November 2012
Abstracts of the QSIT Lunch Seminar, November 1, 2012
Coupling of graphene to a THz-metasurface
Federico Valmorra, Quantum Optoelectronics Group, ETH Zurich
The TeraHertz (THz) frequency region is subject of increasing research for both its technological and fundamental importance. Single-layer graphene, already established as 2-dimensional electronic system, is emerging as an optoelectronic material in the THz range where its intraband absorption frequencies lay.
In this frame, we study the light-matter interaction of single-layer CVD-graphene with THz-radiation via THz Time-Domain Spectroscopy in the frequency range from 200 GHz to 3 THz. A subwavelength Thz-metasurface is used to enhance such interaction thanks to its strong resonances (main mode at about 0.6 THz). Room temperature measurements show that the presence of the graphene redshifts the resonance of the THz-Split-Ring Resonators, decreases the Quality-factor and increases the transmitted amplitude. The gating of the graphene sheet allows then to actively control such interaction: moving the graphene’s Fermi level towards the charge neutrality point tends to restore the Q-factor of the resonance by decreasing the intensity of the transmitted light and shifts back the resonance minimum towards the uncoupled SRR value.
Observing the Drop of Resistance in the Flow of a Superfluid Fermi Gas
David Stadler, Quantum Optics Group, ETH Zurich
The ability of particles to flow with very low resistance is a distinctive character of a superfluid or superconducting state and led to its discovery in the last century. While the particle flow in liquid Helium or superconducting materials is essential to identify superfluidity or superconductivity, an analogous measurement has not been performed with superfluids based on ultracold Fermi gases. Here we report on the direct measurement of the conduction properties of strongly interacting fermions, and the observation of the celebrated drop of resistance associated with the onset of superfluidity. We observe variations of the atomic current over several orders of magnitude by varying the depth of the trapping potential in a narrow channel, which connects two atomic reservoirs. We relate the intrinsic conduction properties to thermodynamic functions in a model-independent way, making use of high-resolution in-situ imaging in combination with current measurements. Our results show that, similar to solid-state systems, current and resistance measurements in quantum gases are a sensitive probe to explore many-body physics. The presented method is closely analogous to the operation of a solid-state field-effect transistor. It can be applied as a probe for optical lattices and disordered systems, and paves the way towards the modeling of complex superconducting devices.