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arXiv · 2204.12124

Collective magnetic Higgs excitation in a pyrochlore ruthenate

Dirk Wulferding·Junkyeong Kim·Mi Kyung Kim·Yang Yang·Jae Hyuck Lee·Dongjoon Song·Dongjin Oh·Heung-Sik Kim·Li Ern Chern·Yong Baek Kim·Minji Noh·Hyunyong Choi·Sungkyun Choi·Natalia B. Perkins·Changyoung Kim·Seung Ryong Park
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Abstract

The emergence of scalar Higgs-type amplitude modes in systems where symmetry is spontaneously broken has been a highly successful, paradigmatic description of phase transitions, with implications ranging from high-energy particle physics to low-energy condensed matter systems. Here, we uncover two successive high temperature phase transitions in the pyrochlore magnet Nd$_2$Ru$_2$O$_7$ at $T_{\mathrm{N}} = 147$ K and $T^* = 97$ K, that lead to giant phonon instabilities and culminate in the emergence of a highly coherent excitation. This coherent excitation, distinct from other phonons and from conventional magnetic modes, stabilizes at a low energy of 3 meV. We assign it to a collective Higgs-type amplitude mode, that involves bond energy modulations of the Ru$_4$ tetrahedra. Its striking two-fold symmetry, incompatible with the underlying crystal structure, highlights the possibility of multiple entangled broken symmetries.

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Dirk Wulferding, Junkyeong Kim, Mi Kyung Kim, Yang Yang, Jae Hyuck Lee, Dongjoon Song, Dongjin Oh, Heung-Sik Kim, Li Ern Chern, Yong Baek Kim, Minji Noh, Hyunyong Choi, Sungkyun Choi, Natalia B. Perkins, Changyoung Kim, Seung Ryong Park. 2023-07-23. Collective magnetic Higgs excitation in a pyrochlore ruthenate. https://doi.org/10.1038/s41535-023-00572-9

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