凝縮系における原子分布凾数についてIII:Bethe 近似について

Masuo Suzuki

Seiichiro Katsura, M. Takizawa

Statistical mechanics of the Bethe lattice was developed. Properties of the Bethe lattice, i. e., (of the Bethe approximation) were summarized including low temperature susceptibilities of ferro- and antiferromagnets. The equivalence of the exact solution of the Bethe lattice to the Bethe approximation of the Ising model in the case of the antiferromagnetism was shown. The partition function of the Bethe lattice with first and second neighbor interactions was obtained and the critical temperature was calculated in the case where the first neighbor interaction is ferromagnetic.

Xin Zhang, Andreas Klümper, Vladislav Popkov

We investigate special solutions to the Bethe Ansatz equations (BAE) for open integrable XXZ Heisenberg spin chains containing phantom (infinite) Bethe roots. The phantom Bethe roots do not contribute to the energy of the Bethe state, so the energy is determined exclusively by the remaining regular excitations. We derive the phantom Bethe roots criterion and focus on BAE solutions for mixtures of phantom roots and regular (finite) Bethe roots. We prove that in the presence of phantom Bethe roots, all eigenstates are split between two invariant subspaces, spanned by chiral shock states. Bethe eigenstates are described by two complementary sets of Bethe ansatz equations for regular roots, one for each invariant subspace. The respective ``semiphantom'' Bethe vectors are states of chiral nature, with chirality properties getting less pronounced when more regular Bethe roots are added. For the easy plane case, ``semiphantom'' Bethe states carry nonzero magnetic current and are characterized by quasiperiodic modulation of the magnetization profile, the most prominent example being the spin helix states (SHS). We illustrate our results investigating ``semiphantom'' Bethe states generated by one regular Bethe root (the other Bethe roots being phantom), with a simple structure of the invariant subspace, in all details. We obtain the explicit expressions for Bethe vectors and calculate the simplest correlation functions, including the spin-current for all the states in the single-particle multiplet.

Masuo Suzuki

Johnson Leow

In the present thesis, we will review various Bethe ansatz techniques such as the (nested)\nalgebraic Bethe ansatz and the (nested) coordinate Bethe ansatz using well known models\nlike the XXX model and the Hubbard model. Subsequently, the Bethe ansatz techniques\nare applied to obtain Bethe ansatz equations and Bethe vectors of the su(2|2)-invariant S -\nmatrix. Finally, these results are used to obtain the full S -matrix and Bethe equations of the\nstring sigma model.

Yukio Saitō