METHOD FOR MEASURING THE MOMENTUM-DEPENDENT RELATIVE PHASE OF THE SUPERCONDUCTING GAP OF HIGH-TEMPERATURE SUPERCONDUCTORS

The phase variation of the superconducting gap over the (normal) Fermi surface of the high-temperature superconductors remains a significant unresolved question. Is the phase of the gap constant, does it change sign, or is it perhaps complex? A detailed answer to this question wo uld provide important constraints on various pairing mechanisms. Here we propose a new method for measuring the relative gap phase on the Fe rmi surface which is direct, is angle-resolved, and probes the bulk. T he required experiments involve measuring phonon linewidths in the nor mal and superconducting state, with resolution available in current fa cilities. We primarily address La1.85Sr0.15CuO4, but also propose a mo re detailed study of a specific phonon in Bi2Sr2CaCu2O8.