Phase correlation investigated by the Josephson plasma resonance in Bi2(Sr,La)2CuOy

Josephson plasma resonance (JPR) was investigated in the mixed state of Bi 2 (Sr,La) 2 CuO y (Bi-2201). The plasma frequency ω p was studied in detail, as a function of temperature, magnetic field and doping. The peculiar temperature dependence of the resonance field B p ( T ) in underdoped crystals was understood in terms of pinning effect with the assumption that JPR takes place in the whole sample at an average field, which was supported by the experimental data. B p ( T ) under the uniform field application in the temperature swept experiment showed no anomaly at the irreversibility line. ω p 2 ( B ) determined precisely using cavities was found to behave as B − μ with μ smaller than unity, which might suggest that a rather strong long-range correlation exists among vortices even in the liquid state.