Universality is a powerful concept that arises from the divergence of a characteristic length scale. For condensed matter systems, this length scale is typically the correlation length, which diverges at critical points separating two different phases. Few-particle systems exhibit a simpler form of universality when the $s$-wave scattering length diverges. A prominent example of universal phenomena is the emergence of an infinite tower of three-body bound states obeying discrete scale invariance, known as the Efimov effect, which has been subject to extensive research in chemical, atomic, nuclear, and particle physics. In principle, these universal phenomena can also emerge in the excitation spectrum of condensed matter systems, such as quantum magnets [Y. Nishida, Y. Kato, and C. Batista, Nat. Phys. 9, 93 (2013)]. However, the limited tunability of the effective interparticle interaction relative to the kinetic energy has precluded so far their observation. Here, we demonstrate that a high degree of magnetic field induced tunability can also be achieved in quantum magnets with strong spin-orbit coupling: A two-magnon resonance condition can be achieved in ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ with a field of $\sim 13$ T along the [110] direction, which leads to the formation of Efimov states in the three-magnon spectrum of this material. Raman scattering experiments can reveal the field-induced two-magnon resonance, as well as the Efimov three-magnon bound states that emerge near the resonance condition.

• Received 14 March 2020
• Revised 5 June 2020
• Accepted 15 June 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.033024

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Published by the American Physical Society