A key property of the cuprate superconductors

First, some trivia. Neville Mott wrote his seminal paper on the metal-insulator transition while at Bristol University. I am writing this in the H.H. Wills building where he worked and down the corridor from the Mott lecture theatre!

This week Nigel Hussey taught me something important about the cuprate superconductors: the electronic density of states (or effective mass) does not vary significantly with doping. This is inconsistent with the Brinkmann-Rice picture which predicts that the effective mass is inversely proportional to the doping.

This figure is taken from a paper by Wade et al.
The effective mass that one deduces from the optical conductivity also shows a weak doping dependence as summarised in the figure below, taken from this  PRB paper.