Uniform spin-chain physics arising from N-C-N bridges in CuNCN, the nitride analog of the copper oxides

We report on the unexpected uniform spin-chain physics in CuNCN, the insulating nitride analog of copper oxides. Based on full-potential band-structure calculations, we derive the relevant microscopic parameters, estimate individual exchange couplings, and establish a realistic spin model of this compound. The structure of CuNCN contains chains of edge-sharing CuN4 plaquettes. As a surprise and in contrast to analogous [CuO2] chains in edge-sharing cuprates, the leading magnetic interactions J similar or equal to 2500 K run perpendicular to the structural [CuN2] chains via bridging NCN groups. The resulting spin model of a uniform chain is in agreement with the experimentally observed temperature-independent magnetic susceptibility below 300 K. The nearest-neighbor and next-nearest-neighbor interactions along the structural [CuN2] chains are J(1) similar or equal to -500 K and J(2) similar or equal to 100 K, respectively. Despite the frustrating nature of J(1) and J(2), we assign the susceptibility anomaly at 70 K to long-range magnetic ordering, which is likely collinear with antiparallel and parallel arrangement of spins along the c and a directions, respectively. The pronounced one-dimensionality of the spin system should lead to a reduction in the ordered moment and to a suppression of the transition anomaly in the specific heat, thus impeding the experimental observation of the long-range ordering. Our results suggest CuNCN as a promising material for ballistic heat transport within spin chains, while the sizable bandwidth W similar or equal to 3 eV may lead to a metal-insulator transition and other exotic properties under high pressure.