It was recently demonstrated that introducing the shape anisotropy of a skyrmion is an alternative strategy for controlling the skyrmion Hall effect. For example, an elliptical skyrmion shows anisotropic motion under the applied driving force. Here, we study the fundamental properties of an elliptical skyrmion induced by uniaxial in-plane magnetic anisotropy. The dynamics of a skyrmion under various parameters, such as uniaxial in-plane magnetic anisotropy, interfacial Dzyaloshinskii-Moriya interaction, and the damping constant, are investigated, and we employ Thiele's theory and derive expressions for the velocity of the skyrmion that match very well with micromagnetic simulation results. We find that the skyrmion Hall angle can reach ${90}^{\circ }$ with the optimized angle between the in-plane easy axis and the current. Our results offer a potential application of racetrack memory based on skyrmions, and a type of racetrack configuration was designed.

Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder-Tinkham-Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings—that fully determine the magnetoresistance in the normal-conducting state—and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip (“unconventional”) Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double peaks around the singlet-gap energy, reflecting the competition between the singlet and an additionally emerging triplet gap; the latter is an effective superconducting gap that can be ascribed to the formation of triplet Cooper pairs through interfacial spin-flip scatterings (i.e., to the generation of an effective triplet-pairing term in the order parameter). We thoroughly analyze the Andreev reflections' role in connection with superconducting magnetoresistance phenomena, and eventually unravel huge conductance and magnetoresistance magnetoanisotropies—easily exceeding their normal-state counterparts by several orders of magnitude—as another experimentally accessible fingerprint of unconventional Andreev reflections. Our results provide an important contribution to establish superconducting magnetic spin valves as an essential ingredient for future superconducting-spintronics concepts.

Based on evolutionary crystal structure searches in combination with ab initio calculations, we predict an unusual structural phase of the superconducting ${\mathrm{LaH}}_{10}$ that is stable from about 250 to 425 GPa pressure. However, inclusion of phonon zero-point and 300 K free-energy contributions shifts the transition pressures to higher values. This phase belongs to a trigonal $R\overline{3}m$ crystal lattice with an atypical cell angle, ${\alpha }_{\mathrm{rhom}}\sim 24.{56}^{\circ }$. We find that this structure contains three units of ${\mathrm{LaH}}_{10}$ in its primitive cell, unlike the previously known trigonal phase, in which the primitive cell contains only one ${\mathrm{LaH}}_{10}$ unit. In this phase, a 32-H-atom cage encapsulates La atoms, analogous to the lower-pressure fcc phase. However, the hydrogen cages of the trigonal phase consist of quadrilaterals and hexagons, in contrast to the cubic phase, which exhibits squares and regular hexagons. Surprisingly, the shortest H-H distance in this phase is shorter than that of the lower-pressure cubic phase and of atomic hydrogen metal. We find a structural phase transition from trigonal to hexagonal at 492 GPa at 300 K, where the hexagonal crystal lattice coincides with earlier predictions. Solving the anisotropic Migdal-Eliashberg equations, we find that the predicted trigonal phase (for standard values of the Coulomb pseudopotential) is expected to become superconducting at a critical temperature of about 175 K, which is less than $T_c\sim 250$ K measured for cubic ${\mathrm{LaH}}_{10}$.

We investigated the phase boundaries of three ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$-(NBT) based piezoelectric systems NBT-${\mathrm{BaTiO}}_3$ (NBT-BT), NBT-${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$ (NBT-KBT) and NBT-${\mathrm{SrTiO}}_3$ (NBT-ST), all exhibiting similar piezoelectric responses ($d_{33}\sim 180pC/N$). We found that, apart from a qualitative difference (NBT-KBT/NBT-BT exhibits R3c-P4mm interferroelectric instability and NBT-ST does not), the critical compositions of all the three systems exhibit onset of a disordered P4bm phase. Our study suggests that the preponderant effect of this distortion overrides the interferroelectric instability, limiting the piezoelectric response of NBT-BT/NBT-KBT like that in NBT-ST.

The high-pressure synthesis of rhenium nitride pernitride with a crystal structure that is unusual for transition metal dinitrides and high values of hardness and bulk modulus attracted significant attention to this system. We investigate the thermodynamic and electronic properties of the $P{2}_1/c$ phase of ${\mathrm{ReN}}_2$ and compare them with two other polytypes, the $C2/m$ and $P4/mbm$ phases, suggested in the literature. Our calculations of the formation enthalpy at zero temperature show that the former phase is the most stable of the three up to a pressure $p=170$ GPa, followed by the stabilization of the $P4/mbm$ phase at higher pressure. The theoretical prediction is confirmed by diamond anvil cell synthesis of the $P4/mbm$ ${\mathrm{ReN}}_2$ at $\approx 175$ GPa. Considering the effects of finite temperature in the quasiharmonic approximation at $p=100$ GPa we demonstrate that the $P{2}_1/c$ phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of rhenium nitride pernitride shows the presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to the appearance of an electron pocket at the high-symmetry $Y_2$ point of the Brillouin zone while the disruption of the neck takes place slightly off from the $\mathrm{\Gamma }\text{−}A$ line.

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4$[2,6-anthracene-bis(sulfonate)]$·4{\mathrm{H}}_2\mathrm{O}$ with $\frac{1}{4}$-filled band has been measured to investigate the metal-insulator phase transition at $T_{\mathrm{MI}}=85\mathrm{K}$ linked to charge ordering (CO). Vibrational bands related to $\mathrm{C}=\mathrm{C}$ stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen $\mathrm{OH}\cdots \mathrm{O}$ bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-$\mathrm{H}\cdots \mathrm{O}$ bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of ∼24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase transition is a consequence of both Coulomb interactions between charge carriers and donor-acceptor interactions.

Single-crystal synchrotron x-ray diffraction, Raman spectroscopy, and first principles calculations have been used to identify the structure of the high-pressure (HP) phase of molecular methane above 20 GPa up to 71 GPa at room temperature. The structure of the HP phase is trigonal $R3$, which can be represented as a distortion of the cubic phase B, previously documented at 7–15 GPa and confirmed here. The positions of hydrogen atoms in the HP phase have been obtained from first principles calculations, which also demonstrated the stability of this structure above 260 K at 25 GPa. The molecules occupy four different crystallographic sites in phase B and 11 sites in the HP phase, which result in splitting of molecular stretching modes detected in Raman spectroscopy and assigned here based on a good agreement with the Raman spectra calculated from the first principles. Our study points out to a single HP phase unlike up to three HP phases proposed previously based on the Raman spectroscopy results only.

A stochastic model of electric-field-driven polarization reversal in orthorhombic ferroelectrics is advanced, providing a description of their temporal electromechanical response. The theory accounts for all possible parallel and sequential switching events. Application of the model to the simultaneous measurements of polarization and strain kinetics in a lead-free orthorhombic $\left(\mathrm{K},\mathrm{Na}\right)\mathrm{Nb}{\mathrm{O}}_3$-based ferroelectric ceramic over a wide timescale of seven orders of magnitude allowed identification of preferable polarization switching paths, fractions of individual switching processes, and their activation fields. Particularly, the analysis revealed substantial contributions of coherent non-180° switching events, which do not cause macroscopic strain and thus mimic 180° switching processes.

Spin-charge interconversion in systems with spin-orbit coupling has provided a new route for the generation and detection of spin currents in functional devices for memory and logic such as spin-orbit torque switching in magnetic memories or magnetic-state reading in spin-based logic. Disentangling the bulk (spin Hall effect) from the interfacial (inverse spin galvanic effect) contribution has been a common issue to properly quantify the spin-charge interconversion in these systems, being the case of Au paradigmatic. Here, we obtain a large spin-charge interconversion at a highly conducting Au/Cu interface which is experimentally shown to arise from the inverse spin galvanic effect of the interface and not from the spin Hall effect of bulk Au. We use two parameters independent of the microscopic details to properly quantify the spin-charge interconversion and the spin losses due to the interfacial spin-orbit coupling, providing an adequate benchmarking to compare with any spin-charge interconversion system. The good performance of this metallic interface, not based in Bi, opens the path to the use of much simpler light/heavy-metal systems.

We theoretically investigate spin-lattice coupling (SLC) effects on the in-field ordering properties of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. Here, we use the two possible simplified models describing the effect of local lattice distortions on the spin ordering via the SLC, the bond-phonon and site-phonon models. It is found by means of Monte Carlo simulations that in both models, the $\frac{1}{2}$ plateau shows up in the magnetization curve being relatively robust against the breathing bond alternation, although magnetic long-range orders (LRO's) are realized only in the site-phonon model. In the bond-phonon model, additional further neighbor interactions are necessary to induce a magnetic LRO. In the site-phonon model, it is also found that in addition to the low-field, middle-field $\frac{1}{2}$ plateau, and high-field phases appearing on both the uniform and breathing pyrochlore lattices, various types of unconventional phases which can be viewed as LRO's in units of tetrahedron are induced by the breathing bond alternation just below the $\frac{1}{2}$ plateau and the saturation field. The occurrence of these tetrahedron-based orders could be attributed to the nature characteristic of the breathing pyrochlore lattice, i.e., the existence of the nonequivalent small and large tetrahedra. Experimental implications of our result are also discussed.

For semiconductor spintronics, efficient spin generation in semiconductor and spin transfer to ferromagnetic metal (FM) are essentially required. Two-dimensional electron gas (2DEG) of semiconductor quantum wells is a promising system for generating spin via the Rashba-Edelstein effect (REE) because of its strong inversion symmetry breaking. In this study, we investigate spin accumulation through REE and spin Hall effect (SHE) in the 2DEG of an InAs quantum well. We use spatial- and polarization-resolved measurements of spin, which reveals that REE dominates SHE in 2DEG. Furthermore, REE in 2DEG induces a spin-orbit torque on FM in a 2DEG/insulator/FM heterostructure. Using direction- and time-resolved measurements of FM magnetization, we determine a sizeable fieldlike torque, which is attributed to the phonon-mediated spin transport from 2DEG to FM.

The discovery of unidirectional magnetoresistance (UMR) in ferromagnet/heavy metal heterostructures provides one possibility to detect the magnetization direction with a simple two-terminal geometry. We investigated the temperature and thickness dependence of UMR in single-crystalline Fe/Pt bilayers and discovered an obvious sign reversal when increasing the Fe thickness at low temperature. Meanwhile, the same phenomenon is observed when increasing temperature for thick Fe samples. All the UMR mechanisms are quantitatively analyzed and the UMR contribution induced by the thermal effect shows a thickness-dependent sign change, thus the sign reversal of UMR is attributed to the competition between the contributions on UMR from the thermal effect and the spin accumulation induced by the spin Hall effect in the Pt layer. Our results emphasize the thermal contribution to the UMR in ferromagnet/heavy metal heterostructures and suggest a promising way to tune the UMR, which will promote its application in information storage devices.

We discuss the charge and the spin tunneling currents between two Bardeen-Cooper-Schrieffer (BCS) superconductors, where one density of states is spin-split by the proximity of a ferromagnetic insulator. In the presence of a large temperature bias across the junction, we predict the generation of a spin-polarized thermoelectric current. This thermospin effect is the result of a spontaneous particle-hole symmetry breaking in the absence of any polarizing tunnel barrier. The two spin components, which move in opposite directions, generate a spin current larger than the purely polarized case when the thermoactive component dominates over the dissipative one.

We analyze the influence of the Fermi surface (FS) shape on magnetotransport properties, particularly on the Hall effect in the MnAs compound. Evidence of opposite conduction polarities for different crystal directions (goniopolarity) and a strong dependence of carrier type has been observed in MnAs films with an applied magnetic field. To understand this behavior, we developed a semiclassical model together with the Boltzmann transport theory that takes into account both the applied magnetic field and the FS shape. The MnAs FS is obtained by means of density functional theory, showing a clear dominance of the hyperboloid shape. Our study corroborates that the specific topology of the Fermi surface gives rise to a goniopolar behavior in the Hall transport. These theoretical results are supported by magnetotransport measurements on MnAs thin layers epitaxially grown on GaAs(001) and on GaAs(111), allowing us to explore the transport characteristics for two different crystal directions of the system.

Local magnetic moment (LMM) and antiferromagnetic (AFM) fluctuation play a critical role in affecting the properties of FeSe superconductor. By constraining the local magnetic moment on Fe atoms using density functional theory, we investigate how LMM in FeSe monolayer alters the total energy, heights of Se atoms, band structure, and the electronic properties, for three different AFM spin arrangements which consist of the checkerboard (CB), collinear (CL), and pair-checkerboard (PC) spin phases. We find that (i) the total energy decreases drastically in all three spin structures when LMM develops, showing that the existence of LMM significantly stabilizes the system. The optimal LMM is found to be 2.23, 2.54, and $2.47{\mu }_B$, respectively, in the CB, CL, and PC spin phases. (ii) The heights of Se atoms increase markedly (and in a quadratic manner) with LMM, demonstrating a strong magnetostriction effect. Also intriguingly, we find that the Se heights are insensitive to spin ordering, displaying a rather universal dependence on LMM in three different AFM spin phases. (iii) LMM is shown to alter substantially the electron band structures and Fermi surfaces. Near their optimal LMM, while both CB and PC phases possess electron pockets and no hole pockets, the CL phase exhibits neither electron pockets nor hole pockets, and interestingly, it becomes a semiconductor of a small gap of 60 meV. These results reveal that there is a rich and interesting physics to be tuned by LMM in FeSe superconductor.

A hydrodynamic flow of electrons driven by an oscillating electric field in Dirac and Weyl semimetals is investigated. It is found that a double-peak profile of the electric current appears and is manifested in a stray magnetic field with peaks in one of the field components. The nontrivial current profile originates from the interplay of viscous and inertial properties of the electron fluid as well as the boundary conditions. Analytical results are supported by numerical calculations in samples of different geometries such as straight channels, nozzles, and cavities. The double-peak profile of the current is found to be qualitatively insensitive to a specific form of the time dependence of the oscillating electric field and is stable with respect to the sample geometry. A phase diagram and criteria for observing the double-peak structure are determined. In addition, it is shown that nozzles and cavities provide an efficient means to locally enhance or reduce the fluid velocity.

Angle-resolved photoelectron spectroscopy utilizing soft x-ray synchrotron radiation was applied to Heusler-type ${\mathrm{Co}}_2\mathrm{MnGa}$ thin films that have a 1-nm Al capping layer. The bulk Fermi surfaces and band structures varied along the out-of-plane momentum, stemming from the three-dimensional crystal structure, in the absence of any in situ surface treatment. In addition, there were characteristic intersecting bands (Weyl cones), with crossing points near the Fermi level, which were consistent with computed results. The Weyl cones are of bulk origin and are responsible for the high anomalous Nernst and the anomalous Hall coefficients. A close comparison of the experimental band structures in ${\mathrm{Co}}_2\mathrm{MnGe}$ and ${\mathrm{Co}}_2\mathrm{MnGa}$ indicated that the rigid band picture is valid in both alloys and that fine carrier tuning is possible by replacing Ga with Ge to improve the anomalous conductivity.

We study properties of thermal transport and quantum many-body chaos in a lattice model with $N\to \infty$ oscillators per site, coupled by strong anharmonic terms. We first consider a model with only optical phonons. We find that the thermal diffusivity $D_{\mathrm{th}}$ and chaos diffusivity $D_L$ (defined as $D_L=v_B^2/{\lambda }_L$, where $v_B$ and ${\lambda }_L$ are the butterfly velocity and the scrambling rate, respectively) satisfy $D_{\mathrm{th}}\approx \gamma D_L$ with $\gamma \gtrsim 1$. At intermediate temperatures, the model exhibits a quantum “phonon fluid” regime, where both diffusivities satisfy $D^{-1}\propto T$, and the thermal relaxation time and inverse scrambling rate are of the order the of Planckian timescale $\hslash /k_{B}T$. We then introduce acoustic phonons to the model and study their effect on transport and chaos. The long-wavelength acoustic modes remain long-lived even when the system is strongly coupled, due to Goldstone's theorem. As a result, for $d=1,2$, we find that $D_{\mathrm{th}}/D_L\to \infty$, while for $d=3, D_{\mathrm{th}}$ and $D_L$ remain comparable.