S 123 b -NQR study of unconventional superconductivity in the filled skutterudite heavy-fermion compound PrOs4 Sb12 under high pressure up to 3.82 GPa

We report S 123 b nuclear-quadrupole-resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs4 Sb12 under high pressure. The temperature dependence of NQR frequency and the spin-lattice relaxation rate 1/ T1 indicate that the crystal-electric-field splitting ΔCEF between the ground-state Γ1 singlet and the first-excited-state Γ4 (2) triplet decreases with increasing pressure. ac-susceptibility measurements indicate that the superconducting transition temperature (Tc) also decreases with increasing pressure. However, above P∼2/GPa, both ΔCEF and Tc do not depend on external pressure up to P=3.82/GPa. These pressure dependences of ΔCEF and Tc suggest an intimate relationship between quadrupole excitations associated with the Γ4 (2) level and unconventional superconductivity in PrOs4 Sb12. In the superconducting state, 1/ T1 below Tc =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 Tc. These data can be well fitted by the gap model Δ (θ) = Δ0 sinθ, with Δ0 =3.08 kB Tc and 3.04 kB Tc for P=1.91 and 2.63 GPa, respectively. The results indicate that there exist point nodes in the gap function.