Found 66 papers in cond-mat Quadratic band touching (QBT) points are widely observed in 2D and 3D
materials, including bilayer graphene and Luttinger semimetals, and attract
significant attention from theory to experiment. However, even in its simplest
form, the 2D checkerboard lattice QBT model, the phase diagram characterized by
temperature and interaction strength still remains unknown beyond the
weak-coupling regime. Intense debates persist regarding the existence of
various interaction-driven insulating states in this system [1-7]. To address
these uncertainties, we employ thermal tensor network simulations, specifically
exponential tensor renormalization group [8], along with density matrix
renormalization group calculations. Our approach enables us to provide a
comprehensive finite-temperature phase diagram for this model and shed light on
previous ambiguities. Notably, our findings consistently reveal the emergence
of a robust bond-nematic Dirac semimetal (BNDS) phase as an intermediate state
between the nematic insulating state and other symmetry broken states. This
previously overlooked feature is found to be ubiquitous in interacting QBT
systems. We also discuss the implications of these results for experimental
systems such as bilayer graphene and iridate compounds.
Electrons on honeycomb or pi-flux lattices obey effective massless Dirac
equation at low energies and at the neutrality point, and should suffer quantum
phase transitions into various Mott insulators and superconductors at strong
two-body interactions. We show that 35 out of 36 such order parameters that
provide Lorentz-invariant mass-gaps to Dirac fermions can be organized into a
single irreducible tensor representation of the $SO(8)$ symmetry of the
two-dimensional Dirac Hamiltonian for the spin-1/2 lattice fermions. The
minimal interacting Lagrangian away from the neutrality point has the $SO(8)$
symmetry reduced to $U(1) \times SU(4)$ by finite chemical potential, and it
allows only two independent interaction terms. When the Lagrangian is nearly
$SO(8)$-symmetric and the ground state insulating at the neutrality point, we
argue it turns superconducting at the critical value of the chemical potential
through a ``flop" between the tensor components. The theory is exactly solvable
when the $SU(4)$ is generalized to $SU(N)$ and $N$ taken large. A lattice
Hamiltonian that may exhibit this transition, parallels with the Gross-Neveu
model, and applicability to related electronic systems are briefly discussed.
In the past decades many density-functional theory methods and codes adopting
periodic boundary conditions have been developed and are now extensively used
in condensed matter physics and materials science research. Only in 2016,
however, their precision (i.e., to which extent properties computed with
different codes agree among each other) was systematically assessed on
elemental crystals: a first crucial step to evaluate the reliability of such
computations. We discuss here general recommendations for verification studies
aiming at further testing precision and transferability of
density-functional-theory computational approaches and codes. We illustrate
such recommendations using a greatly expanded protocol covering the whole
periodic table from Z=1 to 96 and characterizing 10 prototypical cubic
compounds for each element: 4 unaries and 6 oxides, spanning a wide range of
coordination numbers and oxidation states. The primary outcome is a reference
dataset of 960 equations of state cross-checked between two all-electron codes,
then used to verify and improve nine pseudopotential-based approaches. Such
effort is facilitated by deploying AiiDA common workflows that perform
automatic input parameter selection, provide identical input/output interfaces
across codes, and ensure full reproducibility. Finally, we discuss the extent
to which the current results for total energies can be reused for different
goals (e.g., obtaining formation energies).
Optical control of crystal structures is a promising route to change physical
properties including topological nature of a targeting material. Time-resolved
X-ray diffraction measurements using the X-ray free-electron laser are
performed to study the ultrafast lattice dynamics of VTe$_2$, which shows a
unique charge-density-wave (CDW) ordering coupled to the topological surface
states as a first-order phase transition. A significant oscillation of the CDW
amplitude mode is observed at a superlattice reflection as well as Bragg
reflections. The frequency of the oscillation is independent of the fluence of
the pumping laser, which is prominent to the CDW ordering of the first-order
phase transition. Furthermore, the timescale of the photoinduced
1$T^{\prime\prime}$ to 1$T$ phase transition is independent of the period of
the CDW amplitude mode.
Optical dynamics in van der Waals heterobilayers is of fundamental scientific
and practical interest. Based on a time-dependent adiabatic GW approach, we
discover a new many-electron (excitonic) channel for converting photoexcited
intralayer to interlayer excitations and the associated ultrafast optical
responses in heterobilayers, which is conceptually different from the
conventional single-particle picture. We find strong electron-hole interactions
drive the dynamics and enhance the pump-probe optical responses by an order of
magnitude with a rise time of ~300 fs in MoSe$_2$/WSe$_2$ heterobilayers, in
agreement with experiment.
We report a study of the noncentrosymmetric TaReSi superconductor by means of
muon-spin rotation and relaxation ($\mu$SR) technique, complemented by
electronic band-structure calculations. Its superconductivity, with $T_c$ = 5.5
K and upper critical field $\mu_0H_\mathrm{c2}(0)$ $\sim$ 3.4 T, was
characterized via electrical-resistivity- and magnetic-susceptibility
measurements. The temperature-dependent superfluid density, obtained from
transverse-field $\mu$SR, suggests a fully-gapped superconducting state in
TaReSi, with an energy gap $\Delta_0$ = 0.79 meV and a magnetic penetration
depth $\lambda_0$ = 562 nm. The absence of a spontaneous magnetization below
$T_c$, as confirmed by zero-field $\mu$SR, indicates a preserved time-reversal
symmetry in the superconducting state. The density of states near the Fermi
level is dominated by the Ta- and Re-5$d$ orbitals, which account for the
relatively large band splitting due to the antisymmetric spin-orbit coupling.
In its normal state, TaReSi behaves as a three-dimensional Kramers nodal-line
semimetal, characterized by an hourglass-shaped dispersion protected by glide
reflection. By combining non\-triv\-i\-al electronic bands with intrinsic
superconductivity, TaReSi is a promising material for investigating the
topological aspects of noncentrosymmetric superconductors.
Interfacial bond formation during sputter deposition of metal oxide thin
films onto polycarbonate (PC) is investigated by ab initio molecular dynamics
simulations and X-ray photoelectron spectroscopy (XPS) analysis of PC | X
interfaces (X = Al$_2$O$_3$, TiO$_2$, TiAlO$_2$). Generally, the predicted bond
formation is consistent with the experimental data. For all three interfaces,
the majority of bonds identified by XPS are (C-O)-metal bonds, whereas C-metal
bonds are the minority. Compared to the PC | Al$_2$O$_3$ interface, the PC |
TiO$_2$ and PC | TiAlO$_2$ interfaces exhibit a reduction in the measured
interfacial bond density by ~ 75 and ~ 65%, respectively. Multiplying the
predicted bond strength with the corresponding experimentally determined
interfacial bond density shows that Al$_2$O$_3$ exhibits the strongest
interface with PC, while TiO$_2$ and TiAlO$_2$ exhibit ~ 70 and ~ 60% weaker
interfaces, respectively. This can be understood by considering the complex
interplay between the metal oxide composition, the bond strength as well as the
population of bonds that are formed across the interface.
Geometric effects can play a pivotal role in streamlining quantum
manipulation. We demonstrate a geometric diabatic control, that is, perfect
tunneling between spin states in a diamond by a quadratic sweep of a driving
field. The field sweep speed for the perfect tunneling is determined by the
geometric amplitude factor and can be tuned arbitrarily. Our results are
obtained by testing a quadratic version of Berry's twisted Landau-Zener model.
This geometric tuning is robust over a wide parameter range. Our work provides
a basis for quantum control in various systems, including condensed matter
physics, quantum computation, and nuclear magnetic resonance.
Non-Hermitian PT-symmetric systems can be conveniently realized in optical
systems in the classical domain and have been used to explore a plethora of
exotic phenomena like loss-induced lasing and selective propagation of chiral
modes in waveguides. On the other hand, a microcavity exciton-polariton system
is intrinsically non-Hermitian in the quantum regime. However, realization of
such systems in the PT-symmetric phase has not been achieved so far. Here we
show how a pair of nearly orthogonal sets of anisotropic exciton-polaritons can
offer a versatile platform for realizing multiple Eps and propose a roadmap to
achieve a PT-symmetric system. By utilizing the tunability of coupling strength
and energy detuning on the polarization of probe beam, the angle of incidence,
and the orientation of the anisotropic sample, we realise two kinds of Eps:
Polarization-tunable polariton dispersion creates one set of EPs based on
tunable coupling strength, while the rotating the sample reveals Voigt EPs for
specific orientations. Pair of anisotropic microcavity exciton-polaritons can
offer a promising platform not only for fundamental research in non-Hermitian
quantum physics and topological polaritons but also, we have proposed that it
can offer a system to realize zero threshold laser.
We explore a technique for probing energy spectra in synthetic lattices that
is analogous to scanning tunneling microscopy. Using one-dimensional synthetic
lattices of coupled atomic momentum states, we explore this spectroscopic
technique and observe qualitative agreement between the measured and simulated
energy spectra for small two- and three-site lattices as well as a uniform
many-site lattice. Finally, through simulations, we show that this technique
should allow for the exploration of the topological bands and the fractal
energy spectrum of the Hofstadter model as realized in synthetic lattices.
The efficient optical second-harmonic generation (SHG) of two-dimensional
(2D) crystals, coupled with their atomic thickness that circumvents the
phase-match problem, has garnered considerable attention. While various 2D
heterostructures have shown promising applications in photodetectors, switching
electronics, and photovoltaics, the modulation of nonlinear optical properties
in such hetero-systems remains unexplored. In this study, we investigate
exciton sensitized SHG in heterobilayers of transition metal dichalcogenides
(TMDs), where photoexcitation of one donor layer enhances the SHG response of
the other as an acceptor. We utilize polarization-resolved interferometry to
detect the SHG intensity and phase of each individual layer, revealing the
energetic match between the excitonic resonances of donors and the SHG
enhancement of acceptors for four TMD combinations. Our results also uncover
the dynamic nature of interlayer coupling, as evidenced by the dependence of
sensitization on interlayer gap spacing and the average power of the
fundamental beam. This work provides insights into how interlayer coupling of
two different layers can modify nonlinear optical phenomena in 2D
heterostructures.
Enhancing robustness of topological orders against perturbations is one of
the main goals in topological quantum computing. Since the kinetic of
excitations is in conflict with the robustness of topological orders, any
mechanism that reduces the mobility of excitations will be in favor of
robustness. A strategy in this direction is adding frustration to topological
systems. In this paper we consider a frustrated toric code on a kagome lattice,
and show that although increasing the strength of perturbation reduces the
topological order of the system, it cannot destroy it completely. Our
frustrated toric code is indeed a quantum loop gas model with string tension
and pressure which their competition leads to a partially topological phase
(PTP) in which the excitations are restricted to move in particular
sublattices. In this phase the ground state is a product of many copies of
fluctuating loop states corresponding to quasi one dimensional ladders. By
defining a non-local matrix order parameter and studying the behavior of ground
state global entanglement (GE), we distinguish the PTP from the standard
topological phase. The partial mobility of excitations in our system is a
reminiscent of fracton codes with restricted mobility, and therefore our
results propose an alternative way for making such a restriction in three
dimension.
We study surface states in the three-dimensional topological insulators
Bi$_2$Te$_{3-x}$Se$_{x}$ (x = 0, 2, 3) by polarization resolved resonant Raman
spectroscopy. By tracking the spectral intensity of the surface phonon modes
with respect to the incident photon energy, we show that the surface phonons
are qualitatively similar to their bulk counterparts. Using the resonant Raman
excitation profile, we estimated the binding energy of the surface conduction
bands relative to bulk conduction bands. In addition, by analyzing the Fano
interaction between the electronic continuum and the surface phonons as a
function of incident photon energy, we determined the spectral properties of
the electronic continuum excitations between surface and bulk states in
Bi$_2$Se$_3$.
Despite its simple crystal structure, layered boron nitride features a
surprisingly complex variety of phonon-assisted luminescence peaks. We present
a combined experimental and theoretical study on ultraviolet-light emission in
hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of
high-quality samples are measured via cathodoluminescence spectroscopy,
displaying characteristic differences between the two polytypes. These
differences are explained using a fully first-principles computational
technique that takes into account radiative emission from ``indirect'',
finite-momentum, excitons via coupling to finite-momentum phonons. We show that
the differences in peak positions, number of peaks and relative intensities can
be qualitatively and quantitatively explained, once a full integration over all
relevant momenta of excitons and phonons is performed.
We study the two-dimensional motion of a magnetic skyrmion driven by a
ratchetlike polarized electric current that is periodic in both space and time.
Some general cases are considered, in each of which,in the low temperature and
adiabatic limit, regardless of the details of the driving current, the time and
statistical average velocity along any direction is topologically quantized as
a Chern number, multiplied by a basic unit. We make two approaches, one based
on identifying the drift direction, and the other based on the nonhermitian
adiabatic perturbation theory developed for the Fokker-Planck operator. Both
approach applies in the case of periodicity along the direction of the driving
current and homogeneity in the transverse direction, for which the analytical
result is confirmed by our numerical simulation on the constituent spins,and a
convenient experiment is proposed.
The first-principles calculations and measurements of the magnetic
penetration depths, the upper critical field, and the specific heat were
performed for a family of Mo$_5$Si$_{3-x}$P$_x$ superconducotrs.
First-principles calculations suggest the presence of a flat band dispersion,
which gradually shifts to the Fermi level as a function of phosphorus doping
$x$. The flat band approaches the Fermi level at $x\simeq 1.3$, thus separating
Mo$_5$Si$_{3-x}$P$_x$ between the purely steep band and the steep band/flat
band superconducting regimes. The emergence of flat bands lead to an abrupt
change of nearly all the superconducting quantities. In particular, a strong
reduction of the coherence length $\xi$ and enhancement of the penetration
depth $\lambda$ result in nearly factor of three increase of the
Ginzburg-Landau parameter $\kappa=\lambda/\xi$ (from $\kappa\simeq 25$ for
$x\lesssim 1.2$ to $\kappa\simeq 70$ for $x\gtrsim 1.4$) thus initiating the
transition of Mo$_5$Si$_{3-x}$P$_x$ from a moderate to an extreme type-II
superconductivity.
Topological insulators have been studied intensively over the last decades.
Earlier research focused on Hermitian Hamiltonians, but recently, peculiar and
interesting properties were found by introducing non-Hermiticity. In this work,
we apply a quantum geometric approach to various Hermitian and non-Hermitian
versions of the Su-Schrieffer-Heeger (SSH) model. We find that this method
allows one to correctly identify different topological phases and topological
phase transitions for all SSH models, but only when using the metric tensor
containing both left and right eigenvectors. Whereas the quantum geometry of
Hermitian systems is Riemannian, introducing non-Hermiticity leads to
pseudo-Riemannian and complex geometries, thus significantly generalizing from
the quantum geometries studied thus far. One remarkable example of this is the
mathematical agreement between topological phase transition curves and
lightlike paths in general relativity, suggesting a possibility of simulating
space-times in non-Hermitian systems. We find that the metric in non-Hermitian
phases degenerates in such a way that it effectively reduces the dimensionality
of the quantum geometry by one. This implies that within linear response
theory, one can perturb the system by a particular change of parameters while
maintaining a zero excitation rate.
Phonons have provided an ideal platform for a variety of intriguing physical
states, such as non-abelian braiding and Haldane model. It is promising that
phonons will realize the complicated nodal states accompanying with unusual
quantum phenomena. Here, we propose the hybrid nodal surface and nodal line
(NS+NL) phonons beyond the single genre nodal phonons. We categorize the NS+NL
phonons into two-band and four-band situations based on symmetry analysis and
compatibility relationships. Combing database screening with first-principles
calculations, we identify the ideal candidate materials for realizing all
categorized NS+NL phonons. Our calculations and tight-binding models further
demonstrate that the interplay between NS and NL induces unique phenomena. In
space group 113, the quadratic NL acts as a hub of the Berry curvature between
two NSs, generating ribbon-like surface states. In space group 128, the NS
serve as counterpart of Weyl NL that NS-NL mixed topological surface states are
observed. Our findings extend the scope of hybrid nodal states and enrich the
phononic states in realistic materials.
Synthesis of one-dimensional molecular arrays with tailored stereoisomers is
challenging yet has a great potential for application in molecular opto-,
electronic- and magnetic-devices, where the local array structure plays a
decisive role in the functional properties. Here, we demonstrate construction
and characterization of dehydroazulene isomer and diradical units in
three-dimensional organometallic compounds on Ag(111) with a combination of
low-temperature scanning tunneling microscopy and density functional theory
calculations. Tip-induced voltage pulses firstly result in the formation of a
diradical species via successive homolytic fission of two C-Br bonds in the
naphthyl groups, which are subsequently transformed into chiral dehydroazulene
moieties. The delicate balance of the reaction rates among the diradical and
two stereoisomers, arising from an in-line configuration of tip and molecular
unit, allows directional azulene-to-azulene and azulene-to-diradical local
probe isomerization in a controlled manner. Furthermore, we found that the
diradical moiety hosts an open-shell singlet with antiferromagnetic coupling
between the unpaired electrons, which can undergo an inelastic spin transition
of 91 meV to the ferromagnetically coupled triplet state.
We review field theoretical studies dedicated to understanding the effects of
electron-electron interaction in graphene, which is characterized by gapless
bands, strong electron-electron interactions, and emerging Lorentz invariance
deep in the infrared. We consider the influence of interactions on the
transport properties of the system as well as their supposedly decisive
influence on the potential dynamical generation of a gap.
When reformulated as a resource theory, thermodynamics can analyze system
behaviors in the single-shot regime. In this, the work required to implement
state transitions is bounded by alpha-Renyi divergences and so differs in
identifying efficient operations compared to stochastic thermodynamics. Thus, a
detailed understanding of the difference between stochastic thermodynamics and
resource-theoretic thermodynamics is needed. To this end, we study
reversibility in the single-shot regime, generalizing the two-level work
reservoirs used there to multi-level work reservoirs. This achieves
reversibility in any transition in the single-shot regime. Building on this, we
systematically explore multi-level work reservoirs in the nondissipation regime
with and without catalysts. The resource-theoretic results show that two-level
work reservoirs undershoot Landauer's bound, misleadingly implying energy
dissipation during computation. In contrast, we demonstrate that multi-level
work reservoirs achieve Landauer's bound and produce zero entropy.
Topologically protected spin textures, such as skyrmions1,2 and vortices3,4,
are robust against perturbations, serving as the building blocks for a range of
topological devices5-9. In order to implement these topological devices, it is
necessary to find ultra-small topological spin textures at room temperature,
because small size implies the higher topological charge density, stronger
signal of topological transport10,11 and the higher memory density or
integration for topological quantum devices5-9. However, finding ultra-small
topological spin textures at high temperatures is still a great challenge up to
now. Here we find ultra-small topological spin textures in Fe78Si9B13 amorphous
alloy. We measured a large topological Hall effect (THE) up to above room
temperature, indicating the existence of highly densed and ultra-small
topological spin textures in the samples. Further measurements by small-angle
neutron scattering (SANS) reveal that the average size of ultra-small magnetic
texture is around 1.3nm. Our Monte Carlo simulations show that such ultra-small
spin texture is topologically equivalent to skyrmions, which originate from
competing frustration and Dzyaloshinskii-Moriya interaction12,13 coming from
amorphous structure14-17. Taking a single topological spin texture as one bit
and ignoring the distance between them, we evaluated the ideal memory density
of Fe78Si9B13, which reaches up to 4.44*104 gigabits (43.4 TB) per in2 and is 2
times of the value of GdRu2Si218 at 5K. More important, such high memory
density can be obtained at above room temperature, which is 4 orders of
magnitude larger than the value of other materials at the same temperature.
These findings provide a unique candidate for magnetic memory devices with
ultra-high density.
The colloidal 2D materials based on graphene and its modifications are of
great interest when it comes to forming LC phases. These LC phases allow
controlling the orientational order of colloidal particles, paving the way for
the efficient processing of modified graphene with anisotropic properties.
Here, we present the peculiarities of AA functionalization of GO, along with
the formation of its LC phase and orientational behavior in an external
magnetic field. We discuss the influence of pH on the GOLC, ultimately showing
its pH-dependent behavior for GO-AA complexes. In addition, we observe
different GO morphology changes due to the presence of AA functional groups,
namely L-cysteine dimerization on the GO platform. The pH dependency of
AA-functionalized LC phase of GO is examined for the first time. We believe
that our studies will open new possibilities for applications in
bionanotechnologies due to self-assembling properties of LCs and magnificent
properties of GO.
Building on the development of momentum state lattices (MSLs) over the past
decade, we introduce a simple extension of this technique to higher dimensions.
Based on the selective addressing of unique Bragg resonances in matter-wave
systems, MSLs have enabled the realization of tight-binding models with tunable
disorder, gauge fields, non-Hermiticity, and other features. Here, we examine
and outline an experimental approach to building scalable and tunable
tight-binding models in two dimensions describing the laser-driven dynamics of
atoms in momentum space. Using numerical simulations, we highlight some of the
simplest models and types of phenomena this system is well-suited to address,
including flat-band models with kinetic frustration and flux lattices
supporting topological boundary states. Finally, we discuss many of the direct
extensions to this model, including the introduction of disorder and
non-Hermiticity, which will enable the exploration of new transport and
localization phenomena in higher dimensions.
Symmetry-breaking electronic phase in unconventional high-temperature
(high-Tc) superconductors is a fascinating issue in condensed-matter physics,
among which the most attractive phases are charge density wave (CDW) phase with
four unit-cell periodicity in cuprates and nematic phase breaking the C4
rotational symmetry in iron-based superconductors (FeSCs). Recently, pair
density wave (PDW), an exotic superconducting phase with non-zero momentum
Cooper pairs, has been observed in high-Tc cuprates and the monolayer FeSC.
However, the interplay between the CDW, PDW and nematic phase remains to be
explored. Here, using scanning tunneling microscopy/spectroscopy, we detected
commensurate CDW and CDW-induced PDW orders with the same period of lambda =
4aFe (aFe is the distance between neighboring Fe atoms) in a monolayer high-Tc
Fe(Te,Se) film grown on SrTiO3(001) substrate. Further analyses demonstrate the
observed CDW is a smectic order, which breaks both translation and C4
rotational symmetry. Moreover, the smecticity of the CDW order is strongest
near the superconducting gap but weakens near defects and in an applied
magnetic field, indicating the interplay between the smectic CDW and PDW
orders. Our works provide a new platform to study the intertwined orders and
their interactions in high-Tc superconductors.
Two-dimensional materials provide remarkable platforms to uncover intriguing
quantum phenomena and develop nanoscale devices of versatile applications.
Recently, AlSb in the double-layer honeycomb (DLHC) structure was successfully
synthesized exhibiting a semiconducting nature [ACS Nano 15, 8184 (2021)],
which corroborates the preceding theoretical predictions and stimulates the
exploration of new robust DLHC materials. In this work, we propose a Janus DLHC
monolayer Al$_2$SbBi, the dynamical, thermal, and mechanical stabilities of
which are confirmed by first-principles calculations. Monolayer Al$_2$SbBi is
found to be a nontrivial topological insulator with a gap of about 0.2 eV,
which presents large spin splitting and peculiar spin texture in the valence
bands. Furthermore, due to the absence of inversion symmetry, monolayer
Al$_2$SbBi exhibits piezoelectricity and the piezoelectric strain coefficients
d$_{11}$ and d$_{31}$ are calculated to be 7.97 pm/V and 0.33 pm/V,
respectively, which are comparable to and even larger than those of many
piezoelectric materials. Our study suggests that monolayer Al$_2$SbBi has
potential applications in spintronic and piezoelectric devices.
We perform a systematic \textit{ab initio} density functional study of the
superconductivity, electronic and phononic band structures, electron-phonon
coupling and elastic constants of all four possible structures of niobium
nitride $\beta$-Nb$_2$N as well as Nb-rich $\gamma$-Nb$_4$N$_3$ and N-rich
$\beta^\prime$-Nb$_4$N$_5$. First of all, we find that all four structures of
$\beta$-Nb$_2$N are superconductors with superconducting transition
temperatures ($T_c$) ranging from 0.6 K to 6.1 K, depending on the structure.
This explains why previous experiments reported contradicting $T_c$ values for
$\beta$-Nb$_2$N. Furthermore, both $\gamma$-Nb$_4$N$_3$ and
$\beta^\prime$-Nb$_4$N$_5$ are predicted to be superconductors with rather high
$T_c$ of 8.5 K and 15.3 K, respectively. Second, the calculated elastic
constants and phonon dispersion relations show that all the considered niobium
nitride structures are mechanically and dynamically stable. Moreover, the
calculated elastic moduli demonstrate that all the niobium nitrides are hard
materials with bulk moduli and hardness being comparable to or larger than the
well-known hard sapphire. Third, the calculated band structures reveal that the
nitrides possess both type I and type II Dirac nodal points and are thus
topological metals. Finally, the calculated electron-phonon coupling strength,
superconductivity and mechanical property of the niobium nitrides are discussed
in terms of their underlying electronic structures and also Debye temperatures.
The present \textit{ab initio} study thus indicates that $\beta$-Nb$_2$N,
$\gamma$-Nb$_4$N$_3$ and $\beta^\prime$-Nb$_4$N$_5$ are hard superconductors
with nontrivial band topology and are promising materials for exploring exotic
phenomena due to the interplay of hardness, superconductivity and nontrivial
band topology.
Theoretical and experimental studies have verified the existence of ``magic
angles'' in twisted bilayer graphene, where the twist between layers gives rise
to flat bands and consequently highly correlated phases. Narrow bands can also
exist in multilayers with alternating twist angles, and recent theoretical work
suggests that they can also be found in trilayers with twist angles between
neighboring layers in the same direction. We show here that flat bands exist in
a variety of multilayers where the ratio between twist angles is close to
coprime integers. We generalize previous analyses, and, using the chiral limit
for interlayer coupling, give examples of many combinations of twist angles in
stacks made up of three and four layers which lead to flat bands. The technique
we use can be extended to systems with many layers. Our results suggest that
flat bands can exist in graphene multilayers with angle disorder, that is,
narrow samples of turbostatic graphite.
Kagome metals present a fascinating platform of novel quantum phases thanks
to the interplay between the geometric frustration and strong electron
correlation. Here, we propose the emergence of the odd-parity bond-order state
that is closely tied to the intra-unit-cell odd-parity configuration (or
electric toroidal order) in recently discovered kagome metal CsTi3Bi5.The
predicted E1u bond-order is induced by the beyond-mean-field mechanism, that
is, the quantum interference among different sublattice spin fluctuations.
Importantly, the accompanied nematic deformation of the Fermi surface is very
small, while the intensity of the quasiparticle interference signal exhibits
large nematic anisotropy, consistently with the scanning tunneling microscope
measurements in CsTi3Bi5. The present odd-parity order triggers interesting
emergent phenomena, such as the Edelstein effect and reciprocal transport with
finite spin-orbit interaction.
Molecular crystals are a flexible platform to induce novel electronic phases.
Due to the weak forces between molecules, intermolecular distances can be
varied over relatively larger ranges than interatomic distances in atomic
crystals. On the other hand, the hopping terms are generally small, which
results in narrow bands, strong correlations and heavy electrons. Here, by
growing K$_x$C$_{60}$ fullerides on hexagonal layered Bi$_2$Se$_3$, we show
that upon doping the series undergoes a Mott transition from a molecular
insulator to a correlated metal, and an in-gap state evolves into highly
dispersive Dirac-like fermions at half filling, where superconductivity occurs.
This picture challenges the commonly accepted description of the low energy
quasiparticles as appearing from a gradual electron doping of the conduction
states, and suggests an intriguing parallel with the more famous family of the
cuprate superconductors. More in general, it indicates that molecular crystals
offer a viable route to engineer electron-electron interactions.
In 1973, Philip Anderson published a paper introducing the resonating valence
bond state, which can be recognized in retrospect as a topologically ordered
phase of matter - one that cannot be classified in the conventional way
according to its patterns of spontaneously broken symmetry. Steven Kivelson and
Shivaji Sondhi reflect on the impact of this paper over the past 50 years.
We study a Kondo state that is strongly influenced by its proximity to an
w^-1/2 singularity in the metallic host density of states. This singularity
occurs at the bottom of the band of a 1D chain, for example. We first analyze
the non-interacting system: A resonant state e_d, located close to the band
singularity, suffers a strong `renormalization', such that a bound state is
created below the bottom of the band in addition to a resonance in the
continuum. When e_d is positioned right at the singularity, the spectral weight
of the bound state is 2/3, irrespective of its coupling to the conduction
electrons. The interacting system is modeled using the Single Impurity Anderson
Model, which is then solved using the Numerical Renormalization Group method.
We observe that the Hubbard interaction causes the bound state to suffer a
series of transformations, including level splitting, transfer of spectral
weight, appearance of a spectral discontinuity, changes in binding energy (the
lowest state moves farther away from the bottom of the band), and development
of a finite width. When e_d is away from the singularity and in the
intermediate valence regime, the impurity occupancy is lower. As e_d moves
closer to the singularity, the system partially recovers Kondo regime
properties, i.e., higher occupancy and lower Kondo temperature T_K. The
impurity thermodynamic properties show that the local moment fixed point is
also strongly affected by the existence of the bound state. When e_d is close
to the singularity, the local moment fixed point becomes impervious to charge
fluctuations (caused by bringing e_d close to the Fermi energy), in contrast to
the local moment suppression that occurs when e_d is away from the singularity.
We also discuss an experimental implementation that shows similar results to
the quantum wire, if the impurity's metallic host is an armchair graphene
nanoribbon.
We present a systematic study of the nonlinear thermal Hall responses in
bosonic systems using the quantum kinetic theory framework. We demonstrate the
existence of an intrinsic nonlinear boson thermal current, arising from the
quantum metric which is a wavefunction dependent band geometric quantity. In
contrast to the nonlinear Drude and nonlinear anomalous Hall contributions, the
intrinsic nonlinear thermal conductivity is independent of the scattering
timescale. We demonstrate the dominance of this intrinsic thermal Hall response
in topological magnons in a two-dimensional ferromagnetic honeycomb lattice
without Dzyaloshinskii-Moriya interaction. Our findings highlight the
significance of band geometry induced nonlinear thermal transport and motivate
experimental probe of the intrinsic nonlinear thermal Hall response with
implications for quantum magnonics.
The effect of inclusion of the planar phonon anisotropy on thermo-electrical
behavior of graphene is analyzed. Charge transport is simulated by means of
Direct Simulation Monte Carlo technique coupled with numerical solution of the
phonon Boltzmann equations based on deterministic methods.
The definition of the crystal lattice local equilibrium temperature is
investigated as well and the results furnish possible alternative approaches to
identify it starting from measurements of electric current density, with
relevant experimental advantages, which could help to overcome the present
difficulties regarding thermal investigation of graphene.
Positive implications are expected for many applications, as the field of
electronic devices, which needs a coherent tool for simulation of charge and
hot phonon transport; the correct definition of the local equilibrium
temperature is in turn fundamental for the study, design and prototyping of
cooling mechanisms for graphene-based devices.
Assuming any site-potential dependent on two-point correlations, we
rigorously derive a new model for an interlayer potential for incommensurate
bilayer heterostructures such as twisted bilayer graphene. We use the ergodic
property of the local configuration in incommensurate bilayer heterostructures
to prove convergence of an atomistic model to its thermodynamic limit without a
rate for minimal conditions on the lattice displacements. We provide an
explicit error control with a rate of convergence for sufficiently smooth
lattice displacements. For that, we introduce the notion of Diophantine 2D
rotations, a two-dimensional analogue of Diophantine numbers, and give a
quantitative ergodic theorem for Diophantine 2D rotations.
The realization of quantum gates in topological quantum computation still
confronts significant challenges in both fundamental and practical aspects.
Here, we propose a deterministic and fully topologically protected
measurement-based scheme to realize the issue of implementing Clifford quantum
gates on the Majorana qubits. Our scheme is based on rigorous proof that the
single-qubit gate can be performed by leveraging the neighboring Majorana qubit
but not disturbing its carried quantum information, eliminating the need for
ancillary Majorana zero modes (MZMs) in topological quantum computing.
Benefiting from the ancilla-free construction, we show the minimum measurement
sequences with four steps to achieve two-qubit Clifford gates by constructing
their geometric visualization. To avoid the uncertainty of the measurement-only
strategy, we propose manipulating the MZMs in their parameter space to correct
the undesired measurement outcomes while maintaining complete topological
protection, as demonstrated in a concrete Majorana platform. Our scheme
identifies the minimal operations of measurement-based topological and
deterministic Clifford gates and offers an ancilla-free design of topological
quantum computation.
First-principles density functional theory (DFT) calculations of Lu-H-N
compounds reveal low-energy configurations of Fm$\overline{3}$m
Lu$_{8}$H$_{23-x}$N structures that exhibit novel electronic properties such as
flat bands, sharply peaked densities of states (van Hove singularities), and
intersecting Dirac cones near the Fermi energy (E$_F$). These N-doped
LuH$_3$-based structures also exhibit an interconnected metallic hydrogen
network, which is a common feature of high-T$_c$ hydride superconductors.
Electronic property systematics give estimates of T$_c$ for optimally ordered
structures that are well above the critical temperatures predicted for
structures considered previously. The vHs and flat bands near E$_F$ are
enhanced in DFT+U calculations, implying strong correlation physics should also
be considered for first-principles studies of these materials. These results
provide a basis for understanding the novel electronic properties observed for
nitrogen-doped lutetium hydride.
Antiferromagnetic order, being a ground state of a number of exotic quantum
materials, is of immense interest both from the fundamental physics perspective
and for driving potential technological applications. For a complete
understanding of antiferromagnetism in materials, nanoscale visualization of
antiferromagnetic domains, domain walls and their robustness to external
perturbations is highly desirable. Here, we synthesize antiferromagnetic FeTe
thin films using molecular beam epitaxy. We visualize local antiferromagnetic
ordering and domain formation using spin-polarized scanning tunneling
microscopy. From the atomically-resolved scanning tunneling microscopy
topographs, we calculate local structural distortions to find a high
correlation with the distribution of the antiferromagnetic order. This is
consistent with the monoclinic structure in the antiferromagnetic state.
Interestingly, we observe a substantial domain wall change by small temperature
variations, unexpected for the low temperature changes used compared to the
much higher antiferromagnetic ordering temperature of FeTe. This is in contrast
to electronic nematic domains in the cousin FeSe multilayer films, where we
find no electronic or structural change within the same temperature range. Our
experiments provide the first atomic-scale imaging of perturbation-driven
magnetic domain evolution simultaneous with the ensuing structural response of
the system. The results reveal surprising thermally-driven modulations of
antiferromagnetic domains in FeTe thin films well below the Neel temperature.
The Landau-Lifshitz-Gilbert equation for rigid and saturated ferromagnets is
derived using a two-continuum model constructed by H.F. Tiersten for elastic
and saturated ferromagnets. The relevant basic laws of physics are applied
systematically to the two continua or their combination. The exchange
interaction is introduced into the model through surface distributed magnetic
couples. This leads to a continuum theory with magnetization gradients in the
stored energy density. The saturation condition of the magnetization functions
as constraints on the energy density and has implications in the constitutive
relations.
Tight-binding models can accurately predict the band structure and topology
of crystalline systems and they have been heavily used in solid-state physics
due to their versatility and low computational cost. It is quite
straightforward to build an accurate tight-binding model of any crystalline
system using the maximally localized Wannier functions of the crystal as a
basis. In 1D and 2D photonic crystals, it is possible to express the wave
equation as two decoupled scalar eigenproblems where finding a basis of
maximally localized Wannier functions is feasible using standard Wannierization
methods. Unfortunately, in 3D photonic crystals, the vectorial nature of the
electromagnetic solutions cannot be avoided. This precludes the construction of
a basis of maximally localized Wannier functions via usual techniques. In this
work, we show how to overcome this problem by using topological quantum
chemistry which will allow us to express the band structure of the photonic
crystal as a difference of elementary band representations. This can be
achieved by the introduction of a set of auxiliary modes, as recently proposed
by Solja\v{c}i\'c et. al., which regularize the $\Gamma$-point obstruction
arising from transversality constraint of the Maxwell equations. The
decomposition into elementary band representations allows us to isolate a set
of pseudo-orbitals that permit us to construct an accurate
transversality-enforced tight-binding model (TETB) that matches the dispersion,
symmetry content, and topology of the 3D photonic crystal under study.
Moreover, we show how to introduce the effects of a gyrotropic bias in the
framework, modeled via non-minimal coupling to a static magnetic field. Our
work provides the first systematic method to analytically model the photonic
bands of the lowest transverse modes over the entire BZ via a TETB model.
What comprises a global symmetry of a Quantum Field Theory (QFT) has been
vastly expanded in the past 10 years to include not only symmetries acting on
higher-dimensional defects, but also most recently symmetries which do not have
an inverse. The principle that enables this generalization is the
identification of symmetries with topological defects in the QFT. In these
lectures, we provide an introduction to generalized symmetries, with a focus on
non-invertible symmetries. We begin with a brief overview of invertible
generalized symmetries, including higher-form and higher-group symmetries, and
then move on to non-invertible symmetries. The main idea that underlies many
constructions of non-invertible symmetries is that of stacking a QFT with
topological QFTs (TQFTs) and then gauging a diagonal non-anomalous global
symmetry. The TQFTs become topological defects in the gauged theory called
(twisted) theta defects and comprise a large class of non-invertible symmetries
including condensation defects, self-duality defects, and non-invertible
symmetries of gauge theories with disconnected gauge groups. We will explain
the general principle and provide numerous concrete examples. Following this
extensive characterization of symmetry generators, we then discuss their action
on higher-charges, i.e. extended physical operators. As we will explain, even
for invertible higher-form symmetries these are not only representations of the
$p$-form symmetry group, but more generally what are called
higher-representations. Finally, we give an introduction to the Symmetry
Topological Field Theory (SymTFT) and its utility in characterizing symmetries,
their gauging and generalized charges.
Lectures prepared for the ICTP Trieste Spring School, April 2023.
A $z$-matching on a bipartite graph is a set of edges, among which each
vertex of two types of the graph is adjacent to at most $1$ and at most $z$
($\geqslant 1$) edges, respectively. The $z$-matching problem concerns finding
$z$-matchings with the maximum size. Our approach to this combinatorial
optimization problem is twofold. From an algorithmic perspective, we adopt a
local algorithm as a linear approximate solver to find $z$-matchings on any
graph instance, whose basic component is a generalized greedy leaf removal
procedure in graph theory. From a theoretical perspective, on uncorrelated
random bipartite graphs, we develop a mean-field theory for percolation
phenomenon underlying the local algorithm, leading to an analytical estimation
of $z$-matching sizes on random graphs. Our analytical theory corrects the
prediction by belief propagation algorithm at zero-temperature limit in
(Krea\v{c}i\'{c} and Bianconi 2019 \textsl{EPL} \textbf{126} 028001). Besides,
our theoretical framework extends a core percolation analysis of $k$-XORSAT
problems to a general context of uncorrelated random hypergraphs with arbitrary
degree distributions of factor and variable nodes.
Fascination in topological materials originates from their remarkable
response properties and exotic quasiparticles which can be utilized in quantum
technologies. In particular, large-scale efforts are currently focused on
realizing topological superconductors and their Majorana excitations. However,
determining the topological nature of superconductors with current experimental
probes is an outstanding challenge. This shortcoming has become increasingly
pressing due to rapidly developing designer platforms which are theorized to
display very rich topology and are better accessed by local probes rather than
transport experiments. We introduce a robust machine-learning protocol for
classifying the topological states of two-dimensional (2D) chiral
superconductors and insulators from local density of states (LDOS) data. Since
the LDOS can be measured with standard experimental techniques, our protocol
contributes to overcoming the almost three decades standing problem of
identifying the topological phase of 2D superconductors with broken
time-reversal symmetry.
Gapped fracton phases of matter generalize the concept of topological order
and broaden our fundamental understanding of entanglement in quantum many-body
systems. However, their analytical or numerical description beyond exactly
solvable models remains a formidable challenge. Here we employ an exact 3D
quantum tensor-network approach that allows us to study a $\mathbb{Z}_N$
generalization of the prototypical X cube fracton model and its quantum phase
transitions between distinct topological states via fully tractable
wavefunction deformations. We map the (deformed) quantum states exactly to a
combination of a classical lattice gauge theory and a plaquette clock model,
and employ numerical techniques to calculate various entanglement order
parameters. For the $\mathbb{Z}_N$ model we find a family of (weakly)
first-order fracton confinement transitions that in the limit of $N\to\infty$
converge to a continuous phase transition beyond the Landau-Ginzburg-Wilson
paradigm. We also discover a line of 3D conformal quantum critical points (with
critical magnetic flux loop fluctuations) which, in the $N\to\infty$ limit,
appears to coexist with a gapless deconfined fracton state.
Spin torques at topological insulator (TI)/ferromagnet interfaces have
received considerable attention in recent years with a view towards achieving
full electrical manipulation of magnetic degrees of freedom. The most important
question in this field concerns the relative contributions of bulk and surface
states to the spin torque, a matter that remains incompletely understood.
Whereas the surface state contribution has been extensively studied, the
contribution due to the bulk states has received comparatively little
attention. Here we study spin torques due to TI bulk states and show that: (i)
There is no spin-orbit torque due to the bulk states on a homogeneous
magnetisation, in contrast to the surface states, which give rise to a
spin-orbit torque via the well-known Edelstein effect. (ii) The bulk states
give rise to a spin transfer torque (STT) due to the inhomogeneity of the
magnetisation in the vicinity of the interface. This spin transfer torque,
which has not been considered in TIs in the past, is somewhat unconventional
since it arises from the interplay of the bulk TI spin-orbit coupling and the
gradient of the monotonically decaying magnetisation inside the TI. Whereas we
consider an idealised model in which the magnetisation gradient is small and
the spin transfer torque is correspondingly small, we argue that in real
samples the spin transfer torque should be sizable and may provide the dominant
contribution due to the bulk states. We show that an experimental smoking gun
for identifying the bulk states is the fact that the field-like component of
the spin transfer torque generates a spin density with the same size but
opposite sign for in-plane and out-of-plane magnetisations. This distinguishes
them from the surface states, which are expected to give a spin density of a
similar size and the same sign for both an in-plane and out-of-plane
magnetisations.
Non-Hermitian (NH) lattice Hamiltonians display a unique kind of energy gap
and extreme sensitivity to boundary conditions. Due to the NH skin effect, the
separation between edge and bulk states is blurred and the (conventional)
bulk-boundary correspondence is lost. Here, we restore the bulk-boundary
correspondence for the most paradigmatic class of NH Hamiltonians, namely those
with one complex band and without symmetries. We obtain the desired NH
Hamiltonian from the (mean-field) unconditional evolution of driven-dissipative
cavity arrays, in which NH terms -- in the form of non-reciprocal hopping
amplitudes, gain and loss -- are explicitly modeled via coupling to (engineered
and non-engineered) reservoirs. This approach removes the arbitrariness in the
definition of the topological invariant, as point-gapped spectra differing by a
complex-energy shift are not treated as equivalent; the origin of the complex
plane provides a common reference (base point) for the evaluation of the
topological invariant. This implies that topologically non-trivial Hamiltonians
are only a strict subset of those with a point gap and that the NH skin effect
does not have a topological origin. We analyze the NH Hamiltonians so obtained
via the singular value decomposition, which allows to express the NH
bulk-boundary correspondence in the following simple form: an integer value
$\nu$ of the topological invariant defined in the bulk corresponds to $\vert
\nu\vert$ singular vectors exponentially localized at the system edge under
open boundary conditions, in which the sign of $\nu$ determines which edge.
Non-trivial topology manifests as directional amplification of a coherent input
with gain exponential in system size. Our work solves an outstanding problem in
the theory of NH topological phases and opens up new avenues in topological
photonics.
We evaluate the differential conductance measured in a scanning tunneling
microscopy (STM) setting at arbitrary electron transmission between an STM tip
and a two-dimensional (2D) superconductor with arbitrary gap structure. Our
analytical scattering theory accounts for Andreev reflections, which become
prominent at larger transmissions. We show that this provides complementary
information about the superconducting gap structure beyond the tunneling
density of states, strongly facilitating the ability to extract the gap
symmetry and its relation to the underlying crystalline lattice. We use the
developed theory to discuss recent experimental results on superconductivity in
twisted bilayer graphene.
We study flat bands and their topology in 2D materials with quadratic band
crossing points (QBCPs) under periodic strain. In contrast to Dirac points in
graphene, where strain acts as a vector potential, strain for QBCPs serves as a
director potential with angular momentum $\ell=2$. We prove that when the
strengths of the strain fields hit certain ``magic" values, exact flat bands
with $C=\pm 1$ emerge at charge neutrality point in the chiral limit, in strong
analogy to magic angle twisted bilayer graphene. These flat bands have ideal
quantum geometry for the realization of fractional Chern insulators, and they
are always fragile topological. The number of flat bands can be doubled for
certain point group, and the interacting Hamiltonian is exactly solvable at
integer fillings. We further demonstrate the stability of these flat bands
against deviations from the chiral limit, and discuss possible realization in
2D materials.
Quantum convolutional neural networks (QCNNs) have been introduced as
classifiers for gapped quantum phases of matter. Here, we propose a
model-independent protocol for training QCNNs to discover order parameters that
are unchanged under phase-preserving perturbations. We initiate the training
sequence with the fixed-point wavefunctions of the quantum phase and then add
translation-invariant noise that respects the symmetries of the system to mask
the fixed-point structure on short length scales. We illustrate this approach
by training the QCNN on phases protected by time-reversal symmetry in one
dimension, and test it on several time-reversal symmetric models exhibiting
trivial, symmetry-breaking, and symmetry-protected topological order. The QCNN
discovers a set of order parameters that identifies all three phases and
accurately predicts the location of the phase boundary. The proposed protocol
paves the way towards hardware-efficient training of quantum phase classifiers
on a programmable quantum processor.
We consider the dynamics and stationary regime of a capacitively-shunted
transmon-type qubit in front of a mirror, affected by two signals: probe and
dressing signals. By varying the parameters of these signals and then analyzing
the probe signal (reflected by the atom-mirror system), it is possible to
explore the system dynamics, which can be described by the Bloch equation. The
obtained time-dependent occupation probabilities are related to the
experimentally measured reflection coefficient. The study of this type of
dynamics opens up new horizons for better understanding of the system
properties and underlying physical processes, such as
Landau-Zener-Stuckelberg-Majorana transitions.
Two-dimensional (2D) transition metal carbides and nitrides (MXenes) are a
large family of materials actively studied for various applications, especially
in the field of energy storage. To date, MXenes are commonly synthesized by
etching the layered ternary compounds, MAX phases. Here we demonstrate a direct
synthetic route for scalable and atom-economic synthesis of MXenes, including
phases that have not been synthesized from MAX phases, by the reactions of
metals and metal halides with graphite, methane or nitrogen. These directly
synthesized MXenes showed excellent energy storage capacity for Li-ion
intercalation. The direct synthesis enables chemical vapor deposition (CVD)
growth of MXene carpets and complex spherulite-like morphologies. The latter
form in a process resembling the evolution of cellular membranes during
endocytosis.
We present a first-principles scheme for incorporating many-body interactions
into the unified description of the quadratic optical response to light of
noncentrosymmetric crystals. The proposed method is based on time-dependent
current-density response theory and includes the electron-hole attraction
\textit{via} a tensorial long-range exchange-correlation kernel, which we
calculate self-consistently using the bootstrap method. By bridging with the
Wannier-interpolation of the independent-particle transition matrix elements,
the resulting numerical scheme is very general and allows resolving narrow
many-body spectral features at low computational cost. We showcase its
potential by inspecting the second-harmonic generation in the benchmark
zinc-blende semiconductor GaAs, the layered graphitic semiconductor BC$_{2}$N
and the Weyl semimetal TaAs. Our results show that excitonic effects can give
rise to large and sharply localized one- and two-photon resonances that are
absent in the independent-particle approximation. We find overall good
agreement with available experimental measurements, capturing the magnitude and
peak-structure of the spectrum as well as the angular dependence at fixed
photon energy. The implementation of the method in Wannier-based code packages
can serve as a basis for performing accurate theoretical predictions of
quadratic optical properties in a vast pool of materials.
In this paper I present a pedagogical derivation of continuity equations
manifesting exact conservation laws in an interacting electronic system based
on the nonequilibrium Keldysh technique. The purpose of this exercise is to lay
the groundwork for extending the hydrodynamic approach to electronic transport
to strongly correlated systems where the quasiparticle approximation and
Boltzmann kinetic theory fail.
The ground-state properties and excitation energies of a quantum emitter can
be modified in the ultrastrong coupling regime of cavity quantum
electrodynamics (QED) where the light-matter interaction strength becomes
comparable to the cavity resonance frequency. Recent studies have started to
explore the possibility of controlling an electronic material by embedding it
in a cavity that confines electromagnetic fields in deep subwavelength scales.
Currently, there is a strong interest in realizing ultrastrong-coupling cavity
QED in the terahertz (THz) part of the spectrum, since most of the elementary
excitations of quantum materials are in this frequency range. We propose and
discuss a promising platform to achieve this goal based on a two-dimensional
electronic material encapsulated by a planar cavity consisting of ultrathin
polar van der Waals crystals. As a concrete setup, we show that nanometer-thick
hexagonal boron nitride layers should allow one to reach the ultrastrong
coupling regime for single-electron cyclotron resonance in a bilayer graphene.
The proposed cavity platform can be realized by a wide variety of thin
dielectric materials with hyperbolic dispersions. Consequently, van der Waals
heterostructures hold the promise of becoming a versatile playground for
exploring the ultrastrong-coupling physics of cavity QED materials.
A monitored quantum system undergoing a cyclic evolution of the parameters
governing its Hamiltonian accumulates a geometric phase that depends on the
quantum trajectory followed by the system on its evolution. The phase value
will be determined both by the unitary dynamics and by the interaction of the
system with the environment. Consequently, the geometric phase will acquire a
stochastic character due to the occurrence of random quantum jumps. Here we
study the distribution function of geometric phases in monitored quantum
systems and discuss when/if different quantities, proposed to measure geometric
phases in open quantum systems, are representative of the distribution. We also
consider a monitored echo protocol and discuss in which cases the distribution
of the interference pattern extracted in the experiment is linked to the
geometric phase. Furthermore, we unveil, for the single trajectory exhibiting
no quantum jumps, a topological transition in the phase acquired after a cycle
and show how this critical behavior can be observed in an echo protocol. For
the same parameters, the density matrix does not show any singularity. We
illustrate all our main results by considering a paradigmatic case, a spin-1/2
immersed in time-varying a magnetic field in presence of an external
environment. The major outcomes of our analysis are however quite general and
do not depend, in their qualitative features, on the choice of the model
studied.
The use of dissipation for the controlled creation of nontrivial quantum
many-body correlated states is of much fundamental and practical interest. What
is the result of imposing number conservation, which, in closed system, gives
rise to diffusive spreading? We investigate this question for a paradigmatic
model of a two-band system, with dissipative dynamics aiming to empty one band
and to populate the other, which had been introduced before for the dissipative
stabilization of topological states. Going beyond the mean-field treatment of
the dissipative dynamics, we demonstrate the emergence of a diffusive regime
for the particle and hole density modes at intermediate length- and
time-scales, which, interestingly, can only be excited in nonlinear response to
external fields. We also identify processes that limit the diffusive behavior
of this mode at the longest length- and time-scales. Strikingly, we find that
these processes lead to a reaction-diffusion dynamics governed by the
Fisher-Kolmogorov-Petrovsky-Piskunov equation, making the designed dark state
unstable towards a state with a finite particle and hole density.
The quantum geometry in the momentum space of semiconductors and insulators,
described by the quantum metric of the valence band Bloch state, has been an
intriguing issue owing to its connection to various material properties.
Because the Brillouin zone is periodic, the integration of quantum metric over
momentum space represents an average distance between neighboring Bloch states,
of which we call the fidelity number. We show that this number can further be
expressed in real space as a fidelity marker, which is a local quantity that
can be calculated directly from diagonalizing the lattice Hamiltonian. A linear
response theory is further introduced to generalize the fidelity number and
marker to finite temperature, and moreover demonstrates that they can be
measured from the global and local optical absorption power against linearly
polarized light. In particular, the fidelity number spectral function in 2D
systems can be easily measured from the opacity of the material. Based on the
divergence of quantum metric, a nonlocal fidelity marker is further introduced
and postulated as a universal indicator of any quantum phase transitions
provided the crystalline momentum remains a good quantum number, and it may be
interpreted as a Wannier state correlation function. The ubiquity of these
concepts is demonstrated for a variety of topological insulators and
topological phase transitions in different dimensions.
We explore the magnetohydrodynamics of Dirac fermions in neutral graphene in
the Corbino geometry. Based on the fully consistent hydrodynamic description
derived from a microscopic framework and taking into account all peculiarities
of graphene-specific hydrodynamics, we report the results of a comprehensive
study of the interplay of viscosity, disorder-induced scattering,
recombination, energy relaxation, and interface-induced dissipation. In the
clean limit, magnetoresistance of a Corbino sample is determined by viscosity.
Hence the Corbino geometry could be used to measure the viscosity coefficient
in neutral graphene.
The extraordinary properties of materials accompanying their phase
transitions are exciting from the perspectives of scientific research and new
applications. Most recently, Karothu et al.1 described guanidinium nitrate,
[C(NH2)3]+[NO3]-, hereafter GN, as a ferroelectric semiconducting organic
crystal with exceptional actuating properties. However, the ferroelectric and
semiconducting properties of this hybrid organic-inorganic material were not
confirmed by the experimental results, and the reproducibility of the large
stroke associated with the first-order transition is questionable, because the
GN crystals are inherently susceptible to the formation of defects. Besides,
previous extensive studies on GN were not acknowledged.
We present a first principles theoretical study employing nonlinear response
theory to investigate the d.c. photocurrent generated by linearly polarized
light in the type-II Weyl semimetal TaIrTe4. We report the low energy spectrum
of several nonlinear optical effects. At second-order, we consider the shift
and injection currents. Assuming the presence of a built-in static electric
field, at third-order we study the current-induced shift and injection
currents, as well as the jerk current. We discuss our results in the context of
a recent experiment measuring an exceptionally large photoconductivity in this
material [J. Ma et at., Nat. Mater. 18, 476 (2019)]. According to our results,
the jerk current is the most likely origin of the large response. Finally, we
propose means to discern the importance of the various mechanisms involved in a
time-resolved experiment.
We demonstrate that a correlated equilibrium $f$-electron system with
time-reversal symmetry can exhibit a $\mathbb{Z}_2$ non-Hermitian skin effect
of quasi-particles. In particular, we analyze a two-dimensional periodic
Anderson model with spin-orbit coupling by combining the dynamical mean-field
theory (DMFT) and the numerical renormalization group. We prove the existence
of the $\mathbb{Z}_2$ skin effect by explicitly calculating the topological
invariant and show that spin-orbit interaction is essential to this effect. Our
DMFT analysis demonstrates that the $\mathbb{Z}_2$ skin effect of
quasi-particles is reflected on the pseudo-spectrum. Furthermore, we analyze
temperature effects on this skin effect using the generalized Brillouin zone
technique, which clarifies that the skin modes are strongly localized above the
Kondo temperature.
We present a metaheuristic conditional neural-network-based method aimed at
identifying physically interesting metastable states in a potential energy
surface of high rugosity. To demonstrate how this method works, we identify and
analyze spin textures with topological charge $Q$ ranging from 1 to $-13$
(where antiskyrmions have $Q<0$) in the Pd/Fe/Ir(111) system, which we model
using a classical atomistic spin Hamiltonian based on parameters computed from
density functional theory. To facilitate the harvest of relevant spin textures,
we make use of the newly developed Segment Anything Model (SAM). Spin textures
with $Q$ ranging from $-3$ to $-6$ are further analyzed using
finite-temperature spin-dynamics simulations. We observe that for temperatures
up to around 20\,K, lifetimes longer than 200\,ps are predicted, and that when
these textures decay, new topological spin textures are formed. We also find
that the relative stability of the spin textures depend linearly on the
topological charge, but only when comparing the most stable antiskyrmions for
each topological charge. In general, the number of holes (i.e.,
non-self-intersecting curves that define closed domain walls in the structure)
in the spin texture is an important predictor of stability -- the more holes,
the less stable is the texture. Methods for systematic identification and
characterization of complex metastable skyrmionic textures -- such as the one
demonstrated here -- are highly relevant for advancements in the field of
topological spintronics.
In the known field of topological photonics, what remains less so is the
breakdown effect of topological phases deteriorated by perturbation. In this
paper, we investigate the variance on topological invariants for a periodic
Kekul\'e medium perturbed in unit cells, which was a gyromagnetic photonic
crystal holding topological phases induced by \emph{synchronized rotation} of
unit cells. Two parameters for geometric and material perturbation are
respectively benchmarked to characterise the topological degradation. Our
calculation demonstrates that such a periodic perturbation easily destructs the
topological phase, and thus calls for further checkups on robustness under such
unit-cell-perturbation in realization.
We theoretically predict the squeezing-induced point-gap topology together
with a {\it symmetry-protected $\mathbb{Z}_2$ skin effect} in a one-dimensional
(1D) quadratic-bosonic system (QBS). Protected by a time-reversal symmetry,
such a topology is associated with a novel $\mathbb{Z}_2$ invariant (similar to
quantum spin-Hall insulators), which is fully capable of characterizing the
occurrence of $\mathbb{Z}_2$ skin effect. Focusing on zero energy, the
parameter regime of this skin effect in the phase diagram just corresponds to a
{\it real-gap and point-gap coexisted topological phase}. Moreover, this phase
associated with the {\it symmetry-protected $\mathbb{Z}_2$ skin effect} is
experimentally observable by detecting the steady-state power spectral density.
Our work is of fundamental interest in enriching non-Bloch topological physics
by introducing quantum squeezing, and has potential applications for the
engineering of symmetry-protected sensors based on the $\mathbb{Z}_2$ skin
effect.
The ground states of twisted bilayer graphene (TBG) at chiral and flat-band
limit with integer fillings are known from exact solutions, while their
dynamical and thermodynamical properties are revealed by unbiased quantum Monte
Carlo (QMC) simulations. However, to elucidate experimental observations of
correlated metallic, insulating and superconducting states and their
transitions, investigations on realistic, or non-chiral cases are vital. Here
we employ momentum-space QMC method to investigate the evolution of correlated
states in magic-angle TBG away from chiral limit at charge neutrality with
polarized spin/valley, which approximates to an experimental case with filling
factor $\nu=-3$. We find that the ground state evolves from quatum anomalous
Hall insulator into an intriguing correlated semi-metallic state as AA hopping
strength reaches experimental values. Such a state resembles the recently
proposed heavy-fermion representations with localized electrons residing at AA
stacking regions and delocalized electrons itinerating via AB/BA stacking
regions. The spectral signatures of the localized and itinerant electrons in
the heavy-fermion semimetal phase are revealed, with the connection to
experimental results being discussed.
We study mass-type surface defects in a free scalar and Wilson-Fisher (WF)
$O(N)$ theories. We obtain exact results for the free scalar defect, including
its RG flow and defect Weyl anomaly. We classify phases of such defects at the
WF fixed point near four dimensions, whose perturbative RG flow is
investigated. We propose an IR effective action for the non-perturbative regime
and check its self-consistency.

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