Field-induced suppression of the phase transition in Bi_{2}Sr_{2}CaCu_{2}O_{8}

Magnetization measurements in field on a ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ single crystal do not reveal any sharp phase transition between the normal and superconducting states. The M(H) curves for temperatures about ${\mathit{T}}_{\mathit{c}}$ in fields up to 5 T all display diamagnetic behavior. Above the mean-field transition temperature ${\mathit{T}}_{\mathit{c}}^{\mathrm{MF}}$, quasi-two-dimensional diamagnetic fluctuations are responsible. Below ${\mathit{T}}_{\mathit{c}}^{\mathrm{MF}}$ strong deviations from the conventional mean-field (Abrikosov) behavior lead to an apparent ${\mathit{T}}_{\mathit{c}}$(B) inadequacy of the mean-field theory to describe the vortex state below the crossover line ${\mathit{H}}_{\mathit{c}2}$(T).