Found 33 papers in cond-mat The study of topological superconductivity is largely based on the analysis
of simple mean-field models that do not conserve particle number. A major open
question in the field is whether the remarkable properties of these mean-field
models persist in more realistic models with a conserved total particle number
and long-range interactions. For applications to quantum computation, two key
properties that one would like to verify in more realistic models are (i) the
existence of a set of low-energy states (the qubit states) that are separated
from the rest of the spectrum by a finite energy gap, and (ii) an exponentially
small (in system size) bound on the splitting of the energies of the qubit
states. It is well known that these properties hold for mean-field models, but
so far only property (i) has been verified in a number-conserving model. In
this work we fill this gap by rigorously establishing both properties (i) and
(ii) for a number-conserving toy model of two topological superconducting wires
coupled to a single bulk superconductor. Our result holds in a broad region of
the parameter space of this model, suggesting that properties (i) and (ii) are
robust properties of number-conserving models, and not just artifacts of the
mean-field approximation.
Symmetries and their anomalies give strong constraints on renormalization
group (RG) flows of quantum field theories. Recently, the identification of a
theory's global symmetries with its topological sector has provided additional
constraints on RG flows to symmetry preserving gapped phases due to
mathematical results in category and topological quantum field theory. In this
paper, we derive constraints on RG flows from $\mathbb{Z}_2$-valued pure- and
mixed-gravitational anomalies that can only be activated on non-spin manifolds.
We show that such anomalies cannot be matched by a unitary, symmetry preserving
gapped phase without symmetry fractionalization. In particular, we discuss
examples that commonly arise in $4d$ gauge theories with fermions.
Here we systematically investigate the impact of the spin direction on the
electronic and optical properties of transition metal phosphorus
trichalcogenides (MPX$_3$, M=Mn, Ni, Fe; X=S, Se) exhibiting various
antiferromagnetic arrangement within the 2D limit. Our analysis based on the
density functional theory and versatile formalism of Bethe-Salpeter equation
reveals larger exciton binding energies for MPS$_3$ (up to 1.1 eV in air) than
MPSe$_3$(up to 0.8 eV in air), exceeding the values of transition metal
dichalcogenides (TMDs). For the (Mn,Fe)PX$_3$ we determine the optically active
band edge transitions, revealing that they are sensitive to in-plane magnetic
order, irrespective of the type of chalcogen atom. We predict the anistropic
effective masses and the type of linear polarization as an important
fingerprints for sensing the type of magnetic AFM arrangements. Furthermore, we
identify the spin-orientation-dependent features such as the valley splitting,
the effective mass of holes, and the exciton binding energy. In particular, we
demonstrate that for MnPX$_3$ (X=S, Se) a pair of non equivalent K+ and K-
points exists yielding the valley splittings that strongly depend on the
direction of AFM aligned spins. Notably, for the out-of-plane direction of
spins, two distinct peaks are expected to be visible below the absorption
onset, whereas one peak should emerge for the in-plane configuration of spins.
These spin-dependent features provide an insight into spin flop transitions of
2D materials. Finally, we propose a strategy how the spin valley polarization
can be realized in 2D AFM within honeycomb lattice.
Computational materials are pivotal in advancing our understanding of
distinct material classes and their properties, offering valuable insights in
predicting novel structures and complementing experimental approaches. In this
context, Psi-graphene is a stable two-dimensional carbon allotrope composed of
5-6-7 carbon rings theoretically predicted recently. Using density functional
theory (DFT) calculations, we explored its boron nitride counterpart's
mechanical, electronic, and optical characteristics (Psi-BN). Our results
indicate that Psi-BN possesses a band gap of 4.59 eV at the HSE06 level. Phonon
calculations and ab initio molecular dynamics simulations demonstrated that
this material has excellent structural and dynamic stability. Moreover, its
formation energy is -7.48 eV. Psi-BN exhibited strong ultraviolet activity,
suggesting its potential as an efficient UV collector. Furthermore, we
determined critical mechanical properties of Psi-BN, such as the elastic
stiffness constants, Young's modulus (250-300 GPa), and Poisson ratio (0.7),
providing valuable insights into its mechanical behavior.
Recently it has been demonstrated that the cosine chirp microwave pulse
(CCMP) is capable of achieving fast and energy-efficient magnetization-reversal
of a nanoparticle with zero-Temperature. However, we investigate the finite
temperature, $T$ effect on the CCMP-driven magnetization reversal using the
framework of the stochastic Landau Lifshitz Gilbert equation. At finite
Temperature, we obtain the CCMP-driven fast and energy-efficient reversal and
hence estimate the maximal temperature, $T_{max}$ at which the magnetization
reversal is valid. $T_{max}$ increases with increasing the nanoparticle
cross-sectional area/shape anisotropy up to a certain value, and afterward
$T_{max}$ decreases with the further increment of nanoparticle cross-sectional
area/shape anisotropy. This is because of demagnetization/shape anisotropy
field opposes the magnetocrystalline anisotropy, i.e., reduces the energy
barrier which separates the two stable states. For smaller cross-sectional
area/shape anisotropy, the controlling parameters of CCMP show decreasing trend
with temperature. We also find that with the increment easy-plane
shape-anisotropy, the required initial frequency of CCMP significantly reduces.
For the larger volume of nanoparticles, the parameters of CCMP remains constant
for a wide range of temperature which are desired for the device application.
Therefore, The above findings might be useful to realize the CCMP-driven fast
and energy-efficient magnetization reversal in realistic conditions.
Two-dimensional (2D) layered metal dichalcogenides constitute a promising
class of materials for photodetector applications due to their excellent
optoelectronic properties. The most common photodetectors, which work on the
principle of photoconductive or photovoltaic effects, however, require either
the application of external voltage biases or built-in electric fields, which
makes it challenging to simultaneously achieve high responsivities across
broadband wavelength excitation - especially beyond the material's nominal band
gap - while producing low dark currents. In this work, we report the discovery
of an intricate phonon-photon-electron coupling - which we term the
acoustophotoelectric effect - in SnS$_2$ that facilitates efficient
photodetection through the application of 100-MHz-order propagating surface
acoustic waves (SAWs). This effect not only reduces the band gap of SnS$_2$,
but also provides the requisite momentum for indirect band gap transition of
the photoexcited charge carriers, to enable broadband photodetection beyond the
visible light range, whilst maintaining pA-order dark currents - remarkably
without the need for any external voltage bias. More specifically, we show in
the infrared excitation range that it is possible to achieve up to eight orders
of magnitude improvement in the material's photoresponsivity compared to that
previously reported for SnS$_2$-based photodetectors, in addition to exhibiting
superior performance compared to most other 2D materials reported to date for
photodetection.
We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate
1D Dirac fermions. A ternary van der Waals material Nb$_9$Si$_4$Te$_{18}$
incorporates in-plane NbTe$_2$ chains, which produce a 1D Dirac band crossing
Fermi energy. Tunneling conductance of electrons confined within NbTe2 chains
is found to be substantially suppressed at Fermi energy, which follows a power
law with a universal temperature scaling, hallmarking a TLL state. The obtained
Luttinger parameter of ~0.15 indicates strong electron-electron interaction.
The TLL behavior is found to be robust against atomic-scale defects, which
might be related to the Dirac electron nature. These findings, as combined with
the tunability of the compound and the merit of a van der Waals material, offer
a robust, tunable, and integrable platform to exploit non-Fermi liquid physics.
The persistent quasi-planar nematic texture known also as the dowser texture
is characterized by a 2D unitary vector field d. We show here that the dowser
texture is sensitive, in first order, to electric fields. This property is due
to the flexo-electric polarisation P collinear with d expected from R.B.
Meyer's considerations on flexo-electricity in nematics. It is pointed out that
due to the flexo-electric polarisation nematic monopoles can be manipulated by
electric fields of appropriated geometry.
The study of moir\'e engineering started with the advent of van der Waals
heterostructures in which stacking two-dimensional layers with different
lattice constants leads to a moir\'e pattern controlling their electronic
properties. The field entered a new era when it was found that adjusting the
twist between two graphene layers led to strongly-correlated-electron physics
and topological effects associated with atomic relaxation. Twist is now used
routinely to adjust the properties of two-dimensional materials. Here, we
investigate a new type of moir\'e superlattice in bilayer graphene when one
layer is biaxially strained with respect to the other - so-called biaxial
heterostrain. Scanning tunneling microscopy measurements uncover spiraling
electronic states associated with a novel symmetry-breaking atomic
reconstruction at small biaxial heterostrain. Atomistic calculations using
experimental parameters as inputs reveal that a giant atomic swirl forms around
regions of aligned stacking to reduce the mechanical energy of the bilayer.
Tight-binding calculations performed on the relaxed structure show that the
observed electronic states decorate spiraling domain wall solitons as required
by topology. This study establishes biaxial heterostrain as an important
parameter to be harnessed for the next step of moir\'e engineering in van der
Waals multilayers.
Modern Visual-Based Tactile Sensors (VBTSs) use cost-effective cameras to
track elastomer deformation, but struggle with ambient light interference.
Solutions typically involve using internal LEDs and blocking external light,
thus adding complexity. Creating a VBTS resistant to ambient light with just a
camera and an elastomer remains a challenge. In this work, we introduce WStac,
a self-illuminating VBTS comprising a mechanoluminescence (ML) whisker
elastomer, camera, and 3D printed parts. The ML whisker elastomer, inspired by
the touch sensitivity of vibrissae, offers both light isolation and high ML
intensity under stress, thereby removing the necessity for additional LED
modules. With the incorporation of machine learning, the sensor effectively
utilizes the dynamic contact variations of 25 whiskers to successfully perform
tasks like speed regression, directional identification, and texture
classification. Videos are available at: https://sites.google.com/view/wstac/.
We examine the origin of the formation of narrow bands in LK-99
(Pb$_{9}$Cu(PO$_4$)$_6$O) and the parent compound without the Cu doping using
density functional theory calculations and model Hamiltonian studies. Explicit
analytical expressions are given for a nearest-neighbor tight-binding (TB)
Hamiltonian in the momentum space for both the parent and the LK-99 compound,
which can serve as an effective model to study various quantum phenomena
including superconductivity. The parent material is an insulator with the
buckle oxygen atom on the stacked triangular lattice forming the topmost bands,
well-separated from the remaining oxygen band manifold. The $C_3$
symmetry-driven two-band TB model describes these two bands quite well. These
bands survive in the Cu-doped, LK-99, though with drastically altered band
dispersion due to the Cu-O interaction. A similar two-band model involving the
Cu $xz$ and $yz$ orbitals broadly describes the top two valence bands of LK-99.
However, the band dispersions of both the Cu and O bands are much better
described by the four-band TB model incorporating the Cu-O interactions on the
buckled honeycomb lattice. We comment on the possible mechanisms of
superconductivity in LK-99. even though the actual T$_c$ may be much smaller
than reported, and suggest that interstitial Cu clusters leading to broad bands
might have a role to play
Machine learning techniques, in particular the so-called normalizing flows,
are becoming increasingly popular in the context of Monte Carlo simulations as
they can effectively approximate target probability distributions. In the case
of lattice field theories (LFT) the target distribution is given by the
exponential of the action. The common loss function's gradient estimator based
on the "reparametrization trick" requires the calculation of the derivative of
the action with respect to the fields. This can present a significant
computational cost for complicated, non-local actions like e.g. fermionic
action in QCD. In this contribution, we propose an estimator for normalizing
flows based on the REINFORCE algorithm that avoids this issue. We apply it to
two dimensional Schwinger model with Wilson fermions at criticality and show
that it is up to ten times faster in terms of the wall-clock time as well as
requiring up to $30\%$ less memory than the reparameterization trick estimator.
It is also more numerically stable allowing for single precision calculations
and the use of half-float tensor cores. We present an in-depth analysis of the
origins of those improvements. We believe that these benefits will appear also
outside the realm of the LFT, in each case where the target probability
distribution is computationally intensive.
The puzzling question about the floating of the topological surface state on
top of a thick Pb layer, has now possibly been answered. A study of the
interface made by Pb on Bi2Se3 for different temperature and adsorbate coverage
condition, allowed us to demonstrate that the evidence reported in the
literature can be related to the surface diffusion phenomenon exhibited by the
Pb atoms, which leaves the substrate partially uncovered. Comprehensive density
functional theory calculations show that despite the specific arrangement of
the atoms at the interface, the topological surface state cannot float on top
of the adlayer but rather tends to move inward within the substrate.
$\mathrm{MoS_2}$ is an emergent van der Waals material that shows promising
prospects in semiconductor industry and optoelectronic applications. However,
its electronic properties are not yet fully understood. In particular, the
nature of the insulating state at low carrier density deserves further
investigation, as it is important for fundamental research and applications. In
this study, we investigate the insulating state of a dual-gated exfoliated
bilayer $\mathrm{MoS_2}$ field-effect transistor by performing magnetotransport
experiments. We observe positive and non-saturating magnetoresistance, in a
regime where only one band contributes to electron transport. At low electron
density ($\sim 1.4\times 10^{12}~\mathrm{cm^{-2}}$) and a perpendicular
magnetic field of 7 Tesla, the resistance exceeds by more than one order of
magnitude the zero field resistance and exponentially drops with increasing
temperature. We attribute this observation to strong electron localization.
Both temperature and magnetic field dependence can, at least qualitatively, be
described by the Efros-Shklovskii law, predicting the formation of a Coulomb
gap in the density of states due to Coulomb interactions. However, the
localization length obtained from fitting the temperature dependence exceeds by
more than one order of magnitude the one obtained from the magnetic field
dependence. We attribute this discrepancy to the presence of a nearby metallic
gate, which provides electrostatic screening and thus reduces long-range
Coulomb interactions. The result of our study suggests that the insulating
state of $\mathrm{MoS_2}$ originates from a combination of disorder-driven
electron localization and Coulomb interactions.
We study three aspects of work statistics in the context of the fluctuation
theorem for the quantum spin chains by numerical methods based on
matrix-product states. First, we elaborate that the work done on the spin-chain
by a sudden quench can be used to characterize the quantum phase transitions
(QPT). We further obtain the numerical results to demonstrate its capability of
characterizing the QPT of both Landau-Ginzbrug types, such as the Ising chain,
or topological types, such as the Haldane chain. Second, we propose to use the
fluctuation theorem, such as Jarzynski's equality, which relates the real-time
correlator to the ratio of the thermal partition functions, as a benchmark
indicator for the numerical real-time evolving methods. Third, we study the
passivity of ground and thermal states of quantum spin chains under some cyclic
impulse processes. We verify the passivity of thermal states. Furthermore, we
find that some ground states in the Ising-like chain, with less overall spin
order from spontaneous or explicit symmetry breaking, can be active so that
they can be exploited for quantum engines.
Thouless pumping represents a powerful concept to probe quantized topological
invariants in quantum systems. We explore this mechanism in a generalized
Rice-Mele Fermi-Hubbard model characterized by the presence of competing onsite
and intersite interactions. Contrary to recent experimental and theoretical
results, showing a breakdown of quantized pumping induced by the onsite
repulsion, we prove that sufficiently large intersite interactions allow for an
interaction-induced recovery of Thouless pumps. Our analysis further reveals
that the occurrence of stable topological transport at large interactions is
connected to the presence of a spontaneous bond-order-wave in the ground-state
phase diagram of the model. Finally, we discuss a concrete experimental setup
based on ultracold magnetic atoms in an optical lattice to realize the newly
introduced Thouless pump. Our results provide a new mechanism to stabilize
Thouless pumps in interacting quantum systems.
The density functional theory calculations and tight-binding models for the
copper-doped lead apatite support flat bands, which could be susceptible to the
emergence of high-temperature superconductivity. We develop theory for the
geometric contribution of the superfluid weight arising from the momentum-space
topology of the Bloch wave functions of these flat bands, and we compare our
results to the paradigmatic case of $s$-wave superconductivity on an isolated
topological flat band. We show that, in contrast to the standard paradigm of
flat-band superconductivity, there does not exist any lower bound for the
superfluid weight in these models. Moreover, although the nontrivial quantum
geometries of the normal state bands are the same when the superconductivity
appears in the ferromagnetic and paramagnetic phases, the emerging
superconducting phases have very different superfluid weights. In the case of
superconductivity appearing on the spin-polarized bands the superfluid weight
varies a lot as a function of model parameters. On the other hand, if the
superconductivity emerges in the paramagnetic phase the superfluid weight is
robustly large and it contains a significant geometric component.
Ionic liquids provide versatile pathways for controlling the structures and
properties of quantum materials. Previous studies have reported electrostatic
gating of nanometre-thick flakes leading to emergent superconductivity,
insertion or extraction of protons and oxygen ions in perovskite oxide films
enabling the control of different phases and material properties, and
intercalation of large-sized organic cations into layered crystals giving
access to tailored superconductivity. Here, we report an ionic-liquid gating
method to form three-dimensional transition metal monochalcogenides (TMMCs) by
driving the metals dissolved from layered transition metal dichalcogenides
(TMDCs) into the van der Waals gap. We demonstrate the successful
self-intercalation of PdTe$_2$ and NiTe$_2$, turning them into high-quality
PdTe and NiTe single crystals, respectively. Moreover, the monochalcogenides
exhibit distinctive properties from dichalcogenides. For instance, the
self-intercalation of PdTe$_2$ leads to the emergence of superconductivity in
PdTe. Our work provides a synthesis pathway for TMMCs by means of ionic liquid
gating driven self-intercalation.
By using the self-consistent Born approximation, we investigate disorder
effect induced by short-range impurities on the band-gap of a seminal
two-dimensional (2D) system, whose phase diagram contains trivial,
single-band-inverted and double-band-inverted states. Following the
density-of-states (DOS) evolution, we demonstrate multiple closings and
openings of the band-gap with the increase of the disorder strength.
Calculations of the spectral function describing the quasiparticles at the
$\Gamma$ point of the Brillouin zone evidence that the observed band-gap
behavior is unambiguously caused by the topological phase transitions due to
the mutual inversions between the first and second electron-like and hole-like
subbands. We also find that an increase in the disorder strength in the
double-inverted state always leads to the band-gap closing due to the overlap
of the tails of DOS from conduction and valence subbands.
Layer-by-layer assembly of van der Waals (vdW) heterostructures underpins new
discoveries in solid state physics, material science and chemistry. Despite the
successes, all current 2D material (2DM) transfer techniques rely on the use of
polymers which limit the cleanliness, ultimate electronic performance, and
potential for optoelectronic applications of the heterostructures. In this
article, we present a novel polymer-free platform for rapid and facile
heterostructure assembly which utilises re-usable flexible silicon nitride
membranes. We demonstrate that this allows fast and reproducible production of
2D heterostructures using both exfoliated and CVD-grown materials with perfect
interfaces free from interlayer contamination and correspondingly excellent
electronic behaviour, limited only by the size and intrinsic quality of the
crystals used. Furthermore, removing the need for polymeric carriers allows new
possibilities for vdW heterostructure fabrication: assembly at high
temperatures up to 600{\deg}C, and in different environments including
ultra-high vacuum (UHV) and when the materials are fully submerged in liquids.
We demonstrate UHV heterostructure assembly for the first time, and show the
reliable creation of graphene moir\'e superlattices with more than an order of
magnitude improvement in their structural homogeneity. We believe that broad
adaptation of our novel inorganic 2D materials assembly strategy will allow
realisation of the full potential of vdW heterostructures as a platform for new
physics and advanced optoelectronic technologies.
There is a growing interest in searching for topology in fractal dimensions
with the aim of finding different properties and advantages compared to the
integer dimensional case. It has previously been shown that the Laughlin state
can be adapted to fractal lattices. A key element in doing so is to replace the
uniform background charge by a background charge that resides only on the
lattice sites. This motivates the study of Hofstadter type models on fractal
lattices, in which the magnetic field is present only at the lattice sites.
Here, we study such models for hardcore bosons on finite lattices derived from
the Sierpinski carpet and on square lattices with open boundary conditions. We
find that the system sizes that we can investigate with exact diagonalization
are generally too small to judge whether these local models are topological or
not. Studying the particle densities on the lattices derived from the
Sierpinski carpet, we find that the densities tend to accumulate in the regions
that are locally similar to a square lattice. Such accumulation seems to be
incompatible with the uniform densities in fractional quantum Hall systems,
which might suggest that the models are not topological. Our computations
provide guidance for future searches for topology in finite systems. We also
propose a scheme to implement both fractal lattices and our proposed local
Hamiltonian with ultracold atoms in optical lattices, which could allow for
quantum simulators to go beyond the numerical results presented here.
Topological defects have strong impact on both elastic and inelastic
properties of materials. In this article, we investigate the possibility to
controllably inject topological defects in quantum simulators of solid state
lattice structures. We investigate the quench dynamics of a Frenkel-Kontorova
chain, which is used to model discommensurations of particles in cold atoms and
trapped ionic crystals. The interplay between an external periodic potential
and the inter-particle interaction makes lattice discommensurations, the
topological defects of the model, energetically favorable and can tune a
commensurate-incommensurate structural transition. Our key finding is that a
quench from the commensurate to incommensurate phase causes a controllable
injection of topological defects at periodic time intervals. We employ this
mechanism to generate quantum states which are a superposition of lattice
structures with and without topological defects. We conclude by presenting
concrete perspectives for the observation and control of topological defects in
trapped ion experiments.
The investigation of topological state transition in carefully designed
photonic lattices is of high interest for fundamental research, as well as for
applied studies such as manipulating light flow in on-chip photonic systems.
Here, we report on topological phase transition between symmetric topological
zero modes (TZM) and antisymmetric TZMs in Su-Schrieffer-Heeger (SSH) mirror
symmetric waveguides. The transition of TZMs is realized by adjusting the
coupling ratio between neighboring waveguide pairs, which is enabled by
selective modulation of the refractive index in the waveguide gaps.
Bi-directional topological transitions between symmetric and antisymmetric TZMs
can be achieved with our proposed switching strategy. Selective excitation of
topological edge mode is demonstrated owing to the symmetry characteristics of
the TZMs. The flexible manipulation of topological states is promising for
on-chip light flow control and may spark further investigations on
symmetric/antisymmetric TZM transitions in other photonic topological
frameworks.
Topological phases play a crucial role in the fundamental physics of
light-matter interaction and emerging applications of quantum technologies.
However, the topological band theory of waveguide QED systems is known to break
down, because the energy bands become disconnected. Here, we introduce a
concept of the inverse energy band and explore analytically topological
scattering in a waveguide with an array of quantum emitters. We uncover a rich
structure of topological phase transitions, symmetric scale-free localization,
completely flat bands, and the corresponding dark Wannier states. Although
bulk-edge correspondence is partially broken because of radiative decay, we
prove analytically that the scale-free localized states are distributed in a
single inverse energy band in the topological phase and in two inverse bands in
the trivial phase. Surprisingly, the winding number of the scattering textures
depends on both the topological phase of inverse subradiant band and the
odevity of the cell number. Our work uncovers the field of the topological
inverse bands, and it brings a novel vision to topological phases in
light-matter interactions.
In topological phase transitions involving a change in topological invariants
such as the Chern number and the $\mathbb{Z}_2$ topological invariant, the gap
closes, and the electric polarization becomes undefined at the transition. In
this paper, we show that the jump of polarization across such topological phase
transitions in two dimensions is described in terms of positions and monopole
charges of Weyl points in the intermediate Weyl semimetal phase. We find that
the jump of polarization is described by the Weyl dipole at $\mathbb{Z}_2$
topological phase transitions and at phase transitions without any change in
the value of the Chern number. Meanwhile, when the Chern number changes at the
phase transition, the jump is expressed in terms of the relative positions of
Weyl points measured from a reference point in the reciprocal space.
We report on a detailed investigation of the shell-filling sequence in
electrostatically defined elliptic bilayer graphene quantum dots (QDs) in the
regime of low charge carrier occupation, $N \leq 12$, by means of
magnetotransport spectroscopy and numerical calculations. We show the necessity
of including both short-range electron-electron interaction and
wavefunction-dependent valley g-factors for understanding the overall fourfold
shell-filling sequence. These factors lead to an additional energy splitting at
half-filling of each orbital state and different energy shifts in out-of-plane
magnetic fields. Analysis of 31 different BLG QDs reveals that both valley
g-factor and electron-electron interaction induced energy splitting increase
with decreasing QD size, validating theory. However, we find that the
electrostatic charging energy of such gate-defined QDs does not correlate
consistently with their size, indicating complex electrostatics. These findings
offer significant insights for future BLG QD devices and circuit designs.
Superconducting pairing symmetry are crucial in understanding the microscopic
superconducting mechanism of a superconductor. Here we report the observation
of a twofold superconducting gap symmetry in an interfacial superconductor
Bi$_{2}$Te$_{3}$/FeTe, by employing quasiparticle interference (QPI) technique
in scanning tunneling microscopy and macroscopic magnetoresistance
measurements. The QPI patterns corresponding to energies inside and outside the
gap reveal a clear anisotropic superconducting gap. Furthermore, both the
in-plane angle-dependent magnetoresistance and in-plane upper critical field
exhibit a clear twofold symmetry. This twofold symmetry align with the Te-Te
direction in FeTe, which weakens the possible generation by bi-collinear
antiferromagnetism order. Our finding provides key information in further
understanding of the topological properties in Bi$_{2}$Te$_{3}$/FeTe
superconducting system and propels further theoretical interests in the paring
mechanism in the system.
In this paper, the electronic structure and bond properties of MoSS$_2$,
MoSeS$_2$ and MoTeS$_2$ are studied. Density functional theory (DFT) calculates
combined with the binding energy and bond-charge (BBC) model to obtain
electronic structure, binding energy shift and bond properties. It is found
that electrostatic shielding by electron exchange is the main cause of density
fluctuation. A method for calculating the density of Green's function with
energy level shift is established. It provides new methods and ideas for the
further study of the binding energy, bond states and electronic properties of
nanomaterials.
The recent discovery of the Weyl semimetal CeAlSi with simultaneous breaking
of inversion and time-reversal symmetries has opened up new avenues for
research into the interaction between light and topologically protected bands.
In this work, we present a comprehensive examination of shift current and
injection current responsible for the circular photogalvanic effect in CeAlSi
using first-principles calculations. Our investigation identifies a significant
injection current of 1.2 mA/V$^2$ over a broad range in the near-infrared
region of the electromagnetic spectrum, exceeding previously reported findings.
In addition, we explored several externally controllable parameters to further
enhance the photocurrent. A substantial boost in the injection current is
observed when applying uniaxial strain along the $c$-axis of the crystal $-$ a
5% strain results in a remarkable 64% increment. The exceptional photocurrent
response in CeAlSi suggests that magnetic non-centrosymmetric Weyl semimetals
may provide promising opportunities for novel photogalvanic applications.
Optical spectroscopy of quantum materials at ultralow temperatures is rarely
explored, yet it may provide critical characterizations of quantum phases not
possible using other approaches. We describe the development of a novel
experimental platform that enables optical spectroscopic studies, together with
standard electronic transport, of materials at millikelvin temperatures inside
a dilution refrigerator. The instrument is capable of measuring both bulk
crystals and micron-sized two-dimensional van der Waals materials and devices.
We demonstrate the performance by implementing photocurrent-based Fourier
transform infrared spectroscopy on a monolayer WTe$_2$ device and a multilayer
1T-TaS$_2$ crystal, with a spectral range available from the near-infrared to
the terahertz regime and in magnetic fields up to 5 T. In the far-infrared
regime, we achieve spectroscopic measurements at a base temperature as low as ~
43 mK and a sample electron temperature of ~ 450 mK. Possible experiments and
potential future upgrades of this versatile instrumental platform are
envisioned.
We introduce density imbalanced electron-hole bilayers at a commensurate 2 :
1 density ratio as a platform for realizing novel phases involving electrons,
excitons and trions. Three length scales are identified which characterize the
interplay between kinetic energy, intralayer repulsion, and interlayer
attraction. By a combination of theoretical analysis and numerical calculation,
we find a variety of strong-coupling phases in different parameter regions,
including quantum crystals of electrons, excitons, and trions. We also propose
an "excitonic supersolid" phase that features electron crystallization and
exciton superfluidity simultaneously. The material realization and experimental
signature of these phases are discussed in the context of semiconductor
transition metal dichalcogenide bilayers.
III-nitride wide bandgap semiconductors exhibit large exciton binding
energies, preserving strong excitonic effects at room temperature. On the other
hand, semiconducting two-dimensional (2D) materials, including MoS$_2$, also
exhibit strong excitonic effects, attributed to enhanced Coulomb interactions.
This study investigates excitonic interactions between surface GaN quantum well
(QW) and 2D MoS$_2$ in van der Waals heterostructures by varying the spacing
between these two excitonic systems. Optical property investigation first
demonstrates the effective passivation of defect states at the GaN surface
through MoS$_2$ coating. Furthermore, a strong interplay is observed between
MoS$_2$ monolayers and GaN QW excitonic transitions. This highlights the
interest of the 2D material/III-nitride QW system to study near-field
interactions, such as F\"orster resonance energy transfer, which could open up
novel optoelectronic devices based on such hybrid excitonic structures.
Strongly enhanced electron-electron interaction in semiconducting moir\'e
superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers
has led to a plethora of intriguing fermionic correlated states. Meanwhile,
interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e
superlattice, with electrons and holes separated in different layers, inherit
this enhanced interaction and strongly interact with each other, promising for
realizing tunable correlated bosonic quasiparticles with valley degree of
freedom. We employ photoluminescence spectroscopy to investigate the strong
repulsion between interlayer excitons and correlated electrons in a WS2/WSe2
moir\'e superlattice and combine with theoretical calculations to reveal the
spatial extent of interlayer excitons and the band hierarchy of correlated
states. We further find that an excitonic Mott insulator state emerges when one
interlayer exciton occupies one moir\'e cell, evidenced by emerging
photoluminescence peaks under increased optical excitation power. Double
occupancy of excitons in one unit cell requires overcoming the energy cost of
exciton-exciton repulsion of about 30-40 meV, depending on the stacking
configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization
of the excitonic Mott insulator state is enhanced by nearly one order of
magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a
promising platform for engineering and exploring new correlated states of
fermion, bosons, and a mixture of both.

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