Investigating the Photo-Induced Ultrafast Insulator-Metal Phase Transition in Organic Cu(DCNQI)2 Salts by Ultrafast Electron Diffraction
December 4th, 2017 BART SMITH Stellenbosch University

Among functional organic materials, low-dimensional molecular metals represent an intriguing class of solids due to their tunable electronic, dielectric, magnetic and structural ground states. The radical anion salt Cu(DCNQI)2 (DCNQI: Dicyanoquinonediimine) marks a promising candidate for controlled and ultrafast optical switching between a Peierls insulating ground state and anisotropic metallic conductivity. To gain insight into the dynamics and mechanisms for optically driving structural insulator-to-metal phase transitions in organic solids, we studied the collective motion of atoms at the reversed Peierls transition of Cu(Me,Br-DCNQI)2 by ultrafast electron diffraction (UED). We observe the photo-induced lifting of the three-fold lattice periodicity within 2 ps in the entire crystal volume. Converting diffraction data into real space reveals two distinctly different molecular motions responsible for this transition: In general, a translational movement of the cyano groups initiates the phase transition. However, an additional internal pressure relieve by a translational motion of methyl and bromine groups at the quinone ring is required to optically lock the metallic state beyond 100 ps. Hence, we disclosed two microscopic pathways to optically drive the structural Peierls transition in Cu(DCNQI)2 which points out the tailored response to external stimuli available in this unique class of complex functional materials.

Seminar, December 4, 2017, 12:00. ICFO’s Seminar Room

Hosted by Prof. Jens Biegert