Flexible control of magnetization switching by electrical means is crucial for applications of spin orbitronics. Besides a switching current that is parallel to an applied field, a bias current that is normal to the switching current is introduced to tune the magnitude of effective damping-like and field-like torques and further electrically control magnetization switching. Symmetrical and asymmetrical control over the critical switching current by the bias current with opposite polarities is realized in both Pt/Co/MgO and $\alpha$-Ta/CoFeB/MgO systems, respectively. This research not only identifies the influences of field-like and damping-like torques on the switching process, it also demonstrates an electrical method to control it.

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic x-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phases, which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$ point depend critically on the Fe magnetic moment and atomic volume. Analyzing these nonvibrational modes leads to the discovery of a stable monoclinic ground-state structure, which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy, and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling.

A key feature of topological insulators is the robustness of the electron energy spectrum. At a surface of a topological insulator, the Dirac point is protected by the characteristic symmetry of the system. The breaking of the symmetry opens a gap in the energy spectrum. Therefore, topological insulators are very sensitive to magnetic fields, which can open a gap in the electronic spectrum. Concerning “internal” magnetic effects, for example, the situation with doped magnetic impurities, is not trivial. A single magnetic impurity is not enough to open the band gap, while in the case of a ferromagnetic chain of deposited magnetic impurities the Dirac point is lifted. However, a much more interesting case is when localized magnetic impurities form a chiral spin order. Our first principle density functional theory calculations have shown that this is the case for Fe deposited on the surface of a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ topological insulator. But not only magnetic impurities can form a chiral helicoidal spin texture. An alternative way is to use chiral multiferroics (prototype material is ${\mathrm{LiCu}}_2{\mathrm{O}}_2$) that induce a proximity effect. The theoretical approach we present here is valid for both cases. We observed that opposite to a ferromagnetically ordered case, a chiral spin order does not destroy the Dirac point. We also observed that the energy gap appears at the edges of the new Brillouin zone. Another interesting result concerns the spin dynamics. We derived an equation for the spin density dynamics with a spin current and relaxation terms. We have shown that the motion of the conductance electron generates a magnetic torque and exerts a certain force on the helicoidal texture.

Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron-pnictide superconducting family, $\mathrm{Ca}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$. We reanalyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin correlations in the electron-doped ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ system. These NMR data indicate that FM fluctuations exist in general in iron-pnictide superconducting families and thus must be included to capture the phenomenology of the iron pnictides.

Motivated by a growing interest in multiorbital superconductors with spin-orbit interactions, we perform the group-theoretical classification of various unconventional superconductivity emerging in symmorphic $O, D_4$, and $D_6$ space groups. The generalized Cooper pairs, which we here call “multipole” superconductivity, possess spin-orbital coupled (multipole) degrees of freedom, instead of the conventional spin singlet/triplet in single-orbital systems. From the classification, we obtain the following key consequences, which have never been focused in the long history of research in this field: (1) A superconducting gap function with ${\mathrm{\Gamma }}_9\otimes {\mathrm{\Gamma }}_9$ in $D_6$ possesses nontrivial momentum dependence different from the usual spin-$\frac{1}{2}$ classification. (2) Unconventional gap structure can be realized in the BCS approximation of purely local (onsite) interactions irrespective of attraction/repulsion. It implies the emergence of an electron-phonon (e-ph) driven unconventional superconductivity. (3) Reflecting symmetry of orbital basis functions there appear not symmetry protected but inevitable line nodes/gap minima, and thus, anisotropic $s$-wave superconductivity can be naturally explained even in the absence of competing fluctuations.

Spin dephasing by the Dyakonov-Perel mechanism in metallic films deposited on insulating substrates is revealed, and quantitatively examined by means of density functional calculations combined with a kinetic equation. The surface-to-substrate asymmetry, probed by the metal wave functions in thin films, is found to produce strong spin-orbit fields and a fast Larmor precession, giving a dominant contribution to spin decay over the Elliott-Yafet spin relaxation up to a thickness of 70 nm. The spin dephasing is oscillatory in time with a rapid (subpicosecond) initial decay. However, parts of the Fermi surface act as spin traps, causing a persistent tail signal lasting 1000 times longer than the initial decay time. It is also found that the decay depends on the direction of the initial spin polarization, resulting in a spin-dephasing anisotropy of 200% in the examined cases.

Ab initio electronic structure calculations reveal that tetragonal distortion has a dramatic effect on the relative stability of the various magnetic structures (C-, A-, G-, ${\mathrm{A}}^{\prime }$-AFM, and FM) of FeRh giving rise to a wide range of novel stable/metastable structures and magnetic phase transitions between these states. We predict that the cubic G-AFM structure, which was believed thus far to be the ground state, is metastable and that the tetragonally expanded G-AFM is the stable structure. The low energy barrier separating these states suggests phase coexistence at room temperature. We propose an ${\mathrm{A}}^{\prime }$-AFM phase to be the global ground state among all magnetic phases which arises from the strain-induced tuning of the exchange interactions. The results elucidate the underlying mechanism for the recent experimental findings of electric-field control of magnetic phase transition driven via tetragonal strain. The magnetic phase transitions open interesting prospects for exploiting strain engineering for the next-generation memory devices.

By means of magnetization, specific heat, and muon-spin relaxation measurements, we investigate newly synthesized high-pressure oxidized ${\mathrm{Cu}}_{0.75}{\mathrm{Mo}}_{0.25}{\mathrm{Sr}}_2{\mathrm{YCu}}_2{\mathrm{O}}_{7.54}$, in which overdoping is achieved up to $p\sim 0.46$ hole/Cu, well beyond the $T_c\text{−}p$ superconducting dome of cuprates, where Fermi-liquid behavior is expected. Surprisingly, we find bulk superconductivity with $T_c=84$ K and superfluid density similar to those of optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. These results point at an unusual high-$T_c$ phase with a large fraction of unpaired holes. Further experiments may assess the Fermi-liquid properties of the present phase, which would put into question the paradigm that the high $T_c$ of cuprates originates from a non-Fermi-liquid ground state.

We use variable-temperature x-ray thermal diffuse scattering and inelastic scattering to investigate the lattice dynamics in single crystals of multiferroic ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ which undergo a series of orbital and magnetic transitions at low temperatures. Upon approaching the charge and orbital ordering temperature $T_o=250$ K from above, we observe intense diffuse scattering features and a pronounced optical phonon softening centered around the superstructure reflections of the incommensurately modulated structure. The phonon anomaly appears well above $T_o$ and continuously increases upon cooling, following a canonical power-law temperature dependence that confirms the transition at $T_o$ to be of second order and related to a soft-phonon lattice instability. Microscopic mechanisms for the incommensurate charge and orbital ordering based on competing interactions and on momentum-dependent electron-phonon coupling could both account for the observed extended momentum width of the phonon softening. Our results highlight the importance of the lattice interactions in the physics of this magnetically induced ferroelectric system.

Hydrostatic pressure studies of the isothermal magnetization and volume changes up to 7 GPa of magnetic composite containing MnP clusters in an insulating $\mathrm{CdGe}{\mathrm{P}}_2$ matrix are presented. Instead of alleged superparamagnetic behavior, a pressure-induced magnetization process was found at zero magnetic field, showing gradual enhancement in a low-field regime up to $H⩾5$ kOe. The simultaneous application of pressure and magnetic field reconfigures the MnP clusters with antiferromagnetic alignment, followed by onset of a field-induced metamagnetic transition. An unusual hysteresis in magnetization after pressure cycling is observed, which is also enhanced by application of the magnetic field, and indicates reversible metamagnetism of MnP clusters. We relate these effects to the major contribution of structural changes in the composite, where limited volume reduction by 1.8% is observed at $P\sim 5.2$ GPa.

We survey the electronic structure and magnetic properties of two-dimensional (2D) $M\mathrm{P}{X}_3$ ($M=\text{V},\text{Cr},\text{Mn},\text{Fe},\text{Co},\text{Ni},\text{Cu},\text{Zn}$, and $X=\text{S},\text{Se},\text{Te}$) transition-metal chalcogenophosphates to shed light on their potential role as single-layer van der Waals materials that possess magnetic order. Our ab initio calculations predict that most of these single-layer materials are antiferromagnetic semiconductors. The band gaps of the antiferromagnetic states decrease as the atomic number of the chalcogen atom increases (from S to Se to Te), leading in some cases to half-metallic ferromagnetic states or to nonmagnetic metallic states. We find that the competition between antiferromagnetic and ferromagnetic states can be substantially influenced by gating and by strain engineering. The sensitive interdependence we find between magnetic, structural, and electronic properties establishes the potential of this 2D materials class for applications in spintronics.

We investigate spin dynamics in antiferromagnetic multiferroic ${\mathrm{TbMnO}}_3$ using optical-pump, terahertz (THz)-probe spectroscopy. Photoexcitation results in a broadband THz transmission change, with an onset time of 25 ps at 10 K that becomes faster at higher temperatures. We attribute this time constant to spin-lattice thermalization. The excellent agreement between our measurements and previous ultrafast resonant x-ray diffraction measurements on the same material confirms that our THz pulse directly probes spin order. Furthermore, the presence of a pump-probe signal above the magnetic ordering temperatures suggests that, unlike resonant x-ray probing, THz transmission is sensitive to the dynamics of short-range spin order, which is known to be of importance in spin-spiral multiferroics like ${\mathrm{TbMnO}}_3$.

The ferromagnetic topological insulator $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted and exhibits a high density of states near the Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.

The energetics and the electronic structure of methylammonium lead bromine ($\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbB}{\mathrm{r}}_3$) perovskite (001) surfaces are studied based on density functional theory. By examining the surface grand potential, we predict that the $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Br}$-terminated (001) surface is energetically more favorable than the $\mathrm{PbB}{\mathrm{r}}_2$-terminated (001) surface, under thermodynamic equilibrium conditions of bulk $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbB}{\mathrm{r}}_3$. The electronic structure of each of these two different surface terminations retains some of the characteristics of the bulk, while new surface states are found near band edges which may affect the photovoltaic performance in the solar cells based on $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbB}{\mathrm{r}}_3$. The calculated electron affinity of $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{PbB}{\mathrm{r}}_3$ reveals a sizable difference for the two surface terminations, indicating a possibility of tuning the band offset between the halide perovskite and adjacent electrode with proper interface engineering.

Recent advances in nanophotonics have brought about coherent light sources with chaotic circular polarization; a low-dimensional chaotic evolution of optical spin was evidenced in laser diodes. Here we propose a mechanism that gives rise to light with a spatiotemporal spin chaos resembling turbulent states in hydrodynamics. The spin-chaotic radiation is emitted by exciton polaritons under resonant optical pumping in arbitrarily sized planar microcavities, including, as a limiting case, pointlike systems with only three degrees of freedom. The underlying mechanism originates in the interplay between spin symmetry breakdown and scattering into pairs of Bogolyubov excitations. As a practical matter, it opens up the way for spin modulation of light on the scale of picoseconds and micrometers.

Attosecond streaking spectroscopy of solids provides direct observation of the dynamics of electron excitation and transport through the surface. We demonstrate the crucial role of the exciting field in electron propagation and establish that the lattice scattering of the outgoing electron during the optical pumping leads to the wave packet moving faster than with the group velocity and faster than the free electron. We solve the time-dependent Schrödinger equation for a model of laser-assisted photoemission, with inelastic scattering treated as electron absorption and alternatively by means of random collisions. For a weak lattice scattering, the phenomenological result that the photoelectron moves with the group velocity $dE/d\hslash k$ and traverses on average the distance equal to the mean-free path is proved to hold even at very short traveling times. This offers a novel interpretation of the delay time in streaking experiment and sheds new light on tunneling in optoelectronic devices.

Topological surface states have been extensively observed via optics in thin films of topological insulators. However, in typical thick single crystals of these materials, bulk states are dominant and it is difficult for optics to verify the existence of topological surface states definitively. In this Rapid Communication, we study the charge dynamics of the newly formulated bulk-insulating Sn-doped ${\mathrm{Bi}}_{1.1}{\mathrm{Sb}}_{0.9}{\mathrm{Te}}_2\mathrm{S}$ crystal by using time-domain terahertz spectroscopy. This compound shows much better insulating behavior than any other bulk-insulating topological insulators reported previously. The transmission can be enhanced an amount which is $5\%$ of the zero-field transmission by applying magnetic field to 7 T, an effect which we believe is due to the suppression of topological surface states. This suppression is essentially independent of the thicknesses of the samples, showing the two-dimensional nature of the transport. The suppression of surface states in field allows us to use the crystal slab itself as a reference sample to extract the surface conductance, mobility, charge density, and scattering rate. Our measurements set the stage for the investigation of phenomena out of the semiclassical regime, such as the topological magnetoelectric effect.

We demonstrate that in confined plasmonic metal structures subject to ultrafast laser excitation, electron thermal diffusion (ETD) can provide spatial redistribution of excess energy faster than its transfer to the lattice. This relaxation occurs after the excitation of nanometer-sized hot spots in the confined structure, changing sensitively the optical parameters in these regions. The changes become essential when the plasmonic resonance condition is met for both excitation and detection, as evidenced by a pump-probe experiment on plasmonic gold lattices: Subpicosecond relaxation with characteristic times well described by a two-temperature model involving ETD is observed. The results suggest that the dynamical optical response in plasmonic structures can be tuned by the selection of the structural geometry as well as the choice of wavelength and polarization of the excitation and detection light.

We use time- and energy-resolved optical spectroscopy to investigate the coupling of electron-hole excitations to the magnetic environment in the relativistic Mott insulator ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. We show that, on the picosecond time scale, the photoinjected electron-hole pairs delocalize on the hexagons of the Ir lattice via the formation of quasimolecular orbital excitations and the exchange of energy with the short-range-ordered zigzag magnetic background. The possibility of mapping the magnetic dynamics, which is characterized by typical frequencies in the THz range, onto high-energy (1–2 eV) charge excitations provides a platform to investigate, and possibly control, the dynamics of magnetic interactions in correlated materials with strong spin-orbit coupling, even in the presence of complex magnetic phases.