Angular Dependence of the Nonlinear Transverse Magnetic Moment of YBa₂Cu₃O_6.95 in the Meissner State

The angular dependence of the nonlinear transverse magnetic moment of untwinned high-quality single crystals of YBa₂Cu₃O_6.95 has been studied at a temperature of 2.5 K using a low frequency ac technique. The absence of any signature at angular period 2π/4 is analyzed in light of the numerical predictions of such a signal for a pure d_x²-y² order parameter with line nodes. Implications of this null result for the existence of a nonzero gap at all angles on the Fermi surface are discussed.