Evidence for Point Nodes in the Superconducting Gap Function in the Filled Skutterudite Heavy-Fermion Compound PrOs₄Sb₁₂: ¹²³Sb-NQR Study under Pressure

Abstract

We report 123 Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs 4 Sb 12 under high pressures of 1.91 and 2.34 GPa. The temperature dependence of NQR frequency and the spin-lattice relaxation rate 1/ T 1 indicate that the crystal-electric-field splitting Δ CEF between the ground state Γ 1 singlet and the first excited state Γ 4 (2) triplet decreases with increasing pressure. The 1/ T 1 below T c = 1.55 K at P = 1.91 GPa shows a power-law temperature variation and is proportional to T 5 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) = Δ 0 sin θ with Δ 0 = 3.08 k B T c . The relation between the superconductivity and the quadrupole fluctuations associated with the Γ 4 (2) state is discussed.