Motivated by recent experiments on spin chain materials K$_2$CuSO$_4$Cl$_2$ and K$_2$CuSO$_4$Br$_2$, we theoretically investigate the problem of weakly coupled spin chains (chain exchange $J$, interchain $J'$) subject to a $\textit{ staggered between chains}$, but $\textit{ uniform}$ within a given chain, Dzyaloshinskii-Moriya (DM) interaction of magnitude $D$. In the experimentally relevant limit $J' \ll D \ll J$ of strong DM interaction the spins on the neighboring chains are forced to rotate in opposite directions, effectively resulting in a cancelation of the interchain interaction between components of spins in the plane normal to the vector ${ \pmb D}$. This has the effect of promoting two-dimensional collinear spin density wave (SDW) state, which preserves U(1) symmetry of rotations about the $\pmb D$-axis. We also investigate response of this interesting system to an external magnetic field ${ \pmb h}$ and obtain the $h-D$ phase diagrams for the two important configurations, ${ \pmb h} \parallel { \pmb D}$ and ${ \pmb h} \perp { \pmb D}$.
Categories: cond-mat.str-el