Spontaneous macroscopic magnetization at the superconducting transition temperature of YBa2Cu3O7-delta

A noteworthy feature of the high-temperature superconductors is the unconve ntional symmetry of the superconducting order parameter. Several experiment s(1-3) have established that the order parameter has a four-fold d(x2-y2) s ymmetry under rotation of the lattice (the order parameter of conventional superconductors is, in contrast, isotropic). An intriguing and much debated possibility is that, in certain cases, an additional imaginary component m ight be present, having an isotropic s-wave(4-6) or d(xy) symmetry(7-1)0. A consequence of a complex order parameter of the form d(x2-y2) + id(xy) is that it would break both reflection (parity, P) symmetry and time-reversal (T) symmetry, a clear signature of which would be the spontaneous appearanc e of a macroscopic magnetization at the superconducting transition temperat ure. Broken T symmetry has been reported(5,11), but searches for the effect s of combined P and T symmetry breaking have so far yielded null results(12 -15). Here we report the observation of a weak (similar to 10(-5) gauss) ma gnetic field that appears spontaneously at the superconducting transition t emperature of epitaxial thin films of YBa2Cu3O7-delta. The magnetic signal originates near the edges of the samples. One interpretation for this obser vation is that the order parameter carries an intrinsic angular momentum, r elated to the breaking of P and T symmetries, but other possibilities canno t yet be excluded.