article published yes A. Liebman-Peláez author S. J. Garratt author Veronika Sunko author 23cb1cf6-2c7a-11ef-91a4-f72fc19f20b30000-0003-2724-3523 Y. Sun author J. R. Soh author D. Prabhakaran author A. T. Boothroyd author J. Orenstein author Magnets with isotropic easy-plane symmetry host Goldstone modes that can be leveraged for efficient spin transport. Here, we present a time-resolved optical polarimetry technique that allows us to detect and characterize such low-frequency modes, and use it to observe the Goldstone mode in the multi-Q broken helix phase of EuIn2As2. The strength of our technique comes from the ability to distinguish between nematic and magnetization dynamics in order to yield information about the mode structure, in addition to its frequency. We find that the nearly uniform spin precession characteristic of a Goldstone mode is realized only when a small magnetic field is used to unpin the broken helix from local strain generated during crystal growth. In this regime, the mode frequency scales linearly with the applied field due to the ground state C2z symmetry of the broken helix. Our work shows how optical polarimetry can be used to study the Goldstone modes of complex magnets. American Physical Society2026 eng 2469-9950 2469-996910.1103/b48p-kw5l 11322 yes Liebman-Peláez A, Garratt SJ, Sunko V, et al. Observation of a Goldstone mode in the broken helix by time-resolved optical polarimetry. <i>Physical Review B</i>. 2026;113(22). doi:<a href="https://doi.org/10.1103/b48p-kw5l">10.1103/b48p-kw5l</a> A. Liebman-Peláez <i>et al.</i>, “Observation of a Goldstone mode in the broken helix by time-resolved optical polarimetry,” <i>Physical Review B</i>, vol. 113, no. 22. American Physical Society, 2026. Liebman-Peláez, A., Garratt, S. J., Sunko, V., Sun, Y., Soh, J. R., Prabhakaran, D., … Orenstein, J. (2026). Observation of a Goldstone mode in the broken helix by time-resolved optical polarimetry. <i>Physical Review B</i>. American Physical Society. <a href="https://doi.org/10.1103/b48p-kw5l">https://doi.org/10.1103/b48p-kw5l</a> Liebman-Peláez, A., et al. “Observation of a Goldstone Mode in the Broken Helix by Time-Resolved Optical Polarimetry.” <i>Physical Review B</i>, vol. 113, no. 22, 224401, American Physical Society, 2026, doi:<a href="https://doi.org/10.1103/b48p-kw5l">10.1103/b48p-kw5l</a>. A. Liebman-Peláez, S.J. Garratt, V. Sunko, Y. Sun, J.R. Soh, D. Prabhakaran, A.T. Boothroyd, J. Orenstein, Physical Review B 113 (2026). Liebman-Peláez, A., S. J. Garratt, Veronika Sunko, Y. Sun, J. R. Soh, D. Prabhakaran, A. T. Boothroyd, and J. Orenstein. “Observation of a Goldstone Mode in the Broken Helix by Time-Resolved Optical Polarimetry.” <i>Physical Review B</i>. American Physical Society, 2026. <a href="https://doi.org/10.1103/b48p-kw5l">https://doi.org/10.1103/b48p-kw5l</a>. Liebman-Peláez A, Garratt SJ, Sunko V, Sun Y, Soh JR, Prabhakaran D, Boothroyd AT, Orenstein J. 2026. Observation of a Goldstone mode in the broken helix by time-resolved optical polarimetry. Physical Review B. 113(22), 224401. 221162026-06-22T08:52:01Z2026-06-24T09:49:27Z