Ultrawide direct band gap semiconductors hold great promise for deep ultraviolet optoelectronic applications. Here we evaluate the potential of ${\mathrm{MgSiN}}_2$-GaN alloys for this purpose. Although ${\mathrm{MgSiN}}_2$ itself has an indirect gap $\sim 0.4$ eV below its direct gap of $\sim 6.5$ eV, its different sign lattice mismatch from GaN in two different basal plane directions could avoid the tensile strain which limits ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ on GaN for high $x$. Two octet-rule-preserving structures (with space groups $Pmn{2}_1$ and $P1n1$) of a 50% alloy of ${\mathrm{MgSiN}}_2$ and GaN are investigated and are both found to have gaps larger than 4.75 eV using quasiparticle self-consistent $GW$ calculations. Both are nearly direct gap in the sense that the indirect gap is less than 0.1 eV lower than the direct gap. Their mixing energies are positive yet small, with values of 8 (31) meV/atom for $Pmn{2}_1$ ($P1n1$), indicating only a small driving force toward phase separation.

For monolayer ${\mathrm{MoS}}_2$, a magnetic field causes the degeneracy of the $K$ and $K^{\prime }$ valleys to be lifted, owing to the valley Zeeman effect. Thus, in a monolayer ${\mathrm{MoS}}_2$ nanoribbon system with a magnetic barrier, electrons in these valleys exhibit different conductance behaviors when passing through the barrier. One valley shows a strong dip in conductance caused by Fano resonance, i.e., quantum interference between continuum states and Landau levels in the magnetic barrier. This dip can be modulated by changing the strength and length of the magnetic barrier. By calculating the asymmetric conductance for the $K$ and $K^{\prime }$ valleys as a function of incident energy with various values of the physical parameters, optimized conditions for spin and valley polarization can be found. The results presented here provide useful information to enable implementation of spin- and valley-polarized currents in future devices.

The generation of high-order harmonics in bulk systems is a young and fast-growing area of research. Beginning in 2011 with the pioneering studies of high-order harmonic generation in bulk dielectrics, significant progress has already been made in understanding the details of the microscopic mechanisms behind this phenomenon, such as the role of intra- and interband polarization, the contribution of the electronic band structure and dipole moments to the harmonic spectrum, and the effects of structural and electronic symmetry. However, the role of electron-electron correlations in the excited system is much less understood. In this work, we study the role of these effects in the high-order harmonic spectrum by using the Time-Dependent Density Functional Theory (TDDFT) approach with the Dynamical Mean-Field Theory (DMFT) exchange-correlation kernel. In this approximation, one takes into account the time-resolved on-site electron-electron interactions. As we demonstrate, the correlation effects significantly affect the high-harmonic spectrum, most importantly through ultrafast modification of the interband polarization of the system.

We study non-Hermitian many-body physics in the interacting Hatano-Nelson model with an open boundary condition. The violation of reciprocity, resulting from an imaginary vector potential, induces the non-Hermitian skin effect and causes exponential localization for all single-particle eigenfunctions in the noninteracting limit. Nevertheless, the density profile of the interacting system becomes only slightly tilted relative to the average filling. The Friedel oscillations exhibit a beating pattern due to the modification of the Fermi wave number. The probability distribution of particles over any finite interval is the normal distribution, whose mean scales with the imaginary vector potential and the variance is symmetric to the center of the chain. This is confirmed by several numerically exact methods even for relatively small systems. These features are expected to be generic not only for fermions, which naturally repel each other due to Pauli's exclusion principle, but for interacting bosons as well. Our findings indicate that many-body effects can significantly alter and conceal the single-particle properties and the skin effect in non-Hermitian systems.

We explore matrix product state approximations to wave functions which have spontaneously broken symmetries or are critical. We are motivated by the fact that symmetries, and their associated conservation laws, lead to block-sparse matrix product states. Numerical calculations which take advantage of these symmetries run faster and require less memory. However, in symmetry-broken and critical phases the block-sparse ansatz yields less accurate energies. We characterize the role of conservation laws in matrix product states and determine when it is beneficial to make use of them.

The breakdown of the lattice Kondo effect in local-moment metals can lead to nontrivial forms of quantum criticality and a variety of non-Fermi-liquid phases. Given indications that Kondo breakdown transitions involve criticality not only in the spin but also in the charge sector, we investigate the interplay of Kondo breakdown and strong valence fluctuations in generalized Anderson lattice models. We employ a parton mean-field theory to describe the transitions between deconfined fractionalized Fermi liquids and various confined phases. We find that rapid valence changes near Kondo breakdown can render the quantum transition first order. This leads to phase-separation tendencies which, upon inclusion of longer-range Coulomb interactions, will produce intrinsically inhomogeneous states near Kondo breakdown transitions. We connect our findings to unsolved aspects of experimental data.

Noise is a detrimental issue for nitrogen-vacancy (NV) centers in diamond, causing line broadening and decreasing the coherence time $\left(T_2\right)$. Following our previous electric and magnetic field noise work, we investigate noise caused by the diamond surface roughness, which is a source for charge density fluctuations and incoherent photon scattering. We find that the varying surface charge density noise source is prevalent throughout the entire NV dynamical decoupling frequency range, while the photon scattering noise is almost negligible. Next, we combine the results from various noise sources to perform comprehensive analyses on $T_2$ and how it varies with NV depth. At a given NV depth of 5 $\mathrm{nm}$ below a hydrogen- or fluorine-terminated surface, we find that these magnetic nuclei reduce the NV coherence time the most, followed by the surface electric field noise sources. The photon scattering and bulk magnetic field noise effects on $T_2$ are weak compared to the varying charge density, electric dipole, and surface impurity noise. However, with oxygen surface termination, the surface electric field noise sources are comparable to the surface magnetic field noise. Our calculated values of $T_{2,\text{Hahn}}$ (few $\mu \mathrm{s}$ to 10 $\mu \mathrm{s}$) are in good agreement with the experimental values reported elsewhere. Finally, we calculate an anticipated signal-to-noise ratio (SNR) for NV AC magnetometry of external nuclear spins. In our simplified assessment, where some depth-dependent parameters (e.g., NV conversion efficiency) are held constant, we find that shallower NV layers should yield the best SNR, which is consistent with experimental findings.

Non-Hermitian skin effect (NHSE) is a unique phenomenon studied intensively in non-Hermitian systems during the past few years. In this work, we discuss the energy dependence of NHSE by introducing nonreciprocity beyond the nearest-neighboring hopping in the one-dimensional lattices. The direction of NHSE reverses as the eigenenergy of the system under open boundary conditions (OBCs) sweeps across some critical energies. To characterize such a phenomenon, we introduce the concept of non-Hermitian skin effect edges, which separate the eigenstates localized at opposite ends of the lattice in the OBC spectrum. We find the skin effect edges are determined by the self-intersections in the spectrum under periodic boundary conditions (PBCs), which are topological as the winding number of the PBC spectrum changes sign when crossing them. Moreover, the NHSE edges will disappear when the self-intersections merge into a single point by tuning the system parameters. Our work reveals the intricate interplay between NHSE and nonreciprocal hopping in non-Hermitian systems.

We investigate the adiabatic evolution of the thermal state in nonreciprocal many-body systems coupled to their environment and subject to periodic drivings. In such systems we show that besides the dynamical phase a geometrical phase can exist and it drives the relaxation dynamic of the system. In contrast to the dynamical phase which always pushes the system toward its equilibrium state we show that the geometric phase can speed up or reduce the speed of the relaxation process. These results could have applications in the field of thermal management of complex systems.

We show the presence of Majorana edge modes in an interacting fermionic ladder with spin in a number-conserved setting. The interchain single-particle hopping is suppressed and only a pair hopping is present between the different chains of the ladder. Additionally, the hopping along the chains is spin imbalanced and a transverse magnetic field is applied breaking time-reversal invariance. We study the robustness of the topological phase with respect to an on-site interaction between the spin-up and spin-down fermions and the spin-dependent imbalance of the hopping. The main result of the present work is that the topological phase survives for a finite region in the parameter space in the presence of interactions. The localized Majorana edge modes seem to be more stable in the case when the on-site interaction is an attraction.

Rare-earth doped crystals have long coherence times and the potential to provide quantum interfaces between microwave and optical photons. Such applications benefit from a high cooperativity between the spin ensemble and a microwave cavity—this motivates an increase in the rare-earth ion concentration which in turn impacts the spin coherence lifetime. We measure spin dynamics of two rare-earth spin species, ${}^{145}\mathrm{Nd}$ and Yb, doped into ${\mathrm{Y}}_{\text{2}}{\mathrm{SiO}}_{\text{5}}$, coupled to a planar microwave resonator in the high-cooperativity regime, in the temperature range $1.2\mathrm{K}$ to $14\mathrm{mK}$. We identify relevant decoherence mechanisms, including instantaneous diffusion arising from resonant spins and temperature-dependent spectral diffusion from impurity electron and nuclear spins in the environment. We explore two methods to mitigate the effects of spectral diffusion in the Yb system in the low-temperature limit, first, using magnetic fields of up to 1 T to suppress impurity spin dynamics and, second, using transitions with low effective $g$ factors to reduce sensitivity to such dynamics. Finally, we demonstrate how the “clock transition” present in the ${}^{171}\mathrm{Yb}$ system at zero field can be used to increase coherence times up to $T_2=6\left(1\right)\mathrm{ms}$.

We propose an all-optical scheme of topological lasing and switching based on the Aubry-André-Harper (AAH) model of an exciton-polariton chain. We theoretically show that the phase parameter of the optical potential, with a tunable effective quasimomentum, allows the system to exhibit nontrivial topological properties which are attributed to higher dimensions. The topological modes emerging within the bulk band gaps are spatially localized at the edges of the polariton lattice, and their topological properties are characterized by the nonzero Chern numbers of the bulk bands. Polariton lasing in topological edge modes exhibits a higher efficiency and better robustness than in bulk modes, and can be switched between two opposite edges of the lattice by nonresonant excitation, which paves a way for topologically protected optical circuits.

We consider dressing of excitonic properties by strongly correlated electrons in gate-controlled twisted homobilayer heterostructures. The combined effect of the moiré potential and the Coulomb interaction supports the formation of different strongly correlated phases depending on the filling, including charge-ordered metals or incompressible insulators at integer occupation. The coupling between excitons and electrons results in a splitting of the excitonic resonance into an attractive and a repulsive polaron peak. Analyzing the properties of the exciton-polarons across the different phases of the system, we reveal a discontinuous evolution of the spectrum with the formation of a double-peak structure in the repulsive polaron branch. The double-peak structure emerges for noninteger fillings and it is controlled by the energy separation between the quasiparticle states close to the Fermi level and the high-energy doublons excitations. Our results demonstrate that exciton-polarons carry a clear hallmark of the electronic correlations and, thus, provide a direct signature of the formation of correlation-driven insulators in gate-controlled heterostructures.

In this paper we propose that quantized and nearly half-quantized intrinsic anomalous Hall effect can be induced by in-plane external magnetic field through the Zeeman coupling in nonmagnetic two-dimensional (2D) systems with sizable spin-orbital coupling but without twofold rotational symmetry. An analytical result is derived for the 2D electron gas model with $C_{3v}$ symmetry. Further, based on the $\mathit{k}·\mathit{p}$ Hamiltonian derived from first principle calculations, we find that the quantized and nearly half-quantized Hall conductance can be observed in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ thin film in the clean limit with strong in-plane magnetic field $B>20\mathrm{T}$ and low temperature $T<100\mathrm{mK}$.

Some puzzling and unexplained observations regarding the cubic to “tetragonal” phase transition at $T_{\mathrm{S}}=282\mathrm{K}$ in the almost-multiferroic perovskite ${\mathrm{EuTiO}}_3$ have been reanalyzed to obtain deeper insight into the true structure and magnetic activity evolving below $T_{\mathrm{S}}$. For this purpose, birefringence and high-quality synchrotron x-ray-diffraction (XRD) data have been used where zero magnetic field data are compared to data under the influence of a direction-dependent magnetic field. The birefringence data have been analyzed with respect to the angle dependence of the domain orientation from which the dominance of the monoclinic symmetry is unambiguously proven. The XRD structural data have been taken as input to derive the magnetic exchange constants, the related Néel temperatures $T_N$, and the energies of the possible magnetic ground states. Taking both results together we conclude that below $T_{\mathrm{S}}$ the symmetry cannot be tetragonal, but only monoclinic followed by another symmetry lowering transition around $T^{*}\approx 210\mathrm{K}$. In addition, the involved magnetic exchange interactions have been calculated based on the experimental lattice constants, and it is shown that these support the conclusion that any kind of magnetism stems from a competition of the various ground-state energies and consequently induces a highly frustrated magnetically active state near room temperature.

We present a frequency-domain study of the dynamic behavior of a magnetic vortex core within a single Permalloy disk by means of electrical detection and micromagnetic simulations. When exciting the vortex core dynamics in a nonlinear regime, the lineshape of the rectified dc signal reveals a resonance peak splitting which depends on the excitation amplitude. Using micromagnetic simulations, we show that at high excitation power the peak splitting originates from the nanosecond time scale quasiperiodic switching of the vortex core polarity. Using lock-in detection, the rectified voltage is integrated over a ms time scale, so that the net signal detected between the two resonant peaks for a given range of parameters cancels out. The results are in agreement with the reported effects of the in-plane static field magnitude on the gyration dynamics, and complement them by detailed analysis of the effects of the rf current amplitude and the azimuthal angle of the in-plane bias magnetic field. Systematic characterization shows that a transition from linear to nonlinear dynamical regime can be controlled by rf current as well as by varying the magnitude and the direction of the bias magnetic field.

Metallic Mn-based alloys with a nearest-neighbor Mn-Mn distance greater than 0.4 nm exhibit large, well-localized magnetic moments. Here we investigate the magnetism of tetragonal ${\mathrm{Au}}_4\mathrm{Mn}$ with a Curie temperature of 385 K, where manganese has a spin moment of $4.1{\mu }_{\mathrm{B}}$ and its orbital moment is quenched. Since 80% of the atoms are gold, the spin-orbit interaction is strong and ${\mathrm{Au}}_4\mathrm{Mn}$ exhibits uniaxial magnetocrystalline anisotropy with surface maze domains at room temperature. The magnetic hardness parameter of 1.0 is sufficient to maintain the magnetization along the $c$ axis for a sample of any shape. Au also reduces the spin moment of Mn through $5d-3d$ orbital hybridization. An induced moment of $0.05{\mu }_{\mathrm{B}}$ was found on Au under a pulsed field of 40 T. Density functional theory calculations indicate that the Mn-Mn exchange is mediated by spin-polarized gold $5d$ and $6p$ electrons. The distance dependence shows that it is ferromagnetic or zero for the first ten shells of Mn neighbors out to 1.041 nm (64 atoms), and very weak and oscillatory thereafter.

We report on the theoretical investigation of the topological properties of a periodically quenched one-dimensional dimerized lattice where a piecewise constant Hamiltonian switches from $h_1$ to $h_2$ at a partition time $t_p$ within each driving period $T$. We examine different dimerization patterns for $h_1$ and $h_2$ and the interplay with the driving parameters that lead to the emergence of topological states both at zero energy and at the edge of the Brillouin-Floquet quasienergy zone. We illustrate different phenomena, including the occurrence of both edge states in a semimetal spectrum, the topological transitions, and the generation of zero-energy topological states from trivial snapshots. The role of the different symmetries in our results is also discussed.

Cerium-based intermetallics are currently attracting much attention as highly promising alternatives to conventional permanent magnets that contain a scarce rare earth element like neodymium. In the present work we apply a charge fully self-consistent approach combining density functional theory and dynamical mean-field theory (DFT $+$ DMFT) to investigate the magnetization and electronic structure of the ${\mathrm{CeCo}}_5$ system. We treat simultaneously the correlation effects in Ce-$4f$ and Co-$3d$ orbitals while taking the spin polarization into account. The calculated magnetic moment corresponding to the Ce-$4f$ shell using the DFT $+$ DMFT method is found to be drastically reduced as compared to the DFT results. Moreover, the Ce-$4f$ valence state fluctuations are evaluated and compared within ${\mathrm{CeCo}}_5$ and $\mathrm{Ce}{\left({\mathrm{Co}}_{0.8}{\mathrm{Cu}}_{0.2}\right)}_5$ on account of the trivalent Ce in ${\mathrm{CeCu}}_5$. Regarding the Cu substitutions at two Wyckoff positions of Co (2c and 3g), the substitution at 3g sites slightly enhances the magnetic moment of Ce while the substitution at 2c sites leads to nearly vanishing Ce and Co moments. Such a contrast may contribute to the experimentally reported nonmonotonic change of the magnetic anisotropy with increasing Cu alloying content in $\mathrm{Ce}{\left({\mathrm{Co}}_{1-x}{\mathrm{Cu}}_x\right)}_5$ alloys.