Using high-level quantum chemistry calculations, we predict various ultrafast laser-induced spin dynamics scenarios in clusters ${\mathrm{Fe}}_m{\mathrm{Bz}}_n$ ($m,n=1,2$) based on the nonadiabatic $\mathrm{\Lambda }$ process—an indirect transition channel from the initial state to the final state through the participation of several spin-mixed intermediate states. The geometry-dependent electronic structures of the four clusters are found to exhibit distinct characteristics, and prove to strongly affect the laser parameters and dynamical behavior of the spin-flip scenarios. For the two Fe-dimer clusters, a charge-transfer state involving spin-transfer scenario is achieved in ${\mathrm{Fe}}_2\mathrm{Bz}$, which turns to be much faster than the ordinary one obtained in ${\mathrm{Fe}}_2{\mathrm{Bz}}_2$, and thus is considered to be promising for future spintronics applications. Furthermore, to provide valuable information for the measurement and implementation of the ultrafast magnetism response, the effects of the full width at half maximum (FWHM) of the laser pulse and the magnitude of the external magnetic field on the proposed spin scenarios are investigated. The analysis of the stability and sensitivity of each scenario shows that the spin-transfer scenarios have smaller tolerance values than spin-flip scenarios with respect to the laser FWHM, and a desired spin scenario is only possible when the magnetic field strength lies in a moderate region. The latter effect enables us to obtain the lateral resolution of the device sample for its possible memory usage and to reach the complementary metal oxide semiconductor scale, beyond the optical resolution limit. All the results in this paper contribute a further step toward the experimental realization of our spin dynamics and their future nanospintronics applications.

The phase difference $\varphi$, between the superconducting terminals in superconductor–normal metal–superconductor tunnel junctions (SINIS), incorporates the phase differences ${\chi }_{1,2}$ across thin interfaces of constituent SIN junctions and the phase incursion $\phi$ between the side faces of the central electrode of length $L$. It is demonstrated here that ${\chi }_{1,2}$ pass through over their proximity-reduced domain twice, there and back, while $\varphi$ changes over the single period. Two corresponding solutions, that describe the double-valued order-parameter dependence on ${\chi }_{1,2}$, jointly form the single-valued dependence on $\varphi$, operating in two adjoining regions of $\varphi$. The phase incursion $\phi$ plays a crucial role in creating such a behavior. The current-phase relation $j(\varphi ,L)$ is composed of the two solutions and, at a fixed small $L$, is characterized by the phase-dependent effective transmission coefficient.

We report a temperature- and magnetic-field-dependent Raman-scattering study of the spin-ladder compound ${\mathrm{BaFe}}_2{\mathrm{Se}}_2\mathrm{O}$. Temperature evolution of the $B_{1g}$ mode self-energies revealed anomalous behavior at about 100 and 240 K with strong temperature-dependent Fano asymmetry. Furthermore, the $A_g$ modes integrated intensity exhibits an additional change in tendency at about 50 K. All the observed anomalies can be traced back to spin-phonon interaction contributions as well as multiple magnetic phase transitions present in ${\mathrm{BaFe}}_2{\mathrm{Se}}_2\mathrm{O}$ also detected in the Raman continuum induced by the spin fluctuations. Moreover, the absence of magnetic-field dependence of the magnetic mode observed at $436{\mathrm{cm}}^{-1}$ and the small linewidth and high intensity are different from the magnetic modes at about $650{\mathrm{cm}}^{-1}$. This suggests a two-magnon continuum and a two-magnon bound state resonance for this mode.

While the pure elements tend to exhibit type-I rather than type-II superconductivity, nearly all compound superconductors are type-II, with only a few known exceptions. We report single-crystal growth and physical characterization of the rhenium aluminide ${\mathrm{Al}}_6\mathrm{Re}$, which we conclude is a type-I superconductor based on magnetization, ac susceptibility, and specific-heat measurements. This detection of superconductivity, despite the strong similarity of ${\mathrm{Al}}_6\mathrm{Re}$ to a family of W and Mo aluminides that do not superconduct, suggests that these aluminides are an ideal testbed for identifying the relative importance of valence electron count and inversion symmetry in determining whether a material will superconduct.

Current-induced spin polarization and the resulting spin-orbit torque (SOT) in a ferromagnet/topological insulator (FM/TI) bilayer have been investigated by taking into account the hexagonal warping spectrum of topological surface states. We find that the usually ignored in-plane FM magnetization plays an important role to the spin polarization. The resulting spin polarization and spin torque significantly depend on azimuthal angle of the magnetization, which has not been reported theoretically before in the linear dispersion TI model. These interesting results arise from the combination effect of in-plane magnetization and warping effect by modifying the Berry curvature and impurity scattering. Based on Matsubara-Green function approach, we derive the formula of SOT and analyze the results analytically and numerically, including the contribution from intrabands and interbands, and intrinsic and extrinsic contribution. More importantly, it is found that the hexagonal warping can prominently enhance the antidamping SOT if there exists the in-plane FM magnetization, which provides a new perspective to understand the recent giant SOT effect.

We propose a surface-wave dispersion retrieval method and synthesis technique that applies to bianisotropic metasurfaces that are embedded in symmetric or asymmetric environments. Specifically, we use general zero-thickness sheet transition conditions to relate the propagation constants of surface-wave modes to the bianisotropic susceptibility components of the metasurface, which can themselves be directly related to its scattering parameters. It is then possible to either obtain the metasurface dispersion diagram from its known susceptibilities or, alternatively, compute the susceptibilities required to achieve a desired surface-wave propagation. The validity of the method is demonstrated by comparing its results to those obtained with exact dispersion relations of well known structures such as the propagation of surface plasmons on thin metallic films. In particular, this work reveals that it is possible to achieve surface-wave propagation only on one side of the metasurface either by superposition of symmetric and asymmetric modes in the case of anisotropic metasurfaces or by completely forbidding the existence of the surface wave on one side of the structure using bianisotropic metasurfaces.

The combination of strong spin-orbit coupling and strong correlations holds tremendous potential for interesting physical phenomena as well as applications in spintronics and quantum computation. In this context, we here study the interplay between the Rashba spin-orbit coupling (RSOC) and the Kondo screening in noncentrosymmetric $f$-electron materials. We show that the Kondo coupling of the $f$ electrons becomes anisotropic at high temperatures due to the RSOC. However, an isotropic Kondo effect is restored at low temperature, which leads to a complete Kondo screening. We furthermore demonstrate that the Kondo effect has influence on the Rashba splitting in the band structure, which becomes temperature dependent. Although the $f$ electrons are localized at high temperature, a helical spin polarization of the conduction band emerges due to the scattering with the $f$ electrons. With decreasing temperature, the Kondo screening occurs, which leads to drastic changes in the band structure. Remarkably, these changes in the band structure depend on the helical spin polarization. For strong RSOC, we observe that the hybridization gap of one of the helical bands is closed at low temperature and a helical half metal is formed.

Weyl semimetals (WSM) are a new class of topological materials that exhibit a bulk Hall effect and a chiral magnetic effect. The topological contribution of these unusual electromagnetic responses can be characterized by an axion term $\theta \mathbf{E}·\mathbf{B}$ with space and time dependent axion angle $\theta (\mathbf{r},t)$. In this paper we compute the electromagnetic fields produced by an electric charge near a topological Weyl semimetal with two Weyl nodes, in the equilibrium state, at zero electric chemical potential, and with broken time-reversal symmetry. We find that, as in ordinary metals and dielectrics, outside the WSM the electric field is mainly determined by the optical properties of the material. The magnetic field is, on the contrary, of topological origin due to the magnetoelectric effect of topological phases. We show that the magnetic field exhibits an interesting behavior above the WSM as compared with that induced above a topological insulator: the field lines begin at the surface and then end at the surface (but not at the same point). This distinctive behavior of the magnetic field is an experimentally observable signature of the anomalous Hall effect in the bulk of the WSM. We discuss two experimental setups for testing our predictions of the induced magnetic field.

Graphene grown epitaxially on SiC, close to the charge neutrality point (CNP), in an orthogonal magnetic field shows an ambipolar behavior of the transverse resistance accompanied by a puzzling longitudinal magnetoresistance. When injecting a transverse current at one end of the Hall bar, a sizable nonlocal transverse magnetoresistance is measured at low temperature. While Zeeman spin effect seems not to be able to justify these phenomena, some dissipation involving edge states at the boundaries could explain the order of magnitude of the nonlocal transverse magnetoresistance but not the asymmetry when the orientation of the orthogonal magnetic field is reversed. As a possible contribution to the explanation of the measured nonlocal magnetoresistance, which is odd in the magnetic field, we derive a hydrodynamic approach to transport in this system, which involves particle and hole Dirac carriers, in the form of charge and energy currents. We find that thermal diffusion can take place on a large distance scale, thanks to long recombination times, provided a noninsulating bulk of the Hall bar is assumed, as recent models seem to suggest in order to explain the appearance of the longitudinal resistance. In presence of the local source, some leakage of carriers from the edges generates an imbalance of carriers of opposite sign, which are separated in space by the magnetic field and diffuse along the Hall bar generating a nonlocal transverse voltage.

We theoretically investigate the indirect signatures of the tilted anisotropic Dirac cones on Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in a two-dimensional polymorph of boron atoms. Unlike the case of isotropic nontilted Dirac material graphene, here we observe that the tilting of the Dirac cones exhibits a significant impact on the RKKY exchange interaction in terms of the suppression of oscillation frequency. The reason can be attributed to the behavior of the Fermi level and the corresponding density of states with respect to the tilting parameter. The direct measurement of the period of the RKKY interaction can thus be a possible probe of the tilt parameter associated with the tilted Dirac cones. We also obtain the direction dependent analytical expressions of the RKKY exchange interaction, in terms of Meijer G-function. However, the effects due to tilting of the Dirac cones on the RKKY interaction depend on the spatial alignments of the two magnetic impurities with respect to the direction of tilting.

In the presence of strong enough disorder one-dimensional systems of interacting spinless fermions at nonzero filling factor are known to be in a many-body localized phase. When represented in ‘Fock space,’ the Hamiltonian of such a system looks like that of a single ‘particle’ hopping on a Fock lattice in the presence of a random disordered potential. The coordination number of the Fock lattice increases linearly with the system size $L$ in one dimension. Thus in the thermodynamic limit $L\to \infty$, the disordered interacting problem in one dimension maps on to an Anderson model with infinite coordination number. Despite this, this system displays localization which appears counterintuitive. A close observation of the on-site disorder potentials on the Fock lattice reveals a large degree of correlation among them as they are derived from an exponentially smaller number of on-site disorder potentials in real space. This indicates that the correlations between the on-site disorder potentials on a Fock lattice has a strong effect on the localization properties of the corresponding many-body system. This intuition is also consistent with studies of quantum random energy model where the typical mid-spectrum states are ergodic and the on-site potentials in Fock space are completely uncorrelated. In this work we perform a systematic quantitative exploration of the nature of correlations of the Fock space potential required for localization. We study different functional variations of the disorder correlation in Fock lattice by analyzing the eigenspectrum obtained through exact diagonalization. Without changing the typical strength of the on-site disorder potential in Fock lattice we show that changing the correlation strength can induce thermalization or localization in systems. From among the various forms of correlations we study, we find that only the linear variation of correlations with Hamming distance in Fock space is able to drive a thermal-MBL phase transition where the transition is driven by the correlation strength. Systems with the other forms of correlations we study are found to be ergodic.

We study the dynamical behavior of doped electronic systems subject to a global ramp of the repulsive Hubbard interaction. We start with formulating a real-time generalization of the fluctuation-exchange approximation. Implementing this numerically, we investigate the weak-coupling regime of the Hubbard model both in the electron-doped and hole-doped regimes. The results show that both local and nonlocal (momentum-dependent) observables evolve toward a thermal state, although the temperature of the final state depends on the ramp duration and the band filling. We further reveal a momentum-dependent relaxation rate of the distribution function in doped systems and trace back its physical origin to the anisotropic self-energies in the momentum space.

It is difficult to unambiguously confirm the existence of Majorana zero modes (MZMs) due to the absence of smoking-gun signatures in charge transport measurements. Recent studies suggest that the spin degree of freedom of MZMs may provide an alternative detection method. We study the spin properties of the superconducting state in Majorana nanowires and the associated unconventional Josephson effect with realistic experimental parameters taken from Zhang et al., Nature (London) 556, 74 (2018). We start from a general discussion of the polarized spin-triplet pairing, characterized by the superconducting spin polarization, which is fundamentally different from the spin polarization of electrons. For a superconducting thin film with in-plane polarized spin-triplet pairing, an out-of-plane electric field can lead to the spin-obit coupling which generates a supercurrent perpendicular to both the superconducting spin polarization and the electric field, so we name this phenomena as spin-orbit coupling driven transverse supercurrent (SOCDTS). In a Majorana nanowire, the regime with finite superconducting spin polarization almost coincides with the helical regime, which includes the topological regime. We further study the effects of polarized spin-triplet pairing in two types of Josephson junctions. One dramatic finding is that SOC can induce an anomalous supercurrent at zero phase difference only in the U-shape junction, which has been proposed as a basic ingredient of scalable topological quantum computation with the system in the topological regime. This can be viewed as a consequence of the SOCDTS. Our work reveals that the unique superconducting spin properties of MZMs can affect the unusual topological Josephson effect and is indeed helpful for detecting MZMs.

We investigate the origin of overshoots in the exciton spin dynamics after resonant optical excitation. As a material system, we focus on diluted magnetic semiconductor quantum wells as they provide a strong spin-flip scattering for the carriers. Our study shows that overshoots can appear as a consequence of radiative decay even on the single-particle level in a theory without any memory. The magnitude of the overshoots in this case depends strongly on the temperature as well as the doping fraction of the material. If many-body effects beyond the single-particle level become important so that a quantum-kinetic description is required, a spin overshoot appears already without radiative decay and is much more robust against variations of system parameters. We show that the origin of the spin overshoot can be determined either via its temperature dependence or via its behavior for different doping fractions. The results can be expected to apply to a wide range of semiconductors as long as radiative decay and a spin-flip mechanism are present.

First-principles simulations were used to investigate water (${\mathrm{H}}_2\mathrm{O}$) decomposition induced by a femtosecond laser with high flux $\sim 1\times {10}^{20}\mathrm{photons}/\left(\sec \mathrm{c}{\mathrm{m}}^2\right)$. One goal of our research is to find metamaterials that locally enhance the laser field to reduce the threshold laser intensity required to decompose ${\mathrm{H}}_2\mathrm{O}$ molecules. In this work, small-diameter (6.3 Å) single-walled carbon nanotubes were found to reduce the threshold power by 90% compared with the power required to decompose ${\mathrm{H}}_2\mathrm{O}$ in the gas phase. The present results suggest a strategy for the design of materials with high energy efficiency for ${\mathrm{H}}_2\mathrm{O}$ decomposition based on polarizability and morphology (curvature) to enhance the local field. We demonstrate that carbon nanotubes enhance the local field resulting in a power enhancement of approximately eight times.

We present neutron scattering measurements of the phonon-roton and layer modes of liquid helium confined in 28 Å diameter nanopores of FSM-16. The goal is to determine the energy, lifetime, and intensity of the modes as a function of temperature. It is particularly to determine the highest temperature, denoted $T_{PR}$, at which well-defined phonon-roton modes are observed at higher wave vector ($Q>0.8$ ${\AA }^{-1}$) in the nanopores. The temperature $T_{PR}$, which can be identified with loss of Bose-Einstein condensation (BEC), can be compared with the superfluid to normal liquid transition temperature, $T_O$, and other transition temperatures of ${}^4\mathrm{He}$ in the nanopores. The aim is to identify the nature of BEC in a narrow nanopore. Two pressures are investigated, saturated vapor pressure (SVP) and $p=26$ bars. We find that well-defined P-R modes are observed up to temperatures much higher than the conventional superfluid to normal liquid transition temperature, $T_O$, observed in torsional oscillator measurements, i.e., $T_{PR}>T_O$. At SVP, $T_{PR}=1.8$ K and $T_O=0.9$ K. This supports the interpretation that BEC exists in a localized or partially localized form in the temperature range $T_O<T<T_{PR}$; i.e., there is a localized BEC region lying between the superfluid and fully normal liquid phase, as observed in some other porous media. At close to full filling, the P-R mode energies in FSM-16 are similar to those in bulk liquid ${}^4\mathrm{He}$. However, a substantial P-R mode width at $T\to 0$ K and at higher temperatures is observed.

We discuss devices for detection of the topological insulator phase based on the two-path electron interference. For that purpose we consider buckled silicene, for which a local energy gap can be opened by a vertical electric field to close one of the interference paths and for which the quantum spin Hall insulator conditions are controlled by the Fermi energy. In quantum spin Hall phase the interference is absent due to the separation of the spin currents, and the conductance of the devices include sharp features related to localized resonances. In the normal transport conditions the two-path interference produces regular Aharonov-Bohm oscillations in the external magnetic field.

The melting curve and stability of ammonia (${\mathrm{NH}}_3$) is investigated up to 40 GPa and 3500 K by x-ray diffraction and Raman spectroscopy in the laser-heated diamond anvil cell. The ${\mathrm{NH}}_3$ samples were directly heated by the $10.6\mu \mathrm{m}$ radiation of a ${\mathrm{CO}}_2$ laser to reduce the risks of chemical reactions. Melting was unambiguously detected by the appearance of the liquid diffraction signal upon temperature increase. The melting temperature of ${\mathrm{NH}}_3$ is found to steadily increase with pressure up to 40 GPa, and the previously reported turnover is not observed. As a result, the melting line of ${\mathrm{NH}}_3$ is expected to cross the isentropes of Neptune and Uranus in the pressure range 55–65 GPa, implying the possible presence of superionic solid ${\mathrm{NH}}_3$ in these planets. Our x-ray and Raman measurements confirm the appearance of ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ upon heating the liquid phase from 6 to 40 GPa. But while the equilibrium $2{\mathrm{NH}}_3\rightleftharpoons {\mathrm{N}}_2+3{\mathrm{H}}_2$ balances towards the dissociated elements at low pressure and high temperature, ammonia is found to the more stable species in the range 20–40 GPa, 300–3000 K.

The interfaces in hybridized structures usually give rise to rich phenomena, which open the way to novel devices with extraordinary performance. Here, we investigate the interface-related spin transport properties in $\mathrm{Fe}\text{−}\mathrm{Au}$ bilayers based on first-principles calculations. We find that the spin Hall current in the Au side near the interface flows in the opposite direction to the bulk spin Hall current with the magnitude sensitive to the magnetization direction of Fe. This interfacial contribution is attributed to the spin-dependent transmission within a few atomic layers, where a strong interfacial Rashba spin-orbit coupling exists. Surprisingly, the interfacial spin Hall currents are found to be not confined at the interface but extend tens of nanometers. This length scale is found to be much shorter than the spin diffusion length and determined by momentum relaxation, instead of spin relaxation. The resistivity dependence of the bulk spin Hall angle implies the coexistence of different spin Hall mechanisms. In addition, the interfacial swapping spin currents, as a consequence of the spin precession under the interfacial Rashba field, are also obtained from our calculation and complete the full spin transport picture with all nonvanishing components. Our results suggest the importance of the interface engineering in spin-related transport in ferromagnetic-nonmagnetic heterostructures and the possibility of manipulating the interfacial transport by the magnetization orientation of the magnetic layer.