Previous Research Nuggets

Non-equilibrium BCS superconductors

(July 8, 2005)

Non equilibrium dynamics in superconductors
One liner: The out of equilibrium infrared response of BCS superconductors shows a gap shift due to excess quasiparticles induced by breaking Cooper pairs with light.

More details: In a superconductor all the electrons form pairs below its Fermi level [panel (a)]. The absorption of a photon will excite pairs across the gap and produce electrons above the Fermi energy [panel (b)]. These "unstable" excited electrons want to recombine into a pair. The energy gain occurring when electrons form a pair must be emitted, usually as a crystal lattice vibration (phonon). However this emitted phonon can break another pair and produce two new excited electrons. This subtle competition between electrons willing to recombine and phonons trying to break pairs eventually ends when phonons become unavailable by either relaxing to very low energies or simply escaping the material. The whole process of relaxation is schematized on panel (c).

Far-infared non-equilibirum transmission of Pb.

We studied BCS superconductors (Pb, Nb, NbN, ...) out of equilibrium utilizing a Ti:Sapphire pulsed laser synchronized to the electron bunches of the NSLS-VUV ring at BNL. The laser pulses (pump) break pairs and the synchrotron far-infrared light probes the changes in the optical response of the superconductor. By delaying the laser pulses with respect to the synchrotron bunches, we can obtain the time dependent optical response for the specimen.

The panel at the right shows the measurement of the non equilibrium infrared spectrum, is the first direct determination of the superconducting gap shrinkage expected for superconductors with excess unpaired electrons.

Publication: Photoinduced time-resolved electrodynamics of superconducting metals and alloys, R.P.S.M. Lobo, J.D. LaVeigne, D.H. Reitze, D.B. Tanner, Z.H. Barber, E. Jacques, P. Bosland, M.J. Burns, and G.L. Carr, Phys. Rev. B 72, 024510 (10 pp.) (2005). [PRB page]