FIZIKA A 8 (1999) 4 , 333-344

GROUND STATE SUPERCONDUCTING PHASE FLUCTUATIONS AS A PRECURSOR FOR STRONG CRITICAL FLUCTUATIONS IN HIGH-T_c SUPERCONDUCTORS

J. R. COOPER^a, D. BABIĆ^b,c, J. W. LORAM^a, WAI LO^a,d and D. A. CARDWELL^a

^aInterdisciplinary Research Centre in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom

^dTexas Center for Superconductivity, University of Houston, Houston,U.S.A.

Dedicated to Professor Boran Leontić on the occasion of his 70^th birthday

Received 26 November 1999; Accepted 28 February 2000

We analyse the reversible magnetisation and heat capacity of YBa₂Cu₃O_7-d in the "vortex liquid" state and find that both properties are reasonably well described by the 3D XY critical-fluctuation model. The free-energy density in the "vortex liquid" state has a particularly simple form over a wide range of fields (H) and temperatures (T). This leads us to a picture in which the presence of critical fluctuations in high-T_c superconductors is directly linked to the remarkably small number of overlapping Cooper pairs at T=0 and H=0 rather than low dimensionality or high temperatures.

PACS numbers: 74.72.-h; 74.25.Ha; 74.25.Bt; 74.40.+k
UDC 538.945

Keywords: high-T_c materials, magnetisation, heat capacity, critical fluctuations

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