Disorder-Induced Mimicry of a Spin Liquid in YbMgGaO4

Abstract

We suggest that a randomization of the pseudo-dipolar interaction in the spin-orbit-generated lowenergy Hamiltonian of YbMgGaO4 due to an inhomogeneous charge environment from a natural mixing of Mg 2+ and Ga 3+ can give rise to orientational spin disorder and mimic a spin-liquid-like state. In the absence of such quenched disorder, 1/S and density matrix renormalization group calculations both show robust ordered states for the physically relevant phases of the model. Our scenario is consistent with the available experimental data and further experiments are proposed to support it.PACS numbers: 75.10. Jm, 75.40.Gb, 78.70.Nx Dating back to Wannier's pioneering study of the Ising model [1], triangular lattice models and materials with frustrating antiferromagnetic interactions have served as fertile playgrounds for new ideas [2][3][4][5][6][7][8][9][10]. These systems continue to draw significant experimental [11][12][13][14][15] and theoretical interest because they exhibit many intriguing novel ordered states [16][17][18][19][20][21][22] and unusual continuumlike spectral features [23][24][25][26][27][28][29][30][31] and especially because they provide a setting for spin-liquid states [32][33][34][35][36][37][38][39][40][41][42].Among the latest experimental discoveries [14,15], a rare-earth triangular-lattice antiferromagnet YbMgGaO 4 has recently emerged as a new candidate for a quantum spin liquid of the effective spin-1/2 degrees of freedom of Yb 3+ ions [43,44]. It has been argued that the spin-orbit origin of its magnetic properties and the pseudo-spin nature of the low-energy states with highly anisotropic effective spin interactions may potentially open a new route to realizing quantum spin liquids [44][45][46]. While the lack of ordering, anomalous specific heat, and especially continuum-like excitations in inelastic neutron scattering [45,47] all provide strong support to the idea of an intrinsic spin liquid, other experimental findings are increasingly at odds with this picture.First, in magnetization vs field measurements, there is no sharpening of the transition to the saturated phase upon lowering the temperature, and the lack of the upward curvature in M (H) at the lowest T 's [43,44] is indicative of low quantum fluctuations in the ground state [48]. Second, in the high-field polarized phase, neutron scattering shows that continuum-like excitations persist, with significant smearing of magnon lines that are expected to be sharp [47]. In addition, an apparent absence of any detectable contribution of spin excitations to thermal conductivity down to the lowest temperatures, accompanied by a strong deviation of the phonon part from the ballistic T 3 form [49], both suggest strong scattering effects. These, combined with the anomalously broadened higher-energy Yb 3+ doublet structure [47, 50] and a ubiquitous mixing of Mg 2+ and Ga 3+ ions in the non-magnetic layers [43,47], implicate disorder as a key contributor to the observed properties [50].In this Letter, we first argue that a hypothetical,...

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

“…Because of their nonspin-conserving nature, dominant pseudodipolar interactions should manifest as an asymptotic approach to saturated magnetization, as recently observed in the candidate Kitaev spin liquid α-RuCl 3 [81]. Finally, we note that the role and extent of CEF and exchange disorder remains an outstanding issue in YbMgGaO 4 with possible impact ranging from the mimicry of a spin liquid [47,48,82,83] to disorder-induced entanglement [84][85][86].…”

“…Moreover, an analysis of TDTS linewidths further constrains the anisotropic exchange interactions. In the context of prevailing models our results suggest that both NNN and off-diagonal anisotropic exchange interactions are present in YbMgGaO 4 -with pseudodipolar terms subleading compared with the XXZ part of the model-and that this nominal set of exchanges may lie closer to a spin-liquid regime than previously thought [47,61]. When combined with the likely presence of exchange disorder, the extent of which remains to be determined, our work strongly constrains the nature of the YbMgGaO 4 ground state and points towards a more intricate underlying scenario than originally reported [62].…”

“…Experiments on various spin-S triangular-lattice antiferromagnets have overwhelmingly confirmed this picture [27][28][29][30], with a few noteworthy exceptions [31][32][33]. Several perturbations from the pure TLHAF have been proposed to enhance quantum fluctuations: next-nearest neighbor (NNN) interactions [34][35][36][37][38], ring-exchange terms [39,40], and anisotropic exchange [41][42][43][44], although it remains theoretically unclear if the latter mechanism alone can stabilize a QSL [45][46][47][48]. Nevertheless, anisotropic exchange interactions [49,50] are known to generate new physics in rare-earth pyrochlores such as Yb 2 Ti 2 O 7 [49,[51][52][53], making it worthwhile to investigate their effect in other lattice geometries.…”

“…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].…”

“…Additionally, the disordered occupancy of the intertriangular layers by Mg 2þ and Ga 3þ ions [54] appears to affect profoundly the Yb 3þ ions with broadened crystal electric-field (CEF) levels [61,64], a distribution of g tensors [64], and a broadened magnetic excitation spectrum at high fields [61]. The impact of disorder on the YbMgGaO 4 exchange Hamiltonian and, therefore, whether the ground state is a QSL or not, remains an outstanding issue [47,48]. In fact, the nature of the dominant exchange interactions in YbMgGaO 4 is also controversial.…”