Special CPM Seminar

Vibrating vortices as a probe of quasiparticles in unconventional superconductors

David Broun

SFU

In a beautiful set of experiments carried out on superfluid helium-3 in the microkelvin regime, Pickett and coworkers at Lancaster used small, vibrating-wire viscometers to study the dynamical behaviour of helium quasiparticles under a wide range of conditions [1-3]. A superconducting analog of this experiment would be immensely interesting, but is hampered by the fact that the electron fluid of the superconductor is entrained within a mechanically rigid crystal lattice: to have any chance of succeeding, the "wire" would need to be formed from the electron fluid itself. Superconducting vortices, which have macroscopic stability due to their topology and can be driven dynamically by the application of external electromagnetic fields, present one possibility. However, unlike solid wires in superfluid helium, the quasi-normal core of a vortex typically dominates its dynamical response, as described by the Bardeen-Stephen model. This theory is applicable to conventional superconductors for two reasons: the vortex cores are large and contain a nearly continuous spectrum of single-particle states; and s-wave pairing symmetry results in a low density of extended states surrounding the vortex cores. In cuprate and other unconventional superconductors the opposite situation often holds: small vortex cores contain at most a few discrete states, with a continuum of low-lying states outside the vortex cores due to nodes in the superconducting energy gap. Using precision microwave measurements of low-lying states outside the vortex cores due to nodes in the superconducting energy gap. Using precision microwave measurements of vortex viscosity, we have recently observed a breakdown of the Bardeen-Stephen model in cuprate and heavy fermion superconductors, in which the frictional forces on the flux lines instead appear to be caused by interactions with extended quasiparticle states outside the cores. This suggests that we may be entering a regime analogous to that in the helium-3 measurements, in which the interaction of moving vortices with a dilute gas of bulk quasiparticle excitations can be employed as a useful new dynamical probe of a superconductor.

[1] S. N. Fisher et al., PRL 69, 1073 (1992).
[2] M. P. Enrico et al., PRL 70, 1846 (1993).
[3] C. Bauerle et al., Nature 382, 332 (1995).