Energy-dependent kinetic model of photon absorption by superconducting tunnel junctions

Abstract

Articles you may be interested inNumerical analysis of detection-mechanism models of superconducting nanowire single-photon detector We describe a model for photon absorption by superconducting tunnel junctions in which the full energy dependence of all the quasiparticle dynamic processes is included. The model supersedes the well-known Rothwarf-Taylor approach, which becomes inadequate for a description of the small gap structures that are currently being developed for improved detector resolution and responsivity. In these junctions relaxation of excited quasiparticles is intrinsically slow so that the energy distribution remains very broad throughout the whole detection process. By solving the energy-dependent kinetic equations describing the distributions, we are able to model the temporal and spectral evolution of the distribution of quasiparticles initially generated in the photoabsorption process. Good agreement is obtained between the theory and experiment.

Dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions

“…18 The quasiparticles are produced in this small volume and reach nearly the Ta gap energy before significant diffusion occurs. This situation differs from that described in 8 where a sandwich structure of Ta/Al films forms the absorber and tunnel junction. For this structure, the fast equilibration occurs in the volume from which the tunneling current originates.…”

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

“…In the BCS model, the latter is singular, leading to an enhanced detrapping rate in comparison with that proposed in Ref. 20. A realistic description of both the density of states in the vicinity of the local trap as well as of the detrapping rate requires an accurate model for the local trap.…”

“…The simulation begins at an initial instant of time when the infinitesimally narrow initial distribution containing N 0 = E / 1.75⌬ qps, where E is the deposited energy, is taken at an arbitrary energy below 3⌬, to avoid any further qp generation. The exact energy and shape of the initial distribution are of no importance 20 because after a small number of tunnel events, the qp spectral distribution converges very rapidly to a stable shape, which remains unchanged during the remainder of the charge acquisition process, with only the total number of qps decreasing with time through losses. Hence, calculating the current flowing through the STJ according to Eq.…”

“…In Ref. 20, detrapping rate was proposed to be proportional to that of qp absorption from the initial state at the edge, ⌬, into a final state above this level corresponding to the trap depth. This assumption has never been tested experimentally before and needs refinement before a quantitative modeling can be carried out.…”

“…We previously used this approach to model successfully the nonequilibrium qp dynamics for a BCS superconductor in the stationary regime 12,13 and for the general situation of a proximized structure with timeevolving distributions. 20,21 However, the latter scheme was incomplete since it did not take account of the qp selfgeneration and used an oversimplified model of qp detrapping at localized traps. In addition, the effects of the nonequilibrium subgap phonon distribution were not considered.…”