We theoretically and numerically study current-induced modification of ferrimagnetic spin-wave dynamics when an electrical current generates adiabatic and nonadiabatic spin-transfer torques. We find that the sign of the Doppler shift depends on the spin-wave handedness because the sign of spin polarization carried by spin waves depends on the spin-wave handedness. It also depends on the sign of the adiabatic-torque coefficient, originating from unequal contributions from two sublattices. For a positive nonadiabaticity of spin current, the attenuation lengths of both right- and left-handed spin waves increase when electrons move in the same direction with spin-wave propagation. Our result establishes a way to simultaneously measure important material parameters of a ferrimagnet, such as angular momentum compensation point, spin polarization, and nonadiabaticity using current-induced control of ferrimagnetic spin-wave dynamics.

We report a $\mu \mathrm{SR}$ investigation of a noncentrosymmetric superconductor (${\mathrm{LaNiC}}_2$) in single-crystal form. Compared to previous $\mu \mathrm{SR}$ studies of noncentrosymmetric superconducting polycrystalline and powder samples, the unambiguous orientation of single crystals enables a simultaneous determination of the absolute value of the magnetic penetration depth and the vortex core size from measurements that probe the magnetic field distribution in the vortex state. The magnetic field dependence of these quantities unambiguously demonstrates the presence of two nodeless superconducting energy gaps. In addition, we detect weak internal magnetic fields in the superconducting phase, confirming earlier $\mu \mathrm{SR}$ evidence for a time-reversal symmetry-breaking superconducting state. Our results suggest that Cooper pairing in ${\mathrm{LaNiC}}_2$ is characterized by an interorbital equal-spin pairing model introduced to unify the pairing states of ${\mathrm{LaNiC}}_2$ and the centrosymmetric superconductor ${\mathrm{LaNiGa}}_2$.

We develop a theory for the three-terminal nonlocal conductance in Majorana nanowires as existing in the superconductor-semiconductor hybrid structures in the presence of superconducting proximity, spin-orbit coupling, and Zeeman splitting. The key question addressed is whether such nonlocal conductance can decisively distinguish between trivial and topological Majorana scenarios in the presence of chemical potential inhomogeneity and random impurity disorder. We calculate the local electrical as well as nonlocal electrical and thermal conductance of the pristine nanowire (good zero-bias conductance peaks), the nanowire in the presence of quantum dots and inhomogeneous potential (bad zero-bias conductance peaks), and the nanowire in the presence of large disorder (ugly zero-bias conductance peaks). The local conductance by itself is incapable of distinguishing the trivial states from the topological states since zero-bias conductance peaks are generic in the presence of disorder and inhomogeneous potential. The nonlocal conductance, which in principle is capable of providing the bulk gap closing and reopening information at the topological quantum phase transition, is found to be far too weak in magnitude to be particularly useful in the presence of disorder and inhomogeneous potential. Therefore, we focus on the question of whether the combination of the local, nonlocal electrical, and thermal conductance can separate the good, bad, and ugly zero-bias conductance peaks in finite-length wires. Our paper aims to provide a guide to future experiments, and we conclude that a combination of all three measurements would be necessary for a decisive demonstration of topological Majorana zero modes in nanowires—positive signals corresponding to just one kind of measurements are likely to be false positives arising from disorder and inhomogeneous potential.

We have recently shown that the logarithmic growth of the entanglement entropy following a quantum quench in a many-body localized phase is accompanied by a slow growth of the number entropy $S_N\sim lnlnt$. Here we provide an in-depth numerical study of $S_N\left(t\right)$ for the disordered Heisenberg chain and show that this behavior is not transient and persists even for very strong disorder. Calculating the truncated Rényi number entropy $S_N^{\left(\alpha \right)}\left(t\right)={(1-\alpha )}^{-1}ln{\sum }_n{p}^{\alpha }\left(n\right)$ for $\alpha \ll 1$ and $p\left(n\right)>p_c$—which is sensitive to large number fluctuations occurring with low probability—we demonstrate that the particle number distribution $p\left(n\right)$ in one half of the system has a continuously growing tail. This indicates a slow but steady increase in the number of particles crossing between the partitions in the interacting case and is in sharp contrast to Anderson localization for which we show that $S_N^{(\alpha \to 0)}\left(t\right)$ saturates for any cutoff $p_c>0$. We show, furthermore, that the growth of $S_N$ is not the consequence of rare states or rare regions but rather represents typical behavior. These findings indicate that the interacting system is never fully localized even for very strong but finite disorder.

Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasiharmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ as a function of $x$ exhibits a smoother U shape than in crystalline mass-disordered alloys. The main contribution to the initial drop of thermal conductivity at low Ge concentration stems from the localization of otherwise extended modes that make up the lowest 8% of the population by frequency. Contributions from intermediate frequency modes are decreased more gradually with increasing Ge to reach a broad minimum thermal conductivity between concentrations of Ge from $x=0.25$ to 0.75. Modal analysis unravels the correlations among localization, line broadening, and the contribution to thermal transport of modes within different frequency ranges.

Exploration and understanding of the spinterface between magnetic electrodes and organic semiconductors are vital to get an insight into interfacial science and develop spintronic devices. Intrigued by the fact that the modification of chemical states may affect interfacial exchange coupling, magnetic interactions between a series of metallic phthalocyanines ($M\mathrm{Pcs}, M=\mathrm{Cr}$, Mn, Fe, Co) and surface oxidized Co were studied. Interestingly, these four $M\mathrm{Pcs}$ are all antiferromagnetically coupled with the Co substrate via the interfacial O atomic layer, but the coupling strength strongly relies on the type of metallic ion in the $M\mathrm{Pcs}$. The physical mechanism for different exchange strengths is explored and discussed in terms of a superexchange model via O atoms. It is also noted that the adsorption energy and work function of the surface oxidized Co were sensitive to the metallic ions of $M\mathrm{Pcs}$. The valence and spin state of central $3d$ metallic ions could be tuned by the surface oxidized Co. Very interestingly, high negative spin-polarized spinterfaces are obtained in these $M\mathrm{Pcs}$. The spin polarizations are up to −86% and −80% for CrPc and MnPc, respectively. Additionally, high spin polarization can be maintained beyond room temperature because of the very large interfacial coupling energy, demonstrating a promising application of $M\mathrm{Pcs}$ in spintronic devices.

Motivated by the recent experiments suggesting the importance of nematicity in the phase diagrams of iron-based and cuprate high-$T_c$ superconductors, we study the influence of nematicity on the collective modes inside the superconducting state in a nonequilibrium. In particular, we consider the signatures of collective modes in short-time dynamics of a system with competing nematic and $s$- and $d$-wave superconducting orders. In the rotationally symmetric state, we show that the Bardasis-Schrieffer mode, corresponding to the subdominant pairing, hybridizes with the nematic collective mode and merges into a single in-gap mode, with the mixing vanishing only close to the phase boundaries. For the $d$-wave ground state, we find that nematic interaction suppresses the damping of the collective oscillations in the short-time dynamics. Additionally, we find that even inside the nematic $s+d$-wave superconducting state, a Bardasis-Schrieffer-like mode leads to order parameter oscillations that strongly depend on the competition between the two pairing symmetries. We discuss the connection of our results to the recent pump-probe experiments on high-$T_c$ superconductors.

Migdal-Eliashberg theory describes the properties of the normal and superconducting states of electron-phonon-mediated superconductors based on a perturbative treatment of the electron-phonon interactions. It is necessary to include both electron and phonon self-energies self-consistently in Migdal-Eliashberg theory in order to match numerically exact results from determinantal quantum Monte Carlo in the adiabatic limit. In this work we provide a method to obtain the real-axis solutions of the Migdal-Eliashberg equations with electron and phonon self-energies calculated self-consistently. Our method avoids the typical challenge of computing cumbersome singular integrals on the real axis and is numerically stable and exhibits fast convergence. Analyzing the resulting real-frequency spectra and self-energies of the two-dimensional Holstein model, we find that self-consistently including the lowest-order correction to the phonon self-energy significantly affects the solution of the Migdal-Eliashberg equations. The calculation captures the broadness of the spectral function, renormalization of the phonon dispersion, enhanced effective electron-phonon coupling strength, minimal increase in the electron effective mass, and the enhancement of superconductivity which manifests as a superconducting ground state despite strong competition with charge-density-wave order. We discuss surprising differences in two common definitions of the electron-phonon coupling strength derived from the electron mass and the density of states, quantities which are accessible through experiments such as angle-resolved photoemission spectroscopy and electron tunneling. An approximate upper bound on $2\mathrm{\Delta }/T_c$ for conventional superconductors mediated by retarded electron-phonon interactions is proposed.

The discovery of signatures of topological superconductivity in superconducting bulk materials with topological surface states has attracted intensive research interest recently. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, here, we demonstrate the existence of topological nodal-line states and drumheadlike surface states in centrosymmetric superconductor $\mathrm{Sn}\mathrm{Ta}{\mathrm{S}}_2$, which is a type-II superconductor with a critical transition temperature of about 3 K. The valence bands from Ta $5d$ orbitals and the conduction bands from Sn $5p$ orbitals cross each other, forming two nodal lines in the vicinity of the Fermi energy without the inclusion of spin-orbit coupling (SOC), protected by the spatial-inversion symmetry and time-reversal symmetry. The nodal lines are gapped out by SOC. The drumheadlike surface states, the typical characteristics in nodal-line semimetals, are quite visible near the Fermi level. Our findings indicate that $\mathrm{Sn}\mathrm{Ta}{\mathrm{S}}_2$ offers a promising platform for exploring the exotic properties of the topological nodal-line fermions and helps in the study of topological superconductivity.

The nitrogen-vacancy center (NV center) in diamond at magnetic fields corresponding to the ground-state level anticrossing (GSLAC) region gives rise to rich photoluminescence (PL) signals due to the vanishing energy gap between the electron spin states, which enables for a broad variety of environmental couplings to have an effect on the NV center's luminescence. Previous works have addressed several aspects of the GSLAC photoluminescence, however, a comprehensive analysis of the GSLAC signature of NV ensembles in different spin environments at various external fields is missing. Here we employ a combination of experiments and recently developed numerical methods to investigate in detail the effects of transverse electric and magnetic fields, strain, P1 centers, NV centers, and the ${}^{13}\mathrm{C}$ nuclear spins on the GSLAC photoluminescence. Our comprehensive analysis provides a solid ground for advancing various microwave-free applications at the GSLAC, including but not limited to magnetometry, spectroscopy, dynamic nuclear polarization (DNP), and nuclear magnetic resonance (NMR) detection. We demonstrate that not only the most abundant ${}^{14}\mathrm{NV}$ center but the ${}^{15}\mathrm{NV}$ can also be utilized in such applications.

We applied a many-body Wannier functions method to theoretically calculate the excitonic optical conductivity spectrum and energy structure in a one-dimensional (1D) Mott insulator at absolute zero temperature with a large system size. Focusing on full charge fluctuations associated with holon and doublon pairs, we employ a charge model, which is interpreted as an effective model for investigating the photoexcitations of a 1D extended Hubbard model under a half-filling of the spin-charge separation. As a result, theoretical spectra with the appropriate broadenings qualitatively reproduce the recent experimental data of ET-${\mathrm{F}}_2\mathrm{TCNQ}$ at 294 K with and without a modulated electric field. Regarding the excitonic energy structure, we found that the excitons, particularly for even-parity, are weakly bound by many-body effects. This is also consistent with the fitting parameters reported in a recent experiment. Thus, the theoretical method presented in this paper is useful for understanding the physical roles of the charge fluctuations in many-body excited states of a 1D Mott insulator.

We report the emergence or broadening of exotic magnetic metallic phases upon explicit breaking of $SU\left(2\right)$ symmetry by introduction of mass imbalance in a variant of Hubbard model, known as the ionic Hubbard model in the presence of frustration at half filling on a square lattice. The ionic Hubbard model has in addition to hopping ($\sim t$) and onsite Coulomb repulsion ($\sim U$), a staggered ionic potential ($\sim \mathrm{\Delta }$) which breaks translational symmetry of the underlying lattice. In the low to intermediate ranges of $U$ and $\mathrm{\Delta }$, we use unrestricted Hartree-Fock theory to construct the phase diagram in the $U-\mathrm{\Delta }$ plane for a fixed mass imbalance ($\sim \eta$). Where as in the limit where both $U,\mathrm{\Delta }$ are comparable and much larger than the first $(\sim t_{\sigma })$ and second $(\sim t_{\sigma }^{\prime })$ neighbor hopping amplitudes, we employ the technique of generalized Gutzwiller approximation and subsequently use renormalized mean field theory to construct the phase diagram for varying values of hopping asymmetry. In both cases, starting from a correlated band insulator with weak antiferromagnetic spin density wave order, if we tune $U/\mathrm{\Delta }$, we observe the opening of novel magnetically ordered metallic phases such as spin imbalanced ferromagnetic metal, ferrimagnetic metal, and antiferromagnetic half metal. We also study singlet superconductivity in the $d$-wave and extended $s$-wave channels in the strong coupling limit of this model in the presence of mass imbalance.

Transition metal oxides in the $d^0$ perovskite class, defined broadly to include $\mathrm{W}{\mathrm{O}}_3$, have shown promise as light absorbers for photocatalytic applications. These compounds are highly tunable in their composition and atomic structure, and consequently, in their electronic structure and properties. Through the lens of atomic orbital interactions and density functional theory calculations, the present work explores how and why certain structural distortions tune the band gaps and band edges of $\mathrm{W}{\mathrm{O}}_3$ and $d^0$ perovskite oxides. It is determined that, because an individual distortive mode (e.g., polar distortion along or octahedral rotation around a cubic perovskite axis) changes the character of only some band-edge orbitals, combinations of distortive modes may tune band gaps much more significantly than the sum of individual modes. The implications of this tunability are discussed for $\mathrm{W}{\mathrm{O}}_3$ as a photocatalyst, and for $d^0$ perovskite oxides more generally.

We study the backward scatterings of plane waves by reciprocal scatterers and reveal that $n$-fold ($n\ge 3$) rotation symmetry is sufficient to secure invariant backscattering for arbitrarily polarized incident plane waves. It is further demonstrated that the same principle is also applicable for infinite periodic structures in terms of reflection, which simultaneously guarantees the transmission invariance if there are neither Ohmic losses nor extra diffraction channels. In the presence of losses, extra reflection symmetries (with reflection planes either parallel or perpendicular to the incident direction) can be incorporated to ensure simultaneously the invariance of transmission and absorption. The principles we have revealed are protected by fundamental laws of reciprocity and parity conservation, which are fully independent of the optical or geometric parameters of the photonic structures. The optical invariance obtained is intrinsically robust against perturbations that preserve reciprocity and the geometric symmetries, which could be widely employed for photonic applications that require stable backscatterings or reflections.

Transition-metal dichalcogenides are characterized by a layered lattice structure, facilitating the fabrication of two-dimensional crystals. Concerning their electronic properties, these monolayers differ fundamentally from the bulk material, exhibiting direct band gaps and significantly higher exciton binding energies. Hence, to shed light on how the crystal thickness influences the relevant excited states and in particular their dynamics, we performed time-resolved optical pump-probe spectroscopy on monolayer and bulk samples of molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}_2$) for a broad range of excitation fluences. The observed transient spectra result from photoinduced shifts in the band structure. Here, we find evidence for a localized nature of the excited species as opposed to the common model of band-gap renormalization. At high excitation densities, strong collisional broadening of the shifted absorption occurs. Within the first picosecond after excitation, the free carriers thermalize, while in the monolayer, additionally, electrons and holes pair to form excitons. On longer time scales up to several nanoseconds, the excited populations decay. In the bulk sample, the corresponding signal reduction could be described in terms of a defect-assisted Auger recombination of electrons and holes. For monolayer $\mathrm{Mo}{\mathrm{S}}_2$, in contrast, two-dimensional diffusion leads to recombination of the excitons at defect sites.