26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Revealing interactions in the charge density wave material 1T-TiSe$_2$ with multi-dimensional coherent spectroscopy

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Condensed Matter & Materials (CMM)

Speaker

N. Stavrias (Swinburne University of Technology)

Description

The macroscopic presentation of ordered phases in strongly correlated materials such as superconductivity and charge density waves (CDW) are intricately linked to the interactions within and between the various elementary excitations. Ultrafast spectroscopy with temperature and fluence control is a powerful technique for disentangling these interactions.

1T-TiSe$_2$ is a CDW system which undergoes a structural phase transition below ~200 K leading to a periodic lattice distortion (PLD) and modulation in charge density. The Brillouin zone folds, new phonons appear, and the separation between the top of the valence band and the bottom of the conduction band increases, referred to as the CDW gap.

We measured 1T-TiSe$_2$ using multidimensional coherent spectroscopy (MDCS), a phase sensitive pump-probe technique. Using the four-wave mixing signal amplitude we explored the electron population and the coherent phonon dynamics across the photoinduced phase transition. The energy transfer within the material after the laser pulse was modelled using a 3 temperature models, where the hot electrons relax via multiple phonon systems. Consistent with previous results, as the fluence increased 3 regions were observed: i) where the CDW and PLD are present, ii) where the CDW has disappeared and the electron-phonon interaction is weakening and iii) where the PLD had also melted.

Resolving the phase of the MDCS signal revealed the dynamics of the CDW gap, which shifted in energy and included coherent oscillations. Good agreement was found between the experimental results and the order parameter in a time dependent Ginzburg-Landau model.

Disentangling the population, coherent phonon, and gap dynamics is critical to developing more detailed and complete models of this complex system where interactions play a key role in the macroscopic properties. Through measurements of 1T-TiSe$_2$ and other similar strongly correlated materials we hope to consolidate an understanding of this difficult but intriguing class of materials.