Friday 18.12.2009, 13:00-14:00 SE 1.


Electronic structure of the layered dichalcogenide 1T-VS_2 as measured by soft X-ray angle-resolved photoemission

Mattia Mulazzi

Spring 8, Japan

In this seminar I will show you our photoemission measurements on the charge-density-wave (CDW) material VS_2. This material crystallises in the 1T structure having a distorted octahedral coordination. It was proved that VS_2 undergoes a CDW transition at 305K accompanied by the partial gapping of the Fermi surface, but the origin of the transition is unknown to date. By X-ray absorption and core-level photoemission we could infer that the electrons that build up the Fermi surface are derived only from the Vanadium atoms which are in a 3d1 configuration half filling the 3d_z2 orbitals. Employing angle-resolved photoemission (ARPES) excited by soft X-rays we could map the complete band dispersion and the Fermi surface of VS_2 throughout the whole Brillouin zone finding a strong degree of three-dimensionality. From the Fermi surface measurements it is possible to find a nesting vector joining straight sections of the Fermi surface. However, the nesting vector is different from tne CDW vector determined by diffraction measurements. This disagreement casts doubts on the nesting as the origin of the CDW transition for VS_2. At the end of the seminar I will discuss teh relevance of the electron-phonon coupling for the system and the momentum dependence of the electron-phonon coupling constant.