Anisotropic superconducting gap in the spin-triplet superconductor Sr2RuO4:: Evidence from a Ru-NQR study

We have investigated a gap structure in the spin-triplet superconductor Sr2RuO4 through the measurement of the Ru-101 nuclear spin-lattice relaxation rate (101)(1/T-1) down to 0.09 K at zero magnetic field. In the superconducting state, 1/T-1 in a high-quality sample with T-c similar to 1.5 K exhibits a sharp decrease without the coherence peak, followed by a T-3 behavior down to 0.15 K. This result is in marked contrast to the behavior observed below similar to 0.4 K in samples with lower T-c, where T1T is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in Sr2RuO4 is significantly anisotropic, consistent with line-node-like models.