We introduce a machine-learning (ML) interatomic potential for Mg-H system based on Behler-Parrinello approach. In order to fit the complex bonding conditions in the cluster structure, we combine multiple sampling strategies to obtain training samples that contain a variety of local atomic environments. First-principles calculations based on density functional theory (DFT) are employed to get reference energies and forces for training the ML potential. For the calculation of bulk properties, phonon dispersion, gas-phase ${\mathrm{H}}_2$ interactions, and the potential energy surface (PES) for ${\mathrm{H}}_2$ dissociative adsorption on Mg(0001) surfaces, our ML potential has reached DFT accuracy at the level of GGA-PBE, and can be extended by combining the DFT-D3 method to describe van der Waals interaction. Moreover, through molecular dynamics (MD) simulations based on the ML potential, we find that for ${\mathrm{Mg}}_n{\mathrm{H}}_m$ clusters, $\mathrm{Mg}/\mathrm{Mg}{\mathrm{H}}_x$ phase separation occurs when $m<2n$, and for a cluster with a diameter of about 4 nm, the Mg part of the cluster forms a hexagonal close-packed (hcp) nanocrystalline structure at low temperature. Also, the calculated diffusion coefficients reproduce the experimental values and confirm an Arrhenius type temperature dependence in the range of 400 to 700 K.

A moiré pattern occurs when two periodic structures in a system have a slightly mismatched period, resulting in the coexistence of distinct phases in different large-scale spatial regions of the same system. The periodic structures can arise from the intrinsic crystalline order or periodic external field. We investigate the moiré pattern using a dimerized Kitaev spin model with a periodic transverse field, which can be mapped onto the system of dimerized spinless fermions with $p$-wave superconductivity or a quasi-one-dimensional Su-Schrieffer-Heeger Majorana lattice. The exact solution for the staggered field demonstrated that the ground state has two distinct phases: a Néel magnetic phase for the nonzero field and a spin liquid phase due to the emergence of an isolated flat Bogoliubov–de Gennes band for the vanishing field. We compute the staggered magnetization and local density of state for the field with a slightly different period of the lattice. The numerical simulation demonstrates that two such phases appear alternatively along the system with a long beat period. Additionally, we propose a dynamic scheme, via a evanescently coupled-waveguide array, to detect the moiré fringes based on the measurement of the Loschmidt echo in the presence of local perturbations.

We studied the stability of skyrmion formation and its abnormal dynamic motions in magnetic nanotubes with perpendicular magnetic anisotropy with respect to the tube surface. Unlike planar geometry, skyrmions, which are formed in an elliptical shape, are stabilized, to a greater or lesser degree, by the curved geometry, depending on the sign of the Dzyaloshinskii-Moriya Interaction (DMI) constant. The positive and negative signs of the DMI constant play a crucial role in the formation of skyrmions on the curved surface. These effects become stronger as the diameter of the tube decreases, quantitatively evidencing that the curvature-induced DM-like interaction also contributes to the stability of skyrmion formation. The dynamic modes of the skyrmion in the tubes are found to be as follows: two in-plane gyration modes in the counterclockwise (CCW) and clockwise (CW) rotation senses, and one out-of-plane breathing mode. On the other hand, in contrast to the planar geometry, the initial CCW gyration motion turns out to be a linear translational motion in the circumferential direction of the tube for cases where the strength of axial AC magnetic fields is greater than a threshold field strength. The direction of the translational motions is determined by the skyrmion helicity governed by the sign of the interfacial DMI constant. This work provides further physical insight into the static and dynamic properties of skyrmions formed in curved-geometry systems and also suggests its potential application to information processing devices.

${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ has recently emerged as a ferromagnetic Weyl semimetal. Theoretical investigation of the spin-split bands predicted half metallicity in the compound. Here, we report the detection of a spin-polarized supercurrent through a $\mathrm{Nb}/{\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ point contact where Andreev reflection is seen to be large, indicating a large deviation from half metallicity. In fact, analysis of the Andreev reflection spectra reveals very small spin polarization at different points with the degree of spin polarization ranging from 20% to 50% at the Fermi level of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. Our theoretical calculations of electronic density of states reveal a spin-depolarizing effect near the Fermi energy when the role of spin-orbit coupling is included. The inclusion of spin-orbit coupling also reveals particle-hole asymmetry that explains a large asymmetry observed in our experimental Andreev reflection spectra.

Magnetization switching in a fine-structured ferromagnet of nanoscale by the spin-transfer torque excited via the spin Hall effect has attracted much attention because it enables us to manipulate the magnetization without directly applying current to the ferromagnet. However, the switching mechanism is still unclear in regard to the ferromagnet having an in-plane easy axis parallel to the current. Here we develop an analytical theory of the magnetization switching in this type of ferromagnet, and reveal the threshold current formulas for a deterministic switching. It is clarified that the current should be in between a certain range determined by two threshold currents because the spin-transfer torque due to a large current outside the range brings the magnetization in an energetically unstable state, and causes magnetization precession around the hard axis.

Spin waves in antiferromagnetic materials have great potential for next-generation magnonic technologies. However, their properties and their dependence on the type of ground-state antiferromagnetic structure are still open questions. Here, we investigate theoretically spin waves in one- and two-dimensional model systems with a focus on noncollinear antiferromagnetic textures such as spin spirals and skyrmions of opposite topological charges. We address in particular the nonreciprocal spin excitations recently measured in bulk antiferromagnet $\alpha -{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ utilizing inelastic neutron scattering experiments [Phys. Rev. Lett. 119, 047201 (2017)], where we help to characterize the nature of the detected spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya interaction can lift the degeneracy of the spin-wave modes in antiferromagnets, resembling the electronic Rashba splitting. We consider the spin-wave excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss the features of their inelastic scattering spectra. We demonstrate that antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya interaction in certain antiferromagnets.

We present a real-time ab initio description of optical orientation in bulk GaAs due to the coupling with an ultrashort circularly polarized laser source. The injection of spin-polarized electrons in the conduction band is correctly reproduced, and a nonvanishing spin polarization $\mathbf{P}$ parallel to the direction of propagation of the laser ($z$) emerges. A detailed analysis of the generation and the evolution of $\mathbf{P}\left(t\right)$ is given. The single $\mathbf{k}$-point dynamics is a motion of precession around a fixed axis with constant $\left|\mathbf{P}\right|$ and fixed frequency. Instead, the $\mathbf{k}$-integrated signal shows only a time-dependent $P_z\left(t\right)$ and decays a few picoseconds after the end of the laser pump due to decoherence. Decoherence emerges since the individual contributions activated by the pump give rise to destructive interference. We interpret the results in terms of the free induction decay mechanism proposed some years ago [M. W. Wu and C. Z. Ning, Eur. Phys. J. B 18, 373 (2000)]. We are able to reproduce such an effect in a full ab initio fashion, giving a quantitative estimate of the associated decay time. Our result also shows a possible explanation for the time decay of spin magnetization observed in many real-time ab initio simulations.

The ground states of bosons have been classified into superfluids, bosonic quantum Hall states, Mott insulators, and Bose glass. Recent experiments in two-dimensional clean superconductors under an external magnetic field $B$ strongly suggest the existence of the fourth quantum state of Cooper pairs, i.e., Bose metal or quantum metal, where the resistivity remains constant at lowest temperature. However, its theoretical understanding remains unsettled. In this paper, we propose a model where the vortices behave as quantum bosons subject to dissipation at the normal core. We discuss that those bosons remain metallic even at zero temperature by generalizing the Feynman picture of superfluidity in the first quantization formulation and also by a perturbative calculation for a field theoretical model in the second quantization formalism. This result indicates that the resistivity $\rho$ of Bose metal at zero temperature behaves as $\rho \sim {\rho }_n(B/B_{c2})$ with ${\rho }_n$ being the residual resistivity in the normal state and $B_{c2}$ the second upper critical field. It also predicts that the Bose metal is missing in superclean superconductors and at the dirty limit or in granular superconductors.

It has been predicted theoretically and indirectly confirmed experimentally that single-layer $\mathrm{Cr}{X}_3$ ($X$ = Cl, Br, I) might be the prototypes of topological magnetic insulators (TMI). In this work, by using first-principles calculations combined with atomistic spin dynamics, we provide a complete picture of the magnetic interactions and magnetic excitations in $\mathrm{Cr}{X}_3$. The focus is here on the two most important aspects for the actual realization of TMI, namely the relativistic magnetic interactions and the finite-size (edge) effects. We compute the full interaction tensor, which includes both Kitaev and Dzyaloshinskii-Moriya (DM) terms, which are considered as the most likely mechanisms for stabilizing topological magnons. First, we instigate the properties of bulk ${\mathrm{CrI}}_3$ and compare the simulated magnon spectrum with the experimental data [Phys. Rev. X 8, 041028 (2018)]. Our results suggest that a large size of topological gap, seen in experiment ($\approx 4$ meV), cannot be explained by considering pair-wise spin interactions only. We identify several possible reasons for this disagreement. The magnetic interactions in the monolayers of $\mathrm{Cr}{X}_3$ are also investigated. The strength of the anisotropic interactions is shown to scale with the position of halide atom in the periodic table, the heavier the element the larger is the anisotropy, in agreement with prior studies. Comparing the magnons for the bulk and single-layer ${\mathrm{CrI}}_3$, we find that the size of the topological gap becomes smaller in the latter case. The obtained next nearest-neighbor DM vector is oriented primarily in-plane of the monolayer and has relatively small $z$ component, which results in a small value of the topological gap. Finally, we investigate finite-size effects in monolayers and demonstrate that the anisotropic couplings between Cr atoms close to the edges are much stronger than those in ideal periodic structure. This should have impact on the dynamics of the magnon edge modes in Cr halides.

Magnesium (Mg) atoms in interstitial positions of a silicon host lattice form double donor centers. The binding energy of the neutral $\mathrm{M}{\mathrm{g}}^0$ state corresponds to midinfrared wavelengths. Due to its interstitial character, the outer orbitals of Mg atoms can form specific bonds with different trace elements in the silicon crystal. These occur as single and double donors that become detectable by infrared spectroscopy when the concentration of either Mg or the dopant coupled to it is sufficiently large. Infrared absorption spectroscopy at high magnetic fields allows us to observe the Zeeman effect of several shallow Mg-related donor complexes and neutral Mg donors, very similar to those observed for hydrogenlike centers. The quadratic Zeeman effect indicates a hydrogenlike center structure of the excited states and spatial localization of complexes with a single excess electron in its outer shell, which are formed by Mg with an impurity atom. In contrast, Mg complexes formed by bonds with two electrons are closer to substitutional double donors in silicon.

We develop a theory for the nonlinear time-domain response of a semimetal driven by an ultrafast optical pulse. At quadratic order in the driving field we find that the response near a band inversion transition contains a coherent oscillating component proportional to field intensity with a frequency that can be tuned over a wide range (from terahertz to the near IR) selected by the chemical potential. We illustrate the effect by calculating the induced current as a function of time for a two-band system with time-reversal symmetry but broken parity symmetry to study semimetals where large Berry curvature near a band inversion transition promotes this nonlinear response.

We study the system of a transition metal dichalcogenide (TMD) monolayer placed in an optical resonator, where the strong light-matter coupling between excitons and photons is achieved. We present a quantitative theory of the nonlinear optical response for exciton-polaritons for the case of a doped TMD monolayer, and analyze in detail two sources of nonlinearity. The first nonlinear response contribution stems from the Coulomb exchange interaction between excitons. The second contribution comes from the reduction of Rabi splitting that originates from phase space filling at increased exciton concentration and the composite nature of excitons. We demonstrate that both nonlinear contributions are enhanced in the presence of free electrons. As free electron concentration can be routinely controlled by an externally applied gate voltage, this opens a way of electrical tuning of the nonlinear optical response.

Based on ab initio relativistic $\mathbf{k}·\mathbf{p}$ theory, we derive an effective two-band model for surface states of three-dimensional topological insulators up to seventh order in $\mathbf{k}$. It provides a comprehensive description of the surface spin structure characterized by a nonorthogonality between momentum and spin. We show that the oscillation of the nonorthogonality with the polar angle of $\mathbf{k}$ with a $\pi /3$ periodicity can be seen as due to effective sixfold symmetric spin-orbit magnetic fields with a quintuple and septuple winding of the field vectors per single rotation of $\mathbf{k}$. Owing to the dominant effect of the classical Rashba field, there remains a single-winding helical spin structure but with a periodic few-degree deviation from the orthogonal locking between momentum and spin.

${\mathrm{WTe}}_2$ is a multifunctional quantum material exhibiting numerous emergent phases in which tuning of the carrier density plays an important role. Here, we demonstrate two nonmonotonic changes in the electronic structure of ${\mathrm{WTe}}_2$ upon in situ electron doping. The first phase transition is interpreted in terms of a shear displacement of the top ${\mathrm{WTe}}_2$ layer, which realizes a local crystal structure not normally found in bulk ${\mathrm{WTe}}_2$. The second phase transition is associated with stronger interactions between the dopant atoms and the host, both through hybridization and electric field. These results demonstrate that electron doping can drive structural and electronics changes in bulk ${\mathrm{WTe}}_2$ with implications for realizing nontrivial band-structure changes in heterointerfaces and devices.

So far, the circular photogalvanic effect (CPGE) is the only possible quantized signal in Weyl semimetals. With inversion and mirror symmetries broken, Weyl and multifold fermions in band structures with opposite chiralities can stay at different energies and generate a net topological charge. Such a kind of net topological charge can present as a quantized signal in the circular polarized light-induced injection current. According to current theoretical understanding, RhSi and its counterparts are believed to be the most promising candidates for the experimental observation of the quantized CPGE. However, a real quantized signal has yet to be experimentally observed. Since previous theoretical studies for the quantized CPGE were based on an effective model but not realistic band structures, it should lose some crucial details that influence the quantized signal. The current status motivates us to perform a realistic ab initio study for the CPGE. Our result shows that RhSi and PtAl in chiral multifold semimetals are alternative materials for obtaining the quantized CPGE which is very easy to be interfered with by trivial band-related optic transitions, and a fine tuning of the chemical potential by doping is essential for the observation of the quantized CPGE. We perform an ab initio analysis for the quantized CPGE based on a realistic electronic band structure and provide an effective way to solve the current problem for the given materials.

We demonstrate that ${\mathbb{Z}}_2$ gauge transformations and lattice deformations in Kitaev's honeycomb lattice model can have the same description in the continuum limit in terms of a chiral gauge field. The chiral gauge field is coupled to the Majorana fermions that satisfy the Dirac dispersion relation in the non-Abelian sector of the model. For particular values, the effective chiral gauge field becomes equivalent to the ${\mathbb{Z}}_2$ gauge field, enabling us to associate effective fluxes to lattice deformations. Motivated by this equivalence, we consider Majorana-bounding $\pi$ vortices and Majorana-bounding lattice twists and demonstrate that they are adiabatically connected to each other. This equivalence opens the possibility for novel encoding of Majorana-bounding defects that might be easier to realize in experiments.

In this work we compute the torque field present in a ferromagnet in contact with a metallic nanowire when a skyrmion is present. If the nanowire is narrow enough, then the current is carried by a single conduction band. In this regime the classical torque model breaks down and we show that a skyrmion driven by spin transfer torque moves in a different direction than predicted by the classical model. However, the amount of charge current required to move a skyrmion with a certain velocity in the single-band regime is similar to a classical model of torque where it is implicitly assumed current transport by many conduction bands. The single-band regime is more efficient creating spin current from charge current because of the perfect polarization of the single band but is less efficient creating torque from spin current. Nevertheless, it is possible to take profit of the single-band regime to move skyrmions even with no net charge or spin current flowing between the device contacts. We have also been able to recover the classical limit considering an ensemble of only a few electronic states. In this limit we have discovered that electron diffusion needs to be considered even in ballistic nanowires due the effect of the skyrmion structure on the electron current.

Defects and impurities strongly dictate the physical properties of materials by tuning conductivity, modulating magnetic response, and changing optical properties. Here we analyze how these entities behave in the prototype quantum material ${\mathrm{FeGa}}_3$. We have studied its intrinsic defects (vacancies, antisites, and interstitials) as well as different types of dopants such as Zn, Mn, Ge, Co, In, and Ru. Intrinsic defects are shown to have large formation energies, whereas dopants have a larger solubility. Different electronic and magnetic states, including localized polarons, were found to be induced by specific defect charge states, significantly altering the properties of the host material.

We examine the validity of the widely used T-matrix approximation for treating phonon-disorder scattering by implementing an unfolding algorithm that allows simulation of disorder up to tens of millions of atoms. The T-matrix approximation breaks down for low-energy flexure phonons that play an important role in thermal transport in two-dimensional materials. Furthermore, insights are developed into the success of the T-matrix approximation in describing maximally mass disordered systems. To achieve this, the phonon unfolding formalism is generalized to describe mass disorder and strongly nonperturbative features of the spectrum are connected to the Boltzmann quasiparticle picture.