In-plane magnetoelectric response in bilayer graphene

A graphene bilayer shows an unusual magnetoelectric response whose magnitude is controlled by the valley-isospin density, making it possible to link magnetoelectric behavior to valleytronics. Complementary to previous studies, we consider the effect of static homogeneous electric and magnetic fields that are oriented parallel to the bilayer's plane. Starting from a tight-binding description and using quasi-degenerate perturbation theory, the low-energy Hamiltonian is derived including all relevant magnetoelectric terms whose prefactors are expressed in terms of tight-binding parameters. We confirm the existence of an expected axion-type pseudoscalar term, which turns out to have the same sign and about twice the magnitude of the previously obtained out-of-plane counterpart. Additionally, small anisotropic corrections to the magnetoelectric tensor are found that are fundamentally related to the skew interlayer hopping parameter $\gamma_4$. We discuss possible ways to identify magnetoelectric effects by distinctive features in the optical conductivity.Comment: 14 pages, 7 figure

Weak (anti)localization in tubular semiconductor nanowires with spin-orbit coupling

We compute analytically the weak (anti)localization correction to the Drude conductivity for electrons in tubular semiconductor systems of zinc blende type. We include linear Rashba and Dresselhaus spin-orbit coupling (SOC) and compare wires of standard growth directions $\langle100\rangle$, $\langle111\rangle$, and $\langle110\rangle$. The motion on the quasi-two-dimensional surface is considered diffusive in both directions: transversal as well as along the cylinder axis. It is shown that Dresselhaus and Rashba SOC similarly affect the spin relaxation rates. For the $\langle110\rangle$ growth direction, the long-lived spin states are of helical nature. We detect a crossover from weak localization to weak anti-localization depending on spin-orbit coupling strength as well as dephasing and scattering rate. The theory is fitted to experimental data of an undoped $\langle111\rangle$ InAs nanowire device which exhibits a top-gate-controlled crossover from positive to negative magnetoconductivity. Thereby, we extract transport parameters where we quantify the distinct types of SOC individually.Comment: 17 pages, 9 figure

Control of Spin Relaxation in Disordered Quantum Wells and Nanowires

A central theme in semiconductor spintronics is the control of spin-polarized charge carriers. In order to utilize the spin for information processing, a long spin lifetime is essential. In this thesis, we address this issue for important types of semiconductor systems: disordered quantum wells and nanowires. Typically, the most prominent process that limits the spin lifetime in inversion-asymmetric systems is the D'yakonov-Perel' mechanism. It results from the random spin precessions due to the combined effect of impurity scattering and spin-orbit coupling. One very convenient experimental tool to gather information about the spin-relaxation properties, as well as transport parameters, are magnetoconductance measurements of the weak (anti)localization. In particular, the latter is mainly determined by the spin lifetime of the long-lived spin states. After giving a comprehensive introduction of the underlying theoretical fundamentals, we identify spin-preserving symmetries in quantum wells of zinc-blende structure. First, we focus on electron systems and prove that persistent spin states can be found due to the interplay of Rashba and Dresselhaus spin-orbit coupling if at least two growth-direction Miller indices agree in modulus. Additionally, a general closed-form expression for the weak (anti)localization is provided to enable an experimental verification. Secondly, we show that also in [001]-oriented hole systems such symmetries can be realized if in addition uniaxial shear strain is present. Semiconductor nanowires can have very distinct mesoscopic characteristics. We concentrate on three important kinds of nanowires with diffusive transport channels: (i) tubular zinc-blende nanowires as well as cylindrical (ii) zinc-blende and (iii) wurtzite nanowires. In each of these cases, the impact of a gate-induced Rashba effect is taken into account which allows an external manipulation of the spin relaxation. Employing a Cooperon-based approach, we theoretically analyze the spin-relaxation features, identify the long-lived spin states, and compute the weak (anti)localization correction. The obtained expressions for both types of zinc-blende nanowires are fitted to the experimental data of magnetoconductance measurements of InAs nanowires. We find good agreement between theory and experiment and extract reasonable transport parameters. Regarding the spin relaxation, in contrast to the cylindrical counterpart, the tubular zinc-blende nanowires exhibit a growth-direction dependence. In the cylindrical channels, the spin relaxation is sensitive to the wire diameter and suppressed in narrow wires due to boundary-induced motional narrowing. The suppression is particularly pronounced in wurtzite nanowires as the relaxation due to the intrinsic linear-in-momentum spin-orbit terms becomes ineffective. Yet, the corresponding long-lived spin states possess a complex helical spin texture which is difficult to realize. This can yield very dissimilar values for the spin lifetimes when extracted from distinct experiments. We explicitly demonstrate the arising discrepancies for optical and magnetotransport measurements

Das Langzeitverhalten ein- und zweiteiliger Zirkoniumdioxid-Implantatsysteme im Frontzahnbereich - eine in vitro Untersuchung

In dieser Dissertation wurden das Langzeitverhalten und die Bruchfestigkeit von einteiligen und zweiteiligen, verklebten Zirkoniumdioxid-Implantatsystemen untersucht. Die in vitro Studie nimmt Bezug auf die spezielle Belastungssituation im Frontzahngebiet. Zwei Gruppen von zweiteiligen, verklebten Zirkoniumdioxid-Implantatsystemen (ZZB), vier Gruppen von einteiligen Zirkoniumdioxid-Implantatsystemen (Z) und zwei Gruppen von zweiteiligen Titan-Implantatsystemen (TTS, Referenz) wurden mit identischen monolithischen Zirkoniumdioxid-Kronen prothetisch versorgt (n = 10 / Gruppe). Acht Exemplare pro Gruppe wurden im Kausimulator der Universität Regensburg 18000 thermischen Zyklen (TC, 5 ° / 55 °) und simultan 3,6 x 106 mechanischen Belastungen (ML, 100 N) ausgesetzt, wobei durch einen Belastungswinkel von 135° eine anteriore Situation simuliert wurde. Man bestimmte den individuellen Bruchwiderstand und die maximale Biegespannung sowohl für diejenigen Probekörper, die die Alterung überlebten, als auch für zwei Referenzen pro Gruppe nach einer 24-stündigen Wasserlagerung. Zur Fehleranalyse sind REM-Bilder angefertigt worden. Eine statistische Datenanalyse erfolgte im Anschluss (ANOVA, Post-hoc Bonferroni, Kaplan-Meier-Log-Rank, α = 0,05). Ein einteiliges Zirkoniumdioxid- und ein zweiteiliges Titan-Implantatsystem überlebten die Kausimulation ohne Ausfälle. Alle Probekörper der zweiteiligen, verklebten Zirkoniumdioxid-Implantatsysteme und eines einteiligen Zirkoniumdioxid-Implantatsystems frakturierten (Bruch des Abutments oder Implantats). Die Ausfallraten der anderen Systeme variierten zwischen 1 × (1 Gruppe) und 5 × (2 Gruppen). Die Überlebensraten unterschieden sich signifikant (Log-Rank-Test: p = 0,000). Die maximalen Bruchkräfte / Biegespannungen variierten signifikant (ANOVA: p = 0,000) zwischen 188,00 ± 44,80 N / 381,02 ± 80,15 N / mm2 und 508,67 ± 107,00 N / 751,45 ± 36,73 N / mm2. Die mittleren Bruchwerte nach 24-stündiger Wasserlagerung und Kausimulation unterschieden sich nicht signifikant. Zirkoniumdioxid-Implantatsysteme zeigten teilweise Materialdefekte oder auch ein unzureichendes Design der Abutment-Implantat Verbindung. Zweiteilige, verklebte Implantatsysteme hatten höhere Ausfallraten und niedrigere Bruchfestigkeiten als einteilige Implantate. Individuelle Zirkoniumdioxid-Implantatsysteme können im Frontzahngebiet mit Einschränkungen angewendet werden

Identification of the Drosophila melanogaster homolog of the human spastin gene

The human SPG4 locus encodes the spastin gene, which is responsible for the most prevalent form of autosomal dominant hereditary spastic paraplegia (AD-HSP), a neurodegenerative disorder. Here we identify the predicted gene product CG5977 as the Drosophila homolog of the human spastin gene, with much higher sequence similarities than any other related AAA domain protein in the fly. Furthermore we report a new potential transmembrane domain in the N-terminus of the two homologous proteins. During embryogenesis, the expression pattern of Drosophila spastin becomes restricted primarily to the central nervous system, in contrast to the ubiquitous expression of the vertebrate spastin genes. Given this nervous system-specific expression, it will be important to determine if Drosophila spastin loss-of-function mutations also lead to neurodegeneratio

Conserved spin quantity in strained hole systems with Rashba and Dresselhaus spin-orbit coupling

We derive an effective Hamiltonian for a (001)-confined quasi-two-dimensional hole gas in a strained zinc-blende semiconductor heterostructure including both Rashba and Dresselhaus spin-orbit coupling. In the presence of uniaxial strain along the axes, we find a conserved spin quantity in the vicinity of the Fermi contours in the lowest valence subband. In contrast to previous works, this quantity meets realistic requirements for the Luttinger parameters. For more restrictive conditions, we even find a conserved spin quantity for vanishing strain, restricted to the vicinity of the Fermi surface

Signatures of spin-preserving symmetries in two-dimensional hole gases

We investigate ramifications of the persistent spin helix symmetry in two-dimensional hole gases in the conductance of disordered mesoscopic systems. To this end we extend previous models by going beyond the axial approximation for III-V semiconductors. For heavy-hole subbands we identify an exact spin-preserving symmetry analogous to the electronic case by analyzing the crossover from weak antilocalization to weak localization and spin transmission as a function of extrinsic spin-orbit interaction strength.Comment: 7 pages, 3 figures; reference adde

Spin relaxation in wurtzite nanowires

We theoretically investigate the D'yakonov-Perel' spin-relaxation properties in diffusive wurtzite semiconductor nanowires and their impact on the quantum correction to the conductivity. Although the lifetime of the long-lived spin states is limited by the dominant k-linear spin-orbit contributions in the bulk, these terms show almost no effect in the finite-size nanowires. Here, the spin lifetime is essentially determined by the small k-cubic spin-orbit terms and nearly independent of the wire radius. At the same time, these states possess in general a complex helical structure in real space that is modulated by the spin-precession length induced by the k-linear terms. For this reason, the experimentally detected spin relaxation largely depends on the ratio between the nanowire radius and the spin-precession length as well as the type of measurement. In particular, it is shown that while a variation of the radius hardly affects the magnetoconductance correction, which is governed by the long-lived spin states, the change in the spin lifetime observed in optical experiments can be dramatic. We compare our results with recent experimental studies on wurtzite InAs nanowires