A quantum-kinetic picture of Shockley-Read-Hall-type (SRH) defect-mediated recombination is derived within the nonequilibrium Green's function formalism for an optoelectronic device with selectively contacted, current-carrying extended states and a localized deep defect state in the energy gap. The theory is first tested for recombination from bulk band states and then implemented for defective bipolar homo- and heterojunction thin-film devices with realistic spatial variation of the band edge profile. While the quantum-kinetic treatment reproduces the semiclassical characteristics for a bulk absorber in flat-band and quasiequilibrium conditions, for which the conventional SRH picture is valid, it reveals nonclassical features such as recombination enhancement by tunneling into field-induced subgap states in the presence of large fields, or the complex recombination current flow at heterointerfaces. Being fully compatible with the rigorous treatment of electron-photon and electron-phonon scattering in the nonequilibrium Green's function (NEGF) formalism, the approach enables a consistent inclusion of defect-mediated nonradiative recombination in comprehensive NEGF simulations of nanostructure-based quantum optoelectronic devices such as quantum well lasers, LEDs and solar cells.

In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely used classical molecular dynamics force fields. We present a modification of the embedded atom method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This “density readjusting EAM” (DR-EAM) preserves nearly all of the useful qualities of traditional EAM, including bulk elastic properties and surface energies. However, it also allows valence electron density to migrate through the metal in response to external perturbations. We show that DR-EAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DR-EAM also captures some of the behavior of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. We further show that it predicts charge transfer between the constituent atoms in alloys, leading to novel predictions about unit cell geometries in layered $\mathrm{L}{1}_0$ structures.

We study the critical behavior of dc magnetization in the uranium ferromagnet URhSi around the paramagnetic to ferromagnetic phase transition at $T_{\mathrm{C}}\sim 10$ K with a modified Arrott plot, a Kouvel-Fisher plot, the critical isotherm analysis, and the scaling analysis. URhSi is isostructural to uranium ferromagnetic superconductors URhGe and UCoGe. The critical exponent $\beta$ for the temperature dependence of the spontaneous magnetization below $T_{\mathrm{C}}$, $\gamma$ for the magnetic susceptibility, and $\delta$ for the magnetic isotherm at $T_{\mathrm{C}}$ in URhSi have been determined as $\beta =0.300\pm 0.002$, $\gamma =1.00\pm 0.02$, and $\delta =4.38\pm 0.04$ by the scaling analysis and the critical isotherm analysis. These critical exponents fulfill the Widom scaling law $\delta =1+\gamma /\beta$. Magnetization has strong uniaxial magnetic anisotropy in the ferromagnetic state of URhSi. However, the universality class of the ferromagnetic transition does not belong to the three-dimensional Ising system with short-range exchange interactions between magnetic moments ($\beta =0.325$, $\gamma =1.241$, and $\delta =4.82$). The obtained exponents in URhSi are similar to those in the uranium ferromagnetic superconductors ${\mathrm{UGe}}_2$ and URhGe, and uranium ferromagnets UIr and $\mathrm{U}({\mathrm{Co}}_{0.98}{\mathrm{Os}}_{0.02}$)Al. We have previously reported the unconventional critical behavior of magnetization in the uranium ferromagnetic superconductors [N. Tateiwa et al., Phys. Rev. B 89, 064420 (2014)]. The universality class of the ferromagnetic transition in URhSi may belong to the same one in the uranium ferromagnetic superconductors and the uranium ferromagnets. The unconventional critical behavior of the magnetization in the uranium compounds cannot be understood with previous theoretical interpretations of critical phenomena. The absence of the superconductivity in URhSi is discussed from several viewpoints. The improvement of the sample quality in URhSi could provide a good opportunity to gain a deeper understanding of the ferromagnetic superconductivity in the uranium ferromagnets.

We present a model compound for the $S=1/2$ ferromagnetic Heisenberg chain composed of the verdazyl-based complex $\left[\mathrm{Zn}{\left(\mathrm{hfac}\right)}_2\right][4-\mathrm{Cl}-o-\mathrm{Py}-\mathrm{V}-{(4-\mathrm{F})}_2]$. $Abinitio$ MO calculations indicate a predominant ferromagnetic interaction forming an $S=1/2$ ferromagnetic chain. The magnetic susceptibility and specific heat indicate a phase transition to an antiferromagnetic (AF) order due to the finite interchain couplings. We explain the magnetic susceptibility and magnetization curve above the phase-transition temperature based on the $S=1/2$ ferromagnetic Heisenberg chain. The magnetization curve in the ordered phase is described by a conventional AF two-sublattice model. Furthermore, the obtained magnetic specific heat reproduces the almost temperature-independent behavior of the $S=1/2$ ferromagnetic Heisenberg chain. In the low-temperature region, the magnetic specific heat exhibits a $\sqrt{T}$ dependence, which is attributed to the energy dispersion in the ferromagnetic chain.

We demonstrate that numerical linked cluster expansions (NLCE) yield a powerful approach to calculate time-dependent correlation functions for quantum many-body systems in one dimension. As a paradigmatic example, we study the dynamics of the spin current in the spin-$1/2 XXZ$ chain for different values of anisotropy, as well as in the presence of an integrability-breaking next-nearest neighbor interaction. For short to intermediate time scales, we unveil that NLCE yields a convergence towards the thermodynamic limit already for small cluster sizes, which is much faster than in direct calculations of the autocorrelation function for systems with open or periodic boundary conditions. Most importantly, we show that the range of accessible cluster sizes in NLCE can be extended by evaluating the contributions of larger clusters by means of a pure-state approach based on the concept of dynamical quantum typicality (DQT). Even for moderate computational effort, this combination of DQT and NLCE provides a competitive alternative to existing state-of-the-art techniques, which may be applied in higher dimensions as well.

The heavy fermion systems $\mathrm{Ce}M{\mathrm{In}}_5$ with $M=\mathrm{Co}$, Rh, Ir, the “115” family, provide a prototypical example of strange superconductivity with unconventional $d$-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductivity and its link to antiferromagnetic quantum criticality of the strange metal state are still long-standing open issues. We propose a microscopic mechanism for strange superconductors, based on the coexistence and competition between the Kondo correlation and the quasi-$2d$ short-ranged antiferromagnetic resonating-valence-bond spin-liquid near the antiferromagnetic quantum critical point via a large-$N$ Kondo-Heisenberg model and renormalization group analysis beyond the mean-field level. The interplay of these two effects provides a qualitative understanding on how superconductivity emerges from the strange metal state and the observed superconducting phase diagrams for $\mathrm{Ce}M{\mathrm{In}}_5$.

We theoretically investigate which experimental observations enable discrimination between thermal and nonthermal melting in femtosecond laser pulse-irradiated semiconductors. We identify that coherent phonon excitation, visible in the oscillations of various diffraction peaks, provides an opportunity to observe ongoing modifications of interatomic potential. Decoupling the effects of anharmonicity, caused by thermal heating, from the effects of evolution of the interatomic potential, due to electronic excitation, potentially enables differentiation between the two damage channels.

By using the dyadic Green's matrix spectral method, we demonstrate that aperiodic deterministic Vogel spirals made of electric dipoles support a light localization transition in three dimensions, an effect that does not occur in traditional uniform random media. We discover a light localization transition in Vogel spiral arrays embedded in three-dimensional space by evaluating the Thouless conductance, the level spacing statistics, and by performing a finite-size scaling. We probe light localization in the plane of the array by analyzing the behavior of the scattering resonances in three-dimensional space. This light localization transition is different from the Anderson transition because Vogel spirals are aperiodic deterministic structures characterized by nonuniform geometries. Moreover, this transition occurs when the vector character of light is fully taken into account, in contrast to what is expected for traditional uniform random media of pointlike scatterers. We show that light localization in Vogel arrays is a collective phenomenon that involves the contribution of multiple length scales. Vogel spirals are suitable photonic platforms to localize light thanks to their distinctive structural correlation properties that enable collective electromagnetic excitations with strong light-matter coupling. Our results unveil the importance of aperiodic correlations for the engineering of photonic media with strongly enhanced light-matter coupling compared to the traditional periodic and homogeneous random media.

We construct a class of period-$n$-tupling discrete time crystals based on ${\mathbb{Z}}_n$ clock variables, for all the integers $n$. We consider two classes of systems where this phenomenology occurs: disordered models with short-range interactions and fully connected models. In the case of short-range models, we provide a complete classification of time-crystal phases for generic $n$. For the specific cases of $n=3$ and $n=4$, we study in detail the dynamics by means of exact diagonalization. In both cases, through an extensive analysis of the Floquet spectrum, we are able to fully map the phase diagram. In the case of infinite-range models, the mapping onto an effective bosonic Hamiltonian allows us to investigate the scaling to the thermodynamic limit. After a general discussion of the problem, we focus on $n=3$ and $n=4$, representative examples of the generic behavior. Remarkably, for $n=4$ we find clear evidence of a crystal-to-crystal transition between period $n$-tupling and period $n/2$-tupling.

Using high resolution resonant inelastic x-ray scattering measurements, we have observed that the orbital excitations of the quasi-1D spin chain compound ${\mathrm{CuGeO}}_3$ has nontrivial and noticeable orbital mixing effects from $3d$ valence spin-orbit coupling. In particular, the SOC leads to a significant correction of $d_{z^2}$ state, which has a direct interplay with the low energy physics of cuprates. Guided by atomic multiplet based modeling, our results strongly support a $3d$ spin-orbit mixing scenario and explore in detail the nature of these excitations.

In the semiclassical regime, the chiral anomaly of the Weyl semimetals is demonstrated by the quadratic longitudinal magnetoconductivity. Recently the planar Hall effect is proposed in the same systems, where the Hall conductivity depends quadratically on the magnetic field. If the Weyl cones are tilted, both the chiral chemical potential and the anomalous velocity contribute a linear term to the conductivities. We investigate the electronic transport of a tilted Weyl semimetal in the presence of an in-plane magnetic field. The results show that both the longitudinal and the Hall conductivities contain a linear term in magnetic field besides the quadratic terms and the angular dependence of them is different from the case without tilting. The correction of the phase space volume factor is also studied, which shows effects on both the angular dependence as well as the amplitudes of the conductivities. The angular dependence of the conductivities, showing the signature of the planar Hall effect, is experimentally verifiable.

In this work, we study interacting electrons on a square lattice in the presence of strong Rashba spin-orbit interaction. The spin-orbit term forces the time-reversal electron states to be paired in even Cooper channels. For concreteness, we only consider the repulsive onsite Hubbard and nearest-neighbor Coulomb interactions, the so-called extended Hubbard model. To examine the superconducting instability, we obtain the effective interaction between electrons within the random phase approximation and treat the pairing instabilities driven by charge and spin fluctuations and their combined effects. We mapped out the phase diagram of the model in terms of interactions and electron fillings and found that while the $d_{xy}$ and $d_{x^2-y^2}$ symmetries are the most likely pairing symmetries driven by charge and spin fluctuations, respectively, the strong effect of both fluctuations yields higher angular momentum Cooper instabilities. The possibility of topological superconductivity and triplet pairing is also discussed.

The optically induced long-lived spin polarization in the bulk diluted magnetic semiconductor (Cd,Mn)Te with small manganese concentration is studied by picosecond pump-probe Kerr rotation. At temperatures below 6 K and in transversal magnetic field the Kerr rotation signal contains three components: two oscillating components, corresponding to the Larmor precession of manganese spins and spins of photoexcited electrons, and a long-lived (up to 15 ns) nonoscillating component. The latter one is provided by optical orientation of equilibrium hole magnetic polarons involving holes bound to acceptors. The origin of the anisotropy controlling the orientation and the spin dynamics of the acceptor-bound hole magnetic polaron in bulk (Cd,Mn)Te is discussed.

In this work, the resonant coupling behaviors between the transverse-electric (TE) and transverse-magnetic (TM) Bloch surface waves (BSWs) on a dielectric multilayer have been theoretically studied. Due to the different penetration depths in the dielectric multilayer, the TM BSWs and TE BSWs can act as the radiative and dark electromagnetic modes, respectively. By using a rectangular grating on the dielectric multilayer, both Rabi splitting and Fano resonance phenomena based on the coupling of the two BSW modes were demonstrated, through tuning the period of the grating and the azimuthal angle of the incoming wave. Furthermore, by using the temporal coupled-mode theory, we show that the anti-Hermitian coupling between the two BSW modes contributes to the enhanced diffraction and the huge polarization transformation efficiency of incoming waves in the weak coupling regime.

An effective tight-binding (TB) Hamiltonian for monolayer ${\mathrm{GeSnH}}_2$ is constructed which has an inversion-asymmetric honeycomb structure. The low-energy band structure of our TB model agrees very well with previous ab initio calculations even under biaxial tensile strain. Our model predicts a phase transition at 7.5% biaxial tensile strain in agreement with DFT calculations. Upon 8.5% strain the system exhibits a band gap of 134 meV, suitable for room temperature applications. It is shown that an external applied magnetic field produces a special phase which is a combination of the quantum Hall (QH) and quantum spin Hall (QSH) phases; and at a critical magnetic field strength the QSH phase completely disappears. The topological nature of the phase transition is confirmed from: (1) the calculation of the ${\mathbb{Z}}_2$ topological invariant, and (2) quantum transport properties of disordered ${\mathrm{GeSnH}}_2$ nanoribbons which allows us to determine the universality class of the conductance fluctuations. The application of an external applied magnetic field reduces the conductance fluctuations by a factor of $\sqrt{2}$.

Flat electronic bands can accommodate a plethora of interaction-driven quantum phases, since kinetic energy is quenched therein and electronic interactions therefore prevail. Twisted bilayer graphene, near the so-called “magic angles”, features slow Dirac fermions close to the charge-neutrality point that persist up to high energies. Starting from a continuum model of slow but strongly interacting Dirac fermions, we show that with increasing chemical doping away from the charge-neutrality point, a time-reversal symmetry breaking, valley pseudospin-triplet, topological $p+ip$ superconductor gradually sets in, when the system resides at the brink of an antiferromagnetic ordering (due to Hubbard repulsion), in qualitative agreement with recent experimental findings. The $p+ip$ paired state exhibits quantized spin and thermal Hall conductivities, and polar Kerr and Faraday rotations. Our conclusions should also be applicable for other correlated two-dimensional Dirac materials.

Gallium metal possesses a complex phase diagram and it has been the subject of many experimental and theoretical studies. Nevertheless, hyperfine properties of its phases requiring higher pressure beyond the liquid-I-II triple point were seldom examined. In this work, hyperfine parameters of liquid and solid gallium metal under pressure are investigated by nuclear magnetic resonance (NMR) measurements and ab initio calculations. The electric field gradient and NMR shift of the Ga-III phase are both measured and calculated and their relation to electronic structure is interpreted. Further, calculations of pressure dependencies of the hyperfine parameters of several other solid gallium phases are presented.

Topological phases of matter are defined by their nontrivial patterns of ground-state quantum entanglement, which is irremovable so long as the excitation gap and the protecting symmetries, if any, are maintained. Recent studies on noninteracting electrons in crystals have unveiled a peculiar variety of topological phases, which harbor nontrivial entanglement that can be dissolved simply by the the addition of entanglement-free, but charged, degrees of freedom. Such topological phases have a weaker sense of robustness than their conventional counterparts, and are therefore dubbed “fragile topological phases.” In this work, we show that fragile topology is a general concept prevailing beyond systems of noninteracting electrons. Fragile topological phases can generally occur when a system has a $\mathrm{U}\left(1\right)$ charge conservation symmetry, such that only particles with one sign of the charge are physically allowed (e.g., electrons but not positrons). We demonstrate that fragile topological phases exist in interacting systems of both fermions and of bosons.

The Monte Carlo method is very useful for studying various model states proposed for the fractional quantum Hall systems. In this paper, we introduce a lattice Monte Carlo method based on an exact lattice formalism for quantum Hall problems defined on a torus. This “lattice representation” is applied to study the many body Berry phase of the composite-Fermi liquid phase in a half-filled Landau level. The Monte Carlo result agrees with the exact numerical result found on small systems sizes and is consistent with the prediction from the Dirac fermion low-energy effective theory. Many other aspects of other model states, including the structure factor, Coulomb energy and particle-hole symmetry, are studied and discussed.