Models and Simulations of Superconducting Nanowires
Time: Fri 2025-04-11 13.00
Location: FB55, Roslagstullsbacken 21, Stockholm
Language: English
Subject area: Physics, Theoretical Physics
Doctoral student: Robert Vedin , Kondenserade materiens teori, KTH Royal Institute of Technology
Opponent: Professor James F Annett, University of Bristol
Supervisor: Associate Professor Jack Lidmar, Fysik, KTH Royal Institute of Technology
QC 2025-03-21
Abstract
Superconducting nanowires are an important component in modern photon detectors, such as the superconducting nanowire single photon detector (SNSPD). In the first three parts of this thesis work I have used simulations of the Ginzburg-Landau (GL) model in order to study how the effect of current crowding, material inhomogeneities, and thermal fluctuations can impact the critical current of such superconducting wires. The critical current is an important parameter for the SNSPD in order to achieve a single photon sensitivity, and a high detection efficiency.
The effect of Ic reduction due to current crowding is found to be less pronounced in the GL model, as compared to previous calculations using the London model.However it remains a relevant factor for meander designs that use very sharp turns in order to achieve a high fill factor. We show how an applied magnetic field may be used as a probe to identify the presence of current crowding in a turn design.We also propose a modified meander design which allows for arbitrarily high fill factor of the meander lines, without suffering Ic reduction due to current crowding.
The study of inhomogeneities shows how even in a straight wire the presence of inhomogeneities can result in a statistical variation of the Ic, leading to a reduction of the effective critical current. We have also demonstrated how the effective critical current in this way can acquire a characteristic dependence on the length of the wire.
Similarly, even for an otherwise homogeneous wire thermal fluctuations may excite vortices to enter into the wire. Such random vortex entry can lead to breakdown of the superconducting state, and thereby a random variation of the critical current. We show how the rate of such switching events can be calculated by combining critical current distributions sampled using different sweep rates of the current, or different wire lengths.
The fourth work in this thesis focuses instead on vortex fluctuations in superconducting thin film with a regular lattice of nanometre sized pores. The addition of nanopores to the film is seen, experimentally, to reduce the superfluid stiffness.We use Monte Carlo simulations of a 2D XY model to investigate this dependence for a number of different pore configurations.