4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu2(BO3)2

5 years ago

4-spin plaquette singlet state in the Shastry–Sutherland compound SrCu2(BO3)2

K. Conder, R. A. Sadykov, Th. Strässle, C. Panagopoulos, Ch. Rüegg, M. Boehm, D. F. McMorrow, H. M. Rønnow, M. Ellerby, M. Jiménez–Ruiz, A. M. Läuchli, M. Stingaciu, E. Pomjakushina, G. Hamel, A. Schneidewind, S. Klotz, M. E. Zayed, S. S. Saxena, J. Larrea J., V. Pomjakushin

In the field of quantum magnetism, geometrically frustrated lattices generally imply major difficulties in analytical and numerical studies. For very few particular topologies, however, it has been shown that the ground state, at least, can be calculated exactly as for the Majumdar–Ghosh model¹⁵ that solves the J₁ − J₂ zigzag chain when J₁ = 2J₂. In two dimensions, the Shastry–Sutherland model² consisting of an orthogonal dimer network of spin S = 1/2 was developed to be exactly solvable. For an inter-dimer J′ to intra-dimer J exchange ratio α ≡ J′/J ≤ 0.5 the ground state is a product of singlets on the strong bond J. Numerical calculations have further shown that this remains valid up to α ≤ ~0.7 and for small values of three-dimensional (3D) couplings J′′ between dimer layers. At the other end, for ~0.9 ≤ α ≤ ∞ the system approaches the well-known 2D square lattice, which is antiferromagnetically (AFM) ordered, albeit with significant quantum fluctuations that are believed to include resonating singlet correlations resulting in fractional excitations¹⁶. The phase diagram of the Shastry–Sutherland model, both with and without applied magnetic field, has been intensively studied by numerous theoretical and numerical approaches⁴. In the presence of magnetic field, magnetization plateaus at fractional values of the saturation magnetization corresponding to Mott insulator phases of dimer states, as well as possible superfluid and supersolid phases have been extensively studied^{7, 17, 18, 19}. At zero field, the main unsolved issue is the existence and nature of an intermediate phase for ~0.7 ≤ α ≤ ~0.9. A variety of quantum phases and transitions between them have been predicted depending on the theoretical technique used: a direct transition from dimer singlet phase to AFM order^{2, 6, 7}, or an intermediate phase with helical order⁵, columnar dimers¹¹, valence bond crystal^{Publisher URL: http://dx.doi.org/10.1038/nphys4190
DOI: 10.1038/nphys4190}