Atomic motions in the layered copper pseudochalcogenide CuNCN indicative of a quantum spin-liquid scenario

Abstract

We explore the thermodynamic properties of the layered copper(II) carbodiimide CuNCN by heat-capacity measurements and investigate the corresponding thermal atomic motions by means of neutron powder diffraction as well as inelastic neutron scattering. The experiments are complemented by a combination of density-functional calculations, phonon analysis and analytic theory. The existence of a soft flexural mode—bending of the layers, characteristic for the material structure—is established in the phonon spectrum of CuNCN by giving characteristic temperature-dependent contributions to the heat capacity and atomic displacement parameters. The agreement with the neutron data allows us to extract a residual—on top of the lattice—presumably spinon contribution to the heat capacity , speaking in favor of the spin-liquid picture of the electronic phases of CuNCN.