Quasiparticle Dynamics in Disordered Superconductors
S.A.H. de Rooij (TU Delft - Tera-Hertz Sensing)
J.J.A. Baselmans – Promotor (TU Delft - Tera-Hertz Sensing, SRON–Netherlands Institute for Space Research)
P.J. de Visser – Promotor (SRON–Netherlands Institute for Space Research, TU Delft - Tera-Hertz Sensing)
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Abstract
In 1911, Heike Kamerlingh Onnes and his research team measured a steep drop to zero in the resistance of mercury when cooling it below4.2 K: superconductivity was discovered. Forty-six years later, the microscopic theory to describe superconductivity was formulated. It predicts that disorder (the increase of electron scattering due to impurities, defects or other causes), to first order, does not influence the superconducting state. From an application perspective, disordered superconductors are interesting as they have a high kinetic inductance: they strongly - and non-linearly - oppose the change in current due to the inertia of the superconducting charge carriers, which are paired electrons, or Cooper pairs. Therefore, disordered superconductors can be used in superconducting circuits as highly inductive, nonlinear elements to realize, for example, quantum bits, quantum-limited amplifiers and single-photon detectors. In addition, the high normal state resistance of disordered superconductors allows an efficient, broadband absorption of light, which enables single photon counting, energy resolving microwave kinetic inductance detectors (MKIDs) for visible and near-infrared light.
These applications are, however, hindered by an unexplained microwave loss and, for MKID applications, an enhanced decay rate of the elementary excitations in the superconductor. Both these effects become stronger with increasing disorder. The elementary excitations are called quasiparticles and are essentially broken Cooper pairs. Their density decays as they recombine pair-wise into Cooper pairs. Since quasiparticles also induce microwave loss, these observations point towards an effect of disorder on the quasiparticle dynamics. It is however unclear how disorder affects the quasiparticle dynamics exactly.....