Forum DQMP

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Day / Time
Date(s) - 28/10/2014
13 h 00 min - 14 h 00 min

Category


Auditoire Stückelberg – Ecole de physique

 

Andreev resonances contribution to the supercurrent at energies above the superconducting gap E > Δ

Sandra Šopić (group of Prof. Morpurgo)

We present a spectroscopic study of how electronic states at different energies contribute to the supercurrent inside superconductor-normal metal-superconductor (SNS) Josephson junctions. The states responsible for the non-dissipative transport are typically confined inside Δ and known as Andreev bound states (ABS). However, already the first theoretical paper predicted an appearance of Andreev resonances at E > Δ that carry the supercurrent. In contrast to ABS, the Andreev resonances and their contribution to the supercurrent remained unobserved until now. The experiment relies on multi-terminal Josephson junctions, where the contribution of state to the supercurrent is determined by modifying the electron distribution in the junction. Devices, made in past, were fabricated with conventional metals that limited the supercurrent to E << Δ. In order to investigate the supercurrent at E > Δ, we use graphene as normal conductor. At subgap energies, the change of the electron distribution progressively suppresses the ABS contribution. When the ABS contribution is completely removed we measure a considerable supercurrent still flowing through the junction. This remained current is due to the Andreev resonances occurring in the continuum. Our experimental results confirm importance of the Andreev resonances in the superconducting transport and show how the use of graphene provides a new platform to investigate unexplored aspects of superconducting proximity effect.

 

Spin and lattice dynamics in low dimensional quantum systems

Björn Wehinger (group of Prof. Rüegg)

The investigation of quantum spin systems continues to reveal fundamental insights about the ground state of quantum magnetic systems and the development of quantum criticality. Prominent examples are dimer and spin ladder materials with structural and physical properties of 2D and 1D quantum systems. Recent progress in neutron spectroscopy and numerical methods provides the necessary tools to quantitatively study correlations of such low dimensional systems. The combination of these methods with inelastic and diffuse x-ray scattering allow disentangling lattice and spin dynamics thus providing a novel tool to quantify quantum critical effects. I will present first results on the spin dynamics in a 1D quantum ladder system, address the complex mechanism of magnetic frustration in a 2D magnetic insulator and outline a new research approach for the study of disorder and spin lattice coupling in quantum magnets.

 

Coffee and tea will be available from12h50 at the entrance of the Auditoire 

Forum Committee :
Laura Foini
Céline Lichtensteiger
Nicolas Ubrig

Réalisation : Sur Mesure concept