The roughness of solids is crucial for interactions between bodies at short separations due to capillary or van der Waals–Casimir forces and for contact mechanics. Specifically, it is critical for the fabrication and operation of microelectromechanical systems, for which functional materials are deposited using thin film coating technologies. Here, it is demonstrated that the materials deposited by magnetron sputtering or thermally evaporated on a cold Si substrate reveal a significantly larger number of high asperities than that predicted by the normal distribution. Such asperities define the distance between the solids in contact that is the key parameter for many problems. The effect is related to the nonequilibrium deposition conditions and is suppressed if the material is deposited on a hot substrate or annealed. The high asperity tails can be described by the extreme value distribution or in some cases by the exponential distribution.

A one-dimensional (1D) mechanical model for nanogranular films, based on a structural interface, is presented. The analytical dispersion relation for the frequency and lifetimes of the acoustics breathing modes is obtained in terms of the interface layer thickness and porosity. The model is successfully benchmarked both against three-dimensional finite element method simulations and experimental photoacoustic data on a paradigmatic system available from the literature. A simpler 1D model, based on an homogenized interface, is also presented and its limitations and pitfalls discussed at the light of the more sophisticated pillar model. The pillar model captures the relevant physics responsible for acoustic dissipation at a disordered interface. Furthermore, the present findings furnish to the experimentalist an easy-to-adopt, benchmarked analytical tool to extract the interface layer physical parameters upon fitting of the acoustic data. The model is scale invariant and may be deployed, other than the case of granular materials, where a patched interface is involved.

Carrier multiplication (CM), which generates multiexcitons from a single photon absorption, is advantageous for increasing the optoelectronic device efficiency. However, CM is intrinsically inefficient in conventional semiconductors, and enhancing CM has been a long-standing challenge. Here, we propose that multisubbands in nanostructures can significantly enhance CM by opening up the intersubband CM transitions, which circumvent the strict restrictions enforced by the energy and momentum conservations. Using real-time time-dependent density functional theory, we demonstrate the mechanism in graphene nanoribbons as an example of a multisubband system. The CM mechanism provides a pathway for developing efficient optoelectronic devices.

Odd-parity magnetic and magnetic toroidal multipoles in the absence of both spatial-inversion and time-reversal symmetries are sources of multiferroic and nonreciprocal optical phenomena. We investigate electronic states caused by an emergent odd-parity magnetic quadrupole (MQ) as a representative example of magnetic odd-parity multipoles. It is shown that spontaneous ordering of the MQ leads to an antisymmetric spin-orbital polarization in momentum space, which corresponds to a spin-orbital-momentum locking at each wave vector. By a symmetry argument, we show that the orbital or sublattice degree of freedom is indispensable to give rise to the spin-orbital-momentum locking. We demonstrate how the electronic band structures are modulated by the MQ ordering in the three-orbital system, in which the MQ is activated by the spin-dependent hybridization between the orbitals with different spatial parities. The spin-orbital-momentum locking is related to the microscopic origin of cross-correlated phenomena, e.g., the magnetic-field-induced symmetric and antisymmetric spin polarization in the band structure, the current-induced distortion, and the magnetoelectric effect. We also discuss similar spin-orbital-momentum locking in an antiferromagnet where the MQ degree of freedom is activated through the antiferromagnetic spin structure in a sublattice system.

Nonlocal kinetic energy functionals with a density-dependent kernel are the most accurate functionals available for carrying out orbital-free density functional theory simulations. Among them, the Huang and Carter (HC) functional [Huang and Carter, Phys. Rev. B 81, 045206 (2010)] is the most accurate for bulk semiconductors. A major hurdle in applying HC to nonbulk systems (such as clusters and surfaces which have at least one nonperiodic dimension where the density decays to zero) lies in its numerical instability for large values of the reduced density gradient, $s\left(\mathbf{r}\right)\propto \frac{\left|\mathbf{\nabla }\mathbf{n}\right(\mathbf{r}\left)\right|}{{\mathbf{n}}^{\mathbf{4}/\mathbf{3}}\left(\mathbf{r}\right)}$, where $n$ is the electron density. We propose a revision to the HC functional, revHC, that allows it to achieve dramatically improved numerical stability, efficiency (in terms of timing to solution), and applicability. Not only does revHC reproduce all previously presented results for HC, but it extends them to a crucially important class of materials: surfaces. We show that surface energy trends of clean-cut and reconstructed surfaces of group IV and III-V semiconductors are recovered and, where available semiquantitatively, reproduce the experimental results.

The $5f$-based ferromagnet $\mathrm{U}{\mathrm{Ga}}_2$ with the Curie temperature $T_{\mathrm{C}}=125\mathrm{K}$ was investigated by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) experiments at the $\mathrm{U}–M_{4,5}$ and Ga–K edges. The position of the $\mathrm{U}–M_4$ white line, determined in the high-energy resolution fluorescence detection XAS, suggests that $\mathrm{U}{\mathrm{Ga}}_2$ is neither a localized $5f^2$ nor an itinerant system with $5f$ occupancy close to $n_{5f}=3$. The analysis of the acquired $M_{4,5}$ XANES and XMCD spectra indicates the $5f$ occupancy close to 2.5 and a large orbital magnetic moment of the uranium $5f$ states (3.18 ${\mu }_{\mathrm{B}}$) that is partly compensated by the antiparallel spin moment (1.31 ${\mu }_{\mathrm{B}}$). Thus, the total $5f$ magnetic moment of 1.87 ${\mu }_{\mathrm{B}}$ is obtained, which is smaller than the known bulk magnetization of 3.0 ${\mu }_{\mathrm{B}}$ per formula unit, while the magnetic moments of the Ga atoms are negligible. Several methods based on density-functional theory were applied and the obtained results were compared with XAS spectral features, the Sommerfeld coefficient of the electronic specific heat, and the size of the U moments and $5f$ occupancies. A clear correlation is revealed between the $\mathrm{U}–M_4$ white-line position of three metallic uranium compounds and the calculated uranium ionicity. It is demonstrated that only electronic structure methods taking appropriate care of orbital magnetism and related atomic multiplet effects can successfully describe all considered properties.

Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from $n$-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with $1.45\mathrm{meV}$ and $1.59\mathrm{meV}$ energy spacing after H and He irradiation and increase linearly in intensity with fluence—reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect.

A recent experiment reported the first rare-earth binary oxide superconductor LaO ($T_c\sim 5\mathrm{K}$) with a rock-salt structure [K. Kaminaga et al., J. Am. Chem. Soc. 140, 6754 (2018)]. Correspondingly, the underlying superconducting mechanism in LaO needs theoretical elucidation. Based on first-principles calculations on the electronic structure, lattice dynamics, and electron-phonon coupling of LaO, we show that the superconducting pairing in LaO belongs to the conventional Bardeen-Cooper-Schrieffer (BCS) type. Remarkably, the electrons and phonons of the heavy La atoms, instead of those of the light O atoms, contribute most to the electron-phonon coupling. We further find that both the biaxial tensile strain and the pure electron doping can enhance the superconducting $T_c$ of LaO. With the synergistic effect of electron doping and tensile strain, the $T_c$ could be even higher, for example, 11.11 K at a doping of 0.2 electrons per formula unit and a tensile strain of $4\%$. Moreover, our calculations show that the superconductivity in LaO thin film remains down to the trilayer thickness with a $T_c$ of 1.4 K.

IV–VI materials are some of the most efficient bulk thermoelectric materials due to their proximity to soft-mode phase transitions, which leads to low lattice thermal conductivity. It has been shown that the lattice thermal conductivity of PbTe can be considerably reduced by bringing PbTe closer to the phase transition, e.g., via lattice expansion. However, the effect of soft phonon modes on the electronic thermoelectric properties of such a system remains unknown. Using first-principles calculations, we show that the soft zone-center transverse optical phonons do not deteriorate the electronic thermoelectric properties of PbTe driven closer to the phase transition via lattice expansion due to external stress, and thus they enhance the thermoelectric figure of merit. We find that the optical deformation potentials change very weakly as the proximity to the phase transition increases, but the population and scattering phase space of soft phonon modes increase. Nevertheless, scattering between electronic states near the band edge and soft optical phonons remains relatively weak even very near the phase transition.

We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy (NV) center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by constructing dynamical matrices of large supercells containing tens of thousands of atoms. The main goal of the paper is to calculate luminescence and absorption lineshapes due to coupling to vibrational degrees of freedom. The coupling to symmetric $a_1$ modes is computed via the Huang-Rhys theory. Importantly, to include a nontrivial contribution of $e$ modes we develop an effective methodology to solve the multimode $E\otimes e$ Jahn-Teller problem. Our results show that for NV centers in diamond a proper treatment of $e$ modes is particularly important for absorption. We obtain good agreement with experiment for both luminescence and absorption. Finally, the remaining shortcomings of the theoretical approach are critically reviewed. The presented theoretical approach will benefit identification and future studies of point defects in solids.

We experimentally investigate thermoelectric response of a 100-nm-thick SnSe single crystal flake under the current-induced dc electric field. Thermoelectric response appears as a second-harmonic transverse voltage $V_{xy}^{2\omega }$, which reflects temperature gradient across the sample due to the Joule heating by harmonic ac excitation current $I_{\mathrm{ac}}$. In addition to strongly nonmonotonous dependence $V_{xy}^{2\omega }$, we observe that dc field direction controls the sign of the temperature gradient in the SnSe flake. We provide arguments, that electrocaloric effect is the mostly probable reason for the results obtained. Thus, our experiment can be understood as demonstration of the possibility to induce electrocaloric effect by in-plane electric field in conducting ferroelectric crystals and to detect it by thermoelectric response.

Radiative thermal diodes based on two-element structures rectify heat flows thanks to a temperature dependence of material optical properties. The heat transport asymmetry through these systems, however, remains weak without a significant change in material properties with the temperature. Here we explore the heat transport in three-element radiative systems and demonstrate that a strong asymmetry in the thermal conductance can appear because of many-body interactions, without any dependence of optical properties on the temperature. The analysis of transport in three-body systems made with polar dielectrics and metallic layers reveals that rectification coefficients exceeding 50% can be achieved in the near-field regime with temperature differences of about 200 K. This work paves the way for compact devices to rectify near-field radiative heat fluxes over a broad temperature range and could have important applications in the domain of nanoscale thermal management.

We report a first-principles investigation of the atomic and electronic properties at the perfect and defective (001) surfaces of the antiperovskite ${\mathrm{Sr}}_3\mathrm{Sn}\mathrm{O}$. We first performed a thermodynamical study of the atomic structure terminations and demonstrated that SrSn-terminated surfaces should be the most stable one, either with a perfect $(1\times 1$) structure or with a $(2\times 1)$ reconstruction induced by the formation of Sn vacancies. We detailed the surface gap states obtained for these surfaces, which we compare with those of other surface terminations, also having relatively low energies. These gap states, located near the Fermi level, could have a major contribution to the transport properties. Due to the lack of inversion symmetry associated with the surface, we predict that they also experience spin splittings, an important property for spin-orbitronic applications. Finally, we found that ${\mathrm{Sr}}_2\mathrm{O}$-terminated surfaces could display a ferromagnetic ordering resulting from the population of $4d$ orbitals of Sr atoms at the surface and that this could lead to the formation of a spin-polarized two-dimensional electron gas.

We reexamine the Kosterlitz-Thouless phase transition in the ground state $|{\mathrm{\Psi }}_0\rangle$ of an antiferromagnetic spin-$\frac{1}{2}$ Heisenberg chain with nearest- and next-nearest-neighbor interactions $\lambda$ from a different perspective: After defining winding number (topological charge) $W$ in the basis of resonating valence bond states, the finite-size scaling of $\langle {\mathrm{\Psi }}_0\left|W\right|{\mathrm{\Psi }}_0\rangle , \langle {\mathrm{\Psi }}_0\left|W\right|{\partial }_{\lambda }{\mathrm{\Psi }}_0\rangle , \langle {\partial }_{\lambda }{\mathrm{\Psi }}_0\left|W\right|{\partial }_{\lambda }{\mathrm{\Psi }}_0\rangle$ leads to the accurate value of critical coupling ${\lambda }_c=0.2412\pm 0.0007$ and to the value of subleading critical exponent $\nu =2.000\pm 0.001$. This approach should be useful when examining the topological phase transitions in all systems described in the basis of resonating valence bonds.

The experimental realization of high-temperature superconductivity in compressed hydrides ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$ under high pressures over 150 GPa has aroused great interest in reducing the stabilization pressure of superconducting hydrides. For cerium hydride ${\mathrm{CeH}}_9$ recently synthesized at $80–100$ GPa, our first-principles calculations reveal that the strongly hybridized electronic states of $\mathrm{Ce}\text{−}4f$ and $\mathrm{H}\text{−}1s$ orbitals produce the topologically nontrivial Dirac nodal lines around the Fermi energy $E_F$, which are protected by crystalline symmetries. By hole doping, $E_F$ shifts down toward the symmetry-driven van Hove singularity to increase the density of states, which in turn significantly raises a superconducting transition temperature $T_c$. We show that hole doping with ${\mathrm{Ce}}^{3+}$ ions can be very electronically miscible in ${\mathrm{CeH}}_9$ because both ${\mathrm{Ce}}^{3+}$ and Ce behave similarly as cations. Therefore, the interplay of crystalline symmetry, band topology, and hole doping contributes to enhance $T_c$ in compressed ${\mathrm{CeH}}_9$, which can also be demonstrated in another superconducting rare-earth hydride, ${\mathrm{LaH}}_{10}$.

We investigate the effect of correlated disorder on Majorana zero modes (MZMs) bound to magnetic vortices in two-dimensional topological superconductors. By starting from a lattice model of interacting fermions with a $p_x\pm i{p}_y$ superconducting ground state in the disorder-free limit, we use perturbation theory to describe the enhancement of the Majorana localization length at weak disorder and a self-consistent numerical solution to understand the breakdown of the MZMs at strong disorder. We find that correlated disorder has a much stronger effect on the MZMs than uncorrelated disorder and that it is most detrimental if the disorder correlation length $\ell$ is on the same order as the superconducting coherence length $\xi$. In contrast, MZMs can survive stronger disorder for $\ell \ll \xi$ as random variations cancel each other within the length scale of $\xi$, while an MZM may survive up to very strong disorder for $\ell \gg \xi$ if it is located in a favorable domain of the given disorder realization.

The relaxation dynamics of hot carriers in silicon (100) is studied via a holistic approach based on phase-resolved transient absorption spectroscopy with few-cycle optical pulses. After excitation by a sub-5-fs light pulse, strong electron-electron coupling leads to an ultrafast single electron momentum relaxation time of 10 fs. The thermalization of the hot carriers is visible in the temporal evolution of the effective mass and the collision time as extracted from the Drude model. The optical effective mass decreases from $0.3m_{\mathrm{e}}$ to about $0.125m_{\mathrm{e}}$ with a time constants of 58 fs, while the collision time increases from 3 fs for the shortest timescales with a saturation at approximately 18 fs with a time constant of 150 fs. The observation shows that both Drude parameters exhibit different dependences on the carrier temperature. The presented information on the electron mass dynamics as well as the momentum-, and electron-phonon scattering times with unprecedented time resolution is important for all hot-carrier optoelectronic devices.

In the work by Tarnopolsky et al. [Phys. Rev. Lett. 122, 106405 (2019)] they obtained the magic angles for superconductivity in twisted bilayer graphene (TBG) with a fundamental continuum model for TBG which captures the physics of the flat band. In this Letter we study the continuum model in detail and demonstrate that the system of TBG described by the continuum model is equivalent to a dynamical system that is composed of three parts of effective interactions: the coupling between two effective non-Abelian gauge fields originating from the periodic hopping potential and two effective massless fermion fields which have 1/2 lattice pseudospin and a twist-angle-dependent effective charge, the coupling between a pseudomagnetic field created by gauge fields and a layer pseudospin, and an effective pseudo-Rashba effect in an equivalent unrotated single layer with twist-angle-dependent Rashba coupling strength. The effective Rashba effect is shown to be also the sum of the pseudo-Rashba effects in two effective twisted single layers with a relative twist angle $\pi -\theta$. The longitudinal component ${\widehat{\tau }}^z$ of the layer pseudospin vector is revealed to have the physical meaning of an interlayer electric field. Due to the relative twist between the two layers it geometrically leads to the nonvanishing curvature on a vector bundle in a two-dimensional effective hexagonal lattice. The geometric curvature on the vector bundle creates a pseudomagnetic field physically and interacts with the layer pseudospin.

We use first-principles molecular dynamics simulations coupled with the thermodynamic integration method to study the hexagonal close-packed (hcp) to body-centered cubic (bcc) transition and melting of beryllium up to a pressure of 1600 GPa. We derive the melting line by equating solid and liquid Gibbs free energies and represent it by a Simon-Glatzel fit $T_m=1564\text{K}{[1+P/\left(15.6032\text{GPa}\right)]}^{0.383}$, which is in good agreement with previous two-phase simulations <6000 K. We also derive the hcp-bcc solid-solid phase boundary and show that the quasiharmonic approximation underestimates the stability of the hcp structure, predicting lower transition pressures between hcp and bcc phases. Our results are consistent with the stability regime predicted by the phonon quasiparticle method. We also predict that the hcp-bcc-liquid triple point is located at 164.7 GPa and 4314 K. In addition, we compute the shock Hugoniot curve and show that it is in good agreement with experiments, intersecting our derived melting curve at $\sim 235$ GPa and 4900 K. Finally, we make predictions for future ramp compression experiments. Starting with an isentropic compression of the liquid, we predict the path to intersect the melting line at low pressure and temperature, then to continue along the melting line over a large temperature interval of 7000 K as the sample remains in the mixed solid-liquid state before it enters the solid phase.

We exploit the quantum cluster variational method (QCVM) to study the $J_1\text{−}{J}_2$ model for quantum Ising spins. We first describe the QCVM and discuss how it is related to other mean field approximations. The phase diagram of the model is studied at the level of the Kikuchi approximation in square lattices as a function of the ratio between $g=J_2/J_1$, the temperature and the longitudinal and transverse external fields. Our results show that quantum fluctuations may change the order of the transition and induce a gap between the ferromagnetic and the stripe phases. Moreover, when both longitudinal and transverse fields are present, thermal fluctuations and quantum effects contribute to the appearance of a nematic phase.