Collective magnetic Higgs excitation in a pyrochlore ruthenate

TL;DR

This paper reports the discovery of a collective Higgs-type amplitude mode in the pyrochlore magnet Nd$_2$Ru$_2$O$_7$, arising from successive high-temperature phase transitions and involving bond energy modulations.

Contribution

It uncovers a novel Higgs-like excitation in a condensed matter system, linked to multiple broken symmetries and giant phonon instabilities.

Findings

Identification of a 3 meV Higgs-type mode

Observation of two successive phase transitions at 147 K and 97 K

Detection of symmetry-incompatible two-fold excitation

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.