15:30-16:15

Probing the Dynamics of Andreev States in Coherent Normal/Superconducting ring

A dissipationless current is known to flow through a thin (subnanometer) insulating barrier between two superconductors S with a superconducting phase difference, the well-known Josephson effect. This supercurrent may also flow through a long (micrometer) non-superconducting metal wire at low temperatures, a spectacular consequence of the quantum phase coherence throughout the normal metal. The way the supercurrent responds to a dc phase difference, the current-phase relation, was only recently measured with a Hall probe^[1]. It reflects the phase dependence of the Andreev bound states (ABS) , entangled electron-hole states which form in the normal metal as a consequence of the superconducting mirror-like boundary conditions. However little is known about the dynamics of ABS and their relaxation. In order to investigate the evolution of the current phase relation in such superconductor/normal metal/superconductor (SNS) junctions with high frequency phase driving, we have inductively coupled one NS ring to a multimode superconducting resonator^[2]. The in-phase (c') and out-of-phase (c'') ac magnetic susceptibility of the ring is deduced from the dc flux dependence of the resonance frequency and quality factor of the resonances from 300 MHz up to 6 GHz. Whereas at very low temperatures we essentially measure the phase derivative of the supercurrent, at higher temperature we find a strong frequency dependence in the current response of the ring: the ABS do not follow adiabatically the phase modulation. Indeed, different behaviors are observed for frequencies below and above the inverse of the diffusion time through the normal wire. These results are compared to recent theoretical findings and stimulate future similar investigations on more exotic junctions. This experiment also illustrates a new tool to probe the fundamental time scales of phase coherent systems that are decoupled from macroscopic normal contacts and thermal baths.

References

[1] M. Fuechsle et al., Phys. Rev. Lett., 102, 127001 (2009).

[2] F. Chiodi et al., Nature Scientific Reports 1, 3 (2011)

Speaker

Prof. Meydi Ferrier, Laboratoire de Physique des Solides, Solid State Physics Laboratory, Université Paris Sud, Orsay, France

Chair

Dr. Katsuyoshi Komatsu, Postdoctoral research associate, π-electronics research group, MANA, NIMS