The mysterious normal state of TiSe2

The layered material TiSe2 is famous for several reasons, one of which is the discovery of a new state of matter “excitonium” [1]. This state involves the spontaneous formation and Bose-Einstein condensation of excitons, resulting in an elemental electronic crystal in which excitons play the role of atoms.


The formation of excitonium has been experimentally verified, but many of its physical properties are still unknown. Much of this enduring mystery is rooted in the normal state of TiSe2, found at high temperatures where no excitons exist. This “parent” phase of excitonium has been reported to be a semiconductor by some, and a semimetal by others, while yet others find it to be a normal metal.


The project

Supervisors: Dr. J. van Wezel and Dr. E. van Heumen

We will use optical spectroscopy [1] to probe the electronic structure of TiSe2 [2] over a broad energy range, and develop a theoretical model to interpret the data. The theoretical analysis will be based on a general order parameter (hydrodynamic) analysis as well as a more detailed microscopic model in the tight-binding approximation [3]. Obtaining predictions for the experimentally relevant response functions will involve the design and application of some quantum field theoretical techniques [4].


Relevant literature

  1. Previous optical experiments
  2. Discovery of excitonium.
  3. Tight binding model description.
  4. Comparison to ARPES