Studies of the High Transition Temperature Superconductor BISMUTH(2) STRONTIUM(2) Calcium COPPER(2) Oxygen(x) Using Low Temperature Scanning Tunneling Microscopy

A low temperature STM has been used to study superconducting single crystals of Bi_2Sr_2CaCu _2O_{x}. Topographical and atomic resolution images have been obtained at 4.2K. The crystal lattice parameter was found to be about 5.0 A, consistent with the observation at room temperature. We have observed surface steps caused by different termination along the c-axis due to weak bonding between the two BiO layers. We have also observed a superstructure with a 27 A period along the a(b)-axis. The vacuum tunneling conductance was a linear function of voltage when the STM tip was positioned over a region of flat surface. When the tip was placed over a region of faceted surface, BCS-like characteristics were observed. The apparent values of the gap were found to be distributed over a range from 29 meV up to 50 meV. This distribution is discussed in terms of directional tunneling of quasiparticles. The characteristics with the smallest apparent gap values had the largest conductance peaks, smallest peak broadening and lowest zero bias conductance, and for these the ratio 2Delta/k _{B}T_{c}=8.2. A significant feature of the tunneling characteristics was a small, but well defined, peak found at bias voltage of about 90 meV in all of the characteristics exhibiting a gap feature. A histogram of the positions of the second peak was centered about 90 meV with a spread of about 2 meV. These peaks may be caused by the bosonic spectrum of particle-hole pairs as described in the marginal fermi liquid theory. In the "soft" point contact studies, the piezo capability of the STM was utilized to delicately control the depth of penetration of the STM tip into the sample's outmost layers. Over a small range, we observed oscillations in the conductance vs. bias voltage superimposed on a linear conductance background. These features, which were found to be reproducible for different samples, and in measurements using different tips, cannot be explained as Coulomb staircase or multiple superconducting gap effects. These features are discussed using a model based on the idea of compound geometrical resonances.