1110.0761 (C. Liu et al.)

Two-orbital quantum spin model of magnetism in the iron pnictides    [PDF]

C. Liu, D. -X. Yao, A. W. Sandvik

We study a two-orbital spin model to describe (pi,0) stripe
antiferromagnetism in the iron pnictides. The "double-spin" model has an
on-site Hunds's coupling and inter-site interactions extending to second
neighbors (inter- and intra-orbital) on the square lattice. Using a variational
method based on a cluster decomposition, we optimize wave functions with up to
8 cluster sites (up to 2^16 variational parameters). We focus on the
anomalously small ordered moments in the stripe state of the pnictides. To
account for it, and large variations among different compounds, we show that
the second-neighbor cross-orbital exchange constant should be ferromagnetic,
which leads to "partially hidden" stripe order, with a moment that can be
varied over a large range by small changes in the coupling constants. In a
different parameter region, we confirm the existence of a canted state
previously found in spin-wave theory. We also identify several other phases of
the model.

View original: http://arxiv.org/abs/1110.0761