Flux Phase in Bilayer $t-J$ Model: Time-Reversal Symmetry Breaking Surface State without Spontaneous Magnetic Field

We study surface states of high-$T_C$ cuprate superconductor YBCO using the bilayer $t-J$ model. Calculations based on the Bogoliubov de Gennes method show that a flux phase that breaks time-reversal symmetry (${\cal T}$) may arise near a (110) surface where the $d_{x^2-y^2}$-wave superconductivity is strongly suppressed. It is found that the flux phase in which spontaneous magnetic fields in two layers have opposite directions may be stabilized in a wide region of doping rate, and split peaks in the local density of states appear. Near the surface, spontaneous magnetic field may not be observed experimentally, because the contributions from two layers essentially cancel out. This may explain the absence of local magnetic filed near the (110) surface of YBCO, for which the sign of ${\cal T}$ violation has been detected.

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