1. Field-induced bound-state condensation and spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ revealed by neutron scattering up to 25.9 T
- Author
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Fogh, Ellen, Nayak, Mithilesh, Prokhnenko, Oleksandr, Bartkowiak, Maciej, Munakata, Koji, Soh, Jian-Rui, Turrini, Alexandra A., Zayed, Mohamed E., Pomjakushina, Ekaterina, Kageyama, Hiroshi, Nojiri, Hiroyuki, Kakurai, Kazuhisa, Normand, Bruce, Mila, Frédéric, and Rønnow, Henrik M.
- Subjects
Condensed Matter - Strongly Correlated Electrons - Abstract
Bose-Einstein condensation (BEC) underpins exotic forms of order ranging from superconductivity to superfluid 4 He. In quantum magnetic materials, ordered phases induced by an applied magnetic field can be described as the BEC of magnon excitations. With sufficiently strong magnetic frustration, exemplified by the system SrCu$_2$(BO$_3$)$_2$ , no clear magnon BEC is observed and the complex spectrum of multi-magnon bound states may allow a different type of condensation, but the high fields required to probe this physics have remained a barrier to detailed investigation. Here we exploit the first purpose-built high-field neutron scattering facility to measure the spin excitations of SrCu$_2$(BO$_3$)$_2$ up to 25.9 T and use cylinder matrix-product-states (MPS) calculations to reproduce the experimental spectra with high accuracy. Multiple unconventional features point to a condensation of $S = 2$ bound states into a spin-nematic phase, including the gradients of the one-magnon branches, the presence of many novel composite two- and three-triplon excitations and the persistence of a one-magnon spin gap. This gap reflects a direct analogy with superconductivity, suggesting that the spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ is best understood as a condensate of bosonic Cooper pairs. Our results underline the wealth of unconventional states yet to be found in frustrated quantum magnetic materials under extreme conditions.
- Published
- 2023
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