Regensburg 2019 – wissenschaftliches Programm

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MA: Fachverband Magnetismus

MA 6: Frustrated Magnets - Spin Liquids (joint session TT/MA)

MA 6.12: Vortrag

Montag, 1. April 2019, 18:00–18:15, Theater

Low-energy spin excitations in the triangular-lattice quantum spin liquid candidate YbMgGaO₄ — •Yuesheng Li, Alexander Tsirlin, and Philipp Gegenwart — Experimental Physics VI, Center for Electronic Correlations and Magnetism, University of Augsburg, 86159 Augsburg, Germany

YbMgGaO₄ was first proposed as a perfect triangular-lattice rare-earth quantum spin liquid (QSL) candidate in 2015. After that, several scenarios, such as the spin-liquid mimicry, valence bond (VB) glass, and spin-glass, were reported in the presence of the site-mixing disorder between nonmagnetic Mg²⁺ and Ga³⁺. Here, we critically test these scenarios by probing the low-energy spin excitations of YbMgGaO₄ based on the low-T magnetization and triple-axis inelastic neutron scattering (INS) experiments. Our magnetization data measured down to 40 mK speak against any conventional freezing and reinstate YbMgGaO4 as a QSL candidate. The low-energy (E ≤ J₀ ∼ 0.2 meV) part of the INS continuum presents at low temperatures, but completely disappears upon warming the system above T ≫ J₀/k_B. In contrast to the high-energy part at E > J₀ that is rooted in the breaking of nearest-neighbor VBs and persists to temperatures well above J₀/k_B, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the QSL candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of QSLs.