Quantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide

Abstract

Solid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.

Ultrafast X-ray imaging of the light-induced phase transition in VO2

“…To address the role of nanoscale heterogeneity and phase separation, we use time-and spectrally resolved resonant soft X-ray coherent imaging at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) [28][29][30][31] to image the light-induced phase transition on the ultrafast timescale with nanometre spatial resolution. The power of this technique lies in the fact that it is a wide-field imaging technique that can exploit resonant X-ray spectroscopy both to provide a contrast mechanism between phases 32 and to enable the extraction of quantitative spectral information to aid phase identification on the nanoscale 33 . We report time-resolved imaging using two modes of operation, Fourier transform holography (FTH) and coherent diffractive imaging (CDI).…”

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

“…Key to resolving this issue will be to understand how inhomogeneity both before 35 and after excitation could impact interpretations based on TDGL theory. To this end, the need for techniques that can image the initial inhomogeneity as well as their dynamics 36 , 37 will be increasingly important 38 – 41 . Alternatively, disorder and fluctuations may stabilize the equilibrium high temperature phase in both VO 2 42 and LSMO 22 as both systems show large fluctuations above T c , and systems that are stabilized by entropy may show fundamentally different dynamics to those driven by soft modes.…”

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

“…The reversible semiconductor-to-metal transition, often referred to as metal–insulator transition (MIT), can be triggered by various stimuli, such as heat, light, mechanical pressure, and electric and magnetic fields, which opens the possibility for realizing switching devices between well-defined off (insulating) and on (metallic) states. In order to better understand the microscopic origin of the MIT, VO 2 has been investigated with state-of-the-art (pump–probe) spectroscopy techniques employing radiation all across the electromagnetic spectrum from hard and soft X-rays, to XUV, infrared, THz, microwave, and radio frequency, as well as electron microscopy. , The interested reader is referred to the review by Shao et al Our current understanding is that the electronic phase transition of VO 2 is inherently coupled to an accompanying structural phase transition from a monoclinic crystal structure at low temperature to a tetragonal structure at high temperature. The monoclinic phase is defined by a characteristic dimerization and tilt motif of the vanadium ions, whereas the tetragonal phase is isostructural to rutile TiO 2 .…”