ESR of coupled spin-1/2 chains: unveiling zig-zag-type interchain   correlations
  By means of electron spin resonance investigations we revealed the crucial role of the interchain coupling in the spin dynamics of the spin-1/2 Heisenberg antiferromagnetic (AF) chain material copper-pyrazine-dinitrate, Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$. We found that the dominating interchain interaction is of a zig-zag type. This interaction gives rise to geometrical frustration effects and strongly influences the character of AF ordering. Combining our experimental findings with the results of a quasiclassical approach we argue that at low temperatures the system orders in an incommensurate spiral state. 
A. A. Validov, M. Ozerov, J. Wosnitza, S. A. Zvyagin, M. M. Turnbull,   C. P. Landee and G. B. Teitel'baum
cond-mat.str-el