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DATE | 2011-03-31 |

TIME | 14:10 |

PLACE | R203, 2F, NCTS, NCKU |

FIELD | Computational Materials Research |

SPEAKER | Prof. Philippe Sindzingre - Laboratoire de Physique Théorique de la Matière Condensée, Université Pierre et Marie Curie, France |

TITLE | SOME EXAMPLES OF T=0 PHASES OF QUANTUM ANTIFERROMAGNETS IN TWO DIMENSIONS |

ABSTRACT | The investigation of the nature of ground-state an low-lying excitations in insulating quantum antiferromagnetic systems has received a large interest in the last decades in relation to the physics of high Tc superconductors, the quatum Hall effect, systems of cold atoms localized on lattices by crossed laser beams, quantum computations... In these systems, quantum fluctations, which are the largest when the spin S of the ions are the smallest, i.e., when S=1/2, and when the dimensionality is reduced, e.g. for 2D layers or quasi 1D weakly coupled chains, can be responsible, in presence of competing interactions (frustration), of a variety of effects. They may destabilize the classical magnetic (Neel) orders or, when the system would have a degenerate ground-state for classical spins (S equal to infinity), lift this degeneracy or even more interestingly induce phases that are not present classically, with "quantum orders". The T=0 phase diagrams of mny models (possibly relevant to experiments) has been studied in past years by various analytical or numerical approaches. In this talk I illustrate some of the possible physic that can be encountered in these systems froms results of "exact diagonalizations" of two model Hamiltonians~: the J1-J2-J3 model on the square lattice and the Heisenberg model on the kagome lattice. |