Direct characterization of photo-induced lattice dynamics in BaFe₂As₂

Ultrafast light pulses can modify electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved X-ray scattering to measure the lattice dynamics of photoexcited BaFe$_{2}$As$_{2}$. On optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A$_{1g}$ mode that modulates the Fe–As–Fe bond angle. We directly quantify the coherent lattice dynamics and show that even a small photoinduced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands

Ultrafast AMO physics at the LCLS x-ray FEL

The Linac Coherent Light Source at the SLAC National Accelerator Laboratory, Stanford University, began operation in 2009 as the world's first hard x-ray free electron laser. Early experiments have concentrated on atomic physics, and have demonstrated several key features of the ultrafast high field x-ray-atom interaction. This paper reviews some of these early results

Ultrafast AMO physics at the LCLS x-ray FEL

The Linac Coherent Light Source at the SLAC National Accelerator Laboratory, Stanford University, began operation in 2009 as the world's first hard x-ray free electron laser. Early experiments have concentrated on atomic physics, and have demonstrated several key features of the ultrafast high field x-ray-atom interaction. This paper reviews some of these early results

Ultrafast AMO physics at the LCLS x-ray FEL

The Linac Coherent Light Source at the SLAC National Accelerator Laboratory, Stanford University, began operation in 2009 as the world's first hard x-ray free electron laser. Early experiments have concentrated on atomic physics, and have demonstrated several key features of the ultrafast high field x-ray-atom interaction. This paper reviews some of these early results

Ultrafast myoglobin structural dynamics observed with an X-ray free-electron laser

Light absorption can trigger biologically relevant protein conformational changes. The light-induced structural rearrangement at the level of a photoexcited chromophore is known to occur in the femtosecond timescale and is expected to propagate through the protein as a quake-like intramolecular motion. Here we report direct experimental evidence of such 'proteinquake' observed in myoglobin through femtosecond X-ray solution scattering measurements performed at the Linac Coherent Light Source X-ray free-electron laser. An ultrafast increase of myoglobin radius of gyration occurs within 1 picosecond and is followed by a delayed protein expansion. As the system approaches equilibrium it undergoes damped oscillations with a ∼3.6-picosecond time period. Our results unambiguously show how initially localized chemical changes can propagate at the level of the global protein conformation in the picosecond timescale. © 2015 Macmillan Publishers Limited. All rights reserved156611sciescopu

Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho

Using femtosecond time resolved resonant magnetic x ray diffraction at the Ho L3 absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x ray energy to the electric dipole E1, 2p amp; 8594;5d or quadrupole E2, 2p amp; 8594;4f transition allows us to selectively and independently study the spin dynamics of the itinerant 5d and localized 4f electronic subsystems via the suppression of the magnetic 2 1 3 amp; 8722; amp; 964; satellite peak. We find demagnetization time scales very similar to ferromagnetic 4f systems, suggesting that the loss of magnetic order occurs via a similar spin flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra atomic 4f amp; 8722;5d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto striction leads to a transient shift of the magnetic satellite pea

The Photoactive Excited State of the B12-Based Photoreceptor CarH

7 pags., 5 figs.We have used transient absorption spectroscopy in the UV-visible and X-ray regions to characterize the excited state of CarH, a protein photoreceptor that uses a form of B12, adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.This work was supported by grants from the National Science Foundation NSF-CHE 1464584 and NSFCHE 1836435 to R.J.S., NSF-CHE 1565795 to K.J.K., NSFCHE 1608553 and NSF-CHE-1904759 to E.N.G.M., and NSF-CHE 1945174 to M.K.; from the Agencia Estatal de Investigación (AEI)-Spain and the European Regional Development Fund (FEDER) grants PGC2018-094635-BC21 (to M.E.-A.) and PGC2018-094635-B-C22 (to S.P.); and from the Fundacion Seneca (Murcia)-Spain grant 20992/PI/ ́ 18 (to M.E.-A.). Portions of this work were carried out in the Laboratory for Ultrafast Multidimensional Optical Spectroscopy (LUMOS) supported by NSF-CHE 1428479. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515