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Direct time-domain determination of electron-phonon coupling strengths in chromium

机译：直接时域确定铬中电子 - 声子耦合强度

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摘要

We report the results of an ultrafast, direct structural measurement of optically pumped phonons in a Cr thin film using ultrashort x-ray pulses from a free-electron laser. In addition to measuring and confirming the known long-wavelength dispersion relation of Cr along a particular acoustic branch, we are able to determine the relative phase of the phonons as they are generated. The Cr sample exhibits two generation mechanisms for the phonons: the releasing of a preexisting charge density wave at higher frequencies, and the creation of an acoustic strain pulse via laser heating that dominates at lower frequencies. For the latter mechanism, we are able to measure the frequency dependence of the time required to generate the phonons. To explain the observed magnitude and slope of the delays, we perform first-principles simulations in the framework of density functional perturbation theory and ab initio molecular dynamics to fit anharmonic phonon models. These results show that the wave-vector dependence of the electron-phonon coupling is the driving mechanism behind the delay times: Phase-space limitation leads to higher times near the zone center. The absolute magnitudes of the delay limes measured are found to be much shorter than the equilibrium electron-phonon coupling times we compute, indicating that the coupling strength is greatly enhanced when the electronic system is out of equilibrium with the lattice, as has been seen in bismuth and other systems.
机译：我们报告了超快，使用来自自由电子激光的超短X射线脉冲在Cr薄膜中的光学泵浦声子的直接结构测量。除了测量和确认CR沿着特定声学分支的已知的长波长色散关系之外，我们能够根据生成而确定声子的相对相位。 CR样品对声子表现出两代机制：在较高频率下释放预先存在的电荷密度波，并通过激光加热产生声学应变脉冲，其在较低频率下占主导地位。对于后一种机制，我们能够测量生成声子所需的时间的频率依赖性。为了解释观察到的延迟的幅度和斜率，我们在密度函数扰动理论框架中执行第一原理模拟，并在AB Initio分子动力学框架中进行适应的Anharmonic源音质。这些结果表明，电子 - 声子耦合的波矢量依赖性是延迟时间后面的驱动机构：相空间限制导致区域中心附近的更高次。发现测量的延迟晶粒的绝对幅度比我们计算的平衡电子 - 声子耦合时间短得多，表明当电子系统与晶格中的平衡有大大提高的耦合强度，如已经看到的那样铋和其他系统。

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  • 来源
    《Physical review》 |2020年第4期|041101.1-041101.6|共6页
  • 作者

    J. Wingert; A. Singer; S. K. K. Patel; R. Kukreja; Matthieu J. Verstraete; Aldo H. Romero; V. Uhlir; S. Festersen; D. Zhu; J. M. Glownia; H. T. Lemke; S. Nelson; M. Kozina; K. Rossnagel; B. M. Murphy; O. M. Magnussen; E. E. Fullerton; O. G. Shpyrko;

  • 作者单位

    Department of Physics University of California San Diego La Jolla California 92093 USA;

    Department of Physics University of California San Diego La Jolla California 92093 USA Department of Materials Science and Engineering Cornell University Ithaca NY 14853;

    Department of Physics University of California San Diego La Jolla California 92093 USA Center for Memory and Recording Research University of California San Diego La Jolla California 92093 USA;

    Department of Physics University of California San Diego La Jolla California 92093 USA Center for Memory and Recording Research University of California San Diego La Jolla California 92093 USA Department of Materials Science and Engineering University of California Davis 1 Shields Avenue Davis California 95618 USA;

    nanomat/Q-MAT//CESAM Universite de Liege and European Theoretical Spectroscopy Facility allee du 6 aout 19 B-4000 Liege Belgium;

    Department of Physics and Astronomy West Virginia University Morgantown West Virginia 26505-6315 USA;

    Center for Memory and Recording Research University of California San Diego La Jolla California 92093 USA CEITEC BUT Brno University of Technology Purkynova 123 CZ-612 00 Brno Czech Republic;

    Institute for Experimental and Applied Physics Kiel University D-24098 Kiel Germany;

    LCLS SLAC National Accelerator Laboratory Menlo Park California 94025 USA;

    LCLS SLAC National Accelerator Laboratory Menlo Park California 94025 USA;

    LCLS SLAC National Accelerator Laboratory Menlo Park California 94025 USA Paul Scherrer Institut Forschungsstrasse 111 CH-5232 Villigen Switzerland;

    LCLS SLAC National Accelerator Laboratory Menlo Park California 94025 USA;

    LCLS SLAC National Accelerator Laboratory Menlo Park California 94025 USA;

    Institute for Experimental and Applied Physics Kiel University D-24098 Kiel Germany Ruprecht Haensel Laboratory Kiel University D-24098 Kiel Germany 12 Deutsches Elektronen-Synchrotron DESY D-22607 Hamburg Germany;

    Institute for Experimental and Applied Physics Kiel University D-24098 Kiel Germany Ruprecht Haensel Laboratory Kiel University D-24098 Kiel Germany;

    Institute for Experimental and Applied Physics Kiel University D-24098 Kiel Germany Ruprecht Haensel Laboratory Kiel University D-24098 Kiel Germany;

    Center for Memory and Recording Research University of California San Diego La Jolla California 92093 USA;

    Department of Physics University of California San Diego La Jolla California 92093 USA;

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