We explore the evolution of superconductivity in La${}_{2-x}$Ba${}_x$CuO${}_4$ with $x=0.095$ in magnetic fields of up to 35 T applied perpendicular to the CuO${}_2$ planes. Previous work on this material has shown that perpendicular fields enhance both charge- and spin-stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fields sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large-amplitude but phase-disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.

The Green function approach to the Bardeen-Cooper-Schrieffer theory of superconductivity is used to study nanofilms. We go beyond previous models and include effects of confinement on the strength of the electron-phonon coupling as well as on the electronic spectrum and on the phonon modes. Within our approach, we find that in ultrathin films, confinement effects on the electronic screening become very important. Indeed, contrary to what has been advanced in recent years, the sudden increases of the density of states when new bands start to be occupied as the film thickness increases, tend to suppress the critical temperature rather than to enhance it. On the other hand, the increase of the number of phonon modes with increasing number of monolayers in the film leads to an increase in the critical temperature. As a consequence, the superconducting critical parameters in such nanofilms are determined by these two competing effects. Furthermore, in sufficiently thin films, the condensate consists of well-defined subcondensates associated with the occupied bands, each with a distinct coherence length. The subcondensates can interfere constructively or destructively giving rise to an interference pattern in the Cooper pair probability density.

We report a ${}^{23}$Na and ${}^{27}$Al nuclear magnetic resonance (NMR) investigation of low-silica X (LSX) zeolite with chemical formula Na${}_{12}$Al${}_{12}$Si${}_{12}$O${}_{48}$ (Na${}_{12}$-LSX) loaded with $n$ additional guest sodium atoms. Na${}_n$/Na${}_{12}$-LSX exhibits an insulator-to-metal transition around $n=11.6$, which is accompanied by a significant enhancement of bulk magnetic susceptibility. Paramagnetic moments are in the metallic Na${}_{12}$/Na${}_{12}$-LSX thermally activated with an activation energy of around 0.1 eV. Simultaneously a new shifted component appears in the ${}^{23}$Na NMR, whose large and positive NMR shift scales with bulk magnetic susceptibility. Its spin-lattice relaxation rate $1/T_1$ is governed by the local-field fluctuations characterized by the same activation energy as obtained from the bulk magnetic susceptibility data. The time scale of these fluctuations is typical for atomic motions, which suggest strong electron-phonon coupling, a hallmark of polaron states. The insulator-to-metal transition in Na${}_n$/Na${}_{12}$-LSX is thus discussed within a polaron model.

Giant Rashba spin splitting was recently discovered in bismuth tellurohalide (BiTeI) [K. Ishizaka et al., Nat. Mater. 10, 521 (2011)]. In this work, biaxial strain is used to modify the interatomic interaction in BiTeI, and the strain dependence of spin splitting is determined theoretically by first-principles density functional calculations. Large tunability is found for the Rashba energy $E_{\mathrm{R}}$, momentum offset $k_{\mathrm{R}}$, and coupling constant ${\alpha }_R$, showing that the spin splitting in BiTeI can be significantly modified by atom-atom interaction in a solid. The maximum values of $E_{\mathrm{R}}$ and ${\alpha }_R$ occur at strain $\eta =-1.5\%$. Furthermore, tensile strain is shown to favor the increase of $k_{\mathrm{R}}$, and compressive strain favors the increase of $E_{\mathrm{R}}$. Moreover, contrast of the spin splittings between the electron at CBM and the hole at VBM reveals that $E_{\mathrm{R}}^e$ of the electron and $E_{\mathrm{R}}^h$ of the hole display drastically different behaviors under tensile strains. However, under compressive strains, the strain dependencies of $E_{\mathrm{R}}^e$ and $E_{\mathrm{R}}^h$ are interestingly similar (so are the strain dependencies of $k_{\mathrm{R}}^e$ and $k_{\mathrm{R}}^h$). By examining microscopic electron states, the anomalous Bi-I interaction is found to be important for the spin splitting in BiTeI, and qualitative explanation for the strain dependence of the spin splitting is provided.

The properties of the entanglement entropy (EE) of two-particle excited states in a one-dimensional ring are studied. For a clean system we show analytically that as long as the momenta of the two particles are not close, the EE is twice the value of the EE of any single-particle state. For almost identical momenta the EE is lower than this value. The introduction of disorder is numerically shown to lead to a decrease in the median EE of a two-particle excited state, while interactions (which have no effect for the clean case) mitigate the decrease. For a ring which is of the same size as the localization length, interaction increases the EE of a typical two-particle excited state above the clean system EE value.

A large spin Hall effect is calculated in doped silicon. The effect is determined using first principle calculations for the extrinsic spin Hall effect due to skew scattering at substitutional impurities. It is shown that the applied method accounts accurately for experimental results on B-doped Si. Here, the effect is weak but can be tuned significantly with heavy impurities. In the case of Si(Pt) and Si(Bi) a spin Hall angle is calculated comparable to those found in metals. Furthermore, the calculated spin relaxation times give physical insight to the different effect of electron and hole doping in Si. Experimentally, spin relaxation times for the electron-doped regime were found three orders of magnitude larger than for the hole-doped systems. Our calculations reproduce this finding which can be understood in terms of the electronic band structure of bulk Si.

The enthalpy of formation for 23 metal oxides, which include groups I and II as well as two transition metals (Ti and Ru), are calculated using random phase approximation (RPA). Compared to the PBE xc functional, the RPA reduces the mean absolute error (MAE) per oxygen from 0.44 to 0.15 eV. The calculated deviations from experiments are separated into two parts: a systematic and uniform error related to the reference energy of O and the errors specific to diffferent oxidation states O${}^{2-}$, O${}_2^{2-}$, and O${}_2^{-}$. Our results show that the RPA improves both the reference energy and the three oxidation states.

We present a theoretical analysis of the finite-temperature equilibrium surface reconstruction stability of GaAs(001) from first principles, encompassing the As-rich regime relevant to low-temperature grown GaAs. Experimental evidence points to the thermodynamic stability of a $(4\times 3)$ reconstruction in this regime, but density functional theory (DFT) calculations predict all $(4\times 3)$ reconstructions to be metastable relative to the $\beta 2(2\times 4)$ and $c(4\times 4)$ reconstructions. We employ statistical mechanical simulations, parameterized by DFT to study the combined effects of configurational disorder and vibrational excitations on surface phase stability. The calculated finite-temperature surface free energies of the various reconstructions indicate that, if a small constant energy shift is used to enforce stability of the lowest-energy $(4\times 3)$, the resultant phase diagram is consistent with experiment with the $c(4\times 4)$ overwhelming $(4\times 3)$ at high temperatures. This behavior is due to competition between configurational entropy, which favors $c(4\times 4)$, and vibrational entropy, which favors $(4\times 3)$.

We discuss the implementation of a beam splitter for electron waves in a quantum Hall bilayer. Our architecture exploits interlayer tunneling to mix edge states belonging to different layers. We discuss the basic working principle of the proposed coherent edge mixer, possible interferometric implementations based on existing semiconductor-heterojunction technologies, and advantages with respect to canonical quantum Hall interferometers based on quantum point contacts.

A set of LaAlO${}_3$/SrTiO${}_3$ (LAO-STO) interfaces has been probed by x-ray photoemission spectroscopy in order to contrast and compare the effects of LAO overlayer thickness and of the growth conditions on the electronic properties of these heterostructures. These effects are tracked by considering the band offset and the density of Ti${}^{+3}$ states, respectively. It is shown that the dominant effects on the local electronic properties are determined by the O${}_2$ partial pressure during the growth. In particular, a low $P_{{\text{O}}_2}$ yields Ti${}^{+3}$ states with higher density and lower binding energy compared to the sample grown at high $P_{{\text{O}}_2}$ or to the bare STO reference sample. Band-offset effects are all below about 0.7 eV, but a careful analysis of Ti $2p$ and Sr $3d$ peaks shows that valence-band offsets can be at the origin of the observed peak width. In particular, the largest offset is shown by the conducting sample, which displays the largest Ti $2p$ and Sr $3d$ peak widths.

The inelastic mean free path (IMFP) of electrons is derived using a new approach based on the low-energy electron reflectivity from ultrathin films. The thickness-dependent quantum size oscillations as a function of electron energy observed in the reflectivity of slow electrons are modeled using an absorbing Fabry-Pérot interferometer consisting of vacuum, film, and substrate. The absorbing properties of the film are represented by the imaginary part of the complex refractive index associated with the IMFP which determines the amplitude of the electron reflectivity oscillations. Using this formalism for an Fe film on W(110), the IMFP in Fe is found in the energy range from 4 to 18 eV above the vacuum level. In contrast to the common notion, the IMFP in Fe is shown to have a very weak energy dependence at low energy. The results are in good agreement with independent IMFP measurements found in thickness-dependent photoemission experiments.

We investigate 1D quantum systems that support Majorana bound states at interfaces between topologically distinct regions. In particular, we show that there exists a duality between particle-hole and spin degrees of freedom in certain spin-orbit-coupled 1D platforms such as topological insulator edges. This duality results in a spin analog of previously explored “fractional Josephson effects”—that is, the spin current flowing across a magnetic junction exhibits $4\pi$ periodicity in the relative magnetic field angle across the junction. Furthermore, the interplay between the particle-hole and spin degrees of freedom results in unconventional magneto-Josephson effects, such that the Josephson charge current is a function of the magnetic field orientation with periodicity $4\pi$.

The thermal free energy and pressure of dispersion interaction between two graphene sheets described by the Dirac model are calculated using the Lifshitz formula with reflection coefficients expressed via the polarization tensor. The obtained results for a pristine graphene are found to be in agreement with computations using Coulomb coupling between density fluctuations. For a graphene with nonzero mass gap parameter a qualitatively different behavior for the free energy and pressure is obtained. The Lifshitz formula with reflection coefficients expressed via the polarization tensor is used as a test for different computational approaches proposed in the literature for modeling the response function and conductivity of graphene at both zero and nonzero temperature.

We describe Majorana edge states of a semi-infinite wire using the complex band structure approach. In this method the edge state at a given energy is built as a superposition of evanescent waves. It is shown that the superposition can not always satisfy the required boundary condition, thus restricting the existence of edge modes. We discuss purely 1D and 2D systems, focusing in the latter case on the effect of the Rashba mixing term.

Thallium bromide (TlBr) is an ionic semiconductor that has shown great capacity for accurate radiation detection. Its application to this end, however, has been hampered by degradation of performance over time, in a process called polarization. This effect has been traditionally assigned to a build-up of ions at the electrodes, which would counteract an applied electrical bias field. Here, we estimate the ionic mobility in TlBr and its possible association with the polarization effect using parameter-free quantum simulations. Our results indicate that in samples with up to moderate levels of impurities, ions cannot traverse distances large enough to generate zones of accumulation and depletion in the crystal, suggesting different causes for the polarization effect.

By measuring the photoluminescence linewidth of a GaAs quantum well under perpendicular electric fields, we have traced the variation of the effective radius $a^{*}$ of a charged exciton (trion) as a function of electron density. The $a^{*}$ increases sharply above a critical density $n_s=2\times {10}^{14}$ m${}^{-2}$, which is consistent with the decrease of the screening length predicted by nonlinear-screening theory. Our analysis shows that the crossover from trions to the two-dimensional electron gas plus hole is generated by Coulomb screening in the high-electron-density regime, eliminating the possible mechanism by the trion localization effect due to the single-electron localization.

We report magnetotransport measurements in a HgTe quantum well with an inverted band structure, which is expected to be a two-dimensional (2D) topological insulator. A small magnetic field perpendicular the 2D layer breaks the time-reversal symmetry and thereby suppresses the edge state transport. A linear magnetoresistance is observed in low magnetic fields when the chemical potential moves through the bulk gap. That magnetoresistance is well described by numerical calculations of the edge state magnetotransport in the presence of nonmagnetic disorder. With the magnetic field increasing, both the local and nonlocal resistances first sharply decrease and then increase again in disagreement with the existing theories.

An electronic state with zero-differential conductance is found in nonlinear response to an electric field $E$ applied to two dimensional Corbino discs of highly mobile carriers placed in quantizing magnetic fields. The state occurs above a critical electric field $E>E_{th}$ at low temperatures and is accompanied by an abrupt dip in the differential conductance. The proposed model considers a $local$ instability of the electric field $E$ as the origin of the observed phenomenon. Comparison between the observed electronic state and the state with zero differential resistance, occurring in Hall bar geometry, indicates that the nonlinear response of edge states and/or skipping orbits is not essential in the studied samples. The result confirms that quantal heating is the dominant nonlinear mechanism leading to electronic states with both zero differential resistance and conductance.

We calculate the time-dependent nonequilibrium current through a single-level quantum dot strongly coupled to a vibrational mode. The nonequilibrium real-time dynamics caused by an instantaneous coupling of the leads to the quantum dot is discussed using an approximate method. The approach, which is specially designed for the strong polaronic regime, is based on the so-called polaron tunneling approximation. Considering different initial dot occupations, we show that a common steady state is reached after times much larger than the typical electron tunneling times due to a polaron blocking effect in the dot charge. A direct comparison is made with numerically exact data, showing good agreement for the time scales accessible by the diagrammatic Monte Carlo simulation method.

A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO${}_3$ on SrTiO${}_3$ is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti $3d$ states and the relative energetic alignment—and hence internal electric fields—within the LaAlO${}_3$ layer. First, the Ti $2p$ core-level spectra clearly show occupation of Ti $3d$ states already for two unit cells of LaAlO${}_3$. Second, the LaAlO${}_3$ core levels were seen to shift to lower binding energy as the LaAlO${}_3$ overlayer thickness, $n$, was increased, agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only $\sim 300$ meV between $n=2$ (insulating interface) and $n=6$ (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell-by-unit-cell shifts within the LaAlO${}_3$ block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO${}_3$ valence bands above the SrTiO${}_3$ conduction bands, resulting in charge transfer only for $n\ge 4$. We discuss effects which could frustrate this elegant and simple charge-transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO${}_3$ surface at the $25\%$ level reverses the direction of the internal field in the LaAlO${}_3$. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until $n=4$ and already for $n=2$ mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO${}_3$ to near-interfacial states in the SrTiO${}_3$ substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.