We study exciton radiative decay in a two-dimensional material, taking into account large thermal population in the nonradiative states, from which excitons are scattered into the radiative states by acoustic phonons. We find an analytical solution of the kinetic equation for the nonequilibrium distribution function of excitons in the radiative states. Our estimates for bright excitons in transition-metal dichalcogenides indicate a strong depletion of radiative state population due to insufficient exciton-phonon scattering rate at low temperatures.

Motivated by the resurgence of electronic and optical property design in ordered fluoride and oxyfluoride compounds, we present a density functional theory (DFT) study of 19 materials with structures, ranging from simple to complex, and variable oxygen-to-fluorine ratios. We focus on understanding the accuracy of the exchange-correlation potentials $\left(V_{xc}\right)$ to DFT for the prediction of structural, electronic, and lattice dynamical properties at four different levels of theory, i.e., the local density approximation (LDA), generalized gradient approximation (GGA), metaGGA, and hybrid functional level which includes fractions of exact exchange. We investigate in detail the metaGGA functionals MS2 [Sun et al., Phys. Rev. Lett. 111, 106401 (2013)] and SCAN [Sun et al., Phys. Rev. Lett. 115, 036402 (2015)], and show that although the metaGGAs show improvements over the LDA and GGA functionals in describing the electronic structure and phonon frequencies, the static structural properties of fluorides and oxyfluorides are often more accurately predicted by the GGA-level Perdew-Burke-Ernzerhof functional for solids, PBEsol. Results from LDA calculations are unsatisfactory for any compound, regardless of oxygen concentration. The PBEsol and Heyd-Scuseria-Ernzerhof (HSE06) functionals give good performance in all-oxide or all-fluoride compounds. For the oxyfluorides, PBEsol is consistently more accurate for structural properties across all oxygen concentrations; however, we stress the need for detailed property assessment with various functionals for oxyfluorides with variable composition. The “best” functional is anticipated to be more strongly dependent on the property of interest. Our study provides useful insights in selecting an $V_{xc}$ that achieves optimal performance compromises, enabling more accurate predictive design of functional fluoride-based materials with density functional theory.

The $\mathrm{Xe}\text{−}{\mathrm{N}}_2$ binary phase diagram was determined at 296 K from the pressure evolution of 14 different concentrations. The properties of $\mathrm{Xe}\text{−}{\mathrm{N}}_2$ mixtures were characterized using visual observation, Raman spectroscopy, and powder x-ray diffraction. Above 4.9 GPa, the $\mathrm{Xe}({\mathrm{N}}_2{)}_2$ van der Waals compound is stable and adopts the $\mathrm{MgC}{\mathrm{u}}_2$-type Laves phase structure $\left(Fd\text{−}3m\right)$ with ${\mathrm{N}}_2$ molecules orientationally disordered. At 10 GPa, this cubic lattice undergoes a martensitic phase transition into a tetragonal $(I{4}_1/amd)$ unit cell. This transition is associated with a partial ordering of the ${\mathrm{N}}_2$ molecules, possibly due to the growing ${\mathrm{N}}_2\text{−}{\mathrm{N}}_2$ quadrupole-quadrupole interaction with density. No other phase transition was detected up to 154 GPa, even after heating the sample to 2000 K. Above 30 GPa, a softening of the ${\mathrm{N}}_2$ vibron mode with pressure reveals a weakening of the ${\mathrm{N}}_2$ intramolecular bond that suggests an electronic redistribution between ${\mathrm{N}}_2\text{−}{\mathrm{N}}_2$ and $\mathrm{Xe}\text{−}{\mathrm{N}}_2$ entities. These interactions could explain the great stability of the $\mathrm{Xe}({\mathrm{N}}_2{)}_2$ compound. However, no xenon nitride was observed.

We study the temperature dependence of the charge density wave (CDW) in a chromium thin film using x-ray diffraction. We exploit the interference between the CDW satellite peaks and Laue oscillations to determine the amplitude, the phase, and the period of the CDW. We find discrete half-integer periods of CDW in the film and switching of the number of periods by one upon cooling/heating with a thermal hysteresis of 20 K. The transition between different CDW periods occurs over a temperature range of 30 K, slightly larger than the width of the thermal hysteresis. A comparison with simulations shows that the phase transition occurs as a variation of the volume fraction of two distinct phases with well-defined periodicities. The phase of the CDW is constant for all temperatures, and we attribute it to strong pinning of the CDW by the mismatch-induced strain at the film-substrate interface.

Sodium silicate glasses exhibit a characteristic luminescence with a maximum at about 3.4 eV, which is thought to be determined by optical excitation of local glass structures, called L centers. To investigate the atomic and electronic structures of these centers, we calculated the electronic properties of the ground and excited states of a sodium silicate glass using classical and ab initio methods. Classical molecular dynamics was used to generate glass models of ${\mathrm{Na}}_2\mathrm{O}-3{\mathrm{SiO}}_2$ molar composition, and the density functional theory (DFT), with hybrid functionals, was used to identify and characterize the geometric and electronic structures of L centers. The ground and excited ${\mathrm{L}}^{*}$ center states are studied, and their calculated excitation and luminescence transition energies are in good agreement with experimental data. The results confirm that the lowest triplet excited states in sodium silicate glass are associated with small clusters of Na ions and nonbridging oxygen atoms. These clusters serve as structural precursors for the localization of the excited states, and the broad distribution of the luminescence energies is correlated with the short-range order of the Na cations. The atomic and electronic structures of the electron ${\mathrm{E}}_1^{-}$ and hole ${\mathrm{H}}_1^{+}$ centers are also studied. These results provide a more detailed insight into the atomistic structure of localized states in these important glasses.

Negative thermal expansion is an uncommon phenomenon of theoretical interest. Multiple hypotheses regarding its microscopic origins have been suggested. In this paper, the thermal expansion of a representative semiconductor, Si, and a representative metal, Ti, are calculated ab initio using density-functional perturbation theory. The phonon modes' contributions to the thermal expansion are analyzed and the negative thermal expansion is shown to be dominated by negative mode Grüneisen parameters at specific points on the Brillouin zone boundaries. Thus, the elastic (Debye) theory for negative thermal expansion is shown to be irrelevant for these phenomena. The anomalous behavior of these modes in Ti is shown to be unaffected by an electronic topological transition as previously suggested, instead it arises from complex interplay of atomic displacements of the anomalous mode.

Three-terminal devices, where the same ferromagnetic electrode is used for electrical spin injection and detection, is a very easy and powerful tool to probe the spin properties in nonmagnetic materials. For instance, it has been intensively used to study spin injection and detection in silicon. However the interpretation of the magnetoresistance signals observed experimentally is still under debate. In particular, a controversy has been raised about the experimental spin signal which is orders of magnitude larger than the predicted value. Recently, Song et al. [Phys. Rev. Lett. 113, 047205 (2014)] proposed that the magnetoresistance signal measured using the Hanle effect in a three-terminal geometry is due to defects or impurities in the tunnel barrier separating the ferromagnetic electrode from the silicon channel. It has also been supported by the experimental work of Txoperena et al. [Phys. Rev. Lett. 113, 146601 (2014)]. In this study, we perform electrical spin injection/detection measurements using three-terminal devices in different silicon films and study the role of defects. For this purpose, we use the tunneling inelastic spectroscopy to measure the Hanle effect and control the presence of defects in the tunnel barrier. Contrary to previous reports, we demonstrate that defects have no significant contribution to the spin signal. From a comparison with capacitance-voltage measurements in $n$-doped germanium in which interface states contribute to the spin signal, we also conclude on the presence of interface states in silicon.

To clarify how the electronic state of ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{RuO}}_3$ evolves with La doping, we conducted photoemission (PES) experiments using soft x rays. The spectral shape of the Ru $4d$-derived peak near the Fermi level changes significantly with increasing $x$. This variation indicates that a spectral weight transfer from the coherent to the incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La $3d_{5/2}$ edge have confirmed that there is no significant contribution of the La $5d$ state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean-free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to $x=0.5$, reflecting that the Ru $4d$ state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{RuO}}_3$ and that the Ru $4d$ electrons possess both local and itinerant characteristics.

The response of nonlinear metamaterials and superconducting electronics to two-tone excitation is critical for understanding their use as low-noise amplifiers and tunable filters. A new setting for such studies is that of metamaterials made of radio frequency superconducting quantum interference devices (rf-SQUIDs). The two-tone response of self-resonant rf-SQUID meta-atoms and metamaterials is studied here via intermodulation (IM) measurement over a broad range of tone frequencies and tone powers. A sharp onset followed by a surprising strongly suppressed IM region near the resonance is observed. Using a two time scale analysis technique, we present an analytical theory that successfully explains our experimental observations. The theory predicts that the IM can be manipulated with tone power, center frequency, frequency difference between the two tones, and temperature. This quantitative understanding potentially allows for the design of rf-SQUID metamaterials with either very low or very high IM response.

For Josephson junctions based on $s$-wave superconductors, time-reversal symmetry is known to allow for powerful relations between the normal-state junction properties, the excitation spectrum, and the Josephson current. Here we provide analogous relations for Josephson junctions involving one-dimensional time-reversal-invariant topological superconductors supporting Majorana-Kramers pairs, considering both topological-topological and $s$-wave–topological junctions. Working in the regime where the junction is much shorter than the superconducting coherence length, we obtain a number of analytical and numerical results that hold for arbitrary normal-state conductance and the most general forms of spin-orbit coupling. The signatures of topological superconductivity we find include the fractional ac Josephson effect, which arises in topological-topological junctions provided that the energy relaxation is sufficiently slow. We also show, for both junction types, that robust signatures of topological superconductivity arise in the dc Josephson effect in the form of switches in the Josephson current due to zero-energy crossings of Andreev levels. The junction spin-orbit coupling enters the Josephson current only in the topological-topological case and in a manner determined by the switch locations, thereby allowing quantitative predictions for experiments with the normal-state conductance, the induced gaps, and the switch locations as inputs.

Superconducting properties of three series of amorphous ${\mathrm{W}}_x\mathrm{S}{\mathrm{i}}_{1-x}$ films with different thickness and stoichiometry were investigated by dc transport measurements in a magnetic field up to 9 T. These amorphous ${\mathrm{W}}_x\mathrm{S}{\mathrm{i}}_{1-x}$ films were deposited by magnetron cosputtering of the elemental source targets onto silicon substrates at room temperature and patterned in the form of bridges by optical lithography and reactive ion etching. Analysis of the data on magnetoconductivity allowed us to extract the critical temperatures, superconducting coherence lengths, magnetic penetration depths, and diffusion constants of electrons in the normal state as functions of film thickness for each stoichiometry. Two basic time constants were derived from transport and time-resolving measurements. A dynamic process of the formation of a hotspot was analyzed in the framework of a diffusion-based vortex-entry model. We used a two-stage diffusion approach and defined a hotspot size by assuming that the quasiparticles and normal-state electrons have the same diffusion constant. With this definition and these measured material parameters, the hotspot in the 5-nm-thick ${\mathrm{W}}_{0.85}\mathrm{S}{\mathrm{i}}_{0.15}$ film had a diameter of 107 nm at the peak of the number of nonequilibrium quasiparticles.

We performed ${}^{17}\mathrm{O}$ nuclear magnetic resonance measurements on superconducting (SC) ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ under in-plane magnetic fields. We found that no new signal appears in the SC state and that the ${}^{17}\mathrm{O}$ Knight shifts obtained from the double-site measurements remain constant across the first-order phase-transition line, as well as across the second-order phase-transition line as already reported. The present results indicate that the SC spin susceptibility does not decrease in the high-field region, although a magnetization jump in the SC state was reported at low temperatures. Because the spin susceptibility is unchanged in the SC state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we suggest that the first-order phase transition across the upper critical field should be interpreted as a depairing mechanism other than the conventional Pauli-paramagnetic effect.

A equation of state (EOS) experimental technique that enables the study of thermodynamic derivatives into the TPa regime is described and applied to boron carbide (${\mathrm{B}}_4\mathrm{C}$). Data presented here are principal Hugoniot sound speed measurements reported using a laser-driven shock platform, providing a means to explore the high-pressure off-Hugoniot response of opaque materials. The extended ${\mathrm{B}}_4\mathrm{C}$ Hugoniot suggests the presence of a high-pressure phase, as recently predicted by molecular dynamics simulations, adding to the complexity of the existing phase diagram.

The manipulation of atomic-scale magnetization is important from both a fundamental and a practical perspective. Using first-principles density-functional-theory calculations within the hybrid functional approach, we systematically study spin-lattice coupling effects for isolated $3d^4–3d^7$ transition-metal dopants in a nonmagnetic, ferroelectric ${\mathrm{PbTiO}}_3$ host material. When present at the B-site, a low-spin (or intermediate-spin) to high-spin crossover induces marked ferroelectric-like distortions in the local geometry, characterized by a shift of the dopant ion with respect to the surrounding ${\mathrm{O}}_6$ octahedral cage. The origins of this microscopic multiferroic effect are discussed in terms of the pseudo-Jahn-Teller theory for ferroelectricity. The possibility to exploit this phenomenon to achieve strong magnetoelectric coupling, including controlled spin switching, is also investigated. These results provide a further understanding of ferroelectricity and multiferroicity in perovskite oxides, and they suggest a possible pathway to manipulate single atomic spins in semiconductor solid solutions.

The recently discovered ${\mathrm{BaPt}}_2{\mathrm{As}}_2$ shows a structural distortion at around 275 K, followed by the emergence of superconductivity at lower temperatures. Here we identify the presence of charge-density-wave order at room temperature and ambient pressure using single-crystal x-ray diffraction, with both a superlattice and an incommensurate modulation, where there is a change of the superlattice structure below $\simeq 275$ K. Upon applying pressure, ${\mathrm{BaPt}}_2{\mathrm{As}}_2$ shows a rich temperature-pressure phase diagram with multiple pressure-induced transitions at high temperatures, the emergence or disappearance of which are correlated with sudden changes in the superconducting transition temperature $T_c$. These findings demonstrate that ${\mathrm{BaPt}}_2{\mathrm{As}}_2$ is a promising system for studying competing interactions and the relationship between high-temperature electronic instabilities and superconductivity.

The nematic-superconductor state is an example of a quantum liquid crystal that breaks gauge as well as rotation invariance. It was conjectured to exist in the pseudogap regime of the cuprates high-$T_c$ superconductors. The nematic-superconductor state is characterized by two complex order parameters: one of them is related to superconductivity and the other one describes a nematic order. It supports two main classes of topological defects: half vortices and disclinations. In this paper we present a Ginzburg-Landau approach to study the structure of these topological defects. Due to a geometrical coupling between the superconductor and the nematic order parameters, we show that vortices are strongly coupled with disclinations. We have found a restoring force between vortices and disclinations that produces harmonic excitations whose natural frequency depends on the geometrical coupling constant and the superconductor condensation energy. Moreover, in a regime with high density of defects, we have found a structural phase transition between vortex-disclination lattices with different symmetries.

Insulating ${\mathbb{Z}}_2$ spin liquids are a phase of matter with bulk anyonic quasiparticle excitations and ground-state degeneracies on manifolds with nontrivial topology. We construct a time-reversal symmetric ${\mathbb{Z}}_2$ spin liquid in two spatial dimensions using an array of quantum wires. We identify the anyons as kinks in the appropriate Luttinger-liquid description, compute their mutual statistics, and construct local operators that transport these quasiparticles. We also present a construction of a fractionalized Fermi liquid (FL*) by coupling the spin sector of the ${\mathbb{Z}}_2$ spin liquid to a Fermi liquid via a Kondo-like coupling.

We investigated the ferroelectric perovskite material ${\mathrm{BaTiO}}_3$ by resonant inelastic x-ray scattering (RIXS) at the Ti ${\mathrm{L}}_3$ edge. We observe with decreasing temperature a transfer of spectral weight from the elastic to the charge-transfer spectral features, indicative of increasing Ti $3d$-O $2p$ hybridization. When the incident photon energy selects transitions to the Ti $3d{e}_g$ manifold, the quasielastic RIXS response exhibits a tail indicative of phonon excitations. A fit of the spectral line shape by a theoretical model allows us to estimate the electron-phonon coupling strength $M\sim 0.25$ eV, which places ${\mathrm{BaTiO}}_3$ in the intermediate coupling regime.

We study the slow time evolution of localized states of the open-boundary Lieb lattice when a magnetic flux is applied perpendicularly to the lattice and increased linearly in time. In this system, Dirac cones periodically disappear, reappear, and touch the flat band as the flux increases. We show that the slow time evolution of a localized state in this system is analogous to that of a zero-energy state in a three-level system whose energy levels intersect periodically and that this evolution can be mapped into a classical precession motion with a precession axis that rotates as times evolves. Beginning with a localized state of the Lieb lattice, as the magnetic flux is increased linearly and slowly, the evolving state precesses around a state with a small itinerant component and the amplitude of its localized component oscillates around a constant value (below but close to 1), except at multiples of the flux quantum where it may vary sharply. This behavior reflects the existence of an electric field (generated by the time-dependent magnetic field) which breaks the $C_4$ symmetry of the constant flux Hamiltonian.