Dimensionality switching in electric conduction in FeS

Orbital degree of freedom plays an essential role in the properties of strongly correlated systems, and will be a key function in the next generation. The orbital state, namely, shape of electron cloud, can affect some macroscopic properties such as transport properties. We have indeed observed a dimensionality change in electric conduction in stoichiometric iron sulfide, FeS, as a consequence of change in orbital states. FeS crystallizes in a NiAs-based troilite structure and is an antiferromagnetic semiconductor below $T_{\rm N}$ = 600 K. At $T_{s} \sim$ 400 K, spin-axis transition, the rotation of antiferromagnetically ordered spins of iron atoms, occurs; the spins are directed along $c$-axis at low temperatures, and they rotate and lie in $c$-plane at $T_{s}$. This transition involves a change in orbital states of ${\rm Fe^{2+}}$ ions. The outermost electron of ${\rm Fe^{2+}}$ ions spreads in basal-plane below $T_{s}$, while it elongates in $c$-direction above $T_{s}$. This change strongly couples onto the electric conduction and induces its dimensionality change from two-dimensional below Ts to three-dimensional above $T_{s}$. We will discuss this change in orbital states and its appearance in electric conduction.