In the family of rare-earth nickel dicarbides, RNiC₂ (R = rare earth), a large variety of charge density wave (CDW) and magnetic states is found depending on the incorporated R. The observed changes in the characteristic charge ordering and its interplay with magnetism upon substitution are in part attributed to the chemical pressure exerted throughout the rare-earth series. Hence, the investigation of the effect of hydrostatic pressure on the RNiC₂ compounds is of particular interest to elucidate the role of pressure in this system.

SmNiC₂ exhibits an incommensurate CDW-1 with propagation vector q_CDW-1 = (½, ½+η, 0), η ≈ 0.02, below 148 K, evident from X-ray powder diffraction [1]. At T_C = 17.7 K, SmNiC₂ undergoes a first-order phase transition to a ferromagnetic state, coupled to an abrupt suppression of the CDW [1]. This strong interplay of the charge order and the magnetic order indicates a weakening of the Fermi surface nesting conditions responsible for the CDW formation in favor of the ferromagnetic ordering [2,3]. The existence of a second, commensurate CDW-2 with q_CDW-2 = (½, ½, ½) is implied by diffuse X-ray scattering up to room temperature [1].

To investigate the charge ordering in SmNiC₂, we have performed single-crystal X-ray diffraction experiments at simultaneously low temperatures and high pressures at the ESRF. Our findings show the suppression of CDW-1 at increased pressures and confirm the full formation of CDW-2. In addition, we observe a CDW with propagation vector q_CDW-3 = (½, ½, ξ), ξ ≈ 0.06, coexisting with CDW-1 and CDW-2, that has not been reported in the system RNiC₂ until now.

[1] S. Shimomura et al., Phys. Rev. Lett. 102, 076404, 2009

[2] J. Laverock et al., Phys. Rev. B 80, 125111, 2009

[3] J.N. Kim, C. Lee & J.-H. Shim, New J. Phys. 15, 123018, 2013