Thickness independence of field-induced time-reversal symmetry breaking inYBa2Cu3O7−δthin films

We have measured the field dependence of the Zero Bias Conductance Peak (ZBCP) of tunnel junctions prepared on (110) oriented Y1Ba2Cu3O7-d 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.