Fluctuation-enhanced electron-phonon coupling in FeSe

summary

The gist

The gist: Uniaxial strain acts as a tunable symmetry-breaking control parameter to probe the intrinsic lattice response in FeSe, revealing that its vibrational properties are directly governed by its

In short

The study used uniaxial strain to probe how lattice vibrations (phonons) interact with electronic states in FeSe near its nemato-structural transition temperature ($T_s$). They found that strain reveals a subtle asymmetry in a specific phonon mode ($A_{1g}^{ph}$) that appears due to fluctuations right above the transition. This suggests that ordering fluctuations strongly enhance electron-phonon coupling, making the lattice highly sensitive to local symmetry breaking.

Key concepts

Nematic-structural transition ($T_s$): This is a phase change in FeSe where its crystal symmetry changes from tetragonal to orthorhombic. It happens at 89.1 K and is a critical point where electronic ordering begins to affect the lattice structure, making it a key area for studying material interactions.
Electron-phonon coupling (EPC): This describes the interaction where vibrations in the crystal lattice (phonons) influence or are influenced by the movement of electrons. In this study, strong EPC is expected near $T_s$ because electronic fluctuations are maximal, leading to enhanced phonon behavior.
Uniaxial Strain: This is a controlled physical method used in the experiment to intentionally break the crystal's symmetry by applying stress along specific crystallographic directions ($ ext{e.g., } ext{B}1g$ or $ ext{B}2g$). It acts as a tunable knob to see how FeSe's lattice responds differently based on which direction the strain is applied.
Phonon Modes ($A_{1g}^{ph}$ and $A'_{1g}$): These are specific vibrational states (quantized waves) within the FeSe crystal structure. The study focuses on how their energy and shape change when subjected to strain or near the transition, revealing details about the underlying electronic interactions that govern these vibrations.

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This episode discusses

The paper

Fluctuation-enhanced electron-phonon coupling in FeSe · Read on arXiv

Jovan Blagojevi´c, Ana Milosavljevi´c, Tea Belojica, Marko Opaˇci´c, Andrijana Solaji´c ˇ, Jelena Peˇsi´c, Enrico Di Lucente, Novica Paunovi´c, Milorad V. Miloˇsevi´c, Emil S. Bozin, Aifeng Wang, Cedomir Petrovic, Zoran V. Popovi´c, Rudi Hackl, Bojana Viˇsi´c, Nenad Lazarevi´c

Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade · Department of Applied Physics and Applied Mathematics, Columbia University, New York, USA · COMMIT, Department of Physics, University of Antwerp · Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame · Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory · Shanghai Advanced Research in Physical Sciences (SHARPS), Shanghai 201203, China · Center for High Pressure Science & Technology Advanced Research (HPSTAR) - Beijing · Department of Nuclear and Plasma Physics, Vinca Institute of Nuclear Sciences, University of Belgrade · Serbian Academy of Sciences and Arts · School of Natural Sciences, Department of Physics E51, Technische Universit¨at M¨unchen, 85748 Garching, Germany · IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany · Department of Condensed Matter Physics, Jozef Stefan Institute

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 fine temperature resolution 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 1g ph 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 enhance susceptibility for phonon-electron-phonon coupling in the vicinity of the X and R points of the Brillouin zone and promote two-phonon scattering close to the A 1g ph mode. 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.

DOI: 10.1103/yhzg-7n5t

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: I'm Kai, and with me are Mira and Lev, guest researcher.

Mira: Today's paper: "Fluctuation-enhanced electron-phonon coupling in FeSe".

Kai: The gist: Uniaxial strain acts as a tunable symmetry-breaking control parameter to probe the intrinsic lattice response in FeSe,

Mira: First, who's behind it and why it matters.

Paper summary: Kai: So, we're diving deeper into how this whole study on "Fluctuation-enhanced electron-phonon coupling in FeSe" works. The paper’s main thesis is that uniaxial strain is a tunable parameter used to probe the intrinsic lattice response of FeSe without adding disorder, which helps us understand these fundamental couplings.

Mira: They are specifically looking at how the temperature evolution of phonon excitations behaves right around the nemato-structural transition temperature, T s, which they set at eighty-nine point one K for this crystal under normal conditions. The core claim is that a subtle asymmetry in the A 1g ph mode shows up in the unstrained crystal near T s, and this feature gets much clearer when strain is applied <ref:2602.20756#pg1>.

Lev: So, what’s the real significance of seeing that asymmetry? Is it just an interesting phonon feature, or does it tell us something deeper about the underlying physics of how electrons and phonons talk to each other in iron-based systems?

Kai: It tells us that this anomaly is actually caused by modifications in the coupling between lattice and electronic degrees of freedom, which are being driven by those ordering fluctuations that happen right above the nematic transition. These fluctuations are what amplify the interaction.

Mira: They elaborate on what happens physically: these fluctuations boost susceptibility for phonon-electron-phonon coupling near specific parts of the Brillouin zone, like the X and R points. Plus, they promote two-phonon scattering close to that A 1g ph mode in this critical area <ref:2602.20756#pg1>.

Lev: If we take that two-phonon scattering claim seriously, it means the system is not just reacting linearly; it’s undergoing more complex interactions where multiple lattice vibrations are involved simultaneously due to the proximity of those electronic fluctuations. That adds a layer of complexity to the dynamics.

Kai: And what they found was that this two-phonon scattering isn't uniform; its presence depends on both the strength and the direction of the applied strain, which points directly to how sensitive FeSe is to local symmetry breaking.

Mira: This whole paper, "Fluctuation-enhanced electron-phonon coupling in FeSe," matters because it moves beyond looking at simple defects or substitutions. It shows that even applying controlled structural perturbations like uniaxial strain can reveal intrinsic lattice responses governed by the correlated electronic state near a phase transition.

Lev: For someone building quantum hardware, this means that if you’re trying to engineer a system where electron-phonon coupling is strongly influenced by symmetry, you need to be aware of these fluctuation-driven effects and how they depend on the strain direction.

Kai: So, the gist is that strain lets them see how critical fluctuations near T s dictate the lattice dynamics in FeSe. This sets a benchmark for probing these complex couplings using structural control rather than just relying on disorder.

Conclusion: Kai: So, looking at the full picture with this paper, "Fluctuation-enhanced electron-phonon coupling in FeSe," it’s really about mapping out the relationship between structure, electronic order, and lattice vibrations in iron selenide. The authors are showing that controlling strain lets them see how critical fluctuations amplify the way those vibrations couple to the electrons.

Mira: Essentially, what this means for us is that we can use controlled symmetry breaking—like applying strain—to isolate and study these specific coupling mechanisms that govern how structure influences electronic excitations near phase boundaries in these materials. It’s about separating out the driving forces from the resulting material properties.

Lev: I think the most important implication for error correction is that it gives us a clearer picture of where the noise originates. If you know it's driven by fluctuations near T s and strain-dependent, you can tailor your hardware to mitigate those specific types of dynamic noise.

Kai: Exactly. It moves the focus from just looking at static properties to understanding how dynamic, fluctuating electronic states dictate the behavior of the lattice itself under controlled stress. That sensitivity is a huge piece of information for anyone trying to design new quantum materials.

Mira: Ultimately, it confirms that in FeSe, the coupling isn't straightforward; it’s deeply intertwined with critical fluctuations near transitions and highly dependent on how you break that symmetry.

Lev: So, this work contributes to showing that these materials are incredibly responsive to small structural changes in a way that’s dictated by the electronic environment right at those critical points. It gives us concrete data on what we can expect when we push the lattice near its limits.

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