Frustrated magnetism, magnetization plateaux, and the mystery of SrCu2(BO3)2

Orateur : Frédéric MILA (Ecole Polytechnique Fédérale de Lausanne, Switzerland)

Frustration is at the origin of many strange properties in quantum magnets. It dramatically enhances quantum fluctuations, often preventing systems from developing long-range magnetic order. This opens the way to original alternatives such as algebraic or gapped spin liquids. Even the way a system polarizes in an external field is qualitatively modified by frustration, which often induces strange anomalies such as magnetization jumps or magnetization plateaux. The most spectacular example is the orthogonal dimer system SrCu2(BO3)2, with its improbable sequence of plateaux at 1/8, 2/15, 1/6, 1/4, 1/3 and 1/2. After reviewing the explanations put forward since 1999, which range from some form of fractional quantum Hall effect to localized triplets, I will argue that the low magnetization plateaus are in fact crystals of two-triplet bound states, a rather exotic possibility that had escaped everybody’s imagination until unbiased numerical simulations based on a tensor network algorithm finally resolved this 15-year old mystery.

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