Thickness independence of field-induced time-reversal symmetry breaking in YBa₂Cu₃O_7-δ thin films

We have measured the field dependence of the zero-bias conductance peak (ZBCP) of tunnel junctions prepared on (110) oriented YBa₂Cu₃O_7-δ films of various thickness. We have found that the field splitting of the ZBCP, believed to be due to the time reversal symmetry breaking (TRSB) of Andreev surface-bound states, is independent of film thickness. This rules out that Meissner screening currents are at the origin of TRSB. We conclude that TRSB must, instead, be due to a field-induced modification of the order parameter. Different possibilities involving an imaginary component are discussed.