Evolution of a metal to insulator transition in ${\mathrm{Ca}}_{2-x}{\mathrm{Na}}_x\mathrm{Cu}{\mathrm{O}}_2{\mathrm{Cl}}_2$ as seen by angle-resolved photoemission

We present angle resolved photoemission data on Na-doped ${\mathrm{Ca}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2.$ We demonstrate that the chemical potential shifts upon doping the system across the insulator to metal transition. The resulting low-energy spectra reveal a gap structure which appears to deviate from the canonical $d_{x^2-y^2}\propto |\cos {(k}_{x}a\left)-\cos {(k}_{y}a\right)|$ form. To reconcile the measured gap structure with d-wave superconductivity one can understand the data in terms of two gaps, a very small one contributing to the nodal region and a very large one dominating the antinodal region. The latter is a result of the electronic structure observed in the undoped antiferromagnetic insulator. Furthermore, the low-energy electronic structure of the metallic sample contains a two component structure in the nodal direction, and a change in velocity of the dispersion in the nodal direction at roughly 50 meV. We discuss these results in connection with photoemission data on other cuprate systems.