Specific heat of cuprate superconductors

This thesis examines the magnetic field dependence of the low-temperature specific heat of two cuprate superconductors, YBa{dollar}rmsb2Cusb3Osb{lcub}6.95{rcub}{dollar} and Bi{dollar}rmsb2Srsb2CaCusb2Osb8{dollar}. Specific heat is sensitive to all excitations, including phonons and impurities as well as electronic excitations. The relaxation method for measuring the specific heat of milligram-sized single crystals is described in chapter 2. The use of single crystals allows the measurement of the anisotropy of the magnetic field dependence of the specific heat. The anisotropy can only be determined on oriented materials, and proves to be a valuable handle for distinguishing between different types of excitations.; Chapter 3 includes measurements on single crystals of YBa{dollar}rmsb2Cusb3Osb{lcub}6.95{rcub}{dollar} grown at the University of British Columbia. These materials have a zero-field linear-T term in the low-temperature specific heat which is small compared to other YBa{dollar}rmsb2Cusb3Osb{lcub}6.95{rcub}{dollar} samples. With a magnetic field H applied perpendicular to the copper oxide planes, the specific heat includes an increased linear-T term whose coefficient has a sqrt(H) dependence on field. As discussed in chapter 4, this sqrt(H)T term agrees semiquantitatively with G. Volovik's prediction for a superconductor with lines of nodes in the gap function, suggesting an exotic vortex structure and an unconventional pairing state.; Preliminary measurements on the specific heat of Zn-doped YBa{dollar}rmsb2Cusb3Osb{lcub}6.95{rcub}{dollar} and Bi{dollar}rmsb2Srsb2CaCusb2Osb8{dollar} are presented in chapters 5 and 6. The change in the specific heat of YBa{dollar}rmsb2Cusb3Osb{lcub}6.95{rcub}{dollar} with Zn doping is consistent with resonant scattering for lines of nodes. There is evidence for low-energy excitations in Bi{dollar}rmsb2Srsb2CaCusb2Osb8{dollar}, but it is difficult to determine the nature of these excitations from the low-temperature specific heat. The increase in the specific heat in a magnetic field is significantly smaller for Bi{dollar}rmsb2Srsb2CaCusb2Osb8{dollar} than for YBa{dollar}rmsb2Cusb3Osb{lcub}6.95{rcub}{dollar}.; In chapter 7 we discuss the possibility of specific heat contributions associated with vibrations of the vortex lattice. Finally, in chapter 8 we summarize and compare the specific heat of different cuprate superconductors, and suggest directions for future research.