Low-frequency chain and in-plane optical conductivities of detwinned YBa2Cu3Oy:: Slave-boson mean-field analysis of the t-J model

Based on a scenario in which the CuO2 plane and CuO chain is coupled together, we study theoretically the low-frequency chain and in-plane optical conductivities of detwinned YBa2Cu3Oy. Using the slave-boson mean-field approach to the t-J model and including the antiferromagnetic spin fluctuations in the plane, we elaborate that the chain is superconducting for y>6.67 while insulating for y < 6.67 due to the competition between the plane-chain coupling and the antiferromagnetic order in the chain, corresponding to a superconductor-insulator transition. Stemming also from the coupling between the plane and chain, a new peak emerges at a low frequency in the in-plane spectra in the superconducting state, while it disappears in the normal state. These results are consistent with very recent experiments.