Observation of 0 to pi transition in YBCO/Gallium oxide multilayers
  When two superconductors are coupled via an insulator the phase difference between them is expected to be zero in the ground state and such Josephson junctions are known as "0" type Josephson junctions . But in the presence of magnetic impurities in the barrier or ferromagnetism, the phase difference in the ground state could become pi; such Josephson junctions are known as "pi" type Josephson junctions1-4. Experimental results have confirmed the observation of a 0 to pi transition as a function of temperature in Josephson junctions such as Nb/Cu1-xNix/Nb based on ferromagnetic weak links with low exchange energy5. Recently, observation of possible magnetic behaviour in oxides has been reported6,7, and it would, therefore, be interesting to study the Josephson coupling in YBCO/gallium oxide multilayers. This work reports the observation of a very anomalous superconducting behaviour in YBCO/gallium oxide multilayers. A transition to the superconducting state has been observed with an enhanced superconducting transition temperature of 98 K in the YBCO/gallium oxide multilayer followed by a second transition temperature at 90 K with the multilayer losing its superconductivity at intermediate temperatures. This behaviour is explained by assuming that the Josephson coupling between the superconducting YBCO layers in the multilayer changes from pi type to 0 type. This is the first experimental observation of the 0 to pi transition in HTSC/gallium oxide/HTSC thin film structures. 
L. S. Vaidhyanathan, M. P. Janawadkar and D. K. Baisnab
cond-mat.supr-con