Rare-Earth Triangular Lattice Spin Liquid: A Single-Crystal Study ofYbMgGaO4

Abstract

YbMgGaO4, a structurally perfect two-dimensional triangular lattice with an odd number of electrons per unit cell and spin-orbit entangled effective spin-1/2 local moments for the Yb(3+) ions, is likely to experimentally realize the quantum spin liquid ground state. We report the first experimental characterization of single-crystal YbMgGaO4 samples. Because of the spin-orbit entanglement, the interaction between the neighboring Yb(3+) moments depends on the bond orientations and is highly anisotropic in the spin space. We carry out thermodynamic and the electron spin resonance measurements to confirm the anisotropic nature of the spin interaction as well as to quantitatively determine the couplings. Our result is a first step towards the theoretical understanding of the possible quantum spin liquid ground state in this system and sheds new light on the search for quantum spin liquids in strong spin-orbit coupled insulators.

Hierarchy of Exchange Interactions in the Triangular-Lattice Spin Liquid YbMgGaO4

“…The newly discovered rare-earth triangular-lattice antiferromagnet YbMgGaO 4 [54,55] appears to fulfill precisely this promise. The magnetic Yb 3þ ions carry effective spin-1=2 moments in a symmetry environment allowing anisotropic exchange interactions [41,47] in the absence of antisymmetric (Dzyaloshinsky-Moriya) terms and magnetic defects, both of which are present in other two-dimensional QSL candidates such as herbertsmithite [56][57][58].…”

“…We note that the randomness of the interlayer Mg=Ga atoms in YbMgGaO 4 causes displacements of the Yb atoms [55] leading to modulations (disorder) in the g tensors [64] and exchange interactions [66]. Thus, the values of the exchange parameters we determine in this work are averaged over these intrinsic disorder effects.…”

“…Thus, the values of the exchange parameters we determine in this work are averaged over these intrinsic disorder effects. Previous high-field INS measurements were limited to wave vectors around the antiferromagnetic zone boundary [61] while previous X-band electron spin resonance (ESR) measurements [55] were intrinsically limited to small fields (≲0.4 T) below the field-polarized regime. In contrast, high-resolution TDTS (see, e.g., Ref.…”

“…The magnetic Yb 3þ ions carry effective spin-1=2 moments in a symmetry environment allowing anisotropic exchange interactions [41,47] in the absence of antisymmetric (Dzyaloshinsky-Moriya) terms and magnetic defects, both of which are present in other two-dimensional QSL candidates such as herbertsmithite [56][57][58]. The immediate availability of single crystals [55] uncovered a QSL phenomenology in YbMgGaO 4 characterized by the absence of spin ordering or freezing down to T ¼ 100 mK in muon spin relaxation measurements [59], much lower than the Curie-Weiss temperature θ W ≈ −4 K, and a power-law behavior for the magnetic specific heat at low temperatures [55,60]. Perhaps the strongest evidence for a QSL in YbMgGaO 4 came from inelastic neutron scattering measurements in zero field that unraveled a broad continuum of magnetic excitations across the entire Brillouin zone [61][62][63].…”

“…In fact, the nature of the dominant exchange interactions in YbMgGaO 4 is also controversial. While the overall planar anisotropy is clear from susceptibility measurements [55,62], both antiferromagnetic next-nearest-neighbor terms (J 2 ) [61] and nearest-neighbor anisotropic offdiagonal exchanges (so-called J AEAE 1 and J zAE 1 ) [43,65] have been proposed as extensions from the XXZ model. Comprehensively determining the exchange interactions in YbMgGaO 4 is of fundamental importance in deciphering the nature of its ground state.…”