Electronic structure of the low-dimensional transition metal oxyhalide VOCl

In the quest for RVB-like superconductivity, layered oxyhalides of the form MOX (M=Ti,V; X=Cl,Br) have been discussed some fifteen years ago as possible candidates. This was due to their low-dimensional crystal structure involving frustrated triangular lattice planes. While no superconducting state in these Mott insulators was observed yet, other interesting phenomena, e.g. an unconventional spin-Peierls transition, have been found in TiOX. The isostructural material VOCl has a 3d2 configuration and shows antiferromagnetic ordering below 150K; magnetic susceptibility measurements show an anisotropy along the a-axis. Electronically, however, the degree of one-dimensionality observed by photoemission is reduced compared to the well-studied 3d1 TiOX systems. LDA+U calculations indicate that VOCl is a Mott insulator despite its two 3d electrons. Upon n-doping with alkali metals new states appear in the gap without having quasi-particle character, i.e. no evidence for a metallic phase is found. A comparison between VOCl and TiOX is expected to yield new insights into the importance of one-dimensionality and multi-band Mott-Hubbard physics in the oxyhalides.