Ph.D Student | Negri Ori |
---|---|

Subject | Noise in Normal Metal-High Temperature Superconductor Junctions |

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 G^{2} , 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 *q*_{eff} of the noise spectral density, *S=2q _{eff}I*,
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 *id*_{xy}. 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.