We demonstrate that elastic interactions between nanomagnets in a periodic array can determine the magnetic response according to the nanoscale array geometry. These findings are attributed to magnetoelastic coupling of the spin dynamics with simultaneously excited surface acoustic waves. Specifically, we observe three manifestations of this effect: pinning of the magnetic resonance frequency over an extended range of applied magnetic fields, generation of additional modes whose frequency can differ by more than 100% from the intrinsic element response, and an enhancement of the Fourier amplitude of the magnetic mode at crossovers with mechanical modes. Simulations of the dynamics in the presence of coupling between magnetic and mechanical degrees of freedom are in good agreement with the experiment. This suggests that magnetization dynamics can be controlled by rational structural design on the nanoscale even when the magnetostatic interactions are negligible.

We theoretically investigate the tunneling anisotropic magneto-Seebeck effect in a realistically modeled $\mathrm{CoPt}\text{/}\mathrm{MgO}\text{/}\mathrm{Pt}$ tunnel junction using coherent transport calculations. For comparison we study the tunneling magneto-Seebeck effect in $\mathrm{CoPt}\text{/}\mathrm{MgO}\text{/}\mathrm{CoPt}$ as well. We find that the magneto-Seebeck ratio of $\mathrm{CoPt}\text{/}\mathrm{MgO}\text{/}\mathrm{Pt}$ exceeds that of $\mathrm{CoPt}\text{/}\mathrm{MgO}\text{/}\mathrm{CoPt}$ for small barrier widths, reaching $175\%$ at room temperature. This provides a sharp contrast to the magnetoresistance, in which $\mathrm{CoPt}\text{/}\mathrm{MgO}\text{/}\mathrm{CoPt}$ performs better by one order of magnitude for all barrier widths. Thus, by switching from two ferromagnetic layers to one (so that spin-orbit coupling alone governs the magnetic transport anisotropy), the magnetoresistance ratio diminishes while the magneto-Seebeck ratio remains comparable or improves considerably. We therefore demonstrate that magnetic tunability can increase when caused solely by spin-orbit coupling. This result sheds light on the role that spin-orbit coupling plays in magnetically tuning the properties of tunnel junctions.

An iron-selenide ${\mathrm{Na}}_{0.8}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$ single crystal has been successfully synthesized using a self-flux method. The electrical resistivity measurement shows that this material exhibits semiconducting behavior in the whole temperature range, with an anomalous increment of resistivity at $T_s\sim 595$ K. By varying the concentrations of Na and Fe, a small volume of superconducting phase could be achieved with a critical temperature of $T_c\sim 34$ K. Structural characterization shows that, similarly to ${\mathrm{K}}_{0.8}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$, the ${\mathrm{Na}}_{0.8}{\mathrm{Fe}}_{1.6}{\mathrm{Se}}_2$ phase exhibits clear superstructure with a modulation wave vector of $q=(3/5,1/5,0)$ caused by the Fe-vacancy order within the a-b plane.

We present temperature dependent powder x-ray diffraction measurements of the spinel $A{\mathrm{V}}_2{\mathrm{O}}_4$ ($A=\mathrm{Cd}, \mathrm{Mg}, \mathrm{Zn}$, and ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x{\mathrm{V}}_2{\mathrm{O}}_4$). The result shows that even though the V-V distance is an important governing parameter, the $A$-site magnetism also has a significant effect on the physical properties of $A{\mathrm{V}}_2{\mathrm{O}}_4$. This is demonstrated by comparing the structural and transport properties of nonmagnetic $A$-site ions $A{\mathrm{V}}_2{\mathrm{O}}_4$ ($A$ $=\mathrm{Cd}, \mathrm{Mg}, \mathrm{Zn}$) with the magnetic $A$-site ion substitutional sequence ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x{\mathrm{V}}_2{\mathrm{O}}_4$.

Structural, electronic, and magnetic properties of ${\mathrm{Ti}}_5{\mathrm{O}}_9$ have been studied by ab initio methods in low-, intermediate-, and high-temperature phases. We have found the charge and orbital order in all three phases to be nonunique, and the formation of ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ bipolaronic states less likely as compared to ${\mathrm{Ti}}_4{\mathrm{O}}_7$. Several quasidegenerate magnetic configurations were calculated to have different width of the band gap, suggesting that the reordering of the unpaired spins at ${\mathrm{Ti}}^{3+}$ ions might at least partially be responsible for the changes in conductivity of this material.

We demonstrate experimentally the frequency upconversion of a terahertz (THz) pulse by the relativistic Doppler reflection from a counterpropagating charge-carrier plasma front in high-resistivity Si. The plasma front is generated via interband excitation with an ultrashort optical pump pulse. Spectral components extending to $\sim 28\mathrm{THz}$ are observed, using an input THz pulse with a bandwidth of $\sim 20\mathrm{THz}$ obtained from a two-color gas plasma emitter. We model the experiment using one-dimensional finite-difference time-domain simulations with realistic values of the Drude parameters for the plasma front, including effects due to excitation from a weak prepulse structure on the pump pulse.

Using first-principles calculations, we study the diffusions of interstitial defects Cd, Cu, Te, and Cl in CdTe. We find that the diffusion behavior is strongly correlated with the electronic structure of the interstitial diffuser. For Cd and Cu, because the defect state is the nondegenerated $s$-like state under the $T_d$ symmetry, the diffusions are almost along the $\langle 111\rangle$ directions between the tetrahedral sites, although the diffusion of Cu shows some deviation due to the coupling between the Cu $d$ and host $s$ orbitals. The diffusions of the neutral and charged Cd and Cu follow similar paths. However, for Te and Cl atoms, because the defect state is the degenerated $p$-like state under the $T_d$ symmetry, large distortions occur. Therefore the diffusion paths are very different from those of the interstitial Cd and Cu atoms, and depend strongly on the charge states of the interstitial atoms. For Te, we find that the distortion is mostly stabilized by the crystal-field splitting, but for Cl, the exchange splitting plays a more important role.

Noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) have become important tools for nanotechnology; however, their contrast mechanisms on the atomic scale are not entirely understood. Here we used chlorine vacancies in NaCl bilayers on Cu(111) as a model system to investigate atomic contrast as a function of applied voltage, tip height, and tip functionalization. We demonstrate that the AFM contrast on the atomic scale decisively depends on both the tip termination and the sample voltage. On the contrary, the local contact potential difference acquired with KPFM showed the same qualitative contrast for all tip terminations investigated, which resembled the contrast of the electrostatic field of the sample. We find that the AFM contrast stems mainly from electrostatic interactions but its tip dependence cannot be explained by the tip dipole alone. With the aid of a simple electrostatic model and by density functional theory, we investigate the underlying contrast mechanisms.

We present a surface x-ray analysis of the atomic structure of the (0001) surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, which was grown as a single crystal and as an ultrathin film on Si(111) using molecular beam epitaxy (MBE). In general we find that the top Se-Bi layer spacing is expanded between 2% and 17% relative to the bulk, while deeper layers and the first van der Waals gap are unrelaxed. The top layer expansion is directly related to the amount of surface contamination by carbon and oxygen. The near-surface structures of the single crystal and the MBE-grown thin film differ in the degree of (static) disorder: for the former an overall Debye parameter ($B$) per quintuple layer (QL) of $5{\AA }^2$ is found to decrease slowly with depth. MBE-grown ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films exhibit the opposite scenario, characterized by an increase in $B$ from about $10{\AA }^2$ for the topmost QL to values of $B=20–40 {\AA }^2$ for the fourth QL. This is attributed to the lattice misfit to the Si(111) surface. Ab initio calculations reveal carbon to act as an $n$-dopant, while the first interlayer spacing expansion induces a shift of the Dirac point towards the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ bulk conduction band minimum.

We derive a general procedure for finding the electromagnetic normal modes in layered structures. We apply this procedure to planar, spherical, and cylindrical structures. These normal modes are important in a variety of applications. They are the only input needed in calculations of Casimir interactions. We present an explicit expression for the condition for modes and Casimir energy for a large number of specific geometries. The layers are allowed to be two-dimensional so graphene and graphenelike sheets as well as two-dimensional electron gases can be handled within the formalism. Also, forces on atoms in layered structures are obtained. One side result is the van der Waals and Casimir-Polder interaction between two atoms.

We predict a magnetoresistance induced by the interfacial Rashba spin-orbit coupling in normal metal/ferromagnetic insulator bilayers. It depends on the angle between current and magnetization directions as found for the “spin Hall magnetoresistance” mechanism, i.e., the combined action of spin Hall and inverse spin Hall effects. By the identical phenomenology it is not obvious whether the magnetoresistance reported by Nakayama et al. [Phys. Rev. Lett. 110, 206601 (2013)] is a bulk metal or interface effect. The interfacial Rashba-induced magnetoresistance may be distinguished from the bulk metal spin Hall magnetoresistance by its dependence on the metal film thickness.

Recent numerical simulations of flat-band models with interactions which show clear evidence of fractionalized topological phases in the absence of a net magnetic field have generated a great deal of interest. We provide an explanation for these observations by showing that the physics of these systems is the same as that of conventional fractional quantum Hall phases in the lowest Landau level under certain ideal conditions which can be specified in terms of the Berry curvature and the Fubini-Study or quantum metric of the topological band. In particular, we show that when these ideal conditions hold, the density operators projected to the topological band obey the $W_{\infty }$ algebra. Our approach leads us to propose a way of testing the suitability of topological bands for hosting fractionalized phases.

Topological insulators are known for their metallic surface states, a result of strong spin-orbit coupling, that exhibit unique surface transport phenomenon. However, these surface transport phenomena are buried in the presence of metallic bulk conduction. We synthesized very high quality ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ single crystals by using a modified Bridgman method that possess high bulk resistivity of $>20$ $\Omega \mathrm{cm}$ below 20 K, whereas the bulk is mostly inactive and surface transport dominates. The temperature dependence of resistivity follows an activation law like a gap semiconductor in temperature range 20–300 K. To extract the surface transport from that of the bulk, we designed a special measurement geometry to measure the resistance and found that single-crystal ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ exhibits a crossover from bulk to surface conduction at 20 K. Simultaneously, the material also shows strong evidence of weak antilocalization in magnetotransport owing to the protection against scattering by conducting surface states. This simple geometry facilitates finding evidence of surface transport in topological insulators, which are promising materials for future spintronic applications.

High-precision measurements of the electric resistance, thermopower, and volume of TmS, TmSe, and TmTe under hydrostatic pressures up to 8.5 GPa were conducted. Comparison of the behavior of the electron-transport characteristics and volume of TmTe and SmTe in the electron transition region demonstrates a complete analogy up to the quantitative coincidence. We found that the thermopower of all samarium and thulium chalcogenides in the lattice-collapse region and during the subsequent reconstruction of the electronic spectrum obeys the universal dependence, which corresponds to the intersection of the Fermi level with the peak of the electron density of states. The results obtained testify in favor of the exciton nature of the intermediate valence state in chalcogenides of the rare-earth metals.

Solar cells based on conventional semiconductors have low efficiency in converting solar energy into electricity because the excess energy beyond the gap of an incident solar photon is converted into heat by phonons. Here we show by ab initio methods that the presence of strong Coulomb interactions in strongly correlated insulators (SCIs) causes the highly photoexcited electron-hole pair to decay fast into multiple electron-hole pairs via impact ionization (II). We show that the II rate in the insulating $M_1$ phase of vanadium dioxide (chosen for this study as it is considered a prototypical SCI) is 2 orders of magnitude higher than in Si and much higher than the rate of hot electron-hole decay due to phonons. Our results indicate that a rather broad class of materials may be harnessed for an efficient solar-to-electrical energy conversion that has been not considered before.

Using tight-binding models and first-principles calculations, we demonstrate the possibility to achieve a quantum anomalous Hall (QAH) phase on a two-dimensional square lattice, which can be realized in monolayers of double perovskites. We show that effective intersite spin-orbit coupling between $e_g$ orbitals can be induced perturbatively, giving rise to a QAH state. Moreover, the effective spin-orbit coupling can be enhanced by octahedral rotations. Based on first-principles calculations, we propose that this type of QAH state could be realized in ${\mathrm{La}}_2{\mathrm{MnIrO}}_6$ monolayers, with the size of the gap as large as 26 meV in the ideal case. We observe that the electronic structure is sensitive to structural distortions, and that an enhanced Hubbard $U$ tends to stabilize the nontrivial gap.

We study hot-electron cooling by acoustic and optical phonons in monolayer ${\mathrm{MoS}}_2$. The cooling power $P$ ($P_e=P/n$) is investigated as a function of electron temperature $T_e$ (0–500 K) and carrier density $n({10}^{10}–{10}^{13} {\mathrm{cm}}^{-2}$) taking into account all relevant electron-phonon (el-ph) couplings. We find that the crossover from acoustic phonon dominated cooling at low $T_e$ to optical phonon dominated cooling at higher $T_e$ takes place at $T_e\sim 50–75$ K. The unscreened deformation potential (DP) coupling to the TA phonon is shown to dominate $P$ due to acoustic phonon scattering over the entire temperature and density range considered. The cooling power due to screened DP coupling to the LA phonon and screened piezoelectric (PE) coupling to the TA and LA phonons is orders of magnitude lower. In the Bloch-Grüneisen (BG) regime, $P\sim T_e^4\left(T_e^6\right)$ is predicted for unscreened (screened) el-ph interaction and $P\sim n^{-1/2}(P_e\sim n^{-3/2})$ for both unscreened and screened el-ph interaction. The cooling power due to optical phonons is dominated by zero-order DP couplings and the Fröhlich interaction, and is found to be significantly reduced by the hot-phonon effect when the phonon relaxation time due to phonon-phonon scattering is large compared to the relaxation time due to el-ph scattering. The $T_e$ and $n$ dependence of the hot-phonon distribution function is also studied. Our results for monolayer ${\mathrm{MoS}}_2$ are compared with those in conventional two-dimensional electron gases (2DEGs) as well as monolayer and bilayer graphene.

The structural, magnetic, and thermodynamic properties of the Au(001)/Fe(001) interface are investigated as a function of the in-plane strain using density functional theory calculations for two different Au slab thicknesses: 2 and 8 monolayers. The structural and magnetic properties are analyzed by studying the interlayer distance in the direction perpendicular to the interface and the atomic magnetic moments of Fe atoms, as a function of the in-plane strain. The structural study evidences both the bulk elastic and surface and interface contributions. The atomic magnetic moments of Fe atoms are essentially dependent on their local environment (number and distance of the Fe first neighbors). Thermodynamic properties of the interface are investigated through the calculation of the interface energy and interface stress. These thermodynamic quantities are subsequently used in a simple model to evaluate the strain state of an ideal spherical symmetric Fe@Au core-shell nanoparticle. The surface elastic effects are found to be significant for nanoparticles of diameter smaller than $\sim 20$ nm and predominant for diameters smaller than $\sim 2.3$ nm. Interface elastic effects are weaker than surface elastic effects but can not be neglected for very small nanoparticles ($\lesssim 1.9$ nm) or for thin shells.

The (001) surfaces of cleaved ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ samples were investigated using low-temperature scanning tunneling microscopy and density functional theory calculations. Intrinsic defects are not created during cleaving. This experimental observation is consistent with calculations, where the formation energy for a Sr and O vacancy, 4.19 eV and 3.81 eV, respectively, is significantly larger than that required to cleave the crystal, 1.11 eV/(1 × 1) unit cell. Surface oxygen vacancies can be created through electron bombardment, however, and their appearance is shown to vary strongly with the imaging conditions. Point defects observed on as-cleaved surfaces result from bulk impurities and adsorption from the residual gas.

Due to its $p$-like character, the valence band in GaAs-based heterostructures offers rich and complex spin-dependent phenomena. One manifestation is the large anisotropy of Zeeman spin splitting. Using undoped, coupled quantum wells (QWs), we examine this anisotropy by comparing the hole-spin dynamics for high- and low-symmetry crystallographic orientations of the QWs. We directly measure the hole $g$ factor via time-resolved Kerr rotation, and for the low-symmetry crystallographic orientations (110) and ($113a$), we observe a large in-plane anisotropy of the hole $g$ factor, in good agreement with our theoretical calculations. Using resonant spin amplification, we also observe an anisotropy of the hole-spin dephasing in the (110)-grown structure, indicating that crystal symmetry may be used to control hole-spin dynamics.