The interface structure of hydrogen-intercalated graphene/SiC(0001), formed by annealing SiC substrates with the buffer layer at high temperature under atmospheric molecular hydrogen, was investigated by core-level photoelectron spectroscopy. The investigation of C 1s spectra, captured before and after the annealing at various temperatures in a vacuum, indicates that residual materials (hydrocarbon and hydrogen) stayed at the interface on the as-treated sample and remained there until annealing at around 700 °C. These residual materials would cause distortion of the graphene. The analysis of Si 2p photoelectron spectra reveals insufficient termination of Si-dangling bonds at the interface by hydrogen and that significant interface states remained. The interface states of Si dangling bonds are plausible origins of Fermi level pinning and would act as charged impurities. These distortion and charged impurities should degrade the electronic performance of graphene in this system.

We present calculations of free energy barriers and diffusivities as functions of temperature for the diffusion of hydrogen in $\alpha$-Fe. This is a fully quantum mechanical approach since the total energy landscape is computed using a self-consistent, transferable tight binding model for interstitial impurities in magnetic iron. Also the hydrogen nucleus is treated quantum mechanically and we compare here two approaches in the literature both based in the Feynman path integral formulation of statistical mechanics. We find that the quantum transition state theory which admits greater freedom for the proton to explore phase space gives result in better agreement with experiment than the alternative which is based on fixed centroid calculations of the free energy barrier. This will have an impact on future modeling and the simulation of hydrogen trapping and diffusion.

${\text{Sm}}_{1-x}{\text{Y}}_x{\text{MnO}}_3$ (SYM x) is one of a class of multiferroic manganites that has seen significant recent interest due to the intimate connection between cycloidal magnetic order and ferroelectricity in these materials. SYM shows sequential transitions between paramagnetic, sinusoidally ordered antiferromagnetic and cycloidally ordered antiferromagnetic phases with decreasing temperature. As in the other members of the family, the magnetic spin cycloid induces ferroelectricity, although whether there is any elastic coupling involved in this process is not known. In this work, resonant ultrasound spectroscopy (RUS) is used to examine the stiffness and dissipation in SYM 0.4 as the magnetic transitions are traversed. It is found that there are only very small signatures of the transitions in the elastic properties of the material, indicating the weakness of the magnetoelastic (and electroelastic) coupling. The mechanical loss does show a significant decrease upon cooling below $T_{N1}=\sim 50$ K, indicating the freezing of some loss mechanism near the temperature where magnetic order is achieved. The strain at these magnetic transitions in a related material, ${\text{Eu}}_{1-x}{\text{Y}}_x{\text{MnO}}_3$, is examined from data published in the literature, and very low shear strain is observed, along with a more significant volume strain effect. This correlates well with the observations from RUS, as the peak frequencies are more sensitive to shear effects than bulk effects. These results suggest that the weak coupling of the magnetic transitions with shear may be a more general behavior in multiferroic perovskite-related manganites.

The means by which charge is exchanged between two surfaces that have been brought into contact is perhaps the longest standing unsolved problem in physics. To this day, it is debated as to whether charging is due to electrons or ions. Contact electrification is such a singular process that it lies beyond the scope of density functional theory and other ab initio theories of material structure. We present a new method for studying the fundamental processes that underlie triboelectrification, based upon the structure of x-ray emission from the interface of separating surfaces. Our measurement of the x-ray spectrum emitted from lead rolling against various thicknesses of unplasticized polyvinyl chloride (uPVC) on top of a grounded conductor indicate that triboelectrification, like turbulence, extends over a broad range of length scales. We observe millimeter-scale charge-patching, which indicates the feasibility of building MEMS x-ray sources.

The spin-orbit splitting of graphene $\pi$ states can be strongly enhanced by external influences such as corrugation or proximity to heavier atoms. Here we investigate experimentally and theoretically whether such strong enhancement is possible for graphene on SiC(0001). By spin- and angle-resolved photoemission we found for two independently grown samples no resolvable spin-orbit splitting with an upper limit of our analysis of 20 meV. Our ab initio calculations predict a low spin-orbit splitting of 0.05 meV with small anisotropy but a local tenfold enhancement where hybridization with SiC substrate bands occurs.

A computational study of the self-organization of heteroepitaxial ultrathin metal films is presented. By means of a continuum complex field model, the relationship of the equilibrium surface patterns of the film to the adsorbate-substrate adhesion energy, as well as to the mismatch between the adsorbate and the substrate bulk lattice parameters, are obtained in both the tensile and the compressive regimes. Our approach captures pattern periodicities over large length scales, up to several hundreds of nm, retaining atomistic resolution. Thus, the results can be directly compared with experimental data, in particular for systems such as Cu/Ru(0001) and Ag/Cu(111). Three nontrivial, stable superstructures for the overlayer, namely, stripe, honeycomb, and triangular, are identified that closely resemble those observed experimentally. Simulations in nonequilibrium conditions are performed as well to identify metastable structural configurations and the dynamics of ordering of the overlayer.

The Casimir force between two finite-thick topological insulator slabs and its dependence on the gap size between the slabs are investigated in detail. Two typical substrate materials including semi-infinite vacuum and silicon are used in the study, in which the Casimir force can always change from attractive to repulsive when the gap size decreases. The gap width at transition is a function of the slab thickness and also depends strongly on the electric permittivity and topological magnetoelectric polarizability of the slabs. In particular, in the absence of a substrate, this width increases with decreasing slab thickness and for thin slabs with large internal and external topological magnetoelectric polarizability the Casimir force is always repulsive where the surface topological magnetoelectric effect of the slabs plays a dominant role. In the presence of a semi-infinite silicon substrate, however, the attractive role of the silicon substrate becomes increasingly important, and thus the transition gap decreases with decreasing thickness of the slab. The characteristic features of the Casimir force may be detected experimentally through exploring its gradient with a certain dynamical method.

First-principles techniques are used to investigate the behavior of BiFeO${}_3$/LaFeO${}_3$ perovskite oxide superlattices epitaxially grown on a (001)-SrTiO${}_3$ substrate. The calculations show that 1/1 superlattices exhibit a $Pmc{2}_1$ ground state combining a trilinear coupling of one polar and two oxygen rotational lattice modes, and weak ferromagnetism. The microscopic mechanism allowing one to manipulate the magnetization with an electric field in such systems is presented and its dependence on strain and chemical substitution is discussed. BiFeO${}_3$/LaFeO${}_3$ artificial superlattices appear to be good candidates to achieve electric switching of magnetization at room temperature.

Electron-phonon coupling plays a central role for time-dependent phenomena in condensed matter, for example, in photoexcitation experiments. We use the continuous-time quantum Monte Carlo method to study the real-time evolution of charge and spin correlation functions of a Peierls insulator after a quench to a noninteracting Hamiltonian. This approach gives exact results, and fully takes into account quantum phonon effects without relying on a Hilbert space truncation. It is also free from a dynamical sign problem. The observed time dependence is compared to free-fermion time evolution starting from a dimerized state. Our exact results provide a benchmark for more realistic calculations and may be directly applicable to experiments with cold atoms or trapped ions.

Dipolar-controlled quantum deflagration going over into quantum detonation in the elongated Mn${}_{12}$ Ac molecular magnet in a strong transverse field has been considered within the full three-dimensional model. It is shown that within the dipolar window around tunneling resonances the deflagration front is nonflat. With increasing bias, dipolar instability makes the front turbulent, while its speed reaches sonic values, which is a signature of detonation.

The manipulation and movement of Dirac points in the Brillouin zone by the electron-electron interaction is considered within leading order perturbation theory. At the merging point, an infinitesimal interaction is shown to cause opening of the gap or splitting of the Dirac points, depending on the inter- or intrasublattice nature of the merging and the sign of the interaction. The topology of the spectrum can therefore be efficiently changed by simply tuning the interaction between particles, as opposed to the usual careful band structure engineering. This is illustrated around the merging transition of one, two, and three dimensional Dirac-Weyl fermions. A simple Weyl-like Hamiltonian that describes the quadratic band crossing in three dimensions is also proposed, and its stability under interactions is addressed.

Cluster dynamical mean field methods are used to calculate the superconductivity-induced changes in the interplane conductivity and Raman scattering cross section of the two-dimensional Hubbard model. When superconductivity emerges from the pseudogap, the superconducting response is found to be diminished in amplitude, broadened, and, in the case of the interplane conductivity, shifted to higher frequency. The results are in agreement with data on high-temperature copper-oxide superconductors indicating that the Hubbard model contains the essential low-energy physics of the pseudogap and its interplay with superconductivity in the cuprates.

Dispersion (van der Waals) forces play an important role in determining the structural properties of the systems where dispersion is crucial, for example, layerlike crystals (CdI${}_2$ type). Thus, to contribute to the understanding of the role of van der Waals (vdW) corrections for the Pt$X_2$ ($X=\text{O}$, S, Se) compounds, we report a density functional theory (DFT) investigation within nonlocal vdW corrections for the atomic structure and electronic properties of bulk Pt$X_2$. From our calculations, we identified the lowest energy structures, CaCl${}_2$ type for PtO${}_2$ and CdI${}_2$ type for both PtS${}_2$ and PtSe${}_2$, i.e., CdI${}_2$ type is not the lowest energy structure for PtO${}_2$. We reported the structural changes for higher energy Pt$X_2$ systems and the changes in the stability order compared with plain DFT-PBE calculations. The structural changes induced by vdW corrections affects the electronic structure of the layered CdI${}_2$-type structures (PtS${}_2$ and PtSe${}_2$), where a metallic behavior is obtained in contrast to PBE calculations, which yields an energy separation between the unoccupied and occupied states.

We generalize the hidden symmetry-breaking picture of symmetry-protected topological (SPT) order developed by Kennedy and Tasaki in the context of the Haldane phase. Our generalization applies to a wide class of SPT phases in one-dimensional spin chains, protected by an on-site representation of a finite Abelian group. This generalization takes the form of a nonlocal unitary map that relates local symmetry-respecting Hamiltonians in an SPT phase to local Hamiltonians in a symmetry-broken phase. Using this unitary, we establish a relation between the two-point correlation functions that characterize fully symmetry-broken phases with the string-order correlation functions that characterize the SPT phases, therefore establishing the perspective in these systems that SPT phases are characterized by hidden symmetry breaking. Our generalization is also applied to systems with continuous symmetries, including $\mathrm{SO}(2k+1)$ and $\mathrm{SU}\left(k\right)$.

Since many years, the contribution of vacancies ($V$) and self-interstitials ($I$) to silicon (Si) self-diffusion is a matter of debate. Native defects and their interaction among themselves and with foreign atoms influence the processes taking place during device fabrication, starting with the growth of Si single crystals and ending with doping of nanosized electronic devices. Considering this relevance, it is remarkable that present data about the properties of native point defects in Si are still limited and controversial. This work reports experiments on self-diffusion in Si for temperatures between 650${}^{\circ }$C and 960${}^{\circ }$C to verify recent results of Shimizu et al. [Phys. Rev. Lett. 98, 095901 (2007)] that give rise to inconsistencies in $V$-mediated self- and dopant diffusion. Two different structures of isotopically controlled epitaxial layers of Si are used for the diffusion study. One structure consisting of 20 bilayers of ${}^{29}$Si/${}^{28}$Si was grown by molecular beam epitaxy (MBE). The other structure with a ${}^{28}$Si layer sandwiched between natural Si was grown by means of chemical vapor deposition. Self-diffusion in (${}^{29}$Si/${}^{28}$Si)${}_{20}$ multilayers (ML) was analyzed by means of secondary ion mass spectrometry (SIMS) and neutron reflectometry, whereas self-diffusion in ${}^{\mathrm{nat}}$Si/${}^{28}$Si/${}^{\mathrm{nat}}$Si sandwich (SW) structures was measured with SIMS only. Analysis of the experimental profiles reveals an enhanced self-diffusion in ML compared to SW structures. The enhanced diffusion is ascribed to the dissolution of $V$- and $I$-related defect clusters grown-in during MBE. On the other hand, self-diffusion in the SW structures accurately confirms the data of Shimizu et al. that are considered to represent data for thermal equilibrium conditions. The temperature dependence of self-diffusion is described by $V$- and $I$-mediated contributions with temperature-dependent thermodynamic properties of $V$. This interpretation can solve the inconsistency between self- and dopant diffusion in Si, but further experiments are required to verify this concept.

Due to random dopant fluctuations, the device-to-device variability is a serious challenge to emerging nanoelectronics. In this work we present theoretical formalisms and numerical simulations of quantum-transport variability, based on the Green's function technique and the multiple-scattering theory. We have developed a general formalism using the diagrammatic technique within the coherent-potential approximation that can be applied to a wide range of disorder concentrations. In addition, we have developed a method by using a perturbative expansion within the low-concentration approximation that is extremely useful for typical nanoelectronic devices having low dopant concentration. Applying both formalisms, transport fluctuations due to random impurities can be predicted without lengthy brute force computation of ensembles of device structures. Numerical implementations of the formalisms are demonstrated using both tight-binding models and first-principles models.

The phase transitions in the host-guest composite structure of potassium have been determined between 19 and 54 GPa by a combination of x-ray powder and single-crystal diffraction. The guest component of the incommensurate composite phase K-III undergoes an “intraphase” transition at 30.0(7) GPa from a $C$-centered tetragonal structure to an $A$-centered orthorhombic form. At 39.7(5) GPa, the guest component undergoes a reentrant transition back to the $C$-centered tetragonal form. This is the first such reentrant transition observed in the composite structures of the elements. The transition in the guest at 30.0(7) GPa is preceded by a loss of the $c$-glide symmetry of the host component of the structure, and this is not recovered until some 3 GPa after the reentrant transition at 39.7 GPa.

Recent theoretical efforts aimed at understanding the nature of antiferromagnetic ordering in GdNi${}_2$B${}_2$C predicted double-$q$ ordering. Here we employ resonant elastic x-ray scattering to test this theory against the formerly proposed, single-$q$ ordering scenario. Our study reveals a satellite reflection associated with a mixed-order component propagation wave vector, viz., ($q_a,2q_b,0$) with $q_b=q_a\approx 0.55$ reciprocal lattice units, the presence of which is incompatible with single-$q$ ordering but is expected from the double-$q$ model. A ($3q_a,0,0$) wave vector (i.e., third-order) satellite is also observed, again in line with the double-$q$ model. The temperature dependencies of these along with that of a first-order satellite are compared with calculations based on the double-$q$ model and reasonable qualitative agreement is found. By examining the azimuthal dependence of first-order satellite scattering, we show the magnetic order to be, as predicted, elliptically polarized at base temperature and find the temperature dependence of the “out of $a$-$b$ plane” moment component to be in fairly good agreement with calculation. Our results provide qualitative support for the double-$q$ model and thus in turn corroborate the explanation for the “magnetoelastic paradox” offered by this model.