Imaging dissipative current in superconducting Niobium film using scanning SQUID susceptometry

The quasiparticle density of states is a strong diagnostic of the superconducting gap and may be observed as dissipation in two coil mutual inductance experiments. Mutual inductance experiments have typically been limited to millimeter spatial resolution and therefore measure nonzero impedance resulting from motion of vortices. Susceptibility measurements with greater spatial resolution may be able to detect the additional dissipation resulting from scattering of excited quasiparticles. Scanning Superconducting QUantum Interference Device (SQUID) susceptibility measurements offer micron scale spatial resolution with the in-phase component measuring superfluid density and the quadrature component measuring dissipation. In this talk, we report on micron scale scanning SQUID susceptometry measurements on a Niobium film in an effort to observe the frequency and temperature dependence of quasiparticle scattering. Successfully observing the temperature dependence of quasiparticle excitations in Niobium would be a proof of concept to measure dissipation in unconventional superconductors with micron scale spatial resolution.