We have investigated the structure of macroscopic suction flows in superfluid ${}^4\mathrm{He}$. In this study, we primarily analyze the structure of the quantized vortex bundle that appears to play an important role in such systems. Our study is motivated by a series of recent experiments conducted by a research group at Osaka City University [Yano et al., J. Phys.: Conf. Ser. 969, 012002 (2018)]; they created a suction vortex using a rotor in superfluid ${}^4\mathrm{He}$. They also reported that up to ${10}^4$ quantized vortices accumulated in the central region of the rotating flow. The quantized vortices in such macroscopic flows are assumed to form a bundle structure; however, the mechanism has not yet been fully investigated. Therefore, we prescribe a macroscopic suction flow to the normal fluid and discuss the evolution of a giant vortex (i.e., one with a circulation quantum number exceeding unity) and a bundle of singly quantized vortices from a small number of seed vortices. Then, using numerical simulations, we discuss several possible characteristic structures of the bundle in such a flow, and we suggest that the actual steady-state bundle structure in the experiment can be verified by measuring the diffusion constant of the vortex bundle after the macroscopic normal flow has been switched off. By applying extensive knowledge of the superfluid ${}^4\mathrm{He}$ system, we elucidate a type of macroscopic superfluid flow and identify a structure of quantized vortices.

We consider twisted bilayer graphene on a transition metal dichalcogenide substrate, where proximity-induced spin-orbit coupling significantly alters the eight flat bands which occur near the magic angle. The resulting band structure features a pair of extremely flat bands across most of the mini-Brillouin zone. Further details depend sensitively on the symmetries of the heterostructure; we find semiconducting band structures when all twofold rotations around the in-plane axis are broken, and semimetallic band structures otherwise. We calculate the Chern numbers of the different isolated bands, and identify the parameter regimes and filling factors where valley Chern insulators and topological insulators are realized. Interestingly, we find that for realistic values of the proximity-induced terms, it is possible to realize a topological insulator protected by time-reversal symmetry by doping two holes or two electrons per superlattice unit cell into the system.

We study the current response of an ac-driven dissipative Mott-insulator system, a normal quantum dot array, using an analytical Keldysh field-theory approach. Deep in the Mott-insulator regime, the nonequilibrium steady state (NESS) response resembles a resistively shunted Josephson array with a nonequilibrium Mott-insulating to conductor transition as the drive frequency $\mathrm{\Omega }$ is increased. The diamagnetic component of the NESS in the conducting phase is anomalous, implying negative inductance, strikingly reminiscent of the $\eta$-pairing phase of a Josephson array with negative phase stiffness. However, in the presence of an additional dc field the signature of supercurrent—Shapiro steps—is completely absent. We interpret these properties as number-phase fluctuation effects shared with Josephson systems rather than superconductivity.

Recently a generic class of three-dimensional band structures was identified that host twofold line degeneracies meeting at threefold or triple point degeneracies, which resist the usual topological characterization of isolated point degeneracies as in Dirac/Weyl semimetals. For these so-called “Nexus” fermions which lie beyond Dirac/Weyl fermions, we lay out several concepts to characterize the wave function geometry and spell out its topology. Our approach is based on an understanding of the analyticity properties of Nexus wave-functions building on a two-dimensional analog studied recently by us. We use this to write down a homological characterization of various Nexus triple point degeneracies in three dimensions.

Higher-rank electric/magnetic multipole moments are attracting attention these days as candidate order parameters for exotic material phases. However, the quantum mechanical formulation of those multipole moments is still an ongoing issue. In this paper, we propose a thermodynamic definition of electric quadrupole moments as a measure of symmetry breaking, following previous studies of orbital magnetic dipole moments and magnetic quadrupole moments. The obtained formulas are illustrated with a model of orbital-ordered nematic phases of iron-based superconductors.

Materials with reduced dimensionality often exhibit exceptional properties that are different from their bulk counterparts. Here, we report the emergence of a commensurate $2\times 2$ charge density wave (CDW) in monolayer and bilayer ${\mathrm{SnSe}}_2$ films by scanning tunneling microscopy. The visualized spatial modulation of the CDW phase becomes prominent near the Fermi level, which is pinned inside the semiconductor band gap of ${\mathrm{SnSe}}_2$. We show that both CDW and Fermi level pinning are intimately correlated with band bending and virtual induced gap states at the semiconductor heterointerface. Through interface engineering, the electron-density-dependent phase diagram is established in ${\mathrm{SnSe}}_2$. Fermi surface nesting between symmetry inequivalent electron pockets is revealed to drive the CDW formation and to provide an alternative CDW mechanism that might work in other compounds.

The hydrodynamic regime of electron transport has been recently realized in conductors with ultralow densities of defects. Although relaxation processes in two-dimensional (2D) fluids have been studied in many theoretical works, the viscosity of the realistic Fermi gas of 2D electrons having a quadratic energy spectrum and interacting by Coulomb's law has not been reliably determined either in theory or in experiment. Here, we construct a theory of viscosity and thermal conductivity in such a system. We compare the calculated viscosity of the 2D electron Fermi gas and the previously known viscosity of a 2D Fermi liquid with available experimental data extracted from the hydrodynamic negative magnetoresistance of the best-quality GaAs quantum wells. Based on this comparison, we argue that measurements of the temperature dependence of the viscosity can allow us to trace the transition between an electron Fermi liquid and a Fermi gas.

The low-lying states of graphene contain exciting topological properties that depend on the interplay of different symmetry-breaking terms. The corresponding energy gaps remained unexplored until recently due to the low-energy scale of the terms involved (few tens of $\mu \mathrm{eV}$). These low-energy terms include sublattice splitting, the Rashba coupling, and the intrinsic spin-orbit coupling, whose balance determines the topological properties. In this work, we unravel the contributions arising from the sublattice and the intrinsic spin orbit splitting in graphene on hexagonal boron-nitride. Employing resistively detected electron spin resonance, we identify a sublattice splitting of the order of 20 $\mu \mathrm{eV}$, and we confirm an intrinsic spin orbit coupling of approximately 45 $\mu \mathrm{eV}$. The dominance of the latter suggests a topologically nontrivial state, involving fascinating properties. Electron spin resonance is a promising route toward unveiling the intriguing band structure at low-energy scales.

Low-dimensional charge density wave (CDW) systems, which undergo successive so-called Peierls phase transitions at characteristic temperatures associated with corresponding changes in electronic structure, are of great importance for exploring novel quantum physics. Here, we report systematic research of the magnetotransport properties for the quasi-two-dimensional CDW conductor $\eta \text{−}\mathrm{M}{\mathrm{o}}_4{\mathrm{O}}_{11}$ under high magnetic fields up to 59 T. Measurements of in-plane and out-of-plane magnetoresistance and Hall resistance have been carried out to determine the Fermi surface (FS) structure at low temperatures and high magnetic fields. Beyond the quantum limit (QL), fast quantum oscillations (QOs) with complex periods have been found in both MR and Hall resistance, representing a direct measurement of the area of the FS. In addition, we observe a conspicuous phase transition at 45 T. The QO frequency and carrier density successively increase above the QL and phase transition fields. Consequently, we propose that such remarkable feature of the magnetic field dependence of the fast QOs for $\eta \text{−}\mathrm{M}{\mathrm{o}}_4{\mathrm{O}}_{11}$ arises from the field-induced density wave transition and FS reconstruction. We have further discussed the mechanism and provided the evolution on the geometry of the FS with magnetic field in the second CDW state based on the field-dependent FS model.

We present methods for analyzing and designing cylindrical electromagnetic metasurfaces with noncircular cross sections based on conformal transformations. It can be difficult to treat surfaces with noncanonical geometries since they generally do not admit straightforward solutions to the Helmholtz wave equation subject to the appropriate boundary conditions. This leads to the reliance on full wave numerical techniques which are only suitable for the analysis, but not the synthesis, of these surfaces. We address this issue by employing conformal transformations to map the physical space into a computational space in which the surface coincides with a circular cylinder. The electromagnetic boundary conditions on the surface remain intact under the transformations due to their angle-preserving nature. However, they are much more easily enforced. As a result, analytical modal solutions for the scattered fields are readily obtainable, which facilitates closed-form analysis and synthesis equations for general noncircular cylindrical metasurfaces. One important utility enabled by the proposed framework is the efficient identification of electromagnetic field distributions that satisfy local power conservation. This leads to passive and lossless surface designs, which are highly desirable in practice as they do not require active and/or lossy components.

Metasurfaces, the two-dimensional counterparts of three-dimensional metamaterials, have recently attracted much attention due to their interesting properties, such as negative refraction, hyperbolic dispersion, and the ability to manipulate the evanescent spectrum. In this work, we propose a theoretical model for near-field radiative heat transfer between two multilayered systems consisting of anisotropic metasurfaces. The choice of metasurface is graphene, with an adjustable drift current, since this provides an ideal platform to support a high density of modes around the plasmon frequency. In this configuration, multiple nonreciprocal surface plasmon polaritons are excited, providing a strong way for near-field energy transport. The resulting heat transfer, assisted by the graphene's multilayered structure and with a high drift-current velocity, is more than 36 times stronger than that of a graphene monolayer structure without a drift current for the same vacuum gap. By adjusting the vacuum gap and the thickness of a dielectric spacer, this enhanced effect can be modulated over a large range, and can even turn into a suppression. Our findings provide a powerful way to enhance and regulate energy transport, and in turn, open up a way to enrich the moiré physics inherent to the anisotropic optical properties of a metasurface.

A nonlinear scheme of high-harmonic generation in a wide class of time-reversal invariant materials with broken spatial inversion symmetry (for which, recently, the nonlinear Hall effect was established) due to the nontrivial topology of bands is proposed. A microscopic quasiclassical theory describing the nonperturbative optical response of pseudorelativistic electrons with nonzero Berry curvature of bands to a strong laser field is developed. We analyze the harmonic content of the induced current and show that one can decouple induced laser harmonics solely by the Berry curvature of bands. We also study the dependence of the nonlinear response on the driving wave and system parameters.

The vertical electric field response of dark- and bright-excitonic fine structures in self-assembled quantum dots remains largely unexplored. Using an atomistic tight-binding model, combined with a configuration-interaction approach, we show that the fine structure of both bright and dark excitons can be effectively tuned with a vertical field. The dark-exciton splitting reveals parabolic evolution under an applied electric field, contrary to linearlike trends for the bright-exciton splitting, with a linear change rate of the latter related to the bright-dark splitting. Atomistic results are further investigated in terms of the hole-band-mixing term, which reverses its sign under the electric field leading to a vanishing bright-exciton fine structure, and a minimum of the dark-exciton splitting in a nonalloyed case. Surprisingly, we find that the dark-exciton optical activity is also highly tunable with the electric field, despite the quantum dot's cylindrical shape, with potential implications for applications involving dark excitons. Finally, we study different random realizations of the same alloyed quantum dot, showing that mere alloy randomness substantially affects the bright-exciton splitting both at zero field and at the splitting minimum.

Feynman path-integral deep potential molecular dynamics (PI-DPMD) calculations have been employed to study both light (${\mathrm{H}}_2\mathrm{O}$) and heavy water (${\mathrm{D}}_2\mathrm{O}$) within the isothermalisobaric ensemble. In particular, the deep neural network is trained based on ab initio data obtained from the strongly constrained and appropriately normed (SCAN) exchange-correlation functional. Because of the lighter mass of hydrogen than deuteron, the properties of light water are more influenced by nuclear quantum effect than those of heavy water. Clear isotope effects are observed and analyzed in terms of hydrogen-bond structure and electronic properties of water that are closely associated with experimental observables. The molecular structures of both liquid ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ agree well with the data extracted from scattering experiments. The delicate isotope effects on radial distribution functions and angular distribution functions are well reproduced as well. Our approach demonstrates that deep neural network combined with SCAN functional based ab initio molecular dynamics provides an accurate theoretical tool for modeling water and its isotope effects.

A two-dimensional (2D) spin-1/2 antiferromagnetic Heisenberg model with a specific kind of quenched disorder is investigated, using the first-principles nonperturbative quantum Monte Carlo calculations (QMCs). The employed disorder distribution has a tunable parameter $p$ which can be considered as a measure of randomness ($p=0$ correponds to the clean model). Through large-scale QMCs, the dynamic critical exponents $z$, the ground-state energy densities $E_0$, and the Wilson ratios $W$ of various $p$ are determined with high precision. Interestingly, we find that the $p$ dependencies of $z$ and $W$ are likely to be complementary to each other. For instance, while the $z$ values of $0.4\le p\le 0.9$ match well among themselves and are statistically different from that of $p=0$, the $W$ values for $p<0.7$ are in reasonably good agreement with $W\sim 0.1243$ of the clean case. Surprisingly, our study indicates that a threshold of randomness, $p_W$, associated with $W$ exists. In particular, beyond this threshold the magnitude of $W$ grows with $p$. This is somehow counterintuitive since one expects the spin correlations should diminish accordingly. Similarly, there is a threshold $p_z$ related to $z$ after which a constant value is obtained for $z$. The results presented here are not only interesting from a theoretical perspective but also can serve as benchmarks for future related studies.

We present a technique for an exact solution of the linearized Boltzmann equation for the electrical and thermal transport coefficients in metals in the low-temperature limit. This renders unnecessary an uncontrolled approximation that has been used in all previous solutions of the integral equations for the transport coefficients. Applications include electron-phonon scattering in nonmagnetic metals, as well as the magnon contribution to the electrical and thermal conductivities, and to the thermopower, in metallic ferromagnets, antiferromagnets, and helimagnets. In this paper, the first of a pair, we set up the technique and apply it to the scattering of electrons by phonons, antiferromagnons, and helimagnons. We show that the Bloch $T^5$ law for the electrical resistivity, the $T^2$ law for the thermal resistivity, and the $T$ law for the thermopower due to phonon and antiferromagnon scattering are exact and determine the prefactors exactly. The corresponding exact results for helimagnons are $T^{5/2}$, $T^{1/2}$, and $T$, respectively. In a second paper we will consider the scattering by ferromagnons.

In a previous paper (Paper I) we developed a technique for exactly solving the linearized Boltzmann equation for the electrical and thermal transport coefficients in metals in the low-temperature limit. Here we adapt this technique to determine the magnon contribution to the electrical and thermal conductivities, and to the thermopower, in metallic ferromagnets. For the electrical resistivity $\rho$ at asymptotically low temperatures we find $\rho \propto exp(-T_{\text{min}}/T)$, with $T_{\text{min}}$ an energy scale that results from the exchange gap and a temperature-independent prefactor of the exponential. The corresponding result for the heat conductivity is ${\sigma }_h\propto T^{3}exp(T_{\text{min}}/T)$, and thermopower is $S\propto T$. All of these results are exact, including the prefactors.

We present an analytical framework to investigate the acoustic performance of an array of closely spaced spherical cavities embedded in a thin soft medium submerged in water. Each layer of cavities is approximated as a homogenized layer with effective properties. Strong monopole resonance of the cavities and multiple scattering of waves between cavities in proximity are taken into account. Analytical results for the metasurface with and without a rigid backing are compared with numerical simulations as well as with experimental results from the literature.

A major impediment towards realizing technologies based on the emerging principles of antiferromagnetic spintronics is the shortage of suitable materials. In this paper, we propose a design of $\mathrm{polar}|\mathrm{nonpolar}$ heterostructures of perovskite oxides, where a single unit cell of ${\mathrm{SrIrO}}_3$ is sandwiched between a thin film of ${\mathrm{LaAlO}}_3$ and a substrate of ${\mathrm{SrTiO}}_3$. Our calculations within the framework of density-functional theory + Hubbard $U$ + spin-orbit coupling reveal a two-dimensional conducting layer with electron and hole pockets at the interface, exhibiting a strong anisotropic Rashba-Dresselhaus effect along with noncollinear antiferromagnetism, indicating the possibility of realizing a spin-orbit torque. An insightful physical model for the anisotropic Rashba-Dresselhaus effect nicely interprets our results, providing an estimate for the Rashba-Dresselhaus coefficients and illustrating pseudospin orientation. We also observe a proximity-induced prominent Rashba-like effect for Ti $3d$ empty bands. Our results suggest that the heterostructure may possess the essential ingredients for antiferromagnetic spintronics, deserving experimental verification.