Berlin 2005 – scientific programme

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TT: Tiefe Temperaturen

TT 21: Symposium Frustrated Systems

TT 21.2: Invited Talk

Monday, March 7, 2005, 14:25–14:50, TU H104

2D Quantum Antiferromagnets from Néel-Ordered Phases to Spin Liquids — •Claire Lhuillier — Laboratoire de Physique Théorique des Liquides, Université P. et M. Curie and UMR 7600 of CNRS, Case 121, 4 Place Jussieu, 75252 Paris Cédex, France

2D quantum magnets display a large variety of low energy phases. The spin-1/2 next neighbor 2-dimensional Heisenberg model on Bravais lattices (square, triangular) is Néel ordered. Frustration, small coordination number, competition between interactions can lead directly, or step by step, to various gapful quantum phases without magnetic long range order. The variety of long ranged ordered gapless phases is larger than usually expected: some examples will be given of exotic phases (nematic order) or "paradoxical" effects (T ≠ 0 phase transitions in 2D magnets with SU(2) invariant interactions). In 2D the spin-gapped phases may be Valence Bond Crystals (VBC) with long range order in singlets or Spin Liquids (SL). There are major physical differences between these two kinds of spin-gapped phases. Propagating modes of integer spin can describe 99 SL excitations are much more exotic: there are both fractionnalized and topological ones (these last excitations being good candidates for quantum bits, with very low rate of quantum decoherence).