At low temperature, we have used thermal conductivity as a directional probe of the residual normal fluid in two superconductors, UPd2Al 3 (a heavy fermion) and YBa2Cu3O7-delta (a high-Tc cuprate). By extrapolating our measurements to zero temperature, we can shed light on zero energy quasiparticles and the structure of the superconducting gap. / For both superconductors, we review measurements pertaining to the density of states. In the case of the heavy fermion superconductor UPd2Al 3, we have found a finite anisotropy between b axis and c axis heat conduction, which excludes those gap structures with only zeroes along c or in the equatorial plane of a spherical Fermi surface; however, our results are consistent for two line nodes equidistant from the equatorial plane, as in the A 1g gap. Comparisons to theory developed for UPt3 show qualitative agreement with two hybrid gaps with strong spin-orbit coupling, of E2u and E 1g symmetry. / For YBa2Cu3O7-delta, because the gap symmetry has been established as dx2-y2 , we can go much further as regards a quantitative analysis. The anisotropy in the thermal conductivity was measured along both high symmetry directions. A residual T-linear term in kappa(T) was observed in both directions. In the CuO2 planes (J∥ a) the magnitude of the residual normal fluid conduction is perfectly consistent with the temperature dependence of the penetration depth, within the theory for a d-wave superconductor. The value for J∥b is slightly larger, yielding an anisotropy ratio of 1.3 +/- 0.3. This is considerably weaker than that observed in the normal state resistivity, pointing to a suppressed heat conduction by quasiparticles in the chains, either due to strong defect scattering or a gapped excitation spectrum. / With the application of an external magnetic field (up to 8 T), we can study the effect of vortices on quasiparticle transport. The residual linear term increases with field, directly reflecting the occupation of extended quasiparticle states. A study for different Zn impurity concentrations reveals a good agreement with recent calculations for a d-wave gap. The magnitude of the suppression indicates that Zn impurity scattering needs to be treated in the resonant impurity scattering limit, until now an unverified assumption. Together with specific heat measurements, we obtain a quantitative measure of the gap near the nodes.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.35992 |
| Date | January 1999 |
| Creators | Chiao, May. |
| Contributors | Taillefer, Louis (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Physics.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001687503, proquestno: NQ55313, Theses scanned by UMI/ProQuest. |
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