Experimental Physics III

Quantum Transport

The research area of the Quantum Transport group is the experimental investigation of quantum phenomena in the transport properties of nano-structured semiconductor materials.

Currently the research is focused on the transport properties of topological insulators and related meterials which are realised in HgTe quantum well and bulk materials.

Topological Insulators (TI)

Topological insulators (TI) are a new class of materials which consits of an insulating bulk and metallic surface states. In contrast to ordinary surface states the surface states of a TI are protected by time reversal symmetry. These surface states exhibit distinct spin properties and back scattering becomes suppressed.

The TI state was first experimentally observed by our group in a two-dimensional (2D) electron system of a HgTe quantum well (QW). In case of a 2D system the surface states are counter-propagating one-dimensional (1D) channel with opposite spin polarisation (see figure).
The dissipationless spin transport properties of such structures are subject of intens experimental investigations.

The surface states of a HgTe 3D TI are single Dirac cones. Theoretical investigation predicted here the experimental realization of many long-sought effects, phases and paticle states like for expample magnetic monopoles or Majorana Fermions. The demonstration of an odd number of chiral Majorana edge modes in a topologicalinsulator is the key point for the realization of states with non-Abelian statistics. With such a system it becomes possible to create quantum gates for quantum computing processes. In contrast to recently followed routes, namely the 5/2 state in the fractional quantum Hall regime, the new approach may realize this topological phase at much higher temperatures and also does not require a magnetic field, which are important advantages for potential device application. Moreover, the experimental observation of Majorana fermion will also attract great fundamental interest, since a Majorana fermion is a particle proposed in particle physics long time ago but has never been observed (c.f. F Wilczek, Majorana returns, Nat. Phys. 5 614-618 (2009)). Several proposals have been made to realize Majorana fermions in condensed matter systems but no experimental realization has been made.

HgTe 2D and 3D TI are grown within the QT group by molecular beam epitaxy (MBE).
Nano- and micro-structures are fabricated by optical and electron-beam lithography using the nano-structuring facilities of the Lehrstuhl EP3.
Transport characterization measurements are done in the temperature range between 5 mK and room temperature and in magneic fields up to 18 T.