123Sb-NQR study of unconventional superconductivity in the filled skutterudite heavy-fermion compound PrOs4Sb12 under high pressure up to 3.82 GPa

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 the 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) triplet decreases with increasing pressure. ac-susceptibility measurements indicate that the superconducting transition temperature (T-c) also decreases with increasing pressure. However, above P similar to 2 GPa, both Delta(CEF) and T-c do not depend on external pressure up to P =3.82 GPa. These pressure dependences of Delta(CEF) and T-c suggest an intimate relationship between quadrupole excitations associated with the Gamma((2))(4) level and unconventional superconductivity in PrOs4Sb12. In the superconducting state, 1/T-1 below T-c= 1.55 and 1.57 K at P= 1.91 and 2.63 GPa shows a power-law temperature variations and is proportional to T-5 at temperatures considerably below T-c. These data can be well fitted by the gap model Delta(theta) = Delta(0) sin theta, with Delta(0)= 3.08k(B)T(c) and 3.04k(B)T(c) for P= 1.91 and 2.63 GPa, respectively. The results indicate that there exist point nodes in the gap function.