SPIN LIQUID STATE AND THE TIME-REVERSAL SYMMETRY BREAKING EFFECT OBSERVED IN Bi₂Sr₂CaCu₂O_8+δ

The unusual properties of a doped high-T_c oxide can be attributed to its unique electronic structure, here we put emphasis on importance of the O2p band for the origin of magnetism and superconductivity. Antiferromagnetism of a charge transfer type insulator should be described by the Kramers-Anderson theory, the superexchange between localized Cu3d spins is mediated by O2p electrons. A doped hole resided in the O2p orbital in a Cu-O-Cu bond effectively suppresses the superexchange between Cu3d spins creating a local moment or a "Kondo impurity" in the magnetically ordered CuO₂ layer. The interaction between a local moment and O2p electrons yields an effective spin coupling between O2p electrons and pairing. Accordingly, itinerant O2p spins may condense into a paired state or a spin liquid state via local exchange interactions. On the contrary, Cu3d spins merely provide a magnetic background, the existing local field breaks the time-reversal symmetry in a photoemission process. We use a semiclassical approach to estimate the difference between the intensities of photocurrents generated by the left and the right circularly polarized photons observed in Bi₂Sr₂CaCu₂O_8+δ by Kaminski et al.