High Tc: Top Down or Bottom Up?

Abstract

We present and discuss recent tunneling results, which shed some light on the two contradicting approaches to the high-Tc problem. The “top-down” approach starts from the ansatz that the anomalous properties of the cuprates in their normal state above Tc need to be understood first, before the problem of superconductivity can be tackled successfully. On the other hand, in the “bottom-up” approach, both high Tc and the anomalous normal-state properties come about because of the vicinity of a quantum critical point. Our new tunneling results indicate that in the superconducting state, at some critical doping, the order parameter goes from a pure d-wave on the underdoped side to a complex one on the overdoped one. This symmetry breaking lends support to the hypothesis that there does exist a quantum critical point and, therefore, favors the “bottom-up” approach.