Found 32 papers in cond-mat We show that the dissipation of energy in quasi-adiabatically driven quantum
systems weakly coupled to a heat bath admits a description in terms of
trajectories on a manifold characterized by the quantum geometry of the
problem. For two-level systems, this description involves the quantum metric,
further implying a connection between energy dissipation and the Berry
curvature. As a consequence, we demonstrate that in systems slowly driven by a
two-tone incommensurate drive, the dissipation rate has a lower bound
proportional to an integer describing topological energy conversion between the
two tones (provided certain symmetry conditions are respected). These results
provide a design principle towards developing optimal driving protocols.
In nature, $\alpha$-quartz crystals frequently form contact twins - two
adjacent crystals with the same chemical structure but different
crystallographic orientation, sharing a common lattice plane. As
$\alpha$-quartz crystallises in a chiral space group, such twinning can occur
between enantiomorphs with the same handedness or with opposite handedness.
Here, we use first-principle methods to investigate the effect of twinning and
chirality on the bulk and surface phonon spectra, as well as on the topological
properties of phonons in $\alpha$-quartz. We demonstrate that, even though the
dispersion appears identical for all twins along all high-symmetry lines and at
all high-symmetry points in the Brillouin zone, the dispersions can be distinct
at generic momenta for some twin structures. Furthermore, when the twinning
occurs between different enantiomorphs, the charges of all Weyl nodal points
flip, which leads to mirror symmetric isofrequency contours of the surface
arcs. We show that this allows negative refraction to occur at interfaces
between certain twins of $\alpha$-quartz.
The shape of the Fermi surface, the effective mass of carriers, and the
topologically non-trivial nature of electronic bands of kagome magnet GdV6Sn6
are investigated using de Haas van Alphen (dHvA) oscillations measurements. Our
temperature and angle dependent torque magnetometry measurements reveal at
least seven different frequencies ranging from ~90 T up to ~9000 T. These
frequencies correspond to extremal areas of Fermi surface ranging from ~1% up
to 50% of the first Brillouin zone, qualitatively consistent with electronic
structure calculations. The angle dependent dHvA oscillations frequencies
indicate that all pockets of Fermi surface are mostly two-dimensional. We also
find evidence of the presence of lighter (0.58 m0) as well as heavier (2.25 m0)
electrons through the analysis of the temperature dependence of dominant
frequencies, reflecting the features of correlated and Dirac like dispersions
in the electronic structure. The estimation of the Berry phase indicates the
topologically non-trivial nature of the lowest frequency band containing
lighter electrons. This is consistent with the presence of Dirac-like linear
dispersion in the electronic structure.
We have investigated the structure of hydrogen-intercalated
quasi-free-standing monolayer graphene (QFMLG) grown on 6H-SiC(0001) by
employing total-reflection high-energy positron diffraction (TRHEPD). At least
nine diffraction spots of the zeroth order Laue zone were resolved along
<11-20> and three along <1-100>, which are assigned to graphene, SiC and higher
order spots from multiple diffraction on both lattices. We further performed
rocking curve analysis based on the full dynamical diffraction theory to
precisely determine the spacing between QFMLG and the SiC substrate. Our study
yields a spacing of d = 4.18(6)\r{A} that is in excellent agreement with the
results from density-functional theory (DFT) calculations published previously.
The study of waveguide propagating modes is essential for achieving
directional electronic transport in two-dimensional materials. Simultaneously,
exploring potential gaps in these systems is crucial for developing devices
akin to those employed in conventional electronics. Building upon the
theoretical groundwork laid by Hartmann et al., which focused on implementing
waveguides in pristine graphene monolayers, this work delves into the impact of
a waveguide on two-dimensional gapped Dirac systems. We derive exact solutions
encompassing wave functions and energy-bound states for a secant-hyperbolic
attractive potential in gapped graphene, with a gap generated by sublattice
asymmetry or a Kekul\'e-distortion. These solutions leverage the inherent
properties and boundary conditions of the Heun polynomials. Our findings
demonstrate that the manipulation of the number of accessible energy-bound
states, i.e., transverse propagating modes, relies on factors such as the width
and depth of the potential as well as the gap value of the two-dimensional
material.
We theoretically investigate high-order harmonic generation (HHG) in graphene
under mid-infrared (MIR) and terahertz (THz) fields based on a quantum master
equation. Numerical simulations show that MIR-induced HHG in graphene can be
enhanced by a factor of 10 for fifth harmonic and a factor of 25 for seventh
harmonic under a THz field with a peak strength of 0.5 MV/cm by optimizing the
relative angle between the MIR and THz fields. To identify the origin of this
enhancement, we compare the fully dynamical calculations with a simple
thermodynamic model and a nonequilibrium population model. The analysis shows
that the enhancement of the high-order harmonics mainly results from a coherent
coupling between MIR- and THz-induced transitions that goes beyond a simple
THz-induced population contribution.
The excitations in the Kitaev spin liquid (KSL) can be described by Majorana
fermions, which have characteristic field dependence of bulk gap and
topological edge modes. In the high-field state of layered honeycomb magnet
$\alpha$-RuCl$_3$, experimental results supporting these Majorana features have
been reported recently. However, there are challenges due to sample dependence
and the impact of inevitable disorder on the KSL is poorly understood. Here we
study how low-energy excitations are modified by introducing point defects in
$\alpha$-RuCl$_3$ using electron irradiation, which induces site vacancies and
exchange randomness. High-resolution measurements of the temperature dependence
of specific heat $C(T)$ under in-plane fields $H$ reveal that while the
field-dependent Majorana gap is almost intact, additional low-energy states
with $C/T=A(H)T$ are induced by introduced defects. At low temperatures, we
obtain the data collapse of $C/T\sim H^{-\gamma}(T/H)$ expected for a
disordered quantum spin system, but with an anomalously large exponent
$\gamma$. This leads us to find a new power-law scaling of the coefficient
$A(H)$ with the field-sensitive Majorana gap. These results imply that the
disorder induces low-energy linear Majorana excitations, which may be
considered as a weak localization effect of Majorana fermions in the KSL.
The K\"ahler-Dirac fermion, recognized as an elegant geometric approach,
offers an alternative to traditional representations of relativistic fermions.
Recent studies have demonstrated that symmetric mass generation (SMG) can
precisely occur with two copies of K\"ahler-Dirac fermions across any spacetime
dimensions. This conclusion stems from the study of anomaly cancellation within
the fermion system. Our research provides an alternative understanding of this
phenomenon from a condensed matter perspective, by associating the interacting
K\"ahler-Dirac fermion with the boundary of bosonic symmetry-protected
topological (SPT) phases. We show that the low-energy bosonic fluctuations in a
single copy of the K\"ahler-Dirac fermion can be mapped to the boundary modes
of a $\mathbb{Z}_2$-classified bosonic SPT state, protected by an inversion
symmetry universally across all dimensions. This implies that two copies of
K\"ahler-Dirac fermions can always undergo SMG through interactions mediated by
these bosonic modes. This picture aids in systematically designing SMG
interactions for K\"ahler-Dirac fermions in any dimension. We present the exact
lattice Hamiltonian of these interactions and validate their efficacy in
driving SMG.
Bi-layer Kagome compounds provide an exciting playground where the interplay
of topology and strong correlations can give rise to exotic phases of matter.
Motivated by recent first principles calculation on such systems (Phys. Rev.
Lett 125, 026401), reporting stabilization of a Chern metal with topological
nearly-flat band close to Fermi level, we build minimal models to study the
effect of strong electron-electron interactions on such a Chern metal. Using
approriate numerical and analytical techniques, we show that the topologically
non-trivial bands present in this system at the Fermi energy can realize
fractional Chern insulator states. We further show that if the time-reversal
symmetry is restored due to destruction of magnetism by low dimensionality and
fluctuation, the system can realize a superconducting phase in the presence of
strong local repulsive interactions. Furthermore, we identify an interesting
phase transition from the superconducting phase to a correlated metal by tuning
nearest-neighbor repulsion. Our study uncovers a rich set of non-trivial phases
realizable in this system, and contextualizes the physically meaningful regimes
where such phases can be further explored.
The discovery of (4x4) silicene formation on Ag(111) raised the question on
whether silicene maintains its Dirac fermion character, similar to graphene, on
a supporting substrate. Previous photoemission studies indicated that the
{\pi}-band forms Dirac cones near the Fermi energy, while theoretical
investigations found it shifted at deeper binding energy. By means of
angle-resolved photoemission spectroscopy and density functional theory
calculations we show instead that the {\pi}-symmetry states lose their local
character and the Dirac cone fades out. The formation of an interface state of
free-electron-like Ag origin is found to account for spectral features that
were theoretically and experimentally attributed to silicene bands of
{\pi}-character.
The similarities of quantum turbulence with classical hydrodynamics allow
quantum fluids to provide essential models of their classical analogue, paving
the way for fundamental advances in physics and technology. Recently,
experiments on 2D quantum turbulence observed the clustering of same-sign
vortices in strong analogy with the inverse energy cascade of classical fluids.
However, self-similarity of the turbulent flow, a fundamental concept in the
study of classical turbulence, has so far remained largely unexplored in
quantum systems. Here, thanks to the unique features of exciton-polaritons, we
measure the scale invariance of velocity circulations and show that the cascade
process follows the universal scaling of critical phenomena in 2D. We
demonstrate this behaviour from the statistical analysis of the experimentally
measured incompressible velocity field and the microscopic imaging of the
quantum fluid. These results can find wide application in both quantum and
classical 2D turbulence.
Spin-triplet superconductors provide an ideal platform for realizing
topological superconductivity with emergent Majorana quasiparticles. The
promising candidate is the recently discovered superconductor UTe$ _2$, but the
symmetry of the superconducting order parameter remains highly controversial.
Here we determine the superconducting gap structure by the thermal conductivity
of ultra-clean UTe$ _2$ single crystals. We find that the $a$ axis thermal
conductivity divided by temperature $\kappa/T$ in zero-temperature limit is
vanishingly small for both magnetic fields $\mathbf{H}||a$ and $\mathbf{H}||c$
axes up to $H/H_{c2}\sim 0.2$, demonstrating the absence of any types of nodes
around $a$ axis contrary to the previous belief. The present results, combined
with the reduction of the NMR Knight shift in the superconducting state,
indicate that the superconducting order parameter belongs to the isotropic
$A_u$ representation with a fully gapped pairing state, analogous to the B
phase of superfluid $ ^3$He. These findings reveal that UTe$ _2$ is likely to
be a long-sought three-dimensional (3D) strong topological superconductor
characterized by a 3D winding number, hosting helical Majorana surface states
on any crystal plane.
The recently discovered ATi$_3$Bi$_5$ (A=Cs, Rb) exhibit intriguing quantum
phenomena including superconductivity, electronic nematicity, and abundant
topological states, which provide promising platforms for studying kagome
superconductivity, band topology, and charge orders. In this work, we
comprehensively study various properties of ATi$_3$Bi$_5$ including
superconductivity under pressure and doping, band topology under pressure,
thermal conductivity, heat capacity, electrical resistance, and spin Hall
conductivity (SHC) using first-principles calculations. Calculated
superconducting transition temperature ($\mathrm{ T_{c}}$) of CsTi$_3$Bi$_5$
and RbTi$_3$Bi$_5$ at ambient pressure are about 1.85 and 1.92K. When subject
to pressure, $\mathrm{ T_{c}}$ of CsTi$_3$Bi$_5$ exhibits a special valley and
dome shape, which arises from quasi-two-dimensional to three-dimensional
isotropic compression within the context of an overall decreasing trend.
Furthermore, $\mathrm{ T_{c}}$ of RbTi$_3$Bi$_5$ can be effectively enhanced up
to 3.09K by tuning the kagome van Hove singularities (VHSs) and flat band
through doping. Pressure can also induce abundant topological surface states at
the Fermi energy ($\mathrm{E}_{\mathrm{F}}$) and tune VHSs across
$\mathrm{E}_{\mathrm{F}}$. Additionally, our transport calculations are in
excellent agreement with recent experiments, confirming the absence of charge
density wave. Notably, SHC of CsTi$_3$Bi$_5$ can reach as large as 226$
\hbar\cdot (e\cdot \Omega \cdot cm) ^{-1} $ at $\mathrm{E}_{\mathrm{F}}$. Our
work provides a timely and detailed analysis of the rich physical properties
for ATi$_3$Bi$_5$, offering valuable insights for further explorations and
understandings on these intriguing superconducting materials.
We investigate the Casimir-Lifshitz force (CLF) between two identical
graphene strip gratings, laid on finite dielectric substrate. By using the
scattering matrix (S-matrix) approach derived from the Fourier Modal Method
with local basis functions (FMM-LBF), we fully take into account the high-order
electromagnetic diffractions, the multiple scattering and the exact 2D feature
of the graphene strips. We show that the non-additivity, which is one of the
most interesting features of the CLF in general, is significantly high and can
be modulated in situ without any change in the actual material geometry, by
varying the graphene chemical potential. This study can open the deeper
experimental exploration of the non-additive features of CLF with micro- or
nano-electromechanical graphene-based systems.
The van der Waals (vdWs) forces between monolayers has been a unique
distinguishing feature of exfoliable materials since the first isolation of
graphene. However, the vdWs interaction of exfoliable materials with their
substrates and how this interface force influences their interaction with the
environment is yet to be well understood.Here, we experimentally and
theoretically unravel the role of vdWs forces between the recently rediscovered
wide band gap p-type vdW semiconductor violet phosphorus (VP), with various
substrates (including, SiO$_2$, mica, Si, Au) and quantify how VP stability in
air and its interaction with its surroundings is influenced by the interface
force.Using a combination of infrared nanoimaging and theoretical modeling we
find the vdWs force at the interface to be a main factor that influences how VP
interacts with its surroundings.In addition, the hydrophobicity of the
substrate and the substrate surface roughness modify the vdWs force there by
influencing VP stability. Our results could guide in the selection of
substrates when vdW materials are prepared and more generally highlight the key
role of interface force effects that could significantly alter physical
properties of vdWs materials.
Strong correlations lead to emergent excitations at low energies. When
combined with symmetry constraints, they may produce topological electronic
states near the Fermi energy. Within this general framework, here we address
the topological features in iron-based superconductors. We examine the effects
of orbital-selective correlations on the band inversion in the iron
chalcogenide FeSe$_{x}$Te$_{1-x}$ near its doping of optimal superconductivity,
within a multiorbital model and using a $U(1)$ slave spin theory. The orbital
selectivity of the quasiparticle spectral weight, along with its counterpart of
the energy level renormalization, leads to a band inversion and Dirac node
formation pinned to the immediate vicinity of the Fermi energy. Our work
demonstrates both the naturalness and robustness of the topological properties
in FeSe$_{x}$Te$_{1-x}$, and uncovers a new setting in which strong
correlations and space-group symmetry cooperate in generating strongly
correlated electronic topology.
Quaternary mixed-metal chalcohalides (Sn$_2$BCh$_2$X$_3$) are emerging as
promising lead-free perovskite-inspired photovoltaic absorbers. Motivated by
recent developments of a first Sn$_2$BCh$_2$X$_3$-based device, we used density
functional theory to identify lead-free Sn$_2$BCh$_2$X$_3$ materials that are
structurally and energetically stable within Cmcm, Cmc2$_1$ and P2$_1$/c space
groups and have a band gap in the range of 0.7 to 2.0 eV to cover out- and
indoor photovoltaic applications. A total of 27 Sn$_2$BCh$_2$X$_3$ materials
were studied, including Sb, Bi, In for B-site, S, Se, Te for Ch-site and Cl,
Br, I for X-site. We identified 12 materials with a direct band gap that meet
our requirements, namely: Sn$_2$InS$_2$Br$_3$, Sn$_2$InS$_2$I$_3$,
Sn$_2$InSe$_2$Cl$_3$, Sn$_2$InSe$_2$Br$_3$, Sn$_2$InTe$_2$Br$_3$,
Sn$_2$InTe$_2$Cl$_3$, Sn$_2$SbS$_2$I$_3$, Sn$_2$SbSe$_2$Cl$_3$,
Sn$_2$SbSe$_2$I$_3$, Sn$_2$SbTe$_2$Cl$_3$, Sn$_2$BiS$_2$I$_3$ and
Sn$_2$BiTe$_2$Cl$_3$. A database scan reveals that 9 out of 12 are new
compositions. For all 27 materials, P2$_1$/c is the thermodynamically preferred
structure, followed by Cmc2$_1$. In Cmcm and Cmc2$_1$ mainly direct gaps occur,
whereas mostly indirects in P2$_1$/c. To open up the possibility of band gap
tuning in the future, we identified 12 promising
Sn$_2$B$_{1-{a}}$B$'_{a}$Ch$_{2-{b}}$Ch$'_{b}$X$_{3-{c}}$X$_{c}$ alloys which
fulfill our requirements and additional 69 materials by combining direct and
indirect band gap compounds.
We apply the next-nearest-neighbor-interaction model to estimate the band
gaps of the polytypes of group IV elements (C, Si, and Ge)and binary compounds
of groups: IV-IV (SiC, GeC, and GeSi), and III-V (nitride, phosphide, and
arsenide of B, Al, and Ga). The band gap models are based on reference values
of the simplest polytypes comprising 2-6 bilayers calculated with the hybrid
density functional approximation, HSE06. We report four models capable of
estimating band gaps of nine polytypes containing 7 and 8 bilayers with an
average error of less than ~0.05 eV. We apply the best model with an error of <
0.04 eV to predict the band gaps of 497 polytypes with up to 15 bilayers in the
unit cell, providing a comprehensive view of the variation in the electronic
structure with the degree of hexagonality of the crystal structure. Within our
enumeration, we identify four rhombohedral polytypes of SiC -- 9R, 12R, 15R(1),
and 15R(2) -- and perform detailed stability and band structure analysis. Of
these, 15R(1) that has not been experimentally characterized has the widest
band gap (> 3.4 eV); phonon analysis and cohesive energy reveal 15R(1)-SiC to
be metastable. Additionally, we model the energies of valence and conduction
bands of the rhombohedral phases of SiC at the high-symmetry points of the
Brillouin zone and predict band structure characteristics around the Fermi
level. The models presented in this study may aid in identifying polytypic
phases suitable for various applications, such as wide-gap SiC relevant to
high-voltage applications. In particular, the method holds promise for
forecasting electronic properties of long-period and ultra-long-period
polytypes for which accurate first-principles modeling is computationally
challenging.
We consider a Su-Schrieffer-Heeger chain to which we attach a semi-infinite
undimerized chain (lead) to both ends. We study the effect of the openness of
the SSH model on its properties. A representation of the infinite system using
an effective Hamiltonian allows us to examine its low-energy states in more
detail. We show that, as one would expect, the topological edge states
hybridize as the coupling between the systems is increased. As this coupling
grows, these states are suppressed, while a new type of edge state emerges from
the trivial topological phase. These new states, referred to as phase-inverted
edge states, are localized low-energy modes very similar to the edge states of
the topological phase. Interestingly, localization occurs on a new shifted
interface, moving from the first (and last) site to the second (and second to
last) site. This suggests that the topology of the system is strongly affected
by the leads, with three regimes of behavior. For very small coupling the
system is in a well-defined topological phase; for very large coupling it is in
the opposite phase; in the intermediate region, the system is in a transition
regime.
Living systems are chiral on multiple scales, from constituent biopolymers to
large scale morphology, and their active mechanics is both driven by chiral
components and serves to generate chiral morphologies. We describe the
mechanics of active fluid membranes in coordinate-free form, with focus on
chiral contributions to the stress. These generate geometric `odd elastic'
forces in response to mean curvature gradients but directed perpendicularly. As
a result, they induce tangential membrane flows that circulate around maxima
and minima of membrane curvature. When the normal viscous force amplifies
perturbations the membrane shape can become linearly unstable giving rise to
shape instabilities controlled by an active Scriven-Love number. We describe
examples for spheroids, membranes tubes and helicoids, discussing the relevance
and predictions such examples make for a variety of biological systems from the
sub-cellular to tissue level.
The interplay of electronic and structural degrees of freedom in solids is a
topic of intense research. Experience and intuition suggest that structural
changes drive conduction electron behavior, because the large number of valence
electrons dominate the structural properties. As part of a seminal paper
written over sixty years ago, Lifshitz discussed an alternative possibility:
lattice softening driven by conduction electrons at topological Fermi surface
transitions. The effect he predicted, however, was small, and has not been
convincingly observed. Using measurements of the stress-strain relationship in
the ultra-clean metal Sr$_{2}$RuO$_{4}$, we reveal a huge softening of the
Young's modulus at a Lifshitz transition of a two-dimensional Fermi surface,
and show that it is indeed entirely driven by the conduction electrons of the
relevant energy band.
The unambiguous identification of Majorana zero modes (MZMs) is one of the
most outstanding problems of condensed matter physics. Thermal transport
provides a detection tool that is sensitive to these chargeless quasiparticles.
We study thermoelectric transport between metallic leads transverse to a
Josephson junction. The central double quantum dot hosts conventional or
topological Andreev states that depend on the phase difference $\phi$. We show
that the presence of MZMs can be identified by a significant amplification of
both the electrical and thermal conductance at $\phi \approx \pi$ as well as
the Seebeck coefficient at $\phi \approx 0$. We further investigate the
robustness of our results against Cooper pair splitting processes.
Real-world networks are neither regular nor random, a fact elegantly
explained by mechanisms such as the Watts-Strogatz or the Barabasi-Albert
models, among others. Both mechanisms naturally create shortcuts and hubs,
which while enhancing network's connectivity, also might yield several
undesired navigational effects: they tend to be overused during geodesic
navigational processes -- making the networks fragile -- and provide suboptimal
routes for diffusive-like navigation. Why, then, networks with complex
topologies are ubiquitous? Here we unveil that these models also entropically
generate network bypasses: alternative routes to shortest paths which are
topologically longer but easier to navigate. We develop a mathematical theory
that elucidates the emergence and consolidation of network bypasses and measure
their navigability gain. We apply our theory to a wide range of real-world
networks and find that they sustain complexity by different amounts of network
bypasses. At the top of this complexity ranking we found the human brain, which
points out the importance of these results to understand the plasticity of
complex systems.
Black holes are considered among the most fascinating objects that exist in
our universe, since in the classical formalism nothing, even no light, can
escape from their vicinity due to gravity. The gravitational potential causes
the light to bend towards the hole, which is known by gravitational lensing.
Here we present a synthetic realization of this phenomenon in a lab-scale
two-dimensional network of mechanical circuits, based on analogous condensed
matter formalism of Weyl semimetals with inhomogeneous nodal tilt profiles.
Some of the underlying network couplings turn out as unstable and
non-reciprocal, and are implemented by embedded active feedback interactions in
an overall stabilized structure. We demonstrate the lensing by propagating
mechanical wavepackets through the network with a programmed funnel-like
potential, achieving wave bending towards the circle center. We then
demonstrate the versatility of our platform by reprogramming it to mimic
quantum tunneling of particles through the event horizon, known by Hawking
radiation, achieving an exceptional correspondence to the original mass loss
rate within the hole. The network couplings and the potential can be further
reprogrammed to realize other curvatures and associated relativistic phenomena.
Ongoing advances in force field and computer hardware development enable the
use of molecular dynamics (MD) to simulate increasingly complex systems with
the ultimate goal of reaching cellular complexity. At the same time, rational
design by high-throughput (HT) simulations is another forefront of MD. In these
areas, the Martini coarse-grained force field, especially the latest version
(i.e. v3), is being actively explored because it offers enhanced
spatial-temporal resolution. However, the automation tools for preparing
simulations with the Martini force field, accompanying the previous version,
were not designed for HT simulations or studies of complex cellular systems.
Therefore, they become a major limiting factor. To address these shortcomings,
we present the open-source Vermouth python library. Vermouth is designed to
become the unified framework for developing programs, which prepare, run, and
analyze Martini simulations of complex systems. To demonstrate the power of the
Vermouth library, the Martinize2 program is showcased as a generalization of
the martinize script, originally aimed to set up simulations of proteins. In
contrast to the previous version, Martinize2 automatically handles protonation
states in proteins and post-translation modifications, offers more options to
fine-tune structural biases such as the elastic network, and can convert
non-protein molecules such as ligands. Finally, Martinize2 is used in two
high-complexity benchmarks. The entire I-TASSER protein template database as
well as a subset of 200,000 structures from the AlphaFold Protein Structure
Database are converted to CG resolution and we illustrate how the checks on
input structure quality can safeguard high-throughput applications.
The magnetic structure, magnetoresistance, and Hall effect of
non-centrosymmetric magnetic semimetal NdAlGe are investigated revealing an
unusual magnetic state and anomalous transport properties that are associated
with the electronic structure of this non-centrosymmetric compound. The
magnetization and magnetoresistance measurements are both highly anisotropic
and indicate an Ising-like magnetic system. The magnetic structure is complex
in that it involves three magnetic ordering vectors including an incommensurate
spin density wave and commensurate ferrimagnetic state in zero field. We have
discovered a large anomalous Hall conductivity that reaches = 430
{\Omega}-1cm-1 implying that it originates from an intrinsic Berry curvature
effect stemming from Weyl nodes found in the electronic structure. These
electronic structure calculations indicate the presence of nested Fermi surface
pockets with nesting wave vectors similar to the measured magnetic ordering
wavevector and the presence of Weyl nodes in proximity to the Fermi surface. We
associate the incommensurate magnetic structure with the large anomalous Hall
response to be the result of the combination of Fermi surface nesting and the
Berry curvature associated with Weyl nodes.
In neuronal systems, inhibition contributes to stabilizing dynamics and
regulating pattern formation. Through developing mean field theories of
neuronal models, using complete graph networks, inhibition is commonly viewed
as one ``control parameter'' of the system, promoting an absorbing phase
transition. Here, we show that for low connectivity sparse networks, inhibition
weight is not a control parameter of the transition. We present analytical and
simulation results using generic stochastic integrate-and-fire neurons that,
under specific restrictions, become other simpler stochastic neuron models
common in literature, which allow us to show that our results are valid for
those models as well. We also give a simple explanation about why the
inhibition role depends on topology, even when the topology has a
dimensionality greater than the critical one. The absorbing transition
independence of the inhibitory weight may be an important feature of a sparse
network, as it will allow the network to maintain a near-critical regime,
self-tuning average excitation, but at the same time, have the freedom to
adjust inhibitory weights for computation, learning, and memory, exploiting the
benefits of criticality.
Recent experimental study unveiled highly unconventional phenomena in the
superconducting twisted bilayer graphene (TBG) with ultra flat bands, which
cannot be described by the conventional BCS theory. For example, given the
small Fermi velocity of the flat bands, the predicted superconducting coherence
length accordingly to BCS theory is more than 20 times shorter than the
measured values. A new theory is needed to understand many of the
unconventional properties of flat band superconductors. In this work, we
establish a Ginzburg-Landau (GL) theory from a microscopic flat band
Hamiltonian. The GL theory shows how the properties of the physical quantities
such as the critical temperature, the superconducting coherence length, the
upper critical field and the superfluid density are governed by the quantum
metric of the Bloch states. One key conclusion is that the superconducting
coherence length is not determined by the Fermi velocity but by the size of the
optimally localized Wannier functions which is limited by quantum metric.
Applying the theory to TBG, we calculated the superconducting coherence length
and the upper critical fields. The results match the experimental ones well
without fine tuning of parameters. The established GL theory provides a new and
general theoretical framework for understanding flat band superconductors with
quantum metric.
Circuits provide ideal platforms of topological phases and matter, yet the
study of topological circuits in the strongly nonlinear regime, has been
lacking. We propose and experimentally demonstrate strongly nonlinear
topological phases and transitions in one-dimensional electrical circuits
composed of nonlinear capacitors. Nonlinear topological interface modes arise
on domain walls of the circuit lattices, whose topological phases are
controlled by the amplitudes of nonlinear voltage waves. Experimentally
measured topological transition amplitudes are in good agreement with those
derived from nonlinear topological band theory. Our prototype paves the way
towards flexible metamaterials with amplitude-controlled rich topological
phases and is readily extendable to two and three-dimensional systems that
allow novel applications.
Atomic gases confined in curved geometries display distinctive features that
are absent in their flat counterparts, such as periodic boundaries, local
curvature, and nontrivial topologies. The recent experiments with shell-shaped
quantum gases and the study of one dimensional rings point out that the
manifold of a quantum gas could soon become a controllable feature, thus
allowing to address the fundamental study of curved many-body quantum systems.
Here, we review the main geometries realized in the experiments, analyzing the
theoretical and experimental status on their phase transitions and on the
superfluid dynamics. In perspective, we delineate the study of vortices, the
few-body physics, and the search for analog models in various curved geometries
as the most promising research areas.
A comparative theoretical study is presented for the rhombohedral R3 and R3m
phase HfO2, of two possible forms in its heavily Zr-doped ferroelectric thin
films found recently in experiments. Their structural stability and
polarization under the in-plane compressive strain are comprehensively
investigated. We discovered that there is a phase transition from R3 to R3m
phase under the biaxial compressive strain. Both the direction and amplitude of
their polarization can be tuned by the strain. By performing a symmetry mode
analysis, we are able to understand its improper nature of the
ferroelectricity. These results may help to shed light on the understanding of
the hafnia ferroelectric thin films.
A non-iterative method is presented to calculate the closest Wannier
functions (CWFs) to a given set of localized guiding functions, such as atomic
orbitals, hybrid atomic orbitals, and molecular orbitals, based on minimization
of a distance measure function. It is shown that the minimization is directly
achieved by a polar decomposition of a projection matrix via singular value
decomposition, making iterative calculations and complications arising from the
choice of the gauge irrelevant. The disentanglement of bands is inherently
addressed by introducing a smoothly varying window function and a greater
number of Bloch functions, even for isolated bands. In addition to atomic and
hybrid atomic orbitals, we introduce embedded molecular orbitals in molecules
and bulks as the guiding functions, and demonstrate that the Wannier
interpolated bands accurately reproduce the targeted conventional bands of a
wide variety of systems including Si, Cu, the TTF-TCNQ molecular crystal, and a
topological insulator of Bi$_2$Se$_3$. We further show the usefulness of the
proposed method in calculating effective atomic charges. These numerical
results not only establish our proposed method as an efficient alternative for
calculating WFs, but also suggest that the concept of CWFs can serve as a
foundation for developing novel methods to analyze electronic structures and
calculate physical properties.

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