A NEW BUFFER LAYER FOR HIGH-QUALITY HTSC ULTRATHIN-FILM FABRICATION

A new structure of buffer layer for ultrathin YBCO film is offered. Th is structure represents a dielectric material composed of insulator gr ains and HTSC grains. The latter serve as perfect nuclei while ultrath in HTSC YBCO film is grown on this buffer layer. The absence of lattic e mismatching between nuclei and an incipient film yield at the early stage of film growth a continuously connected structure that exhibits transport properties with full superconducting transition temperature higher than 80 K. As such a buffer layer we use Nb-doped YBCO material (YBCNO) composed of the elements (Y:Ba:Cu:Nb:O) in the proportion (1: 2:3 - x:x:7), where x > 0.29. Using this buffer layer a 3-unit-cell-th ick YBCO film with superconducting transition temperatures Delta T = 9 0-80 K and a metal-insulator-superconductor structure with a 4-unit-ce ll-thick YBCO film showing Delta T = 92-85 K were fabricated. To our k nowledge this is the best result obtained so far.