We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.
The ultrafast photoinduced insulator-metal transition in VO 2 is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.
The competition between electron localization and de-localization in Mott insulatorsunderpins the physics of strongly-correlated electron systems. Photo-excitation, which redistributes charge between sites, can control this many-body process on the ultrafast In Mott insulators, conductivity at low energies is prevented by repulsion among electrons.This state is fundamentally different from that of conventional band insulators, in which Bragg scattering from the lattice opens gaps in the single particle density of states. The electronic structure of Mott insulators is, therefore, sensitive to doping. Photo-excitation, in analogy to static doping, can trigger large changes in the macroscopic properties viii .However, the coherent physics driving these transitions has not been fully observed because the many-body electronic dynamics are determined by hopping and correlation processes that only persist for a few femtoseconds.We report measurements of coherent many-body dynamics with ultrafast optical spectroscopy in the one-dimensional Mott insulator ET-F2TCNQ. Several factors make this possible: ET-F2TCNQ has a narrow bandwidth (~ 100 meV), which corresponds to hopping times of tens of femtoseconds; the material has a weak electron-lattice interaction; we use a novel optical device producing pulses of 9 fs at the 1.7 m Mott gap; we study this physics in a one-dimensional system, allowing the evolution of the many-body wavefunction to be calculated and compared with experimental data. The characteristics of this new peak are time dependent, as visualized in Fig. 2c, where we have normalized the reflectivity at each time step. Two contours are shown in Fig. 2c. On the blue side, a prompt red-shift and recovery of the resonance is observed, whereas the red side shows a longer-lived component, containing a damped oscillatory response at 25 THz. StaticRaman data on ET-F2TCNQ does not show any equivalent features, strongly suggesting that the oscillation is not due to coherent phonons, but of an electronic origin xiv .To investigate such dynamics, we used a one-dimensional Mott-Hubbard Hamiltonian for a half-filled chain, with N = 10 sites, with electron hopping, t, and onsite and nearest neighbourCoulomb repulsion U and V, where, c †l, and cl, are the creation and annihilation operators for an electron at site l with spin , nl, is the number operatorand nl = nl, + nl,. We described the initial state, where represents a many-body wavefunction with one electron per site and total spin-vector . This reflects the fact that, at room temperature, charges are localized, but posses no magnetic ordering.We calculate the static optical conductivity (see methods section) to find values of U, V and t that provide the best fit to the experimental results. The best fit, shown in Fig. 3c (t = -200 fs), gave U = 820 meV, V = 100 meV and t = 50 meV. It was not possible to fit the optical conductivity using U and t alone and inter-site correlation energy, V, was needed xv .These static parameters were used to fit to the...
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