Strongly type-II superconductivity at high magnetic fields: Breakdown of thermodynamic scaling

The Bragg-chain model of the two-dimensional (2D) vortex state at high magnetic field is extended to an array of coupled superconducting (SC) layers. Application to YNi2B2C and MgB2 yields good quantitative agreement with high-field magnetization measurements, indicating that the smeared transitions observed in these materials are, at least in great part, due to SC fluctuations. The analysis indicates, however, that the thermodynamic properties of these materials, as well as of other strongly type-II superconductors, at low magnetic fields near the SC transition, are not connected to those at high magnetic fields (and low T) with a single scaling function, since the SC energy gap near T-c0 significantly underestimates the amplitude of the SC order parameter at high magnetic fields.