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Methods for Reducing Response Times in Superconducting Devices Governed by Quasiparticle Relaxation and Recombination

IP.com Disclosure Number: IPCOM000059950D
Original Publication Date: 1986-Feb-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 3 page(s) / 27K

Publishing Venue

IBM

Related People

Epperlein, PW: AUTHOR

Abstract

Treating the surface of the top film of a superconductive device, so as to remove phonons, increases speed of response. Treatments include evaporation of the top film in oxygen background, adsorption of impurities or condensed gas on the top film surface, and thallium chloride or indium overlay on top film. These treatments reduce response times in superconducting devices governed by quasiparticle relaxation and recombination, by removing pair- breaking phonons and thus reducing the effective quasiparticle lifetimes. Quasiparticle (QP) excess concentrations, created in superconducting films by tunnel injection or by phonon or photon absorption, decay with a characteristic time teff, which is determined by the coupled system of quasiparticles, phonons and Cooper pairs.

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Methods for Reducing Response Times in Superconducting Devices Governed by Quasiparticle Relaxation and Recombination

Treating the surface of the top film of a superconductive device, so as to remove phonons, increases speed of response. Treatments include evaporation of the top film in oxygen background, adsorption of impurities or condensed gas on the top film surface, and thallium chloride or indium overlay on top film. These treatments reduce response times in superconducting devices governed by quasiparticle relaxation and recombination, by removing pair- breaking phonons and thus reducing the effective quasiparticle lifetimes. Quasiparticle (QP) excess concentrations, created in superconducting films by tunnel injection or by phonon or photon absorption, decay with a characteristic time teff, which is determined by the coupled system of quasiparticles, phonons and Cooper pairs. QP's with an energy at the gap edge of E Z W recombine into the ground state by emitting 2W-phonons. It is known that the intrinsic QP recombination time tR is enhanced by repeated 2W-phonon reabsorption and remission. This process leads to a constant "oscillation" of the excitation energy between the QP- and 2W-phonon systems. Since the phonon lifetime tB against pair breaking usually is much shorter than tR, the energy exists most of the time in the form of the potential energy of the QP excitations. This mechanism would result in an infinitely long, effective QP lifetime teff (includes phonon contributions), if there were no phonon loss processes (described by tq) as in the bulk (phonon- QP scattering, spontaneous phonon decay by lattice anharmonicity) or by escape across the sample surface. An approximate phonon escape rate usually is written as

(Image Omitted)

where t = film thickness, Ct = transversal phonon velocity and <Ti d>d . weighted acoustical transmission probability at the boundaries to the adjacent media i. For weak overinjection wN/Nth << 1 (wN = N - Nth, Nth = thermal QP density), the "phonon-dressed" lifetime is

(Image Omitted)

For dominating phonon loss by escape and t > GB, the phonon reabsorption mean free path, it is

(Image Omitted)

To get fast response times in injection-type devices, teff has to be low. The figure shows a superconducting device of the QUITERON or Phonon-Mediated Quasiparticle Injection Tra...