Order by Disorder and Topology in Frustrated Magnetic Systems

摘要: The phenomenon of order by disorder in frustrated magnetic systems is reviewed. Thermal, quantum, and even quenched noise may sometimes increase ordering where energetics ensure a nontrivially degenerate classical ground state. In the number variables parametrizing manifold states not macroscopic, fluctuations remove degeneracy reduce symmetry to that Ising model. We concentrate on kagome antiferromagnet, whose has macroscopic degrees freedom. this system, thermal around lead entropically driven local spin nematic at low T. are so strong no single state or finite set selected. derive an effective Hamiltonian, giving description terms fluctuating surface. A novel phenomenon, quasi-nonergodicity, arising perfectly lattice, briefly discussed.