Vanadium dioxide (VO2) is a material that undergoes an insulator–metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron–electron correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state.
Bibliographical noteFunding Information:
We acknowledge the valuable discussions about optical design and principles of operation for customized terahertz instruments with Dr. Ian Gregory of TeraView Limited, Cambridge. Research at ETRI is supported by a principal project (18ZB1320). Work at UC-San Diego and Columbia University is supported by ARO-W911nf-17-1-0543. D.N. B. is the Gordon and Betty Moore Foundation’s EPiQS Initiative investigator, Grant GBMF4533. The development of THz nano-imaging is supported by ONR-DURIP: N00014-18-1-2737. The development of the cryogenic scanner and rapid sample exchange is supported by DE-SC-0012375 and DE-SC0018218. M.R. acknowledges the support by public grants from the French National Research Agency (ANR), project LACUNES No. ANR-13-BS04-0006-01, and the French–US Associated International Laboratory on Nanoelectronics funded by CNRS.
© 2018, The Author(s).