Thermally Activated Dissipation in Superconducting Thin Films of Bismuth-Strontium

The resistance as a function of temperature of a wide variety of superconducting Bi_2Sr _2CaCu_2O _{8+delta} thin films has been measured in applied magnetic fields ranging from 0.01 to 15 tesla. The magnetic fields were applied parallel to the average c-axis of the highly -textured Bi_2Sr_2 CaCu_2O_{8+ delta} films. The lowest resistance portions of the transitions, 10^{-2 } rho_{n} <=q rho <=q rho_{n}, were found to be thermally activated with an activation energy well approximated by U(H,T) ~ A(1 - T/T_{c})/sqrt{H } over more than three orders of magnitude in magnetic field. Values of A varied from 890 K to 1760 K depending on the type of film measured. The field and temperature dependence of the activation energy, as well as its overall magnitude, were found to be consistent with theoretical models of thermally activated plastic deformations of the vortex lattice. The qualitative shapes of the transitions were the same for all films measured suggesting that this activation energy may be intrinsic to all Bi_2 Sr_2CaCu_2 O_{8+delta} films. Sputtered films were characterized by x-ray diffraction, scanning electron microscopy, scanning tunneling microscopy, and transport measurements of their resistive transitions, and critical current densities. All critical current densities were defined using an electric field criterion of 1 muV/cm. X-ray diffraction indicated that all films were highly-textured with the average c-axis normal to the (100) SrTiO _3 or MgO substrates. Films deposited at ambient temperature and annealed ex-situ were extremely rough with non-superconducting second phase surface precipitates and ~100 nm voids. These films had zero-field transition temperatures of 80-84 K and critical current densities of 5-7 times 10 ^4 A/cm^2 at 4.2 K. Films deposited at elevated temperatures were extremely smooth, with rms surface roughnesses of less than 5 nm over significant fractions of the films. These films had zero-field transition temperatures of 40-50 K and critical current densities in excess of 10 ^6 A/cm^2 at 4.2 K. The transition temperatures of films deposited at elevated temperatures could be increased to ~ 80 K by an ex-situ anneal with only a slight increase in the surface roughness. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.) (Abstract shortened with permission of school.).