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Acta Physica Polonica A

Three-Diagonal Blocks of Two-Magnon Hamiltonians with Specified Wave Numbers for Heisenberg Rings

M. Łabuz, Jan Milewski

Published Jun 1, 2019• 1 citation
Publisher

Abstract

We demonstrate the method of creating blocks of Hamiltonian matrices for Heisenberg rings with N nodes and r = 2 overturned spins, depending on total quasimomentum k.Initial problem of dimension N 2 reduces, approximately, N -tuply, depending on N and k numbers.We consider block Hamiltonians using a particular basis, called wavelet basis.In this basis, these blocks take three-diagonal form.

DOI
10.12693/aphyspola.135.1255

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We demonstrate the method of creating blocks of Hamiltonian matrices for Heisenberg rings with N nodes and r = 2 overturned spins, depending on total quasimomentum k.Initial problem of dimension N 2 reduces, approximately, N -tuply, depending on N and k numbers.We consider block Hamiltonians using a particular basis, called wavelet basis.In this basis, these blocks take three-diagonal form.
Finite spin models, applicable to investigations of mesoscopic rings, give rise to eigenproblems of very large dimensions.Efficient, and as exact as possible, solutions of such eigenproblems are very difficult.A method leading to block diagonalization of Hamiltonian matrix is proposed in this paper.For a given symmetry group of a Heisenberg Hamiltonian commuting with the total spin projection (i.e. with the total magnetization being a good quantum number) appropriate combinatorial and group-theoretical structures (partitions, orbits, stabilizers etc.) are introduced and briefly discussed.Generation of these structures can be performed by means of algorithms being modifications of standard ones.Main ideas are presented in this paper, whereas the actual form of algorithms will be discussed elsewhere.
Abstract We propose some operator techniques, applicable in the procedure of an immediate diagonalization for the Heisenberg chain, such as projection operators for subspaces with definite quasimomentum, total or partial spin, quasimomenta of strings etc. These techniques yield, in particular, matrices of appropriate Clebsch–Gordan coefficients, and reduce significantly the size of an effective Hamiltonian matrix. We apply the basis of wavelets to exploit the translational symmetry, along the Schwinger scheme of unitary geometry, applied to a single N‐dimensional magnon space (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Abstract This paper presents a numerical method for solving Schrödinger equations with arbitrary potentials with the adoption of the qunatum diagonalization scheme and basis functions in the discrete wavelet transform. The quantum diagonalization scheme, developed to solve the Hubbard model and so on, is known to allow the diagonalization of a Hamiltonian matrix of a much large size because it does not require to maintain the entire matrix in memory. And, the adoption of wavelet basis functions enables representing eigenstates efficiently using a number of bases much smaller than the number of grid points to integral and reducing the computation time to perform the diagonalization process. With the help of another additional techniques, the method allows to find solutions of a Schrödinger equation much exactly and efficiently. The validity of the method and its efficiency depending on several conditions are proven and surveyed by applying it to several problems for which the exact solutions are known.
The study of quantum models for magnetism requires the diagonalization of large matrices. For electrons occupying a lattice of sixteen sites, the dimension of these matrices is 65,536 x 65,536. I have written a program in Mathematica to exploit symmetries of the model and render the diagonalization of the Hamiltonian matrix tractable. Specifically, the program utilizes four different operators: translational in the x and y directions (Tx and Ty), total spin in one direction (Sz), and total spin squared (S2). As symmetries, all of these operators commute with the Hamiltonian and therefore the eigenstates of the Hamiltonian are simultaneously eigenstates of these operators. In practice, the eigenstates of these symmetry operators are easier to discover then those of the Hamiltonian. The most difficult symmetry to exploit is the S2 symmetry. In order to generate this symmetry, I reference Clebsch-Gordan coefficient tables. These tables exist to aid in the addition of angular momentum under the laws of quantum mechanics. The tables are generally used to couple two spins. When adding sixteen spins, the coefficients are multiplied together iteratively, adding one spin at a time up to the desired number. The products of these coefficients are then employed to construct all 65,536 eigenstates of the S2 operator. Combined with the remaining symmetry operators, these states can be used to put the Hamiltonian into a simpler, block diagonal form that is readily diagonalized.
Employing the method of a symmetry lowering (form the permutation group Pn to the dihedral group Cnv), the eigenstates of the ring Heisenberg Hamiltonian H=-2J ∑ni=1Si·Si+1 are classified not only by total spin S and its z-component M, but also by wave number k=2 πL/n, where L= ±0, ±1, ±2, ∼, ±p for n=2p+1 or 2p. By the characters of irreducible representation of Pn and Cnv, we express the sum of the energy eigenvalues at fixed S, M and L, together with the number of the eigenstates contained in this sum. With respect to the spin configurations in the ring, it is revealed that there exist invariant subspaces under all symmetry operations of Cnv in the space of the spin-functions specified by M. By connecting this fact with the Wigner coefficients, the eigenfunctions are easily obtained.
A general treatment of the symmetry for quantum spin systems is presented with the use of permutational operators. Symmetry operations which are commutable with a Hamiltonian form the symmetry group. Complete sets of spin functions which are specified by the total spin ( S tot ) are projected onto the irreducible representations of the symmetry group, using the character table. We show from symmetry consideration that a minimum dimension exists for the ground state of the Heisenberg Hamiltonian with finite N spins. The diagonalization of the Hamiltonian with the use of symmetry consideration can be effectively performed for the complete sets of S tot =0 by the method proposed in this paper. Numerical results of the ground state and all the excited states of S tot =0 are presented for spin systems up to N =20.