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Femtosecond NEXAFS of Photoinduced Insulator-Metal Transition in VO2 Print

The grand goal motivating femtosecond studies of condensed-matter dynamics is to directly measure the structural pathways that connect different crystallographic, electronic, and magnetic phases of solids, as well as the short-lived transition states between reactants and products in chemical and biochemical reactions. Researchers from Berkeley Lab and the Université du Québec have taken a big step forward by adding femtosecond x-ray spectroscopy to the experimental toolkit with their first use of the laser-slicing technique to study the photoinduced metal–insulator phase transition in vanadium dioxide (VO2).

Ultrafast X-Ray Science

Only a moment’s reflection is enough to confirm that the world around us is in constant flux. That change is everywhere and ever present yields the conclusion that extracting the full story requires time-resolved experiments in which one can trace in detail on the time scale on which atoms move the pathways (including transient intermediate states) by which matter changes from one form to another (phase transitions). For example, the making and breaking of chemical bonds and the rearrangement of atoms, which occur on the fundamental time scale of a vibrational period (about 100 femtoseconds), ultimately determine the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes.

One of the areas of research in which femtosecond x rays are opening new horizons is the physics of phase transitions in correlated electron systems (i.e., those in which interactions between electrons cannot be ignored). Cavalleri et al. have examined a general question—how do electronic properties relate to the geometrical rearrangement of the atoms—in the context of the transition from an insulator that does not conduct electricity to a metal that does in the compound vanadium dioxide (VO2). Rather than varying external parameters such as temperature, pressure, or magnetic field, they studied the nonequilibrium pathway of these phase transitions after the material is impulsively excited with light.

In a series of experiments, the group has been investigating the VO2 transition in which an insulator with a monoclinic crystal lattice becomes a metal with a rutile structure when heated above 340 K. The controversial nature of the low-T, insulating phase, as well as substantial uncertainty over the roles of structural motion and electronic correlations in driving this phase transition, have fueled an enduring debate over several decades.

Schematic of the photoinduced insulator-to-metal transition in VO2. Following hole photodoping into the 3d|| band, the band gap collapses and a metallic phase is formed.

Rather than varying the temperature, the group studied the nonequilibrium pathway of the phase transition after impulsively exciting the material with light. Such “photodoping” can, in analogy with chemical substitution, favor relaxation of the system into a competing state (in this case a metal). The relevant electronic states for the phase transition derive from vanadium 3d states split in energy by crystal-field effects, period doubling in the crystal unit cell (dimerization) in the low-T insulator, and electron–electron correlations.

In their experiments, prompt photodoping by exciting electrons out of (and hence holes into) the valence band (formed from 3d|| states) of the low-T insulator and delocalization of the charge carriers in the spatially extended conduction band cause an ultrafast transition to the metallic state as the valence and conductions bands once separated by an energy gap now overlap. During this process, the structural dimerization of the low-T insulator coherently relaxes. Both processes occur on the 100-fs time scale.

Left: Atomic rearrangements in the transition from the insulating monoclinic phase of VO2 to the metallic rutile phase emphasizes pairing and tilting along the c axis in the period-doubled, insulating structure. Right: Static NEXAFS spectrum of VO2, as measured with 100-meV (dashed curve) and 4-eV resolution (full curve). The blue part of the spectrum refers to transitions from core vanadium 2p3/2 and 2p1/2 states; the red part of the spectrum refers to transitions from the oxygen 1s core levels.

To date, the lack of femtosecond x-ray sources that are tunable over a broad spectral range has hindered development of femtosecond x-ray spectroscopy. The technique of laser slicing now coming into use at the ALS is currently the only proven method to generate broadband x-ray pulses of femtosecond duration. With soft x-ray near-edge x-ray absorption spectroscopy (NEXAFS), the electronic states near the Fermi level that participate in the insulator-to-metal phase transition can be distinguished by measuring absorption from symmetry-selective core levels.

In their optical-pump/x-ray-probe experiments at bend-magnet Beamline 5.3.1, they used a flat-field imaging spectrometer to spectrally disperse the soft x rays transmitted through the sample, thereby capturing the entire absorption spectrum at once for each time delay between the laser excitation and the measurement. Earlier picosecond NEXAFS measurements in VO2 uncovered a red shift of the L3 edge to lower energy, corresponding to a collapse of the unoccupied states of 3d symmetry, and followed the quasi-equilibrium kinetics of the transition as metallic layer grew into the material.

Femtosecond x-ray experiments performed with laser-sliced x-ray pulses. Femtosecond laser pulses from the same titanium:sapphire (Ti:Sa) oscillator are used for sample excitation and for slicing the electron beam after separate amplification, ensuring absolute synchronization between pump and probe. A flat-field imaging spectrometer placed after the sample measures the spectra as a function of the pump–probe time delay.

Femtosecond NEXAFS measurements made at the vanadium 2p3/2 edge revealed a prompt increase in absorption immediately after photoexcitation, recovering within a few picoseconds. At the oxygen 1s resonance, the absorption coefficient also initially increased synchronously with that at the vanadium 2p3/2 resonance but then decreased (bleached) before relaxing on the same few-picosecond time scale as the signal at the vanadium edge.

Femtosecond dynamics of photodoping and of the ultrafast phase transition, as measured at the vanadium 2p3/2 and oxygen 1s resonances.

The observed behavior likely results from a combination of hole and electron photodoping, band-structure rearrangement, and dynamic shift of the core levels. Full, high-resolution x-ray absorption spectra at the higher-flux undulator Beamline 6.0.1 now under construction will make it possible to clarify the nature of this complicated interplay. For the moment, these data represent the first successful measurement of x-ray absorption on the fundamental time scale where the phase transition occurs.

Research conducted by A. Cavalleri, M. Rini, H.H.W. Chong, and R.W. Schoenlein (Berkeley Lab); S. Fournaux (Université du Québec, Institut National de la Recherche Scientifique, Canada); and T.E. Glover and P.A. Heimann (ALS).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); Natural Sciences and Engineering Research Council, Canada; and Canada Research Chair Program. Operation of the ALS is supported by BES.

Publications about this research: A. Cavalleri, M. Rini, H.H.W. Chong, S. Fourmaux, T.E. Glover, P.A. Heimann, J.C. Kieffer, and R.W. Schoenlein, “Band-selective measurement of electronic dynamics in VO2 using femtosecond near edge x-ray absorption,” Phys. Rev. Lett. 95, 067405 (2005); A. Cavalleri, H.H.W. Chong, S. Fourmaux, T.E. Glover, P.A. Heimann, J.C. Kieffer, B.S. Mun, H.A. Padmore, and R.W. Schoenlein, “Picosecond soft x-ray absorption measurement of the photoinduced insulator-to-metal transition in VO2,” Phys. Rev. B 69, 153106 (2004).