We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa(2)Cu(3)O(7-delta) films through T(c). This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20 K/s, gave null results. In the present experiment, the quench rate was increased to >10(8) K/s. The amount of spontaneous flux increases weakly with the cooling rate.

We describe observations of spontaneous flux generation inside a YBa(2)Cu(3)O(7-delta) loop made of 214 Josephson junctions in series. The flux is generated spontaneously during cooldown into the superconducting state. The experiment is motivated by the Kibble-Zurek scenario of formation of topological defects in condensed matter systems. The transition from decoupled superconducting segments into a coherent loop is determined by the strength of thermal fluctuations in the junctions. Values of the flux measured at the end of each cooldown follow a normal distribution, and are consistent with the instantaneous phase differences across the junctions adding up as the loop becomes coherent.

We describe an experimental search for spontaneous formation of flux lines during a rapid quench of thin YBa 2 Cu 3 O 7−δ films through T c . This effect is expected according to the Kibble-Zurek mechanism of a creation of topological defects of the order parameter during a symmetry breaking phase transition.Spontaneously formed vortices were previously observed in superfluid 3 He, while a similar experiment in superfluid 4 He gave negative results. Using a high T c SQUID, we measured both the magnetic flux in the sample during a quench with a sensitivity of 20 φ 0 /cm 2 , and the field noise which one would expect from flux lines pinned in the film. The sensitivity was sufficient to detect spontaneous flux at a level corresponding to 10 −3 of the prediction.Within our resolution, we saw no evidence for this effect.

A noteworthy feature of the high-temperature superconductors is the unconventional symmetry of the superconducting order parameter. Several experiments have established that the order parameter has a four-fold d(x2 - y2) symmetry 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 might be present, having an isotropic s-wave or d(xy) symmetry. 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 appearance of a macroscopic magnetization at the superconducting transition temperature. Broken T symmetry has been reported, but searches for the effects of combined P and T symmetry breaking have so far yielded null results. Here we report the observation of a weak (approximately 10(-5) gauss) magnetic field that appears spontaneously at the superconducting transition temperature of epitaxial thin films of YBa2Cu3O(7-delta). The magnetic signal originates near the edges of the samples. One interpretation for this observation is that the order parameter carries an intrinsic angular momentum, related to the breaking of P and T symmetries, but other possibilities cannot yet be excluded.

We have imaged spontaneously created arrays of vortices (magnetic flux quanta), generated in a superconducting film quenched through its transition temperature at rates around 10(9)   K/s. The spontaneous appearance of vortices is predicted by the Kibble-Zurek and by the Hindmarsh-Rajantie models of phase transitions under nonequilibrium conditions. Differentiating between these models requires a measurement of the internal correlations within the emerging vortex array. In addition to short-range correlations predicted by Kibble and Zurek, we found unexpected long-range correlations which are not described by any of the existing models.