ISBN 9783843939669

978-3-8439-3966-9, Reihe Physik

Sebastian Heedt
Spin Coherence and Quantum Transport in Semiconductor Nanowires

231 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2019), Softcover, A5

Zusammenfassung / Abstract

In this dissertation, quantum transport and spin-orbit coupling in InAs nanowires are investigated. An important aspect is the search for compelling signatures of one-dimensional transport in semiconductor nanowires. Quantum point contacts formed in a nanowire with intrinsic lateral confinement are profoundly different from their counterparts in conventional 2D electron gases. Here, quantized conductance, the hallmark of ballistic transport, is observed and the confinement geometry, subband spacing, and Zeeman splitting of the subbands are determined. In the adiabatic transport regime quantum point contacts are used to selectively occupy certain subbands and demonstrate the suppression of backscattering. Magnetoconductance measurements are employed to provide evidence for the formation of spatially separated edge channels at large magnetic fields.

One-dimensional transport entails exciting new physics due to the dominant character of electron-electron interactions which can manifest themselves in collective excitations. Several indications of strong interactions are found, namely features of the 0.7 conductance anomaly, a density-dependent Landé g factor, and most notably signatures of an interaction-induced helical gap in the energy dispersion. This feature is a vital precursor of Majorana bound states that form in a 1D nanowire in proximity to a superconductor. These quasiparticle excitations that emerge as pairs at opposite nanowire ends are promising candidates for topological quantum computing. The energy gap protecting these modes is determined by the spin-orbit coupling strength.

Here, three distinct approaches are pursued to obtain the spin-orbit properties of the InAs nanowires. In the quantum dot regime spin-orbit coupling is determined from the avoided level crossing between even-parity ground and excited states at finite magnetic field. In the 1D transport regime it is extracted from the location of the helical gap feature at the first quantized conductance plateau. Moreover, Rashba and Dresselhaus parameters are derived from phase-coherent transport measurements and the impact of the doping level on the spin relaxation rate is investigated. It exhibits a pronounced gate-dependence and a transition from the D'yakonov-Perel' to the Elliott-Yafet spin relaxation mechanism is observed for large doping levels. These findings are also beneficial from a spintronics point of view for spin-polarized transport and spin manipulation.