Evidence for a structurally-driven insulator-to-metal transition inVO2: A view from the ultrafast timescale

Abstract

We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly correlated electron system. We discuss the case of the model system VO 2 , a prototypical nonmagnetic compound that exhibits cell doubling, charge localization, and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-T insulator. The insulator-to-metal transition is, however, delayed with respect to hole injection, exhibiting a bottleneck time scale, associated with the phonon connecting the two crystallographic phases. This structural bottleneck is observed despite faster depletion of the d bands and is indicative of important bandlike character for this controversial insulator. Correlated electron materials exhibit remarkable effects, ranging from metal-insulator transitions to nonconventional (high temperature) superconductivity. The subtle interplay between atomic structure, charge, spin, and orbital dynamics is responsible for many of the critical phenomena observed. 1 Importantly, because "simultaneous" changes in more than one degree of freedom are often observed as chemical doping or external parameters are tuned across critical values, time-integrated spectroscopies are unable to uniquely assign cause-effect relationships.Here, we demonstrate that time-resolved spectroscopy can instead be applied to overcome such ambiguities. We study the case of nonmagnetic VO 2 , a controversial, strongly correlated compound that exhibits cell doubling in "concomitance" with electron localization and a metal-insulator transition below 340 K 2 (see Fig. 1). The issue is whether the insulating behavior in the low-T phase derives directly from the Peierls distortion 3 or from electron localization and the consequent increase in electron-electron repulsion. 4,5 Recently, a theoretical study by Wentzcovitch et al. has revived attention into this four-decade-long debate, 6 suggesting that the former mechanism may be dominant, i.e., the low-T phase may be bandlike and the transition structurally driven. New controversy has resulted 7,8 and the problem is yet to be settled experimentally.Previous time-resolved optical 9 and x-ray diffraction 10 experiments in this compound demonstrated that impulsive photoexcitation of the low-T monoclinic insulator causes an ultrafast transition in both the electronic properties and the atomic structural arrangement. However, it was not clear whether the system becomes metallic due to the change in symmetry of the unit cell or to the prompt creation of holes, causing the closure of a Mott gap. We have now performed optical experiments with 15 fs resolution, and we report evidence of a limiting structural time scale for the formation of the metallic phase. This delay is observed despite much faster hole doping into the correlated d band. Such bottleneck time originates from the coherent optical-phonon distortions in the excited state of the system, mapping onto the crystal...

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

“…This value is in striking agreement with transition times t TF from thin-film studies 5 (Fig. 4b). s-SNOM imaging.…”

“…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.…”

“…While insulating VO 2 is clearly not a band insulator 16 , the importance of electron correlations and electron-phonon coupling for its properties remain unclear after more than half a century of study, and experimental results conflict with both Mott or Peierls explanations 7,[16][17][18][19] . For example, degenerate pump-probe studies 5 revealed a limiting transition timescale of 75 fs, suggesting a phonon bottleneck and therefore a structurally limited transition. In contrast, the observation of coherent phonon oscillations above the apparent threshold for triggering the photoinduced phase transition 7 indicates that the photoinduced IMT is decoupled from the structural transition.…”

Ultrafast changes in lattice symmetry probed by coherent phonons

“…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.…”

“…However, measurements of the rising edge of the optical transient suggested that the phase transition occurred on a timescale limited by the phonon modes of M 1 -phase VO 2 (ref. 21). Figure 1a shows the transient reflectivity of VO 2 measured using 800 nm, 40 fs pump and probe laser pulses for multiple pump fluences at room temperature.…”