Nematic-Fluctuation-Mediated Superconductivity Revealed by Anisotropic Strain in $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$

Nematic-Fluctuation-Mediated Superconductivity Revealed by Anisotropic Strain in $Ba ({Fe}_{1 - x} {Co}_{x})_{2} {As}_{2}$

Jean-Côme Philippe, Alexis Lespinas, Jimmy Faria, Anne Forget, Dorothée Colson, Sarah Houver, Maximilien Cazayous, Alain Sacuto, Indranil Paul, and Yann Gallais

Phys. Rev. Lett. 129, 187002 – Published 25 October 2022

Abstract

Anisotropic strain is an external field capable of selectively addressing the role of nematic fluctuations in promoting superconductivity. We demonstrate this using polarization-resolved elasto-Raman scattering by probing the evolution of nematic fluctuations under strain in the normal and superconducting state of the paradigmatic iron-based superconductor $Ba ({Fe}_{1 - x} {Co}_{x})_{2} {As}_{2}$ . In the parent compound ${BaFe}_{2} {As}_{2}$ we observe a strain-induced suppression of the nematic susceptibility which follows the expected behavior of an Ising order parameter under a symmetry breaking field. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the critical temperature $T_{c}$ , indicating a significant contribution of nematic fluctuations to electron pairing. Our results validate theoretical scenarios of enhanced $T_{c}$ near a nematic quantum critical point.

Received 27 April 2022
Revised 10 September 2022
Accepted 5 October 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.187002

Physics Subject Headings (PhySH)

Pairing mechanisms Quantum phase transitions Strain Superconducting phase transition

High-temperature superconductors Iron-based superconductors

Inelastic light scattering Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jean-Côme Philippe ¹, Alexis Lespinas¹, Jimmy Faria¹, Anne Forget ², Dorothée Colson², Sarah Houver¹, Maximilien Cazayous¹, Alain Sacuto¹, Indranil Paul¹, and Yann Gallais ^1,*

¹Université Paris Cité, Matériaux et Phénomènes Quantiques, UMR CNRS 7162, Bátiment Condorcet, 75205 Paris Cedex 13, France
²Service de Physique de l’Etat Condensé, DSM/DRECAM/SPEC, CEA Saclay, Gif-sur-Yvette 91191, France

^*yann.gallais@u-paris.fr

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Vol. 129, Iss. 18 — 28 October 2022

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Figure 1
Strain effect on the nematic ( $B_{2 g}$ ) Raman response for the parent compound ${BaFe}_{2} {As}_{2}$ ( $x = 0$ ). (a) to (f) Raman response obtained at 188 [(a),(b)], 145 [(c),(d)] and 118 K [(e),(f)], upon negative [(a),(c),(e)] or positive [(b),(d),(f)] strain. The sketch in the upper right displays the directions of the incident and scattered polarizations (blue arrows) with respect to the Fe atom square lattice. (g) Relative variation of $χ_{nem}$ upon strain with respect to the zero strain susceptibility at fixed temperature. No data were obtained at 154 K under tensile strain as the sample broke during the experiment. The dashed lines are guides to the eye using quadratic law [from Eq. (3)]. (h) Temperature dependence of $χ_{nem}$ at fixed strain, renormalized by the high-temperature susceptibility denoted $χ_{nem, T >}$ (taken here at 220 K under large strain). The dashed lines correspond to the expected theoretical behavior of $χ_{nem} (T)$ under weak and strong external fields [34]. For (g) and (h), the error bars take into account the uncertainty on the Drude-like low energy extrapolation of the spectra but not the uncertainty related to changes in spot or optical alignment that likely dominate the uncertainty in the temperature dependence [34].
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Figure 2
Strain effect on the nematic ( $B_{2 g}$ ) Raman response for $Ba ({Fe}_{1 - x} {Co}_{x})_{2} {As}_{2}$ ( $x = 0.07$ ). (a)–(d) Raman response obtained at 9 [(a),(b)] and 26 K [(c),(d)], upon negative [(a)–(c)] or positive [(b)–(d)] strain. (e) Relative variation of $χ_{nem}$ upon strain with respect to the zero strain susceptibility at fixed temperature. The dashed lines are guides to the eye obtained by following quadratic laws. The error bars take into account the uncertainty on the low energy extrapolation of the spectra [34]. For the 9 K spectra, the maximum of $χ_{nem}$ occurs at a nominally small compressive strain of approximately $- 0.5 \times 10^{- 3}$ , whereas for all other probed temperatures for the two samples the maximum is located very close to nominally zero strain. The offset at 9 K is likely due to plastic deformation in the epoxy glue which occurred just before this measurement [42]. Therefore the 9 K data have been shifted by an offset of $+ 0.5 \times 10^{- 3}$ on $ε_{x^{'} x^{'}}^{nom}$ .
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Figure 3
Scaling between the suppression of $χ_{nem}$ in the superconducting state measured through elasto-Raman spectroscopy and the fall of $T_{c}$ measured in transport on a crystal with similar doping $x$ . The $T_{c} (ε_{x^{'} x^{'}}^{nom})$ data are taken from Ref. [29]. The estimated strain transmission ratios have been taken into account for both datasets [29, 34]. The empty symbols stand for the data under positive strain, the filled ones under negative strain. The offset between positive and negative strain data points is partly attributed to uncertainties in the exact location of the zero strain state, but could also possibly reflect an intrinsic asymmetry between tensile and compressive strain [34].
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Physical Review Letters

Nematic-Fluctuation-Mediated Superconductivity Revealed by Anisotropic Strain in Ba(Fe1−xCox)2As2

Jean-Côme Philippe, Alexis Lespinas, Jimmy Faria, Anne Forget, Dorothée Colson, Sarah Houver, Maximilien Cazayous, Alain Sacuto, Indranil Paul, and Yann Gallais

Phys. Rev. Lett. 129, 187002 – Published 25 October 2022

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Nematic-Fluctuation-Mediated Superconductivity Revealed by Anisotropic Strain in $Ba ({Fe}_{1 - x} {Co}_{x})_{2} {As}_{2}$