Quasiparticle Recombination Research Articles

Direct Measurement of Quasiparticle-Lifetime Broadening in a Strong-Coupled Superconductor

We have measured the quasiparticle recombination time in the strong-coupled superconductor ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$ directly by measuring the lifetime-broadened energy gap edge. This is done by measuring the $I\ensuremath{-}V$ characteristics of a superconducting tunnel junction of the type ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$-insulator-${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$. Agreement with the calculated value is excellent.

Gap suppression by self-injection of quasiparticles in tin—tin-oxide—tin tunnel junctions

A decrease of the superconducting energy gap has been observed in high-current-density tin---tin-oxide---tin tunnel junctions. This effect cannot be adequately accounted for by simple heating and is attributed to the presence of a nonequilibrium excess quasiparticle population created by the tunnel current. Analysis using a phenomenological model of the nonequilibrium superconductor yields a determination of the phonon-trapped quasiparticle recombination lifetime in tin in good agreement with the results of other experiments. A size-dependent enhancement of the phonon-emission efficiency of very narrow tunnel junctions is also observed and explained.

Superconducting lead: Quasiparticle recombination time and Kapitza resistance

We deconvolve the system impulse response from the observed quasiparticle pulse transients of Hu et al., to determine the quasiparticle recombination time. Deconvolution of the accompanying phonon transient yields a phonon-helium-bath relaxation time from which we determine the Kapitza resistance of lead.

Quasiparticle Propagation and Recombination in Bulk, Superconducting Pb

The propagation characteristics of quasiparticles in bulk superconducting Pb single crystals is studied. A transition from quasiparticle diffusion to diffusion of the combined gas of quasiparticles and phonons is observed as the temperature is increased. The intrinsic quasiparticle recombination time, as well as the decay time of the quasiparticle density, is determined. The latter is found to be at least an order of magnitude longer than this quasiparticle recombination time.

Intrinsic and experimental quasiparticle recombination times in superconducting films

Experimental quasiparticle recombination lifetime data for superconducting Al, Sn, and Pb films are compared with calculations based on a ray acoustic model taking account of the film thickness dependence of the reabsorption of recombination phonons. Information on the true or intrinsic quasiparticle recombination lifetime obtained from these and other data is discussed.

Quasiparticle recombination and 2Δ-phonon-trapping in superconducting tunneling junctions

The experimental recombination lifetime ref f of quasiparticles in superconducting films in general exceeds the intrinsic recombination lifetime vR by phonon trapping. On the basis of geometric acoustic propagation and reabsorption of phonons emitted in quasiparticle recombination, %ff is calculated as a function of film thickness d taking into account longitudinal and transverse phonon reabsorption, bulk loss processes and acoustical phonon transmission into the substrate. With increasing thickness d three characteristic ranges are found: range 1 with film thickness d small compared to the phonon reabsorption mean free path A~, range 2 with d larger than A w and dominating boundary losses, and range 3, also with d larger than Aw but with dominating bulk losses. For very small d the relation between reff and rR, the intrinsic recombination lifetime, contains only the limiting angle of total reflection of phonons within the superconducting film. Therefore, rR can be directly obtained by reef measurements and from the sound velocities of the film-substrate system. Range 2 is characterized by a linear dependence of %ff on d. In this range it is not possible to obtain za from %fr measurements, however, reef allows a determination of the phonon boundary transmission. Range 3 shows no thickness dependence of %ff on d in the limit of large d values. In this range a further method for obtaining ~R from reff values is suggested.

Quasiparticle and phonon lifetimes in superconducting Pb films

The effective quasiparticle recombination lifetime in superconducting Pb films has been measured using Pb-oxide-Pb tunnel junctions and is consistent with the assumption that phonon lifetimes dominate the quasiparticle density relaxation. No evidence is found of the inelastic phonon processes suggested by Schuller and Gray.

Phonon and Quasiparticle Dynamics in Superconducting Aluminum Tunnel Junctions

The effect of the $2\ensuremath{\Delta}$ recombination phonons in modifying the effective lifetime ${\ensuremath{\tau}}_{\mathrm{eff}}$ for quasiparticle recombination has been determined for the first time in a double-junction experiment by comparing measurements taken both with and without a helium film covering the junctions. For a double junction made from evaporated aluminum films 300 \AA{} thick deposited on a glass substrate, we find $\frac{{\ensuremath{\tau}}_{\mathrm{eff}}}{{\ensuremath{\tau}}_{R}}=8.6\ifmmode\pm\else\textpm\fi{}1.1$ in the absence of the helium film, and, at $\frac{\ensuremath{\Delta}}{\mathrm{kT}}=6$, ${\ensuremath{\tau}}_{R}=1.4\ifmmode\pm\else\textpm\fi{}0.1$ \ensuremath{\mu}sec.

Photoexcitation of Quasiparticles in Nonequilibrium Superconductors

We report measurements on superconducting tunnel junctions illuminated with optical radiation, which confirm the model of nonequilibrium superconductors proposed by Owen and Scalapino. Measurements of the chemical potential ${\ensuremath{\mu}}^{*}$ for quasiparticle excitations and of the temperature dependence of the quasiparticle recombination time are presented.

Dependence of quasiparticle recombination time on film thickness

We find that the measured recombination time of quasiparticles in superconducting aluminum films increases approximately linearly with film thickness as predicted by Rothwarf and Taylor, the lifetime for a thickness of 1000 Å being approximately twice the extrapolated value at zero thickness. They suggest that the reabsorption of phonons, emitted during the recombination of quasiparticles, will lead to an enhancement of the measured quasiparticle recombination time proportional to the film thickness, if the main process competing with phonon reabsorbtion is the direct propagation of phonons into the substrate. Our results are in semi-quantitative agreement with a simple model based on their discussion. The enhancement is expected to depend on the phonon transmission coefficient. We observed essentially no effect when layers of WO 3 or CeO 2, having very high acoustic impedances, were interposed between the aluminum film and substrate. We have, at present, no convincing explanation for this result which casts considerable doubt on the above interpretation of our data.

A calculation of the quasiparticle recombination rate in superconducting aluminium

Abstract A calculation of the quasiparticle recombination rate in superconducting aluminium is presented. Details of the Fermi surface are taken into account in determining the rate due to umklapp processes, which turns out to be approximately equal to the rate due to normal processes. At low temperatures in bulk aluminium (δo = 0·176 mev) the recombination rate is 9·45 × 10−12 N(T) sec−1, where N(T) is the quasiparticle density per cm3 at the temperature T.

Recombination Time of Quasiparticles in Superconducting Aluminum

The quasiparticle recombination time ${\ensuremath{\tau}}_{\mathcal{r}}$, has been measured in superconducting aluminum films. The temperature dependence of ${\ensuremath{\tau}}_{\mathcal{r}}$ at low temperatures was found to be in good agreement with theoretical expectations.

Measurement of Recombination Lifetimes in Superconductors

It is shown that the experimentally measured quasiparticle recombination lifetime in a superconductor is not the same as the previously calculated theoretical lifetime. A simple expression relating the two is derived.