We perform an exact diagonalization of the Heisenberg Hamiltonian for the magnetic ring consisting of N=6 nodes, compare the result with the Bethe Ansatz (BA) approach and verify the hypothesis of strings. We find and explain the failure of BA method in a single case of the highest weight eigenvector at the boundary of the Brillouin zone for the case of two spin deviations. The explanation involves the orbit structure of the corresponding set of magnetic configurations. We demonstrate that the finite size effects for N=6 only slightly deform the shape of asymptotic strings.
We perform an exact diagonalization of the Heisenberg Hamiltonian for the magnetic ring consisting of N=6 nodes, compare the result with the Bethe Ansatz (BA) approach and verify the hypothesis of strings. We find and explain the failure of BA method in a single case of the highest weight eigenvector at the boundary of the Brillouin zone for the case of two spin deviations. The explanation involves the orbit structure of the corresponding set of magnetic configurations. We demonstrate that the finite size effects for N=6 only slightly deform the shape of asymptotic strings.
The main purpose of this report is a thorough analysis of completeness of solutions of the one-dimensional Heisenberg Hamiltonian through the hypothesis of strings. A somehow astonishing conclusion emerges from studying of the structure of the classical configuration space of this system. Namely, all allowed information concerning quantum states, which are exact solutions of the Bethe equations, encoded in quantum numbers, are predictable via a bijection between the set of the magnetic configurations and the string configurations. This startling and beautiful observation constitutes the proof of the completeness of the eigenstates of the Heisenberg Hamiltonian, deduced in a purely combinatorial way. We interpret the set of all magnetic configurations with a fixed number r of spin deviations as the classical configuration space of a hypothetic system of r Bethe pseudoparticles, which move, in a stroboscopic manner, on the magnetic ring. The geometry of this configuration space, induced by the action of Heisenberg Hamiltonian and the translation symmetry group of the ring, implies the structure of a locally r -dimensional hypercubic lattice with well defined F -dimensional boundaries, 1 ⩽ F ⩽ r . We demonstrate that rigged string configurations originate from these boundaries, depending upon the island structure of spin deviations. We show that a relatively simple combinatoric definition of rigged strings reproduces completely exact results of Bethe Ansatz. It is expressed in terms of a combined bijection: Robinson-Schensted with Kerov- Kirillov-Reshetikhin (RSKKR) which produces a geography of exact Bethe Ansatz solutions on the classical configuration space.
The Bethe Ansatz can be treated using two approaches: The Co-ordinate Bethe Ansatz (CBA) and The Algebraic Bethe Ansatz (ABA). In this paper the relation between the solutions given by these two methods is addressed. The considerations are based on the finite Heisenberg magnet with N=8 nodes and for r=4 reversed spins s=1/2. Both methods provide consistent solutions in the entire Brillouin zone (BZ). Moreover there is a special point — a border of the BZ — for both approaches, in which the solutions are not well-defined.
We demonstrate that the seminal one-dimensional model of the Heisenberg magnet, consisting of N spins 1/2 with the nearest-neighbour isotropic interaction, solved exactly by Bethe ansatz, admits an interpretation of a system of r = N/2 -M pseudoparticles (spin deviations) which are indistinguishable, have hard cores and move on the chain by local hoppings.Such an approach allows us to construct a manifold with some boundaries, which is genericly r-dimensional, and whose F -dimensional regions, 0 < F < r, point out all l-strings.The latter classify, in terms of rigged string configurations of Kerov, Kirillov and Reshetikhin, all exact Bethe eigenfunctions.In this way, we interpret these eigenfunctions in terms of the classical configuration space, in particular on the structure of islands of adjacent spin deviations, in a way independent of the size N .
We have derived an explicit formula for a complete set of commuting operators for the XXX model of the Heisenberg magnetic ring, using an algebraic Bethe ansatz approach of Faddeev and Takhtajan. Each operator turns out to be the sum of increasing l -cycles in the symmetric group acting on the set of nodes of the ring. It is demonstrated that the resulting algebra of operators encloses the total spin S , the Hamiltonian and the quasimomentum. We point out that it is a maximal Abelian subalgebra in the algebra of the symmetric group, associated with the basis of exact Bethe ansatz eigenstates, the latter classified by rigged string configurations of Kerrov, Kirillov and Reshetikhin. This algebra is also conjugated to the Jucys–Murphy algebra, responsible for the Young orthogonal basis of standard tableaux along the Schur–Weyl duality.
We sketch a reciprocal space analogue of the combinatorial bijection of Robinson- Schensted and Kerov-Kirillov-Reshetikhin (RSKKR) between magnetic configurations (the initial basis for quantum calculations of the eigenproblem of the Heisenberg Hamiltonian for a one-dim finite Heisenberg chain), and rigged string configurations (the classification labels for the exact results of Bethe Ansatz). Existence of such a bijection admits an interpretation of the exact quantum numbers of riggings as quasimomenta of l -strings. The extended size of an l -string results in selection rules for these quasimomenta, and thus in a division of the Brillouin zone into compact subzones of forbidden and allowed states of the system of coupled Bethe pseudoparticles. The forbidden Brillouin subzone for a particular l -string is evidently the effect of kinematical restrictions for motions of constituent Bethe pseudoparticles. These restrictions can be easily predicted in a combinatorially unique way due to completness of the RSKKR bijection.