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 ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_{6+\mathit{x}}$, κ-(BEDT- TTF${)}_2$Cu(NCS${)}_2$, and ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ 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≲${\mathit{k}}_{\mathit{BT}\mathit{c}}$ at low temperatures and zero field, where Ω is the coherence volume, which probably includes most cuprate superconductors.