Continuous excitations of the triangular-lattice quantum spin liquid YbMgGaO 4

A quantum spin liquid (QSL) is an exotic state of matter in which electrons' spins are quantum entangled over long distances, but do not show magnetic order in the zero-temperature limit. The observation of QSL states is a central aim of experimental physics, because they host collective excitations that transcend our knowledge of quantum matter; however, examples in real materials are scarce. Here, we report neutron-scattering experiments on YbMgGaO 4 , a QSL candidate in which Yb 3+ ions with effective spin-1/2 occupy a triangular lattice. Our measurements reveal a continuum of magnetic excitations-the essential experimental hallmark of a QSL-at very low temperature (0.06 K). The origin of this peculiar excitation spectrum is a crucial question, because isotropic nearest-neighbour interactions do not yield a QSL ground state on the triangular lattice. Using measurements in the field-polarized state, we identify antiferromagnetic next-nearest-neighbour interactions, spin-space anisotropies, and chemical disorder between the magnetic layers as key ingredients in YbMgGaO 4 . © 2017 Macmillan Publishers Limited.