Towards the Optical Control of Resonantly Bonded Materials: An Ultrafast X-ray Study

Max Planck Quantum Matter Seminar

  • Date: Dec 10, 2021
  • Time: 11:00 AM - 12:00 PM (Local Time Germany)
  • Speaker: Yijing Huang
  • Stanford University
  • Location: SR I/II/III and online via Zoom
  • Host: Michael Först
There has been growing interest in using ultrafast light pulses to drive materials into nonequilibrium states with novel properties. In my study, I focus on a specific class of materials, the resonantly bonded materials with various functional properties including ferroelectricity, high thermoelectric figure of merit [1], and large change of optical constants upon crystallization and amorphization (or phase change materials). More importantly, this class of materials host a number of structural phases that are sensitive to external parameters including temperature, pressure and chemical doping. It will be very interesting to structurally probe these materials under photoexcitation and explore possible new functionalities. The large polarizability in resonant bonded materials means pronounced coupling between phonons and electronic states. Therefore, on top of probing the structural dynamics, we want to understand the photoexcited interatomic forces that drive the atomic motion, and quantify the electron phonon coupling.

Using time-resolved X ray scattering, I demonstrated that SnSe, one of the IV-VI resonantly bonded compounds [2], hosts a novel photo-induced lattice instability associated with an orthorhombic distortion of the rock-salt structure [3]. This lattice instability is distinct from the one associated with the high-temperature phase, providing a counterexample of the conventional wisdom that laser pump pulse serves as a heat dump. I will explain the driving mechanism for this new lattice instability. The results suggest that above-gap excitation can give selectivity for driving lattice instabilities, and motivate further studies to tune the pump-photon energy to material specific inter-band transitions aiming to transiently manipulate materials' functional properties, or even generate long-lived metastable states.

Furthermore, I will show results of non-zone-center measurements of time-resolved X-ray scattering [4], from which I extracted interatomic force constants in the photoexcited states. The results show the nonlinear pump fluence dependence of the interatomic forces, which can reveal information about the relaxation process of the carriers and the nature of the photoinduced lattice instability. This is only the second direct photoexcited interatomic force measurements to our knowledge, after the first one applied to a much simpler few-parameter dimer-like system of photoexcited bismuth [5]. Combining time-resolved X-ray diffraction with time-resolved ARPES (angular resolved photoemission spectroscopy) on Bi2Te3 and Bi2Se3 (V2-VI3 resonantly bonded materials) I extract specific deformation potentials, which are important parameters that quantifies the electron-phonon coupling [6].

[1] Lee, Sangyeop, et al. "Resonant bonding leads to low lattice thermal conductivity." Nature communications 5.1 (2014): 1-8.
[2] Behnia, Kamran. "Finding merit in dividing neighbors." Science 351.6269 (2016): 124-124.
[3] Huang, Yijing, et al. "Photoinduced Lattice Instability in SnSe." arXiv preprint arXiv:2106.07863 (2021).
[4] Trigo, Mariano, et al. "Fourier-transform inelastic X-ray scattering from time-and momentum-dependent phonon–phonon correlations." Nature Physics 9.12 (2013): 790-794.
[5] Teitelbaum, Samuel W., et al. "Measurements of nonequilibrium interatomic forces using time-domain x-ray scattering." Physical Review B 103.18 (2021): L180101.
[6] Gerber, S., et al. "Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser." Science 357.6346 (2017): 71-75.
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