X-ray study of the charge-density-wave transition in single-layer TiSe2

Synchrotron x-ray studies of single-layer TiSe2 reveal displacements of the Ti and Se atoms as a function of temperature. The measurements, with a high sensitivity of 0.001 angstrom, show a (2 x 2) charge-density-wave (CDW) structure at temperatures below a critical temperature of T-C1 = 233 K. The temperature dependence follows a BCS-like second-order mean-field behavior. A five-layer TiSe2 film also exhibits a CDW transition of the same character but at a lower transition temperature of T-C5 = 204 K, which is the same as that for bulk TiSe2. The results demonstrate that lattice distortion is an integral part of the CDW transition that must also involve renormalization of the electronic structure.