Persistent photoconductivity in high-temperature superconductors with nano-scale fragmented copper-oxide chains

We present studies of the relaxation of persistent photoconductivity in 60-K YB2C3Ox. By using thin films grown on vicinal SrTiO3 substrates, measurements of the in-plane (a–b plane) conductivity and of the out-of-plane (c-axis) conductivity were performed. The photo-induced enhancements of both conductivities showed the well-known stretched-exponential relaxation. We observed a small anisotropy of the relaxation rates of the two conductivity components. The relaxation of the c-axis conductivity appeared somewhat delayed compared to the a–b plane conductivity. In a simple thermal relaxation picture this finding implies different energy barriers for the c-axis and a–b plane relaxations, which is not consistent with generally accepted models of persistent photoconductivity in YB2C3Ox. We interpret the observed anisotropy as a consequence of the structural modifications of the copper-oxide chains that are induced by light illumination. Structural changes such as the re-ordering of the oxygen ions are slow processes compared to electronic excitations. Hence, the c-axis conductivity, which is more influenced by (small) structural modifications of the chain layer, needs a longer time to return to the equilibrium state after the cessation of the illumination.