Point-contact tunneling study of high temperature superconductors

Tunneling measurements have been an important probe of high-temperature superconductors (HTS), providing a direct measure of the superconducting energy gap and giving insight into the quasiparticle density of states. The advantage and validity of point-contact tunneling on HTS will be addressed. We used this method to fabricate tunnel junctions on recently discovered polycrystalline HgBa{dollar}sb2{dollar}CuO{dollar}sb{lcub}rm 4+x{rcub}{dollar} superconductors with a T{dollar}sb{lcub}rm c{rcub}{dollar} onset of 97 K. Both superconductor-insulator-normal metal (SIN) and superconductor-insulator-superconductor (SIS') junctions have been studied using Au and Nb tips, respectively and consistent results are obtained. The data exhibit low and flat subgap conductances and are fit using a BCS density of states with a weak smearing parameter, {dollar}Gamma{dollar}. The ratio {dollar}Gamma{dollar}/{dollar}Delta{dollar} is typically in the range of 5%-7%, making this the lowest, reproducible value found on any cuprate superconductor. Typical gap parameter values are {dollar}Delta{dollar} = 13-16 meV. The data are more compatible with an s-wave pairing state, but d-wave cannot be ruled out due to the possibility of directional tunneling.; The point-contact tunneling study on the infinite-layer compound Sr{dollar}sb{lcub}rm 1-x{rcub}{dollar}Nd{dollar}sb{lcub}rm x{rcub}{dollar}CuO{dollar}sb2{dollar} showed that this electron-doped cuprate behaves like other electron-doped cuprates, such as Nd{dollar}sb{lcub}rm 2-x{rcub}{dollar}Ce{dollar}sb{lcub}rm x{rcub}{dollar}CuO{dollar}sb{lcub}rm 4-gamma{rcub}{dollar} and Pr{dollar}sb{lcub}rm 2-x{rcub}{dollar}Th{dollar}sb{lcub}rm x{rcub}{dollar}CuO{dollar}sb{lcub}rm 4-y{rcub}.{dollar} First, the observation of a similar linear background conductance in all of the electron-doped cuprates suggests a similar inelastic tunneling contributions from a broad continuum of states. Secondly, the energy gap of Sr{dollar}sb{lcub}rm 1-x{rcub}{dollar}Nd{dollar}sb{lcub}rm x{rcub}{dollar}CuO{dollar}sb2{dollar} is found consistently to be 6 meV, leading to 2{dollar}Delta{dollar}/k{dollar}sb{lcub}rm B{rcub}{dollar}T{dollar}sb{lcub}rm c{rcub}{dollar} = 4.0. The reproducibility of the measured gap values is similar to that found in other electron-doped cuprates and is consistent with the apparent s-wave symmetry found in Nd{dollar}sb{lcub}rm 2-x{rcub}{dollar}Ce{dollar}sb{lcub}rm x{rcub}{dollar}CuO{dollar}sb{lcub}rm 4-y{rcub}.{dollar} Finally, the gap region tunneling data of the Sr{dollar}sb{lcub}rm 1-x{rcub}{dollar}Nd{dollar}sb{lcub}rm x{rcub}{dollar}CuO{dollar}sb2{dollar} junctions all display subgap conductances, but there is no evidence of the Nb gap in the SIS{dollar}spprime{dollar} junctions, suggesting that the subgap conductance is of extrinsic origin.; As an extension of the above method, proximity-effect tunneling was utilized to study metal/superconductor bilayers and some preliminary results are presented.