Synthesis of novel strontium-based cuprate superconducting thin films, and the relationship between their crystal structures and electrical properties

Novel Sr-based cuprate thin films were investigated to explore their potential as next generation superconducting materials. Thin films of infinite-layer compound (Sr,Ca)CuO2 (no blocking layer), cuprate oxycarbonate Sr2CuO2(CO3) (carbonate blocking layer), and Tl(Sr,Ba)2Can−1CunOy (n = 2 and 3) (thin blocking layer) were synthesized using metal-organic chemical vapor deposition. The structure and defect chemistry of the blocking layers of these cuprate compounds were found to have profound effects on the transport properties both in the normal state and the superconducting state.;By annealing in air, the infinite-layer compound SrCuO2 thin films reacted with the CO2 in air to generate Sr 2CuO2(CO3) thin films. Upon formation of carbonate blocking layers, charger carriers were introduced into the Sr2CuO 2(CO3) thin films through the partial substitution of carbon by copper or boron in the SrCO3 blocking layers. After oxygen annealing or upon boron substitution, the carrier concentration increased up to 10 21 cm−3. A superconducting onset temperature of 34K and a zero resistivity temperature of 20K have been observed for Sr 2CuO2(C1−xBx)O3 thin films. A critical carrier density of 0.10∼0.12 holes/Cu was required to render superconductivity.;The effect of crystal structure on the critical current density was investigated by measuring the vortex pinning energies of Tl2Ba2CaCu 2Oy (Tl-2212) and Tl(Sr,Ba)2Ca Cu2O y (Tl-(Sr,Ba)1212) thin films. A model based on thermally activated flux flow (TAFF) was developed to describe the vortex line motion. The activation energies of the TAFF process were extracted from the magnetoresistivity measurements. Because of the thinner blocking layers, the Tl-(Sr,Ba)-1212 thin film has a higher activation energy of 77 meV compared to 27 meV of the Tl-2212 thin film at 77K and 1 tesla. The activation process was ascribed to the formation of double kinks in the flux lines. The activation energies for Tl-based and Bi-based superconducting cuprates were found to decrease exponentially with the increase of blocking layer thickness.;Phase pure, epitaxial infinite-layer compound (Sr1−xCa x)CuO2 thin films were deposited on SrTiO3(100) substrates. However, these films were always semiconducting with resistivities of the order of 1 ohm-cm and with carrier concentrations of 1017∼10 19cm−3, which is two to four orders of magnitude lower than the typical superconducting cuprates. The low carrier concentration was attributed to the absence of blocking layers containing a sufficient concentration of charged defects. Transport was via variable range hopping conduction.