Using ab initio density functional theory applied to semiconductor nanoclusters, we show that optical experiments where absorption is involved probe vibrations that are not necessarily the ones with large exciton-phonon matrix elements. The vibrations involved in these experiments must satisfy more stringent symmetry constraints than given by the selection rules of the exciton-phonon matrix elements. This resolves a long-standing observed discrepancy, while offering a viable theoretical approach to properly account for the experimental situation.

In this paper we investigate the high-temperature structure of ${\mathrm{Co}}_3{\mathrm{O}}_4$, a compound that has been studied extensively over the last 60 years due to its unresolved high-temperature structure. In situ thermal analysis and x-ray diffraction confirm previously reported high-temperature structural changes and show that these changes are unrelated to the high-temperature decomposition to CoO. Raman-active peaks are also extinguished over the same temperature range. By considering the changing lattice parameter, $A$-O, and $B$-O bond lengths as well as cation size we are able to calculate the degree of inversion which reaches a maximum of 0.6. To further study the structure in this experimentally inaccessible range we quench samples and perform ex situ measurements including redox titration, x-ray photoelectron spectroscopy, and neutron diffraction. We do not observe any evidence of large oxygen vacancy concentrations or octahedral ${\mathrm{Co}}_B{}^{3+}$ ions with high spin state. However, we do show an evolution in the magnetic moment from magnetic structure refinement from ($2.4{\mu }_{\text{B}}$) to ($2.7{\mu }_{\text{B}}$) that coincides exactly with the high-temperature anomaly and suggests partial inversion (0.46) of the spinel structure in fairly good agreement with the inversion calculated from bond lengths.

We theoretically study spin-transfer torques in magnetic tunnel junctions (MTJs) with an antiferromagnetic insulator (AFI) as the tunnel barrier. When a finite voltage bias is applied to the MTJ, the energy relaxation of the tunnel electrons leads to asymmetric heating of two metallic layers. Consequently, there would be a magnon current flowing across the AFI layer, resulting a magnon transfer torque in addition to the electron spin-transfer torque. Comparing to MTJs with a nonmagnetic insulator which prohibits the magnon transmission, we find the magnon transfer torque with an AFI barrier could be several times larger than the conventional spin-transfer torque of the tunnel electrons. This study presents a potential method to realize more efficient switching in MTJs and provides a motivation of experimental search for AFI-based MTJs.

Exact formulas for the Hall coefficient [A. Auerbach, Phys. Rev. Lett. 121, 066601 (2018).], modified Nernst coefficient, and thermal Hall coefficient of metals are derived from the Kubo formula. These coefficients depend exclusively on equilibrium susceptibilities, which are significantly easier to compute than conductivities. For weak isotropic scattering, Boltzmann theory is recovered. For strong scattering, well-controlled methods for thermodynamic functions are available. As an example, the Hall sign reversals of lattice bosons near the Mott insulator phases are determined. Appendices include mathematical supplements and instructions for calculating the coefficients.

Systematic measurements on the impact of interdiffusion between a metal overlayer and adhesion layer on the thermal interface conductance ($G$) at the metal bilayer-dielectric interface are reported. Composition depth profiles quantify the interdiffusion of a Au-Cu bilayer as a function of Cu adhesion layer thickness (0–10 nm), annealing time, and annealing temperature. Optical pump/probe measurements of $G$ quantify the effect of Au-Cu interdiffusion on thermal transport across the (Au-Cu)-$\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interface. The enhancement of $G$ between Au and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ through the addition of a Cu adhesion layer decreases as Au-Cu interdiffusion occurs. For example, annealing a 49-nm Au film with a 4.7-nm Cu adhesion layer on $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ at 520 K for 30 min, results in a $57\pm 15\%$ drop in $G$. An analytical model of the composition profile is derived with inputs of annealing time, temperature dependent permeabilities of the Au-Cu interface to each species, and the initial thicknesses of the Au and Cu layers. Integrating this model with a diffuse mismatch model defines a methodology for the prediction of $G$ that accounts for interdiffusion in metal bilayers on dielectric substrates, and can be used to evaluate the degradation of $G$ over a device's lifetime.

The jump distribution, a property of the motion of adsorbates on a corrugated surface, contains crucial information on adsorbate-substrate energy dissipation processes. To provide a means to study jump distributions in a honeycomb array of adsorption sites, we derive analytical expressions for the intermediate scattering function (ISF) describing jump diffusion taking into account jumps up to fourth-nearest neighbor in length. To enable testing the analytical expressions against experimental or simulated data, we develop a global fitting routine that can be applied to experimental or simulated ISFs to infer multiple jumps. We demonstrate the analysis method by studying the jump distribution arising from classical Langevin molecular dynamics simulations of two model systems, cyclopentadienyl (Cp) on Cu(111), and deuterium (D) on Pd(111). The simulations and analysis confirm that diffusion of Cp/Cu(111) at a surface temperature $T_s=135$ K takes place in a regime of predominantly single jumps. Classical simulations of D/Pd(111) at $T_s=350$ K, with a realistic Langevin friction, suggest that the diffusion of D/Pd(111) involves a high proportion of multiple jumps. The parameters that apply to D/Pd(111) are typical of the interaction of hydrogen atoms with close-packed transition metal surfaces, suggesting that long jumps are a general feature of the high temperature surface diffusion of hydrogen.

In electronic band structures, nodal lines may arise when two (or more) bands contact and form a one-dimensional manifold of degeneracy in the Brillouin zone. Around a nodal line, the dispersion for the energy difference between the bands is typically linear in any plane transverse to the line. Here, we explore the possibility of higher-order nodal lines, i.e., lines with higher-order dispersions, that can be stabilized in solid-state systems. We reveal the existence of quadratic and cubic nodal lines, and we show that these are the only possibilities (besides the linear nodal line) that can be protected by crystalline symmetry. We derive effective Hamiltonians to characterize the novel low-energy fermionic excitations for the quadratic and cubic nodal lines, and explicitly construct minimal lattice models to further demonstrate their existence. Their signatures can manifest in a variety of physical properties such as the (joint) density of states, magnetoresponse, transport behavior, and topological surface states. Using ab initio calculations, we also identify possible material candidates that realize these exotic nodal lines.

We study topological transport in the steady state of a quantum particle hopping on a one-dimensional lattice in the presence of dissipation. The model exhibits a rich phase structure, with the average particle velocity in the steady state playing the role of a nonequilibrium order parameter. Within each phase, the average velocity is proportional to a topological winding number and to the inverse of the average time between quantum jumps. While the average velocity depends smoothly on system parameters within each phase, nonanalytic behavior arises at phase-transition points. We show that certain types of spatial boundaries between regions where different phases are realized host a number of topological bound states which is equal to the difference between the winding numbers characterizing the phases on the two sides of the boundary. These topological bound states are attractors for the dynamics; in cases where the winding number changes by more than one when crossing the boundary, the subspace of topological bound states forms a dark, decoherence free subspace for the dissipative system. Finally, we discuss how the dynamics we describe can be realized in a simple cavity or circuit QED setup, where the topological boundary mode emerges as a robust coherent state of the light field.

Optical binding allows creation of mechanically stable nanoparticle configurations owing to formation of self-consistent optical trapping potentials. While the classical diffraction limit prevents achieving deeply subwavelength arrangements, auxiliary nanostructures enable tailoring optical forces via additional interaction channels. Here, a dimer configuration next to a metal surface was analyzed in detail and the contribution of surface plasmon polariton waves was found to govern the interaction dynamics. It is shown that the interaction channel, mediated by resonant surface waves, enables achieving subwavelength stable dimers. Furthermore, the vectorial structure of surface modes allows binding between two dipole nanoparticles along the direction of their dipole moments, contrary to vacuum binding, where a stable configuration is formed in the direction perpendicular to the polarization of the dipole moments. In addition, the enhancement by one order of magnitude of the optical binding stiffness is predicted owing to the surface plasmon polariton interaction channel. These phenomena pave the way for developing new flexible optical manipulators, allowing for control over a nanoparticle trajectory on subwavelength scales and opening opportunities for optical-induced anisotropic (i.e., with different periods along the field polarization as well as perpendicular to it) organization of particles on a plasmonic substrate.

Motivated by recent experimental progress, we study the effect of mechanical deformations on the superconducting pairing symmetries in monolayer black phosphorus (MBP). Starting with phonon-mediated intervalley spin-singlet electron-electron pairing and making use of realistic band parameters obtained through first-principles calculations, we show that the application of weak mechanical strain in the plane of MBP can change the effective $s$-wave and $p$-wave symmetry of the superconducting correlations into effective $d$-wave and $f$-wave symmetries, respectively. This prediction of a change in the pairing symmetries might be experimentally confirmed through angular dependence high-resolution tunneling spectroscopy, the Meissner effect, and critical temperature experiments. The idea of manipulating the superconducting symmetry class by applying planar mechanical strain can be extended to other anisotropic materials as well and may help in providing important information of the symmetries of the order parameter, perhaps even in some high-$T_c$ superconductors.

The wave numbers and efficiencies of the first-order lattice vibrational modes of the isomorphous solid solutions of $\mathrm{LiNb}{\mathrm{O}}_3\text{−}\mathrm{LiTa}{\mathrm{O}}_3$ have been studied by Raman and infrared spectroscopies. The substitution of niobium by tantalum induces a continuous shift of wave numbers and intensities of phonon modes indicating a one-phonon behavior. In general, the wave numbers of vibrations at low frequencies $(<325\mathrm{c}{\mathrm{m}}^{-1})$ soften with the increased atomic mass. The frequencies of most of the high-frequency modes increase with the Ta/Nb ratio owing to enhanced force constants. A qualitative relation between the shift of transverse and longitudinal modes in the solid solutions has been found on the basis of an overlap matrix. The exceptional softening of several high-frequency modes with the replacement of Nb by Ta originates from the strong interaction of these modes with modes at low frequencies according to the overlap matrix. The intensities of the Raman modes are strongly modulated by the electronic properties with Ta/Nb substitution and this emphasizes their role in the physical properties of $\mathrm{LiNb}{\mathrm{O}}_3$ and $\mathrm{LiTa}{\mathrm{O}}_3$.

The intense search for topological superconductivity is inspired by the prospect that it hosts Majorana quasiparticles. We explore in this work the optimal design for producing topological superconductivity by combining a quantum Hall state with an ordinary superconductor. To this end, we consider a microscopic model for a topologically trivial two-dimensional $p$-wave superconductor exposed to a magnetic field and find that the interplay of superconductivity and Landau level physics yields a rich phase diagram of states as a function of $\mu /t$ and $\mathrm{\Delta }/t$, where $\mu ,t$, and $\mathrm{\Delta }$ are the chemical potential, hopping strength, and the amplitude of the superconducting gap. In addition to quantum Hall states and topologically trivial $p$-wave superconductor, the phase diagram also accommodates regions of topological superconductivity. Most importantly, we find that application of a nonuniform, periodic magnetic field produced by a square or a hexagonal lattice of $h/e$ fluxoids greatly facilitates regions of topological superconductivity in the limit of $\mathrm{\Delta }/t\to 0$. In contrast, a uniform magnetic field, a hexagonal Abrikosov lattice of $h/2e$ fluxoids, or a one-dimensional lattice of stripes produces topological superconductivity only for sufficiently large $\mathrm{\Delta }/t$.

We propose a doped two-leg spin-$1/2$ diagonal ladder model (or a composite spin model) to simulate a doped spin-1 chain. By using the density matrix renormalization group method for an open chain, we find that the ground state hosts a dominant singlet $d_{xy}$-wave superconducting correlation with features similar to those of the spin-1 Haldane phase. Meanwhile, we apply the renormalized mean-field theory to the model and obtain a $d_{xy}$-wave superconducting state. The superconducting order exhibits a domelike shape as a function of hole concentration $\delta$. Our work provides a physical understanding of the origin of $d_{xy}$ pairing symmetry, which originates from antiferromagnetic Heisenberg interaction in the diagonal direction, and the intrachain hopping term makes it stable.

The recent discovery of quasi-one-dimensional chromium-based superconductivity has generated much excitement. We study in this work the superconducting instabilities of a representative compound, the newly synthesized ${\mathrm{KCr}}_3{\mathrm{As}}_3$ superconductor. Based on inputs from density functional theory calculations, we first construct an effective multiorbital tight-binding Hamiltonian to model its low-energy band structure. We then employ standard random-phase approximation calculations to investigate the superconducting instabilities of the resultant multiorbital Hubbard model. We find various pairing symmetries in the phase diagram in different interaction parameter regimes, including the triplet $f$-wave, $p_z$-wave, and singlet $s^{\pm }$-wave pairings. We argue that the singlet $s^{\pm }$-wave pairing, which emerges at intermediate interaction strength, may be realized in this material. This singlet pairing is driven by spin-density wave fluctuations enhanced by Fermi-surface nesting. We point out that phase-sensitive measurement can distinguish the $s$-wave pairing in ${\mathrm{KCr}}_3{\mathrm{As}}_3$ from the $p_z$ wave previously proposed for a related compound ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. The $s^{\pm }$-wave pairing in ${\mathrm{KCr}}_3{\mathrm{As}}_3$ shall also exhibit a subgap spin resonance mode near the nesting vector, which can be tested by inelastic neutron scattering measurements. Another intriguing property of the $s^{\pm }$ pairing is that it can induce time-reversal invariant topological superconductivity in a semiconductor wire with large Rashba spin-orbit coupling via proximity effect. Our study shall be of general relevance to all superconductors in the family of $A{\mathrm{Cr}}_3{\mathrm{As}}_3(A=\text{K},\text{Rb},\text{Cs})$.

The magnetic propagation vector in delafossite ${\mathrm{CuCrO}}_2$ with classical Heisenberg spins is calculated analytically as a function of exchange interactions up to fourth-nearest neighbors. Exchange interactions are estimated by a series of density functional theory calculations for several values of lattice distortion. Our calculations show that the magnetic propagation vector is directly affected by the considered distortions providing different stable commensurate or incommensurate magnetic configurations. A realistic set of exchange interactions corresponding to a 0.1% lattice distortion yields the experimental ground state with an incommensurate propagation vector q $\sim$ (0.329, 0.329, 0). We find that a very weak antiferromagnetic interlayer interaction favors an incommensurate ordering even in the absence of lattice distortion. Moreover, the exchange energy of a magnetic configuration of a finite crystal of ${\mathrm{CuCrO}}_2$ with periodic boundary conditions is derived analytically. Based on that, highly accurate Monte Carlo simulations performed on ${\mathrm{CuCrO}}_2$ confirm both the proposed analytical calculations and the density functional theory estimations, where we obtain excellent convergence toward the experimental ground state with a magnetic propagation vector q = (0.3288, 0.3288, 0).