Evidence for point nodes in the superconducting gap function in the filled skutterudite heavy-fermion compound PrOs₄Sb₁₂: ¹²³Sb-NQR study under pressure

We report ¹²³Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavyfermion superconductor PrOs₄Sb ₁₂ under high pressures of 1.91 and 2.34 GPa. The temperature dependence of NQR frequency and the spin-lattice relaxation rate 1/T₁ indicate that the crystal-electric-field splitting Δ_CEF between the ground state Γ₁ singlet and the first excited state Γ₄⁽²⁾ triplet decreases with increasing pressure. The 1/T₁ below T_c = 1.55 K at P = 1:91 GPa shows a power-law temperature variation and is proportional to T⁵ at temperatures considerably below T_c, which indicates the existence of point nodes in the superconducting gap function. The data can be well fitted by the gap model Δ(θ) = Δ₀ sinθ with Δ₀ = 3.08k_BT_c. The relation between the superconductivity and the quadrupole fluctuations associated with the Γ₄⁽²⁾ state is discussed.