Identification of topologically different valence bond states in spin ladders

We discuss relation between three different methods to identify topologically distinct short-range valence-bond ground states in spin-$\frac12$ two-leg ladders: the string order parameters, the level-crossing method, and the ground-state expectation value of the twist operator. For this purpose we reexamine a phase transition in the frustrated spin ladder. We propose a proper bosonization of the string order parameters. Then, these three methods are shown to be equivalent reflecting the $Z_2\times Z_2$ symmetry breaking of the order and the disorder fields of the two-dimensional Ising model.Comment: 2 pages, Proceedings for LT2

Novel Order Parameter to Characterize Valence-Bond-Solid States

We propose an order parameter to characterize valence-bond-solid (VBS) states in quantum spin chains, given by the ground-state expectation value of a unitary operator appearing in the Lieb-Schultz-Mattis argument. We show that the order parameter changes the sign according to the configuration of the valence bonds. This allows us to determine the phase transition point in between different VBS states accurately. We demonstrate this theory in the bond-alternating Heisenberg chain and in the frustrated spin ladder.Comment: 4 pages, 1 figure, Proceedings for Yukawa-Institute-for-Theoretical-Physics International Workshop "Order, Disorder, and Dynamics in Quantum Spin Systems" (Nov. 2001), See also cond-mat/011237

Orbital Magnetism and Transport Phenomena in Two Dimensional Dirac Fermions in Weak Magnetic Field

We discuss the orbital magnetism and the Hall effect in the weak magnetic field in two dimensional Dirac fermion systems with energy gap. This model is related to the graphene sheet, organic conductors, and $d$-density wave superconductors. We found the strong diamagnetism and finite Hall conductivity even in gapped systems. We also discuss the relation between the weak-magnetic field formalism and the Landau quantization with the Euler-Maclaurin expansion.Comment: 4pages,3figure