Interfacial thermal conduction is traditionally described by the transmission of traveling phonons in their bulk counterparts. However, recent experiments suggest that localized phonons at an interface may exert a substantial influence on interfacial thermal conductance. By considering quantum phonon scattering processes at a weakly interacting interface, we analyze the role of localized phonons at the interface involved in the interfacial thermal conduction and discover that there is a coupling between traveling phonons and the even orders of the localized phonons at the interface. Using fourth-order localized phonons as an example, we show that the traveling phonons can collide with the localized phonons at the interface. Such collisions can significantly change the transmission probability of the traveling phonons and thus affect the interfacial thermal conductance, and importantly, such effects become increasingly significant with increasing temperature.

Recently, Lee et al. [Nano Lett. 21, 4305 (2021)] synthesized the monochalcogenide GeSe in a polar phase, referred to as the $\gamma$-phase. Motivated by this work, we study the shift current of $\gamma$-GeSe and its tunability via an in-plane uniaxial strain. Using first-principles calculations, we uncover the electronic structure of strained $\gamma$-GeSe systems. We then calculate the frequency-dependent shift current conductivities at various strains. The tunability increases the shift current to $\sim 40$ $\mu {\mathrm{A/V}}^2$ for visible light. Moreover, the direction of the shift current can be inverted by a light strain. Markedly, noticeable behavior is found in the zero-frequency limit, which can be indicative of band inversion and electronic phase transition driven by the strain. Our results suggest that the shift current can be tangible proof of bulk electronic states of $\gamma$-GeSe.

Nonreciprocity in superconductors is attracting much interest owing to its fundamental importance as well as the potential applicability to engineering. In this paper, we generalize the previous theories of the intrinsic superconducting diode effect (SDE) and microscopically elucidate its relationship with the nonreciprocity of the transition lines under supercurrent. We derive a general formula for the intrinsic SDE by using the phenomenological Ginzburg-Landau theory and thereby show that the SDE is determined by the relative angle between the magnetic field and an effective anti-symmetric spin-orbit coupling defined from the Ginzburg-Landau coefficients. The obtained formula offers a convenient criterion to obtain a finite SDE. We also study the SDE and the nonreciprocal phase transitions of the $s$-wave and $d$-wave superconductors by using the mean-field theory. It is established that the sign reversal of the SDE accompanied by the crossover of the helical superconductivity is a general feature irrespective of the system details. We study the phase transition lines in the temperature-magnetic-field phase diagram under the supercurrent, and clarify that the sign reversal of the SDE generally accompanies the crossings of the transition lines under positive and negative current directions. Furthermore, the superconducting phases under the supercurrent even become re-entrant under moderate strength of the electric current, implying the current-induced first-order phase transitions. Our findings establish the electric current as the control parameter and the powerful probe to study the superconducting properties related to the finite-momentum Cooper pairs.

We propose a novel heterostructure to achieve chiral topological superconductivity in two dimensions. A substrate with a large Rashba spin-orbit coupling energy is brought in proximity to a twisted bilayer of thin films exfoliated from a high-temperature cuprate superconductor. The combined system is then exposed to an out-of-plane magnetic field. The rare $d+id$ pairing symmetry expected to occur in such a system allows for nontrivial topology; specifically, in contrast to the case of the twisted bilayer in isolation, the substrate induces an odd Chern number. The resulting phase is characterized by the presence of a Majorana zero mode in each vortex.

A spectral neighbor analysis (SNAP) machine learning interatomic potential (MLIP) has been developed for simulations of carbon at extreme pressures (up to $5\mathrm{TPa}$) and temperatures (up to 20 000 K). This was achieved using a large database of experimentally relevant quantum molecular dynamics (QMD) data, training the SNAP potential using a robust machine learning methodology, and performing extensive validation against QMD and experimental data. The resultant carbon MLIP demonstrates unprecedented accuracy and transferability in predicting the carbon phase diagram, melting curves of crystalline phases, and the shock Hugoniot, all within 3% of QMD. By achieving quantum accuracy and efficient implementation on leadership-class high-performance computing systems, SNAP advances frontiers of classical MD simulations by enabling atomic-scale insights at experimental time and length scales.

We augment the time-linear formulation of the Kadanoff-Baym equations for systems of interacting electrons and quantized phonons or photons with the $G\tilde{W}$ approximation, the Coulomb interaction $\tilde{W}$ being dynamically screened by both electron-hole pairs and bosonic particles. We also show how to combine different approximations to include simultaneously multiple correlation effects in the dynamics. The final outcome is a versatile framework comprising $2^{12}$ distinct diagrammatic methods, each scaling linearly in time and preserving all fundamental conservation laws. The dramatic improvement over current state-of-the-art approximations brought about by $G\tilde{W}$ is demonstrated in a study of the correlation-induced charge migration of the glycine molecule in an optical cavity.

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft x-ray pulses from the free electron laser FLASH. We collect Se $3d$ photoelectron diffraction patterns over a wide angular range from optically excited $\mathrm{Bi}{}_2\mathrm{Se}{}_3$ with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the $A_{1g}$ optical phonons. Using a fluence of $4.2\mathrm{mJ}/{\mathrm{cm}}^2$ from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 0.01 Å.

Beryllium oxide (BeO) is a promising host for quantum defects because of its ultrawide band gap. We conducted comprehensive first-principles investigations of the native point defects in BeO using density functional theory with a hybrid functional. We found that the beryllium and oxygen vacancies are the most stable defects, whereas other native defects such as interstitials or antisites have high formation energies. We investigate the point defects as candidates for quantum defects by examining spin states and internal optical transitions. The oxygen vacancy ($V_{\mathrm{O}}^{+}$) emerges as a suitable spin qubit or single-photon emitter; we also find its stability can be enhanced by forming a ${\left(V_{\mathrm{O}}\text{−}{\mathrm{Li}}_{\mathrm{Be}}\right)}^0$ complex with a Li acceptor. The ${\mathrm{O}}_{\mathrm{Be}}^{-}$ antisite also has desirable optical and spin properties. Overall, because of its desirable properties as a host material, BeO could be an excellent host for quantum defects, with $V_{\mathrm{O}}^{+}$, ${\left(V_{\mathrm{O}}\text{−}{\mathrm{Li}}_{\mathrm{Be}}\right)}^0$, and ${\mathrm{O}}_{\mathrm{Be}}^{-}$ as prime candidates.

Standard superconductors display a ubiquitous discontinuous jump in the electronic specific heat at the critical superconducting transition temperature. In a growing class of unconventional superconductors, however, a second order parameter component may get stabilized and produce a second heat capacity jump at a lower temperature, typically associated with the spontaneous breaking of time-reversal symmetry. The splitting of the two specific heat discontinuities can be controlled by external perturbations such as chemical substitution, hydrostatic pressure, or uniaxial strain. We develop a theoretical quantitative multiband framework to determine the ratio of the heat capacity jumps, given the band structure and the order parameter momentum structure. We discuss the conditions of the gap profile which determine the amplitude of the second jump. We apply our formalism to the case of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, and using the gap functions from a microscopic random phase approximation calculation, we show that recently proposed accidentally degenerate order parameters may exhibit a strongly suppressed second heat capacity jump. We discuss the origin of this result and consider also the role of spatial inhomogeneity on the specific heat. Our results provide a possible explanation of why a second heat capacity jump has so far evaded experimental detection in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Van der Waals (vdW) layered materials have attracted extensive attention and become a platform for investigating various exotic physical properties of interest. Considering experimentally synthesized vdW layered ${\mathrm{SiTe}}_2$ and monolayer Bi, here we propose a feasible way to obtain topological superconductivity by intercalating monolayer Bi into the bulk ${\mathrm{SiTe}}_2$. Using Allen-Dynes-modified McMillan equation based on the first principles calculations, we predict the bulk ${\mathrm{SiTe}}_2$ to be a superconductor with superconducting transition temperature ${\mathrm{T}}_C$ of about 2.9 K, which could be effectively modulated by hydrostatic and uniaxial pressures. Intercalating a two-dimensional topological insulator Bi into the bulk ${\mathrm{SiTe}}_2$, we find that the system exhibits both superconductivity with ${\mathrm{T}}_C$ of about 1.7 K and topological surface states near the Fermi energy. In addition, the ${\mathrm{T}}_C$ of ${\mathrm{SiTe}}_2$ monolayer remains almost intact when exfoliated from its bulk. Our work paves a new way to realize topological superconductivity by intercalating topological matter into vdW layered materials.

In this paper, we examine the carbon defects associated with peaks of infrared (IR) absorption and Raman scattering appearing in GaN crystals at carbon (${}^{12}\mathrm{C}$) doping in the range of concentrations from $3.2\times {10}^{17}$ to $3.5\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$. Here, 14 unique vibrational modes of defects are observed in GaN samples grown by HVPE (hydride vapor phase epitaxy) and then compared with defect properties predicted from first-principles calculations. The vibrational frequency shift in two ${}^{13}\mathrm{C}$-enriched samples related to the effect of the isotope mass indicates six distinct configurations of the carbon-containing point defects. The effect of the isotope replacement is well reproduced by the density functional theory (DFT) calculations. Specific attention is paid to the most pronounced defects, namely, tricarbon complexes (${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$) and carbon substituting for nitrogen (${\mathrm{C}}_{\mathrm{N}}$). The position of the transition level $\left(+/0\right)$ in the bandgap found for ${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$ defects by DFT at 1.1 eV above the valence band maximum suggests that ${({\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}})}^{+}$ provides compensation of ${\mathrm{C}}_{\mathrm{N}}^{-}$. Here, ${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$ defects are observed to be prominent yet have high formation energies in DFT calculations. Regarding ${\mathrm{C}}_{\mathrm{N}}$ defects, it is shown that the host Ga and N atoms are involved in the delocalized vibrations of the defect and significantly affect the isotopic frequency shift. Much more faint vibrational modes are found from di-atomic carbon-carbon and carbon-hydrogen (C–H) complexes. Also, we note changes of vibrational mode intensities of ${\mathrm{C}}_{\mathrm{N}}, {\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$, C–H, and ${\mathrm{C}}_{\mathrm{N}}–{\mathrm{C}}_i$ defects in the IR absorption spectra upon irradiation in the defect-related ultraviolet/visible absorption range. Finally, it is demonstrated that the resonant enhancement of the Raman process in the range of defect absorption $>2.5\mathrm{eV}$ enables the detection of defects at carbon doping concentrations as low as $3.2\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$.

Recent findings of large anomalous Hall signal in nonferromagnetic and nonferrimagnetic materials suggest that the magnetization of the system is not a critical component for the realization of the anomalous Hall effect (AHE). Here, we present a combined theoretical and experimental study demonstrating the evolution of different mechanisms of AHE in a cubic Heusler system ${\mathrm{MnPt}}_{1-x}{\mathrm{Ir}}_x\mathrm{Sn}$. With the help of magnetization and neutron diffraction studies, we show that the substitution of nonmagnetic Ir in place of Pt significantly reduces the net magnetic moment from 4.17 ${\mu }_B$/f.u. in MnPtSn to 2.78 ${\mu }_B$/f.u. for ${\mathrm{MnPt}}_{0.5}{\mathrm{Ir}}_{0.5}\mathrm{Sn}$. In contrast, the anomalous Hall resistivity is enhanced by nearly three times from 1.6 $\mathrm{\mu }\mathrm{\Omega }$ cm in MnPtSn to about 5 $\mathrm{\mu }\mathrm{\Omega }$ cm for ${\mathrm{MnPt}}_{0.5}{\mathrm{Ir}}_{0.5}\mathrm{Sn}$. The power law analysis of the Hall resistivity data suggests that the extrinsic contribution of AHE that dominates in the case of the parent MnPtSn almost vanishes for ${\mathrm{MnPt}}_{0.5}{\mathrm{Ir}}_{0.5}\mathrm{Sn}$, where the intrinsic mechanism plays the major role. The experimental results are well supported by our theoretical study, which shows a considerable enhancement of the spin-orbit coupling when Ir is introduced into the system. Our finding of a crossover of the anomalous Hall effect with chemical engineering is a major contribution toward the recent interest in controlling the band topology of topological materials, both in bulk and thin-film forms.

We probe the effect on $T_{\text{c}}$ of in-plane uniaxial stress applied to the high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.67}$. We find a highly anisotropic response. Under compression along the $b$ axis, which reduces the orthorhombicity of the ${\mathrm{CuO}}_2$ planes, $T_{\text{c}}$ is broadly flat for stresses up to at least 1.7 GPa. Under compression along the $a$ axis, $T_{\text{c}}$ decreases steeply. For stresses beyond $\approx 1$ GPa the decrease is quasilinear. We hypothesize that superconductivity is suppressed by competition with uniaxial charge density wave order, which has been found to onset at $\approx 1$ GPa.

We present a solution for the nonequilibrium dynamics of an interacting disordered system. The approach adapts the combination of the equilibrium dynamical mean-field theory and the equilibrium coherent potential approximation methods to the nonequilibrium many-body formalism, using the Kadanoff-Baym-Keldysh complex time contour, for the dynamics of interacting disordered systems away from equilibrium. We use our time domain solution to obtain the equilibrium density of states of the disordered interacting system described by the Anderson-Hubbard model, bypassing the necessity for the cumbersome analytical continuation process. We further apply the nonequilibrium solution to the interaction quench problem for an isolated disordered system. Here, the interaction is abruptly changed from zero (noninteracting system) to another constant (finite) value at which it is subsequently kept. We observe via the time dependence of the potential, kinetic, and total energies the effect of disorder on the relaxation of the system as a function of final interaction strength. The real-time approach has the potential to shed light on the fundamental role of disorder in the nonequilibrium dynamics of interacting quantum systems.

We study the axion in three-dimensional topological insulators with magnetic impurities under finite temperature. We find a stable antiferromagnetic ground state and ferromagnetic metastable state. In both magnetic states, the mass of the axion is found to be up to eV scale and it approaches zero near the phase boundary of the magnetic state. This result applies to both normal and topological insulator phases, i.e., the axion mass is insensitive to the topological states and it will have a direct impact on the targeted mass range of particle axion dark matter in future experiments.

Frustration and quantum entanglement are two exotic quantum properties in quantum many-body systems. However, despite several efforts, an exact relationship between them remains elusive. In this work, we explore the relationship between frustration and quantum entanglement in a physical model describing strongly correlated ultracold bosonic atoms in optical lattices. In particular, we consider the one-dimensional Bose-Hubbard model comprising both nearest-neighbor ($t_1$) and frustrated next-nearest neighbor ($t_2$) hoppings and examine how the interplay of on-site interaction ($U$) and hoppings results in different quantum correlations dominating the ground state of the system. We then analyze the behavior of quantum entanglement in the model. In particular, we compute genuine multipartite entanglement as quantified through the generalized geometric measure and make a comparative study with bipartite entanglement and other relevant order parameters. We observe that genuine multipartite entanglement has a very rich behavior throughout the considered parameter regime and frustration does not necessarily favor generating a high amount of it. Moreover, we show that in the region with strong quantum fluctuations, the particles remain highly delocalized in all momentum modes and share a very low amount of both bipartite and multipartite entanglement. Our work illustrates the necessity to give separate attention to dominating ordering behavior and quantum entanglement in the ground state of strongly correlated systems.

Halide perovskites (HPs) are widely viewed as promising photovoltaic and light-emitting materials for their suitable band gaps in the visible spectrum. Density functional theory (DFT) calculations employing (semi)local exchange-correlation functionals usually underestimate the band gaps for these systems. Accurate descriptions of the electronic structures of HPs often demand higher-order levels of theory such as the Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional and $GW$ approximations that are much more computationally expensive than standard DFT. Here, we investigate three representative types of HPs, $AB{X}_3$ halide perovskites, vacancy-ordered double perovskites, and bond disproportionated halide perovskites (BDHPs), using $\mathrm{DFT}+U+V$ with onsite $U$ and intersite $V$ Hubbard parameters computed self-consistently without a priori assumption. The inclusion of Hubbard corrections improves the band gap prediction accuracy for all three types of HPs to a similar level of advanced methods. Moreover, the self-consistent Hubbard $U$ is a meaningful indicator of the true local charge state of multivalence metal atoms in HPs. The inclusion of the intersite Hubbard $V$ is crucial to properly capture the hybridization between valence electrons on neighboring atoms in BDHPs that have breathing-mode distortions of halide octahedra. In particular, the simultaneous convergence of both Hubbard parameters and crystal geometry enables a band gap prediction accuracy superior to HSE for BDHPs but at a fraction of the cost. Our work highlights the importance of using self-consistent Hubbard parameters when dealing with HPs that often possess intricate competitions between onsite localization and intersite hybridization.

Recently, a two-dimensional (2D) orthorhombic silver-organic framework, ${\mathrm{Ag}}_3{\mathrm{C}}_{20}$ monolayer, was synthesized by assembling organic molecules linked with multiple aryl-metal bonds. Using first-principles calculation, herein we demonstrate that, owing to the unique bonding feature, ${\mathrm{Ag}}_3{\mathrm{C}}_{20}$ monolayer not only exhibits strong mechanical anisotropy, but also possesses rich orientation-dependent Dirac states allowing for modulation via external means. Around the Fermi level below, the intrinsic Dirac points form two quasi-type-III nodal lines protected by mirror symmetry, which can further evolve into hybrid nodal loops under tiny strains. Intriguingly, a peculiar semi-Dirac state near the Fermi level above emerges under a critical strain by merging two type-I Dirac cones, which harbors direction-dependent strongly localized fermions, normal massive carries, and ultrafast Dirac fermions at the same time. These findings suggest that the mechanically sensitive ${\mathrm{Ag}}_3{\mathrm{C}}_{20}$ monolayer is a promising 2D material to realize the interesting Dirac physics and multiple carrier transport with high anisotropy.