Fluctuation-enhanced electron-phonon coupling in FeSe

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Abstract
The interactions among lattice, charge, and spin degrees of freedom fundamentally shape material properties. In FeSe, symmetry-breaking perturbations serve as highly sensitive probes of these couplings. Previous work has shown that defects and isoelectronic substitution can substantially alter these interactions, giving rise to additional phonon modes. In this study, uniaxial strain is employed as a tunable symmetry-breaking control parameter to probe the intrinsic lattice response in the absence of disorder. The temperature evolution of phonon excitations was examined with a temperature resolution of 3 K in the vicinity of the nemato-structural transition temperature T s , under strain applied along the 〈 110 〉 and 〈 100 〉 crystallographic directions. A subtle asymmetry of the A 1 g p h mode appears in the unstrained crystal within a narrow temperature window around T s , originating from the emergence of an additional mode in the fully symmetric channel. With applied strain, this feature becomes more distinctly resolved. The anomaly is attributed to modifications of the coupling between lattice and electronic degrees of freedom driven by the ordering fluctuations right above the nematic transition. These fluctuations may enhance the susceptibility of the electron-lattice system and enable two-phonon Raman scattering involving the acoustic mode near the X and R points. The presence of this two-phonon scattering depends on both the strength and the direction of the applied strain, indicating a high sensitivity of FeSe to local symmetry breaking.
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Funding Information
  • Institute of Physics Belgrade
  • Ministarstvo Prosvete, Nauke i Tehnološkog Razvoja (F-134)
  • Serbian Academy of Sciences and Arts
  • Science Fund of the Republic of Serbia
  • European Commission (101185375)
  • Deutscher Akademischer Austauschdienst (57703419)
  • The Slovenian Research and Innovation Agency (P1-0099)
  • Science and Technology Commission of Shanghai Municipality (22JC1410300)
  • Johannes Kepler Universität Linz
  • Basic Energy Sciences
  • Materials Sciences and Engineering Division
  • U.S. Department of Energy (DE-SC0012704)
  • Brookhaven National Laboratory

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