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Flux phase as possible time-reversal symmetry breaking surface states of high-$T_C$ cuprate superconductors

At a (110) surface of a $d_{x^2-y-2}$-wave superconductor, superconducting order is strongly suppressed. In such a situation, ordered states that are forbidden in the bulk may arise. This problem is studied for high-$T_C$ cuprate superconductors by treating the $t-J$ model with extended transfer integrals using the Bogoliubov de Gennes method. It is found that a flux phase with staggered currents along the surface, or an antiferromagnetic state can occur near the surface. Stability of the emergent surface states is different from system to system depending on the shapes of their Fermi surfaces. Possible relation to the experiments on the Kerr effect that suggest time-reversal symmetry breaking is discussed.