|M.Sc Student||Mor Yoash|
|Subject||Quantum Tunneling of Vortices in the High Temperature|
|Department||Department of Physics||Supervisor||Professor Emeritus Gad Koren|
|Full Thesis text|
Very soon after the discovery of the high temperature superconductors (HTSC), the resistive transition was observed to broaden under magnetic fields, instead of shifting to lower temperatures as was observed in conventional superconductors. This was explained as due to thermally activated vortex motion at high temperatures just below Tc, which gave rise to an induced voltage across the superconductor and thereby to a flux flow resistance Rff. At any given magnetic field in this regime, this resistance has the form of an Arrhenius law, Rff ~ exp(-U0/kBT), where U0 is the activation energy. Generally, flux flow and flux creep are possible at high temperatures where the pinning is relatively weak compared to the thermal energy. At low temperatures, where the pinning is stronger and thermal activation is much weaker, the dominant mechanism for flux motion is via quantum tunneling. Vortex tunneling in a two dimensional (2D) superconductor at temperatures much lower than the transition temperature Tc, and low magnetic fields, was discussed theoretically by Auerbach, Arovas, and Ghosh (AAG). They found that this tunneling occurs via variable range hoping (VRH) and results in flux tunneling resistivity which depends on temperature as exp[-(T0/T)1/3].
Cuprate films offer a unique opportunity to observe vortex tunneling effects due to their unusually low superfluid density and short coherence length. Therefore, in the present study, thin underdoped films of YBa2Cu3O7-x (YBCO) were patterned into a very long and narrow meander lines. The meander lines magnetoresistance (MR) at various magnetic fields was measured using a standard four point technique, and its dependence on the temperature was studied. Right below the resistive transition an Arrhenius activation behavior is obtained, while at low temperatures, the MR shows a significant deviation from this usual activation process. Instead, the data are found to be consistent with the AAG prediction of two dimensional variable range hopping (VRH) of single vortices which behave as, MR~exp[-(T0/T)1/3]. The VRH temperature scale T0 depends on the vortex tunneling rates between pinning sites. Using the MR temperature dependence, we were able to estimate the magnitude of T0, which was about 55 K at 2 T, and calculate the pinning site density per copper in a single CuO2 plane, which was found to be ~0.001.