APRES Study of Spin Spiral States in TlCo$_{2}$Se$_{2}$*

The electronic structure of TlCo$_{2}$Se$_{2}$ has been measured using high resolution angle resolved photoemission spectroscopy (ARPES). TlCo$_{2}$Se$_{2}$ is thought to be a spin spiral system, but experimental evidence from the electronic structure supporting the spin spiral states has been lacking. The original indication of a spin spiral state came from neutron powder diffraction experiments, and was confirmed in a subsequent neutron diffraction study of single crystals. However, it was not possible to distinguish between the helical structure and the sine-modulated moments since the intensity of the magnetic pattern satellites from neutron diffraction was very low. We used high resolution ARPES to study the electronic structure of single crystals of TlCo$_{2}$Se$_{2}$ cleaved in ultra high vacuum. Our experimental results clearly show the existence of band crossings near the Fermi level that would support spin spiral states, and reveals the predicted quasi-two dimensional electronic structure. The Boston University program is supported in part by the Department of Energy under DE-FG02-98ER45680.