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arXiv · 2306.07389

Field-induced bound-state condensation and spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ revealed by neutron scattering up to 25.9 T

Ellen Fogh·Mithilesh Nayak·Oleksandr Prokhnenko·Maciej Bartkowiak·Koji Munakata·Jian-Rui Soh·Alexandra A. Turrini·Mohamed E. Zayed·Ekaterina Pomjakushina·Hiroshi Kageyama·Hiroyuki Nojiri·Kazuhisa Kakurai·Bruce Normand·Frédéric Mila·Henrik M. Rønnow
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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.

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Ellen Fogh, Mithilesh Nayak, Oleksandr Prokhnenko, Maciej Bartkowiak, Koji Munakata, Jian-Rui Soh, Alexandra A. Turrini, Mohamed E. Zayed, Ekaterina Pomjakushina, Hiroshi Kageyama, Hiroyuki Nojiri, Kazuhisa Kakurai, Bruce Normand, Frédéric Mila, Henrik M. Rønnow. 2023-06-12. Field-induced bound-state condensation and spin-nematic phase in SrCu$_2$(BO$_3$)$_2$ revealed by neutron scattering up to 25.9 T. https://doi.org/10.1038/s41467-023-44115-z

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