances, as occurs in real systems
under pressure. The system under study is self-similar at D/ J = constant, and
its properties are defined by the single variable J / T , where T is
temperature. The existence of the first order phase transition critically
depends on the ratio D / J. A variation of J strongly affects the phase
transition temperature and width of the fluctuation region (the hump) as
follows from the system self-similarity. The high-pressure behavior of the spin
system depends on the evolution of the interaction constants J and D on
compression. Our calculations are relevant to the high pressure phase diagrams
of helical magnets MnSi and Cu2OSeO3. | cond-mat_str-el |
Fermionic Monte Carlo study of a realistic model of twisted bilayer
graphene: The rich phenomenology of twisted bilayer graphene (TBG) near the magic angle
is believed to arise from electron correlations in topological flat bands. An
unbiased approach to this problem is highly desirable, but also particularly
challenging, given the multiple electron flavors, the topological obstruction
to defining tight binding models and the long-ranged Coulomb interactions.
While numerical simulations of realistic models have thus far been confined to
zero temperature, typically excluding some spin or valley species, analytic
progress has relied on fixed point models away from the realistic limit. Here
we present for the first time unbiased Monte Carlo simulations of realistic
models of magic angle TBG at charge-neutrality. We establish the absence of a
sign problem for this model in a momentum space approach, and describe a
computationally tractable formulation that applies even on breaking chiral
symmetry and including band dispersion. Our results include (i) the emergence
of an insulating Kramers inter-valley coherent ground state in competition with
a correlated semi-metal phase, (ii) detailed temperature evolution of order
parameters and electronic spectral functions which reveal a `pseudogap' regime,
in which gap features are established at a higher temperature than the onset of
order and (iii) predictions for electronic tunneling spectra and their
evolution with temperature. Our results pave the way towards uncovering the
physics of magic angle graphene through exact simulations of over a hundred
electrons across a wide temperature range. | cond-mat_str-el |
Inelastic neutron scattering studies of the quantum frustrated magnet
clinoatacamite, $γ$-Cu2(OD)3Cl, a proposed valence bond solid (VBS): The frustrated magnet clinoatacamite, $\gamma$-Cu$_2$(OH)$_3$Cl, is
attracting a lot of interest after suggestions that at low temperature it forms
an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised
Cu$^{2+}$ ($S=1/2$) spins.\cite{Lee_clinoatacamite} Key to the arguments
surrounding this proposal were suggestions that the kagom\'e planes in the
magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing
the system to behave as a quasi-2-dimensional magnet. This was reasoned from
the near 95$^\circ$ angles made at the bridging oxygens that mediate exchange
between the Cu ions that link the kagom\'e planes.
Recent work pointed out that this exchange model is inappropriate for
$\gamma$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a
$\mu_3$-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron
powder diffraction to show that the low temperature magnetic structure ($T<6$
K) was canted and involved significant spin ordering on all the Cu$^{2+}$
spins, which is incompatible with the interpretation of simultaneous VBS and
N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted
pyrochlore magnet. In this report we show detailed inelastic neutron scattering
spectra and revisit the responses of this frustrated quantum magnet. | cond-mat_str-el |
Modified kagome physics in the natural spin-1/2 kagome lattice systems -
kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2: The recently discovered natural minerals Cu3Zn(OH)6Cl2 and Cu3Mg(OH)6Cl2 are
spin 1/2 systems with an ideal kagome geometry. Based on electronic structure
calculations, we develop a realistic model which includes couplings across the
kagome hexagons beyond the original kagome model that are intrinsic in real
kagome materials. Exact diagonalization studies for the derived model reveal a
strong impact of these couplings on the magnetic ground state. Our predictions
could be compared to and supplied with neutron scattering, thermodynamic and
NMR data. | cond-mat_str-el |
Freezing out of a low-energy bulk spin exciton in SmB6: The Kondo insulator SmB6 is purported to develop into a robust topological
insulator at low temperature. Yet there are several puzzling and unexplained
physical properties of the insulating bulk. It has been proposed that bulk spin
excitons may be the source of these anomalies and may also adversely affect the
topologically-protected metallic surface states. Here, we report muon spin
rotation measurements of SmB6 that show thermally-activated behavior for the
temperature dependences of the transverse-field (TF) relaxation rate below 20 K
and muon Knight shift below 5-6 K. Our data are consistent with the freezing
out of a bulk low-energy (~ 1 meV) spin exciton concurrent with the appearance
of metallic surface conductivity. Furthermore, our results support the idea
that spin excitons play some role in the anomalous low-temperature bulk
properties of SmB6. | cond-mat_str-el |
Rigorous Wilsonian Renormalization Group for impurity models with a
spectral gap: The Anderson impurity model (AIM) has long served as a cornerstone in the
study of correlated electron systems. While numerical renormalization group
(RG) offers great flexibility for metallic reservoirs, it becomes impossible in
an unbiased way when a spectral gap $\Delta$ opens up in the tunneling density
of states. The only known exception is provided by the superconducting bath. In
this paper, we lift these limitations by a novel numerical RG procedure that
employs a discretization of the gapped tunneling densities of states into
patches which accumulate at the gap edges. This reveals an unusual double
scaling which is a shared behavior by the superconducting and the scalar gapped
AIMs. Moreover, it requires a special iterative diagonalization procedure with
an alternating scheme for discarding states only every second iteration. The
discretization and the diagonalization scheme form together, what we refer to
as, the log-gap numerical RG. It is successfully applied to the superconducting
and to the scalar gapped AIM. Consequently, it reveals that both models belong
to the same RG equivalence class which manifests physically in common
singlet-doublet quantum phase transitions accompanied by in-gap bound states of
given parities. While superconducting AIM is mainly used for benchmarking of
the log-gap numerical RG, we also rigorously confirm the phenomenon of in-gap
states escaping into the continuum, which was recently indirectly considered in
Ref. [1]. The gapped AIM is then tackled in a first ever exact numerical RG
approach and confirms quantitatively assertions based on models with auxiliary
metallic leads [2-5]. Moreover, it reveals that calculations performed in Refs.
[6-8] are of strictly approximate nature. | cond-mat_str-el |
Quantization of fractional corner charge in $C_n$-symmetric higher-order
topological crystalline insulators: In the presence of crystalline symmetries, certain topological insulators
present a filling anomaly: a mismatch between the number of electrons in an
energy band and the number of electrons required for charge neutrality. In this
paper, we show that a filling anomaly can arise when corners are introduced in
$C_n$-symmetric crystalline insulators with vanishing polarization, having as
consequence the existence of corner-localized charges quantized in multiples of
$\frac{e}{n}$. We characterize the existence of this charge systematically and
build topological indices that relate the symmetry representations of the
occupied energy bands of a crystal to the quanta of fractional charge robustly
localized at its corners. When an additional chiral symmetry is present,
$\frac{e}{2}$ corner charges are accompanied by zero-energy corner-localized
states. We show the application of our indices in a number of atomic and
fragile topological insulators and discuss the role of fractional charges bound
to disclinations as bulk probes for these crystalline phases. | cond-mat_str-el |
Unconventional density wave in CeCoIn_5?: Very recently large Nernst effect and Seebeck effect were observed above the
superconducting transition temperature 2.3K in a heavy fermion superconductor
CeCoIn_5. We shall interpret this large Nernst effect in terms of
unconventional density wave (UDW), which appears around T=18K. Also the
temperature dependence of the Seebeck coefficient below T=18K is described in
terms of UDW. Another hallmark for UDW is the angular dependent
magnetoresistance, which should be readily accessible experimentally. | cond-mat_str-el |
Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically
Robust Heavy Fermion System SmOs4Sb12: Elastic properties of the filled skutterudite compound SmOs$_4$Sb$_{12}$ have
been investigated by ultrasonic measurements. The elastic constant
$C_{11}(\omega)$ shows two ultrasonic dispersions at $\sim$15 K and $\sim$53 K
for frequencies $\omega$ between 33 and 316 MHz, which follow a Debye-type
formula with Arrhenius-type temperature-dependent relaxation times, and remain
unchanged even with applied magnetic fields up to 10 T. The corresponding
activation energies were estimated to be $E_2$ = 105 K and $E_1$ = 409 K,
respectively. The latter, $E_1$, is the highest value reported so far in the
Sb-based filled skutterudites. The presence of magnetically robust ultrasonic
dispersions in SmOs$_4$Sb$_{12}$ implies a possibility that an emergence of a
magnetically insensitive heavy fermion state in this system is associated with
a novel local charge degree of freedom which causes the ultrasonic dispersion. | cond-mat_str-el |
Dynamical spin susceptibility in La2CuO4 studied by resonant inelastic
x-ray scattering: Resonant inelastic X-ray scattering (RIXS) is a powerful probe of elementary
excitations in solids. It is now widely applied to study magnetic excitations.
However, its complex cross-section means that RIXS has been more difficult to
interpret than inelastic neutron scattering (INS). Here we report
high-resolution RIXS measurements of magnetic excitations of La2CuO4, the
antiferromagnetic parent of one system of high-temperature superconductors. At
high energies (~2 eV), the RIXS spectra show angular-dependent dd orbital
excitations which are found to be in good agreement with single-site multiplet
calculations. At lower energies (<0.3 eV), we show that the
wavevector-dependent RIXS intensities are proportional to the product of the
single-ion spin-flip cross section and the dynamical susceptibility of the
spin-wave excitations. When the spin-flip crosssection is dividing out, the
RIXS magnon intensities show a remarkable resemblance to INS data. Our results
show that RIXS is a quantitative probe the dynamical spin susceptibility in
cuprate and therefore should be used for quantitative investigation of other
correlated electron materials. | cond-mat_str-el |
Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons
in YbFe2Al10: We present magnetization, specific heat, and 27Al NMR investigations on
YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic
susceptibility at low temperatures is strongly enhanced at weak magnetic
fields, accompanied by a ln(T0/T) divergence of the low-T specific heat
coefficient in zero field, which indicates a ground state of correlated
electrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the
Yb valence at 50 K is evaluated to be 2.38. The system displays valence
fluctuating behavior in the low to intermediate temperature range, whereas
above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of
about 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic
scaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static
susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T)
simultaneously tracks the valence fluctuations from the 4f -Yb ions in the high
temperature range and field dependent antiferromagnetic correlations among
partially Kondo screened Fe 3d moments at low temperature, the latter evolve
out of an Yb 4f admixed conduction band. | cond-mat_str-el |
Dynamics of a bond-disordered $S=1$ quantum magnet near $z=1$
criticality: Neutron scattering is used to study
NiCl$_{2-2x}$Br$_{2x}\cdot$4SC(NH$_2$)$_2$, $x=0.06$, a bond-disordered
modification of the well-known gapped $S=1$ antiferromagnetic quantum spin
system NiCl$_{2}\cdot$4SC(NH$_2$)$_2$. The magnetic excitation spectrum
throughout Brillouin zone is mapped out at $T=60$ mK using high-resolution
time-of-flight spectroscopy. It is found that the dispersion of spin excitation
is renormalized, as compared to that in the parent compound. The lifetime of
excitations near the bottom of the band is substantially decreased. No
localized states are found below the gap energy $\Delta\simeq0.2$ meV. At the
same time, localized zero wave vector states are detected above the top of the
band. The results are consistent with a more or less continuous random
distribution of bond strengths, and a discrete, possibly bimodal, distribution
of single-ion anisotropies in the disordered material. | cond-mat_str-el |
Competing Correlated Insulators in multi-orbital systems coupled to
phonons: We study the interplay between electron-electron interaction and a
Jahn-Teller phonon coupling in a two-orbital Hubbard model. We demonstrate that
the e-ph interaction coexists with the Mott localization driven by the Hubbard
repulsion U, but it competes with the Hund's coupling J. This interplay leads
to two spectacularly different Mott insulators, a standard high-spin Mott
insulator with frozen phonons which is stable when the Hund's coupling
prevails, and a low-spin Mott-bipolaronic insulator favoured by phonons, where
the characteristic features of Mott insulators and bipolarons coexist. The two
phases are separated by a sharp boundary along which an intriguing intermediate
solution emerges as a kind of compromise between the two solutions. | cond-mat_str-el |
Orbital Disordering and metal-insulator transition with hole-doping in
perovskite-type vanadium oxides: Filling-control metal-insulator transitions (MITs) and related electronic
phase diagrams have been investigated for hole-doped vanadium oxides,
Pr_{1-x}Ca_xVO_3, Nd_{1-x}Sr_xVO_3 and Y_{1-x}Ca_xVO_3, with perovskite
structure. The increase of the doping level x causes the melting of the G-type
(and C-type) orbital order, prior to or concomitantly with the MIT, due partly
to the doped-hole motion and partly to the ramdom potential arising from the
quenched disorder. In particular, the G-type spin- and C-type orbital-ordered
phase present in Y_{1-x}Ca_xVO_3 disappears immediately upon hole doping,
around x=0.02. On the other hand, the critical doping level x for MIT is
governed by the electron-correlation strength of the undoped parent compound. | cond-mat_str-el |
Quantum criticality and the formation of a putative electronic liquid
crystal in Sr3Ru2O7: We present a brief review of the physical properties of Sr3Ru2O7, in which
the approach to a magnetic-field-tuned quantum critical point is cut off by the
formation of a novel phase with transport characteristics consistent with those
of a nematic electronic liquid crystal. Our goal is to summarize the physics
that led to that conclusion being drawn, describing the key experiments and
discussing the theoretical approaches that have been adopted. Throughout the
review we also attempt to highlight observations that are not yet understood,
and to discuss the future challenges that will need to be addressed by both
experiment and theory. | cond-mat_str-el |
Gapless edges of 2d topological orders and enriched monoidal categories: In this work, we give a precise mathematical description of a fully chiral
gapless edge of a 2d topological order (without symmetry). We show that the
observables on the 1+1D world sheet of such an edge consist of a family of
topological edge excitations, boundary CFT's and walls between boundary CFT's.
These observables can be described by a chiral algebra and an enriched monoidal
category. This mathematical description automatically includes that of gapped
edges as special cases. Therefore, it gives a unified framework to study both
gapped and gapless edges. Moreover, the boundary-bulk duality also holds for
gapless edges. More precisely, the unitary modular tensor category that
describes the 2d bulk phase is exactly the Drinfeld center of the enriched
monoidal category that describes the gapless/gapped edge. We propose a
classification of all gapped and fully chiral gapless edges of a given bulk
phase. In the end, we explain how modular-invariant bulk conformal field
theories naturally emerge on certain gapless walls between two trivial phases. | cond-mat_str-el |
An anomalous butterfly-shaped magnetoresistance loop in an alloy,
Tb4LuSi3: Magnetic-field (H) induced first-order magnetic transition and the assiciated
electronic phase-separation phenomena are active topics of research in
magnetism. Magnetoresistance (MR) is a key property to probe these phenomena
and, in literature, a butterfly-shaped MR loop has been noted while cycling the
field, with the envelope curve lying below the virgin curve in MR versus H
plots of such materials. Here, we report an opposite behavior of MR loop for an
alloy, Tb4LuSi3, at low temperatures (<<20 K) in the magnetically ordered
state. Such an anomalous curve reveals unexpected domination of higher
resistive high-field phase in electronic conduction, unlike in other materials
where conducion is naturally by low-resistive high-field phase that follows
first-order transition. The observed features reveal an unusual electronic
phase separation, namely involving high-resistive high-field phase and
low-resistive virgin phase. | cond-mat_str-el |
Spin-fluctuation mechanism of superconductivity in cuprates: The theory of superconductivity within the t-J model, as relevant for
cuprates, is developed. It is based on the equations of motion for projected
fermionic operators and the mode-coupling approximation for the self-energy
matrix. The dynamical spin susceptibility at various doping is considered as an
input, extracted from experiments. The analysis shows that the
superconductivity onset is dominated by the spin-fluctuation contribution. We
show that T_c is limited by the spin-fluctuation scale $\Gamma$ and shows a
pronounced dependence on the next-nearest-neighbor hopping t'. The latter can
offer an explanation for the variation of T_c among different families of
cuprates. | cond-mat_str-el |
Quantum and thermal effects in the double exchange ferromagnet: The physics of the ferromagnetic phase of the ``double exchange'' model has
been widely discussed in the context of the CMR manganites. Usually, the double
exchange ferromagnet is treated is classically, by mapping it onto an effective
Heisenberg model. However this mapping does not permit a correct treatment of
quantum or thermal fluctuation effects, and the results obtained lack many of
the interesting features seen in experiments on the manganites. Here we outline
a new analytic approach to systematically evaluating quantum and thermal
corrections to the magnetic and electronic properties of the double exchange
ferromagnet. | cond-mat_str-el |
Interacting Anisotropic Dirac Fermions in Strained Graphene and Related
Systems: We study the role of long-range electron-electron interactions in a system of
two-dimensional anisotropic Dirac fermions, which naturally appear in
uniaxially strained graphene, graphene in external potentials, some strongly
anisotropic topological insulators, and engineered anisotropic graphene
structures. We find that while for small interactions and anisotropy the system
restores the conventional isotropic Dirac liquid behavior, strong enough
anisotropy can lead to the formation of a quasi-one dimensional electronic
phase with dominant charge order (anisotropic excitonic insulator). | cond-mat_str-el |
Elastic Properties and Magnetic Phase Diagrams of Dense Kondo Compound
Ce0.75La0.25B6: We have investigated the elastic properties of the cubic dense Kondo compound
Ce0.75La0.25B6 by means of ultrasonic measurements. We have obtained magnetic
fields vs temperatures (H-T) phase diagrams under magnetic fields along the
crystallographic [001], [110] and [111] axes. An ordered phase IV showing the
elastic softening of c44 locates in low temperature region between 1.6 and 1.1
K below 0.7 T in all field directions. The phase IV shows an isotropic nature
with regard to the field directions, while the antiferro-magnetic phase III
shows an anisotropic character. A remarkable softening of c44 and a spontaneous
trigonal distortion εyz+εzx+εxy recently reported by Akatsu et
al. [J. Phys. Soc. Jpn. 72 (2003) 205] in the phase IV favor a ferro-quadrupole
(FQ) moment of Oyz+Ozx+Oxy induced by an octupole ordering. | cond-mat_str-el |
Magnetism and fine electronic structure of UPd2Al3 and NpPd2Al3: We claim the existence of the f3 (U3+) configuration in UPd2Al3. It is in
agreement with inelastic-neutron-scattering (INS) excitations and is consistent
with the trivalent neptunium configuration in NpPd2Al3. We have derived set of
CEF parameters for the U3+ state that reproduces the INS excitations and
temperature dependence of the heat capacity. On basis of the crystal-field
theory, extended to Quantum Atomistic Solid State Theory we argue that the
magnetic moment of the uranium moment amounts at 0 K to 1.7-1.8 muB.
Keywords: Crystalline Electric Field, Heavy fermion magnetism, UPd2Al3,
NpPd2Al3
PACS: 71.70.E, 75.10.D | cond-mat_str-el |
Temperature-dependent $f$-electron evolution in CeCoIn$_5$ via a
comparative infrared study with LaCoIn$_5$: We investigated CeCoIn$_5$ and LaCoIn$_5$ single crystals, which have the
same HoCoGa$_5$-type tetragonal crystal structure, using infrared spectroscopy.
However, while CeCoIn$_5$ has 4$f$ electrons, LaCoIn$_5$ does not. By comparing
these two material systems, we extracted the temperature-dependent electronic
evolution of the $f$ electrons of CeCoIn$_5$. We observed that the differences
caused by the $f$ electrons are more obvious in low-energy optical spectra at
low temperatures. We introduced a complex optical resistivity and obtained a
magnetic optical resistivity from the difference in the optical resistivity
spectra of the two material systems. From the temperature-dependent average
magnetic resistivity, we found that the onset temperature of the Kondo effect
is much higher than the known onset temperature of Kondo scattering ($\simeq$
200 K) of CeCoIn$_5$. Based on momentum-dependent hybridization, the periodic
Anderson model, and a maximum entropy approach, we obtained the hybridization
gap distribution function of CeCoIn$_5$ and found that the resulting gap
distribution function of CeCoIn$_5$ was mainly composed of two (small and
large) components (or gaps). We assigned the small and large gaps to the
in-plane and out-of-plane hybridization gaps, respectively. We expect that our
results will provide useful information for understanding the
temperature-dependent electronic evolution of $f$-electron systems near Fermi
level. | cond-mat_str-el |
Vortices in the presence of a nonmagnetic atom impurity in 2D XY
ferromagnets: Using a model of nonmagnetic impurity potential, we have examined the
behavior of planar vortex solutions in the classical two-dimensional XY
ferromagnets in the presence of a spin vacancy localized out of the vortex
core. Our results show that a spinless atom impurity gives rise to an effective
potential that repels the vortex structure. | cond-mat_str-el |
Antiferromagnetic spin-1/2 chains in (NO)Cu(NO3)3: a microscopic study: We report on the microscopic model of the recently synthesized
one-dimensional quantum magnet (NO)Cu(NO3)3. Applying density functional theory
band structure calculations, we obtain a leading antiferromagnetic exchange
coupling J ~ 200 K, which runs via NO3 groups forming spin chains along the b
direction. Much weaker couplings J' ~ 2 K link the chains into layers in a
non-frustrated manner. Our calculations do not support the earlier conjecture
on an anisotropic frustrated square lattice physics in (NO)Cu(NO3)3. In
contrast, the model of uniform spin chains leads to a remarkably good fit of
the experimental magnetic susceptibility data, although the low-temperature
features of the intrinsic magnetic susceptibility measured by electron spin
resonance might call for extension of the model. We outline possible
experiments to observe the suggested long-range magnetic ordering in
(NO)Cu(NO3)3 and briefly compare this compound to other spin-1/2 uniform-chain
systems. | cond-mat_str-el |
Anisotropic pseudogap in the half-filling 2-d Hubbard model at finite T: We have studied the pseudogap formation in the single-particle spectra of the
half-filling two-dimensional Hubbard model. Using a Green's function with the
one-loop self-energy correction of the spin and charge fluctuations, we have
numerically calculated the self-energy, the spectral function, and the density
of states in the weak-coupling regime at finite temperature. Pseudogap
formations have been observed in both the density of states and the spectral
function at the Fermi level. The pseudogap in the spectral function is
explained by the non-Fermi-liquid-like nature of the self-energy. The anomalous
behavior in the self-energy is caused by both the strong antiferromagnetic spin
fluctuation and the nesting condition on the non-interacting Fermi surface. In
the present approximation, we find a logarithmic singularity in the integrand
of the imaginary part of the self-energy. Concerning the energy dependence of
the spectral function and the self-energy, two theorems are proved. They give a
necessary condition in the self-energy to produce the pseudogap at the Fermi
level. The pseudogap in the spectral function is highly momentum dependent on
the Fermi surface. It opens initially in the $(\pm \pi,0)$, $(0,\pm \pi)$
regions as the normal state pseudogap observed in the high-$T_c$
superconductors and if the interaction is increased, it spreads to other Fermi
surface sectors. The anisotropy of the pseudogap is produced by the low-energy
enhancement of the spin excitation around ${\bf Q}=(\pi,\pi)$ and the flatness
of the band dispersion around the saddle point. | cond-mat_str-el |
Fermionic superfluidity: From high Tc superconductors to ultracold Fermi
gases: We present a pairing fluctuation theory which self-consistently incorporates
finite momentum pair excitations in the context of BCS--Bose-Einstein
condensation (BEC) crossover, and we apply this theory to high $T_c$
superconductors and ultracold Fermi gases. There are strong similarities
between Fermi gases in the unitary regime and high Tc superconductors. Here we
address key issues of common interest, especially the pseudogap. In the Fermi
gases we summarize recent experiments including various phase diagrams (with
and without population imbalance), as well as evidence for a pseudogap in
thermodynamic and other experiments. | cond-mat_str-el |
Renormalized perturbation calculations for the single impurity Anderson
model: We illustrate the renormalized perturbation expansion method by applying it
to a single impurity Anderson model. Previously, we have shown that this
approach gives the {\it exact} leading order results for the specific heat,
spin and charge susceptibilities and leading order temperature dependence of
the resistivity for this model in the Fermi-liquid regime, when carried out to
second order in the renormalized interaction $\tilde U$. Here we consider the
effects of higher order quasi-particle scattering and calculate the third order
contributions to the $H^3$ term in the impurity magnetization for the symmetric
model in a weak magnetic field $H$. The result is asymptotically exact in the
weak coupling regime, and is very close to the exact Bethe ansatz result in the
Kondo regime. We also calculate the quasi-particle density of states in a
magnetic field, which is of interest in relation to recent experimental work on
quantum dots. | cond-mat_str-el |
Phonon-induced disorder in dynamics of optically pumped metals from
non-linear electron-phonon coupling: The non-equilibrium dynamics of matter excited by light may produce
electronic phases that do not exist in equilibrium, such as laser-induced
high-transition-temperature superconductivity. Here we simulate the dynamics of
a metal driven at initial time $t=0$ by a spatially uniform pump that excites
dipole-active vibrational modes which couple quadratically to electrons. We
study in detail the evolution of electronic and vibrational observables and
their coherences. We provide evidence for enhancement of local electronic
correlations, including double occupancy, accompanied by rapid loss of spatial
structure, which we interpret as a signature of emergent effective disorder in
the dynamics. This effective disorder, which arises in absence of quenched
randomness, dominates the electronic dynamics as the system evolves towards a
correlated electron-phonon long-time state, possibly explaining why transient
superconductivity is not observed. The pumped electron-phonon systems studied
here, which are governed by non-linear coupling, exhibit a much more
substantial dynamical response than linearly coupled models relevant in
equilibrium, thus presenting a pathway to new modalities for out-of-equilibrium
phases. Our results provide a basis within which to understand correlation
dynamics in current pump-probe experiments of vibrationally coupled electrons,
highlight the importance of the evolution of phase coherence, and demonstrate
that pumped electron-phonon systems provide a means of approximately realizing
recently proposed scenarios of dynamically induced disorder in
translation-invariant systems. | cond-mat_str-el |
Complex pressure-temperature structural phase diagram of honeycomb
iridate Cu$_2$IrO$_3$: $\mathrm{Cu_2IrO_3}$ is among the newest layered honeycomb iridates and a
promising candidate to harbor a Kitaev quantum spin liquid state. Here, we
investigate the pressure and temperature dependence of its structure through a
combination of powder x-ray diffraction and x-ray absorption fine structure
measurements, as well as $ab$-$initio$ evolutionary structure search. At
ambient pressure, we revise the previously proposed $C2/c$ solution with a
related but notably more stable $P2_1/c$ structure. Pressures below 8 GPa drive
the formation of Ir-Ir dimers at both ambient and low temperatures, similar to
the case of $\mathrm{Li_2IrO_3}$. At higher pressures, the structural evolution
dramatically depends on temperature. A large discontinuous reduction of the Ir
honeycomb interplanar distance is observed around 15 GPa at room temperature,
likely driven by a collapse of the O-Cu-O dumbbells. At 15 K, pressures beyond
20 GPa first lead to an intermediate phase featuring a continuous reduction of
the interplanar distance, which then collapses at 30 GPa across yet another
phase transition. However, the resulting structure around 40 GPa is not the
same at room and low temperatures. Remarkably, the reduction in interplanar
distance leads to an apparent healing of the stacking faults at room
temperature, but not at 15 K. Possible implications on the evolution of
electronic structure of $\mathrm{Cu_2IrO_3}$ with pressure are discussed. | cond-mat_str-el |
Magnetic transition and spin fluctuations in the unconventional
antiferromagnetic compound Yb3Pt4: Muon spin rotation and relaxation measurements have been carried out on the
unconventional antiferromagnet Yb_3Pt_4. Oscillations are observed below T_N =
2.22(1) K, consistent with the antiferromagnetic (AFM) Neel temperature
observed in bulk experiments. In agreement with neutron diffraction experiments
the oscillation frequency omega_ mu(T) follows a S = 1/2 mean-field temperature
dependence, yielding a quasistatic local field 1.71(2) kOe at T = 0. A crude
estimate gives an ordered moment of ~0.66 mu_B at T = 0, comparable to 0.81
mu_B from neutron diffraction. As T approaches T_N from above the dynamic
relaxation rate lambda_d exhibits no critical slowing down, consistent with a
mean-field transition. In the AFM phase a T-linear fit to lambda_d(T),
appropriate to a Fermi liquid, yields highly enhanced values of lambda_d/T and
the Korringa constant K_ mu^2 T/lambda_d, with K_ mu the estimated muon Knight
shift. A strong suppression of lambda_d by applied field is observed in the AFM
phase. These properties are consistent with the observed large
Sommerfeld-Wilson and Kadowaki-Woods ratios in Yb_3Pt_4 (although the data do
not discriminate between Fermi-liquid and non-Fermi-liquid states), and suggest
strong enhancement of q ~ 0 spin correlations between large-Fermi-volume band
quasiparticles in the AFM phase of Yb_3Pt_4. | cond-mat_str-el |
Quantum kagome antiferromagnet: ZnCu3(OH)6Cl2: Herbertsmithite, ZnCu3(OH)6Cl2, is the prototype candidate for a spin liquid
behavior on a geometrically perfect kagome lattice. Its discovery and the
absence of any evidence for spin-freezing down to the lowest probed temperature
to-date enable one to explore the properties of kagome-related physics in an
unprecedented temperature range. We review its properties and discuss some open
issues. A tentative comparison to models is also performed. | cond-mat_str-el |
Construction of Localized Basis for Dynamical Mean Field Theory: Many-body Hamiltonians obtained from first principles generally include all
possible non-local interactions. But in dynamical mean field theory the
non-local interactions are ignored, and only the effects of the local
interactions are taken into account. The truncation of the non-local
interactions is a basis dependent approximation. We propose a criterion to
construct an appropriate localized basis in which the truncation can be carried
out. This involves finding a basis in which a functional given by the sum of
the squares of the local interactions with appropriate weight factors is
maximized under unitary transformations of basis. We argue that such a
localized basis is suitable for the application of dynamical mean field theory
for calculating material properties from first principles. We propose an
algorithm which can be used for constructing the localized basis. We test our
criterion on a toy model and find it satisfactory. | cond-mat_str-el |
Structural and dynamical study of moment localization in
beta-Mn(1-x)In(x): We have used neutron scattering and muon spin relaxation (muSR) to
investigate the structural and magnetic properties of the beta-phase of
elemental manganese doped with dilute amounts of indium. beta-Mn is an example
of a topologically frustrated antiferromagnetically correlated metal - but
which remains paramagnetic at all temperatures. The addition of In to beta-Mn
results in a vast volume expansion of the lattice, and would therefore be
expected to have a major effect on the stability and localization of the Mn
moment - as observed in, for example, Ru and Al doped beta-Mn alloys. We find
that In doping in beta-Mn results in a short-range ordered spin-glass like
ground state, similar to that of Al-doped beta-Mn but with residual low
frequency spin fluctuations. This is in contrast to Ru doping which results in
the stabilization of a long-range ordered Mn moment | cond-mat_str-el |
Terahertz parametric amplification as a reporter of exciton condensate
dynamics: Condensates are a hallmark of emergence in quantum materials with
superconductors and charge density wave as prominent examples. An excitonic
insulator (EI) is an intriguing addition to this library, exhibiting
spontaneous condensation of electron-hole pairs. However, condensate
observables can be obscured through parasitic coupling to the lattice.
Time-resolved terahertz (THz) spectroscopy can disentangle such obscurants
through measurement of the quantum dynamics. We target $Ta_{2}NiSe_{5}$, a
putative room-temperature EI where electron-lattice coupling dominates the
structural transition ($T_{c}$=326 K), hindering identification of excitonic
correlations. A pronounced increase in the THz reflectivity manifests following
photoexcitation and exhibits a BEC-like temperature dependence. This occurs
well below the $T_{c}$, suggesting a novel approach to monitor exciton
condensate dynamics. Nonetheless, dynamic condensate-phonon coupling remains as
evidenced by peaks in the enhanced reflectivity spectrum at select
infrared-active phonon frequencies. This indicates that parametric reflectivity
enhancement arises from phonon squeezing, validated using Fresnel-Floquet
theory and density functional calculations. Our results highlight that coherent
dynamics can drive parametric stimulated emission with concomitant
possibilities, including entangled THz photon generation. | cond-mat_str-el |
Scaling of THz-conductivity at metal-insulator transition in doped
manganites: Magnetic field and temperature dependence of the Terahertz conductivity and
permittivity of the colossal magnetoresistance manganite
Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the
metal-to-insulator transition (MIT) from the insulating side. In the
charge-ordered state of PCSMO both conductivity and dielectric permittivity
increase as function of magnetic field and temperature. Universal scaling
relationships between the changes in permittivity and conductivity are observed
in a broad range of temperatures and magnetic fields. Similar scaling is also
seen in La_{1-x}Sr_xMnO_3 for different doping levels. The observed
proportionality points towards the importance of pure ac-conductivity and
phononic energy scale at MIT in manganites. | cond-mat_str-el |
Ground-state selection and spin-liquid behaviour in the classical
Heisenberg model on the breathing pyrochlore lattice: Magnetic pyrochlore oxides, including the spin ice materials, have proved to
be a rich field for the study of geometrical frustration in 3 dimensions.
Recently, a new family of magnetic oxides has been synthesised in which the
half of the tetrahedra in the pyrochlore lattice are inflated relative to the
other half, making an alternating array of small and large tetrahedra. These
"breathing pyrochlore" materials such as LiGaCr4O8, LiInCr4O8 and Ba3Yb2Zn5O11
provide new opportunities in the study of frustrated magnetism. Here we provide
an analytic theory for the ground state phase diagram and spin correlations for
the minimal model of magnetism in breathing pyrochlores: a classical nearest
neighbour Heisenberg model with different exchange coefficients for the two
species of tetrahedra. We find that the phase diagram comprises a Coulombic
spin liquid phase, a conventional ferromagnetic phase and an unusual
antiferromagnetic phase with lines of soft modes in reciprocal space,
stabilised by an order-by-disorder mechanism. We obtain a theory of the spin
correlations in this model using the Self Consistent Gaussian Approximation
(SCGA) which enables us to discuss the development of correlations in breathing
pyrochlores as a function of temperature, and we quantitatively characterise
the thermal crossover from the limit of isolated tetrahedra to the strongly
correlated limit of the problem. We compare the results of our analysis with
the results of recent neutron scattering experiments on LiInCr4O8. | cond-mat_str-el |
My Random Walks in Anderson's Garden: Anderson's Garden is a drawing presented to Philip W. Anderson on the eve of
his 60th birthday celebration, in 1983. This cartoon (Fig. 1), whose author is
unknown, succinctly depicts some of Anderson's pre-1983 works, as a blooming
garden. As an avid reader of Anderson's papers, random walk in Anderson's
garden had become a part of my routine since graduate school days. This was of
immense help and prepared me for a wonderful collaboration with the gardener
himself, on the resonating valence bond (RVB) theory of High Tc cuprates and
quantum spin liquids, at Princeton. The result was bountiful - the first (RVB
mean field) theory for i) quantum spin liquids, ii) emergent fermi surfaces in
Mott insulators and iii) superconductivity in doped Mott insulators. Beyond
mean field theory - i) emergent gauge fields, ii) Ginzbuerg Landau theory with
RVB gauge fields, iii) prediction of superconducting dome, iv) an early
identification and study of a non-fermi liquid normal state of cuprates and so
on. Here I narrate this story, years of my gardening attempts and end with a
brief summary of my theoretical efforts to extend RVB theory of
superconductivity to encompass the recently observed very high Tc ~ 203 K
superconductivity in molecular solid H2S at high pressures ~ 200 GPa. | cond-mat_str-el |
Magnetic-field induced triplet superconductivity in the Hubbard model on
a triangular lattice: We propose theoretically that a magnetic field can realize spin-triplet
superconductivity in repulsively interacting electron systems having strong
ferromagnetic spin fluctuations. We confirm the general idea for the
low-density Hubbard model on a triangular lattice, whose Fermi surface consists
of disconnected pieces, by calculating the pairing susceptibility in a moderate
magnetic field with the quantum Monte-Carlo method combined with the dynamical
cluster approximation. | cond-mat_str-el |
Phonon Thermal Transport of URu2Si2: Broken Translational Symmetry and
Strong-Coupling of the Hidden Order to the Lattice: A dramatic increase in the total thermal conductivity (k) is observed in the
Hidden Order (HO) state of single crystal URu2Si2. Through measurements of the
thermal Hall conductivity, we explicitly show that the electronic contribution
to k is extremely small, so that this large increase in k is dominated by
phonon conduction. An itinerant BCS/mean-field model describes this behavior
well: the increase in kappa is associated with the opening of a large energy
gap at the Fermi Surface, thereby decreasing electron-phonon scattering. Our
analysis implies that the Hidden Order parameter is strongly coupled to the
lattice, suggestive of a broken symmetry involving charge degrees of freedom. | cond-mat_str-el |
Charge Stripe in an Antiferromagnet: 1d Band of Composite Excitations: With the help of analytical and numerical studies of the $t$-$J_z$ model we
argue that the charge stripe in an antiferromagnetic insulator should be
understood as a system of holon-spin-polaron excitations condensed at the
self-induced antiphase domain wall. The structure of such a charge excitation
is studied in detail with numerical and analytical results for various
quantities being in a very close agreement. An analytical picture of these
excitations occupying an effective 1D stripe band is also in a very good accord
with numerical data. The emerging concept advocates the primary role of the
kinetic energy in favoring the stripe as a ground state. A comparative analysis
suggests the effect of pairing and collective meandering on the energetics of
the stripe formation to be secondary. | cond-mat_str-el |
Josephson current through the SYK model: We calculate the equilibrium Josephson current through a disordered
interacting quantum dot described by a Sachdev-Ye-Kitaev model contacted by two
BCS superconductors. We show that, at zero temperature and at the conformal
limit, i.e. in the strong interacting limit, the Josephson current is
suppressed by $U$, the strength of the interaction, as $\ln(U)/U$ and becomes
universal, namely it gets independent on the superconducting pairing. At finite
temperature $T$, instead, it depends on the ratio between the gap $\Delta$ and
the temperature and goes as $\Delta^2/T^2$ for sufficiently large temperatures.
A proximity effect exists but the self-energy corrections induced by the
coupling with the superconducting leads seem subleading as compared to the
self-energy due to the interaction for large number of particles. | cond-mat_str-el |
Lattice source for charge and spin inhomogeneity in 2D perovskite
cuprates: In the work we highlight the structural features of 2D perovskite cuprates
(tilted CuO$_6$ octahedra with different orientation with respect to spacer
rocksalt layers), where sources of charge and spin inhomogeneity can be hidden.
We used the impurity Anderson model with the Jahn-Teller(JT) local cells to
show the charge inhomogeneity arises at any low doping concentration $x$, but
disappears when the doping level exceeds threshold concentration $x_c$, and the
lower the magnitudes $x_c$, the more JT region square. It is expected that
spontaneous chiral symmetry breaking in the dynamic JT state of the stripe
CuO$_2$ layer as a whole can lead to the appearance of the goldstone phonon
mode. As consequence, the giant thermal Hall effect could be observed in the 2D
perovskite cuprates with CuO$_6$ octahedra, rather than with CuO$_4$ squares,
e.g. in Tl-based $n$ layer cuprates or cuprates based on the infinite-layer
CaCuO$_2$ structure. | cond-mat_str-el |
Connecting high-field quantum oscillations to zero-field electron
spectral functions in the underdoped cuprates: The central puzzle of the cuprate superconductors at low hole density is the
nature of the pseudogap regime. It has a number of seemingly distinct
experimental signatures: a suppression of the paramagnetic spin susceptibility
at high temperatures, low energy electronic excitations that extend over arcs
in the Brillouin zone, X-ray detection of charge density wave order at
intermediate temperatures, and quantum oscillations at high magnetic fields and
low temperatures. We show that a model of competing charge density wave and
superconducting orders provides a unified description of the intermediate and
low temperature regimes. We treat quantum oscillations at high field beyond
semiclassical approximations, and find clear and robust signatures of an
electron pocket compatible with existing observations; we also predict
oscillations due to additional hole pockets. In the zero field and intermediate
temperature regime, we compute the electronic spectrum in the presence of
thermally fluctuating charge density and superconducting orders. Our results
are compatible with experimental trends. | cond-mat_str-el |
Minimalist approach to the classification of symmetry protected
topological phases: A number of proposals with differing predictions (e.g. Borel group
cohomology, oriented cobordism, group supercohomology, spin cobordism, etc.)
have been made for the classification of symmetry protected topological (SPT)
phases. Here we treat various proposals on an equal footing and present
rigorous, general results that are independent of which proposal is correct. We
do so by formulating a minimalist Generalized Cohomology Hypothesis, which is
satisfied by existing proposals and captures essential aspects of SPT
classification. From this Hypothesis alone, formulas relating classifications
in different dimensions and/or protected by different symmetry groups are
derived. Our formalism is expected to work for fermionic as well as bosonic
phases, Floquet as well as stationary phases, and spatial as well as on-site
symmetries. As an application, we predict that the complete classification of
3-dimensional bosonic SPT phases with space group symmetry $G$ is $H^4_{\rm
Borel}\left(G;U(1)\right) \oplus H^1_{\rm group}\left(G;\mathbb Z\right)$,
where the $H^1$ term classifies phases beyond the Borel group cohomology
proposal. | cond-mat_str-el |
Realistic Modeling of Complex Oxide Materials: Since electronic and magnetic properties of many transition-metal oxides can
be efficiently controlled by external factors such as the temperature,
pressure, electric or magnetic field, they are regarded as promising materials
for various applications. From the viewpoint of electronic structure, these
phenomena are frequently related to the behavior of a small group of states
close to the Fermi level. The basic idea of this project is to construct a
low-energy model for the states near the Fermi level on the basis of
first-principles density functional theory, and to study this model by modern
many-body techniques. After a brief review of the method, the abilities of this
approach will be illustrated on a number of examples, including multiferroic
manganites and spin-orbital-lattice coupled phenomena in RVO3 (R being the
three-valent element). | cond-mat_str-el |
Intrasubband and Intersubband Electron Relaxation in Semiconductor
Quantum Wire Structures: We calculate the intersubband and intrasubband many-body inelastic Coulomb
scattering rates due to electron-electron interaction in two-subband
semiconductor quantum wire structures. We analyze our relaxation rates in terms
of contributions from inter- and intrasubband charge-density excitations
separately. We show that the intersubband (intrasubband) charge-density
excitations are primarily responsible for intersubband (intrasubband) inelastic
scattering. We identify the contributions to the inelastic scattering rate
coming from the emission of the single-particle and the collective excitations
individually. We obtain the lifetime of hot electrons injected in each subband
as a function of the total charge density in the wire. | cond-mat_str-el |
Decoherence of charge density waves in beam splitters for interacting
quantum wires: Simple intersections between one-dimensional channels can act as coherent
beam splitters for non-interacting electrons. Here we examine how coherent
splitting at such intersections is affected by inter-particle interactions, in
the special case of an intersection of topological edge states. We derive an
effective impurity model which represents the edge-state intersection within
Luttinger liquid theory at low energy. For Luttinger K = 1 / 2 , we compute the
exact time-dependent expectation values of the charge density as well as the
density-density correlation functions. In general a single incoming charge
density wave packet will split into four outgoing wave packets with
transmission and reflection coefficients depending on the strengths of the
tunnelling processes between the wires at the junction. We find that when
multiple charge density wave packets from different directions pass through the
intersection at the same time, reflection and splitting of the packets depend
on the relative phases of the waves. Active use of this phase-dependent
splitting of wave packets may make Luttinger interferometry possible. We also
find that coherent incident packets generally suffer partial decoherence from
the intersection, with some of their initially coherent signal being
transferred into correlated quantum noise. In an extreme case four incident
coherent wave packets can be transformed entirely into density-density
correlations, with the charge density itself having zero expectation value
everywhere in the final state. | cond-mat_str-el |
Influence of phonon-assisted tunneling on the linear thermoelectric
transport through molecular quantum dots: We investigate the effect of vibrational degrees of freedom on the linear
thermoelectric transport through a single-level quantum dot described by the
spinless Anderson-Holstein impurity model. To study the effects of strong
electron-phonon coupling, we use the nonperturbative numerical renormalization
group approach. We also compare our results, at weak to intermediate coupling,
with those obtained by employing the functional renormalization group method,
finding good agreement in this parameter regime. When applying a gate voltage
at finite temperatures, the inelastic scattering processes, induced by
phonon-assisted tunneling, result in an interesting interplay between
electrical and thermal transport. We explore different parameter regimes and
identify situations for which the thermoelectric power as well as the
dimensionless figure of merit are significantly enhanced via a Mahan-Sofo type
of mechanism. We show, in particular, that this occurs at strong
electron-phonon coupling and in the antiadiabatic regime. | cond-mat_str-el |
Muon spin rotation and neutron scattering study of the
non-centrosymmetric tetragonal compound CeAuAl3: We have investigated the non-centrosymmetric tetragonal heavy-fermion
compound CeAuAl3 using muon spin rotation (muSR), neutron diffraction (ND) and
inelastic neutron scattering (INS) measurements. We have also revisited the
magnetic, transport and thermal properties. The magnetic susceptibility reveals
an antiferromagnetic transition at 1.1 K with a possibility of another magnetic
transition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at
1.1 K in zero-filed, which broadens and moves to higher temperature in applied
magnetic field. Our zero-field muSR and ND measurements confirm the existence
of a long-range magnetic ground state below 1.2 K. Further the ND study reveals
an incommensurate magnetic ordering with a magnetic propagation vector k = (0,
0, 0.52) and a spiral structure of Ce moments coupled ferromagnetically within
the ab-plane. Our INS study reveals the presence of two well-defined crystal
electric field (CEF) excitations at 5.1 meV and 24.6 meV in the paramagnetic
phase of CeAuAl3 which can be explained on the basis of the CEF theory.
Furthermore, low energy quasi-elastic excitations show a Gaussian line shape
below 30 K compared to a Lorentzian line shape above 30 K, indicating a
slowdown of spin fluctuation below 30 K. We have estimated a Kondo temperature
of TK=3.5 K from the quasi-elastic linewidth, which is in good agreement with
that estimated from the heat capacity. This study also indicates the absence of
any CEF-phonon coupling unlike that observed in isostructural CeCuAl3. The CEF
parameters, energy level scheme and their wave functions obtained from the
analysis of INS data explain satisfactorily the single crystal susceptibility
in the presence of two-ion anisotropic exchange interaction in CeAuAl3. | cond-mat_str-el |
Magnetic tunneling induced Weyl node annihilation in TaP: Weyl nodes are topological objects in three-dimensional metals. Their
topological property can be revealed by studying the high-field transport
properties of a Weyl semimetal. While the energy of the lowest Landau band
(LLB) of a conventional Fermi pocket always increases with magnetic field due
to the zero point energy, the LLB of Weyl cones remains at zero energy unless a
strong magnetic field couples the Weyl fermions of opposite chirality. In the
Weyl semimetal TaP, we achieve such a magnetic coupling between the
electron-like Fermi pockets arising from the W1 Weyl fermions. As a result,
their LLBs move above chemical potential, leading to a sharp sign reversal in
the Hall resistivity at a specific magnetic field corresponding to the W1 Weyl
node separation. By contrast, despite having almost identical carrier density,
the annihilation is unobserved for the hole-like pockets because the W2 Weyl
nodes are much further separated. These key findings, corroborated by other
systematic analyses, reveal the nontrivial topology of Weyl fermions in
high-field measurements. | cond-mat_str-el |
NaCo2(SeO3)2(OH): Competing Magnetic Ground States of a New Sawtooth
Structure with 3d7 Co2+ Ions: While certain magnetic sublattices have long been known theoretically to give
rise to emergent physics via competing magnetic interactions and quantum
effects, finding such configurations in real materials is often deeply
challenging. Here we report the synthesis and characterization of a new such
material, NaCo2(SeO3)2(OH) which crystallizes with a highly frustrated
sublattice of sawtooth Co2+ chains. Single crystals of NaCo2(SeO3)2(OH) were
synthesized using a low-temperature hydrothermal method. X-ray single crystal
structure analysis reveals that the material crystallizes in orthorhombic space
group of Pnma (no. 62). Its crystal structure exhibits one-dimensional chains
of corner-sharing isosceles triangles that are made of two crystallographically
distinct 3d7 Co2+ sites (Co(1) and Co(2)). The chains run along the b-axis and
are interconnected via [SeO3] groups to form a three-dimensional structure
mediating super-exchange interactions. The temperature dependent magnetization
data show a ferromagnetic-like (FM) transition at 11 K (T1) followed by an
antiferromagnetic (AFM) transition at about 6 K (T2). Neutron-powder
diffraction measurements reveal that at T1 = 11 K only Co(2) site orders
magnetically, forming ferromagnetic zigzag chains along the b-axis. Below T2 =
6 K, both Co(1) and Co(2) sites order in an nearly orthogonal configuration,
with Co(1) moments lying inside the plane of the sawtooth chain while Co(2)
moments cant out of the plane. The canting of the magnetic moments leads to a
net ferromagnetic component along b-axis, parallel to the chain direction. The
ordered moments are fully compensated in the ac-plane. | cond-mat_str-el |
Van der Waals Schottky barriers as interface probes of the correlation
between chemical potential shifts and charge density wave formation in
1T-TiSe$_2$ and 2H-NbSe$_2$: Layered transition metal dichalcogenide (TMD) materials, i.e. 1T-TiSe$_2$ and
2H-NbSe$_2$, harbor a second order charge density wave (CDW) transition where
phonons play a key role for the periodic modulations of conduction electron
densities and associated lattice distortions. We systematically study the
transport and capacitance characteristics over a wide temperature range of
Schottky barriers formed by intimately contacting freshly exfoliated flakes of
1T-TiSe$_2$ and 2H-NbSe$_2$ to \textit{n}-type GaAs semiconductor substrates.
The extracted temperature-dependent parameters (zero-bias barrier height,
ideality and built-in potential) reflect changes at the TMD/GaAs interface
induced by CDW formation for both TMD materials. The measured built-in
potential reveals chemical-potential shifts relating to CDW formation. With
decreasing temperature a peak in the chemical-potential shifts during CDW
evolution indicates a competition between electron energy re-distributions and
a combination of lattice strain energies and Coulomb interactions. These
modulations of chemical potential in CDW systems, such as 1T-TiSe$_2$ and
2H-NbSe$_2$ harboring second-order phase transitions, reflect a corresponding
conversion from short to long-range order. | cond-mat_str-el |
Quantum renormalization of entanglement in an antisymmetric anisotropic
and bond-alternating spin system: The quantum renormalization group method is applied to study the quantum
criticality and entanglement entropy of the ground state of the Ising chain in
the presence of antisymmetric anisotropic couplings and alternating exchange
interactions. The quantum phase transitions can be characterized by the
discontinuity in the second derivative of the energy of renormalized ground
state. The phase diagram is obtained by the critical boundary line. The first
derivative of entanglement entropy also diverges at the same critical points
after enough iterations of the renormalization of coupling constants. The
antisymmetric anisotropy and alternating interaction can enhance the
renormalized entanglement via the creation of quantum fluctuations. The scaling
behavior of the derivative of the entropy around the critical points manifest
the logarithm dependence on the size of the spin system. | cond-mat_str-el |
Photo-induced Tomonaga-Luttinger-like liquid in a Mott insulator: Photo-induced metallic states in a Mott insulator are studied for the
half-filled, one-dimensional Hubbard model with the time-dependent density
matrix renormalization group. An irradiation of strong AC field is found to
create a linear dispersion in the optical spectrum (current-current
correlation) in the nonequilibrium steady state reminiscent of the
Tomonaga-Luttinger liquid for the doped Mott insulator in equilibrium. The spin
spectrum in nonequilibrium retains the des Cloizeaux-Pearson mode with the spin
velocity differing from the charge velocity. The mechanism of the
photocarrier-doping, along with the renormalization in the charge velocity, is
analyzed in terms of an effective Dirac model. | cond-mat_str-el |
Interaction-induced adiabatic cooling and antiferromagnetism of cold
fermions in optical lattices: We propose an interaction-induced cooling mechanism for two-component cold
fermions in an optical lattice. It is based on an increase of the ``spin''
entropy upon localisation, an analogue of the Pomeranchuk effect in liquid
Helium 3. We discuss its application to the experimental realisation of the
antiferromagnetic phase. We illustrate our arguments with Dynamical Mean-Field
Theory calculations. | cond-mat_str-el |
Magnon-Hole Scattering and Charge Order in $Sr_{14-x}Ca_xCu_{24}O_{41}$: The magnon thermal conductivity $\kappa_{\mathrm{mag}}$ of the hole doped
spin ladders in $\rm Sr_{14-x}Ca_xCu_{24}O_{41}$ has been investigated at low
doping levels $x$. The analysis of $\kappa_{\mathrm{mag}}$ reveals a strong
doping and temperature dependence of the magnon mean free path
$l_{\mathrm{mag}}$ which is a local probe for the interaction of magnons with
the doped holes in the ladders. In particular, this novel approach to studying
charge degrees of freedom via spin excitations shows that charge ordering of
the holes in the ladders leads to a freezing out of magnon-hole scattering
processes. | cond-mat_str-el |
Negative local resistance caused by viscous electron backflow in
graphene: Graphene hosts a unique electron system in which electron-phonon scattering
is extremely weak but electron-electron collisions are sufficiently frequent to
provide local equilibrium above liquid nitrogen temperature. Under these
conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic
phenomena similar to classical liquids. Here we report strong evidence for this
transport regime. We find that doped graphene exhibits an anomalous (negative)
voltage drop near current injection contacts, which is attributed to the
formation of submicrometer-size whirlpools in the electron flow. The viscosity
of graphene's electron liquid is found to be ~0.1 m$^2$ /s, an order of
magnitude larger than that of honey, in agreement with many-body theory. Our
work shows a possibility to study electron hydrodynamics using high quality
graphene. | cond-mat_str-el |
Scattering rate, transport and specific heat in a metal close to a
quantum critical point : emergence of a robust Fermi liquid picture ?: We calculate the low temperature one-particle scattering rate and the
specific heat in a weakly disordered metal close to a quantum critical point.
To lowest order in the fluctuation potential, we obtain typical Fermi-liquid
results proportional to T^2 and T respectively, with prefactors which diverge
as a power law of the control parameter upon approaching the critical point.
The Kadowaki-Woods ratio is shown to be independent of the control parameter
only for the case of 3-D FM fluctuations. Our work is relevant for experiments
on CeCoIn$_5$ and Sr_3Ru_2O_7. | cond-mat_str-el |
Nonresonant B1g Raman scattering in the Hubbard model: The numerically exact solution for nonresonant B1g Raman scattering is
presented for the half-filled Hubbard model in infinite dimensions. This
solution illustrates the modifications of the Raman response (in a system tuned
through the quantum-critical point of a metal-insulator transition) due to
Fermi-liquid properties in the metallic phase. In the insulating phase, we
recover the predicted universal behavior, while we find the Raman response is
quite anomalous on the metallic side of the transition. Our calculated results
are similar to those measured in FeSi, SmB6, and underdoped cuprates. | cond-mat_str-el |
Spontaneous Magnetization of an Ideal Ferromagnet: Beyond Dyson's
Analysis: Using the low-energy effective field theory for magnons, we systematically
evaluate the partition function of the O(3) ferromagnet up to three loops.
Dyson, in his pioneering microscopic analysis of the Heisenberg model, showed
that the spin-wave interaction starts manifesting itself in the low-temperature
expansion of the spontaneous magnetization of an ideal ferromagnet only at
order $T^4$. Although several authors tried to go beyond Dyson's result, to the
best of our knowledge, a fully systematic and rigorous investigation of higher
order terms induced by the spin-wave interaction, has never been achieved. As
we demonstrate in the present paper, it is straightforward to evaluate the
partition function of an ideal ferromagnet beyond Dyson's analysis, using
effective Lagrangian techniques. In particular, we show that the
next-to-leading contribution to the spontaneous magnetization resulting from
the spin-wave interaction already sets in at order $T^{9/2}$ -- in contrast to
all claims that have appeared before in the literature. Remarkably, the
corresponding coefficient is completely determined by the leading-order
effective Lagrangian and is thus independent of the anisotropies of the cubic
lattice. We also consider even higher-order corrections and thereby solve --
once and for all -- the question of how the spin-wave interaction in an ideal
ferromagnet manifests itself in the spontaneous magnetization beyond the Dyson
term. | cond-mat_str-el |
Systematic improvement of the Momentum Average approximation for the
Green's function of a Holstein polaron: We show how to systematically improve the Momentum Average (MA)approximation
for the Green's function of a Holstein polaron, bysystematically improving the
accuracy of the self-energy diagrams in such a way that they can still all be
summed efficiently. This allows us to fix some of the problems of the MA
approximation, e.g. we now find the expected polaron+phonon continuum at the
correct location, and a momentum-dependent self-energy. The quantitative
agreement with numerical data is further improved, as expected since the number
of exactly satisfied spectral weight sum rules is increased. The corrections
are found to be larger in lower dimensional systems. | cond-mat_str-el |
Strong in-plane anisotropy in the electronic structure of fixed-valence
$β$-LuAlB$_4$: The origin of intrinsic quantum criticality in the heavy-fermion
superconductor $\beta$-YbAlB$_4$ has been attributed to strong Yb valence
fluctuations and its peculiar crystal structure. Here, we assess these
contributions individually by studying the isostructural but fixed-valence
compound $\beta$-LuAlB$_4$. Quantum oscillation measurements and DFT
calculations reveal a Fermi surface markedly different from that of
$\beta$-YbAlB$_4$, consistent with a `large' Fermi surface there. We also find
an unexpected in-plane anisotropy of the electronic structure, in contrast to
the isotropic Kondo hybridization in $\beta$-YbAlB$_4$. | cond-mat_str-el |
Photoinduced Structural Phase Transitions in Polyacene: There exist two types of structural instability in polyacene: double bonds in
a cis pattern and those in a trans pattern. They are isoenergetic but
spectroscopically distinct. We demonstrate optical characterization and
manipulation of Peierls-distorted polyacene employing both correlated and
uncorrelated Hamiltonians. We clarify the phase boundaries of the cis- and
trans-distorted isomers, elucidate their optical-conductivity spectra, and then
explore their photoresponses. There occurs a photoinduced transformation in the
polyacene structure, but it is one-way switching: The trans configuration is
well convertible into the cis one, whereas the reverse conversion is much less
feasible. Even the weakest light irradiation can cause a transition of
uncorrelated electrons, while correlated electrons have a transition threshold
against light irradiation. | cond-mat_str-el |
Emergent particle-hole symmetry in the half-filled Landau level: We provide an effective description of a particle-hole symmetric state of
electrons in a half-filled Landau level, starting from the traditional approach
pioneered by Halperin, Lee and Read. Specifically, we study a system consisting
of alternating quasi-one-dimensional strips of composite Fermi liquid (CFL) and
composite hole liquid (CHL), both of which break particle-hole symmetry. When
the CFL and CHL strips are identical in size, the resulting state is manifestly
invariant under the combined action of a particle-hole transformation with
respect to a single Landau level (which interchanges the CFL and CHL) and
translation by one unit, equal to the strip width, in the direction transverse
to the strips. At distances long compared to the strip width, we demonstrate
that the system is described by a Dirac fermion coupled to an emergent gauge
field, with an anti-unitary particle-hole symmetry, as recently proposed by
Son. | cond-mat_str-el |
Reduction of ordered moment and Neel temperature of quasi
one-dimensional antiferromagnets Sr2CuO3 and Ca2CuO3: We report elastic neutron diffraction and muon spin relaxation (muSR)
measurements of the quasi one-dimensional antiferromagnets Sr2CuO3 and Ca2CuO3,
which have extraordinarily reduced TN/J ratios. We observe almost
resolution-limited antiferromagnetic Bragg reflections in Sr2CuO3 and obtain a
reduced ordered moment size of ~0.06 \muB. We find that the ratio of ordered
moment size \mu(Ca2CuO3)/\mu(Sr2CuO3)=1.5(1) roughly scales with their Neel
temperatures, which suggests that the ordered moment size of quasi
one-dimensional antiferromagnets decreases continuously in the limit of
vanishing inter-chain interactions. | cond-mat_str-el |
Random singlet-like phase of disordered Hubbard chains: Local moment formation is ubiquitous in disordered semiconductors such as
Si:P, where it is observed both in the metallic and the insulating regimes.
Here, we focus on local moment behavior in disordered insulators, which arises
from short-ranged, repulsive electron-electron interactions. Using density
matrix renormalization group and strong-disorder renormalization group methods,
we study paradigmatic models of interacting insulators: one dimensional Hubbard
chains with quenched randomness. In chains with either random fermion hoppings
or random chemical potentials, both at and away from half-filling, we find
exponential decay of charge and fermion 2-point correlations but power-law
decay of spin correlations that are indicative of the random singlet phase. The
numerical results can be understood qualitatively by appealing to the
large-interaction limit of the Hubbard chain, in which a remarkably simple
picture emerges. | cond-mat_str-el |
Paramagnetism in the kagome compounds
(Zn,Mg,Cd)Cu$_{3}$(OH)$_{6}$Cl$_{2}$: Frustrated magnetism on the kagome lattice has been a fertile ground for rich
and fascinating physics, ranging from experimental evidence of a spin liquid to
theoretical predictions of exotic superconductivity. Among experimentally
realized spin-$\frac{1}{2}$ kagome magnets, herbertsmithite, kapellasite, and
haydeeite [(Zn,Mg)Cu$_{3}$(OH)$_{6}$Cl$_{2}$] are all well described by a
three-parameter Heisenberg model, but they exhibit distinctly different
physics. We address the problem using a pseudofermion functional
renormalization-group approach and analyze the low-energy physics in the
experimentally accessible parameter range. Our analysis places kapellasite and
haydeeite near the boundaries between magnetically ordered and disordered
phases, implying that slight modifications could dramatically affect their
magnetic properties. Inspired by this, we perform \textit{ab initio} density
functional theory calculations of (Zn,Mg,Cd)Cu$_{3}$ (OH)$_{6}$Cl$_{2}$ at
various pressures. Our results suggest that by varying pressure and composition
one can traverse a paramagnetic regime between different magnetically ordered
phases. | cond-mat_str-el |
Local magnetic moment formation and Kondo screening in the half filled
two dimensional single band Hubbard model: We study formation of local magnetic moments in strongly correlated Hubbard
model within dynamical mean field theory and associate peculiarities of
temperature dependence of local charge $\chi_c$ and spin $\chi_s$
susceptibilities with different stages of local moment formation. Local maximum
of temperature dependence of the charge susceptibility $\chi_c$ is associated
with beginning of local magnetic moments formation, while the minimum of the
susceptibility $\chi_c$ and double occupation, as well as low temperature
boundary of the plateau of effective local magnetic moment $\mu_{\rm
eff}^2=T\chi_s$ temperature dependence are connected with full formation of
local moments. We also obtain interaction dependence of the Kondo temperature
$T_K$, which is compared to the fingerprint criterion of Phys. Rev. Lett. 126,
056403 (2021). Near the Mott transition the two criteria coincide, while
further away from Mott transition the fingerprint criterion somewhat
overestimates Kondo temperature. The relation of the observed features to the
behavior of eigenvectors/eigenvalues of fermionic frequency-resolved charge
susceptibility and divergences of irreducible vertices is discussed. | cond-mat_str-el |
Orbital superexchange and crystal field simultaneously at play in YVO3:
resonant inelastic x-ray scattering at the V L edge and the O K edge: We report on the observation of orbital excitations in YVO3 by means of
resonant inelastic x-ray scattering (RIXS) at energies across the vanadium L3
and oxygen K absorption edges. Due to the excellent experimental resolution we
are able to resolve the intra-t2g excitations at 0.1-0.2 eV, 1.07 eV, and 1.28
eV, the lowest excitations from the t2g into the eg levels at 1.86 eV, and
further excitations above 2.2 eV. For the intra-t2g excitations at 0.1-0.2 eV,
the RIXS peaks show small shifts of the order of 10-40 meV as a function of
temperature and of about 13-20 meV as a function of the transferred momentum
q||a. We argue that the latter reflects a finite dispersion of the orbital
excitations. For incident energies tuned to the oxygen K edge, RIXS is more
sensitive to intersite excitations. We observe excitations across the
Mott-Hubbard gap and find an additional feature at 0.4 eV which we attribute to
two-orbiton scattering, i.e., an exchange of orbitals between adjacent sites.
Altogether, these results indicate that both superexchange interactions and the
coupling to the lattice are important for a quantitative understanding of the
orbital excitations in YVO3. | cond-mat_str-el |
Spin-triplet f-wave pairing due to three-site cyclic-exchange
ferromagnetic interactions: Ferromagnetiam and superconductivity in a two-dimensional triangular-lattice
Hubbard model are studied using the density-matrix renormalization group
method. We propose a mechanism of the {\it f}-wave spin-triplet pairing derived
from the three-site cyclic-exchange ferromagnetic interactions. We point out
that a triangular network of hopping integrals, which is required for the
three-site cyclic hopping processes, is contained in the (possibly)
spin-triplet superconducting systems, such as Bechgaard salts (TMTSF)$_2$X,
cobalt oxide Na$_{0.35}$CoO$_2$$\cdot$1.3H$_2$O, and layered perovskite
Sr$_2$RuO$_4$. | cond-mat_str-el |
Orbital Polarization in Strained LaNiO$_{3}$: Structural Distortions and
Correlation Effects: Transition-metal heterostructures offer the fascinating possibility of
controlling orbital degrees of freedom via strain. Here, we investigate
theoretically the degree of orbital polarization that can be induced by
epitaxial strain in LaNiO$_3$ films. Using combined electronic structure and
dynamical mean-field theory methods we take into account both structural
distortions and electron correlations and discuss their relative influence. We
confirm that Hund's rule coupling tends to decrease the polarization and point
out that this applies to both the $d^8\underline{L}$ and $d^7$ local
configurations of the Ni ions. Our calculations are in good agreement with
recent experiments, which revealed sizable orbital polarization under tensile
strain. We discuss why full orbital polarization is hard to achieve in this
specific system and emphasize the general limitations that must be overcome to
achieve this goal. | cond-mat_str-el |
Electronic correlations and magnetic interactions in infinite-layer
NdNiO$_2$: The large antiferromagnetic exchange coupling in the parent high-$T_{\rm c}$
cuprate superconductors is believed to play a crucial role in pairing the
superconducting carriers. The recent observation of superconductivity in
hole-doped infinite-layer (IL-) NdNiO$_2$ brings to the fore the relevance of
magnetic coupling in high-$T_{\rm c}$ superconductors, particularly because no
magnetic ordering is observed in the undoped IL-NdNiO$_2$ unlike in parent
copper oxides. Here, we investigate the electronic structure and the nature of
magnetic exchange in IL-NdNiO$_2$ using state-of-the-art many-body quantum
chemistry methods. From a systematic comparison of the electronic and magnetic
properties with isostructural cuprate IL-CaCuO$_2$, we find that the on-site
dynamical correlations are significantly stronger in IL-NdNiO$_2$ compared to
the cuprate analog. These dynamical correlations play a critical role in the
magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest
neighbor isotropic $J$ of 77 meV between the Ni$^{1+}$ ions within the
$ab$-plane. While we find many similarities in the electronic structure between
the nickelate and the cuprate, the role of electronic correlations is
profoundly different in the two. We further discuss the implications of our
findings in understanding the origin of superconductivity in nickelates. | cond-mat_str-el |
Computation of dynamical correlation functions of Heisenberg chains: the
gapless anisotropic regime: We compute all dynamical spin-spin correlation functions for the spin-1/2
$XXZ$ anisotropic Heisenberg model in the gapless antiferromagnetic regime,
using numerical sums of exact determinant representations for form factors of
spin operators on the lattice. Contributions from intermediate states
containing many particles and string (bound) states are included. We present
modified determinant representations for the form factors valid in the general
case with string solutions to the Bethe equations. Our results are such that
the available sum rules are saturated to high precision. We Fourier transform
our results back to real space, allowing us in particular to make a comparison
with known exact formulas for equal-time correlation functions for small
separations in zero field, and with predictions for the zero-field asymptotics
from conformal field theory. | cond-mat_str-el |
de Haas-van Alphen oscillations in quasi-two-dimensional underdoped
cuprate superconductors in the canonical ensemble: We calculate the de Haas-van Alphen (dHvA) effect waveform using the
canonical ensemble for different Fermi surface scenarios applicable to the
underdoped cuprate superconductor YBa2Cu3O6.5, in which quantum oscillations
have recently been observed. The harmonic content of the dHvA waveform of the
principal F ~ 500 T frequency is consistent with the existence of a second
thermodynamically dominant section of Fermi surface that acts primarily as a
charge reservoir. Oscillations in the charge density to and from this reservoir
are shown to potentially contribute to the observed large quantum oscillations
in the Hall resistance. | cond-mat_str-el |
Magnon-phonon interactions in magnetic insulators: We address the theory of magnon-phonon interactions and compute the
corresponding quasi-particle and transport lifetimes in magnetic insulators
with focus on yttrium iron garnet at intermediate temperatures from anisotropy-
and exchange-mediated magnon-phonon interactions, the latter being derived from
the volume dependence of the Curie temperature. We find in general weak effects
of phonon scattering on magnon transport and the Gilbert damping of the
macrospin Kittel mode. The magnon transport lifetime differs from the
quasi-particle lifetime at shorter wavelengths. | cond-mat_str-el |
Construction of Variational Matrix Product States for the Heisenberg
Spin-1 Chain: We propose a simple variational wave function that captures the correct
ground state energy of the spin-1 Heisenberg chain model to within 0.04\%. The
wave function is written in the matrix product state (MPS) form with the bond
dimension $D=8$, and characterized by three fugacity parameters. The proposed
MPS generalizes the Affleck-Kennedy-Lieb-Tasaki (AKLT) state by dressing it
with dimers, trimers, and general $q$-dimers. The fugacity parameters control
the number and the average size of the $q$-mers. Furthermore, the $D=8$
variational MPS state captures the ground states of the entire family of
bilinear-biquadratic Hamiltonian belonging to the Haldane phase to high
accuracy. The 2-4-2 degeneracy structure in the entanglement spectrum of our
MPS state is found to match well with the results of density matrix
renormalization group (DMRG) calculation, which is computationally much
heavier. Spin-spin correlation functions also find excellent fit with those
obtained by DMRG. | cond-mat_str-el |
A Simple Treatment of Metal-Insulator Transition: Effects of Degeneracy,
Temperature and Applied Magnetic Field: A simple slave-boson representation combined with the Hartree-Fock
approximation for the Hund's rule coupling is introduced for a doubly
degenerate narrow band, which bears a direct relation to that introduced
previously in the nondegenerate case. Namely, one keeps the fermion
representation of the spin operator to recover properly the energy of fermionic
quasiparticles in the presence of an applied magnetic field. A simple
two-parameter mean-field analysis of the metamagnetism is provided, with the
emphasis on the role of the Hund's rule coupling. We also analyse the
appearance of the spin-split effective masses in the applied field and for
nonhalf-filled-band situation. The Mott-Hubbard boundary is determined at
nonzero temperature (T>0); it shifts towards lower interactions with increasing
T and the field signalling the precursory localization effects, explicitly
exhibited in the behavior of the magnetic susceptibility calculated in the
Appendix. We also formulate a more general two-parameter rotationally invariant
approach for an arbitrary degeneracy d of equivalent orbitals and show that the
Mott-Hubbard transition at zero temperature and at any integer filling n>1 is
always discontinuous. A brief overview of experimental situation is also made. | cond-mat_str-el |
Prototypical many-body signatures in transport properties of
semiconductors: We devise a methodology for charge, heat, and entropy transport driven by
carriers with finite lifetimes. Combining numerical simulations with analytical
expressions for low temperatures, we establish a comprehensive and
thermodynamically consistent phenomenology for transport properties in
semiconductors. We demonstrate that the scattering rate (inverse lifetime) is a
relevant energy scale: It causes the emergence of several characteristic
features in each transport observable. The theory is capable to reproduce --
with only a minimal input electronic structure -- the full temperature profiles
measured in correlated narrow-gap semiconductors. In particular, we account for
the previously elusive low-$T$ saturation of the resistivity and the Hall
coefficient, as well as the (linear) vanishing of the Seebeck and Nernst
coefficient in systems, such as FeSb$_2$, FeAs$_2$, RuSb$_2$ and FeGa$_3$. | cond-mat_str-el |
Coexisting charge-ordered states with distinct driving mechanisms in
monolayer VSe$_2$: Thinning crystalline materials to two dimensions (2D) creates a rich
playground for electronic phases, including charge, spin, superconducting, and
topological order. Bulk materials hosting charge density waves (CDWs), when
reduced to ultrathin films, have shown CDW enhancement and tunability. However,
charge order confined to only 2D remains elusive. Here we report a distinct
charge ordered state emerging in the monolayer limit of $1T$-VSe$_2$.
Systematic scanning tunneling microscopy experiments reveal that bilayer
VSe$_2$ largely retains the bulk electronic structure, hosting a
tri-directional CDW. However, monolayer VSe$_2$ -- consistently across distinct
substrates -- exhibits a dimensional crossover, hosting two CDWs with distinct
wavelengths and transition temperatures. Electronic structure calculations
reveal that while one CDW is bulk-like and arises from the well-known Peierls
mechanism, the other is decidedly unconventional. The observed CDW-lattice
decoupling and the emergence of a flat band suggest that the new CDW could
arise from enhanced electron-electron interactions in the 2D limit. These
findings establish monolayer-VSe$_2$ as a host of coexisting charge orders with
distinct origins, and enable the tailoring of electronic phenomena via emergent
interactions in 2D materials. | cond-mat_str-el |
Double-Exchange Model on Triangle Chain: We study ground state properties of the double-exchange model on triangle
chain in the classical limit on $t_{2g}$ spins. The ground state is determined
by a competition among the kinetic energy of the $e_g$ electron, the
antiferromagnetic exchange energy between the $t_{2g}$ spins, and frustration
due to a geometric structure of the lattice. The phase diagrams are obtained
numerically for two kinds of the models which differ only in the transfer
integral being real or complex. The properties of the states are understood
from the viewpoint of the spin-induced Peierls instability. The results suggest
the existence of a chiral glass phase which is characterized by a local spin
chirality and a continuous degeneracy. | cond-mat_str-el |
Bose condensation in a model microcavity: We study the equilibrium properties of a system of dipole-active excitons
coupled to a single photon mode at fixed total excitation. Treating the
presence or absence of a trapped exciton as a two-level system produces a model
that is exactly soluble. It gives a simple description of the physics of
polariton condensation in optical cavities beyond the low-density bosonic
regime. | cond-mat_str-el |
Incommensurate magnetic orders and topological Hall effect in the
square-net centrosymmetric EuGa$_2$Al$_2$ system: Neutron diffraction on the centrosymmetric square-net magnet EuGa$_2$Al$_2$
reveals multiple incommensurate magnetic states (AFM1,2,3) in zero field. In
applied field, a new magnetic phase (A) is identified from magnetization and
transport measurements, bounded by two of the $\mu_0H$~=~0 incommensurate
magnetic phases (AFM1,helical and AFM3, cycloidal) with different moment
orientations. Moreover, magneto-transport measurements indicate the presence of
a topological Hall effect, with maximum values centered in the A phase.
Together, these results render EuGa$_2$Al$_2$ a material with non-coplanar or
topological spin texture in applied field. X-ray diffraction reveals an
out-of-plane (OOP) charge density wave (CDW) below $T_{CDW} \sim$ 50 K while
the magnetic propagation vector lies in plane below $T_N$ = 19.5 K. Together
these data point to a new route to realizing in-plane non-collinear spin
textures through an OOP CDW. In turn, these non-collinear spin textures may be
unstable against the formation of topological spin textures in an applied
field. | cond-mat_str-el |
Magnetic excitations in the low-temperature ferroelectric phase of
multiferroic YMn2O5 using inelastic neutron scattering: We studied magnetic excitations in a low-temperature ferroelectric phase of
the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify
low-energy magnon modes and establish a correspondence between the magnon peaks
observed by INS and electromagnon peaks observed in optical absorption [1].
Furthermore, we explain the microscopic mechanism, which results in the
lowest-energy electromagnon peak, by comparing the inelastic neutron spectral
weight with the polarization in the commensurate ferroelectric phase. | cond-mat_str-el |
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