We perform a theoretical examination of the local electronic structure in the recently discovered bilayer high-temperature superconductor ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. Our method begins with a bilayer two-orbital tight-binding model, incorporating various pairing interaction channels. We determine superconducting order parameters by self-consistently solving the real-space Bogoliubov–de Gennes (BdG) equations, revealing a robust and stable extended $s$-wave pairing symmetry. We investigate the single impurity effect using both self-consistent BdG equations and non-self-consistent $T$-matrix methods, uncovering low-energy in-gap states that can be explained with the $T$-matrix approach. Additionally, we analyze magnetic vortex states using a self-consistent BdG technique, which shows a peak-hump structure in the local density of states at the vortex center. Our results provide identifiable features that can be used to determine the pairing symmetry of the superconducting ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ material.

Andreev molecules consist of two coherently coupled Josephson junctions and permit nonlocal control over supercurrents. By making the barriers magnetic and thus creating a spin valve, we predict that a nonlocal Josephson diode effect occurs that is switchable via the magnetic configuration of the barriers. The diode effect is turned on, off, or changes its sign depending on whether the spin valve is in a parallel, normal, or antiparallel configuration. These results offer a way to exert complete control over a nonlocal Josephson diode effect via the spin degree of freedom.

Finding two-dimensional (2D) materials with ferroelectricity is of great interests towards polarization-related applications and nanosized devices. Despite much theoretical efforts that predict the existence of novel 2D ferroelectrics, only a small portion have been realized in experiments. The well-known 2D transition-metal dichalcogenide molybdenum disulfide (${\mathrm{MoS}}_2$) monolayer was predicted to have a ferroelectric $d1T$ polymorph resulting from condensation of soft phonons from the centrosymmetric $1T$ phase. However, experiments mostly obtain a nonpolar monoclinic $1T^{\prime }$ phase, whereas the $d1T$ phase has not been observed. In this study, we use various first-principles techniques, including density-functional theory total-energy calculations, ab initio molecular dynamics, and the temperature-dependent effective potential method to show that the nonpolar $1T^{\prime }$ phase is thermodynamically more stable than the $d1T$ phase. Furthermore, the $1T$ phase at high temperature is averaged among degenerate $1T^{\prime }$ structures, and the $1T$-to-$1T^{\prime }$ transition upon cooling is predicted to be of first order with a strong order-disorder character. Effect of strain on the stable structure and the likely ground state of other $\mathrm{Mo}{X}_2$ ($X=\text{Se}$, Te) monolayers are also discussed.

Two-dimensional topological superconductors with chiral edge modes are predicted to possess a quantized thermal Hall effect proportional to the Chern number, exactly half that for chiral topological insulators. However, not much work has been done in identifying the quantized heat conductance in the literature, even for some of the standard models of topological superconductivity. Here we introduce a model based on a proximity induced superconducting bismuth bilayer and directly calculate the thermal Hall conductance of this lattice model. This model serves as a demonstration of the state of the art possible in such a calculation, as well as introducing an interesting paradigmatic topological superconductor with a rich phase diagram. We demonstrate the quantized thermal Hall plateaus in several different topological phases and compare this to numerical calculations of the Chern number, as well as analytical calculations of the Chern number's parity invariant. We demonstrate that it is possible to get a reasonable topological phase diagram from the quantized thermal Hall calculations. The technique used can be applied to a wide range of models directly in real space.

The electrical polarization switching on a stoichiometric $\mathrm{GaFe}{\mathrm{O}}_3$ single crystal was measured, and a model of atomic displacements responsible for the polarization reverse was proposed. The widely adapted mechanism of polarization switching in $\mathrm{GaFe}{\mathrm{O}}_3$ can be applied to stoichiometric, perfectly ordered crystals. However, the grown single crystals, as well as thin films of Ga-Fe-O, show pronounced atomic disorder. Using piezoresponse force microscopy, the electrical polarization switching on a crystal surface perpendicular to the electrical polarization direction was demonstrated. Atomic disorder in the crystal was measured by x-ray diffraction and Mössbauer spectroscopy. These measurements were supported by ab initio calculations. Using analysis of atomic disorder and electronic structure calculations, the energies of defects of cations in foreign cationic sites were estimated. The energies of the polarization switch were estimated, confirming the proposed mechanism of polarization switching in $\mathrm{GaFe}{\mathrm{O}}_3$ single crystals.

Electron optics deals with condensed matter platforms for manipulating and guiding electron beams with high efficiency and robustness. Common devices rely on the spatial confinement of the electrons into one-dimensional channels. Recently, there is growing interest in electron optics applications in two dimensions, which heretofore are almost exclusively based on graphene devices. In this work, we study band-inverted systems resulting from particle-hole hybridization and demonstrate their potential for electron optics applications. We develop the theory of interface scattering in an inverted-band $pn$ junction using a scattering matrix formalism and observe negative refraction conditions as well as transmission filtering akin to graphene's Klein tunneling but at finite angles. Based on these findings, we provide a comprehensive protocol for constructing electron optic components, such as focusing and bifurcating lenses, polarizers, and mirrors. We numerically test the robustness of our designs to disorder and finite temperatures, and motivate the feasibility of experimental realization. Our work opens avenues for electron optics in two dimensions beyond graphene-based devices, where a plethora of inverted-band materials in contemporary experiments can be harnessed.

The charge and spin-dependent thermoelectric responses are investigated on a single-stranded helical system possessing a collinear antiferromagnetic spin arrangement with zero net magnetization in the presence of a transverse electric field. Both the short and long-range hopping scenarios are considered. Nonequilibrium Green's function formalism is employed following the Landauer-Buttiker prescription to study the thermoelectric phenomena. The dependence of the thermoelectric quantities on electric field, temperature, parameters associated with the helical geometry, coupling of the helix to electrodes, system size, etc., are elaborately discussed. The charge and spin figure of merits are computed and compared critically. For a more accurate estimation, the phononic contribution towards thermal conductance is also included. The present proposition shows a favorable spin-dependent thermoelectric response compared to the charge counterpart.

Authors of earlier studies on the Kondo insulator ${\mathrm{SmB}}_6$ revealed the presence of a bulk Kondo insulating gap between 30 and 50 K and the emergence of a conducting surface state only $<4\mathrm{K}$. Here, we compare the two-coil mutual inductance pick-up response of a ${\mathrm{SmB}}_6$ single crystal with that of a conventional topological insulator, a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystal. From these studies, we identify three distinct temperature regimes for ${\mathrm{SmB}}_6$, viz., (i) $T\ge T^{*}$($\sim 66\mathrm{K}$), (ii) ($\sim 40\mathrm{K}$) $T_g\le T<T^{*}$, and (iii) $T<T_g$. At $T^{*}$ in ${\mathrm{SmB}}_6$, we observe a peak in the temperature-dependent AC pick-up signal which corresponds to the peak in the broad hump feature in the bulk DC susceptibility measurements and features in the resistivity measurements. A dip in the pick-up signal at $T_g$ in ${\mathrm{SmB}}_6$ correlates with the evidence for the opening of a bulk Kondo gap in transport measurements. In this paper on our study of the pick-up signal in ${\mathrm{SmB}}_6$, we suggest the presence of a thin (submicron-order thickness), high-conducting surface layer from a temperature just below $T_g$. In this $T$ regime in ${\mathrm{SmB}}_6$, the pick-up signal shows a distinct square root frequency ($\nu$) dependence compared with the linear $\nu$ dependence found in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Across all the different $T$ regimes, distinct AC frequency dependence and scaling properties are observed. Our results suggest that, above $T^{*}$, weak exchange interactions cause electrons to scatter from random ion sites. Electronic correlations gradually strengthen with the onset of Kondo-like hybridization, setting in from below $T^{*}$, and at $T_g$, a strongly correlated Kondo gap opens in the bulk of the material. The appearance of the thin, high-conducting surface layer is nearly coincident with the onset of the bulk Kondo insulating state below $T_g$ in ${\mathrm{SmB}}_6$.

Doped ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ is a dilute polaronic metal that exhibits superconductivity and a mid-infrared optical response suggesting parallels with copper oxides. A peculiar feature of ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ is that its plasma frequency, ${\omega }_p$, is highly temperature dependent, increasing by more than a factor of 2 when the system is cooled from 300 to 100 K [F. Gervais et al., Phys. Rev. B 47, 8187 (1993); D. M. Eagles et al., Phys. Rev. B 54, 22 (1996); C. Z. Bi et al., J. Phys.: Condens. Matter 18, 2553 (2006)]. There is still no generally accepted explanation for this dramatic shift. Here, we present momentum-resolved electron energy-loss spectroscopy measurements of ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ at nonzero momentum, $q$. We also calculate the collective excitations of ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ using the random phase approximation (RPA), to assess whether the behavior of the collective modes conforms to established explanations. We find that the plasmon energy and linewidth are momentum independent, in contrast to RPA predictions, and that its shift with temperature takes place everywhere in the Brillouin zone, from $q=0$ to the zone boundary, $q=0.5$ reciprocal lattice units. We also find that the phonon frequencies do not shift with $q$ in the expected way, suggesting the screening properties of the material deviate significantly from RPA predictions. We conclude that a radically different starting point, perhaps based on lattice anharmonicity, may be needed to explain the collective charge excitations of ${\mathrm{SrTi}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$.

Topological interface states in periodic lattices have emerged as valuable assets in the fields of electronics, photonics, and phononics, owing to their inherent robustness against disorder. Unlike electronics and photonics, the linear dispersion relation of hypersound offers an ideal framework for investigating higher-order bandgaps. In this work, we propose a design strategy for the generation and manipulation of topological nanophononic interface states within high-order bandgaps of GaAs/AlAs multilayered structures. These states arise from the band inversion of two concatenated superlattices that exhibit inverted spatial mode symmetries around the bandgap. By adjusting the thickness ratio of the unit cells in these superlattices, we are able to engineer interface states in different bandgaps, enabling the development of versatile topological devices spanning a wide frequency range. Moreover, we demonstrate that such interface states can also be generated in hybrid structures that combine two superlattices with bandgaps of different orders centered around the same frequency. These structures open up avenues for exploring topological confinement in high-order bandgaps, providing a platform for unveiling and better understanding complex topological systems.

Effective medium theory is an integral part of the area of metamaterials providing insights into the physics behind their interaction with light. In this paper, we develop the operator effective medium approximation for spherical and cylindrical multilayer systems. Equating evolution operators (wave propagators) for the actual multilayer and homogenized medium, we establish the effective material parameters, which differ for planar and curvilinear cases. We supplement the criterion for applicability of the effective medium approximation with the assertion that the radii of curvilinear layers must be about or greater than the radiation wavelength. The operator effective medium approximation provides a clear way to approach nonlocal corrections, as well as characterize structures with balanced loss and gain. We envisage that the operator effective medium approximation will be useful for the characterization of metamaterials and other nanophotonic systems of complex shapes.

We study the influence of many-body interactions on the transport properties in a two-site charge Kondo circuit recently implemented in a hybrid metal-semiconductor double-quantum dot device [W. Pouse et al., Nat. Phys. 19, 492 (2023)]. There emerge two principal types of interactions: (i) an intrinsic one, described by the Luttinger liquid model, and (ii) an induced one, which appears due to the coupling of the system to an Ohmic environment. Case (i) could be achieved if the charge Kondo circuit operates in the fractional quantum Hall regime, while case (ii) can be implemented via a finite number of open ballistic channels coupled to both the quantum dots. We demonstrate that the conductance scaling for the case of strong and weak interdot coupling is fully determined by the effective interaction parameter, which is the combination of the fractional filling factor $\nu =1/m$ and the number of transmitting channels. Furthermore, we predict that the fractional filling factor $\nu$ defines a universal Kondo scaling in the vicinity of a special triple-quantum critical point featured by the emergence of a ${\mathbb{Z}}_3$ parafermion.

Photonic topological edge states have shown powerful capabilities to manipulate light propagations. In particular, the all-dielectric structures serve as a promising platform to support the topological states, in which the nontrivial photonic band is usually acquired by engineered shape and lattice with isotropic structures. Here, we propose to manipulate the topological phases in two-dimensional (2D) triangle photonic lattices composed of anisotropic pillars. It is found that the rotation of pillars or unit cells, accompanied by complex coupling effects, can infuse degrees of freedom to switch the photonic band structure between trivial and nontrivial topological phases. We further work out a 2D phase diagram describing the rotation-induced topological transition and demonstrate polarization-dependent robust one-way light propagations in silicon at telecommunication wavelength. This work proposes an alternative scheme to manipulate the topological phases and nontrivial photonic states, which promises more interesting explorations of on-chip light manipulation.

We have investigated effects of boron isotope enrichment of cubic boron nitride (cBN). X-ray Raman scattering spectra of ${}^{10}\mathrm{B}$ and ${}^{11}\mathrm{B}$ enriched samples did not show any chemical shift. meV-resolved inelastic x-ray scattering was used to measure the phonon dispersion. The shift of phonon frequencies between a sample with the natural abundance of B (20% ${}^{10}\mathrm{B}$) and a sample nearly 100% enriched in ${}^{10}\mathrm{B}$ was reasonably explained by treating the mass difference with the same force constants––i.e., purely an isotope effect. The observed shifts were also consistent with first-order perturbation theory which gives the frequency shift, $\mathrm{\Delta }{\omega }_j$, as $\frac{\mathrm{\Delta }{\omega }_j}{{\omega }_j}=-\frac{1}{2}\frac{\mathrm{\Delta }{M}_d}{M_d}{\mathbf{e}}_{jd}^{*}·{\mathbf{e}}_{jd}$, where $M_d, \mathrm{\Delta }{M}_d, {\mathbf{e}}_{jd}$, and ${\omega }_j$ are the atomic mass, change in mass, the eigenvector, and the phonon angular frequency for atom $d$ and phonon mode $j$, respectively.

The vortex in the $(2+1)$-dimensional $\mathrm{O}\left(2\right)$ model is studied via numerical simulations in a fully nonperturbative lattice regularization. We compute the vortex condensate and susceptibility to determine its critical exponents and a renormalized condensate in the continuum limit. Together with recent results on the vortex mass, this gives a complete picture of the scaling behavior of the vortex operator in this model and sheds light on the statistical mechanics of topological excitations.

Quantum kinetically constrained models can exhibit a wealth of dynamical phenomena ranging from anomalous transport to Hilbert-space fragmentation (HSF). We study a class of one-dimensional particle-number-conserving systems in which particle hoppings are subjected to an East-like constraint, akin to facilitated spin models in classical glasses. While such a kinetic constraint leads to HSF, we find that the degree of fragmentation exhibits a sharp transition as the average particle density is varied. Below a critical density, the system transitions from being weakly fragmented where most of the initial states thermalize diffusively to strongly fragmented where the dynamics are frozen and the system fails to thermalize. Remarkably, the East model allows for both efficient numerical simulations and analytic solutions of various diagnostics of the phase transition, from which we obtain a set of exact critical exponents. We find that the freezing transition in particle-conserving East models belongs to the same universality class as dipole-conserving fracton systems. Our results provide a tractable minimal model for filling-induced freezing transitions associated with HSF, which can be readily tested on state-of-the-art quantum platforms.

We conducted a comprehensive study on the electronic structure of a multiferroic ${\mathrm{Lu}}_{0.5}{\mathrm{Sc}}_{0.5}{\mathrm{FeO}}_3$ single crystal using a range of techniques, including x-ray absorption spectroscopy, cluster model calculations, and ab initio analyses. Our x-ray linear dichroism measurements revealed strong hybridization of A-site $d$ orbitals with neighboring O $p$ orbitals. The hybridization strength of Lu $5d$ turns out to be not only much stronger but also more anisotropic than that of Sc $3d$, leading to a huge ligand-field splitting between the out-of-plane $a_{1g}$ orbital state and in-plane $e_g^{\pi }$ one. Based on our findings, we confirmed that Lu has a significantly larger ferroelectric energy gain compared to Sc. By combining our results with a simple phononic potential energy, we were able to quantify the ferroelectric energy landscape, which agrees well with the ab initio calculation result. Through a comparative analysis of Lu $5d$ and Sc $3d$ cases, we revalidate the rehybridization mechanism as the origin of ferroelectricity appearing in the $h$-A(Mn,$\mathrm{Fe}){\mathrm{O}}_3$ family.

We explore the spin density and charge currents arising on the surface of a topological insulator and in a two-dimensional Rashba metal due to magnetization gradients. For topological insulators a single interconversion coefficient controls the generation of both quantities. This coefficient is quantized to a value proportional to the vorticity of the Dirac point which constitutes a hallmark of parity anomaly at finite density. As such, it also unveils a robust route to disentangle and detect the protected states of a topological insulator on a given surface. In stark contrast, Rashba metals do not exhibit such anomalies since they contain an even number of helical branches. Nonetheless, also these are governed by quantized responses which, however, are not protected against weak disorder. Furthermore, we find that for Rashba metals the interconversion coefficients demonstrate discontinuities and a nontrivial interplay upon varying the chemical potential, the strength of the spin-orbit coupling, and a pairing gap. Our results have implications for the binding between magnetic skyrmions and superconducting vortices, the emergence of Majorana zero modes, and pave the way for superconducting diode effects mediated by out-of-plane magnetization gradients.