The electronic structure of EuO under pressure is studied using the self-interaction corrected local spin density approximation. EuO, which at ambient conditions crystallizes in the NaCl (B1) structure, is predicted to undergo an isostructural insulator to metal transition at 48 GPa. This transition is associated with a change of valence from a divalent to an intermediate valent state, with the resulting effective valency of 2.35. The pressure range between 48 and 70 GPa is characterized by the competition between an intermediate valent B1 structured phase and a CsCl (B2) structured phase where both the divalent and intermediate valence configurations are in play. Eventually, at pressures above 70 GPa, the intermediate valent B2 phase prevails. The effective Eu valence in the B2 intermediate valence phase is around 2.28, i.e., a decrease in effective valence occurs. This scenario is in line with the reentrant valence behavior observed in recent pressure experiments.

The single-particle spectral properties near the Mott transition in the two-dimensional Hubbard model with next-nearest-neighbor hopping are investigated by using cluster perturbation theory. Complicated spectral features of this model are simply interpreted, by considering how the next-nearest-neighbor hopping shifts the spectral weights of the two-dimensional Hubbard model. Various anomalous features observed in hole-doped and electron-doped cuprate high-temperature superconductors are explained in a unified manner as properties near the Mott transition in a two-dimensional system whose spectral weights are shifted by next-nearest-neighbor hopping.

Hybrid density functional calculations are carried out to investigate the behavior of holes in ${\mathrm{Sr}\mathrm{Ti}O}_3$. As in many other oxides, it is shown that a hole tend to localize on one oxygen forming an ${\mathrm{O}}^{-}$ anion with a concomitant lattice distortion; therefore a hole polaron. The calculated emission energy from the recombination of the localized hole and a conduction-band electron is about 2.5 eV, in good agreement with experiments. Therefore the localization of the hole or self-trapping is likely to be responsible for the green photoluminescence at low temperature, which was previously attributed to an unknown defect state. Compared to an electron, the calculated hole polaron mobility is three orders of magnitude lower at room temperature. In addition, two ${\mathrm{O}}^{-}$ anions can bind strongly to form an O${}_2{}^{2-}$ peroxide anion. No electronic states associated with the $O_2{}^{2-}$ peroxide anion are located inside the band gap or close to the band edges, indicating that it is electronically inactive. We suggest that in addition to the oxygen vacancy, the formation of the $O_2{}^{2-}$ peroxide anion can be an alternative to compensate acceptor doping in ${\mathrm{Sr}\mathrm{Ti}O}_3$.

A spin-orbit qubit is a hybrid qubit that contains both orbital and spin degrees of freedom of an electron in a quantum dot. Here we study the exchange coupling between two spin-orbit qubits in a nanowire double quantum dot (DQD) with strong spin-orbit coupling (SOC). We find that while the total tunneling in the DQD is irrelevant to the SOC, both the spin-conserved and spin-flipped tunnelings are SOC dependent and can compete with each other in the strong SOC regime. Moreover, the Coulomb repulsion between electrons can combine with the SOC-dependent tunnelings to yield an anisotropic exchange coupling between the two spin-orbit qubits. Also, we give an explicit physical mechanism for this anisotropic exchange coupling.

We investigate the shot noise of an impurity-free graphene flake as a function of the chemical potential. For large width to length ratios, this noise has been predicted and observed to exhibit universal characteristics at the Dirac point. Furthermore, a sharp decrease of the shot noise with increasing carrier density has been predicted. This decrease has also been observed in experiments but with much smaller slope than predicted. We reconcile this discrepancy between theory and experiment by including the effects of the contacts to the graphene ribbon.

Joint experimental and theoretical investigation of image formation in noncontact atomic force microscopy (NC-AFM) of the $c$(6 $\times$ 2) and $p$(2 $\times$ 1) phases of the Cu(110):O surface is presented. We proposed previously that the $c$(6 $\times$ 2) reconstruction of the Cu(110):O surface may serve as a reference system allowing chemical identification of the tip apex atom during the course of NC-AFM experiments. The identification is possible due to the fact that two most likely possible contrasts that could be observed after intermittent contacts of the tip with the surface were found which can be attributed to the tip being either terminated by Cu or O atoms. In this paper the idea of exploiting the Cu(110):O surface in NC-AFM studies is further developed. Specifically, (i) we show that there must be an image contrast inversion when the $c$(6 $\times$ 2) surface reconstruction is scanned depending on the tip-surface distance: at the usual imaging conditions, at tip-sample distances of 2–5 Å, the previously reported contrast is observed; however, an opposite contrast is observed for larger separations with one of the two tip terminations. (ii) We study in detail also the image contrast formation of the $p(2\times 1)$ surface, which is another common surface reconstruction, and show that tip identification for it is not possible. (iii) Finally, we discuss here possible effects of the actual tip atomic structure on the NC-AFM image. In particular, we show that the O-terminated tip will remain such even after picking up a Cu atom from the surface, experimentally a frequently observed process. Hence this type of the tip modification would only affect secondary features in the image.

Understanding the geometric structures of ${\mathrm{TiO}}_2$ surfaces at the atomic level is essential for the development of high-performance photocatalysts with desired properties. In this study, first-principles density functional theory calculations have been performed to detect the stable geometric structures of rutile ${\mathrm{TiO}}_2\left\{111\right\}$ facets, which are key components of rutile ${\mathrm{TiO}}_2$ nanorods for their applications. Based on our theoretical results, the bulk-truncated rutile (111) surfaces with high surface energies can be stabilized through hydroxylation. The stable hydroxylated surface geometries have been rationalized by the photoelectrocatalytic measurements. Using the hydroxylated surface models, some experimental observations of rutile (111) surfaces can therefore be successfully explained.

We present a theoretical analysis of low-frequency dynamics of a single-channel mesoscopic capacitor, which is composed by a quantum dot connected to an electron reservoir via a single quantum channel. At low frequencies, it is known that the Wigner-Smith delay time ${\tau }_W$ plays a dominant role and it can be interpreted as the time delay between the current leaving the dot and the current entering the dot. At higher frequencies, we find that another characteristic time ${\tau }_S$ can also be important. It describes the deformation of the leaving current to the entering one and hence can be referred as the deformation time. At sufficient low temperatures, the deformation time ${\tau }_S$ can be approximated from the second-order derivative of ${\tau }_W$ with respect to the energy via a simple relation ${\tau }_W^{\prime \prime }/{\tau }_S^3=24/{\hslash }^2$. As the temperature increases, this relation breaks down and one has instead ${\tau }_W^{\prime \prime }/{\tau }_S^3\to 0$ in the high-temperature limit. We further show that the deformation time ${\tau }_S$ can have a pronounced influence on the quantum inductance $L_q$ of the mesoscopic capacitor, leading to features different from the ones of the quantum capacitance. The most striking one is that $L_q$ can change its sign as the temperature increases: It can go from positive values at low temperatures to large negative values at high temperatures. The above results demonstrate the importance of the deformation time ${\tau }_S$ on the ac conductance of the mesoscopic capacitor.

We experimentally demonstrate the optical generation of hypersound in a piezoelectric GaAs semiconductor through laser excitation of the THz photo-Dember electric field. Such an ultrashort transient Dember electric field is linked to the spatial separation of photoexcited electrons and holes right after above-band-gap femtosecond laser excitation. Through time-domain coherent Brillouin scattering we demonstrate that photoinduced piezoelectric generation of hypersound can dominate even in the absence of preexisting built-in fields and we observe, with increasing laser fluence, a nonlinear optoacoustic excitation process. These results reveal the onset of the THz photo-Dember electric field and highlight the transition from non-ambipolar flow to ambipolar diffusion of the photoexcited electron-hole plasma.

In epitaxial MnSi/Si(111) films, the in-plane magnetization saturation is never reached due to the formation of specific surface chiral modulations with the propagation direction perpendicular to the film surfaces [Wilson et al., Phys. Rev. B 88, 214420 (2013)]. In this paper we show that the occurrence of such chiral surface twists is a general effect attributed to all bulk and confined magnetic crystals lacking inversion symmetry. We present experimental investigations of this phenomenon in nanolayers of MnSi/Si(111) supported by detailed theoretical analysis within the standard phenomenological model. In magnetic nanolayers with intrinsic or induced chirality, such surface induced instabilities become sizable effects and play a crucial role in the formation of skyrmion lattices and other nontrivial chiral modulations.

We study theoretically a one-dimensional dimerized Kitaev superconductor model which belongs to BDI class with time-reversal, particle-hole, and chiral symmetries. There are two sources of the particle-hole symmetry, i.e., the sublattice symmetry and superconductivity. Accordingly, we define two types of topological numbers with respect to the chiral indices of normal and Majorana fermions, which offers an ideal laboratory to examine the interference between the two different physics within the same symmetry class. Phase diagram, zero-energy bound states, and conductance at normal metal/superconductor junction of this model are unveiled from this viewpoint. Especially, the electron fractionalization to the Majorana fermions showing the splitting of the local density of states is realized at the soliton of the dimerization in this model.

We present NMR measurements of the layered nitride superconductor $\mathrm{Li}{}_x$ZrNCl. The nuclear spin-lattice relaxation rate $1/T_1$ shows that the coherence peak is strongly suppressed in $\mathrm{Li}{}_x$ZrNCl in contrast to conventional BCS superconductors. In the lightly doped region close to the insulating state, the system shows a gaplike behavior, i.e., pseudogap, that is characterized by a reduction in the magnitude of the Knight shift and $1/T_{1}T$. A higher superconducting (SC) transition temperature $T_c$ is achieved by coexisting with the pseudogap state. These unusual behaviors, which deviate from the ordinary BCS framework, are the key ingredients to understanding the SC mechanism of $\mathrm{Li}{}_x$ZrNCl.

A newly discovered topological insulator (TI) $\left({\mathrm{Ag}}_x{\mathrm{Pb}}_{1-x}\mathrm{Se}{)}_5\right({\mathrm{Bi}}_2{\mathrm{Se}}_3{)}_{3m}$ ($m=2$) has a band gap of 0.5 eV, the largest value reported in topological insulators. We present a magnetotransport study of the Dirac electrons of this compound in the quantum diffusion regime. Two-dimensional weak antilocalization due to the destructive interference of the Dirac electrons was observed. The phase coherence length of the Dirac electrons is independent of doping and disorder levels. This provides an indication of the backscattering immunity under the protection of time reversal invariance of the TI. We further report that the homologous compound $\left({\mathrm{Ag}}_x{\mathrm{Pb}}_{1-x}\mathrm{Se}{)}_5\right({\mathrm{Bi}}_2{\mathrm{Se}}_3{)}_{3m}$ ($m=1$) is a superconductor with a transition temperature $T_C=1.7$ K. The related structures of these two phases allow lateral intergrowth of crystals to occur naturally, offering a potential platform to observe the Majorana fermion state at the boundary of two intergrown crystals.

A textured thin film of FeGe was grown by magnetron sputtering with a helimagnetic ordering temperature of $T_{\mathrm{N}}=276\pm 2$ K. From 5 K to room temperature, a variety of scattering processes contribute toward the overall longitudinal and Hall resistivities. These were studied by combining magnetometry and magnetotransport measurements. The high-field magnetoresistance (MR) displays three clear temperature regimes: Lorentz force MR dominates at low temperatures, above $T\approx 80$ K scattering from spin-waves predominates, while finally for $T\gtrsim 200$ K scattering from fluctuating local moments describes the MR. At low fields, where the magnetization is no longer technically saturated, we find a scaling of magnetoresistance with the square of the magnetization, indicating that the MR due to the unwinding of spins in the conical phase arises from a similar mechanism to that in magnetic domain walls. This MR is only visible up to a temperature of about 200 K. No features can be found in the temperature or field dependence of the longitudinal resistivity that belie the presence of the underlying magnetic phase transition at $T_{\mathrm{N}}$: the marked changes in behavior are at much lower temperatures. The anomalous Hall effect has a dramatic temperature dependence in which the anomalous Hall resistivity scales quadratically with the longitudinal resistivity: comparison with anomalous Hall scaling theory shows that our system is in the intrinsic “moderately dirty” regime. Lastly, we find evidence of a topological Hall effect of size $\sim 100\mu \Omega$ cm.

We show how the local temperature of out-of-equilibrium, quantum electron systems can be consistently defined with the help of an external voltage and temperature probe. We determine sufficient conditions under which the temperature measured by the probe (i) is independent of details of the system-probe coupling, (ii) is equal to the temperature obtained from an independent current-noise measurement, (iii) satisfies the transitivity condition expressed by the zeroth law of thermodynamics, and (iv) is consistent with Carnot's theorem. This local temperature therefore characterizes the system in the common sense of equilibrium thermodynamics, but remains well defined even in out-of-equilibrium situations with no local equilibrium.

We discuss the role of spin-flip scattering of electrons from the magnetized edges in graphene nanoribbons. The spin-flip scattering is associated with strong fluctuations of the magnetic moments at the edge. Using the Boltzmann equation approach, which is valid for not too narrow nanoribbons, we calculate the spin-relaxation time in the case of Berry-Mondragon and zigzag graphene edges. We also consider the case of ballistic nanoribbons characterized by very long momentum relaxation time in the bulk, when the main source of momentum and spin relaxation is the spin-dependent scattering at the edges. We found that in the case of zigzag edges, an anomalous spin diffusion is possible, which is related to very weak spin-flip scattering of electrons gliding along the nanoribbon edge.