Novel Ordered Phases in the Orthogonal Dimer Spin System SrCu2(BO3)2

概要

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The magnetic properties of SrCu2(BO3)2, a model spin system on the two-dimensional (2D) Shastry–Sutherland lattice, are reviewed with particular emphasis on the nuclear magnetic resonance (NMR) experiments. SrCu2(BO3)2 has a dimer singlet ground state at zero magnetic field and shows a sequence of magnetization plateaux at high magnetic fields. The plateaux are attributed to the localization of triplets into a superlattice due to mutual repulsion. Such a superstructure has been indeed observed at the 1/8 plateau by NMR experiments. Furthermore, the superstructure persists above the 1/8 plateau, where new phases have been identified by the torque and NMR measurements. SrCu2(BO3)2 also has the anisotropic Dzyaloshinski–Moriya interactions, which significantly change the ground state in magnetic fields. A recent development is the discovery of a new magnetic phase transition under high pressure. At 2.4 GPa, the 11B NMR spectrum exhibits line splitting as a function of temperature in two steps. A gradual splitting below 30 K indicating loss of four-fold symmetry is followed by a further sudden splitting below 4 K marking a clear magnetic phase transition. The low-$T$ phase has a doubled unit cell, yet with no spontaneous magnetic moment. A new type of valence-bond-solid (VBS) order is proposed.
Physical Society of Japanの論文
2010-01-15