|Ph.D Student||Negri Ori|
|Subject||Noise in Normal Metal-High Temperature Superconductor|
|Department||Department of Physics||Supervisor||Professor Michael Reznikov|
|Full Thesis text|
The project was aimed at the search for preformed Cooper pairs above and below the critical temperature (Tc) of high temperature superconductors (HTSC) using shot noise as a tool. In the simplest case of a current passing through a low-transmission barrier, the spectral density of the shot noise is proportional to the charge q of the carriers, S=2qI, therefore making it a tool for the charge measurement. In the case of Cooper pairs passing through a low-transmission barrier, such a current would generate doubled shot noise. However, In s-wave superconductor the current through the barrier would be proportional to G2 , where G is the transmission probability for an electron to pass through the barrier.
The situation is rather different in HTSC, where the gap symmetry is of d-wave type.
In a normal metal-HTSC tunnelling junction on a (110) plane, perpendicular to the nodal direction, the probability for a pair transmission is strongly enhanced. It happens due to the sign change of the pairing potential, which leads to formation of the zero energy bound surface states. These surface states give rise to large zero bias conductance peak (ZBCP). In this geometry, the noise generated by the Andreev reflection process competes with the quasiparticle noise, so that the effective charge qeff of the noise spectral density, S=2qeffI, should be larger than the electron charge e.
We used tunnelling junctions made of gold deposited on (110) and (100) surfaces of single crystal LSCO. We developed a setup for measurements of the current-voltage characteristic of the junctions, and the current noise generated by them at radio frequencies. This setup allows measurement of the differential conductance of the junctions at the same frequency as the noise.
We measured two types of samples: the low-resistance junctions, for which we observed ZBCP, and the high-resistance ones, which exhibit typical v-shape voltage dependence of the conductivity. In the high resistance junctions we observe non-Gaussian excess noise at energies in the vicinity of the superconducting energy gap. We attribute this noise to the instability in the junctions near the superconducting energy gap. At voltages above the gap we observed single-electron shot noise. In the low resistance samples we observed ZBCP, with splitting at temperatures much smaller than Tc. This splitting is related to the existence of additional superconducting order parameter with a symmetry of is, or idxy. At low temperatures we observe large Gaussian current fluctuations, beyond the single-electron shot noise limit. We attribute this noise to the Cooper pair tunneling into the bound surface states. At relatively high temperatures, even below Tc there are no ZBCP and the noise is dominated by 1/f fluctuations. Therefore, we cannot conclude on the existence of Cooper pairs above the transition temperature.