Doping and magnetic field dependence of in-plane tunneling into YBa2Cu3O7-x:: Possible evidence for the existence of a quantum critical point -: art. no. 177004

We present tunneling measurements into (1, 1, 0) YBa2Cu3O7-x films at various doping levels around the optimum. We find that, above a certain doping level near optimum doping, a spontaneous zero bias conductance peak splitting, delta, appears. It increases with doping. It also increases with magnetic field applied along the c axis, for both underdoped and overdoped films. The low field susceptibility chi = d delta /dH\(H-->0) is maximum, possibly diverging when the spontaneous value of delta goes to zero. These results suggest a transition from a pure d(x)2(-y)2 to a d + id(xy) or d + is order parameter.