Fluctuation-enhanced electron-phonon coupling in FeSe

arXiv:2602.20756 · cond-mat.supr-con, cond-mat.str-el · Submitted 2026-02-24 · Read on arXiv

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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.

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

cond-mat.supr-con, cond-mat.str-el

Submitted: 2026-02-24

Updated: 2026-02-24

Journal ref: Phys. Rev. B 114, 235105 (2026)

DOI: 10.1103/yhzg-7n5t

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 79/100

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

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.

Terminology

Summary

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 correlated electronic state near the nemato-structural transition temperature Ts.

Introduction and Motivation

FeSe is an iron-based superconductor exhibiting Tc = 9 K at ambient pressure Superconductivity is substantially enhanced in bulk samples under pressure, with Tc reaching 36 K at 10 GPa Substituting 17% of Se with S raises Tc to 11 K and intercalating alkali ions or molecular layers between the FeSe sheets can boost it above 40 K. These results demonstrate that even small modifications of the structure can lead to substantial changes in the properties of FeSe. Consequently, since superconductivity is a highly sensitive indicator of interactions between electrons and the related excitations of the spin and lattice degrees of freedom, FeSe serves as an excellent platform for disentangling the various types of coupling mechanisms that remain under debate in iron-based systems. Among the different internal and external perturbations, the application of uniaxial stress specifically targets the interplay of the lattice with predominantly orbital degrees of freedom.

Experimental Setup and Strain Configurations

The study employs two strain configurations to impose different symmetry distortions on FeSe (i) strain along a′ (ϵB1g) couples directly to the nematic B1g distortion that describes the transition of the quadratic 1-Fe unit cell into an orthorhombic one, and (ii) strain along a transforms the 1-Fe tetragonal cell into a diamond or rhombohedron (B2g symmetry; see Methods). This approach enables a direct comparison of different symmetry-breaking mechanisms and their impact on the lattice dynamics. The selection rules governing phonon Raman scattering are dictated by the symmetry of the crystallographic 2-Fe unit cell. For example, in the x′x′ configuration, only Aph1g and Bph1g Raman-active modes are observable for the P4/nmm (space group 129) symmetry.

Phonon Response to Strain Near the Transition

The nemato-structural phase transition in FeSe occurs at Ts = 89.1 K with crystal symmetry decreasing from tetragonal P4/nmm to orthorhombic Cmmn symmetry. The analysis focuses on the narrow temperature range around Ts where EPC effects are expected to be strongest. The asymmetry of the Aph1g mode appears in the unstrained crystal within a narrow temperature window around Ts, 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.

Strain-Dependent Mode Evolution

The results show that the direction and symmetry of the applied field dictate the lattice response and its coupling to electronic fluctuations.

  1. For B1g strain, which couples directly to nematic order, an additional A′1g peak appears again at higher temperature (94 – 106 K) compared to the unstrained case.

  2. For B2g strain distortion, the A′1g feature is observed over an extended temperature interval ranging from 88 to 106 K for low-magnitude strain (εlowB2g).

  3. The energy offset of the A′1g mode with respect to the Aph1g mode is controlled by the symmetry of the applied strain field and its coupling to electronic degrees of freedom.

Physical Interpretation

The emergence of the additional features in the A1g scattering channel reflects an interplay between lattice vibrations and electronic degrees of freedom in 11-type iron-based superconductors. In FeSe, the additional mode appears on the low-energy side of the Aph1g mode and vanishes once the system stabilizes into either the fully developed nematic phase below Ts or above the range of strong fluctuations. This is interpreted in terms of an enhanced EPC near the transition, where fluctuations of both electronic and lattice character are maximal. The X-R direction of the phonon dispersion particularly at the X point plays an important role in this behavior. In this critical region, acoustic phonons near the X point become exceptionally susceptible to local symmetry breaking, giving rise to the observed A′1g mode. The sensitivity of the FeSe lattice to the direction of applied strain and reveals that the oscillator strength and coherence of the A′1g mode are governed by the balance between nematic ordering and a modulation of the EPC by lattice distortions.

Conclusion on Coupling Mechanisms

The findings contribute to clarifying how structural, magnetic, and electronic degrees of freedom interrelate in FeSe. The pronounced, direction-dependent lattice response revealed here demonstrates the exceptional sensitivity of the EPC to weak symmetry-breaking perturbations. These results show that while phonon anomalies in FeS and FeSe may be traced back to acoustic branches near the zone boundary, the underlying driving mechanisms differ fundamentally. In FeSe it emerges dynamically from the coupling between critical fluctuations and low-energy lattice modes, amplified by the fluctuating Fermi surface topology near the M point. The pronounced, direction-dependent lattice response revealed here demonstrates the exceptional sensitivity of the EPC to weak symmetry-breaking perturbations.

Methods Summary

The FeSe single crystals were grown using chemical vapor transport (CVT). The measurements in the temperature range between 78 and 300 K were performed in KONTI CryoVac He-flow cryostat capable of a base vacuum of 10−6 mbar. The Raman scattering experiment was performed using a Jobin Yvon T64000 triple spectrometer in back-scattering micro-Raman configuration. For excitation we used a mixed-gas Ar+/Kr+ laser (Coherent Innova 70C) operating at the 514.5 nm emission line. The authors describe the two strain configurations employed in Fig. 1 and described in the Methods section. The Raman spectra were analyzed using Voigt profiles because they were superior to simple Lorentzians since the linewidths are very small. The extracted phonon energies and Lorentzian linewidths of the Aph1g and A′1g modes are presented in Fig. 4. The Grüneisen parameter γ was obtained from Eq. (2) as the only remaining free parameter.

Improvements for AI systems

  1. The AI system can predict phonon mode anomalies based on symmetry-breaking perturbations by analyzing The subtle asymmetry of the Aph1g mode appears in the unstrained crystal within a narrow temperature window around Ts and its subsequent resolution under strain, allowing for real-time characterization of lattice response to external fields.

  2. The system can quantify the directionality of electronic-lattice coupling by correlating strain configurations with phonon energy shifts, specifically noting that B1g strain leads to a blue shift (blue) of the phonon, while B2g strain causes a red shift (orange) with respect to the unstrained case.

  3. The AI can model and predict the evolution of vibrational linewidths under varying strain regimes by employing the standard expression for symmetric anharmonic decay and relating it to experimental observations where under ε low,high B2g, the peak becomes noticeably narrower at lower temperatures than in all other cases.

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 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.

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