Low-temperature increase of resistive critical fields in certain superconductors: A simple fluctuation approach

The resistive critical fields of several types of superconductor such as Tl2Ba2CuO6+x, κ-(BEDT- TTF)2Cu(NCS)2, and Ba1-xKxBiO3 show an unusual increase at low temperatures. We argue that this arises from thermodynamic fluctuations which are strongly enhanced because of the reduction in condensation energy density U(H,T) by a magnetic field. This effect should be significant for any material for which UΩ approx-lt kBTc at low temperatures and zero field, where Ω is the coherence volume, which probably includes most cuprate superconductors