Sb-NQR study of PrOs4Sb12 under high pressure

We report Sb-123 nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs4Sb12 under high pressure. The temperature dependence of NQR frequency and spin-lattice relaxation rate 1/T-1 indicate that the crystal electric field splitting Delta(CEF) between the ground state Gamma(1) singlet and the first excited state Gamma((2))(4) 4 triplet decreases with increasing pressure. However, it is found Delta(CEF) and T-c stay constant above P similar to 2 GPa revealed by Sb-NQR up to P = 3.82 GPa for the first time. Remarkably, the temperature dependence of 1/T1 below T-c = 1.55 K at P = 1.91 GPa shows no coherence peak just below Tc and power-law behavior well below Tc in contrast to the exponential decrease observed at P = 0. Present results indicate the superconductivity in PrOs4Sb12 under pressure has nodes in the superconducting gap function.