4D Visualization of Transitional Structures in Phase Transformations by Electron Diffraction

Baum, Peter; Yang, Ding‐Shyue; Zewail, Ahmed H.

doi:10.1126/science.1147724

Cited by 507 publications

(532 citation statements)

References 35 publications

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“…With the transition times observed below the 75-fs half-cycle period of the o V1,V2 phonon modes, the fastest being 40±8 fs, across a range of fluences, we deduce that the 150-fs timescale for breaking the V-V dimer bonds does not pose a rate-limiting step for the formation of the metallic state of the photoinduced transition, as originally proposed by Cavalleri et al 5,15 . Our range of timescales is similar to that reported by Wall et al 6 shown in Fig.…”

Section: Discussionsupporting

confidence: 80%

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

et al. 2015

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The insulator-metal transition (IMT) of vanadium dioxide (VO 2 ) has remained a longstanding challenge in correlated electron physics since its discovery five decades ago. Most interpretations of experimental observations have implicitly assumed a homogeneous material response. Here we reveal inhomogeneous behaviour of even individual VO 2 microcrystals using pump-probe microscopy and nanoimaging. The timescales of the ultrafast IMT vary from 40±8 fs, that is, shorter than a suggested phonon bottleneck, to 200±20 fs, uncorrelated with crystal size, transition temperature and initial insulating structural phase, with average value similar to results from polycrystalline thin-film studies. In combination with the observed sensitive variations in the thermal nanodomain IMT behaviour, this suggests that the IMT is highly susceptible to local changes in, for example, doping, defects and strain. Our results suggest an electronic mechanism dominating the photoinduced IMT, but also highlight the difficulty to deduce microscopic mechanisms when the true intrinsic material response is yet unclear.

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Section: Discussionsupporting

confidence: 80%

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

et al. 2015

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“…The increased symmetry reduces the number of Raman active phonon modes from 18 in the M 1 phase to 4 in the R-phase, without any significant mode softening near the transition temperature 18 . The same structural transition has been induced on the ultrafast timescale by exciting M 1 -phase VO 2 at room temperature with an intense 800 nm pump laser, using a pump fluence greater than F th~7 mJ cm − 2 , and observed by femtosecond X-ray 6,19 and electron diffraction 7 , as well as through changes in the optical 20,21 and electrical 22,23 properties. These experiments show that the complete transformation to the R-phase, after some initial fast dynamics, is a slow process, taking hundreds of picoseconds to complete.…”

mentioning

confidence: 70%

“…However, unlike bismuth, the transition in VO 2 involves complex changes in multiple phonons and results in a final state that is also an ordered solid. It should be emphasized that the establishment of the R-phase with long-range order is a slow process, occurring over tens of picoseconds 7 , that takes much longer than the observed loss in phonon modes 6,7,19 . The state of VO 2 immediately after photoexcitation is far from equilibrium, and the lattice potential is likely to continue to evolve to that of the R-phase during the thermalization process 30,31 .…”

Section: Discussionmentioning

confidence: 99%

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Ultrafast changes in lattice symmetry probed by coherent phonons

et al. 2012

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The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in Vo 2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

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“…Time resolved transmission electron diffraction (TR-TED) has been used, for example, to study bulk phenomena such as ultrafast laser-induced structural phase transitions [1], cooperative rearrangement of crystalline structures [2], and dynamics of charge density waves [3]. Time resolution of less than 100 fs [4] has been achieved.…”

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confidence: 99%

Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts

Zhou

Streubühr

Kalus

et al. 2013

EPJ Web of Conferences

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Abstract. We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter for the compensation of the velocity mismatch of electrons and light. The laser pulses with tilted fronts were characterized by a spatially resolving cross correlator. The response of the surface upon excitation was observed to be less than 2 ps.Ultrafast electron diffraction is a promising technique for studying the dynamics of the atomic structure on a sub-picosecond time scale. Time resolved transmission electron diffraction (TR-TED) has been used, for example, to study bulk phenomena such as ultrafast laser-induced structural phase transitions [1], cooperative rearrangement of crystalline structures [2], and dynamics of charge density waves [3]. Time resolution of less than 100 fs [4] has been achieved. Time resolved reflection high energy electron diffraction (TR-RHEED), on the other hand, has been used for the investigation of ultrafast surface dynamics, for example the observation of ultrafast heating of surfaces [5], the heat transfer in hetero structures [6] and laser-induced phase transition of the surface reconstructions [7]. However, the time resolution in these experiments was limited to tens of picoseconds because of the different velocities of electrons and light in combination with the grazing angle of incidence of the electron beam in RHEED geometry.This velocity mismatch can be overcome and the time resolution significantly improved by tilting the laser pulse front with respect to the phase fronts. Baum et al. [8] have demonstrated a configuration for pulse front tilting and achieved an improvement of the temporal resolution to less than one picosecond. Here we report the use of a 4f zero dispersion delay line capable of matching the pulse front of the laser to the electron pulse. The improvement of the temporal resolution is demonstrated by measuring the ultrafast lattice heating of Pb(111) islands on a Si(111) surface.Generally speaking, when a laser pulse encounters angular dispersion, the planes of constant amplitude (intensity) will be tilted with respect to the planes of constant phase, for example when the pulse is reflected from a diffraction grating [9]. The pulse then broadens during the free propagation after the dispersive component. The broadening is reversed and the original pulse width restored by passing the pulse through the two lenses in a 4f configuration. Our pulse tilter was designed to compensate the velocity mismatch for 30 keV electrons. Based on the results of our electron source [10] a temporal resolution of better than 700 fs is expected.In order to independently control and characterize the tilted pulse we used a cross-correlator based on second harmonic generation (SHG). The special feature of this cross-correlator is the possibility to perform spatially resolved measurements of the cross-correlation function between the This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, dis...

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4D Visualization of Transitional Structures in Phase Transformations by Electron Diffraction

Cited by 507 publications

References 35 publications

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2

Ultrafast changes in lattice symmetry probed by coherent phonons

Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts

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