May 2012

Abstracts of the QSIT Lunch Seminar, May 3, 2012

Coexistence of Coulomb blockades and the quantum Hall effect in high-quality suspended graphene nanoribbon

Dong-Keun Ki, DPMC and GAP, University of Geneva

By forming a graphene, two-dimensional honeycomb lattice of carbon atoms, into a “long and narrow” nano-ribbon, Dirac electrons can be spatially confined and there many interesting features are expected to appear such as gate-tunable magnetic edge states. Due to high level of disorders in realistic devices, however, even a basic manifestation of Dirac electrons –the presence of zero energy Landau level– has not so far been observed simultaneously with Coulomb blockade effect. Here, by measuring the magneto-transport properties of high-quality suspended graphene nano-ribbon, we present clear experimental evidences of Dirac electrons in a confined geometry: the quantum-Hall effect evolving from Coulomb blockade in magnetic field B. Suspended nano-ribbon was produced by passing a large current through a wide suspended graphene layer, which resulted in a nano-ribbon with level of disorders low enough to show both n=2 quantum-Hall conductance plateaus and a correlation induced insulating state around charge neutrality. Detailed experimental results and discussions will be provided in this talk. This observation demonstrates the potential of high-quality suspended nano-ribbons in investigating intrinsic properties of confined Dirac electrons.

Statics and dynamics of ultracold fermionic gases in harmonic traps with density functional theory

Ilia Zintchenko, Institute for Theoretical Physics, ETH Zurich

Since the landmark work of Hohenberg and Kohn in 1965, density functional theory has been widely used in ab-initio simulations in solids state, materials and bio physics. With novel energy functionals for ultracold fermionic atoms derived from Quantum Monte Carlo calculations of homogeneous systems we can use the extensive tool-set developed for materials also for atomic gases. In particular, we study magnetism for weakly repulsive fermions in harmonic traps and finite temperature properties at unitarity. In addition to serving as guidelines for experiments, these applications also allow us to explore limitations of density functionals and potentially improve simulation
methods for materials.