¹⁹⁵Pt-NMR evidence for opening of partial charge-density-wave gap in layered LAPt₂Si₂ with CaBe₂Ge₂ structure

We carried out ¹⁹⁵Pt-NMR measurements for a single-crystal LaPt₂Si₂, which is expected to show the coexistence of a superconducting state and a charge density wave (CDW) state. LaPt₂Si₂ crystallizes to a quasi-two-dimensional CaBe₂Ge₂-type crystal structure consisting of two types of Pt₂Si₂ layers: Pt1 and Pt2. From both the Knight shift, ¹⁹⁵K, and the NMR relaxation rate divided by the temperature, 1=T₁T, the electronic density of states at the Fermi level, N(ε_F), for only the Pt1 site decreases below the CDW transition temperature T⁺ = 112 K, whereas N(ε_F) for the Pt2 site does not change. These results suggest that the CDW occurs only in the Pt1 layers below 112 K. Furthermore, above T⁺, we also found that both¹⁹⁵K and 1=T₁T decrease upon cooling only for the Pt1 site. The temperature dependences of K and 1=T₁T for the Pt1 site can be explained by assuming a two bands model consisting of a wide band and a narrow band near the Fermi level. The CDW transition is expected to occur only in the narrow band of the Pt1 site, while the wide band of the Pt1-site does not contribute to the CDW transition. The present result strongly indicates that the CDW gap opens partially at the Fermi surface.