Thomas Lee Elifritz

http://arxiv.org/abs/1207.3486

Evolution from a nodeless gap to d_x²-y² form in underdoped La_2-xSr_xCuO₄, E. Razzoli, G. Drachuck, A. Keren, M. Radovic, N. C. Plumb, J. Chang, J. Mesot and M. Shi

To summarize, our main experimental findings are: 1) for highly underdoped LSCO (x = 0:08), in the superconducting state the electronic excitations are gaped along the entire underlying FS; 2) the diagonal gap persists above T_c while increasing the temperature and/or reducing the hole-concentration; 3) whereas the gap size on zone boundary remains constant up to 150 K, the diagonal gap is temperature-dependent; 4) when the diagonal gap decreases to zero, it leaves behind a pure d_x²-y² energy gap, at which point a Fermi arc emerges and its length increases with temperature. Our observations in highly underdoped LSCO could be explained either by a strong fluctuating competing order which is different from the superconducting order parameter [16], or by a mixed d_x²-y² + id_xy gap function occurring when the doping is below a quantum critical point [25]. The entirely gapped Fermi surface in highly underdoped LSCO is a profound departure from the d_x²-y² form observed at moderate to high doping. Further investigations of the origins of this nodeless gap function and its evolution to pure d_x²-y² form promise to shed light on how superconductivity emerges from the Mott insulating state in high-Tc cuprates.

lifeform@charter.net