~(123)Sb-NQR study of unconventional superconductivity in the filled skutterudite heavy-fermion compound PrOs_4Sb_(12) under high pressure up to 3.82 GPa

摘要

We report Sb nuclear-quadrupole-resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs_4Sb_(12) 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 A_(CEF) between the ground-state Γ_1 singlet and the first-excited-state Γ_4~((2) triplet decreases with increasing pressure, ac-susceptibility measurements indicate that the superconducting transition temperature (T_c) also decreases with increasing pressure. However, above P～2 GPa, both Δ_(CEF) and T_c do not depend on external pressure up to P =3.82 GPa. These pressure dependences of Δ_(CEF) and T_c suggest an intimate relationship between quadrupole excitations associated with the Γ_4~((2)) level and unconventional superconductivity in PrOs_4Sb_(12). 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 Δ(θ)=Δ_0 sin θ, with Δ_0=3.08k_BT_c. and 3.04k_BT_c for P= 1.91 and 2.63 GPa, respectively. The results indicate that there exist point nodes in the gap function.