Evidence for surface barriers and their effect on irreversibility and lower-critical-field measurements in Y-Ba-Cu-O crystals

Abstract

We present measurements of the lowest field for flux penetration of untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ crystals near their transition temperature ${\mathit{T}}_{\mathit{c}}$. This field decreases linearly with increasing temperature but exhibits an anomalous change in the slope in close vicinity of ${\mathit{T}}_{\mathit{c}}$. Low-dose electron irradiation reduces dramatically the first field for flux penetration. We discuss these results in terms of Bean-Livingston surface barriers which are proposed to be the origin for retardation in flux entry and for irreversible phenomena in the unirradiated crystals.