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Center for Nonlinear Studies |
Cuprate superconductors have in common that they consist of parallel planes of copper oxide separated by layers whose composition can vary. Being ceramics, these superconductors are poor conductors above the transition temperature, Tc. Below Tc, the parallel Cu–O planes in those materials become superconducting while the layers in between stay poor conductors. As the material is cooled to below Tc, charge and current fluctuations along the now superconducting ab planes should increase. This should lead to an increased van der Waals / Casimir attractive force in between these layers, and this should be associated with a negative van der Waals / Casimir energy. Here, we ask to what extent the lowering of the inter-plane van der Waals/Casimir energy that arises when the parallel Cu–O layers become superconducting could contribute to the superconducting condensation energy. Could this change in energy pay for the energetic expense of the formation of Cooper pairs? This would of course not at all explain how, microscopically, Cooper pairs form. But it may give a hint that the sought-after microscopic mechanism that makes Cooper pairs form could be driven energetically by an effect that operates on a significantly larger scale: the van der Waals / Casimir effect emerges from the interaction of charge and current fluctuations on one layer with the charge and current fluctuations on a neighboring layer. To estimate the size of the effect, the material is here crudely modeled as consisting below Tc of parallel plasma sheets separated by vacuum and as not possessing a significant van der Waals / Casimir effect above Tc. It is found that below Tc, due to the close proximity of the Cu–O planes the system should be in the regime where TM "surface" plasmons, namely TM plasmons that propagate along the superconducting ab planes, dominate the van der Waals/Casimir effect. Within this crude model, the change in the van der Waals/Casimir energy when cooling through Tc is indeed found to be of the same order of magnitude as the superconducting condensation energy. This model implies that High Tc cuprates exhibit a predictable small but potentially measurable contraction in the c-direction as they are cooled through Tc.
Host: Alexander Gutfraind, gfriend@lanl.gov