Ultrathin YBa2Cu3O7−x films were grown on SrTiO3 substrates in a high pressure
oxygen sputtering system to study the superconductor-insulator transition by
electrostatic charging. While backside gating using SrTiO3 as a dielectric induces
only small TC shifts, a clear transition between superconducting and insulating behavior
was realized in a 7 unit cell thick film using an ionic liquid as the dielectric.
Employing a finite size scaling analysis, curves of resistance versus temperature,
R(T), over the temperature range from 6 K to 22 K were found to collapse onto
a single function, which suggests the presence of a quantum critical point. However
the scaling failed at the lowest temperatures indicating the possible presence
of an additional phase between the superconducting and insulating regimes. In
the presence of magnetic field, a cleaner superconductor-insulator transition was
realized by electrostatic charging. A scaling analysis showed that this was a quantum
phase transition. The magnetic field did not change the universality class.
Further depletion of holes caused electrons to be accumulated in the film and the
superconductivity to be recovered. This could be an n-type superconductor. The
carriers were found to be highly localized.
By changing the polarity of the gate voltage, an underdoped 7 unit cell thick
film was tuned into the overdoped regime. This process proved to be reversible.
Transport measurements showed a series of anomalous features compared to chemically
doped bulk samples and an unexpected two-step mechanism for electrostatic
doping was revealed. These anomalous behaviors suggest that there is an electronic
phase transition in the Fermi surface around the optimal doping level.
University of Minnesota Ph.D. dissertation. November 2011. Major: Physics. Advisor: Allen M. Goldman. 1 computer file (PDF); vii, 91 pages, appendix.
Superconductor-insulator transition induced by electrostatic charging in high temperature superconductors..
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