Global and local measures of the intrinsic Josephson coupling in Tl_2Ba_2CuO_6 as a test of the interlayer tunnelling model

摘要

One leading candidate theory of high-temperature superconductivity in the copper oxide systems is the interlayer tunnelling (ILT) mechanism. In this model, superconductivity is created by tunnelling of electron pairs between the copper oxide planes— contrasting with other models in which superconductivity first arises by electron pairing within each plane. The ILT model predicts that the superconducting condensation energy is approximately equal to the gain in kinetic energy of the electron pairs due to tunnelling. Both these energies can be determined independently, providing a quantitative test of the model. The gain in kinetic energy of the electron pairs is related to the interlayer plasma frequency, ω_J, of electron pair oscillations, which can be measured using infrared spectroscopy. Direct imaging of magnetic flux vortices also provides a test, which is performed here on the same samples. In the high-temperature superconductor Tl_2Ba_2CuO_6, both the sample-averaging optical probe and the local vortex imaging give a consistent value of ω_J≈ 28 cm~(-1) which, when combined with the condensation energy, produces a discrepancy of at least an order of magnitude with deductions based on the ILT model.