July 2017

 Abstracts of the QSIT Lunch Seminar, Thursday, July 6, 2017

Giant paramagnetism-induced valley polarization in charge tunable monolayer MoSe₂

Patrick Back – Quantum Photonics Group (Imamoglu group), ETH Zurich

Transition metal dichalcogenide monolayers such as MoSe₂ are strictly two-dimensional direct band-gap semiconductors with a graphene-like honeycomb lattice structure leading to an emergent valley pseudospin degree of freedom. Even though understanding the limits of controllability of the valley pseudospin degree of freedom is of central interest for applications, progress to date has been hindered by the difficulty in obtaining a high-degree of valley polarization of electrons or holes. In this work, we use optical spectroscopy to demonstrate that application of moderate magnetic fields lead to near-complete valley polarization of electrons or holes. Densities as high as 1.6x10¹² cm^-2 are reached for valley polarized electrons under an external magnetic field of 7 T. This unexpected behavior is a direct consequence of super paramagnetic (or valleytronic) response of conduction band electrons. Our experiments pave the way for experiments exploiting the valley degree of freedom of charged carriers.

Lateral p-n Junctions in Inverted InAs/GaSb Double Quantum Wells

Matija Karalic – Nanophysics Group (Ensslin group), ETH Zurich

We have investigated electronic transport across p-n junctions in an inverted InAs/GaSb double quantum well, a material system thought to be a two-dimensional topological insulator or quantum spin Hall insulator. A pair of overlapping  top gates enables us to control carrier type and density in two neighboring regions of a sample, allowing for the creation of a junction when said regions are populated by charge carriers of opposite signs. At zero magnetic field, the p-n junctions show nonlinear current-voltage characteristics, reflecting the underlying inverted band structure. At higher magnetic fields and in the quantum Hall regime we observe reflection, transmission and mixing of spin-polarized edge states at the junctions. Our results may prove useful for designing future, more advanced quantum devices based on topological insulators.