We report near and midinfrared pump, $c$-axis terahertz probe measurements on a superconducting single crystal ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_4$ with $T_c=32$ K. The measurements reveal that the pump-induced change occurs predominantly at the Josephson plasma edge position below $T_c$. Upon excitation by strong near-infrared pulses, the superconducting state is severely disturbed and incoherent quasiparticle excitations develop in the frequency regime above the static plasma edge. However, within a very short time delay $(\sim 1.5$ ps) we observe the reappearance of a very sharp Josephson plasma edge at a frequency lower than the static Josephson plasma edge and the emergence of a new light-induced edge at a higher energy. The results imply that the light can induce new Josephson plasmon modes with different coupling strengths. A similar but weaker effect is observed for the midinfrared pump. No pump-induced effect is detected above $T_c$.

Designing materials by alloying from the melt is a fascinating route but also a big challenge to the scientific community. The difficulty lies not only in the insufficient experimental data of the liquid state, but also in the lack of clear understanding of mechanisms relating to properties. In this work we studied binary La-TM melts as a model system using electrostatic levitation and synchrotron x-ray diffraction. From the measured specific volume and liquid structure, a relative loose packing in La-Cu liquid was found, compared to that of La-Ni and La-Co, with a positive excess volume. We show that this is results from a combination of steep repulsion and anharmonic attraction of the potential. Based on the potential anharmonicity, the deformation process in glasses can be predicted. Our findings not only provide deeper insights into the structural and dynamical properties induced by interatomic interaction, but also shed light on future designing from the characteristics of the constituents.

A strategy to drive skyrmion motion by a combination of an anisotropy gradient and spin-Hall effect has recently been demonstrated. Here, we study the fundamental properties of this type of motion by combining micromagnetic simulations and a generalized Thiele's equation. We find that the anisotropy gradient drives the skyrmion mainly along the direction perpendicular to the gradient, due to the conservative part of the torque. There is some slower motion along the direction parallel to the anisotropy gradient due to damping torque. When an appropriate spin-Hall torque is added, the skyrmion velocity in the direction of the anisotropy gradient can be enhanced. This motion gives rise to acceleration of the skyrmion as it moves to regions of varying anisotropy. This phenomenon should be taken into account in experiments for the correct evaluation of the skyrmion velocity. We employ a Thiele-like formalism and derive expressions for the velocity and the acceleration of the skyrmion that match very well with micromagnetic simulation results.

For exploring the superconductivity mechanisms and seeking potential high-temperature superconductors in hydrogen-rich compounds, we perform a systematical investigation of the phase diagram, crystal structures, electronic properties, and electron-phonon coupling (EPC) of titanium hydrides under high pressures. Strikingly, the low $T_{\mathrm{c}}$ in $\mathrm{Ti}{\mathrm{H}}_3$ (∼4 K) contrast sharply with the high $T_{\mathrm{c}}$ (above 200 K) in $\mathrm{S}{\mathrm{H}}_3$, though they both possess the same stoichiometry and both crystallize in high-symmetrical cubic crystal. The large difference of $T_{\mathrm{c}}$ motivates us to discover the superconductive mechanism by probing the electron-phonon coupling interaction in the cubic crystal of $\mathrm{S}{\mathrm{H}}_3, \mathrm{Ti}{\mathrm{H}}_3, \mathrm{V}{\mathrm{H}}_3$, and $\mathrm{Se}{\mathrm{H}}_3$. The analyses of phonon linewidths and Fermi surface nesting functions reveal that the contrasting $T_{\mathrm{c}}$ is mainly attributed to the disparate intensity of electrons interacting with optic phonons, rather than the contributions from global electronic structures. Furthermore, it is found that the strong dependency of optic phonon frequencies on the wave vector is an essential ingredient for strong EPC λ.

We study theoretically single-particle spectra of Weyl superconductors. Three different superconducting pairing states are addressed, which are the BCS-type states with the $s$-wave pairing symmetry and the $p+ip$-wave pairing symmetry, and the FFLO pairing state. We elaborate that these three states can be resolved well based on the bulk and surface spectral functions as well as the local density of states. The single impurity effect is also explored, which may help us to differentiate the BCS-type pairing states and the FFLO state further.

We propose a method for solving the Schrödinger-Poisson problem that can be efficiently implemented in realistic 3D tight-binding models of semiconductor-based Majorana devices. The method is based on two key ideas. (i) For a given geometry, the Poisson problem is only solved once (for each local orbital) and the results are stored as an interaction tensor; using this Green's function scheme, the Poisson component of the iteration procedure is reduced to a few simple summations. (ii) The 3D problem is mapped into an effective multiorbital 1D problem with molecular orbitals calculated self-consistently as the transverse modes of an infinite wire with the same electrostatic potential as the local electrostatic potential of the finite 3D device. These two ideas considerably simplify the numerical complexity of the full 3D Schrödinger-Poisson problem for the nanowire, enabling a tractable effective theory with predictive power. To demonstrate the capabilities of our approach, we calculate the response of the system to an external magnetic field, the dependence of the effective chemical potential on the work function difference, and the dependence of the effective semiconductor-superconductor coupling on the applied gate potential. We find that, within a wide range of parameters, different low-energy bands are characterized by similar effective couplings, which results in induced gap features characterized by a single energy scale. We also find that electrostatic effects are responsible for a partial suppression of the Majorana energy splitting oscillations. Finally, we show that a position-dependent work function difference can produce a nonhomogeneous effective potential that is not affected by the screening due to the superconductor and is only partially suppressed by the charge inside the wire. In turn, this potential can induce trivial low-energy states that mimic the phenomenology of Majorana zero modes. Thus any position-dependent work function difference (even at the $1\%$ level) along the nanowire must be avoided through carefully engineered semiconductor-superconductor interfaces.

Quantum phase coherence lengths were experimentally measured in nanolithographic wires to investigate the effects of wire length on quantum decoherence, which can be limited by mechanisms such as coupling to an external classical environment. The work demonstrates that device geometry and coupling to the environment have to be taken into account in quantum coherence, of relevance in quantum technologies using electronic nanostructures. The low-temperature measurements of the quantum phase coherence lengths use quantum transport, specifically antilocalization, on wires fabricated from an InGaAs/InAlAs heterostructure. It is observed that longer wire lengths result in longer quantum phase coherence lengths, tending to an asymptotic value in long wires. The results are understood from the observation that longer wires average out the quantum decoherence introduced at the end sections by coupling to the external environment. The experimental results are quantitatively compatible with a model expressing reduced backscattered amplitude due to quantum interference at the wire ends.

We study the structure of the orbital order $\mathrm{\Gamma }=〈d_{xz}^{†}{d}_{xz}-d_{yz}^{†}{d}_{yz}〉$ in FeSe in light of recent scanning tunneling microscopy and angle-resolved photoemission spectroscopy (ARPES) data, which detect the shapes of the hole and electron pockets in the nematic phase. The geometry of the pockets indicates that the sign of $\mathrm{\Gamma }$ is different between the hole and electron pockets (${\mathrm{\Gamma }}_h$ and ${\mathrm{\Gamma }}_e$). We argue that this sign change cannot be reproduced if one solves for the orbital order within the mean-field approximation, as the mean-field analysis yields either no orbital order, or order with the same sign of ${\mathrm{\Gamma }}_e$ and ${\mathrm{\Gamma }}_h$. We argue that another solution with the opposite signs of ${\mathrm{\Gamma }}_e$ and ${\mathrm{\Gamma }}_h$ emerges if we include the renormalizations of the vertices in the $d$-wave orbital channel. We show that the ratio $|{\mathrm{\Gamma }}_e/{\mathrm{\Gamma }}_h|$ is of order one, independent of the strength of the interaction. We also compute the temperature variation of the energy of $d_{xz}$ and $d_{yz}$ orbitals at the center of electron pockets and compare the results with ARPES data.

We study the inelastic scattering rate due to the Coulomb interaction in three-dimensional Dirac/Weyl semimetals at finite temperature. We show that the perturbation theory diverges because of the long-range nature of the interaction, hence, thermally induced screening must be taken into account. We demonstrate that the scattering rate has a nonmonotonic energy dependence with a sharp peak owing to the resonant decay into thermal plasmons. We also consider the Hubbard interaction for comparison. We show that, in contrast to the Coulomb case, it can be well described by the second-order perturbation theory in a wide energy range.

We investigate the emergence of antiferromagnetic ordering and its effect on the helical edge states in a quantum spin Hall insulator, in the presence of strong Coulomb interaction. Using dynamical mean-field theory, we show that the breakdown of lattice translational symmetry favors the formation of magnetic ordering with nontrivial spatial modulation. The onset of a nonuniform magnetization enables the coexistence of spin-ordered and topologically nontrivial states. An unambiguous signature of the persistence of the topological bulk property is the survival of bona fide edge states. We show that the penetration of the magnetic order is accompanied by the progressive reconstruction of gapless states in subperipheral layers, redefining the actual topological boundary within the system.

As a type of fermions without counterpart in high energy physics, triply degenerate fermions show exotic physical properties, which are represented by triply degenerate nodal points in topological semimetals. Here, based on the space group theory analysis, we propose a practical guidance for seeking a topological semimetal with triply degenerate nodal points located at a symmetric axis, which is applicable to both symmorphic and nonsymmorphic crystals. By using this guidance in combination with the first-principles electronic structure calculations, we predict a class of triply degenerate topological semimetals $R{\mathrm{Rh}}_6{\mathrm{Ge}}_4(R=\text{Y},\text{La},\text{Lu})$. In these compounds, the triply degenerate nodal points are located at the $\mathrm{\Gamma }\text{−}A$ axis and not far from the Fermi level. Especially, ${\mathrm{LaRh}}_6{\mathrm{Ge}}_4$ has a pair of triply degenerate nodal points located at just $\sim 3$ meV below the Fermi level. Considering the fact that the single crystals of $R{\mathrm{Rh}}_6{\mathrm{Ge}}_4$ have been synthesized experimentally, the $R{\mathrm{Rh}}_6{\mathrm{Ge}}_4$ class of compounds will be an appropriate platform for studying exotic physical properties of triply degenerate topological semimetals.

The deep double donor levels of substitutional chalcogen impurities in silicon have unique optical properties which may enable a spin/photonic quantum technology. The interstitial magnesium impurity (${\mathrm{Mg}}_i$) in silicon is also a deep double donor but has not yet been studied in the same detail as have the chalcogens. In this paper, we look at the neutral and singly ionized ${\mathrm{Mg}}_i$ absorption spectra in natural silicon and isotopically enriched 28-silicon in more detail. The $1\mathrm{s}({\mathrm{A}}_1$) to $1\mathrm{s}({\mathrm{T}}_2$) transitions, which are very strong for the chalcogens and are central to the proposed spin/photonic quantum technology, could not be detected. We observe the presence of another double donor (${\mathrm{Mg}}_{i*}$) that may result from ${\mathrm{Mg}}_i$ in a reduced symmetry configuration, most likely due to complexing with another impurity. The neutral species of ${\mathrm{Mg}}_{i*}$ reveal unusual low-lying ground state levels detected through temperature dependence studies. We also observe a shallow donor which we identify as a magnesium-boron pair.

We study theoretically the electron spin noise in quantum dots under nonequilibrium conditions caused by the pumping by a train of circularly polarized optical pulses. In such a situation, the nuclear spins are known to adjust in such a way that the electron spin precession frequencies become multiples of the pump pulse repetition frequency. This so-called phase synchronization effect was uncovered in A. Greilich et al. [Science 317, 1896 (2007)] and termed nuclei-induced frequency focusing of electron spin coherence. Using the classical approach to the central spin model, we evaluate the nuclear spin distribution function and the electron spin noise spectrum. We show that the electron spin noise spectrum consists of sharp peaks corresponding to the phase synchronization conditions and directly reveal the distribution of the nuclear spins. We discuss the effects of nuclear spin relaxation after the pumping is over and analyze the corresponding evolution of nuclear spin distributions and electron spin noise spectra.

Free electrons can efficiently absorb or emit plasmons excited in a thin conductor, giving rise to multiple energy peaks in the transmitted electron spectra separated by multiples of the plasmon energy. When the plasmons are chiral, this can also give rise to transfer of orbital angular momentum (OAM). Here, we show that large amounts of OAM can be efficiently transferred between chiral plasmons supported by a thin film and free electrons traversing it. Under realistic conditions, our predictive simulations reveal efficient absorption of a number $\ell \gg 1$ of chiral plasmons of vorticity $m\gg 1$, resulting in an OAM transfer $\ell m\hslash \gg \hslash$. Our work supports the use of chiral plasmons sustained by externally illuminated thin films as a way of generating high-vorticity electrons, resulting in a remarkably large fraction of kinetic energy associated with motion along the azimuthal direction, perpendicular to the incident beam.

We present ab initio calculations of thermal conductivity of InN with vacancies and substitutional defects using a full solution of the Peierls-Boltzmann transport equation. Our parameter-free calculations are in good agreement with experimental measurements demonstrating the predictive power of this approach. Phonon-defect scattering rates are computed from a Green's function methodology that is nonperturbative and includes interatomic force constant variance induced near the defects. Restricting calculations to first-order perturbation approaches can overestimate optic phonon scattering rates by nearly three orders of magnitude. On the other hand, neglecting the force variance weakens the scattering rates by about an order of magnitude, mostly in the low-frequency region below 2 THz. This work elucidates important properties of phonon-defect scattering in thermal transport and demonstrates the predictive power of the coupling of Peierls-Boltzmann transport, Green's function methods, and density functional theory.

An approach to nonadiabatic dynamics of atoms in molecular and condensed matter systems under general nonequilibrium conditions is proposed. In this method interaction between nuclei and electrons is considered explicitly up to the second order in atomic displacements defined with respect to the mean atomic trajectory, enabling one to consider movement of atoms beyond their simple vibrations. Both electrons and nuclei are treated fully quantum-mechanically using a combination of path integrals applied to nuclei and nonequilibrium Green's functions to electrons. Our method is partitionless: initially, the entire system is coupled and assumed to be at thermal equilibrium. Then, the exact application of the Hubbard-Stratonovich transformation in mixed real and imaginary times enables us to obtain, without doing any additional approximations, an exact expression for the reduced density matrix for nuclei and hence an effective quantum Liouville equation for them, both containing Gaussian noises. It is shown that the time evolution of the expectation values for atomic positions is described by an infinite hierarchy of stochastic differential equations for atomic positions and momenta and their various fluctuations. The actual dynamics is obtained by sampling all stochastic trajectories. It is expected that applications of the method may include photoinduced chemical reactions (e.g., dissociation), electromigration, and atomic manipulation in scanning tunneling microscopy, to name just a few.

Frustration in a spin system can give rise to unique ordered states and as a consequence several physical phenomena are expected, such as multiferroics, high temperature superconductors, and anomalous Hall effect. Here we report the “new magnetic orders” induced by anisotropic spin exchanges on pyrochlore spinels as the interplay of spin-orbit coupling and geometrical frustration. Due to complicated superexchange paths of B-site spinels, we claim that anisotropic interaction between next-nearest neighbors play an important role. Based on the systematic studies of the spin model, we argue that several classical spin states can be explored in spinel systems; local XY state, all-in all-out state, Palmer-Chalker state, and coplanar spiral state. In addition, we reveal new types of magnetic phases with finite ordering wave vectors, labeled as octagonal (prism) state and (distorted) cubic states. When the octagonal prism state is stabilized, nonzero scalar spin chirality induces alternating net current in addition to finite orbital current and orbital magnetization even in Mott insulators. Finally, we also discuss the relevance of distorted cubic state to the magnetic order of spinel compound ${\mathrm{GeCo}}_2{\mathrm{O}}_4$.

We investigated the phase-transition kinetics of magnetic skyrmion lattice (SkL) in MnSi by means of stroboscopic small-angle neutron scattering (SANS). Temporal evolutions of SANS patterns were measured with time resolution of 13 ms while sweeping temperature as fast as $50{\mathrm{Ks}}^{-1}$. It turned out that the paramagnetic-to-SkL transition immediately occurs upon traversing the equilibrium phase boundary on the rapid cooling, whereas the SkL-to-conical transition can be kinetically avoided to realize the low-temperature metastable SkL with a long-range magnetic order. The formation of the metastable SkL was found to be strongly dependent not only on cooling rate, but also on magnetic field and trajectory in the $H\text{−}T$ phase diagram.

${}^{195}\mathrm{Pt}$ NMR measurements have been performed on a single crystal of the uranium-based compound UPtGe, which is the only $5f$ system known to possess an incommensurate helical (cycloidal) structure below $T_{\mathrm{N}}=50$ K. Knight shift measurement confirmed the isotropic character of the static spin susceptibilities in hexagonal-like $ac$ crystal planes of the EuAuGe-type crystal structure. The hyperfine coupling constants were also found to be isotropic in the planes, estimated to be 59 kOe/${\mu }_{\mathrm{B}}$. Nuclear relaxation rate ($1/T_1$) measurement revealed the development of the antiferromagnetic spin fluctuations with XY character confined to the hexagonal-like plane below 200 K. The results present a clear contrast to the Ising anisotropy of the fluctuations in ferromagnetic superconductors UCoGe and URhGe.