Interlayer tunneling models of cuprate superconductivity: Implications of a recent experiment

It is shown that, given a few generic assumptions, any theory of high- temperature superconductivity that attributes a substantial fraction of the condensation energy to the saving of c-axis kinetic energy must predict an inequality relating the c-axis penetration depth λ₁ (0) to the zero- temperature superconducting-normal energy difference and the fluctuations of the c-axis kinetic energy around its mean value. Application of this formula to Tl₂Ba₂CuO₆ implies that if λ (0) is greater than 10 micrometers, as suggested by a recent experiment, these fluctuations must have an unusual form.