Vacancy is one of the most frequent defects in metals. We study the impacts of magnetism on vacancy formation properties in fcc Ni, and in bcc and fcc Fe, via density functional theory (DFT) and effective interaction models combined with Monte Carlo simulations. Overall, the predicted vacancy formation energies and equilibrium vacancy concentrations are in good agreement with experimental data, available only at the high-temperature paramagnetic regime. Effects of magnetic transitions on vacancy formation energies are found to be more important in bcc Fe than in fcc Fe and Ni. The distinct behavior is correlated to the relative roles of longitudinal and transversal spin excitations. At variance with the bcc-Fe case, we note a clear effect of longitudinal spin excitations on the magnetic free energy of vacancy formation in fcc Fe and Ni, leading to its steady variation above the respective magnetic transition temperature. Below the Néel point, such effect in fcc Fe is comparable but opposite to the one of the transversal excitations. Regarding fcc Ni, although neglecting the longitudinal spin excitations induces an overestimation of the Curie temperature by 220 K, no additional effect is visible below the Curie point. The distinct effects on the three systems are closely linked to DFT predictions of the dependence of vacancy formation energy on the variation of local magnetic-moment magnitudes and orientations.

We consider the steady-state nonequilibrium behavior of mesoscopic superconducting wires connected to normal-metal reservoirs. Going beyond the diffusive limit, we utilize the quasiclassical theory and perform a self-consistent calculation that guarantees current conservation through the entire system. Going from the ballistic to the diffusive limit, we investigate several crucial phenomena such as charge imbalance, momentum-resolved nonequilbrium distributions, and the current-to-superflow conversion. Connecting to earlier results for the diffusive case, we find that superconductivity can break down at a critical bias voltage $V_{\mathrm{c}}$. We find that $V_{\mathrm{c}}$ generally increases as the interface transparency is reduced, while the dependence on the mean-free path is nonmonotonous. We discuss the key differences of the ballistic and semiballistic regimes to the fully diffusive case.

We consider magic-angle twisted bilayer graphene at filling $\nu =+3$, where experiments have observed a robust quantized anomalous Hall effect. This has been attributed to the formation of a valley- and spin-polarized Chern insulating ground state that spontaneously breaks time-reversal symmetry and is stabilized by a hexagonal boron nitride substrate. We identify three different types of domain wall and study their properties and energetic selection mechanisms via theoretical arguments and Hartree-Fock calculations adapted to deal with inhomogeneous moiré systems. We comment on the implications of these results for transport and scanning probe experiments.

Hybridization between $f$ electrons and conduction bands (c-f hybridization) is a driving force for many unusual phenomena. To provide insight into it, systematic studies of ${\mathrm{CeCoIn}}_5$ heavy fermion superconductor have been performed by angle-resolved photoemission spectroscopy (ARPES) in a large angular range at temperature of $T=6$ K. The used photon energy of 122 eV corresponds to Ce $4d\text{−}4f$ resonance. Calculations carried out with the relativistic multiple scattering Korringa-Kohn-Rostoker method and one-step model of photoemission yielded realistic simulation of the ARPES spectra, indicating that Ce-In surface termination prevails. Surface states, which have been identified in the calculations, contribute significantly to the spectra. Effects of the hybridization strongly depend on wave vector. They include a dispersion of heavy electrons and bands gaining $f$-electron character when approaching Fermi energy. We have also observed a considerable variation of $f$-electron spectral weight at $E_F$, which is normally determined by both matrix element effects and wave vector dependent c-f hybridization. Fermi surface scans covering a few Brillouin zones revealed large matrix element effects. A symmetrization of experimental Fermi surface, which reduces matrix element contribution, yielded a specific variation of $4f$-electron enhanced spectral intensity at $E_F$ around $\overline{\mathrm{\Gamma }}$ and $\overline{\mathrm{M}}$ points. Tight-binding approximation calculations for Ce-In plane provided the same universal distribution of $4f$-electron density for a range of values of the parameters used in the model.

We study the surface states and chiral hinge states of a three-dimensional second-order topological insulator in the presence of an external magnetic gauge field. Surfaces pierced by flux host Landau levels, while surfaces parallel to the applied field are not significantly affected. The chiral hinge modes mediate spectral flow between neighboring surfaces. As the magnetic field strength is increased, the surface Landau quantization deviates from that of a massive Dirac cone. Quantitatively, the $n=0$ Landau level falls inside the surface Dirac gap and not at the gap edge. The $n\ne 0$ levels exhibit a further, qualitative discrepancy: while the massive Dirac cone is expected to produce pairs of levels ($\pm n$) which are symmetric around zero energy, the $n$ and $-n$ levels become asymmetric in our lattice model—one of the pair may even be absent from the spectrum or hybridized with the continuum. In order to resolve the issue, we extend the standard two-dimensional (2D) massive Dirac surface theory by including additional Hamiltonian terms at $O\left({\mathbf{k}}^2\right)$. While these terms do not break particle-hole symmetry in the absence of magnetic field, they lead to the aforementioned Landau level asymmetry once the magnetic field is applied. We argue that similar $O\left({\mathbf{k}}^2\right)$ correction terms are generically expected in lattice models containing gapped Dirac fermions, using the BHZ model of a 2D topological insulator as an example.

Electrides are ionic compounds in which electrons behave as anions in the interior of a positively charged framework. As a layered electride, ${\mathrm{Gd}}_2\mathrm{C}$ receives attention because of its ferromagnetism. Although previous research has focused on the bulk properties of ${\mathrm{Gd}}_2\mathrm{C}$, few studies have focused on ultrathin layers or surfaces for two-dimensional (2D) characteristics. Here, we report a first-principles study of the electronic properties of few-layer ${\mathrm{Gd}}_2\mathrm{C}$ structures. ${\mathrm{Gd}}_2\mathrm{C}$ has a work function of 3.35 eV. When a layered electride is exfoliated, the interstitial layer becomes a surface and may be exposed to the outside. Because the interlayer region has changed to the surface, the properties of the electron gases once located in the interlayer in the past will also change. We found that the surface anionic electrons accounted for about 25% of the number of electrons in the interlayer region in the absence of an external electric field. When we applied an external electric field, the number of surface electrons increased, and the increase was proportional to the square of the field intensity. Since the electronic properties of 2D materials can be understood through scanning tunneling spectroscopy (STS), we also performed the STS simulations. At $-0.9\mathrm{eV}$, the STS image was blurred because of surface anionic electrons. In contrast to the spin-up electron, an interlayer band of the spin-down electron crossed the Fermi level in the ultrathin ${\mathrm{Gd}}_2\mathrm{C}$ layers. Our findings open a possibility that the spin-polarized electronic gas in the few-layer electride could be used for spintronics.

Extreme near-field heat transfer between metallic surfaces is a subject of debate as the state-of-the-art theory and experiments are in disagreement on the energy carriers driving heat transport. In an effort to elucidate the physics of extreme near-field heat transfer between metallic surfaces, this paper presents a comprehensive model combining radiation, acoustic phonon, and electron transport across sub-10-nm vacuum gaps. The results obtained for gold surfaces show that in the absence of bias voltage, acoustic phonon transport is dominant for vacuum gaps smaller than $\sim 2$ nm. The application of a bias voltage significantly affects the dominant energy carriers as it increases the phonon contribution mediated by the long-range Coulomb force and the electron contribution due to a reduced potential barrier. For a bias voltage of 0.6 V, acoustic phonon transport becomes dominant at a vacuum gap of 5 nm, whereas electron tunneling dominates at sub-nm vacuum gaps. The comparison of the theory against experimental data from the literature suggests that well-controlled measurements between metallic surfaces are needed to quantify the contributions of acoustic phonon and electron as a function of the bias voltage.

In this work we develop EPOCH (equilibrium propagator by orthogonal polynomial chain), a computationally efficient method to calculate the time-dependent equilibrium Green's functions, including the anomalous Green's functions of superconductors, to capture the time evolution in large inhomogeneous systems. The EPOCH method generalizes the Chebyshev wave-packet propagation method from quantum chemistry and efficiently incorporates the Fermi-Dirac statistics that is needed for equilibrium quantum condensed matter systems. The computational cost of EPOCH scales only linearly in the system degrees of freedom, generating an extremely efficient algorithm also for very large systems. We demonstrate the power of the EPOCH method by calculating the time evolution of an excitation near a superconductor–normal metal interface in two and three dimensions, capturing transmission as well as normal and Andreev reflections.

We investigate the DC voltage under an applied microwave current for platinum(Pt)/cobalt(Co), tantalum(Ta)/Co, and tungsten(W)/Co bilayers, where the microwave-induced alternating magnetic field ($B_{\mathrm{rf}}$) is parallel to an external static magnetic field ($B_{\mathrm{ext}}$). Whereas spin torque ferromagnetic resonance (ST FMR) signals do not appear because of the parallel configuration of $B_{\mathrm{rf}}$ and the magnetization of the Co layer, we recently found a clear hysteresis signal around $B_{\mathrm{ext}}=0\mathrm{mT}$ only when the microwave frequency ($f_{\mathrm{MW}}$) is low, typically less than 10 GHz. This low-frequency ST FMR (LFST FMR) signal enables us to detect magnetization switching induced by spin-orbit torque (SOT) with high sensitivity. In this study, we found an additional background (BG) signal superimposed on the LSFT FMR signal. The additional BG signal also has hysteresis behavior and appears even at high $f_{\mathrm{MW}}$, where the LSFT FMR signal completely disappears. The sign of the BG signal is changed by changing the nonmagnetic material from Pt to Ta or W. By measuring $f_{\mathrm{MW}}$, the microwave current, the temperature, and the magnetic field angle dependences, we conclude that the BG signal is induced by spin-dependent unidirectional spin Hall magnetoresistance (SD USMR), which is generated by a spin current induced by the spin Hall effect of nonmagnetic metals and spin-dependent electron mobility in ferromagnetic metals. The SD USMR signal appears in wide ranges of $f_{\mathrm{MW}}$ and $B_{\mathrm{ext}}$ because the SD USMR originates from the nonlinearity of current-dependent resistance and is not related to magnetization dynamics. From the magnitude of the BG signal, the spin Hall angles of Pt and Ta are calculated to be 0.026 ± 0.006 and −0.042 ± 0.006, respectively. In addition, we demonstrate SOT magnetization switching using the BG signal.

We report the results of experimental investigations on structural, magnetic, resistivity, and caloric properties of ${\mathrm{Fe}}_2\mathrm{RhZ}$ (Z = Si,Ge) along with ab initio band structure calculations using first-principle simulations. Both alloys are found to crystallize in inverse Heusler structure but with disorder in tetrahedral sites between Fe and Rh. ${\mathrm{Fe}}_2\mathrm{RhSi}$ has a saturation moment of $5.00{\mu }_B$ and while its counterpart has $5.19{\mu }_B$. Resistivity measurement reveals a metallic nature in both of them. Theoretical simulations using generalized gradient approximation (GGA) predict an inverse Heusler structure with ferromagnetic ordering as a ground state for both alloys. However, it underestimates the experimentally observed moments. $\mathrm{GGA}+U$ approach, with Hubbard $U$ values estimated from density functional perturbation theory, helps to improve the comparison of the experimental results. ${\mathrm{Fe}}_2\mathrm{RhSi}$ is found to be a half-metallic ferromagnet while ${\mathrm{Fe}}_2\mathrm{RhGe}$ is not. Varying $U$ values on Fe and Rh sites does not change the net moment much in ${\mathrm{Fe}}_2\mathrm{RhSi}$, unlike in ${\mathrm{Fe}}_2\mathrm{RhGe}$. Relatively small exchange splitting of orbitals in ${\mathrm{Fe}}_2\mathrm{RhGe}$ compared to that of ${\mathrm{Fe}}_2\mathrm{RhSi}$ is the reason for not opening the band gap in the minority spin channel in the former. High ordering temperature and moment make ${\mathrm{Fe}}_2\mathrm{RhSi}$ useful for spintronics applications.

We report a comprehensive study of the temperature evolution of in-plane (IP) and out-of-plane (OOP) effective magnetic anisotropies in compensated ferrimagnetic ${\mathrm{Fe}}_{100-x}{\mathrm{Gd}}_x$ alloy films by employing direct current magnetometry and radiofrequency (RF) transverse susceptibility (TS) measurements. We suggest that our ${\mathrm{Fe}}_{100-x}{\mathrm{Gd}}_x$ system is chemically inhomogeneous and phase segregates into Fe- and Gd-enriched regions. Our IP and OOP magnetometry results indicate that the system undergoes a temperature-driven transformation from an IP-spin-configuration-dominated state to an OOP-spin-configuration-dominated state below a certain temperature (spin reorientation temperature). A two-step reversal behavior emerges in the OOP $M$($H$) loop near compensation, which we attribute to the sequential magnetization reversals of Fe- and Gd-enriched domains. Field-induced spin-flop transitions were also observed near the compensation. Our RF TS measurements indicate that the effective magnetic anisotropy for the OOP configuration dominates over that for the IP configuration below a certain spin reorientation temperature. Both IP and OOP anisotropy fields determined from our TS measurement exhibit a minimum around the compensation temperature, which has been explained in the framework of the Stoner-Wohlfarth model.

We explore the capabilities of the frequency-resolved frozen phonon multislice method introduced in [Phys. Rev. Lett. 124, 025501 (2020)] to model inelastic vibrational scattering in transmission electron microscopy. We review the method in detail and discuss advantages of using a so-called hotspot thermostat instead of the $\delta$ thermostat used in our first report. We apply the method to simulate vibrational electron energy loss spectra of hexagonal boron nitride under plane wave illumination. Simulated spectroscopic information well represents the theoretical phonon band structure of the studied material, both in terms of energies as well as polarization vectors of individual phonon modes.

Interconversion of charge and spin currents via spin Hall effect is one of the key physical phenomena in spintronics. However, the spin polarizations are orthogonal to both the charge and spin flows due to the restricted symmetry conditions. Here, we use noncollinear antiferromagnet ${\mathrm{Mn}}_3\mathrm{Pt}$, which has different magnetic mirror planes in different orientations, to investigate magnetic symmetry dependent atypical spin-orbit torques. We observed weak generation of out of plane spin polarization $\left({\sigma }_z\right)$ in ${\mathrm{Mn}}_3\mathrm{Pt}$/permalloy bilayers when current was applied perpendicular to the magnetic mirror plane of ${\mathrm{Mn}}_3\mathrm{Pt}$, and strong generation of ${\sigma }_z$ when current was applied parallel to the magnetic mirror plane. All three oriented ${\mathrm{Mn}}_3\mathrm{Pt}$ films show the same pattern, indicating the generation of unconventional ${\sigma }_z$ has strong dependence on magnetic symmetry. Controlling spin-orbit torques by changing current directions in differently oriented ${\mathrm{Mn}}_3\mathrm{Pt}$ films provides a strategy for optimizing antiferromagnetic spintronics.

Combining advanced technologies of optics and antiferromagnetic spintronics, we present a method to realize ultrafast spin transport. The optical Barnett effect provokes quasiequilibrium Bose-Einstein condensates (BECs) of magnons associated with the fully spin-polarized state in insulating antiferromagnets (AFs). This optomagnonic Barnett effect enables us to exploit coherent magnons of high frequency over the conventional ones of (sub-) terahertz associated with the Néel magnetic order. We show that the macroscopic coherence of those optical magnon BECs induces a spin current across the junction interface of weakly coupled two insulating AFs, and this optomagnonic Josephson effect realizes ultrafast spin transport. The period of the optomagnonic Josephson oscillation is much shorter than the conventional one of the order of picoseconds. Thus we propose a way to realize ultrafast spin transport in AFs by means of the macroscopic coherence of optical magnon BECs.

We have grown single crystals of ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$, a spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat, and thermal conductivity measurements at ultralow temperatures. The main results are as follows: (i) At zero magnetic field, ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted ${120}^{\circ }$ spin structure, which is in a plane including the crystallographic $c$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $c$ axis. (ii) With increasing field along the $c$ axis, a 1/3 magnetization plateau is observed, which means that the canted ${120}^{\circ }$ spin structure is transformed into an up-up-down (UUD) spin structure. With even higher fields, the UUD phase further evolves into possible V and ${\mathrm{V}}^{\prime }$ phases. (iii) With increasing field along the $a$ axis, the canted ${120}^{\circ }$ spin structure is possibly transformed into an umbrella phase and a V phase. Therefore ${\mathrm{Na}}_2\mathrm{BaNi}{\left({\mathrm{PO}}_4\right)}_2$ is a rare example of a spin-1 ETLAF with single-crystalline form that exhibits easy-axis spin anisotropy and a series of quantum spin state transitions.

Topological quantum computation based on Majorana bound states may enable new paths to fault-tolerant quantum computing. Several recent experiments have suggested that the vortex cores of topological superconductors, such as iron-based superconductors, may host Majorana bound states at zero energy. However, quantum computation with these zero-energy vortex bound states requires precise and fast manipulation of individual vortices, which is difficult to do in a scalable manner. To address this issue, we propose a control scheme based on local heating via, for example, scanning optical microscopy to braid vortex-bound Majorana zero modes in a two-dimensional topological superconductor. First, we derive the conditions required for transporting a single vortex between two defects in the superconducting material by trapping it with a hot spot generated by local optical heating. Equipped with critical conditions for the vortex motion, we then establish the ideal material properties for vortex braiding and describe how transition errors resulting from finite speed and/or temperature can be minimized. Our work paves the way toward optical or microscopic control of zero-energy vortex bound states in two-dimensional topological superconductors.

First-principles calculations using the hybrid density functional are carried out to study the defect levels and photoluminescence of bismuth doped in perovskites $R{\mathrm{AlO}}_3$ ($R=\mathrm{Y}$, Gd, La) and $\mathrm{La}B{\mathrm{O}}_3$ ($B=\mathrm{Al}$, Ga, In). Bismuth dopants are confirmed to be predominantly ${\mathrm{Bi}}^{3+}$ occupying the $R$ site, i.e., ${\mathrm{Bi}}_R^0$ ($R=\mathrm{Y}$, Gd, La). The variation of the electron trap depth ${\mathrm{Bi}}_R\left(0/-1\right)$ is shown mainly due to the shift of the conduction band, while the hole trap level ${\mathrm{Bi}}_R\left(+1/0\right)$ shows the correlation with the shortest $R\text{–}\mathrm{O}$ bond length in hosts. Based on the defect level diagram from the first-principles calculation, the transition types of excitation and emission are predicted. Among the systems considered, the lowest excited level that produces the photoluminescence is the ${}^{3}P_{0,1}$ levels of the $6s^{1}6p^1$ configuration of ${\mathrm{Bi}}^{3+}$, except for ${\mathrm{Bi}}^{3+}$-doped ${\mathrm{LaAlO}}_3$, which is the valence band to ${\mathrm{Bi}}^{3+}$ charge transfer state. The photoluminescence involving ${}^{3}P_{0,1}$ states shows similar Stokes shift in the series, and the excitation and emission energies exhibit almost linear correlation with the band gaps of hosts of a slope $\sim 0.22$. Furthermore, our calculations show that there is no tendency of forming excessive ${\mathrm{Bi}}^{3+}$ pairs in all the five $RB{\mathrm{O}}_3$:${\mathrm{Bi}}^{3+}$, while the ${\mathrm{Gd}}^{3+}\text{−}{\mathrm{Gd}}^{3+}$ coupling in ${\mathrm{GdAlO}}_3$ provides a “bridge” for energy migration from an excited isolated ${\mathrm{Bi}}^{3+}$ ion to the ${\mathrm{Bi}}^{3+}$ pairs that are naturally present due to random distribution, leading to the 495-nm emission being observed uniquely in bismuth-doped ${\mathrm{GdAlO}}_3$ among the series of systems considered. The results lay the basis for manipulating the trap levels and excitation and emission wavelengths in ${\mathrm{Bi}}^{3+}$-doped perovskite oxides.