Thermal conduction in the high temperature superconductors LSCO and TL2201
Subtitle: a field and doping dependent study.
Contributors: Taillefer, Louis (supervisor)
Published: 2005
Language: eng
Number of pages: 163
Subjects: Physics Theses
Description: In striking contrast to Tl2201, in underdoped LSCO kappa is found to decrease with increasing magnetic field in the T &rarr; 0 limit, unlike all previously measured superconductors, and most remarkably YBCO at a comparable low doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied field, we show that a novel thermal metal-to-insulator transition takes place upon going from the super-conducting state to the field-induced normal state. We argue that the doping and field dependence of kappa in underdoped LSCO is a signature of competing superconductivity and spin-density wave order.We measure the low-temperature thermal conductivity, kappa, of single crystals in two high-temperature superconductors, Tl2Ba2CuO 6+delta (Tl2201) and La2-xSr xCuO4 (LSCO), as a function of carrier density (doping) and magnetic field. Thermal conductivity is a bulk probe of the low-energy ground state excitations. By measuring the thermal conductivity we attempt to distinguish ground states in the cuprates, and gain quantitative information on their properties.In the superconducting state, the thermal conductivity (in the  T &rarr; 0 limit) due to d-wave nodal quasiparticles can be directly related to the superconducting gap maximum, Delta0 , with few assumptions and no free parameters. In Tl2201, along with previous measurements on YBCO and Bi2212, we find Delta0 from thermal conductivity to be consistent with spectroscopic measurements of the gap. This argues for a simple d-wave BCS-like superconducting state throughout the phase diagram. In addition, following previous measurements in YBCO and BSCCO, we find kappa in the mixed state of Tl2201 to increase with applied field, qualitatively consistent with semi-classical theories.Finally, we show that the low-temperature (T &lt; 1 K) phonon conductivity, kappaph, increases with increasing doping in both LSCO and Tl2201. We argue that kappa ph is limited at low temperatures by phonon-electron scattering, which is inversely proportional to the electron-phonon coupling, lambda. The doping dependence of kappaph is explained by a decrease in lambda with increasing doping, in agreement with the quasiparticle velocity renormalization observed in angle-resolved photoemission. We speculate on relevance of this result to the mechanism of high-temperature superconductivity.
Notes: Thesis (Ph.D.)--University of Toronto, 2005.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 66-06, Section: B, page: 3187.