Robert W. Schoenlein

Evidence for a structurally-driven insulator-to-metal transition in VO2 : a view from the ultrafast timescale

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

Femtosecond X-ray Pulses at 0.4 Å Generated by 90° Thomson Scattering: A Tool for Probing the Structural Dynamics of Materials

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Control of the electronic phase of a manganite by mode-selective vibrational excitation.

, 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-

Femtosecond Soft X-ray Spectroscopy of Solvated Transition-Metal Complexes: Deciphering the Interplay of Electronic and Structural Dynamics

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Photoinduced phase transition in VO2 nanocrystals: ultrafast control of surface-plasmon resonance

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

Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

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A setup for ultrafast time-resolved x-ray absorption spectroscopy

bands and is indicative of important bandlike character for this controversial insulator.

Optical switching in VO2 films by below-gap excitation

Pulses of x-rays 300 femtoseconds in duration at a wavelength of 0.4 angstroms (30,000 electron volts) have been generated by 90° Thomson scattering between infrared terawatt laser pulses and highly relativistic electrons from an accelerator. In the right-angle scattering geometry, the duration of the x-ray burst is determined by the transit time of the laser pulse across the ∼ 90-micrometer waist of the focused electron beam. The x-rays are highly directed (∼ 0.6° divergence) and can be tuned in energy. This source of femtosecond x-rays will make it possible to combine x-ray techniques with ultrafast time resolution to investigate structural dynamics in condensed matter.

Probing the hydrogen-bond network of water via time-resolved soft X-ray spectroscopy

Controlling a phase of matter by coherently manipulating specific vibrational modes has long been an attractive (yet elusive) goal for ultrafast science. Solids with strongly correlated electrons, in which even subtle crystallographic distortions can result in colossal changes of the electronic and magnetic properties, could be directed between competing phases by such selective vibrational excitation. In this way, the dynamics of the electronic ground state of the system become accessible, and new insight into the underlying physics might be gained. Here we report the ultrafast switching of the electronic phase of a magnetoresistive manganite via direct excitation of a phonon mode at 71 meV (17 THz). A prompt, five-order-of-magnitude drop in resistivity is observed, associated with a non-equilibrium transition from the stable insulating phase to a metastable metallic phase. In contrast with light-induced and current-driven phase transitions, the vibrationally driven bandgap collapse observed here is not related to hot-carrier injection and is uniquely attributed to a large-amplitude Mn–O distortion. This corresponds to a perturbation of the perovskite-structure tolerance factor, which in turn controls the electronic bandwidth via inter-site orbital overlap. Phase control by coherent manipulation of selected metal–oxygen phonons should find extensive application in other complex solids—notably in copper oxide superconductors, in which the role of Cu–O vibrations on the electronic properties is currently controversial.

Tracking the motion of charges in a terahertz light field by femtosecond X-ray diffraction

We present the first implementation of femtosecond soft X-ray spectroscopy as an ultrafast direct probe of the excited-state valence orbitals in solution-phase molecules. This method is applied to photoinduced spin crossover of [Fe(tren(py)3)](2+), where the ultrafast spin-state conversion of the metal ion, initiated by metal-to-ligand charge-transfer excitation, is directly measured using the intrinsic spin-state selectivity of the soft X-ray L-edge transitions. Our results provide important experimental data concerning the mechanism of ultrafast spin-state conversion and subsequent electronic and structural dynamics, highlighting the potential of this technique to study ultrafast phenomena in the solution phase.