Large "forbidden" change in elastic modulus at the superconducting transition of Y1Ba2Cu3O9-x

The torsional modulus of Y1Ba2Cu3O9-x, measured by the composite oscillator method, shows a softening of 1% on cooling through the superconducting transition. The change is 3 to 4 orders of magnitude larger than that in an average modulus for a typical superconductor. For the high symmetry implied by our polycrystalline sample, furthermore, no change is allowed by symmetry for this mode at the superconducting (or any Landau-type) transition. The decrease in modulus is accompanied by an increase in damping.