Bulk phase transition inYBa2Cu3O7−δsingle crystals

Precise magnetization measurements of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals have found evidence for a bulk phase transition a few degrees below the superconducting critical temperature. The phase transition is indicated by a sudden drop of the lower critical field, {ital H}{sub {ital c}1}({ital T}), separating a full coherent state at low fields and temperatures, the Meissner state, from a region at higher temperatures where magnetic excitations are induced at {ital H}=0.