Magneto-optical measurements of the surface step of magnetic induction in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ single crystals: Direct evidence of the influence of the surface barrier

Surface steps of induction were observed at the sample edges in superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ crystals on magneto-optically measured induction profiles. A sharp upturn was found for the temperature dependence of the step value near 35 K. Analysis of the changes of the step value and the flux distribution near the sample edge with variation of external field shows that this upturn is caused by an increase of the effect of the Bean-Livingston surface barrier at low temperatures.