Ultra-thin YBa2Cu3O7−x films with high critical current density

Despite impressive progress in the development of superconducting nanowire single-photon detectors (SNSPD), the main obstacle for the widespread use of such detectors is the low operating temperature required for low-temperature superconductors. The very attractive idea of increasing the operating temperature using high-temperature superconductors for SNSPD fabrication is problematic due to the insufficient quality of ultra-thin films from high-temperature superconductors, which is one of the key requirements for the single-photon detection by superconducting nanowires. In this work, we demonstrate the possibility of fabricating ultra-thin YBa2Cu3O7-x films on SrTiO3 substrates with a surface flatness of +/- 1 unit cell and a high critical current density up to 14 MA cm(-2) at T = 78 K. The critical current density of ultra-thin films had very low value in the first three unit cell layers adjacent to the substrate and reached nearly the bulk value at the fifth layer. 97% of the superconducting current is carried by only two upper layers of a 5-unit-cell thick YBa2Cu3O7-x film. Due to such superconducting current distribution over the film thickness and good surface flatness 5-unit-cell thick YBa2Cu3O7-x films could be promising for the fabrication of single-photon detectors.