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,...