Imaging Nanometer Phase Coexistence at Defects During the Insulator–Metal Phase Transformation in VO₂ Thin Films by Resonant Soft X-ray Holography

Abstract

We use resonant soft X-ray holography to image the insulator-metal phase transition in vanadium dioxide with element and polarization specificity and nanometer spatial resolution. We observe that nanoscale inhomogeneity in the film results in spatial-dependent transition pathways between the insulating and metallic states. Additional nanoscale phases form in the vicinity of defects which are not apparent in the initial or final states of the system, which would be missed in area-integrated X-ray absorption measurements. These intermediate phases are vital to understand the phase transition in VO, and our results demonstrate how resonant imaging can be used to understand the electronic properties of phase-separated correlated materials obtained by X-ray absorption.

Does Vo2 Host a Transient Monoclinic Metallic Phase?

“…Finally, we note that VO 2 , especially in thin films, can be inhomogeneous, with multiple insulating and metallic phases coexisting in equilibrium [25,[46][47][48]. Therefore, a complete understanding of the phase transition will ultimately need to include how these phases compete dynamically in real space during the light-induced transition [50].…”

“…This not only modifies the fluence required to drive the phase transition but can also change the initial phase of the material. Thin films are known to undergo phase separation as a function of temperature [25,[46][47][48]; thus, heating will drive the material away from a purely M 1 initial state. We have shown that there is already a significant volume fraction of a preexisting R phase for high-fluence excitation, but other thermodynamic phases such as M 2 or T [6] could also be present in the sample below our detectable limit.…”

“…The oxygen K-edge of VO 2 has been thoroughly studied, and changes in this spectral region through the phase transition are well understood. These XAS features have been used to highlight the role of electronic correlations [22,23], to identify the presence of other insulating phases [24], and to provide contrast in images of phase separation with high spatial resolution [25]. Dynamics in the soft x-ray region have been previously measured in transmission on picosecond [26] and femtosecond timescales [27] in freestanding samples.…”

Tailoring Vanadium Dioxide Film Orientation Using Nanosheets: a Combined Microscopy, Diffraction, Transport, and Soft X‐Ray in Transmission Study

“…Figure shows a schematic for such an experiment. We have taken the first step toward such experiments and have recently successfully reconstructed first images during the MIT of VO 2 using the holography with extended reference by autocorrelation linear differential operator reconstruction technique …”

Attosecond soft X-ray high harmonic generation

“…Such high fluxes have proved sufficient for diffraction-limited holographic imaging in the XUV [37]. Though the flux required to reach the diffraction limit increases as the wavelength decreases, the elemental, bond and magnetic sensitivity of SXR absorption make even nondiffraction limited SXR holography very valuable, for instance in imaging phase co-existence [38]. Holography and other coherent diffraction imaging schemes will be a key application for the next-generation SXR HHG sources.…”