Metasurfaces based on resonant subwavelength photonic structures enable novel methods of wavefront control and light focusing, underpinning a new generation of flat-optics devices. Recently emerged all-dielectric metasurfaces exhibit high-quality resonances underpinned by the physics of bound states in the continuum that drives many interesting concepts in photonics. Here we suggest an approach to explain the physics of bound photonic states embedded in the radiation continuum. We study dielectric metasurfaces composed of planar periodic arrays of Mie-resonant nanoparticles (“meta-atoms”) which support both symmetry protected and accidental bound states in the continuum, and employ the multipole decomposition approach to reveal the physical mechanism of the formation of such nonradiating states in terms of multipolar modes generated by isolated meta-atoms. Based on the symmetry of the vector spherical harmonics, we identify the conditions for the existence of bound states in the continuum originating from the symmetries of both the lattice and the unit cell. Using this formalism we predict that metasurfaces with strongly suppressed spatial dispersion can support the bound states in the continuum with the wave vectors forming a line in the reciprocal space. Our results provide a method for designing high-quality resonant photonic systems based on the physics of bound states in the continuum.

The present work addresses the problem of thermal conductivity simulation in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and SnSe thermoelectric nanostructures. It first details phonon lifetime calculation in both thermoelectric compounds assuming polynomial dispersion properties and the Debye-Callaway model for the relaxation-time approximation. For both materials, distinct crystallographic directions are considered, i.e., $\mathrm{\Gamma }-Z$ (trigonal axis) and $\mathrm{\Gamma }-X$ (basal plane) for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and $\mathrm{\Gamma }-X$ ($a$ axis), $\mathrm{\Gamma }-Y$ ($b$ axis), and $\mathrm{\Gamma }-Z$ ($c$ axis) for SnSe. On this basis, the lifetime model is parametrized and bulk thermal conductivity is computed through the resolution of the phonon Boltzmann transport equation with a Monte Carlo method. Grüneisen parameter and mass-disorder lifetime are adjusted to fit experimental temperature dependence. The second part of the study addresses the calculation of thermal conductivity for these two thermoelectric materials in the case of thin films (cross-plane case) and nanowires. The main goals of this work are to provide a fully parametric description of heat transport in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and SnSe nanofilms and nanowires showing reliable behavior on an extended size range, from 20 nm to $2\mu \mathrm{m}$, and large temperature range (100–500 K for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$; 200–800 K for SnSe). Comparisons to bulk thermal conductivity calculations and measurements as well as to recent investigations on nanowires, demonstrate the effectiveness of the proposed methodology to deal with nanostructured ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and SnSe.

In this work we study magnetotransport properties in electronic double layers of strongly correlated electron liquids. For sufficiently clean high-mobility samples, the high-temperature regime of transport in these systems can be described in pure hydrodynamic terms. We concentrate on the magnetic field dependence of longitudinal drag effect mediated by the interlayer Coulomb scattering and identify several mechanisms of transresistance which is caused by viscous flows, magnetoplasmon resonance, and dissipative thermal fluxes. In particular, we elucidate how Hall viscosity enters magnetodrag and modifies its temperature dependence in the magnetic field.

Despite the ubiquity of applications of heat transport across nanoscale interfaces, including integrated circuits, thermoelectrics, and nanotheranostics, an accurate description of phonon transport in these systems remains elusive. Here we present a theoretical and computational framework to describe phonon transport with position, momentum, and scattering event resolution. We apply this framework to a single-material nanoparticle for which this multidimensional resolution offers insight into the physical origin of phonon thermalization and the length-scale dependent anisotropy of driven phonon distributions. We extend the formalism to handle interfaces and investigate the specific case of semicoherent materials interfaces by computing the coupling between phonons and interfacial strain resulting from a periodic array of misfit dislocations. We calculate the thermal interface conductance within the technologically relevant Si-Ge heterostructures and obtain $G=173.2\text{MW}{\text{m}}^{-2}{\text{K}}^{-1}$, in good agreement with previous experimental and theoretical work. Finally we comment on future applications of our framework including coherent and driven phonon effects in nanoscale materials, which are increasingly accessible via ultrafast, terahertz, and near-field spectroscopies.

We study the effects of periodic driving on a variant of the Bernevig-Hughes-Zhang (BHZ) model defined on a square lattice. In the absence of driving, the model has both topological and nontopological phases depending on the different parameter values. We also study the anisotropic BHZ model and show that, unlike the isotropic model, it has a nontopological phase which has states localized on only two of the four edges of a finite-sized square. When an appropriate term is added, the edge states get gapped and gapless states appear at the four corners of a square; we have shown that these corner states can be labeled by the eigenvalues of a certain operator. When the system is driven periodically by a sequence of two pulses, multiple corner states may appear depending on the driving frequency and other parameters. We discuss to what extent the system can be characterized by topological invariants such as the Chern number and a diagonal winding number. We have shown that the locations of the jumps in these invariants can be understood in terms of the Floquet operator at both the time-reversal invariant momenta and other momenta which have no special symmetries.

We compute the two-fluid resistivity of a compressed FeSe superconductor, treating electronic correlations within density functional dynamical mean-field theory. Results for the emergent strange and bad metal behavior shows good theory-experiment agreement within the mixed tetragonal and pseudohexagonal structural phases at pressures around 8.1 GPa. Our findings call for more studies on unconventional high-temperature (high-$T_c$) superconductors to unearth the consequences of selective Mott localization as the prominent candidate in governing their strange, $T$-linear resistivity.

Topological orders and associated topological protected excitations with non-Abelian statistics have been widely explored in various platforms. The ${\mathbb{Z}}_3$ parafermions are regarded as the most natural generation of the Majorana fermions to realize these topological orders. Here we investigate the topological phase and emergent ${\mathbb{Z}}_2$ spin phases in an extended parafermion chain. This model exhibits a rich variety of phases, including not only topological ferromagnetic phase, which supports non-Abelian anyon excitation, but also spin-fluid, dimer, and chiral phases from the emergent ${\mathbb{Z}}_2$ spin model. These phases correspond to various metallic and insulator phases with different orders realized by parafermions. We generalize the measurement tools in ${\mathbb{Z}}_2$ spin models to fully characterize these phases and map out the corresponding phase diagram. Surprisingly, all the phase boundaries finally merge to a single supercritical (SC) point with infinite degeneracy. The phases surrounding this SC point can be regarded as phases under different types of symmetry breakings. This criteria may suggest a general principle to search these SC points in the other models. Our phase diagram also reveals the asymptotic behavior of the parafermion towards canonical fermions. Regarding the rather generality of emergent phenomena in parafermion models, this approach opens a wide range of intriguing applications in investigating the various exotic phases using parafermion models.

We investigate the dissipative dynamics yielded by the Lindblad equation within the coexistence region around a first-order phase transition. In particular, we consider an exactly solvable, fully connected quantum Ising model with $n$-spin exchange ($n>2$)—the prototype of quantum first-order phase transitions—and several variants of the Lindblad equations. We show that physically sound results, including exotic nonequilibrium phenomena such as the Mpemba effect, can be obtained only when the Lindblad equation involves jump operators defined for each of the coexisting phases, whether stable or metastable.

Although a rigorous theoretical ground on metasurfaces has been established in recent years on the basis of the equivalence principle, the majority of metasurfaces for converting a propagating wave into a surface wave are developed in accordance with the so-called generalized Snell's law being a simple heuristic rule for performing wave transformations. Recently Tcvetkova et al. [Phys. Rev. B 97, 115447 (2018)] have rigorously studied this problem by means of a reflecting anisotropic metasurface, which is unfortunately difficult to realize, and no experimental results are available. In this paper, we propose an alternative practical design of a metasurface-based converter by separating the incident plane wave and the surface wave in different half-spaces. It allows one to preserve the polarization of the incident wave and substitute the anisotropic metasurface by an omega-bianisotropic one. The problem is approached from two sides: By directly solving the corresponding boundary problem and by considering the “time-reversed” scenario when a surface wave is converted into a nonuniform plane wave. In particular, we reveal that an input surface wave plays an important role in the conversion process, influencing the conversion efficiency. To validate the theory, we develop a practical three-layer metasurface based on a conventional printed-circuit-board technology to mimic the omega-bianisotropic response at the microwave frequency range. The design is verified by full-wave three-dimensional numerical simulations and demonstrates high conversion efficiency. Obtained results are relevant independently of the frequency range and can be generalized to acoustics domain. It enables novel applications, from efficient excitation of waveguide modes in integrated photonic circuits to cloaking of large objects.

The degeneracy of Landau levels flanking charge neutrality in twisted bilayer graphene is known to change from eightfold to fourfold when the twist angle is reduced to values near the magic angle of $\approx 1.{05}^{\circ }$. This degeneracy lifting has been reproduced in experiments by multiple groups and is known to occur even in devices which do not harbor the correlated insulators and superconductors. We propose $C_3$-symmetry breaking as an explanation of such robust degeneracy lifting and support our proposal by numerical results on the Landau-level spectrum in near-magic-angle twisted bilayer graphene. Motivated by recent experiments, we further consider the effect of $C_2$-symmetry breaking on the Landau levels.

The metal-insulator transition (MIT) of bulk rare-earth nickelates is accompanied by a simultaneous charge ordering (CO) transition. We have investigated low-frequency resistance fluctuations (noise) across the MIT and magnetic transition of [${\mathrm{EuNiO}}_3/{\mathrm{LaNiO}}_3$] superlattices, where selective suppression of charge ordering has been achieved by mismatching the superlattice periodicity with the periodicity of charge ordering. We have observed that irrespective of the presence or absence of long-range CO, the noise magnitude is enhanced by several orders with a strong non-$1/f$ ($f=\mathrm{frequency}$) component when the system undergoes a MIT and magnetic transition. The higher-order statistics of resistance fluctuations reveal the presence of strong non-Gaussian components in both cases, further indicating inhomogeneous electrical transport arising from the electronic phase separation. Specifically, we find almost three orders of magnitude smaller noise in the insulating phase of the sample without long-range CO compared to the sample with CO. These findings suggest that digital synthesis can be a potential route to implement electronic transitions of complex oxides for device application.

Free electrons have a uniform kinetic energy density (KED), which evolves into a spatially varying quantity as the electrons respond to the gradual imposition of an external potential. In this paper and a companion paper, we examine two sets of functionals for describing the local, non-negative KED that emerges after such a perturbation. In this paper, we emphasize potential functionals, deriving the first- and second-order deviations from the free-electron KED as functionals of the perturbing potential, also reconsidering the analogous functionals for the local density of states and the electron density. (In the second paper, we use these results to re-express the KED response in terms of functionals of the induced electron density.) We develop reciprocal-space formulations of the response kernels to complement previously known real-space forms. The first-order function is straightforward to obtain, but the second-order function requires considerable effort. To manage the derivations, we relate the KED response to that of the one-electron Green function, and then examine the latter in detail. Finally, we provide extensive validation of the derived response functions based on asymptotic analysis of an integral representation, numerical integration of the same generating integral, and application to the linear potential model.

We present electron-density-based response functionals yielding the non-negative kinetic energy density (KED) of nearly free electron systems. In a previous paper, for a canonical free-electron system perturbed by an external potential, we derived the first- and second-order corrections to the KED as functionals of the potential, providing the response functions in reciprocal space. Here, we formulate the KED response in terms of the electron density by converting the potential-based functionals into density functionals. We also determine the related response of the Pauli KED, which is the KED in excess of the von Weizsäcker KED. We anticipate that the structure of these density functionals will help guide the design of the more sophisticated kinetic energy functionals required for orbital-free density functional theory simulations. We conclude by examining the performance of the first- and second-order density functionals for the KED when applied to electron densities generated from local pseudopotential calculations for Li, Al, and Si crystals.

The feasibility of topological metawaveguides supporting helical propagation in the microwave range has been recently proven. The advantages of unidirectional propagation supported by such waveguides however can only be exploited in real devices if topological modes are endowed with the capability to interact within themselves as well as with trivial modes. Here we show a modal launcher to interface a topological metawaveguide with conventional circular waveguides with negligible reflection, and we exploit the properties of coupled topological modes to show a proof of concept of a topological contradirectional coupler.

Transition-metal oxides are a rich group of materials with very interesting physical properties that arise from the interplay of the charge, spin, orbital, and lattice degrees of freedom. One interesting consequence of this, encountered in systems with orbital degeneracy, is the coexistence of long-range magnetic and orbital order, and the coupling between them. In this paper, we develop and study an effective spin-orbital superexchange model for $e_g^3$ systems and use it to investigate the spectral properties of a charge (hole) injected into the system, which is relevant for photoemission spectroscopy. Using an accurate, semianalytical, magnon expansion method, we gain insight into various physical aspects of these systems and demonstrate a number of subtle effects, such as orbital to magnetic polaron crossover, the coupling between orbital and magnetic order, as well as the orbital order driving the system towards one-dimensional quantum spin-liquid behavior. Our calculations also suggest a potentially simple experimental verification of the character of the orbital order in the system, something that is not easily accessible through most experimental techniques.

We investigate slow quenches in Chern insulators in ribbon geometry. We consider the Qi-Wu-Zhang model and slowly ramp the parameters (large time of the quench $\tau$) from a nontopological (Chern number = 0) to a topological regime (Chern number $\ne$ 0). In contrast to the Haldane model considered in [L. Privitera and G. E. Santoro, Phys. Rev. B 93, 241406(R) (2016)], the in-gap state degeneracy point is pinned to an inversion symmetric momentum, which changes the behavior drastically. The density of excitations in the in-gap states scales with the quench time as ${\tau }^{-1/2}$ as the ramp becomes slow, and the Kibble-Zurek mechanism applies. Despite the slower scaling of the density of in-gap excitations with $\tau$, the Hall conductance after the quench deviates from that of the ground state of the final Hamiltonian by an amount that drops as ${\tau }^{-1}$.

Single layers of transition metal dichalcogenides exhibit tightly bound excitons with large oscillator strength making them to promising materials for future optoelectronic applications. Despite that the linear response of these ultrathin materials is extensively studied, a profound understanding of the excitonic second-order nonlinear optical response, including trigonal warping and Berry curvature effects, is lacking so far. To bridge this gap, in this paper we report a microscopic model for the second-order nonlinear susceptibility tensor of these materials. Based on a $kp$ description of the underlying many-particle Hamiltonian we are able to address excitonic features as well as trigonal warping effects, spin-valley coupling, and also Berry curvature to the susceptibility tensor. We present explicit calculation of the latter for the exemplary materials ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ on silicon substrate and encapsulated in hBN. For instance we predict resonant susceptibility for second-harmonic generation $0.5–1.3\mathrm{nm}/\mathrm{V}$ and double-resonant susceptibility $10–30\mathrm{n}\mathrm{m}/\mathrm{V}$ for the difference and sum-frequency generation processes. Our results are in good agreement with available experimental findings in the literature.

Hole spins in semiconductor quantum dots or bound to acceptor impurities show promise as potential qubits, partly because of their weak and anisotropic hyperfine couplings to proximal nuclear spins. Since the hyperfine coupling is weak, it can be difficult to measure. However, an anisotropic hyperfine coupling can give rise to a substantial spin-echo envelope modulation that can be Fourier analyzed to accurately reveal the hyperfine tensor. Here, we give a general theoretical analysis for hole spin-echo envelope modulation, and apply this analysis to the specific case of a boron acceptor hole spin in silicon. For boron acceptor spins in unstrained silicon, both the hyperfine and Zeeman Hamiltonians are approximately isotropic, leading to negligible envelope modulations. In contrast, in strained silicon, where light-hole spin qubits can be energetically isolated, we find that the hyperfine Hamiltonian and $g$ tensor are sufficiently anisotropic to give spin-echo envelope modulations. We show that there is an optimal magnetic-field orientation that maximizes the visibility of envelope modulations in this case. Based on microscopic estimates of the hyperfine coupling, we find that the maximum modulation depth can be substantial, reaching $\approx 10\%$, at a moderate laboratory magnetic field, $B\lesssim 200\mathrm{mT}$.

The archetype cubic chiral magnet MnSi is home to some of the most fascinating states in condensed matter, such as skyrmions and a non-Fermi-liquid behavior in conjunction with a topological Hall effect under hydrostatic pressure. Using small angle neutron scattering, we study the evolution of the helimagnetic, conical, and skyrmionic correlations with increasing hydrostatic pressure. We show that the helical propagation vector smoothly reorients from $\langle 111\rangle$ to $\langle 100\rangle$ at intermediate pressures. At higher pressures, above the critical pressure, the long-range helimagnetic order disappears at zero magnetic field. Nevertheless, skyrmion lattices and conical spirals form under magnetic fields, in a part of the phase diagram where a topological Hall effect and a non-Fermi-liquid behavior have been reported. These unexpected results shed light on the puzzling behavior of MnSi at high pressures and the mechanisms that destabilize the helimagnetic long-range order at the critical pressure.