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Ultrafast Materials Science/Condensed Matter Physics

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Nonequilibrium electronic and structural dynamics of photo-excited materials: electron-phonon and phonon-phonon coupling, coherent phonon dynamics. photo-induced phase transitions in quantum materials, and related phenomena. Our group is part of the Stanford PULSE Institute and Stanford Institute for Materials and Energy Sciences (SIMES). Here we investigate the dynamics of materials under ultrafast light excitation using x-ray scattering methods to probe the microscopic processes governing their behavior. We are particularly interested in developing new methods to probe the fundamental interactions between low-lying excitations in materials and apply these methods to better understand how to generate and control understand novel states of matter with properties that are inaccessible in equilibrium. To this end, we often make use of the bright femtosecond pulses of hard x-rays from free-electron lasers such as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. Here the combination of the extremely brief pulse duration, short wavelength and high flux allow us to measure, directly in the time-domain, the coherent motion of atoms in the photoexcited materials. This allows us to extract the vibrational spectrum of the material and visualize energy transfer between electrons and the lattice (phonons) as well as phonon-phonon collisions.

Latest LCLS beam time

In a recent beam time, we studied the lattice dynamics on ferroelectric potassium tantalate, KTaO3.In our experiments, ultrafast near-UV light (wavelength 266 nm, 50 fs pulse duration) initiates the dynamics and the X-ray diffuse scattering probes the  hardening of the transverse acoustic phonon branches along Γ to X and Γ to M. Our approach allows us to demonstrate that photoexcitation transfers charges from oxygen 2p derived π-bonding orbitals to Ta 5d derived antibonding orbitals, which suppresses the ferroelectric instability and increases the stability of the cubic, paraelectric structure of KTaO3.

V. Krapivin, M. Gu, D. Hickox-Young, S. W. Teitelbaum, Y. Huang, G. de la Peña, D. Zhu, N. Sirica, M.-C. Lee, R. P. Prasankumar, A. A. Maznev, K. A. Nelson, M. Chollet, J. M. Rondinelli, D. A. Reis, and M. Trigo, Phys. Rev. Lett. 129, 127601