The “90 K” plateau of oxygen deficient YBa2Cu3O7−δ single crystals

Abstract Low field magnetization, resistivity, and specific heat measurements were performed on large, high quality YBa 2 Cu 3 O 7-δ single crystals in the range 0.05 T c goes through a small but well defined maximum near δ =0.12 whereas the jump in the specific heat at T c , Δ C p / T c , dramatically drops by a factor of three between δ =0.10 and 0.20.