New glassy features in high-T_c superconductors

Magnetic measurements on a high-Tc superconductor YBa 2Cu3O7 reveal new spin-glass-like features. At low temperatures and low fields the irreversible part of the field-cooled magnetization Mirr rotates with the sample as a rigid entity. The rigidity is broken and part of Mirr disappears above an angle φ* which decreases with increasing temperature or field. This behavior resembles that of spin glasses and differs qualitatively from the behavior found for Nb.