Quasiparticle self-consistent $GW$ calculations are presented for the band structures of $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_2$ and $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_2$ in the orthorhombic $Pna{2}_1$ tetrahedrally coordinated crystal structures, which are closely related to the wurtzite structure of ZnO. Symmetry labeling of the bands near the gap is carried out, and effective-mass tensors are extracted for the conduction-band minimum and crystal-field split valence-band maxima (VBM) at $\mathrm{\Gamma }$. The gap is found to be direct at $\mathrm{\Gamma }$ and is 5.81 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_2$ and 5.46 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_2$. Electron-phonon coupling zero-point normalization is estimated to lower these gaps by about $0.2\pm 0.1$ eV. Optical response functions are calculated within the independent-particle long-wavelength limit, and they show the expected anisotropy of the absorption onsets due to the crystal-field splitting of the VBM. The results show that both materials are promising candidates as ultrawide-gap semiconductors with wurtzite-based tetrahedrally bonded crystal structures. Direct transitions from the lowest conduction band to higher bands, relevant to $n$-type doped material and transparent conduction applications, are found to start only above 3.9 eV and are allowed for only one polarization, and several higher band transitions are forbidden by symmetry. Alternative crystal structures, such as $R\overline{3}m$ and a rocksalt-type phase with a tetragonally distorted $P4/mmm$ spacegroup, both with octahedral coordination of the cations, are also investigated. They are found to have higher energy but about 20% smaller volume per formula unit. The transition pressures to these phases are determined, and for $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_2$ they are found to be in good agreement with experimental studies. The $R\overline{3}m$ phase also has a comparably high but slightly indirect band gap, while the rocksalt-type phase is found to have a considerably smaller gap of about 3.1 eV in $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_2$ and 1.0 eV in $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_2$.

We aim to understand how the spectrum of semi-Dirac fermions is renormalized due to long-range Coulomb electron-electron interactions at a topological Lifshitz transition where two Dirac cones merge. At the transition, the electronic spectrum is characterized by massive quadratic dispersion in one direction, whereas it remains linear in the other. We have found that, to lowest order, the unconventional ${\log }^2$ (double logarithmic) correction to the quasiparticle mass in bare perturbation theory leads to resummation into strong mass renormalization in the exact full solution of the perturbative renormalization-group equations. This behavior effectively wipes out the curvature of the dispersion and leads to Dirac cone restoration at low energy: The system flows towards Dirac dispersion which is anisotropic but linear in momentum with interaction-depended logarithmic modulation. The Berry phase associated with the restored critical Dirac spectrum is zero—a property guaranteed by time-reversal symmetry and unchanged by renormalization. Our results are in contrast with the behavior that has been found within the large-$N$ approach.

We report an enhancement of the anomalous Nernst effect (ANE) in Ni/Pt (001) epitaxial superlattices. The transport and magnetothermoelectric properties were investigated for the Ni/Pt superlattices with various Ni layer thicknesses ($t$). The anomalous Nernst coefficient was increased up to more than $1\mu \mathrm{V}{\mathrm{K}}^{-1}$ for $2.0\mathrm{nm}\le t\le 4.0\mathrm{nm}$, which was the remarkable enhancement compared to the bulk Ni. It has been found that the large transverse thermoelectric conductivity $\left({\alpha }_{xy}\right)$, reaching ${\alpha }_{xy}=4.8\mathrm{A}{\mathrm{K}}^{-1}{\mathrm{m}}^{-1}$ for $t=4.0\mathrm{nm}$, plays a prime role for the enhanced ANE of the Ni/Pt (001) superlattices.

The topological Hall effect (THE), given by a composite of electric and topologically nontrivial spin texture is commonly observed in magnetic skyrmion crystals. Here we present a study of the THE of electrons coupled to antiferromagnetic skyrmion lattices (AF-SkX). We show that, in the strong Hund coupling limit, topologically nontrivial phases emerge at specific fillings. Interestingly, at low filling an external field controlling the magnetic texture drives the system from a conventional insulator phase to a phase exhibiting the THE. Such behavior suggests the occurrence of a topological transition which is confirmed by a closing of the bulk gap that is followed by its reopening, appearing simultaneously with a single pair of helical edge states. This transition is further verified by the calculation of the Chern numbers and Berry curvature. We also compute a variety of observables in order to quantify the THE, namely, Hall conductivity and the orbital magnetization of electrons moving in the AF-SkX texture.

Metal halide perovskites have emerged as versatile photonic device materials because of their outstanding optical properties. Here, we report the two-photon absorption (TPA) spectra for $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{Br}}_3$ perovskite single crystals under linearly and circularly polarized laser pulses. We experimentally determined the spin-orbit splitting energy from the TPA linear-circular dichroism spectrum and found the higher-energy band around 0.8 eV above the band edge. From a theoretical analysis of the experimental data, we evaluated the exciton binding energy and the exciton reduced mass. Our findings provide essential information on the electronic structures and carrier dynamics of halide perovskites.

We study theoretically second-order topological superconductors characterized by the presence of pairs of zero-energy Majorana corner states. We uncover a quadrupole spin polarization at the system edges that provides a striking signature to identify topological phases, thereby complementing standard approaches based on zero-bias conductance peaks due to Majorana corner states. We consider two different classes of second-order topological superconductors with broken time-reversal symmetry and show that both classes are characterized by a quadrupolar structure of the spin polarization that disappears as the system passes through the topological phase transition. This feature can be accessed experimentally using spin-polarized scanning tunneling microscopes. We study different models hosting second-order topological phases, both analytically and numerically, and using Keldysh techniques we provide numerical simulations of the spin-polarized currents probed by scanning tips.

Floquet higher-order topological insulators (FHOTIs) are a novel topological phase that can occur in periodically driven lattices. An appropriate experimental platform to realize FHOTIs has not yet been identified. We introduce a periodically driven bipartite (two-band) system that hosts FHOTI phases without static counterparts, and predict that this lattice can be realized in experimentally realistic optical waveguide arrays, similar to those previously used to study anomalous Floquet insulators. The model exhibits interesting phase transitions from first-order to second-order topological matter by tuning a coupling strength parameter, without breaking lattice symmetry. In the FHOTI phase, the lattice hosts corner modes at eigenphase 0 or $\pi$, which are robust against disorder in the individual couplings.

A theoretical approach is proposed to describe the spin dynamics in defect color centers. The method explicitly considers the spin dynamics in the ground state and excited state of the defect center as well as spin state dependent transitions involving the ground state, excited state, and an additional intermediate state. The proposed theory is applied to treat spin-dependent phenomena in silicon carbide, namely, in spin-$\frac{3}{2}$ silicon-vacancy centers, termed $V_{\mathrm{Si}}$ or V2 centers. Theoretical predictions of magnetic field dependent photoluminescence intensity and optically detected magnetic resonance spectra demonstrate an important role of level-crossing phenomena in the spin dynamics of the ground state and excited state. The results are in good agreement with previously published experimental data [Phys. Rev. X 6, 031014 (2016)].

Endohedral fullerenes are perfect nanolaboratories for the study of magnetism. The substitution of a diamagnetic scandium atom in ${\mathrm{Dy}}_2\mathrm{ScN}@{\mathrm{C}}_{80}$ with gadolinium decreases the stability of a given magnetization and demonstrates Gd to act as a single atom catalyst that accelerates the reaching of thermal equilibrium. X-ray magnetic circular dichroism at the $M_{4,5}$ edges of Gd and Dy shows that Gd affects the ground state. The Gd magnetic moment follows the sum of the external and the dipolar magnetic field of the two Dy ions and compared to ${\mathrm{Dy}}_2\mathrm{ScN}@{\mathrm{C}}_{80}$ a lower exchange barrier is found between the ferromagnetic and the antiferromagnetic Dy configuration. The Arrhenius equilibration barrier as obtained from superconducting quantum interference device magnetometry is more than one order of magnitude larger, though a much smaller prefactor imposes the faster equilibration in ${\mathrm{Dy}}_2\mathrm{GdN}@{\mathrm{C}}_{80}$. This sheds light on the importance of angular momentum balance and symmetry in magnetic relaxation.

We study the ratchet effect in a narrow pinning-free superconductive ring based on time-dependent Ginzburg-Landau (TDGL) equations. Voltage responses to external dc and ac currents at various magnetic fields are studied. Due to asymmetric barriers for flux penetration and flux exit in the ring-shaped superconductor, the critical current above which the flux-flow state is reached, as well as the critical current for the transition to the normal state, are different for the two directions of applied current. These effects cooperatively cause ratchet signal reversal at high magnetic fields, which has not been reported to date in a pinning-free system. The ratchet signal found here is larger than those induced by asymmetric pinning potentials. Our results also demonstrate the feasibility of using mesoscopic superconductors to employ a superconducting diode effect in versatile superconducting devices.

Using first-principles-based Hamiltonian, we show novel structural behaviors of hexagonal manganite ${\mathrm{BaMnO}}_3$ at moderate low temperatures ($T\mathrm{s}$). The structural change of ${\mathrm{BaMnO}}_3$ occurring at $\sim 130$ K is assigned to an improper transition between the low-$T$ ferroelectric phase with $P{6}_{3}cm$ symmetry and a high-$T$ nonpolar phase with $P{6}_3/mmc$ symmetry, induced by coupled softening of the $K_3$ and ${\mathrm{\Gamma }}_2^{-}$ modes. Herein, the order parameters show unconventional behaviors, with the mode amplitude being order-disorder like and the mode phase being displacive. Optical Goldstone-like mode, implied by the Mexican-hat-like Born-Oppenheimer potential energy surface, is manifested in our simulations via tuning quantum fluctuations and lattice strains. As this mode can be enhanced by lowering the contribution of intersite interactions, we suggest an experimental detecting scheme by inducing a light active $\mathrm{\Gamma }$-point mode strongly coupled to it.

We report the results of the pressure-dependent measurements of the static magnetization and of the ferromagnetic resonance (FMR) of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ to address the properties of the ferromagnetic phase of this quasi-two-dimensional van der Waals magnet. The static magnetic data at hydrostatic pressures up to 3.4 GPa reveal a gradual suppression of ferromagnetism in terms of a reduction of the critical transition temperature, a broadening of the transition width, and an increase of the field necessary to fully saturate the magnetization $M_{\mathrm{s}}$. The value of $M_{\mathrm{s}}\simeq 3{\mu }_{\mathrm{B}}/\mathrm{Cr}$ remains constant within the error bars up to a pressure of 2.8 GPa. The anisotropy of the FMR signal continuously diminishes in the studied hydrostatic pressure range up to 2.39 GPa, suggesting a reduction of the easy-axis-type magnetocrystalline anisotropy energy (MAE). A quantitative analysis of the FMR data gives evidence that up to this pressure the MAE constant $K_{\mathrm{U}}$, although getting significantly smaller, still remains finite and positive, i.e., of the easy-axis type. Therefore, a recently discussed possibility of switching the sign of the magnetocrystalline anisotropy in ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ could only be expected at still higher pressures, if possible at all, due to the observed weakening of the ferromagnetism under pressure. This circumstance may be of relevance for the design of strain-engineered functional heterostructures containing layers of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$.

We study a Kitaev chain model, which is the simplest model of topological superconductors hosting Majorana fermion, appearing as a zero-energy state at the edge. We analytically calculate the Green's function of the semi-infinite Kitaev chain with a delta-function-type impurity potential within the quasiclassical regime to obtain the spatial dependence of the induced odd-frequency pairing. It is found that if the position of the impurity is not far from the edge, the spatial profile of the local density of states (LDOS) and the odd-frequency spin-triplet $s$-wave pair amplitude is tunable as a function of the strength of the impurity potential. Moreover, the zero-energy LDOS and low-frequency odd-frequency pair amplitude are found to have the same spatial dependence. The spatial profile of the zero-energy LDOS is analyzed based on the wave function of Majorana fermions.

We show that under the effect of an external electric field and a gradient of chemical potential, a topological electric current can be induced in Weyl semimetals without inversion and mirror symmetries. We derive analytic expressions for the nonlinear conductivity tensor and show that it is nearly quantized for small tilting when the Fermi levels are close to the Weyl nodes. When the Van Hove point is much larger than the largest Fermi level, the band structure is described by two linearly dispersing Weyl fermions with opposite chiralities. In this case, the electrochemical response is fully quantized in terms of fundamental constants and the scattering time, and it can be used to measure directly the topological charge of Weyl points. We show that the electrochemical chiral current may be derived from an electromagnetic action similar to axion electrodynamics, where the position-dependent chiral Fermi level plays the role of the axion field. This posits our results as a direct consequence of the chiral anomaly.

The working mechanism of optical tweezers is generally believed to be the gradient force arising from the light intensity inhomogeneity of an incident optical field. Here we demonstrate a somewhat counterintuitive phenomenon—optical trapping of chiral particles relying on the gradient force emerging in an optical field with homogeneous light intensity. This type of gradient force is shown to survive even in the Rayleigh limit, where the particle size is far smaller than the illuminating wavelength. By deriving a rigorous analytical expression for the optical force exerted on a chiral particle, the origin of such a gradient force is traced to the gradient of the magnetoelectric energy density, irrespective of the particle size. Besides enriching our understanding of the light-matter interaction, this gradient force is found to have magnitude comparable to the conventional intensity gradient-induced gradient force and to switch its direction for particles with opposite chiralities, rendering it applicable to particle chirality discrimination and identification.

Mostly forsaken, but revived after the emergence of all-dielectric nanophotonics, the Kerker effect can be observed in a variety of nanostructures from high-index constituents with strong electric and magnetic Mie resonances. A necessary requirement for the existence of a magnetic response limits the use of generally nonmagnetic conventional plasmonic nanostructures for the Kerker effect. In spite of this, we demonstrate here the emergence of the lattice Kerker effect in regular plasmonic Al nanostructures. Collective lattice oscillations emerging from the delicate interplay between Rayleigh anomalies and localized surface plasmon resonances both of electric and magnetic dipoles, and electric and magnetic quadrupoles result in suppression of the backscattering in a broad spectral range. Variation of geometrical parameters of Al arrays allows for tailoring the lattice Kerker effect throughout UV and visible wavelength ranges, which is close to impossible to achieve using other plasmonic or all-dielectric materials. It is argued that our results set the ground for wide ramifications in the plasmonics and further application of the Kerker effect.

$\mathrm{SrRu}{\mathrm{O}}_3$ (SRO) is an itinerant ferromagnet with strong coupling between the charge, spin, and lattice degrees of freedom. This strong coupling suggests that the electronic and magnetic behaviors of SRO are highly susceptible to changes in the lattice distortion. Here we show how the spin interaction and resultant magnon formation change with the modification in the crystallographic orientation of SRO thin films. We fabricated SRO epitaxial thin films with (100), (110), and (111) surface orientations, to systematically modulate the spin interaction and spin dimensionality. The reduced spin dimensionality and enhanced exchange interaction in the (111)-oriented SRO thin film significantly suppress magnon formation. Our study comprehensively demonstrates the facile tunability of magnon formation and spin interaction in correlated oxide thin films.

We predict a nonlinear Hall effect in certain Weyl semimetals with broken inversion symmetry. When the energy dispersions about pairs of Weyl nodes are skewed—the Weyl cones are “tilted”—the concerted actions of the anomalous velocity and the chiral anomaly give rise to the nonlinear Hall effect. This Hall conductivity is linear in both electric and magnetic fields and depends critically on the tilting of the Weyl cones. We also show that this effect does not rely on a finite Berry curvature dipole, in contrast with the intrinsic quantum nonlinear Hall effect that was recently observed in type-II Weyl semimetals.