September 2015
Abstracts of the QSIT Lunch Seminar, Thursday, September 3, 2015
Strong interface induced spin-orbit coupling in graphene-on-WS2 heterostructure
Zhe Wang, Quantum Electronics Group, University of Geneva
Disorder-free graphene is the first predicted topological insulator, whose characteristics have not been observed experimentally because the strength of intrinsic spin-orbit interaction (SOI) in graphene is too weak. Here we explore this issue by exploiting interfacial interactions in graphene-on-WS2 heterostructure, whose basic transport characteristics confirm the high device quality. Robust weak anti-localization effect is observed at all accessible carrier density range down to 250 mK, which is the first time in graphene and constitutes unambiguous evidence of strong SOI induced in graphene. The extracted spin-relaxation time is 2-3 order shorter than that in graphene on SiO2 or hexagonal boron nitride (hBN) substrates, and is comparable to the intervalley scattering time. The experimental findings are consistent with first-principle electronic structure calculations, which shows interfacial interactions with WS2 substrate indeed induce strong SOI in graphene. Furthermore, the same analysis also suggests opening of a gap due to SOI which can become a two-dimensional topological insulator. Our work therefore clearly demonstrates strong SOI induced in high-quality graphene using interfacial interactions with WS2, and opens a possible new route to access topological states of matter in graphene-based systems.
On the trace of spin-pairs in single and double quantum dots
Andrea Hofmann, Nanophysics group, ETH Zurich
We study the tunnel dynamics of a GaAs quantum dot coupled to one reservoir. Utilizing a feedback mechanism based on the detection of single electron tunneling events, we are able to measure the tunnel rates far away from the Fermi energy. This allows for exactly determining the tunnel coupling and extracting the degeneracy of the involved quantum dot energy state as well as for detecting excited states and estimating their decay times. The ground and excited state spectroscopy both hint towards doubly degenerate energy levels up to the first eight electrons filled into the quantum dot. We compare these studies with results from single electron counting experiments in a spin-blockaded double quantum dot, where we also find the fingerprint of spin-pairs.