The Heyd-Scuseria-Ernzerhof (HSE) hybrid functional has become a widely used tool for theoretical calculations of point defects in semiconductors. It generally offers a satisfactory qualitative description of defect properties, including the donor/acceptor nature of defects, lowest energy charge states, thermodynamic and optical transition levels, Franck-Condon shifts, photoluminescence (PL) band shapes, and carrier capture cross sections. However, there are noticeable quantitative discrepancies in these properties when compared to experimental results. Some of these discrepancies arise from the presence of self-interaction in various parametrizations of the HSE. Other errors are due to the use of the periodic boundary conditions. In this study, we demonstrate that the error correction scheme based on extrapolation to the dilute limit effectively eliminates the errors due to artificial electrostatic interactions of periodic images and interactions due to the defect state delocalization. This yields parametrizations of HSE that satisfy the generalized Koopmans' condition, essentially eliminating self-interaction from defect state orbitals. We apply this HSE Koopmans tuning individually to a range of cation site acceptors in GaN (${\mathrm{Be}}_{\mathrm{Ga}}, {\mathrm{Mg}}_{\mathrm{Ga}}, {\mathrm{Zn}}_{\mathrm{Ga}}, {\mathrm{Ca}}_{\mathrm{Ga}}, {\mathrm{Cd}}_{\mathrm{Ga}}$, and ${\mathrm{Hg}}_{\mathrm{Ga}}$) and compare the HSE results with experimental data from PL spectra. The Koopmans-compliant HSE calculations show a significantly improved quantitative agreement with the experiment.

A universal topology is predicted for nodal-ring bands formed in planar honeycomb lattices through tight-binding models and topological quantum chemistry theories. In planar honeycomb lattices with $s$ and $p$ orbitals, various nodal-ring bands can occur due to the in-plane mirror symmetry. The band gaps of the nodal rings, opened by spin-orbit coupling, are all found being in quantum spin Hall insulating or topological crystalline insulating states. Two types of the intrinsic topologic states are observed in thallene with first-principles calculations, which has been fabricated in experiments. Our findings provide new routes to generate topologically nontrivial states in honeycomb lattices.

We report on the quantum statistical properties of organic polariton condensates. Our experimental study demonstrates low particle number fluctuations for highly populated condensates at room temperature. We show a quantum noise of the condensate that is $\sim 100$ times the shot-noise limit of an ideal coherent light source, setting the lower limit of the noise for organic polariton devices. Despite the high thermal fluctuations at room temperature, strong dynamical instability, and static disorder intrinsic to organic systems, the condensate exhibits an extremely high degree of second-order coherence $g^{\left(2\right)}(\tau =0)$ down to $1.00034\pm 1\times {10}^{-5}$. We elucidate a nontrivial behavior in the second-order coherence of the condensate with its spatial extension. Unlike a conventional laser, the organic polariton condensate tends to increase its noise level above shot-noise fluctuations due to a complex interplay between polariton thermalization, cavity lifetime, and kinetic losses originating from the quantum fluid properties of polariton condensates.

We report a theoretical study of the effects of electronic correlations, magnetic properties, and chemical bonding in the recently synthesized high-pressure orthorhombic phase of ${\mathrm{FeN}}_2$ using the density functional theory plus dynamical mean-field theory approach. Our analysis documents a complex crystal-chemical behavior of ${\mathrm{FeN}}_2$ characterized by the formation of a strongly covalent N-N bond with an unexpected valence state of Fe ions $3+$ (paramagnetic ferric ${\mathrm{Fe}}^{3+}$ ions in the low-spin state), in agreement with available experimental data. Our results reveal weak (orbital-dependent) correlation effects, which are complicated by the possible emergence of multiple spin density wave states on a microscopic level. This suggests the importance of antiferromagnetic spin fluctuations to explain the properties of ${\mathrm{FeN}}_2$ under pressure.

We study the dc photocurrent induced by linearly polarized light in a multiband Dirac-electron system, focusing on the organic conductor $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$. Utilizing perturbation theory, we predict the dependence of shift current on the frequency of light in photodriven $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$. Our findings demonstrate a strong correlation between the frequency of light and both the magnitude and direction of the shift current. Furthermore, we delve into the nonperturbative effects of nonlinear optical responses using Floquet theory and demonstrate how the sign of the optical response changes with increasing light intensity. Our results unveil remarkable optical phenomena in the multiband Dirac-electron system and are anticipated to be observed in future experiments.

Novel intermetallic ${\mathrm{BaFe}}_2{\mathrm{Al}}_9$ has been of current interest due to indications of the charge-density wave (CDW) behavior associated with the structural phase transition. To further shed light on the nature of the phase transition, we carried out a detailed study by means of the specific-heat and thermoelectric measurements on single-crystalline specimens, mainly focusing on the signatures around the transition temperature $T_{\mathrm{C}}\simeq 100$ K. For comparison, we investigated the isostructural compounds of ${\mathrm{BaCo}}_2{\mathrm{Al}}_9$ and ${\mathrm{SrCo}}_2{\mathrm{Al}}_9$, which showed no indications of the phase transition. While the measured physical quantities in both cobalt-based aluminides varied smoothly with temperature, those in ${\mathrm{BaFe}}_2{\mathrm{Al}}_9$ exhibited profound features with noticeable thermal hysteretic behavior in the vicinity of $T_{\mathrm{C}}$. Based on the observed sudden enhancement in the Seebeck coefficient below $T_{\mathrm{C}}$, we provide evidence for a decreased contribution from the $n$-type carriers undergoing the phase transition. The analysis of the thermal conductivity further indicated that both electronic and phononic origins were responsible for the drastic reduction in the observed thermal conductivity at $T_{\mathrm{C}}$. We thus highlighted that the strong interplay between electronic and structural degrees of freedom gave rise to the peculiar phase transition in ${\mathrm{BaFe}}_2{\mathrm{Al}}_9$. Moreover, we compared the observed signatures with those reported in various CDW systems, and revealed important aspects of the emergence of CDW behavior in ${\mathrm{BaFe}}_2{\mathrm{Al}}_9$.

We study direct current (DC) generation induced by microwave irradiation to ferroelectric materials. The DC generation originates from microwave absorption called dielectric loss due to the delay of a dielectric response. Such current generation can be formulated as the low-frequency limit of the phonon shift current which arises from an increase of electric polarization accompanying photoexcitation of phonons due to the electron-phonon coupling. To study the DC generation by the dielectric loss, we apply the diagrammatic treatment of nonlinear optical responses to photoexcitations of phonons and derive the general formula for phonon shift current. We then study the DC generation in the low-frequency region and find that the current scales as $\propto {\omega }^2$ for the linearly polarized light and time-reversal symmetric systems. We estimate the order of magnitude of the DC generation by dielectric loss, indicating its feasibility for experimental detection in the GHz region.

Efficient manipulation of the optical response of two-dimensional (2D) semiconductors, dominated by excitons due to strong electron-hole Coulomb attraction, could give rise to both discoveries of emergent phenomena and implementation of optoelectronic devices. We propose a manipulation of the optical response of 2D semiconductors through combining magnetic proximity effects and strain, using a Janus type of monolayer (ML) transition-metal dichalcogenides (TMDs) for numerical illustration. Based on an effective Hamiltonian of a strained ML MoSSe on a magnetic substrate and a Bethe-Salpeter equation for an accurate description of excitons, we predict a mergence of $A$ and $B$ excitons at a critical strength of in-plane magnetic exchange field. Additionally, an enhanced tunability of the optical absorption spectra is demonstrated via the combination of magnetic exchange fields and strain. Our approach could be generally extended to other 2D semiconductors subject to magnetic proximity effects and strain.

We present impelling evidence of topological phase transitions induced by electron-phonon (e-ph) coupling in an $\alpha \text{−}{T}_3$ Haldane-Holstein model that facilitates smooth tunability between graphene ($\alpha =0$) and a dice lattice $(\alpha =1)$. The e-ph coupling has been incorporated via the Lang-Firsov transformation which adequately captures the polaron physics in the high-frequency (anti-adiabatic) regime, and yields an effective Hamiltonian through zero phonon averaging at $T=0$. While exploring the signature of phase transitions driven by polaron and its interplay with the parameter $\alpha$, we identify two regions based on the values of $\alpha$, namely, the low to intermediate range $(0<\alpha \le 0.6)$ and larger values of $\alpha (0.6<\alpha <1)$, where the topological transitions host distinct behavior. There exists a single critical e-ph coupling strength for the former, below which the system behaves as a topological insulator characterized by edge modes, finite Chern number, and Hall conductivity, with all of them vanishing above this value, and the system undergoes a spectral gap closing transition. Further, the critical coupling strength depends upon $\alpha$. For the latter case $(0.6<\alpha <1)$, the scenario is more interesting where there are two critical values of the e-ph coupling at which trivial-topological-trivial and topological-topological-trivial phase transitions occur. Our study shows a significant difference with regard to the well-known unique transition occurring at $\alpha =0.5$ (or at 0.7) in the absence of the e-ph coupling, and thus underscores the importance of interaction effects on topological phase transitions.

Ex-so-tic van der Waals heterostructures take advantage of electrically tunable layer polarization to swap proximity exchange and spin-orbit coupling in the electronically active region. Perhaps the simplest example is Bernal bilayer graphene (BBG) encapsulated by a layered magnet from one side and a strong spin-orbit material from the other. Taking ${\mathrm{WS}}_2$/BBG/${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ as a representative ex-so-tronic device, we employ realistic ab initio inspired Hamiltonians and effective electron-electron interactions to investigate the emergence of correlated phases within the random phase approximation. We find that exchange and spin-orbit coupling induced Stoner and intervalley coherence instabilities can be swapped for a given doping level, allowing one to explore the full spectrum of correlated phases within a single device.

Honeycomb-lattice antiferromagnets have attracted wide attention for the exploration of exotic heat transport and their interplay with magnetic excitations. In this work, we have revealed a contrasting behavior in the magnetothermal conductivity (MTC) between two Co-based honeycomb-lattice magnets ${\mathrm{Co}}_4{\mathcal{M}}_2{\mathrm{O}}_9$ ($\mathcal{M}=\mathrm{Nb},\mathrm{Ta}$), despite their identical lattice structures and quite similar magnetism. ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ exhibits enhanced MTC of about $550\%$ at $9\mathrm{T}$ of an in-plane magnetic field, comparable to other honeycomb magnets, while MTC for ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ reaches only $\sim 30\%$. This marked difference is ascribed to distinct features in the field-induced evolution of magnetic excitations that resonantly scatter phonons. This finding sheds light on the implicit impacts of nonmagnetic ions on thermal transport and hints at the potential for broad heat-transport tunability while preserving magnetism and lattice structures.

We show that the coupling of phonons with electrons can have nontrivial consequences to the quantum geometry of an electronic structure, which manifests as oscillations in the Berry curvature dipole and hence have observable nonlinear Hall signatures. Using these, we introduce a vibrational spectroscopy based on the geometry of quantum electronic structure (GQuES) making specific predictions for the transport and radiative GQuES spectra of two-dimensional materials. The selection rule presented here for the GQuES activity of a phonon allows the measurement of acoustic and optic phonons spanning sub-GHz, THz, and infrared frequencies, and is readily generalized to other dynamical excitations. GQuES can be used even for materials having trivial quantum electronic geometry, such as hexagonal boron nitride, through a proximal interaction with substrates such as graphene.

Recent attention to frustrated magnetism has revived interest in most fundamental models of the latter, such as the classical triangular Ising lattice (I-3). Despite the long history, our understanding of the physics of I-3 is still patchy, and even the mean-field magnetic phase diagram has been addressed only in specific cases. Here we present a study considerably more exhaustive than those available in the literature, including arbitrary Heisenberg interaction up to 4th nearest neighbors (NN) identifying all inequivalent magnetic patterns up to the 8th NN (1901 of those), and calculating the phase diagrams in the external magnetic field. We then connect these findings with a real-life material, ${\mathrm{ErGa}}_2$, which our calculations show to be a very good approximation to the I-3, despite its crystallographically 3D character. We calculate, from first principles, the exchange parameters, and show that they reproduce well the observed magnetization steps. We also find a possibility of additional narrow steps, which may be observed in future experiments.

Laser-induced magnetization precession of an in-plane magnetized Pt/Co/Pt film with perpendicular interface anisotropy was studied using time-resolved magneto-optical Kerr effect. An additional $\pi$ shift in the precession phase is needed to describe the measured data if only the demagnetization energy is considered. Based on an augmented microscopic model description of the laser-induced magnetization dynamics, the additional $\pi$ phase is found to be rendered by the dependence on the phonon temperature of the hidden interface anisotropy, in contrast to the dependence on the electron temperature of the demagnetization energy. The observation that the phase of precession is affected by both the electron and the phonon temperature warrants detailed knowledge about the forms of anisotropy present in the system under investigation for a holistic description of laser-induced magnetization precession.

A thorough study of ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ single crystals, which were grown from Sn flux, was performed using both bulk (heat capacity, ac susceptibility, dc magnetization, electrical resistivitivity, magnetoresistance) and microscopic (Mössbauer spectroscopy) techniques. Electrical resistance and magnetic susceptibility were measured also under high pressure conditions (up to 19 and 9.5 GPa, respectively). Further insight into electronic properties and phonons is provided by ab initio calculations. The results indicate that ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ is an antiferromagnet with strong Eu-Eu exchange coupling of ferromagnetic type within the basal plane and weaker antiferromagnetic interaction along the $c$ axis. The Eu magnetic moments are tilted from the basal plane. Hydrostatic pressure strongly affects both magnetic (increase of the Néel temperature) and electronic (suppression of the band gap and semimetallic behavior) properties, indicating a strong interplay of structure with magnetic and electronic degrees of freedom.

Nonlinear optical (NLO) responses of noncentrosymmetric nonmagnets have drawn a lot of attention in the past decades because of their significance in materials characterization, green energy, and device applications. On the other hand, magnetism not only can break the inversion symmetry in centrosymmetric crystals but also introduce additional NLO phenomena in noncentrosymmetric materials, thus enabling the magnetic field control of light-matter interactions. However, the magnetism-induced NLO responses have rarely been studied so far. In this paper, we study the magnetism-induced NLO responses of multiferroic bismuth ferrite (${\mathrm{BiFeO}}_3$) based on density-functional-theory calculations. First, we find that the calculated magnetism-induced second-harmonic generation (SHG) susceptibilities are large and the SHG intensity is tunable with the reversal of magnetization. In particular, the interference between crystallographic SHG and magnetically induced SHG components results in a strong magnetic contrast of the SHG signal of $\sim 440$% at SHG photon energy of 4.82 eV, thus enabling a magnetic control of the SHG in multiferroic ${\mathrm{BiFeO}}_3$. Also, because of the sensitivity of the SHG signal to the direction of the Néel vector, the SHG can be utilized to detect the reversal of the Néel vector in the AFM materials, which is an important issue for AFM spintronics. Second, the calculated bulk photovoltaic effects (BPVE) in ${\mathrm{BiFeO}}_3$ are also strong, being larger than some well-known NLO compounds such as ${\mathrm{BaTiO}}_3$, GaAs, CdS, and CdSe. Finally, we analyze the origins of the prominent features in the NLO response spectra in terms of the calculated quantum geometric quantities. Our interesting findings suggest that the magnetism-driven NLO responses in ${\mathrm{BiFeO}}_3$ are significant, anisotropic, and tunable and that understanding the magnetism-driven components of both SHG and BPVE is essential for their applications in, e.g., multiferroic-based photovoltaic devices and second-harmonic generation.

The amplitude and phase of the third harmonic response in superconductors provide important insights into collective Higgs and Leggett modes, respectively, in the superconducting state. In particular, for twice the incident frequency equal to the binding energy ($2\omega =2\mathrm{\Delta }$), one finds a resonance of the amplitude and a corresponding phase jump in the third harmonic response, respectively. We generalize these concepts to superconductors without an inversion symmetry, which can be effectively described by a two-band model with an order parameter consisting of spin-singlet (even parity) and spin-triplet (odd parity) components. In our work we use an effective action approach for the derivation of the nonlinear response and assign the underlying physical processes to their respective Feynman diagrams. We calculate the third harmonic signal exemplary for the noncentrosymmetric compound ${\mathrm{CePt}}_3\mathrm{Si}$, showing that it contains contributions from three distinguishable sources, namely the Higgs mode, the Leggett mode, and quasiparticles (broken Cooper-pairs). Only in the clean limit diamagnetic Raman-like processes contribute to the third harmonic signal, whereas the quasiparticle contributions dominate the collective modes for all triplet-singlet ratios of the gap structure. In the dirty limit, we find a significant enhancement of the Higgs mode contributions to the third harmonic response, due to the inclusion of nonvanishing paramagnetic diagrams. Finally, we argue that the phase difference between the third harmonic and the fundamental signal might reveal a jump, where its size is dependent on the light-matter coupling.

The structural and electronic transformations of liquid silane as a function of pressure are studied using first-principles molecular dynamics simulations. We present results for two isotherms, 1000 K and 2000 K, in the pressure range from ambient up to 265 GPa. The phases of dense liquid silane bear some similarities with its crystalline phases but there are significant differences as well. Notably, we find the emergence of a mixed polymeric-molecular ${\mathrm{H}}_2$ phase with strong temperature dependence and metallization in the liquid occurring at significantly lower pressure than in the solid. We report dc conductivity, which reaches higher values, and has a different pressure dependence and physical origin than recent analysis of experimental measurements has suggested.