Anisotropic spin fluctuations in the triangular Kondo lattice compound CePtAl 4 Ge 2 probed by site-selective Al 27 NMR

A site-selective ²⁷Al nuclear magnetic resonance (NMR) study is carried out on the Kondo lattice compound CePtAl₄Ge₂, which crystallizes in a rhombohedral lattice with quasi-two-dimensional Ce layers forming a triangular lattice network. Two inequivalent Al sites, Al(1) and Al(2), are unambiguously assigned by comparing measured nuclear quadrupole parameters with electric field gradients obtained from electronic structure calculations. Knight shift analysis yields distinct hyperfine coupling constants, revealing that they arise predominantly from Ruderman-Kittel-Kasuya-Yosida (RKKY)-type transferred hyperfine fields through conduction electrons. Spin-lattice relaxation measurements reveal pronounced anisotropic spin fluctuations, and a comparison of the relaxation rates between the two Al sites clarifies the momentum-space structure of these fluctuations. At low magnetic fields, (T₁T)^-1 is strongly enhanced on cooling toward the N\'eel temperature, indicating the growth of in-plane antiferromagnetic correlations in the paramagnetic state.