Date of Award
Doctor of Philosophy
Dr. Lisa Paulius
The goal of our work is to gain more insight into the vortex dynamics in YBa2Cu307-8 single crystals with point defects induced by 9MeV proton irradiation. The position of the critical points from the first order melting transition line can be shifted in a controlled manner with the induced disorder. In order to accomplish a comparative study, different crystals and irradiation doses were used. The results were correlated using both electrical transport and ac specific heat measurements. Special attention was given to investigating the nature of the vortex state below the lower critical point o f the first order vortex melting transition.
Four YBa2Cu307-8 single crystals were used in the investigation with the purpose of establishing how universal the response to proton irradiation is. While the initial conditions of the crystals varied, we were able to obtain a consistent picture of the evolution of the vortex matter behavior as a function o f irradiation.
The introduction of a sufficiently high density of random point disorder is theoretically predicted to induce the presence of the vortex glass or a vortex molasses in the system. Vortices are predicted to become fixed in a random pattern with no long-range positional order. In this case, the first order melting transition is replaced by a second order phase transition and critical scaling behavior is expected in the vicinity o f transition.
One of the purposes of this study is to determine the nature of the vortex matter below lower critical point. To this end we searched for the signatures of the vortex glass and vortex molasses states. Although our specific heat results indicated the presence of a second order phase transition, our scaling attempts were not consistent with the presence of either the vortex glass or vortex molasses states. Therefore, the nature o f vortex matter in this region of the vortex phase diagram remains elusive.
Undreiu, Lucian M., "Influence of Uncorrelated Defects on the Vortex Phase Diagram in YBA2Cu3O7-6 Superconducting System" (2005). Dissertations. 1072.