The characteristics of orthorhombicity of YBa 2Cu 3O 7− δ in superconducting state

Abstract

The temperature dependence of the lattice constants a, b and c of YBa 2Cu 3O 7−δ (δ=0.5) has been determined in a range 28–300 K by high-angle double-crystal X-ray diffractometry, and examined near the superconducting transition temperature T c=55 K. The angle φ that is related to the orthorhombicity of the crystal as φ=tan −1( a/ b) decreases with decreasing temperature; it is found, however, that at T c the decrease starts to slow down. The difference between φ observed and that extrapolated from the normal state is proportional to (1− T/ T c) ξ . This is new behaviour in connection with the onset of superconductivity, and is discussed in terms of a change in the bond strength around Cu atoms. By comparing the results with that of a specimen with δ∼0.2 (T c ∼90 K) in literature, it is shown that, owing to weaker bond strength around Cu atoms, the crystal with δ=0.5 exhibits the behaviour more clearly.