The reversible magnetisation of YBa2Cu3O7-delta crystals: 3D XY critical fluctuations and a field-dependent correlation volume

The reversible magnetisation of YBa2Cu3O7-delta crystals shows 3D XY scalin g behaviour over a wide doping range (0 less than or equal to delta less th an or equal to 0.35). In an extended region the reversible magnetisation is proportional to H-1/2, this corresponds to a free energy density of k(B) T in a correlation volume V-c(H, T) = xi(ab)(T)xi(c)(T)(Phi(0)/H)(1/2). All the reversible magnetisation curves can be collapsed on to a single curve b y taking into account changes of anisotropy and xi(ab) with delta. We discu ss some possible implications of this new result. (C) 2000 Elsevier Science B.V. All rights reserved.