(HgTe)${}_N$(CdTe)${}_M$(110) and (001) superlattices are studied by means of $ab$ $initio$ calculations versus the thickness of the HgTe quantum wells (QWs). The used approximate quasiparticle theory including spin-orbit coupling (SOC) gives the correct band ordering, band gap, and SOC splitting for bulk HgTe and CdTe. The resulting band discontinuities indicate confinement also for occupied states. In agreement with earlier $\mathbf{k}·\mathbf{p}$ calculations and experiments we find a topological transition from the topological nontrivial quantum spin Hall state into a trivial insulator with decreasing QW thickness. The spatial localization near the interfaces and the spin polarization are demonstrated for the edge states for QWs with thicknesses near the critical one. They do not depend on the QW orientation and are therefore topologically protected. Below the critical QW thickness, the trivial insulator exhibits drastic confinement effects with a significant gap opening. We show that the inclusion of inversion symmetry, the nonaxial rotation symmetry of the QWs, and the real QW barriers lead to some agreement but also significant deviations from the predictions within toy models. The deviations concern the critical thickness, the number and localization of edge states, and the possibility to find QW subbands between edge states.

With the use of band structure calculations we demonstrate that previously reported [A. Prodi et al., Nat. Mater. 3, 48 (2004)] experimental crystal and magnetic structures for ${\mathrm{NaMn}}_7$${\mathrm{O}}_{12}$ are inconsistent with each other. The optimization of the crystal lattice allows us to predict another crystal structure for the low temperature phase, which is qualitatively different from the one presented before. The AFM-CE type of magnetic order stabilizes the structure with elongated, not compressed, Mn${}_B^{3+}$${\mathrm{O}}_6$ octahedra, excluding ${\mathrm{NaMn}}_7$${\mathrm{O}}_{12}$ from the list of anomalous Jahn-Teller systems. The orbital correlations were shown to exist even in the cubic phase, while the charge order appears only in the low temperature distorted phase.

We discuss the possibility of a two-dimensional ordered structure formed upon deposition of Si on metal surfaces. We investigate the atomic and electronic structure of the Si-$(\sqrt{19}\times \sqrt{19})R23.4^{\circ }$/Pt(111) surface reconstruction by means of a set of experimental surface-science techniques supported by theoretical calculations. The theory achieves very good agreement with the experimental results and is corroborating beyond any doubt that this phase is a surface alloy consisting of ${\mathrm{Si}}_3$Pt tetramers that resembles a twisted kagome lattice. These findings render unlikely any formation of silicene or germanene on Pt(111) and other transition-metal surfaces.

We study the general phase diagram of correlated electrons for iridium-based (Ir) compounds on the hyperhoneycomb lattice, a crystal structure where the ${\mathrm{Ir}}^{4+}$ ions form a three-dimensional network with threefold coordination recently realized in the $\beta$-${\mathrm{Li}}_2\mathrm{Ir}{\mathrm{O}}_3$ compound. Using a combination of microscopic derivations, symmetry analysis, and density functional calculations, we determine the general model for the electrons occupying the $j_{\text{eff}}=\frac{1}{2}$ orbitals at the ${\mathrm{Ir}}^{4+}$ sites. In the noninteracting limit, we find that this model allows for both topological and trivial electronic band insulators along with metallic states. The effect of Hubbard-type electron-electron repulsion on the above electronic structure in stabilizing $\mathbf{q}=\mathbf{0}$ magnetic order reveals a phase diagram with continuous phase transition between a topological band insulator and a Néel ordered magnetic insulator.

We consider the effect of dephasing on a quantum dot which injects single electrons on a chiral edge channel of the quantum Hall effect. Dephasing is described by the coupling of the dot to a bosonic bath which represents the electromagnetic environment. Using the input-output formalism of quantum optics, we derive the density matrix of the edge degrees of freedom. Results are illustrated by computing the zero frequency current-current correlations when two such single-electron emitters achieve a collision at the location of a quantum point contact, in the same spirit as the Hong-Ou-Mandel experiment of quantum optics. Such correlations are directly linked to the quantum mechanical purity. We show that, as observed in a recent experiment, the effect of dephasing leads to a lifting of the Hong-Ou-Mandel dip when the time delay between the two electron wave packets is zero. Generalizations to time filtered wave packets as well as to asymmetric, detuned injection between opposite edges are obtained.

We report on spin transport along double-helical molecular systems by considering various contact configurations and asymmetries between the two helical strands in the regime of completely coherent charge transport. Our results reveal that no spin polarization appears in two-terminal molecular devices when coupled to one-dimensional electrodes. The same holds in the case of finite-width electrodes if there is a bottleneck of one single site in the system electrode-molecule-electrode. Then, additional dephasing is necessary to induce spin-filtering effects. In contrast, nonzero spin polarization is found in molecular devices with multiple terminals or with two finite-width electrodes, each of them connected to more than one site of the molecule. The magnitude of spin polarization can be enhanced by increasing the asymmetry between the two strands. We point out that the spin-filtering effects could emerge in double-helical molecular devices at low temperature without dephasing by a proper choice of the electrode number and the connection between the molecule and the electrodes.

We present experimental evidence for differences in surface energy losses between (a) electrons entering a solid from vacuum and (b) electrons leaving the solid into vacuum. Although these so-called in-out asymmetries have been long assumed to exist on theoretical grounds, the present work constitutes a clear experimental observation of the phenomenon. The effect has been exposed by comparing reflection-electron-energy-loss spectra of polycrystalline Al for pairs of conjugate scattering geometries where the directions of the source and the detector were interchanged. Differences of up to 30% in the peak height of surface energy-loss features are observed. The experimentally observed in-out asymmetry has been examined within the semiclassical dielectric formalism using state-of-the-art models for surface scattering of charged projectiles. The theoretical analysis suggests that in-out asymmetry effects are most accentuated for surface-crossing directions close to the surface normal and for high kinetic energies, in good agreement with the observed behavior. The effect is assumed to be present not only for electrons, but in principle for any charged particle.

We show that flexoelectric effect is responsible for the non-Ising character of a 180° ferroelectric domain wall. The wall, long considered being of Ising type, contains both Bloch- and Néel-type polarization components. Using the example of classic ferroelectric ${\mathrm{BaTiO}}_3$, and by incorporating the flexoelectric effect into a phase-field model, it is demonstrated that the flexoelectric effect arising from stress inhomogeneity around the domain wall leads to the additional Bloch and Néel polarization components. The magnitudes of these additional components are two or three magnitudes smaller than the Ising component, and they are determined by the competing depolarization and flexoelectric fields. Our results from phase-field model are consistent with the atomistic scale calculations. The results prove the critical role of flexoelectricity in defining the internal structure of ferroelectric domain walls.

We have studied the effect of pressure on the structural and vibrational properties of lanthanum tritungstate ${\text{La}}_2$$({\text{WO}}_4)$${}_3$. This compound crystallizes under ambient conditions in the modulated scheelite-type structure known as the $\alpha$ phase. We have performed x-ray diffraction and Raman scattering measurements up to a pressure of 20 GPa, as well as ab initio calculations within the framework of the density functional theory. Up to 5 GPa, the three methods provide a similar picture of the evolution under pressure of $\alpha$-${\text{La}}_2$$({\text{WO}}_4)$${}_3$. At 5 GPa, we begin to observe some structural changes, and above 6 GPa we find that the x-ray patterns cannot be indexed as a single phase. However, we find that a mixture of two phases with $C2/c$ symmetry accounts for all diffraction peaks. Our ab initio study confirms the existence of several $C2/c$ structures, which are very close in energy in this compression range. According to our measurements, a state with medium-range order appears at pressures above 9 and 11 GPa, from x-ray diffraction and Raman experiments, respectively. Based upon our theoretical calculations we propose several high-pressure candidates with high cationic coordinations at these pressures. The compound evolves into a partially amorphous phase at pressures above 20 GPa.

High-precision LASER dilatometry with a unique measurement setup was applied to the bulk metallic glass alloy ${\mathrm{Zr}}_{52.5}$${\mathrm{Ti}}_5$${\mathrm{Cu}}_{17.9}$${\mathrm{Ni}}_{14.6}$${\mathrm{Al}}_{10}$ (Vit105) for specific volume studies. Employing small temperature steps of $\Delta T=5$ and 10 K and long measurement times effective heating rates of the order of ${10}^{-2}$ K/min were realized and changes of the specific volume were measured with a resolution of $\Delta V/V={10}^{-6}$ down to 573 K. The temperature regime of these experiments is far below a calorimetrically determined glass transition of $T_g^{\text{calor}}=659$ K (5 K/min). The experimental results strongly support theoretical predictions for the existence of an ideal glass transition that was observed in this study at $T_g=628$ K for this bulk metallic glass model alloy.

We present results of ultrasonic measurements on a single crystal of the distorted diamond-chain compound azurite ${\mathrm{Cu}}_3$(${\mathrm{CO}}_3$)${}_2$(OH)${}_2$. Pronounced elastic anomalies are observed in the temperature dependence of the longitudinal elastic mode $c_{22}$ which can be assigned to the relevant magnetic interactions in the system and their couplings to the lattice degrees of freedom. From a semiquantitative analysis of the magnetic contribution to $c_{22}$ the magnetoelastic coupling $G=\partial J_2$/$\partial {\epsilon }_b$ can be estimated, where $J_2$ is the intradimer coupling constant and ${\epsilon }_b$ the strain along the intrachain $b$ axis. We find an exceptionally large coupling constant of $\left|G\right|\sim$ 3650 K highlighting an extraordinarily strong sensitivity of $J_2$ against changes of the $b$-axis lattice parameter. These results are complemented by measurements of the hydrostatic pressure dependence of $J_2$ by means of thermal expansion and magnetic susceptibility measurements performed both at ambient and finite hydrostatic pressure. We propose that a structural peculiarity of this compound, in which ${\mathrm{Cu}}_2$${\mathrm{O}}_6$ dimer units are incorporated in an unusually stretched manner, is responsible for the anomalously large magnetoelastic coupling.

We study the relaxation of excitations in a system of one-dimensional weakly interacting bosons. Due to residual weak interactions, Bogoliubov quasiparticles in this system have finite lifetimes. As a result of the conservation laws in one dimension, at zero temperature the leading mechanism of decay of a quasiparticle is disintegration into three others. We focus on phonon quasiparticles and find that their decay rate is proportional to the seventh power of momentum. In the integrable case of contact interaction between the bosons, the decay rate vanishes.

Using ab initio calculations we have investigated the lattice thermal conductivity ($\kappa$) of ${\mathrm{CoSb}}_3$ and ${\mathrm{BaCo}}_4$${\mathrm{Sb}}_{12}$. The calculated room temperature $\kappa$ of these compounds are 11.5 and 6.1 W/m K, respectively. The change in $\kappa$ upon filling is mainly due to the reduction of phonon lifetimes limited by the anharmonic scattering, which occurs in the whole phonon spectrum, ruling out the postulated “rattling” model. We show that the harmonic interaction instead of the anharmonic interaction between the filler and the host matrix, depressing the phonon spectrum, is at the origin of the reduction of $\kappa$ upon filling. We believe that the filler plays the same role in other fully filled skutterudites.

The Affleck-Kennedy-Lieb-Tasaki state (or Haldane phase) in a spin-1 chain represents a large class of gapped topological paramagnets that host symmetry-protected gapless excitations on the boundary. In this work, we show how to realize this type of featureless spin-1 state on a generic two-dimensional lattice. These states have a gapped spectrum in the bulk, but they support gapless edge states protected by spin rotational symmetry along a certain direction, and they exhibit the spin quantum Hall effect. Using a fermion representation of integer spins, we show a concrete example of such spin-1 topological paramagnets on a kagome lattice, and we suggest a microscopic spin-1 Hamiltonian that may realize it.

We report on the fabrication and characterization of magnetic tunnel junctions consisting of a single layer of graphene as the tunnel barrier, sandwiched between two metallic ferromagnetic electrodes. We employ a fabrication process chosen to minimize oxidation of the electrode materials at the ferromagnet/graphene interfaces. The devices have low resistance-area products of 1.5–6 Ω μm${}^2$, with low-temperature magnetoresistances of 1.5–3.4%. The temperature and bias dependencies of the resistance confirm that transport is dominated by tunneling processes rather than by any unintended pinholes.

We derive a sum rule to demonstrate that the static magnetoelectric (ME) effect is governed by optical transitions that are simultaneously excited by the electric and magnetic components of light. The ME sum rule is applicable to a broad class of materials lacking the spatial inversion and the time-reversal symmetries, including multiferroic compounds. Due to the dynamical ME effect, the optical excitations in these materials can exhibit directional dichroism, i.e., the absorption coefficient can be different for counter-propagating light beams. According to the ME sum rule, the magnitude of the linear ME effect of a material is mainly determined by the directional dichroism of its low-energy optical excitations. An application of the sum rule to the multiferroic ${\mathrm{Ba}}_2$${\mathrm{CoGe}}_2$${\mathrm{O}}_7$, ${\mathrm{Sr}}_2$${\mathrm{CoSi}}_2$${\mathrm{O}}_7$, and ${\mathrm{Ca}}_2$${\mathrm{CoSi}}_2$${\mathrm{O}}_7$ shows that in these compounds the static ME effect is mostly governed by the directional dichroism of the spin-wave excitations in the giga-terahertz spectral range. On this basis, we argue that the studies of directional dichroism and the application of the ME sum rule promote the synthesis of new materials with large static ME effect.

We present a thermodynamical investigation of the $\alpha \rightleftharpoons \gamma$ transition of Ce using density functional theory within the projector augmented wave framework combined with the dynamical mean field theory. First, we confirm that without spin-orbit coupling no transition appears at zero temperature. Second, we extend the same conclusion to finite temperature with a slight difference: a crossover is observed both in temperature and pressure between the $\alpha$ and $\gamma$ phases. This is obviously visible as a softening of the bulk modulus. Third, we show the leading role of entropy for the description of the equation of state of cerium. Last, we discuss the role of spin coupling, and we argue that neglecting the spin-orbit coupling is roughly equivalent to a renormalization of temperature. Indeed, at 800 K, both our variation of thermodynamical quantities and our spectral functions describe the experimental data.

The DCA${}^{+}$algorithm was recently introduced by Stear, Maier, and Schulthess [Phys. Rev. B 88, 115101 (2013)] to extend the dynamic cluster approximation (DCA) with a continuous lattice self-energy in order to achieve better convergence with cluster size. Here we extend the DCA${}^{+}$algorithm to the calculation of two-particle correlation functions by introducing irreducible vertex functions with continuous momentum dependence consistent with the DCA${}^{+}$self-energy. This enables a significantly more controlled and reliable study of phase transitions than with the DCA. We test the new method by calculating the superconducting transition temperature $T_c$ in the attractive Hubbard model and show that it reproduces previous determinantal quantum Monte Carlo results. We then calculate $T_c$ in the doped repulsive Hubbard model, for which previous DCA calculations could only access the weak-coupling ($U=4t$) regime for large clusters. We show that the new algorithm provides access to much larger clusters and delivers asymptotically converged results for $T_c$ for both the weak ($U=4t$) and intermediate ($U=7t$) coupling regimes, and thereby enables the accurate determination of the exact infinite cluster size result.

Energy functionals which depend explicitly on orbital densities, rather than on the total charge density, appear when applying self-interaction corrections to density-functional theory; this is, e.g., the case for Perdew-Zunger and Koopmans-compliant functionals. In these formulations the total energy is not invariant under unitary rotations of the orbitals, and local, orbital-dependent potentials emerge. We argue that this is not a shortcoming, and that instead these potentials can provide, in a functional form, a simplified quasiparticle approximation to the spectral potential, i.e., the local, frequency-dependent contraction of the many-body self-energy that is sufficient to describe exactly the spectral function. As such, orbital-density-dependent functionals have the flexibility to accurately describe both total energies and quasiparticle excitations in the electronic-structure problem. In addition, and at variance with the Kohn-Sham case, orbital-dependent potentials do not require nonanalytic derivative discontinuities. We present numerical solutions based on the frequency-dependent Sham-Schlüter equation to support this view, and examine some of the existing functionals in this perspective, highlighting the very close agreement between exact and approximate orbital-dependent potentials.