Rare-earth gallates, NdGaO3 and PrGaO3, as substrates lattice-matched to superconducting YBCO thin films are grown using a Czochralski pulling technique. NdGaO3 was twin-free, while PrGaO3 was badly twinned. Twinning in PrGaO3 is presumably due to the first-order phase transition below the melting temperature. On twin-free NdGaO3 substrates, the initial growth stages of YBCO ultrathin films are observed by an atomic force microscope (AFM). AFM reveals that the growth mode is of Stranski-Krastanov, showing two-dimensional lateral growth and subsequent three-dimensional island formation, where the growth unit is one or multiples of the c-axis cell length in height.
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