Thomas Schäpers Tuesday, 14:00 - 14:30
Helical states and spin-orbit coupling in InAs nanowires
Semiconductor nanowires, fabricated by a bottom-up approach, are very promising as building blocks for future nanoscaled electronic and spintronic devices. In addition, they are also very interesting objects for studying fundamental quantum phenomena. In InAs nanowires spin-orbit coupling is relatively strong, owing to the lack of inversion symmetry and the low band gap. Two types of spin-orbit coupling are relevant, i.e. spin-orbit coupling due to structural inversion asymmetry (Rashba effect) and bulk inversion asymmetry (Dresselhaus contribution). By employing a gate electrode the spin precession in the nanowire can be controlled by means of the Rashba effect, which opens up the possibility to realize spintronic devices.
For the realization of spintronic devices based on InAs nanowires a detailed knowledge on spin-orbit coupling is required. In order to gain information on the strength of spin-orbit coupling the weak antilocalization effect is measured [1]. Here, a peak in the magnetoconductance is observed at zero magnetic field which is due to interference effects. By comparing the experimental data to an appropriate theoretical model the relevant parameters for spin-orbit coupling are extracted. Two different approaches are pursued to control the strength of spin-orbit coupling, i.e. gate control and modifying the potential profile by doping. In order to gain more insight into the Dresselhaus contribution, the energy splitting and the spin density of the quantized states are calculated.
On InAs nanowires controlled by a set of top-gate electrodes ballistic transport is investigated. By varying the gate voltage distinct conductance steps due to quantized conductance are observed. By means of bias-dependent measurements at various magnetic fields the g-factor is extracted. We also found indications of a helical gap in the lowest 1D subband of the InAs nanowire. The corresponding dip feature in the first conductance plateau is even observed in the absence of a magnetic fields. This behaviour indicates that exchange interactions have a substantial contribution to the electronic transport.
[1] M. Kammermeier, P. Wenk, J. Schliemann, S. Heedt, and Th. Schäpers, Weak (anti)localization in tubular semiconductor nanowires with spin-orbit coupling, Phys. Rev. B, 93, 205306 (2016).
[2] S. Heedt, N. T. Ziani, F. Crepin, W. Prost, S. Trellenkamp, J. Schubert, D. Grützmacher, B. Trauzettel, and Th. Schäpers, Signatures of interaction-induced helical gaps in nanowire quantum point contacts, Nature Physics, (2017) (doi: 10.1038/nphys4070).