April 2018

Abstracts of the QSIT Lunch Seminar, Thursday,April 12, 2018

Substrate-induced topological mini-bands in graphene

Tobias Wolf – Quantum condensed and coherent systems (Blatter group), ETH Zurich

The honeycomb lattice sets the basic arena for numerous ideas to implement electronic, photonic, or phononic topological bands in (meta-)materials through proper design. Dirac electrons in graphene are predestined for such designed band engineering, e.g., via perturbative potentials as induced by a substrate such as hexa boron-nitride. Making use of the generic form of a substrate potential as dictated by symmetry, we analytically derive the low-energy mini-bands of the superstructure associated with one Dirac cone (or valley) and find the possible arrangements of the low-energy electron- and hole bands in six-dimensional parameter space. We identify the various symmetry planes inducing gap closures and find the sectors hosting topological mini bands, including also complex band crossings generating a higher Chern number. Our map provides a starting point for the systematic design of topological bands by substrate engineering.

Spin and Valley States in Gate-defined Bilayer Graphene Quantum Dots

Marius Eich – Nanophysics (Ensslin group), ETH Zurich

In bilayer graphene, electrostatic confinement can be realized by a suitable design of top and back gate electrodes. We measure electronic transport through a bilayer graphene quantum dot, which is laterally confined by gapped regions and connected to the leads via p-n junctions. Single electron and hole occupancy is realized and charge carriers n = 1, 2, … 50 can be filled successively into the quantum system with charging energies exceeding 10 meV. For the lowest quantum states, we can clearly observe valley and Zeeman splittings with a spin g-factor of g ≈ 2. In the low field-limit, the valley splitting depends linearly on the perpendicular magnetic field and is in qualitative agreement with calculations.