We present a detailed study of the electronic structure and band alignment in LaAlO${}_3$/SrTiO${}_3$ oxide heterostructures by hard x-ray photoelectron spectroscopy. Our spectroscopic measurements find no evidence for the strong potential gradient within the polar LaAlO${}_3$ film predicted by band theory. Due to the high interface sensitivity of the method, we are further able to determine the valence band offset between the LaAlO${}_3$ film and the SrTiO${}_3$ substrate, which is found to be independent of the number of LAO overlayers. Finally, we discuss several explanations for the apparent absence of the built-in field in ex situ prepared LaAlO${}_3$/SrTiO${}_3$ heterostructures.

Density functional theory in the LDA$+U$ approximation is used to calculate the electronic structure of germanium $\delta$ doped with phosphorus and arsenic. We characterize the principal band minima of the two-dimensional electron gas created by $\delta$ doping and their dependence on the dopant concentration. Populated first at low concentrations is a set of band minima at the perpendicular projection of the bulk conduction band minima at $L$ into the ($k$${}_x$,$k$${}_y$) plane. At higher concentrations, band minima at $\Gamma$ and $\Delta$ become involved. Valley splittings and effective masses are computed using an explicit-atom approach, taking into account the effects of disorder in the arrangement of dopant atoms in the $\delta$ plane.

In this paper, we present a comparison of exchange-tuned hybrid density functional calculations with experimental data obtained for the Zn acceptor in GaN. Since this acceptor is one of the few reliably identified defects in GaN, we use Zn-doped GaN as a test case for the widely used HSE06 hybrid functional method of calculations of defect properties in semiconductors. Here, we present the experimental results of luminescence measurements in Zn-doped GaN from which we obtain Zn acceptor defect levels. They are compared with theoretically calculated defect thermodynamic and optical transition levels as well as the zero-phonon line associated with this acceptor. We also analyze the dependence of the results on the exchange-tuning procedure used in the HSE06 hybrid functional. Excellent agreement with the experiment is obtained when the amount of exact exchange in HSE06 is tuned to reproduce the GaN experimental band gap. This favorable comparison with the experimental results for a well-established defect suggests that the exchange-tuned HSE06 hybrid functional yields accurate defect properties in GaN and, therefore, has significant predictive power.

As a contribution to electronic transport within strongly spin-polarized surface states and its modification by adsorption of magnetic impurities we studied the adsorption of Tb (atomic magnetic moment 10 ${\mu }_B$) on epitaxial Bi(111) films by means of surface sensitive (magneto)conductance and low-energy electron diffraction. Surface diffusion turned out to be non-negligible even at substrate temperatures of 10 K. The Tb adatoms finally nucleate at intrinsic defects of the Bi(111) surface, where the Tb impurities act as dopants but not as scatterers. Nevertheless, time-dependent measurements allowed to determine also single-particle Tb scattering properties, as also supported by simulations of adsorption kinetics and time-dependent conductance. The magnetoconductance properties are characterized by small charge transfer (0.05 $e$/atom) and strong spin-orbit scattering, which in this case results only in strong reduction of the weak antilocalization effect but not a reversal to weak localization as for Fe and Co [see Lükermann, Sologub, Pfnür, Klein, Horn-von Hoegen, and Tegenkamp, Phys. Rev. B 86, 195432 (2012)]. Although Tb has a magnetic moment, which is by far higher than for adsorbed Fe and Co, it turns out that the $f$ electrons of Tb play essentially no role in scattering of the conduction electrons, yielding an even smaller scattering cross section than that for Fe and Co. The adatom coordination (interstitial or on the surface) may also play an important role.

We reconsider the problem of the anomalous Hall effect in ferromagnetic SrRuO${}_3$, incorporating insights from the recently developed theory of Weyl semimetals. We demonstrate that SrRuO${}_3$ possesses a large number of Weyl nodes, separated in momentum space, in its band structure. While the nodes normally do not coincide with the Fermi energy, unless the material is doped, we show that even the nodes inside the Fermi sea have a significant effect on the physical properties of the material. In particular, we show that the common belief that (the nonquantized part of) the intrinsic anomalous Hall conductivity of a ferromagnetic metal is entirely a Fermi-surface property, is incorrect: there generally exists a contribution to the anomalous Hall conductivity that arises from topological Fermi-arc surface states, associated with the Weyl nodes, which is of the same order of magnitude as the Fermi-surface contribution.

We have shown that the kink behavior in the spectral function of a heavy fermion can appear during the formation of the Kondo resonance (KR) band and the hybridization gap. We have investigated the heavy fermion compound CeCoGe${}_2$, using a combined approach of the density functional theory and the dynamical mean field theory. Low temperature $T$ spectral functions show dispersive KR states, similarly to the recent experimental observation. During the evolution from the $\text{non-}f$ conduction band state at high $T$ to the dispersive KR band state at low $T$, which have topologically different band shapes, we have found the existence of kinks in the $\text{non-}f$ spectral function near the Fermi level $E_F$. The observation of kink is clearly in correspondence with the multiple temperature scales of the formation of the KR band.

Using a reverse-engineering method, we construct a meta-generalized gradient approximation (meta-GGA) angle-averaged exchange-correlation (XC) hole model which has a general applicability. It satisfies known exact hole constraints and can exactly recover the exchange-correlation energy density of any reasonable meta-GGA exchange-correlation energy functional satisfying a minimal set of exact properties. The hole model is applied to several nonempirical meta-GGA functionals: the Tao-Perdew-Staroverov-Scuseria (TPSS), the revised TPSS (revTPSS), and the recently Balanced LOCalization (BLOC) meta-GGA [L. A. Constantin, E. Fabiano, and F. Della Sala, J. Chem. Theory Comput. 9, 2256 (2013)]. The empirical M06-L meta-GGA functional is also considered. Real-space analyses of atoms and ions as well as wave-vector analyses of jellium surface energies show that the meta-GGA hole models, in particular the BLOC one, are very realistic and can reproduce many features of benchmark XC holes. In addition, the BLOC hole model can be used to estimate with good accuracy the Coulomb hole radius of small atoms and ions. Thus, the proposed meta-GGA hole models provide a valuable tool to validate in detail existing meta-GGA functionals, and can be further used in the development of density functional theory methods beyond the semilocal level of theory.

We take advantage of complementary high-resolution experimental techniques and theoretical tools to get insight into the $\alpha$-Sn/Ge(111) triangular lattice surface consisting of $\mathit{sp}$ electrons. We report a $(3\times 3)$ phase, characterized by a charge ordering settled by electronic correlation, which appears between the known metallic-$(3\times 3)$ and Mott insulator phases. We identify the atomistic mechanism behind the stabilization of this phase and interpret these findings on the basis of theoretical calculations. We disentangle the role of the various degrees of freedom in the stability of the different phases found and describe the stepwise surface changes between the metallic and Mott insulating phases.

We study the influence of phonon scattering on the noise characteristics of a quantum point contact coupled to a two-electron system in a double quantum dot, as proposed for a singlet-triplet measurement scheme in a double-dot system. We point out that at low temperatures phonon-induced relaxation to the ground state suppresses transitions to doubly occupied singlet states which are the source of detectable current fluctuations in this measurement scheme. Thus, for a relatively strong electron-phonon interaction present in the system, the two configurations display the same noise characteristics. In this way, coupling to phonons reduces the distinguishability between the singlet and triplet configurations. Under such conditions, the proposed measurement scheme is no longer valid even though the times of the measurement-induced decoherence of an initial singlet-triplet superposition and of the localization into the singlet or triplet subspace remain essentially unchanged.

The Hall resistance of a homogeneous electron system is well known to be antisymmetric with respect to the magnetic field and the sign of charge carriers. We have observed that such symmetries no longer hold in planar hybrid structures consisting of partly single layer graphene (SLG) and partly bilayer graphene (BLG) in the quantum Hall (QH) regime. In particular, the Hall resistance across the SLG and BLG interface is observed to exhibit quantized plateaus that switch between those characteristic of SLG QH states and BLG QH states when either the sign of the charge carriers (controlled by a back gate) or the direction of the magnetic field is reversed. Simultaneously reversing both the carrier type and the magnetic field gives rise to the same quantized Hall resistances. The observed SLG-BLG interface QH states, with characteristic asymmetries with respect to the signs of carriers and magnetic field, are determined only by the chirality of the QH edge states and can be explained by a Landauer-Büttiker analysis applied to such graphene hybrid structures involving two regions of different Landau level structures.

We describe an implementation of an ab initio method for calculating the electronic structure for systems under finite bias voltage. Our method is based on density functional theory as implemented in the (screened) Korringa-Kohn-Rostoker Green's function method for open systems with two-dimensional translation invariance using the atomic sphere and the full-charge-density approximation. We test our method for metallic lithium separated by vacuum.

We elucidate the microscopic mechanism that causes a suppression of ferroelectricity and an enhancement of octahedral rotations in EuTiO${}_3$ from first principles. We find that the hybridization of the rare-earth Eu 4$f$ states with the B-site Ti cation drives the system away from ferroelectricity. We also show that the magnetic order dependence of this hybridization is the dominant source of spin-phonon coupling in this material. Our results underline the importance of rare-earth $f$ electrons on the lattice dynamics and stability of these transition metal oxides.

We elucidate the role of magnon interaction and spontaneous decays in the spin dynamics of the triangular-lattice Heisenberg antiferromagnet by calculating its dynamical structure factor within the spin-wave theory. Explicit theoretical results for neutron-scattering intensity are provided for spins $S=1/2$ and $S=3/2$. The dynamical structure factor exhibits unconventional features such as quasiparticle peaks broadened by decays, non-Lorentzian lineshapes, and significant spectral weight redistribution to the two-magnon continuum. This rich excitation spectrum illustrates the complexity of the triangular-lattice antiferromagnet and provides distinctive qualitative and quantitative fingerprints for experimental observation of decay-induced magnon dynamics.

Through numerical experiments, we demonstrate an electron tomography method for three-dimensional (3D) structural determination of amorphous materials at atomic resolution. By combining multislice simulations of an aberration-corrected scanning transmission electron microscope with equally sloped tomography, we have determined the 3D atomic structure of a simulated glass particle, consisting of 334 Si and 668 O atoms, from a tilt series of 55 noisy projections. An atomic model refinement method has been implemented to locate the positions of the Si and O atoms in the reconstruction. We expect that the development of this general method will find applications across several disciplines in the physical sciences.

A formation of tetrahedral spin clusters is discovered by Monte Carlo simulation for a spin-ice type double-exchange model on a pyrochlore lattice. The spin-cluster phase is magnetically disordered but breaks spatial inversion symmetry spontaneously by developing noncoplanar four-spin molecules periodically on the pyrochlore lattice. We find that the system exhibits a nonzero spin Hall conductivity in the spin-cluster phase. The result suggests that an intersite-multipole order induces the unconventional spin Hall state without the spin-orbit interaction.

Driven condensed matter systems consistently pose substantial challenges to theoretical understanding. Progress in the study of such systems has been achieved using the Floquet formalism, but certain aspects of this approach are not well understood. In this paper, we consider the exceptionally simple case of the rotating Kekulé mass in graphene through the lens of Floquet theory. We show that the fact that this problem is gauge-equivalent to a time-independent problem implies that the “quasienergies” of Floquet theory correspond to a continuous symmetry of the full time-dependent Lagrangian. We use the conserved Noether charge associated with this symmetry to recover notions of equilibrium statistical mechanics.

The Fano resonance spectra for the $G$ band in metallic carbon nanotubes are calculated as a function of laser excitation energy, in which the origin of the resonance is given by an interference between the continuous electronic Raman spectra and the discrete phonon spectra. We found that the second-order scattering process of the $\mathbf{q}\ne 0$ electron-electron interaction is more relevant to the continuous spectra rather than the $\mathbf{q}=0$ first-order process because the $\mathbf{q}=0$ direct Coulomb interaction vanishes due to the symmetry of the two sublattices of a carbon nanotube.

We present an experimental and theoretical lattice-dynamical study of InN at high hydrostatic pressures. We perform Raman scattering measurements on five InN epilayers, with different residual strain and free electron concentrations. The experimental results are analyzed in terms of ab initio lattice-dynamical calculations on both wurtzite InN (w-InN) and rocksalt InN (rs-InN) as a function of pressure. Experimental and theoretical pressure coefficients of the optical modes in w-InN are compared, and the role of residual strain on the measured pressure coefficients is analyzed. In the case of the LO band, we analyze and discuss its pressure behavior considering the double-resonance mechanism responsible for the selective excitation of LO phonons with large wave vectors in w-InN. The pressure behavior of the $L^{-}$ coupled mode observed in a heavily doped $n$-type sample allows us to estimate the pressure dependence of the electron effective mass in w-InN. The results thus obtained are in good agreement with $\mathbf{k}·\mathbf{p}$ theory. The wurtzite-to-rocksalt phase transition on the upstroke cycle and the rocksalt-to-wurtzite backtransition on the downstroke cycle are investigated, and the Raman spectra of both phases are interpreted in terms of DFT lattice-dynamical calculations.

We study the transport of electrons through a single-mode quantum ring with electric-field induced Rashba spin-orbit interaction that is subject to an in-plane magnetic field and weakly coupled to electron reservoirs. Modeling a ring array by ensemble averaging over a Gaussian distribution of energy-level positions, we predict slow conductance oscillations as a function of the Rashba interaction and electron density due to spin-orbit interaction induced beating of the spacings between the levels crossed by the Fermi energy. Our results agree with experiments by Nitta c.s. [J. Nitta, J. Takagi, F. Nagasawa, and M. Kohda, J. Phys.: Conference Series 302, 012002 (2011) and Nagasawa et al. (unpublished)], thereby providing an interpretation that differs from the ordinary Aharonov-Casher effect in a single ring.

We study the hyperfine interaction between the nuclear spins and the electrons in a HgTe quantum well, which is the prime experimentally realized example of a two-dimensional topological insulator. The hyperfine interaction is a naturally present, internal source of broken time-reversal symmetry from the point of view of the electrons. The HgTe quantum well is described by the so-called Bernevig-Hughes-Zhang (BHZ) model. The basis states of the BHZ model are combinations of both $S$- and $P$-like symmetry states, which means that three kinds of hyperfine interactions play a role: (i) the Fermi contact interaction, (ii) the dipole-dipole-like coupling, and (iii) the electron-orbital to nuclear-spin coupling. We provide benchmark results for the forms and magnitudes of these hyperfine interactions within the BHZ model, which give a good starting point for evaluating hyperfine interactions in any HgTe nanostructure. We apply our results to the helical edge states of a HgTe two-dimensional topological insulator and show how their total hyperfine interaction becomes anisotropic and dependent on the orientation of the sample edge within the plane. Moreover, for the helical edge states, the hyperfine interaction due to the $P$-like states can dominate over the $S$-like contribution in certain circumstances.