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 mea… Show more
“…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].…”
Section: Discussionmentioning
confidence: 98%
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…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.…”
Section: Electronic and Structural Changes At Thresholdmentioning
Ultrafast phase transitions induced by femtosecond light pulses present a new opportunity for manipulating the properties of materials. Understanding how these transient states are different from, or similar to, their thermal counterparts is key to determining how materials can exhibit properties that are not found in equilibrium. In this paper, we reexamine the case of the light-induced insulator-metal phase transition in the prototypical, strongly correlated material VO 2 , for which a nonthermal Mott-Hubbard transition has been claimed. Here, we show that heat, even on the ultrafast timescale, plays a key role in the phase transition. When heating is properly accounted for, we find a single phase-transition threshold corresponding to the thermodynamic structural insulator-metal phase transition, and we find no evidence of a hidden transient Mott-Hubbard nonthermal phase. The interplay between the initial thermal state and the ultrafast transition may have implications for other transient states of matter.
“…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].…”
Section: Discussionmentioning
confidence: 98%
“…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.…”
Section: Discussionmentioning
confidence: 99%
“…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.…”
Section: Electronic and Structural Changes At Thresholdmentioning
Ultrafast phase transitions induced by femtosecond light pulses present a new opportunity for manipulating the properties of materials. Understanding how these transient states are different from, or similar to, their thermal counterparts is key to determining how materials can exhibit properties that are not found in equilibrium. In this paper, we reexamine the case of the light-induced insulator-metal phase transition in the prototypical, strongly correlated material VO 2 , for which a nonthermal Mott-Hubbard transition has been claimed. Here, we show that heat, even on the ultrafast timescale, plays a key role in the phase transition. When heating is properly accounted for, we find a single phase-transition threshold corresponding to the thermodynamic structural insulator-metal phase transition, and we find no evidence of a hidden transient Mott-Hubbard nonthermal phase. The interplay between the initial thermal state and the ultrafast transition may have implications for other transient states of matter.
“…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 …”
Vanadium dioxide (VO 2 ) is a much-discussed material for oxide electronics and neuromorphic computing applications. Here, heteroepitaxy of VO 2 is realized on top of oxide nanosheets that cover either the amorphous silicon dioxide surfaces of Si substrates or X-ray transparent silicon nitride membranes. The out-of-plane orientation of the VO 2 thin films is controlled at will between (011) M1 /(110) R and (−402) M1 /(002) R by coating the bulk substrates with Ti 0.87 O 2 and NbWO 6 nanosheets, respectively, prior to VO 2 growth. Temperature-dependent X-ray diffraction and automated crystal orientation mapping in microprobe transmission electron microscope mode (ACOM-TEM) characterize the high phase purity, the crystallographic and orientational properties of the VO 2 films. Transport measurements and soft X-ray absorption in transmission are used to probe the VO 2 metal-insulator transition, showing results of a quality equal to those from epitaxial films on bulk single-crystal substrates. Successful local manipulation of two different VO 2 orientations on a single substrate is demonstrated using VO 2 grown on lithographically patterned lines of Ti 0.87 O 2 and NbWO 6 nanosheets investigated by electron backscatter diffraction. Finally, the excellent suitability of these nanosheet-templated VO 2 films for advanced lensless imaging of the metalinsulator transition using coherent soft X-rays is discussed.
“…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.…”
Section: Outlook: Towards Attosecond Soft X-ray Spectroscopymentioning
One contribution of 15 to a theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays' .High harmonic generation (HHG) of an intense laser pulse is a highly nonlinear optical phenomenon that provides the only proven source of tabletop attosecond pulses, and it is the key technology in attosecond science. Recent developments in highintensity infrared lasers have extended HHG beyond its traditional domain of the XUV spectral range (10-150 eV) into the soft X-ray regime (150 eV to 3 keV), allowing the compactness, stability and subfemtosecond duration of HHG to be combined with the atomic site specificity and electronic/structural sensitivity of X-ray spectroscopy. HHG in the soft X-ray spectral region has significant differences from HHG in the XUV, which necessitate new approaches to generating and characterizing attosecond pulses. Here, we examine the challenges and opportunities of soft X-ray HHG, and we use simulations to examine the optimal generating conditions for the development of high-flux, attosecond-duration pulses in the soft X-ray spectral range.This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.
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