Anomalous enhancement of nuclear spin relaxation rates of¹⁰⁹Ag and¹¹⁵ in at low temperatures in cubic Γ₃ ground-state system PrAg₂In - First observation of octupole fluctuations of f -electrons

Microscopic properties have been investigated on a cubic nonmagnetic non-Kramers Γ₃ doublet ground-state (GS) system PrAg ₂In by complementarily utilizing ¹¹⁵In (I = 9/2) and ¹⁰⁹Ag (I = 1/2) NMR with particular emphasis on the low-frequency (low-ω) dipole and multipole (octupole and/or quadrupole) fluctuations of f -electrons as probed by the nuclear spin relaxation rates 1/ ¹¹⁵T₁ and 1/¹⁰⁹T₁. We show that 1/¹¹⁵T₁ and 1/¹⁰⁹T₁ are anomalously enhanced respectively below ≅50 K and ≅100 K over those expected for the low-ω dipole fluctuations of the excited magnetic Γ₄ and Γ₅ states in a simple crystalline-electric-field model for a Γ₃ GS system. By comparing 1/(¹¹⁵T₁T) and 1/(¹⁰⁹T₁T) and also by considering an invariant form of the hyperfine and/or quadrupole couplings of Γ₃ octupole and/or quadrupole moments with Ag/In nuclear dipole and/or quadrupole moments, we show that Γ₃ octupole fluctuations dominate 1/ ¹⁰⁹T₁ and quadrupole ones can possibly contribute to 1/¹¹⁵T₁ at low T.