Gapless spin excitations due to feasible nematic correlation in a frustrated zigzag chain of the magnetic semiconductor YbCuS2

We focus on a Yb-based semiconductor YbCuS2, which was recently proposed as the first substance exhibiting peculiar quantum phases due to the frustrated Yb3+ zigzag chain. A first-order phase transition at TO = 0.95 K exhibits reentrant behavior in magnetic fields. The ordered moments of the Yb ions are reduced to 0.3 muB/Yb and are incommensurately aligned on the ac plane, forming an elliptical helix structure. Numerical calculations and effective low-energy theories suggest that a finite anisotropic exchange term converts the spin dimer-singlet state into a coexisting nematic dimer-singlet state, where novel gapless spin excitations are expected. The current proposal uses high-resolution inelastic neutron scattering to directly observe the gapless excitations resulting from the development of spin nematic correlations in the Yb zigzag chain.