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Adv."],"published-print":{"date-parts":[[2026,10,23]]},"abstract":"<jats:p>Breakdown of selection rules and emergence of optical activity in Raman spectroscopy typically signal symmetry-breaking events essential for understanding novel phases of solids. Here, we apply our recently developed rotational anisotropy Raman spectroscopy technique to single-crystal ruthenium dioxide\u2014a compound of recent interest as a candidate workhorse material for altermagnetism yet whose magnetic ground state remains under intense debate. Our data reveal a salient breakdown of selection rules for zone-center phonons, accompanied by clear splitting in the angle-dependent polarimetry plots for the two opposite helicity-nonconserving circularly polarized scattering channels. These observations provide definitive evidence of a previously overlooked symmetry-breaking event, involving either spatial or time-reversal symmetry. In either scenario, the underlying mechanism is likely a bulk hidden order parameter, potentially involving multipole moments of Ruthenium(IV) set by the local crystal field environment. Our work demonstrates how subtle symmetry-breaking events can manifest through lattice dynamics and calls for renewed experimental and theoretical efforts to map the phase diagram of ruthenium dioxide as a system poised near quantum phase transition boundaries.<\/jats:p>","DOI":"10.1126\/sciadv.aeh0887","type":"journal-article","created":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T18:01:42Z","timestamp":1792605702000},"update-policy":"https:\/\/doi.org\/10.34133\/aaas_crossmark","source":"Crossref","is-referenced-by-count":0,"title":["Anomalous phonon selection rules and Raman optical activity in RuO\n                    <sub>2<\/sub>"],"prefix":"10.1126","volume":"12","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-2759-7797","authenticated-orcid":true,"given":"Di","family":"Cheng","sequence":"first","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-0324-6502","authenticated-orcid":true,"given":"Ao","family":"Wu","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5841-4204","authenticated-orcid":true,"given":"Hui","family":"Pan","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3339-3627","authenticated-orcid":true,"given":"Yu-Peng","family":"Zhu","sequence":"additional","affiliation":[{"name":"Department of Physics, State Key Laboratory of Quantum Functional Materials, and Guangdong Basic Research Center of Excellence for Quantum Science, Southern University of Science and Technology (SUSTech), Shenzhen, Guangdong 518055, China."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4432-6603","authenticated-orcid":true,"given":"Shizhuo","family":"Luo","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1431-5025","authenticated-orcid":true,"given":"King Yau","family":"Yip","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2995-9664","authenticated-orcid":true,"given":"Wei","family":"Zhang","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0598-426X","authenticated-orcid":true,"given":"Ariando","family":"Ariando","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7738-743X","authenticated-orcid":true,"given":"Chang","family":"Liu","sequence":"additional","affiliation":[{"name":"Department of Physics, State Key Laboratory of Quantum Functional Materials, and Guangdong Basic Research Center of Excellence for Quantum Science, Southern University of Science and Technology (SUSTech), Shenzhen, Guangdong 518055, China."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jian-Sheng","family":"Wang","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0555-2624","authenticated-orcid":true,"given":"Xinwei","family":"Li","sequence":"additional","affiliation":[{"name":"Department of Physics, National University of Singapore, Singapore 117551, Singapore."}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"221","reference":[{"key":"e_1_3_2_2_2","doi-asserted-by":"publisher","DOI":"10.1080\/00018736400101051"},{"key":"e_1_3_2_3_2","doi-asserted-by":"crossref","unstructured":"M. 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