The polarization field of the ferroelectric oxide lead zirconate titanate [Pb(ZrxTi1-x)O3] was used to tune the critical temperature of the hightemperature superconducting cuprate gadolinium barium copper oxide (GdBa2Cu3O7-x) in a reversible, nonvolatile fashion. For slightly underdoped samples, a uniform shift of several Kelvin in the critical temperature was observed, whereas for more underdoped samples, an insulating state was induced. This transition from superconducting to insulating behavior does not involve chemical or crystalline modification of the material.
We report the observation of a ferroelectric field effect in the conducting oxide SrRuO 3 using Pb͑Zr 0.52 Ti 0.48 )O 3 /SrRuO 3 epitaxial heterostructures. Upon reversing the polarization of the ferroelectric Pb͑Zr 0.52 Ti 0.48 )O 3 layer, we measured a 9% change in the resistance of a nominally 30 Å SrRuO 3 film at room temperature. This change was nonvolatile for a period of several days. Conductivity measurements taken between 4.2 and 300 K are consistent with n-type conduction throughout this temperature range. Hall effect measurements also yield n-type conduction, with nϷ2ϫ10 22 electrons/cm 3 , and furthermore allow us to understand quantitatively the magnitude of the observed resistivity change.
Epitaxial barriers of ferromagnetic SrRuO3 have been used to fabricate high Tc superconductor-normal-superconductor Josephson junctions in the edge junction geometry. At small barrier thicknesses the SrRuO3 junctions follow the behavior of nonferromagnetic but otherwise closely related CaRuO3 junctions. A rapid disappearance of critical current is observed when the barrier thickness is increased to 250 Å. Possible origins of such a large critical thickness are discussed.
We have studied the thermally activated flux-flow resistance in YBa2Cu307/PrBa2Cu307 (YBCO/PrBCO) artificial superlattices in magnetic fields perpendicular to the a-b plane. We find for multilayers with thick enough (> 24 A) insulating PrBCO layers and for YBCO thicknesses smaller than 300 A that the activation energy has a logarithmic magnetic-field dependence and that it is proportional to the YBCO thickness in the multilayer.
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