Persistent photoconductance effects in cuprate-based superconducting tunnel junctions

It is well known that the normal-state conductance and superconducting critical temperature of underdoped YBa₂Cu₃O_7-d (YBCO) can be persistently enhanced upon illumination with visible or UV light. These effects, called persistent photoconductivity (PPC) and photo-superconductivity (PPS), were investigated using YBCO thin films and attracted a great deal of attention over three decades ago. Despite a significant amount of work, a clear picture of the governing microscopic mechanism did not emerge, leaving open an ongoing debate between theories based on doping due to photocarrier excitation and those based on doping by the photoinduced oxygen ordering in the crystal structure. Motivated by this, we studied photoinduced in a different type of device, namely in micrometric tunnel junctions between a degenerate semiconductor transparent to visible and UV light (ITO) and a YBCO electrode. We found that these junctions show a very unique and rich behavior, with different regimes in which either persistent increase or a decrease of the conductance is produced after illumination depending on the electrical and optical history. Interestingly, as it will be discussed, these effects reflect the interplay of various mechanisms in addition to PPC, including photovoltaic effects and photo-activated oxygen migration across the tunnel barrier.