Oxidation of semiconductor surfaces is known to cause defect states at oxide-semiconductor interfaces of various devices. In contrast, effects of the semiconductor interaction with non-oxygen elements at such junctions are still unclear. We present evidence for the interrelationship between a metal (non-oxygen)-semiconductor reaction and formation of the band-gap defect states at a buried oxide-semiconductor interface by investigating well-defined epitaxial $\mathrm{BaO}/\mathrm{Ge}\left(100\right)$ junctions with high-resolution synchrotron-radiation photoelectron spectroscopy. The states that arise from the Ba-Ge interaction lead to Fermi-level pinning at $0.40\mathrm{eV}$ above the valence band maximum, while the defect-free $\mathrm{BaO}/\mathrm{Ge}\left(100\right)$ interface has a flat band structure.

We theoretically study the competition between two possible exotic superconducting orders that may occur in graphene-like systems, assuming dominant nearest-neighbor attraction: the gapless hidden superconducting order, which renormalizes the Fermi velocity, and the Kekule order, which opens a superconducting gap. We perform an analysis within the mean-field theory for Dirac electrons, at finite temperature and finite chemical potential, as well as at half filling and zero temperature, first excluding the possibility of the coexistence of the two orders. In that case, we find the dependence of the critical (more precisely, crossover) temperature and the critical interaction on the chemical potential. As a result of this analysis, we find that the Kekule order is preferred over the hidden order at both finite temperature and finite chemical potential. However, when the coexistence of the two superconducting orders is allowed, by solving the coupled mean-field gap equations, we find that above a critical value of the attractive interaction a mixed phase sets in, in which these orders coexist. We show that the critical value of the interaction for this transition is greater than the critical coupling for the hidden superconducting state in the absence of the Kekule order, implying that there is a region in the phase diagram where the Kekule order is favored as a result of the competition with the hidden superconducting order. The latter, however, eventually sets in and coexists with the Kekule state. According to our mean-field calculations, the transition from the Kekule to the mixed phase is of the second order, but it may become first order when fluctuations are considered. Finally, we investigate whether these phases could be possible in honeycomb superlattices of self-assembled semiconducting nanocrystals, which have been recently experimentally realized with CdSe and PbSe.

We report measurements of current-induced magnetic torques in structures containing a $\sim 1$-nm-thick layer of an antiferromagnet IrMn. We perform ac-electrical measurements in Ta/IrMn/CoFeB and Ta/IrMn structures below and above the Néel temperature of the ultrathin IrMn and compare the results with control experiments in a Ta/CoFeB bilayer. We employ the second harmonics technique to detect current-induced effects and perform symmetry analysis that enables us to separate distinct contributions to the signal. We identify signals generated by a thermal gradient in the sample and signals originating from current-induced torques. In the Ta/CoFeB bilayer we observe a current-induced torque acting on CoFeB consistent with the sign and magnitude of the spin Hall effect in Ta. In the Ta/IrMn/CoFeB trilayer, the opposite sign of spin Hall angles in IrMn and Ta results in a strongly suppressed spin torque acting in CoFeB at high temperatures where IrMn is paramagnetic. In the low temperature antiferromagnetic state of IrMn, the torque acting in CoFeB is enhanced and driven by the spin Hall effect in IrMn with the opposite sign and comparable amplitude to the torque in the Ta/CoFeB bilayer. The spin Hall current from Ta in the Ta/IrMn/CoFeB trilayer at low temperatures is not compensating the spin Hall current from IrMn in the net torque acting on CoFeB but is instead absorbed by the antiferromagnetic moments in IrMn. This is confirmed by measurements in the Ta/IrMn bilayer with no ferromagnetic CoFeB layer in the structure. Here we detect no signatures of a torque in the high-temperature paramagnetic state of IrMn while the second harmonic signal emerges at low temperatures. The signal depends on how the antiferromagnetic moments have been preset by cooling the sample from above the IrMn transition temperature in an applied magnetic field, confirming its magnetic torque origin. When sufficiently below the transition temperature, magnetic field has no effect on the measured signal as expected for an antiferromagnet.

A bond-order potential is presented for the Ru-C system, aiming to model epitaxial graphene on Ru(0001) and ruthenium nanoparticles on such substrates or on graphite. The model has been parametrized on electronic structure calculations and improved to account for long-range London dispersion forces following an approach similar to the Grimme D2 correction scheme, as well as possible nonadditive screening effects that are relevant for epitaxial graphene on metal. The model correctly reproduces a variety of structural properties for different commensurate moiré structures as observed in experiments or predicted by density-functional theory calculations, although limitations are noted for very small adsorbates likely due to the lack of explicit charge transfer. The energetic and thermal stabilities of Ru nanoparticles on graphite and epitaxial graphene have been addressed using local optimizations and molecular dynamics simulations. While the nanoparticles exhibit relatively fast diffusion on the graphite substrate, the corrugation of epitaxial graphene strongly stabilizes them against internal rearrangement and global diffusion. The simulated vibrational spectra of epitaxial graphene show variations with the moiré structure and temperature that provide insight into anharmonicities and emphasize the role of strain.

We report the first-principles study of the correlated behavior of the magnetic anisotropy energy (MAE) and orbital moment anisotropy (OMA) as the functions of the thickness $N$ of the Fe film. The work is motivated by recent experimental studies combining photoemission, x-ray magnetic circular dichroism, and magnetic anisotropy measurements. In agreement with experiment, the correlated oscillations of $\mathrm{MAE}\left(N\right)$ and $\mathrm{OMA}\left(N\right)$ are obtained that have their origin in the formation of the 3d quantum well states (QWS) confined in the films. The main contribution to the oscillation amplitude comes from the surface layer. This is an interesting feature of the phenomenon consisting in the peculiar dependence of the physical quantities on the thickness of the film. We demonstrate that the band structure of the bulk Fe does not reflect adequately the properties of the 3d QWS in thin films and, therefore, does not provide the basis for understanding the oscillations of $\mathrm{MAE}\left(N\right)$ and $\mathrm{OMA}\left(N\right)$. A detailed point-by-point analysis in the two-dimensional (2D) Brillouin zone (BZ) of the film shows that the contribution of the $\mathrm{\Gamma }$ point, contrary to a rather common expectation, does not play an important role in the formation of the oscillations. Instead, the most important contributions come from a broad region of the 2D BZ distant from the center of the BZ. Combining symmetry arguments and direct calculations we show that orbital moments of the electronic states possess nonzero transverse components orthogonal to the direction of the spin magnetization. The account for this feature is crucial in the point-by-point analysis of the OMA. On the basis of the calculations for noncollinear spin configurations we suggest interpretations of two interesting experimental findings: fast temperature decay of the oscillation amplitude in $\mathrm{MAE}\left(N\right)$ and unexpectedly strong spin mixing of the initial states of the photoemission process.

The purple bronze ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ is of interest due to its quasi-one-dimensional electronic structure and the possible Luttinger liquid behavior resulting from it. For sufficiently low temperatures, it is a superconductor with a pairing symmetry that is still to be determined. To shed light on this issue, we analyze a minimal Hubbard model for this material involving four molybdenum orbitals per unit cell near quarter filling, using asymptotically exact perturbative renormalization group methods. We find that spin-triplet odd-parity superconductivity is the dominant instability. Approximate nesting properties of the two quasi-one-dimensional Fermi surfaces enhance certain second-order processes, which play crucial roles in determining the structure of the pairing gap. Notably, we find that the gap has more sign changes than required by the point-group symmetry.

The pyrochlore spin ice compounds ${\mathrm{Dy}}_{\text{2}}{\mathrm{Ti}}_{\text{2}}{\mathrm{O}}_{\text{7}}$ and ${\mathrm{Ho}}_{\text{2}}{\mathrm{Ti}}_{\text{2}}{\mathrm{O}}_{\text{7}}$ are well described by classical Ising models down to low temperatures. Given the empirical success of this description, the question of the importance of quantum effects in these materials has been mostly ignored. We show that the common wisdom that the strictly Ising moments of isolated ${\mathrm{Dy}}^{\text{3+}}$ and ${\mathrm{Ho}}^{\text{3+}}$ ions imply Ising interactions is too naïve; a more complex argument is needed to explain the close agreement between theory and experiment. From a microscopic picture of the interactions in rare-earth oxides, we show that the high-rank multipolar interactions needed to induce quantum effects in these two materials are generated only very weakly by superexchange. Using this framework, we formulate an estimate of the scale of quantum effects in ${\mathrm{Ho}}_{\text{2}}{\mathrm{Ti}}_{\text{2}}{\mathrm{O}}_{\text{7}}$ and ${\mathrm{Dy}}_{\text{2}}{\mathrm{Ti}}_{\text{2}}{\mathrm{O}}_{\text{7}}$, finding it to be well below experimentally relevant temperatures. We discuss the implications of these results for realizing quantum spin ice in other materials.

The electronic transport and spincaloric properties of epitaxial magnetic tunnel junctions with half-metallic ${\mathrm{Co}}_2\mathrm{MnSi}$ Heusler electrodes, MgO tunneling barriers, and different interface terminations are investigated by using first-principles calculations. An approach to spincaloric properties is presented that circumvents the linear response approximation inherent in the Seebeck coefficient and compared to the method of Sivan and Imry. This approach supports two different temperatures in the two electrodes and provides the exact current and/or voltage response of the system. Moreover, it accounts for temperature-dependent chemical potentials in the electrodes and finite-bias effects. We find that especially the former are important for obtaining qualitatively correct results, even if the variations of the chemical potentials are small. It is shown how the spincaloric properties can be tailored by the choice of the growth conditions. We find a large effective and spin-dependent Seebeck coefficient of $-65\mu \mathrm{V}/\mathrm{K}$ at room temperature for the purely Co-terminated interface. We suggest to use such interfaces in thermally operated magnetoresistive random access memory modules, which exploit the magneto-Seebeck effect, to maximize the thermally induced readout voltage.

We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal which is free from doping-introduced inhomogeneities and charged quasiparticle scattering because of its innate superconductivity. The critical current density $J_c$ is found to be almost isotropic and reaches a value of $\sim 3 \times {10}^4 \mathrm{A}/{\mathrm{cm}}^2$ at 2 K (self-field) for both $H\parallel c$ and $ab$. The normalized magnetic relaxation rate $S (=\mid dlnM/dlnt\mid )$ shows a temperature-insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate ($S\sim 0.02$ under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fishtail effect is absent for both $H\parallel c$ and $ab$. Based on this observation, the origin of the fishtail effect is also discussed. Combining the results of $J_c$ and $S$, the vortex motion in the FeSe single crystal is found to be dominated by sparse, strong pointlike pinning from nanometer-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed.

The conducting state of a quasi-two-dimensional electron gas (q2DEG), formed at the heterointerface between the two wide-bandgap insulators $\mathrm{LaAl}{\mathrm{O}}_3$ (LAO) and $\mathrm{SrTi}{\mathrm{O}}_3$, can be made completely insulating by low-energy, 150-eV, $\mathrm{A}{\mathrm{r}}^{+}$ irradiation. The metallic behavior of the interface can be recovered by high-temperature oxygen annealing. The electrical transport properties of the recovered q2DEG are exactly the same as before the irradiation. Microstructural investigations confirm that the transition is not due to physical etching or crystal lattice distortion of the LAO film below its critical thickness. They also reveal a correlation between electrical state, LAO film surface amorphization, and argon ion implantation. The experimental results are in agreement with density functional theory calculations of Ar implantation and migration in the LAO film. This suggests that the metal-insulator transition may be caused by charge trapping in the defect amorphous layer created during the ion irradiation.

In this work, we propose a method for calculating the free energy of anisotropic classical spin systems. We use a Hubbard-Stratonovich transformation to express the partition function of a generic bilinear superexchange Hamiltonian in terms of a functional integral over classical time-independent fields. As an example, we consider an anisotropic spin-exchange Hamiltonian on the cubic lattice as is found for compounds with strongly correlated electrons in multiorbital bands and subject to strong spin-orbit interaction. We calculate the contribution of Gaussian spin fluctuations to the free energy. While the mean-field solution of ordered states for such systems usually has full rotational symmetry, we show here that the fluctuations lead to a pinning of the spontaneous magnetization along some preferred direction of the lattice.

The absorption spectrum represents an important aspect of single semiconductor nanocrystals (NCs), revealing the morphology and orientation of the NCs. By conducting quantitative photoluminescence excitation spectroscopy based on a single-photon counting method, we succeed in clarifying the absorption cross-section spectrum of neutral excitons in single CdSe/ZnS NCs. With the values of the absorption cross section, we reveal that the intensity saturation of the neutral-exciton emission can be explained by considering the Poisson statistics for incident photons and by the small quantum yield of the biexciton emission. We also evaluated quantitatively the biexciton emission contribution and the ionization rate of NCs that appear at increased incident photon fluxes.

We report on the nonlinear optical response of a monochiral sample of (6,5) single-wall carbon nanotubes by means of broadband two-color pump-probe spectroscopy with selective excitation of the $S_{11}$ excitons. By using a moment analysis of the transient spectra, we show that all the nonlinear features can be accurately accounted for by elementary deformations of the linear absorption spectrum. The photogeneration of $S_{11}$ excitons induces a broadening and a blueshift of both the $S_{11}$ and $S_{22}$ excitonic transitions. In contrast, only the $S_{11}$ transition shows a reduction of oscillator strength, ruling out population up-conversion. These nonlinear signatures result from many-body effects, including phase-space filling, wave-function renormalization, and exciton collisions. This framework is sufficient to interpret the magnitude of the observed nonlinearities and stress the importance of intersubband exciton interactions. Remarkably, we show that these intersubband interactions have the same magnitude as the intraband ones and bring the major contribution to the photobleaching of the $S_{22}$ excitonic transition upon $S_{11}$ excitation through energy shift and broadening.

The influence of spin-orbit interaction on an occupied surface state at Ta(110) is investigated with spin- and angle-resolved photoemission and electronic structure calculations. The surface state appears in a symmetry gap at a binding energy of 0.45 eV at $\overline{\mathrm{\Gamma }}$ and exhibits a free-electron-like $E({\mathbf{k}}_{\parallel })$ dispersion with an effective mass $m^{*}/m_e$ of about $-1.35$ along $\overline{\mathrm{\Gamma }}\overline{H}$. Photoemission results for excitation with $s$- and $p$-polarized light confirm the predicted $d_{z^2}$-type symmetry of the state close to $\overline{\mathrm{\Gamma }}$. Spin-resolved data for finite ${\mathbf{k}}_{\parallel }$ reveal a pure Rashba-type spin texture with a Rashba parameter of $0.063\pm 0.007\mathrm{eV}\AA$. These findings clearly prove a sizable impact of spin-orbit coupling on the $d_{z^2}$ surface state and resolve a longstanding disagreement on this issue.

Electron localization and polaron mobility in oxygen-deficient as well as Li-doped monoclinic tungsten trioxide have been studied in the adiabatic limit in the framework of density functional theory. We show that small polarons formed in the presence of oxygen vacancy prefer the bipolaronic ${\text{W}}^{5+}-{\text{W}}^{5+}$ configuration, whereas the ${\text{W}}^{6+}-{\text{W}}^{4+}$ configuration is found to be metastable. Our calculations suggest that bipolarons are tightly bound by the vacancy and therefore largely immobile. On the contrary, polarons formed as a result of Li intercalation can be mobile; the activation energy for polaron jumping in this case varies between 98 and 124 meV depending on the crystallographic direction. The formation of ${\text{W}}^{5+}-{\text{W}}^{5+}$ bipolarons in ${\text{Li-WO}}_3$ is possible. When situated along $\left[001\right]$ the bipolaronic configuration is 8 meV lower in energy than two separate ${\text{W}}^{5+}$ polarons.

We study two-dimensional transport of quasiparticles in bilayer graphene through a modified Pöschl-Teller potential, a potential with hyperbolic profile. Both scattering by well and tunneling through a barrier are investigated. At normal incidence the oscillating and evanescent wave states are completely decoupled, so that only one mode participates in transmission. At oblique incidence a combined contribution of the two modes leads to reduction or enhancement of transmission probability. In both cases the chirality can forbid transition to oscillating modes existing within the well or barrier region, which results in a cloak effect. In the barrier tunneling it appears at normal incidence. In the scattering by the well, however, it arises at an oblique incidence as a consequence of coupling between the positive- and negative-energy states, which we interpret as a secondary cloak effect. When a gap is present in the barrier tunneling, because of the coupling, the transmission probability at and near normal incident angles is greatly enhanced. We also examine the effect of trigonal warping in the barrier tunneling and find valley-dependent transmissions and broadening of resonant-transmission regions. As a result, the resonance curve of conductance has a thicker width and a lower peak-to-bottom contrast than the case without trigonal warping. We demonstrate these phenomena with numerical simulations and explain the results using the chirality-dependent transition probability and semiclassical argument. The numerical results are also compared with those of the corresponding rectangular potential. We observe that the two models display large quantitative differences in their two-dimensional transport.

We report the structure, magnetic, and thermal property measurements on single-crystalline and polycrystalline samples of the Ru-substituted honeycomb lattice iridate ${\mathrm{Na}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_3 (x=0,0.05,0.1,0.15,0.2,0.3,0.5)$. The evolution of magnetism in ${\mathrm{Na}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{\mathrm{O}}_3$ has been studied using dc and ac magnetic susceptibilities and heat-capacity measurements. The parent compound ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is a spin-orbit-driven Mott insulator with magnetic order of reduced moments below $T_N=15\mathrm{K}$. In the Ru-substituted samples the antiferromagnetic long-range state is replaced by a spin-glass-like state even for the smallest substitution suggesting that the magnetic order in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is extremely fragile. We argue that these behaviors indicate the importance of nearest-neighbor magnetic exchange in the parent ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. Additionally, all samples show insulating electrical transport.