Seminars Archive

Inhomogeneity and disorder in the insulator-metal phase transition in VO2: unravelling the phase transition with optics and X-rays

Abstract
Vanadium dioxide typifies the difficulty in understanding how electron-electron interactions and electron-phonon interactions compete to dictate the properties of so-called "quantum" materials. The insulator-metal phase transition, in which both the electronic and lattice structures change, has continued to attract interest, both from an equilibrium and ultrafast perspective, for the last 50 years. In this talk I will review our current understanding of this phase transition from combining equilibrium and non-equilibrium X-ray and optical techniques, which show that sample inhomogeneity and ultrafast disorder play important roles in the equilibrium and non-equilibrium phase transition. Resonant soft-X-ray holographic imaging is used to image the thermally driven phase transition on the nanometre length scale in VO2 thin films, which shows that defects play a key role is generating phase separation in the material (L. Vidas et al. Nano Letters (2018)). I will then discuss the possibilities and difficulties in extending the holographic imaging technique to the time domain. Finally, I will discuss how vanadium disorder occurs with the first few tens of femtoseconds after optical excitation, measured through time-resolved diffuse X-ray scattering (S. Wall et al. Science (2018)). This latter point ultimately limits the ability to coherently control the phase transformation pathway, which may have important implications for our ability to control quantum materials in general.