  We report on the 63/65Cu-nuclear magnetic resonance and nuclear quadrupole
resonance (NQR) studies of trivalent Yb zigzag chain compound YbCuS2. Sharp NQR
signals were observed in the paramagnetic (PM) state. Below TO = 0.95 K, the
multi peaks induced by the internal magnetic fields arising from the
antferromagnetic (AFM) ordered moments appear and coexist with the PM signal
down to 0.85 K, evidencing the first-order AFM phase transition at TO. In
addition, the nuclear spin-lattice relaxation rate 1/T1 abruptly decreases
below TO and shows the T-linear behavior below 0.5 K. The significant large
1/T1T value strongly suggests the presence of the novel gapless spin excitation
in low temperature region.
