Thermal conductivity of impurity-doped superconducting Bi-Sr-Ca-Cu-O whiskers
Abstract
The properties of conventional superconducting elements and alloys are well understood using the BCS model. The BCS model describes phonon mediated coupling of electrons that allows a lower energy state for a pair than for separate electrons. The discovery of superconducting compounds at higher temperatures in 1986 strained the BCS theory and the disagreement between theory and experiment continues 10 years later. Although numerous measurements have been made, a theory to describe high temperature superconductivity (HTS) that is predictive remains elusive. Thermal conductivity is well suited for probing phonon-electron interactions. The lattice and the charge carriers both play an important role in HTS compounds. In addition, thermal conductivity is a transport measurement that remains non-zero below the superconducting transition temperature. This allows useful information to be gathered for both carriers and the lattice through the superconducting transition. To reduce the effects of lattice defects, measurements on high quality single crystals are preferred. We have grown single crystals (whiskers) of Bisb2Srsb2Casb1Cusb2Osbx (Bi-2212) with dimensions of approximately 1000 x 100 x 10 mumsp3. A method of measuring the thermal conductivity of 25 mum wire and of whiskers is described using the classic longitudinal steady-state method. Data from a Pb-doped Bisb2Srsb2Casb1Cusb2Osbx (Bi-2212) whisker is verified with Nb, Pt and Au standards. Careful application of temperature matching shielding and micro-thermocouples have produced an absolute measurement with an error less than 25% over the temperature range 30 to 200 K. A complete accounting of significant errors is reported. Bi-2212 is a well studied cuprate superconductor with a high transition temperature of 84 K. These Bi-2212 whiskers have been doped with the 3d transition elements Ni, Co and Fe for Cu and have been well characterized by electrical transport and electron microprobe. Thermal conductivity results are shown for various doping levels and agreement with theory is discussed.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- November 1997
- Bibcode:
- 1997PhDT.......115V