Thomas Jalabert – Laboratoire PHotonique ELectronique et Ingénierie QuantiqueS (PHELIQS) – Université Grenoble Alpes, CEA, IRIG
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 . 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 . 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  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 . 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.
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