  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.
