Visioconférence
Thomas Jalabert – Laboratoire PHotonique ELectronique et Ingénierie QuantiqueS (PHELIQS) – Université Grenoble Alpes, CEA, IRIG
Abstract
The performances of superconducting devices are often limited or governed by quasi-particles dynamics. For instance, excess quasi-particles are detrimental for superconducting micro coolers and superconducting qubits. On the contrary, the generation of quasi-particles is a prerequisite for the operation of photon detectors. In order to probe the microscopic mechanisms at play, injection of quasiparticles with the help of a tunnel junction has often been employed at the mesoscopic scale [1]. However, lithographed tunneling barriers lack spatial resolution and do not allow to vary the bias voltage and the tunnelling current independently. In order to overcome these two limitations we used a homemade Scanning Tunnelling Microscope (STM) working at very low temperature (50 mK) and monitored the critical current of superconducting nanowires as a function of the tip position and the tunnelling set-point. In NbnAu nanowires, we observed a drastic reduction of the critical current by injecting a tunnelling current of quasiparticles 6 orders of magnitude lower, which demonstrates unam- biguously the existence of quasi-particle overheating in our samples and reconsider the physical interpretation of superconducting FETs [2]. At large energies compared to the superconducting gap, the reduction of the critical current is controlled by the injected power and we develop a heat di-usion model based on standard theory of superconductivity [3] to account for the influence of hot electrons on the out of equilibrium transport in superconductors. It also allows to extract microscopical parameters of the electron-phonon coupling [4]. By contrast, when re- ducing the energy at constant injection rate, the critical current sharply decreases close to the gap energy, signaling the breakdown of the heat di-usion model. We also probed the spectral properties of current carrying nanowires, and induced vortices to create spatial disorder in the density of states. Thus, this experiment opens a new perspective to investigate the competi- tion between di-usion, relaxation and recombination of quasiparticles in strongly disordered superconductors with various applications such as in photon detection.
References
[1] A. Rothwarf and B. N. Taylor, Measurement of Recombination Lifetimes in Superconductors, Physical Review Letters, 19, 27 (1967).[2] G. De Simoni, F. Paolucci, P. Solinas, E. Strambini, and F. Giazotto, Metallic supercurrent -eld- e-ect transistor, Nature Nanotechnology, 13, 802 (2018).
[3] K. D. Usadel, Generalized Di-usion Equation for Superconducting Alloys, Physical Review Letters, 25, 507 (1970).
[4] A. Sergeev and V. Mitin, Electron-Phonon Interaction in Disordered Conductors : Static and Vi- brating Scattering Potentials, Phys. Rev. B, 61, 6041 (2000).
