Weak phonon coupling to nematic quantum critical mode in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>BaFe</mi><mn>2</mn></msub><msub><mrow><mo>(</mo><msub><mi>As</mi><mrow><mn>1</mn><mo>−</mo><mi>x</mi></mrow></msub><msub><mi mathvariant="normal">P</mi><mi>x</mi></msub><mo>)</mo></mrow><mn>2</mn></msub></mrow></math>

S. Wu; D. Ishikawa; A. Q. R. Baron; A. Alatas; A. H. Said; Jiayu Guo; Y. He; X. Chen; Y. Song; J. G. Analytis; Dung-Hai Lee; R. J. Birgeneau

doi:10.1103/dpd1-mbhw

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Weak phonon coupling to nematic quantum critical mode in ${BaFe}_{2} {({As}_{1 - x} P_{x})}_{2}$

S. Wu ^1,2,3,*, D. Ishikawa ^4,5, A. Q. R. Baron^4,5, A. Alatas ⁶, A. H. Said⁶, Jiayu Guo ⁷, Y. He ⁸, X. Chen ^1,9, Y. Song ⁷ et al.

J. G. Analytis ^10,11, Dung-Hai Lee^1,2,†, and R. J. Birgeneau^1,2,‡

¹Department of Physics, University of California, Berkeley, California 94720, USA
²Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
³Department of Physics and Engineering Physics, Santa Clara University, Santa Clara, California 95053, USA
⁴Materials Dynamics Laboratory, RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan
⁵Precision Spectroscopy Division, SPring-8/JASRI, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
⁶Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
⁷Center for Correlated Matter and School of Physics, Zhejiang University, Hangzhou 310058, China
⁸Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA
⁹Center for Neutron Science and Technology, School of Physics, Sun Yat-Sen University, Guangzhou, Guangdong 510275, China
¹⁰Department of Physics, University of California Berkeley, California 94720, USA
¹¹CIFAR Quantum Materials, CIFAR, Toronto, Ontario, Canada M5G 1M1

^*Contact author: swu13@scu.edu
^†Contact author: dunghai@berkeley.edu
^‡Contact author: robertjb@berkeley.edu

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Phys. Rev. B 112, 174302 – Published 3 November, 2025

DOI: https://doi.org/10.1103/dpd1-mbhw

Abstract

In this work, we investigate the softening of the in-plane transverse acoustic phonon driven by electronic nematicity in ${BaFe}_{2} {({As}_{1 - x} P_{x})}_{2}$ using inelastic x-ray scattering, with a focus on the optimally doped sample ( $x = 0.31$ ) sample—a system exhibiting signatures of a putative nematic quantum critical point and minimal disorder among iron pnictides. We observe only a modest softening of the phonon frequency and no evidence of critical damping, suggesting that the nematic quantum critical fluctuations couple only weakly to the lattice from our quantum critical model. Given the close proximity of the structural and magnetic transition temperatures in the underdoped sample—which implies that spin-nematic fluctuations couple strongly to the lattice—we conjecture that the quantum critical nematic fluctuations are predominantly orbital in origin.

Physical Review B

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Weak phonon coupling to nematic quantum critical mode in ${BaFe}_{2} {({As}_{1 - x} P_{x})}_{2}$

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Weak phonon coupling to nematic quantum critical mode in BaFe2(As1−xPx)2

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Weak phonon coupling to nematic quantum critical mode in ${BaFe}_{2} {({As}_{1 - x} P_{x})}_{2}$