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 CuN(4)
squares. As a surprise, in contrast to analogous [CuO(2)] chains in
"edge-sharing" cuprates, the leading magnetic interactions J ~ 2500 K run
perpendicular to the structural [CuN(2)] 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
[CuN(2)] chains are J(1) ~ -500 K and J(2) ~ 100 K, respectively. Despite the
frustrating nature of J(1) and J(2), we assign the 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 ~ 3 eV may lead to
a metal-insulator transition and other exotic properties under high pressure.Comment: 10 pages, 5 figures. Submitted to Phys. Rev.