Ultrafast dynamics in VO2 under high pressures

Vanadium dioxide (VO2) is a classic example of a strongly correlated system demonstrating a sharp insulator-to-metal transition (IMT) at Tc = 340 K. Since the IMT occurs just above room temperature, VO2 has a high potential for applications in optoelectronic and electrical devices. Nevertheless, the microscopic mechanism of the IMT in VO2 is not yet fully understood. In particular, the roles played by the electronic correlation and the lattice distortion during the IMT are still under debate.
A pressure-induced metallization of VO2 above 10 GPa has been reported by Arcangeletti et al. using infrared spectroscopy [1]. Remarkably, high-pressure Raman spectra do not reveal qualitative changes indicating that the pressure-driven IMT in VO2 is not coupled to any structural distortion. Recently, the first time-resolved pump-probe study of VO2 under pressure has, indeed, revealed a nonequilibrium metallic phase with the monoclinic structure inherent to the insulating phase. [2]
Here, we report a systematic study of ultrafast pump-probe response in VO2 under pressure at different excitation fluences in order to clarify the nature of the pressure-induced changes in this material. The pump-probe measurements with 400 nm pump and 800 nm probe wavelengths were performed on single VO2 crystals mounted inside a diamond anvil cell. CsI has been used as a pressure-transmitting medium in order to ensure contact between the sample and the diamond anvil.
At low pressures of a few GPa, the photoexcited electrons relax on a time scale of about 0.5 ps. With increasing pressure, the relaxation time gradually becomes faster reaching the value of about 0.15 ps beyond 10 GPa. Furthermore, the application of pressure reduces the amount of pump energy which is required to induce a metastable metallic state. However, even close to 12 GPa, the sample demonstrates a dynamics typical of the insulating VO2. These preliminary results agree with recent high-pressure resistivity measurements that show a gradual crossover to the metallic state which is reached only above 30 GPa. [3]
References:
[1] E. Arcangeletti et al., Phys. Rev. Lett. 98, 196406 (2007).
[2] W.-P. Hsieh et al., Appl. Phys. Lett. 104, 021917 (2014).
[3] L. Bai et al., Phys. Rev. B. 91, 104110 (2015).