Studies of Dimensional Crossover of the Charge Density Waves in TiSe₂

TiSe₂, a prototypical charge density wave (CDW) system with a 1T structure in the bulk, undergoes a simple (2 × 2 × 2) CDW transition at TB = 205 K in connection with the softening of a phonon mode at the zone boundary. We have successfully grown high quality single- and multi-layer films of TiSe₂ on a bilayer-graphene-terminated SiC surface via molecular beam epitaxy (MBE). Our angle-resolved photoemission spectroscopy (ARPES) and x-ray diffraction (XRD) measurements using synchrotron light sources of the single-layer film show a (2 × 2) CDW transition at T1 = 232 K, which is substantially higher than the bulk transition temperature TB. For the 2-layer case, ARPES shows a mixture of two transitions, one at T1 and the other at TB. The nature of this mixed transition is an interesting question that requires further investigation. We have now performed XRD studies of the 2-layer case. The results show an apparent single transition at T2 = 228 K, which is very close to T1, and no apparent second transition at TB. The results are intriguing, and we are performing theoretical simulations to unravel the underlying physics in terms of the static and dynamic lattice distortions.