Daniel F. Urban

Origin of subgap states in amorphous In-Ga-Zn-O

We present a density functional theory analysis of stoichiometric and nonstoichiometric, crystalline and amorphous In-Ga-Zn-O (c-IGZO, a-IGZO), which connects the recently experimentally discovered electronic subgap states to structural features of a-IGZO. In particular, we show that undercoordinated oxygen atoms create electronic defect levels in the lower half of the band gap up to about 1.5 eV above the valence band edge. As a second class of fundamental defects that appear in a-IGZO, we identify mainly pairs of metal atoms which are not separated by oxygen atoms in between. These defects cause electronic defect levels in the upper part of the band gap. Furthermore, we show that hydrogen doping can suppress the deep levels due to undercoordinated oxygen atoms while those of metal defects just undergo a shift within the band gap. Altogether our results provide an explanation for the experimentally observed effect that hydrogen doping increases the transparency and improves the conductivity of a-IGZO.

Theory of a magnetically controlled quantum-dot spin transistor

We examine transport through a quantum dot coupled to three ferromagnetic leads in the regime of weak tunnel coupling. A finite source-drain voltage generates a nonequilibrium spin on the otherwise non-magnetic quantum dot. This spin accumulation leads to magnetoresistance. A ferromagnetic but current-free base electrode influences the quantum-dot spin via incoherent spin-flip processes and coherent spin precession. As the dot spin determines the conductance of the device, this allows for a purely magnetic transistor-like operation. We analyze the effect of both types of processes on the electric current in different geometries.

Rashba spin-orbit-interaction-based quantum pump in graphene

We present a proposal for an adiabatic quantum pump based on a graphene monolayer patterned by electrostatic gates and operated in the low-energy Dirac regime. The setup under investigation works in the presence of inhomogeneous spin-orbit interactions of intrinsic- and Rashba-type and allows to generate spin polarized coherent currents. A local spin polarized current is induced by the pumping mechanism assisted by the spin-double refraction phenomenon.

Spin-induced charge correlations in transport through interacting quantum dots with ferromagnetic leads

We study the full counting statistics of electronic transport through a single-level quantum dot weakly coupled to two leads, with either one or both of them being ferromagnetic. The interplay of Coulomb interaction and finite spin polarization implies spin-correlation induced charge correlations that give rise to super-Poissonian transport behavior and positive cross correlations of the currents of the two spin species. In the case of two ferromagnetic leads, we analyze the nontrivial dependence of the cumulants on the angle between the polarization directions of the leads. We find diverging second and higher cumulants for spin polarizations approaching unity.

Tunable dynamical channel blockade in double-dot Aharonov-Bohm interferometers

We study electronic transport through an Aharonov-Bohm interferometer with single-level quantum dots embedded in the two arms. The full counting statistics in the shot-noise regime is calculated to first order in the tunnel-coupling strength. The interplay of interference and charging energy in the dots leads to a dynamical channel blockade that is tunable by the magnetic flux penetrating the Aharonov-Bohm ring. We find super-Poissonian behavior with diverging second and higher cumulants when the Aharonov-Bohm flux approaches an integer multiple of the flux quantum.

Coulomb-interaction effects in full counting statistics of a quantum-dot Aharonov-Bohm interferometer

We study the effect of Coulomb interaction on the full counting statistics of an Aharonov-Bohm (AB) interferometer with a single-level quantum dot in one arm in the regime of weak dot-lead and lead-lead tunnel couplings. In the absence of Coulomb interaction, the interference processes are of nonresonant nature with an even AB flux dependence and obey bidirectional Poissonian statistics. For large charging energy, the statistic of these processes changes. In addition, processes of resonant nature with an odd flux dependence appear. In the limit of strongly asymmetric tunnel couplings from the dot to the left and right leads, their statistics is found to be strongly super-Poissonian.

Finite frequency noise properties of the non-equilibrium Anderson impurity model

Physikalisches Institut, Albert{Ludwigs{Universitat Freiburg, D{79104 Freiburg, Germany(Dated: October 2, 2012)We analyze the spectrum of the electric-current autocorrelation function (noise power) in theAnderson impurity model biased by a nite transport voltage. Special emphasis is placed on theinterplay of non-equilibrium e ects and electron-electron interactions. Analytic results are presentedfor a perturbation expansion in the interaction strength U. Compared to the non-interacting setupwe nd a suppression of noise for nite frequencies in equilibrium and an ampli cation in non-equilibrium. Furthermore, we use a diagrammatic resummation scheme to obtain non-perturbativeresults in the regime of intermediate U. At nite voltage, the noise spectrum shows sharp peaks atpositions related to the Kondo temperature instead of the voltage.

Adiabatic pumping in the quasi-one-dimensional triangle lattice

Fraunhofer Institute for Mechanics of Materials IWM, D-79108 Freiburg, Germany(Dated: January 22, 2013)We analyze the properties of the quasi-one-dimensional triangle lattice emphasizing the occurrenceof at bands and band touching via the tuning of the lattice hopping parameters and on-site energies.The spectral properties of the in nite system will be compared with the transmission through a nite piece of the lattice with attached semi-in nite leads. Furthermore, we investigate the adiabaticpumping properties of such a system: depending on the transmission through the lattice, this resultsin nonzero integer charge transfers or transfers that increase linearly with the lattice size.I. INTRODUCTION

Atomic defects and dopants in ternary Z-phase transition-metal nitrides CrMN with M=V , Nb, Ta investigated with density functional theory

A density functional theory study of atomic defects and dopants in ternary Z-phase transition-metal nitrides CrMN with M=V, Nb, or Ta is presented. Various defect formation energies of native point defects and of substitutional atoms of other metal elements which are abundant in the steel as well, are evaluated. The dependence thereof on the thermodynamic environment, i.e. the chemical conditions of a growing Z-phase precipitate, is studied and different growth scenarios are compared. The results obtained may help to relate results of experimental atomic-scale analysis, by atom probe tomography or transmission electron microscopy, to the theoretical modeling of the formation process of the Z phase from binary transition metal nitrides.