Found 30 papers in cond-mat The paper discusses the chiral tunnelling of charge carriers through double
barrier structure in twisted graphene bilayer. The theoretical analysis
investigates the transmission probability for various system parameters under
both symmetric and asymmetric barrier conditions. The results reveal that the
transmission probability of quasiparticles in the $K$ cone is mirror symmetric
to that of $K_{\theta}$ cone about $\varphi =0$. Furthermore, the study shows
that the transmission changes gradually from perfect transmission to perfect
reflection in the normal direction by increasing the incident energy and the
barrier height, which is different from the case of monolayer and AB-stacked
bilayer graphene. It is also found that the double barrier structure remains,
only in certain cases, perfectly transparent for normal or near-normal
incidence. The chiral nature of the quasiparticles in graphene causes the
tunneling to be highly dependent on the direction and also on the double
barrier structure. Interestingly, this characteristic provides additional
parameter that allows us to tune the electronic properties of the twisted
graphene bilayer. Additionally, we found that the transmission exhibits some
sharp resonance peaks, the number and amplitude of which depend on the system
parameters. Our results provide a better understanding of the chiral tunnelling
in twisted graphene bilayer through double barrier structures and can help in
designing efficient electronic devices.
We study phase-controlled planar Josephson junction comprising a
two-dimensional electron gas with strong spin-orbit coupling and d-wave
superconductors, which have an advantage of high critical temperature. We show
that a region between the two superconductors can be tuned into topological
state by the in-plane Zeeman field, and can host Majorana bound states. The
phase diagram as a function of the Zeeman field, chemical potential, and the
phase difference between superconductors exhibits the appearance of robust
Majorana bound states for a wide range of parameters. We further investigate
the behavior of the topological gap and its dependence on the type of d-wave
pairing, i.e., d, d+is, or d+id', and note the difficulties that can arise due
to the presence of gapless excitations in pure d-wave superconductors. On the
other hand, the planar Josephson junctions based on superconductors with d+is
and d+id' pairings can potentially lead to realizations of Majorana bound
states. Our proposal can be realized in twisted bilayer d-wave superconductors
realizable in mechanically exfoliated van der Waals copper oxide
heterostructures.
Magnesium intercalated 'quasi-freestanding' bilayer graphene on 6H-SiC(0001)
(Mg-QFSBLG) has many favorable properties (e.g., highly n-type doped,
relatively stable in ambient conditions). However, intercalation of Mg
underneath monolayer graphene is challenging, requiring multiple intercalation
steps. Here, we overcome these challenges and subsequently increase the rate of
Mg intercalation by laser patterning (ablating) the graphene to form
micron-sized discontinuities. We then use low energy electron diffraction to
verify Mg-intercalation and conversion to Mg-QFSBLG, and X-ray photoelectron
spectroscopy to determine the Mg intercalation rate for patterned and
non-patterned samples. By modeling Mg intercalation with the Verhulst equation,
we find that the intercalation rate increase for the patterned sample is
4.5$\pm$1.7. Since the edge length of the patterned sample is $\approx$5.2
times that of the non-patterned sample, the model implies that the increased
intercalation rate is proportional to the increase in edge length. Moreover, Mg
intercalation likely begins at graphene discontinuities in pristine samples
(not step edges or flat terraces), where the 2D-like crystal growth of
Mg-silicide proceeds. Our laser patterning technique may enable the rapid
intercalation of other atomic or molecular species, thereby expanding upon the
library of intercalants used to modify the characteristics of graphene, or
other 2D materials and heterostructures.
The development of van der Waals heterostructures has introduced
unconventional phenomena that emerge at atomically precise interfaces. For
example, interlayer excitons in two-dimensional transition metal
dichalcogenides show intriguing optical properties at low temperatures. Here we
report on room-temperature observation of interface excitons in
mixed-dimensional heterostructures consisting of two-dimensional tungsten
diselenide and one-dimensional carbon nanotubes. Bright emission peaks
originating from the interface are identified, spanning a broad energy range
within the telecommunication wavelengths. The effect of band alignment is
investigated by systematically varying the nanotube bandgap, and we assign the
new peaks to interface excitons as they only appear in type-II
heterostructures. Room-temperature localization of low-energy interface
excitons is indicated by extended lifetimes as well as small excitation
saturation powers, and photon correlation measurements confirm single-photon
emission. With mixed-dimensional van der Waals heterostructures where band
alignment can be engineered, new opportunities for quantum photonics are
envisioned.
Materials with the mesoscopic scales have provided an excellent platform for
quantum-mechanical studies. Among them, the periodic oscillations of the
electrical resistivity against the direct and the inverse of the magnetic
fields, such as the Aharonov-Bohm effect and the Shubnikov-de Haas effect,
manifest the interference of the wavefunction relevant to the electron motion
perpendicular to the magnetic field. In contrast, the electron motion along the
magnetic field also leads to the magnetic-field periodicity, which is the
so-called Sondheimer effect. However, the Sondheimer effect has been understood
only in the framework of the semiclassical picture, and thereby its
interpretation at the quasiquantum limit was not clear. Here, we show that
thin-film graphite exhibits clear sinusoidal oscillations with a period of
about 1-3 T over a wide range of the magnetic fields (from around 10 T to 30
T), where conventional quantum oscillations are absent. In addition, the sample
with a designed step in the middle for eliminating the stacking disorder effect
verifies that the period of the oscillations is inversely proportional to the
thickness, which supports the emergence of the Sondheimer oscillations in the
quasiquantum limit. These findings suggest that the Sondheimer oscillations can
be reinterpreted as inter-Landau-level resonances even at the field range where
the semiclassical picture fails. Our results expand the quantum oscillation
family, and pave the way for the exploration of the out-of-plane wavefunction
motion.
We study non-Hermitian Josephson junctions formed by conventional
superconductors with a finite phase difference under non-Hermiticity naturally
appearing due to coupling to normal reservoirs. Depending on the structure of
non-Hermiticity, captured here in terms of retarded self-energies, the
low-energy spectrum hosts topologically stable exceptional points either at
zero or finite real energies as a function of the superconducting phase
difference. Interestingly, we find that the corresponding phase-biased
supercurrents acquire divergent profiles at such exceptional points, an
instance that turns out to be a natural and unique non-Hermitian effect that
signals a possible way to enhance the sensitivity of Josephson junctions. Our
work thus opens the way for realizing unique non-Hermitian phenomena due to the
interplay between non-Hermitian topology and the Josephson effect.
Acoustic metamaterials, particularly the topological insulators, exhibit
exceptional wave characteristics that have sparked considerable research
interest. The study of imperfect interfaces affect is of significant importance
for the modeling of wave propagation behavior in topological insulators. This
paper models a soft Rayleigh beam system with imperfect interfaces, and
investigates its topological phase transition process tuned by mechanical
loadings. The model reveals that the topological phase transition process can
be observed by modifying the distance between imperfect interfaces in the
system. When a uniaxial stretch is applied, the topological phase transition
points for longitudinal waves decrease within a limited frequency range, while
they increase within a larger frequency scope for transverse waves. Enhancing
the rigidity of the imperfect interfaces also enables shifting of the
topological phase transition point within a broader frequency range for
longitudinal waves and a confined range for transverse waves. The transition of
topologically protected interface modes in the transmission performance of a
twenty-cell system is verified, which include altering frequencies, switching
from interface mode to edge mode. Overall, this study provides a new approach
and guideline for controlling topological phase transition in composite and
soft phononic crystal systems.
Raman spectroscopy is widely used to assess the quality of 2D materials thin
films. This report focuses on $\rm{PtSe_2}$, a noble transition metal
dichalcogenide which has the remarkable property to transit from a
semi-conductor to a semi-metal with increasing layer number. While
polycrystalline $\rm{PtSe_2}$ can be grown with various cristalline qualities,
getting insight into the monocrystalline intrinsic properties remains
challenging. We report on the study of exfoliated 1 to 10 layers $\rm{PtSe_2}$
by Raman spectroscopy, featuring record linewidth. The clear Raman signatures
allow layer-thickness identification and provides a reference metrics to assess
crystal quality of grown films.
Ab initio calculations for nuclei and nuclear matter are limited by the
computational requirements of processing large data objects. In this work, we
develop low-rank singular value decompositions for chiral three-nucleon
interactions, which dominate these limitations. In order to handle the large
dimensions in representing three-body operators, we use randomized
decomposition techniques. We study in detail the sensitivity of different
three-nucleon topologies to low-rank matrix factorizations. The developed
low-rank three-nucleon interactions are benchmarked in Faddeev calculations of
the triton and ab initio calculations of medium-mass nuclei. Exploiting
low-rank properties of nuclear interactions will be particularly important for
the extension of ab initio studies to heavier and deformed systems, where
storage requirements will exceed the computational capacities of the most
advanced high-performance-computing facilities.
In the presented work we study, by means of numerical simulations, the
behaviour of a suspension of active ring polymers in the bulk and under lateral
confinement. When changing the separation between the confining planes and the
polymers' density, we detect the emergence of a self-organised dynamical state,
characterised by the coexistence of slowly diffusing clusters of rotating disks
and faster rings moving in between them. This system represents a peculiar case
at the crossing point between polymer, liquid crystals and active matter
physics, where the interplay between activity, topology and confinement leads
to a spontaneous segregation of a one component solution.
Weyl semimetals have emerged as a promising quantum material system to
discover novel electrical and optical phenomena, due to their combination of
nontrivial quantum geometry and strong symmetry breaking. One crucial class of
such novel transport phenomena is the diode effect, which is of great interest
for both fundamental physics and modern technologies. In the electrical regime,
giant electrical diode effect (the nonreciprocal transport) has been observed
in Weyl systems. In the optical regime, novel optical diode effects have been
theoretically considered but never probed experimentally. Here, we report the
observation of the nonlinear optical diode effect (NODE) in the magnetic Weyl
semimetal CeAlSi, where the magnetic state of CeAlSi introduces a pronounced
directionality in the nonlinear optical second-harmonic generation (SHG). By
physically reversing the beam path, we show that the measured SHG intensity can
change by at least a factor of six between forward and backward propagation
over a wide bandwidth exceeding 250 meV. Supported by density-functional theory
calculations, we establish the linearly dispersive bands emerging from Weyl
nodes as the origin of the extreme bandwidth. Intriguingly, the NODE
directionality is directly controlled by the direction of magnetization. By
utilizing the electronically conductive semimetallic nature of CeAlSi, we
demonstrate current-induced magnetization switching and thus electrical control
of the NODE in a mesoscopic spintronic device structure with current densities
as small as 5 kA/cm$^2$. Our results advance ongoing research to identify novel
nonlinear optical/transport phenomena in magnetic topological materials. The
NODE also provides a way to measure the phase of nonlinear optical
susceptibilities and further opens new pathways for the unidirectional
manipulation of light such as electrically controlled optical isolators.
Voids develop in crystalline materials under energetic particle irradiation,
as in nuclear reactors. Understanding the underlying mechanisms of void
nucleation and growth is of utmost importance as it leads to dimensional
instability of the metallic materials. In the past two decades, researchers
have adopted the phase-field approach to study the phenomena of void evolution
under irradiation. The approach involves modeling the boundary between the void
and matrix with a diffused interface. However, none of the existing models are
quantitative in nature. This work introduces a thermodynamically consistent,
quantitative diffuse interface model based on KKS formalism to describe the
void evolution under irradiation. The model concurrently considers both
vacancies and self-interstitials in the description of void evolution. Unique
to our model is the presence of two mobility parameters in the equation of
motion of the phase-field variable. The two mobility parameters relate the
driving force for vacancy and self-interstitial interaction to the interface
motion, analogous to dislocation motion through climb and glide processes. The
asymptotic matching of the phase-field model with the sharp-interface theory
fixes the two mobility parameters in terms of the material parameters in the
sharp-interface model. The Landau coefficient, which controls the height of the
double-well function in the phase field variable, and the gradient coefficient
of the phase field variable are fixed based on the interfacial energy and
interface width of the boundary. With all the parameters in the model
determined in terms of the material parameters, we thus have a new phase field
model for void evolution. Simple test cases will show the void evolution under
various defect supersaturation to validate our new phase-field model.
The Zintl compound AIn2X2 (A = Ca, Sr, and X = P, As), as a theoretically
predicted new non-magnetic topological insulator, requires experiments to
understand their electronic structure and topological characteristics. In this
paper, we systematically investigate the crystal structures and electronic
properties of the Zintl compound SrIn2As2 under both ambient and high-pressure
conditions. Based on systematic angle-resolved photoemission spectroscopy
(ARPES) measurements, we observed the topological surface states on its (001)
surface as predicted by calculations, indicating that SrIn2As2 is a strong
topological insulator. Interestingly, application of pressure effectively tuned
the crystal structure and electronic properties of SrIn2As2. Superconductivity
is observed in SrIn2As2 for pressure where the temperature dependence of the
resistivity changes from a semiconducting-like behavior to that of a metal. The
observation of nontrivial topological states and pressure-induced
superconductivity in SrIn2As2 provides crucial insights into the relationship
between topology and superconductivity, as well as stimulates further studies
of superconductivity in topological materials.
Long-range hydrophobic attractions between mesoscopic surfaces in water play
an important role in many colloid and interface phenomena. Despite being
studied by several approaches, the origin of these forces has yet to be
adequately explained. While previous research has focused on solid/water/solid
and solid/water/air scenarios, we investigated a solid/water/liquid situation
to gain additional insight. We directly measured the long-range interactions
between a solid and a hydrophobic liquid separated by water using force
spectroscopy, where colloidal probes were coated with graphene oxide (GO) to
interact with immobilized heptane droplets in water. We detected attractions
with a range of ~0.5 {\mu}m that cannot be explained by standard
Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. When the GO was reduced to rGO
to become more hydrophobic, these forces increased in strength and ranged up to
1.2 {\mu}m. This suggests that the observed attractions result from long-range
hydrophobic forces. Based on our results, we propose air bubbles attached to
the colloidal probe and molecular rearrangement at the water/oil interface as
possible origins of the observed interactions. This knowledge will be useful to
understand and motivate the formation of emulsions using 2D materials and other
amphiphilic/hydrophobic particles.
We consider a one-dimensional driven-dissipative exciton-polariton condensate
under incoherent pump, described by the stochastic generalized Gross-Pitaevskii
equation. It was shown that the condensate phase dynamics maps under some
assumptions to the Kardar-Parisi-Zhang (KPZ) equation, and the temporal
coherence of the condensate follows a stretched exponential decay characterized
by KPZ universal exponents. In this work, we determine the main mechanisms
which lead to the departure from the KPZ phase, and identify three possible
other regimes: (i) a soliton-patterned regime at large interactions and weak
noise, populated by localized structures analogue to dark solitons; (ii) a
vortex-disordered regime at high noise and weak interactions, dominated by
point-like phase defects in space-time; (iii) a defect-free reservoir-textured
regime where the adiabatic approximation breaks down. We characterize each
regime by the space-time maps, the first-order correlations, the momentum
distribution and the density of topological defects. We thus obtain the phase
diagram at varying noise, pump intensity and interaction strength. Our
predictions are amenable to observation in state-of-art experiments with
exciton-polaritons.
We investigate the classical ground state of a large number of charges
confined inside a disk and interacting via the Coulomb potential. By realizing
the important role that the peripheral charges play in determining the lowest
energy solutions, we have successfully implemented an algorithm that allows us
to work with configurations with a desired number of border charges. This
feature brings a consistent reduction in the computational complexity of the
problem, thus simplifying the search of global minima of the energy.
Additionally, we have implemented a divide and conquer approach which has
allowed us to study configurations of size never reached before (the largest
one corresponding to $N=40886$ charges). These last configurations, in
particular, are seen to display an increasingly rich structure of topological
defects as $N$ gets larger.
We introduce a general approach to realize quantum states with holographic
entanglement structure via monitored dynamics. Starting from random unitary
circuits in $1+1$ dimensions, we introduce measurements with a
spatiotemporally-modulated density. Exploiting the known critical properties of
the measurement-induced entanglement transition, this allows us to engineer
arbitrary geometries for the bulk space (with a fixed topology). These
geometries in turn control the entanglement structure of the boundary (output)
state. We demonstrate our approach by giving concrete protocols for two
geometries of interest in two dimensions: the hyperbolic half-plane and a
spatial section of the BTZ black hole. We numerically verify signatures of the
underlying entanglement geometry, including a direct imaging of entanglement
wedges by using locally-entangled reference qubits. Our results provide a
concrete platform for realizing geometric entanglement structures on near-term
quantum simulators.
The topological characteristics of photonic crystals have been the subject of
intense research in recent years. Despite this, the basic question of whether
photonic band topology is rare or abundant -- i.e., its relative prevalence --
remains unaddressed. Here, we determine the prevalence of stable, fragile, and
higher-order photonic topology in the 11 two-dimensional crystallographic
symmetry settings that admit diagnosis of one or more of these phenomena by
symmetry analysis. Our determination is performed on the basis of a data set of
550000 randomly sampled, two-tone photonic crystals, spanning 11 symmetry
settings and 5 dielectric contrasts, and examined in both transverse electric
(TE) and magnetic (TM) polarizations. We report the abundance of nontrivial
photonic topology in the presence of time-reversal symmetry and find that
stable, fragile, and higher-order topology are generally abundant. Below the
first band gap, which is of primary experimental interest, we find that stable
topology is more prevalent in the TE polarization than the TM; is only weakly,
but monotonically, dependent on dielectric contrast; and that fragile topology
is near-absent. In the absence of time-reversal symmetry, nontrivial Chern
phases are also abundant in photonic crystals with 2-, 4-, and 6-fold
rotational symmetries but comparatively rare in settings with only 3-fold
symmetry. Our results elucidate the interplay of symmetry, dielectric contrast,
electromagnetic polarization, and time-reversal breaking in engendering
topological photonic phases and may inform general design principles for their
experimental realization.
Topological defects in active polar fluids can organise spontaneous flows and
influence macroscopic density patterns. Both of them play, for example, an
important role during animal development. Yet the influence of density on
active flows is poorly understood. Motivated by experiments on cell monolayers
confined to discs, we study the coupling between density and polar order for a
compressible active polar fluid in presence of a +1 topological defect. As in
the experiments, we find a density-controlled spiral-to-aster transition. In
addition, biphasic orientational phases emerge as a generic outcome of such
coupling. Our results highlight the importance of density gradients as a
potential mechanism for controlling flow and orientational patterns in
biological systems.
Majorana bound states (MBSs) emerge as zero energy excitations in topological
superconductors. At zero temperature, their presence gives a quantized
conductance in NS junctions and a fractional Josephson effect in Josephson
junctions when the parity is conserved. However, most of current experiments
deviate from the theoretical predictions, yielding for example a non-quantized
conductance or the absence of only few odd Shapiro steps. Although these
results might be compatible with a topological ground state, it is also
possible that a trivial scenario can mimic similar results, by means of
accidental zero energy Andreev bound states (ZEABS) or simply by non-adiabatic
transitions between trivial Andreev bound states. Here, we propose a new
platform to investigate signatures of the presence of MBSs in the Fraunhofer
pattern of Josephson junctions featuring quantum spin Hall edge states on the
normal part and Majorana bound states at the NS interfaces. We use a
tight-binding model to demonstrate a change in periodicity of the Fraunhofer
pattern when comparing trivial and non-trivial regimes. We explain these
results in terms of local and crossed Andreev bound states, which due to the
spin-momentum locking, accumulate different magnetic flux and therefore become
distinguishable in the Fraunhofer periodicity. Furthermore, we introduce a
scattering model that captures the main results of the microscopic calculations
with MBSs and extend our discussion to the main differences found using
accidental ZEABS.
Layered crystalline materials that consist of transition metal atoms on a
kagome network have emerged as a versatile platform to study unusual electronic
phenomena. For example, in the vanadium-based kagome superconductors AV3Sb5
(where A can stand for K, Cs, or Rb) there is a parent charge density wave
phase that appears to simultaneously break both the translational and the
rotational symmetry of the lattice. Here, we show a contrasting situation where
electronic nematic order - the breaking of rotational symmetry without the
breaking of translational symmetry - can occur without a corresponding charge
density wave. We use spectroscopic-imaging scanning tunneling microscopy to
study the kagome metal CsTi3Bi5 that is isostructural to AV3Sb5 but with a
titanium atom kagome network. CsTi3Bi5 does not exhibit any detectable charge
density wave state, but comparison to density functional theory calculations
reveals substantial electronic correlation effects at low energies. Comparing
the amplitudes of scattering wave vectors along different directions, we
discover an electronic anisotropy that breaks the six-fold symmetry of the
lattice, arising from both in-plane and out-of-plane titanium-derived d
orbitals. Our work uncovers the role of electronic orbitals in CsTi3Bi5,
suggestive of a hexagonal analogue of the nematic bond order in Fe-based
superconductors.
Infinite-layer nickelates ($R$NiO$_2$) exhibit some distinct differences as
compared to cuprate superconductors, leading to a debate concerning the role of
rare-earth ions ($R$=La, Pr, Nd) in the low-energy many-body physics. Although
rare-earth $4f$ orbitals are typically treated as inert `core' electrons in
studies, this approximation has been questioned. An active participation of
$4f$ states is most likely for PrNiO$_2$ based on an analogy to cuprates where
Pr cuprates differ significantly from other cuprates. Here, we adopt density
functional plus dynamical mean field theory (DFT+DMFT) to investigate the role
of Pr $4f$ orbitals and more generally the correlated electronic structure of
PrNiO$_2$ and its hole-doped variant. We find that the Pr $4f$ states are
insulating and show no evidence for either a Kondo resonance or Zhang-Rice
singlet formation as they do not have any hybridization channels near the Fermi
energy. The biggest effects of hole doping are to shift the Pr $5d$ and $4f$
states further away from the Fermi energy while enhancing the Ni $3d$ - O $2p$
hybridization, thus reducing correlation effects as the O $2p$ states get
closer to the Fermi energy. We again find no evidence for either Kondo or
Zhang-Rice physics for the $4f$ states upon hole doping. We conclude by
commenting on implications for other reduced valence nickelates.
We investigate electronic states in a two-dimensional network consisting of
interacting quantum wires, a model adopted for twisted bilayer systems. We
construct general operators which describe various scattering processes in the
system. In a twisted bilayer structure, the moir\'e periodicity allows for
generalized umklapp scatterings, leading to a class of correlated states at
certain fractional fillings. We identify scattering processes which can lead to
an insulating gapped bulk with gapless chiral edge modes at fractional
fillings, resembling the quantum anomalous Hall effect recently observed in
twisted bilayer graphene. Finally, we demonstrate that the description can be
useful in predicting spectroscopic and transport features to detect and
characterize the chiral edge modes in the moir\'e-induced correlated states.
We investigate the dynamical quantum phase transition (DQPT) in the
multi-band Bloch Hamiltonian of the one-dimensional periodic Kitaev model,
focusing on quenches from a Bloch band. By analyzing the dynamical free energy
and Pancharatnam geometric phase, we show that the critical times of DQPTs
deviate from periodic spacing due to the multi-band effect, contrasting with
results from two-band models. We propose a geometric interpretation to explain
this non-uniform spacing. Additionally, we clarify the conditions needed for
DQPT occurrence in the multi-band Bloch Hamiltonian, highlighting that a DQPT
only arises when the quench from the Bloch states collapses the band gap at the
critical point. Moreover, we establish that the dynamical topological order
parameter, defined by the winding number of the Pancharatnam geometric phase,
is not quantized but still exhibits discontinuous jumps at DQPT critical times
due to periodic modulation. Additionally, we extend our analysis to mixed-state
DQPT and find its absence at non-zero temperatures.
We study low-frequency linearly-polarized laser-dressing in materials with
valley (graphene and hexagonal-Boron-Nitride), and topological (Dirac- and
Weyl-semimetals), properties. In Dirac-like linearly-dispersing bands, the
laser substantially moves the Dirac nodes away from their original position,
and the movement direction can be fully controlled by rotating the laser
polarization. We prove that this effect originates from band nonlinearities
away from the Dirac nodes. We further demonstrate that this physical mechanism
is widely applicable, and can move the positions of the valley minima in
hexagonal materials to tune valley selectivity, split and move Weyl cones in
higher-order Weyl semimetals, and merge Dirac nodes in three-dimensional Dirac
semimetals. The model results are validated with ab-initio calculations. Our
results directly affect efforts for exploring light-dressed
electronic-structure, suggesting that one can benefit from band nonlinearity
for tailoring material properties, and highlight the importance of the full
band structure in nonlinear optical phenomena in solids.
Kagome metals with magnetic order offer the possibility of tuning topological
electronic states via external control parameters such as temperature or
magnetic field. ErMn$_6$Sn$_6$ (Er$166$) is a member of a group of $R166$,
$R=$~rare earth, compounds hosting ferromagnetic Mn kagome nets whose magnetic
moment direction and layer-to-layer magnetic correlations are strongly
influenced by coupling to $R$ magnetic moments in neighboring triangular
layers. Here, we use neutron diffraction and magnetization data to examine the
temperature-driven transition in Er$166$ from a planar-ferrimagnetic to
distorted-triple-spiral magnetic order. These data inform mean-field
calculations which highlight the fragile, tunable nature of the magnetism
caused by competing Mn-Mn and Mn-Er interlayer magnetic exchange couplings and
Mn and Er magnetic anisotropies. This competition results in the near
degeneracy of a variety of collinear, non-collinear, and non-coplanar magnetic
phases which we show are readily selected and adjusted via changing temperature
or magnetic field. Thermal fluctuations of the Er moment direction provide the
key to this tunability.
We study conservation laws of a general class of quantum many-body systems
subjected to an external time dependent quasi-periodic driving. We show that,
when the frequency of the driving is large enough or the strength of the
driving is small enough, the system exhibits a prethermal state for stretched
exponentially long times in the perturbative parameter. Moreover, we prove the
quasi-conservation of the constants of motion of the unperturbed Hamiltonian
and we analyze their physical meaning in examples of relevance to condensed
matter and statistical physics.
The scope of quantum field theory is extended by introducing a broader class
of discrete gauge theories with fracton behavior in 2+1D. We consider
translation invariant systems that carry special charge conservation laws,
which we refer to as exponential polynomial symmetries. Upon gauging these
symmetries, the resulting $\mathbb{Z}_N$ gauge theories exhibit fractonic
physics, including constrained mobility of quasiparticles and UV dependence of
the ground state degeneracy. For appropriate values of theory parameters, we
find a family of models whose excitations, albeit being deconfined, can only
move in the form of bound states rather than isolated monopoles. For
concreteness, we study in detail the low-energy physics and topological sectors
of a particular model through a universal protocol, developed for determining
the holonomies of a given theory. We find that a single excitation, isolated in
a region of characteristic size $R$, can only move from its original position
through the action of operators with support on $\mathcal{O}(R)$ sites.
Furthermore, we propose a Chern-Simons variant of these gauge theories,
yielding non-CSS type stabilizer codes, and propose the exploration of
exponentially symmetric subsystem SPTs and fracton codes in 3+1D.
The anomalous Hall effect has considerable impact on the progress of
condensed matter physics and occurs in systems with time-reversal symmetry
breaking. Here we theoretically investigate the anomalous Hall effect in
nonmagnetic transition-metal pentatelluride $\mathrm{ZrTe_{5}}$ and
$\mathrm{HfTe}_{5}$. In the presence of Zeeman splitting and Dirac mass, there
is an intrinsic anomalous Hall conductivity induced by the Berry curvature in
the semiclassical treatment. In a finite magnetic field, the anomalous Hall
conductivity rapidly decays to zero for constant spin-splitting and vanishes
for the magnetic-field-dependent Zeeman energy. A semiclassical formula is
derived to depict the magnetic field dependence of the Hall conductivity, which
is beneficial for experimental data analysis. Lastly, when the chemical
potential is fixed in the magnetic field, a Hall conductivity plateau arises,
which may account for the observed anomalous Hall effect in experiments.
The discovery of the Hat, an aperiodic monotile, has revealed novel
mathematical aspects of aperiodic tilings. However, the physics of particles
propagating in such a setting remains unexplored. In this work we study
spectral and transport properties of a tight-binding model defined on the Hat.
We find that (i) the spectral function displays striking similarities to that
of graphene, including six-fold symmetry and Dirac-like features; (ii) unlike
graphene, the monotile spectral function is chiral, differing for its two
enantiomers; (iii) the spectrum has a macroscopic number of degenerate states
at zero energy; (iv) when the magnetic flux per plaquette ($\phi$) is half of
the flux quantum, zero-modes are found localized around the reflected
`anti-hats'; and (v) its Hofstadter spectrum is periodic in $\phi$, unlike for
other quasicrystals. Our work serves as a basis to study wave and electron
propagation in possible experimental realizations of the Hat, which we suggest.

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