Phd in imaging non-equilibrium superconductivity (Closed to applications)

Phd starts in September 2017
STM map of the superconducting order parameter in TiN. We are looking for a motivated candidate for a Phd project preceded by a master's training on imaging non-equilibrium superconductivity. Superconductors with a large normal-state resistivity, such as titanium nitride (TiN), have recently gained a lot of interest from the engineering community. Their large resistivity makes them ideal materials for kinetic inductance photon detectors [1] and superconducting parametric amplifiers [2]. At the same time, this large resistivity gives rise to localization of electrons, which directly competes with superconductivity and leads to a superconductor-to-insulator transition. It has been shown both theoretically and experimentally [3] that close to this transition, the superconducting state becomes intrinsically inhomogeneous, with mesoscopic regions of larger and smaller order parameter.This electronic inhomogeneity should influence the quasiparticle dynamics in such a material. In effect, it has been observed that the electrodynamic response of strongly disordered superconductors is increasingly modified with increasing disorder [4]. Qualitatively, these observations could be explained by a model where quasiparticles are trapped in regions with lower order parameter, intrinsically present in these materials. However, until now the only experiments that allow seeing the dynamics of an inhomogeneous superconducting system, are GHz frequency measurements on superconducting resonators. These experiments are very sensitive to minor changes in the superfluid density and number of quasiparticles, but necessarily probe a macroscopic volume of the superconductor, rendering explanation of the measurements in terms of the inhomogeneity difficult. Scanning tunneling microscopy (STM), on the other hand, is the ideal tool to map the electronic properties of a material on a nanometer scale. We have recently design a STM- compatible environment of a mesoscopic device at 50 mK, which allows studying the quasiparticle dynamics in a superconducting nanowire by using the STM tunneling current as a local quasiparticle injector, and measuring the global consequences of this injection in transport. We named this new STM imaging mode: scanning critical current microscopy. In this technique the pair breaking efficiency of the injection of quasiparticle at different energies by the STM tip is deduced by monitoring the critical current of the superconducting nanowire as a function of the tip position. The aim of the PhD project is to apply this new probe technique to superconducting TiN nanowires in order to unveil the role of inhomogeneities and localized single particle states in the dynamic of quasi- particles in disordered superconductors. Experiments will be also done under magnetic field where the STM tip will be positioned at various distances from a magnetic vortex core in order to understand the competition between the trapping of quasiparticles inside the vortex and the recombination process into Cooper pairs. The results will be used to understand better the position dependence of the photon detection efficiency, and the role of inhomogeneity in the behavior of superconducting detectors. The work will be performed in two laboratories in Grenoble, CEA-INAC and IRAM. The PhD student will grow TiN films at IRAM and perform STM measurements at CEA-INAC. In order to realize the nanowires, he will be trained to electronic lithography at the CEA/CNRS/UJF clean room facility (PTA). The student will also be trained to low temperature techniques at CEA-INAC, in particular to STM imaging and spectroscopy in a dilution refrigerator. The project will provide grounds for collaborations with theorists from CEA-INAC in view of describing aspects out-of-equilibrium properties of strongly disordered superconductors.

[1] H. G. LeDuc et al. Appl. Phys. Lett 97, 102509 (2010)
[2] B. H. Eom et al. Nature Phys 8, 623 (2012)
[3] B. Sacépé et al. Phy. Rev. Lett 101, 157006 (2008)
[4] E. Drissen et al. Phy. Rev. Lett 109, 107003 (2012); P.Coumou et al. Phy. Rev. B 88, 180505(R)) (2013)