We consider the role of coordinate-dependent Fermi velocities, equivalent to effective tetrad (or vierbein) frame fields characterizing momentum space geometry, and torsional Landau levels (LLs) in condensed matter systems with low-energy Weyl quasiparticles. In contrast to their relativistic counterparts, they arise at finite momenta and with an explicit cutoff to the linear spectrum. Via the universal coupling of tetrads to momentum, they experience geometric chiral and axial anomalies with gravitational character. More precisely, at low energy, the fermions experience background fields corresponding to emergent anisotropic Riemann-Cartan and Newton-Cartan space-times, depending on the form of the low-energy dispersion. On these backgrounds, we show how torsion and the Nieh-Yan (NY) anomaly appear in condensed matter Weyl systems with an ultraviolet (UV) parameter with dimensions of momentum. The torsional NY anomaly arises from the spectral flow of torsional LLs and the linear Weyl description with a cutoff. We carefully review the torsional anomaly and spectral flow for relativistic fermions at zero momentum and contrast this with the spectral flow, nonzero torsional anomaly, and the appearance of the dimensionful UV-cutoff parameter in condensed matter systems at finite momentum. We apply this to chiral transport anomalies sensitive to the emergent tetrads in nonhomogeneous chiral superconductors, superfluids, and Weyl semimetals under elastic strain. This leads to previously overlooked suppression of anomalous density at nodes from momentum space geometry, as compared to (pseudo)gauge fields. We also briefly discuss torsion in anomalous thermal transport for nonrelativistic Weyl fermions, which arises via Luttinger's fictitious gravitational field corresponding to thermal gradients.

We present a detailed study of the magnetotransport properties of the charge density wave state in quasi-two-dimensional purple bronze ${\mathrm{Na}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ under pulsed fields up to 62 T. Pronounced Shubnikov–de Haas (SdH) oscillations of three frequencies have been observed in the field induced charge density/spin density wave state of the present compound. The two slow oscillations are attributed to the reconstructed electron and hole pockets near the $M^{\prime }$ point of the Brillouin zone (BZ). The highest-frequency oscillations correspond to an orbit occupying 18.1% of the BZ, which is consistent with the hexagonal electron Fermi surface in the normal state. The appearance of the highest-frequency oscillations can be explained by the magnetic breakdown between the electron pockets at the $M^{\prime }$ point. This magnetic breakdown behavior has also been observed in ${\mathrm{K}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ under very high field range up to 80 T. Based on analyses of the high field SdH oscillations, we propose a possible Fermi surface structure of ${\mathrm{Na}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$.

The stability of the Dirac spin liquid on two-dimensional lattices has long been debated. It was recently demonstrated [Nat. Commun. 10, 4254 (2019) and Phys. Rev. B 93, 144411 (2016)] that the staggered $\pi$-flux Dirac spin-liquid phase on the nonbipartite triangular lattice may be stable in the clean limit. However, quenched disorder plays a crucial role in determining whether such a phase is experimentally viable. For SU(2) spin systems, the effective zero-temperature low-energy description of Dirac spin liquids in $(2+1)$ dimensions is given by the compact quantum electrodynamics $\left({\mathrm{cQED}}_{2+1}\right)$ which admits monopoles. It is already known that generic quenched random perturbations to the noncompact version of ${\mathrm{QED}}_{2+1}$ (where monopoles are absent) lead to strong-coupling instabilities. In this paper we study ${\mathrm{cQED}}_{2+1}$ in the presence of a class of time-reversal invariant quenched disorder perturbations. We show that in this model, random non-Abelian vector potentials make the symmetry-allowed monopole operators more relevant. The disorder-induced underscreening of monopoles, thus, generically makes the gapless spin-liquid phase fragile.

We use the self-consistent Hartree-Fock approximation for numerically addressing the integer quantum Hall effect (IQHE) regime in terms of many-body physics at higher Landau levels (LLs). We investigate the dependence of many-particle interactions on the lateral size of the electron system. We use the exchange enhancement of the $g$-factor for spin-polarized Landau levels as an indicator for the strength of the exchange interaction. The driving force for the $g$-factor enhancement is a Hund's rule behavior for the occupation of spin-split Landau levels that lowers the many-particle ground state energy by arranging as many spins in parallel as possible. By increasing the total number of electrons and total number of available states per LL, it can therefore be expected that the exchange-enhanced spin gap should increase as well. In contrast to the dependence on the magnetic field, an increase of the total number of states by simply increasing the system size at constant magnetic field shows a clear saturation behavior above a lateral system size of 1000 nm. The importance of this result is underlined by an extended introduction, which demonstrates the permanent dominance of many-body interactions in all transport regimes of the IQHE. A modeling of IQHE systems therefore has to include many-body interactions, and our results open a pathway towards many-body modeling of quantum Hall systems of macroscopic size.

Tunable cavities have proven to be highly attractive systems in cavity quantum electrodynamics thanks to their performance and flexibility. The possibility to form a cavity around any emitter while simultaneously spectrally matching the chosen transitions makes these cavities an important tool in photonic quantum technology. In this paper, we report on the experimental realization and theoretical description of a fiber-based resonator with two spatially and spectrally distinct cavity modes. The careful design of the structures was performed via finite-element simulations. Thanks to the intrinsic tunability of the system, one mode can be brought in resonance with the second one, forming a supermode resulting in a hybridized two-mode pattern in the emission spectrum. For its realization, we combine a monolithic bottom cavity, formed by two distributed Bragg reflectors, with a top movable fiber mirror forming an externally tunable cavity mode. When tuning the top cavity in resonance with the monolithic bottom one, the cavity modes exhibit a pronounced anticrossing behavior typical for mode hybridization. Differently from a standard open cavity, when embedding single emitters—in this case, InGaAs quantum dots—in the structure, the Purcell factor is not uniform for each wavelength. Furthermore, we find a strong influence of the simulated Purcell factor, as well as of the measured light extraction, depending on the placement of the emitter in the top or in the bottom cavity. The discussed two-mode cavity could be employed for simultaneously Purcell-enhancing multiple transitions of an emitter while preserving the single-photon nature of the emitters inside the device.

The specific heat of a two-layer ${}^4\mathrm{He}$ film adsorbed on a graphite substrate is estimated as a function of temperature by quantum Monte Carlo simulations. The results are consistent with recent experimental observations [S. Nakamura, K. Matsui, T. Matsui, and H. Fukuyama, Phys. Rev. B 94, 180501(R) (2016)] in that they broadly reproduce their most important features. However, neither the “supersolid” nor the “superfluid hexatic” phases, of which experimental data are claimed to be evidence, are observed. It is contended that heat capacity measurements alone may not be a good predictor of structural and superfluid transitions in this system, as their interpretation is often ambiguous.

We study topological phases in a two-dimensional photonic crystal in a hexagonal lattice composed of an all-dielectric moon-shaped column (MSC) array, which preserves time-reversal symmetry, by calculating band structures, the Wilson loop, edge states, and topological-protected helical surface waves. Breaking the traditional way of changing the position or size of the cylinder, instead we simply rotate the MSCs to achieve topological phase transitions. Furthermore, the Wilson-loop method is used to directly calculate the topological properties of the bulk bands. By observing the eigenvalues of the Wilson loop, we demonstrate that bands in our system display fragile topology behavior. We also simulate the electromagnetic (EM) wave propagation in the designed photonic crystals with a point source at different operating frequencies, showing that the EM waves can propagate unidirectionally along the interface between topological trivial and nontrivial MSCs. Finally, the edge states at different frequencies can be easily tuned by adjusting the size of the MSCs. Our work provides a platform to study fragile topology and achieve frequency-tunable edge states, which may have potential applications for optical devices.

Inelastic x-ray scattering (IXS) was used to examine the valence fluctuation compound YbPd at 300 K in the $\Gamma -X$, $\Gamma -M, X-M$, and X-R directions. The obtained spectra and determined phonon dispersion relations demonstrate lattice instabilities related to the successive transitions and correlations between phonon spectra and valence degrees of freedom in YbPd. The low-lying branches in the phonon dispersion relations as well as the broadened linewidths of the IXS spectra observed around the longitudinal $X$ point provide supporting evidence of correlation between a valence-lattice interaction and the successive transitions in YbPd. X-ray absorption spectroscopy was also performed in order to discuss correlations between lattice instabilities and Yb valence degrees of freedom in YbPd.

The electronic structure of a possible topological Kondo insulator of CeRhAs has been investigated by employing temperature $\left(T\right)$-dependent angle-resolved photoemission spectroscopy (ARPES). Fermi surfaces (FSs) and band structures are successfully measured for three orthogonal crystallographic planes. The measured Fermi-edge states are found to have a three-dimensional (3D) character, contradictory to the proposed topological surface states of the $2\text{D}$ character. The measured FSs due to the Ce $4f$ electrons agree well with those from the density functional theory band structures unfolded into the reduced Ce-only unit cell. The theoretically predicted hourglass-type bulk bands along $\overline{X}\overline{S}\overline{X}$ are not clearly resolved, in spite of the evident existence of those band features. $T$-dependent ARPES measurements reveal that the coherent Ce $4f$ states, having two pseudogap structures of ${\mathrm{\Delta }}_1\sim 80$ meV and ${\mathrm{\Delta }}_2\sim 30$ meV, persist to remain above 200 K, in agreement with the high Kondo temperature of CeRhAs.

Twisted nanoribbons from two-dimensional materials possess many unusual properties tunable by changing the twist angle, for example, electro- and thermal conductivity, chemical properties, and stability against buckling. New interesting phenomena can be observed when a twisted nanoribbon interacts with a substrate. Here we report the results of a classical molecular dynamics study of a graphene nanoribbon twisted about its long axis and interacting with a graphite substrate. Van der Waals interactions with the substrate lead to localization of twisting and the formation of topological solitons called twistons. The topological charge of a twiston obtained with the twist angle $\beta$ multiple of $\pi$ can be defined as $q=\beta /\pi$; it can be positive or negative. Twistons can move along the nanoribbon with very little radiation in the form of small-amplitude phonons. Scenarios of twiston collisions are described depending on their topological charge and on the nanoribbon width. In narrow nanoribbons twistons collide practically elastically, preserving their profiles and speeds after the collision, and inelasticity of collisions increases with increasing nanoribbon width. Our results contribute to an understanding of the behavior of two-dimensional material nanoribbons on a substrate.

Quantum systems can undergo phase transitions and show distinct features in different phases. The corresponding transport properties can also vary significantly due to the underlying quantum phase. We investigate the transport behavior of a two-legged bosonic ladder in a uniform gauge field, which is known to have a Meissner-like phase and a vortex phase in the absence of dissipation. The ladder is coupled to bosonic baths at different temperatures, and we study it using the nonequilibrium Green's function method. In particular, we show the presence of a chiral current and how it is affected by the temperature bias and the dissipation strength. We also demonstrate that the opening of a gap between the lower and upper energy band results in the possibility of tuning heat and particle transport through the ladder. We show that for system parameters for which the ground state is in a vortex phase, the system is more sensitive to external perturbations.

We study the response of a Dirac fluid to electric fields and thermal gradients at finite wave numbers and frequencies in the hydrodynamic regime. We find that nonlocal transport in the hydrodynamic regime is governed by an infinite set of kinetic modes that describe noncollinear scattering events in different angular harmonic channels. The scattering rates of these modes ${\tau }_m^{-1}$ increase as $\left|m\right|$, where $m$ labels the angular harmonics. In an earlier publication, we pointed out that this dependence leads to anomalous, Lévy-flight-like phase space diffusion [Kiselev and Schmalian, Phys. Rev. Lett. 123, 195302 (2019)]. Here, we show how this surprisingly simple, nonanalytic dependence allows us to obtain exact expressions for the nonlocal charge and electronic thermal conductivities. The peculiar dependence of the scattering rates on $m$ also leads to a nontrivial structure of collective excitations: Besides the well-known plasmon, second-sound, and diffusive modes, we find nondegenerate damped modes corresponding to excitations of higher angular harmonics. We use these results to investigate the transport of a Dirac fluid through Poiseuille-type geometries of different widths and to study the response to surface acoustic waves in graphene-piezoelectric devices.

A cornerstone assumption that most literature on discrete time crystals has relied on is that homogeneous Floquet systems generally heat to a featureless infinite temperature state, an expectation that motivated researchers in the field to mostly focus on many-body localized systems. Some works have, however, shown that the standard diagnostics for time crystallinity apply equally well to clean settings without disorder. This fact raises the question whether a homogeneous discrete time crystal is possible in which the originally expected heating is evaded. Studying both a localized and an homogeneous model with short-range interactions, we clarify this issue showing explicitly the key differences between the two cases. On the one hand, our careful scaling analysis confirms that, in the thermodynamic limit and in contrast to localized discrete time crystals, homogeneous systems indeed heat. On the other hand, we show that, thanks to a mechanism reminiscent of quantum scars, finite-size homogeneous systems can still exhibit very crisp signatures of time crystallinity. A subharmonic response can in fact persist over timescales that are much larger than those set by the integrability-breaking terms, with thermalization possibly occurring only at very large system sizes (e.g., of hundreds of spins). Beyond clarifying the emergence of heating in disorder-free systems, our work casts a spotlight on finite-size homogeneous systems as prime candidates for the experimental implementation of nontrivial out-of-equilibrium physics.

The propagation of coherent spin waves in permalloy microstrips was investigated using a heterodyne-magneto-optical Kerr effect technique. The two-dimensional imaging reveals the mode interference with a high-order spin-wave mode along the microstrip width. The spin-wave dispersion relations across the microstrip width were deduced from fast Fourier transformations on the complex spin-wave amplitude of the one-dimensional scan, and multiple spin-wave modes were observed at the edges of the microstrips. These experimental results provide direct evidence for the quantization of spin-wave dispersion due to the finite width of the microstrips and enables us to evaluate the attenuation length for each quantized spin-wave mode.

Work of numerous research groups has shown different outcomes of studies of the transition from the ferroelectric $\alpha$-phase to the high temperature $\beta$-phase of the multiferroic, magnetoelectric perovskite Bismuth Ferrite (${\mathrm{BiFeO}}_3$ or BFO). Using the perturbed angular correlation (PAC) method with ${}^{111m}\mathrm{Cd}$ as the probe nucleus, the $\alpha$ to $\beta$ phase transition was characterized. The phase transition temperature, the change of the crystal structure, and its parameters were supervised with measurements at different temperatures using a six detector PAC setup to observe the $\gamma -\gamma$ decay of the ${}^{111m}\mathrm{Cd}$ probe nucleus. The temperature dependence of the hyperfine parameters shows a change in coordination of the probe ion, which substitutes for the bismuth site, forecasting the phase transition to $\beta$-BFO by either increasing disorder or formation of a polytype transition structure. A visible drop of the quadrupole frequency ${\omega }_0$ at a temperature of about $T_c\approx {820}^{\circ }\mathrm{C}$ indicates the $\alpha -\beta$ phase transition. For a given crystal symmetry, the DFT-calculations yield a specific local symmetry and electric field gradient value of the probe ion. The $Pbnm$ ($\beta$-BFO) crystal symmetry yields calculated local electric field gradients, which very well match our experimental results. The assumption of other crystal symmetries results in significantly different computed local environments not corresponding to the experiment.

We present results of ferromagnetic resonance (FMR) experiments and micromagnetic simulations for a distorted, two-dimensional (2D) kagome artificial spin ice. The distorted structure is created by continuously modulating the 2D primitive lattice translation vectors of a periodic honeycomb lattice, according to an aperiodic Fibonacci sequence used to generate 1D quasicrystals. Experimental data and micromagnetic simulations show that the Fibonacci distortion causes broadening and splitting of FMR modes into multiple branches, which accompany the increasing number of segment lengths and orientations that develop with increasing distortion. When the applied field is increased in the opposite direction to the net magnetization of a segment, spin wave modes appear, disappear, or suddenly shift, to signal segment magnetization reversal events. These results show that the complex behavior of reversal events, as well as well-defined frequencies and frequency-field slopes of FMR modes, can be precisely tuned by varying the severity of the aperiodic lattice distortion. This type of distorted structure could therefore provide a tool for the design of complicated magnonic systems.

We present a consistent fusion of functional renormalization group and mean-field theory which explicitly introduces a bosonic field via a Hubbard-Stratonovich transformation at the critical scale, at which the order sets in. We show that a minimal truncation of the flow equations, that neglects order-parameter fluctuations, is integrable and fulfills fundamental constraints as the Goldstone theorem and the Ward identity connected with the broken global symmetry. To introduce the bosonic field, we present a technique to factorize the most singular part of the vertex, even when the full dependence on all its arguments is retained. We test our method on the two-dimensional attractive Hubbard model at half-filling and calculate the superfluid gap as well as the Yukawa couplings and residual two-fermion interactions in the ordered phase as functions of fermionic Matsubara frequencies. Furthermore, we analyze the gap and the condensate fraction for weak and moderate couplings and compare our results with previous functional renormalization group studies, and with quantum Monte Carlo data. Our formalism constitutes a convenient starting point for the inclusion of order-parameter fluctuations by keeping a full, nonsimplified, dependence on fermionic momenta and/or frequencies.

We construct the thermodynamics of energy magnetization in the presence of gravitomagnetic field. We show that the free energy must be modified to account for the modification of the energy current operator in the presence of a confining potential. The explicit expression of the energy magnetization is derived for a periodic system, and the Streda formula for the thermal Hall conductivity is rigorously established. We demonstrate our theory of the energy magnetization and the Streda formula in a Chern insulator.

Quantum Boltzmann formalism is employed to study the transport properties of strongly-coupled double layer systems that enable the formation of interlayer excitons and exciton condensation. The importance of exciton formation, dissociation, and condensation is highlighted in the context of thermoelectric power generation, and this mathematical inquiry provides an alternative methodology to calculate the thermoelectric efficiency given the conditions of exciton formation. The Onsager relation for the Coulomb drag resistivity is shown to be valid even when exciton condensation is present. In addition, it is found that the traditional thermoelectric figure of merit is no longer sufficient to predict the efficiency of thermoelectric power generation in the presented situations. This inquiry offers insights for designing double layer systems, including their interlayer interactions, with enhanced thermoelectric energy conversion efficiency.

The electrostatic properties of defects of the Si(111)-$(\sqrt{3}\times \sqrt{3})R^{\circ }30$ surface are studied using noncontact atomic force microscopy and Kelvin probe force microscopy with subnanometer resolution and subelementary charge sensitivity. We identify nonparabolicities in the frequency-voltage spectroscopy of single dangling bonds (DBs), which reveal the transition from empty to single electronically occupied DBs. Kelvin probe imaging reveals that the DB charging, however, occurs with a ring shape with radius ∼500 pm located along the circumference of the DB wave function. The ring charging is explained by a tip-induced modulation of the hole recombination rate at the DB-substrate interface.