Observation of a New Mechanism of Spontaneous Generation of Magnetic Flux in a Superconductor

0031-9007= We report the discovery of a new mechanism of spontaneous generation of a magnetic flux in a superconductor cooled through Tc. Values of the spontaneous flux appear random from one cooldown to the next, following a Gaussian distribution. The width of the distribution increases with the size of the temperature gradient in the sample. Our observations appear inconsistent with the well-known mechanisms of flux generation. The dependence on the temperature gradient suggests that the flux may be generated through an instability of the thermoelectric superconducting-normal quasiparticle counterflow.