Ferromagnetic instabilities in atomically-thin lithium and sodium wires

Using density functional theory the ground state structural, electronic, and magnetic properties of monatomic lithium and sodium chains with low average density are investigated. A metallic, zigzag ground state structure is predicted but, most interestingly, stable equilibria for chains under tension are predicted to be {\it ferromagnetic}, which can be traced to exchange effects arising from occupation of the second subband as a function of the interatomic distance.