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Journal de physique

Density of states in the reciprocal lattice for a one-dimensional periodic Heisenberg magnet

Tadeusz Lulek

Published Jan 1, 1984• 14 citations
Publisher

Abstract

A general formula for the number of linearly independent states from (2 s + 1)N-dimensional Hilbert space for a linear chain of N spins s corresponding to a wave number k has been derived. As shown, this density is inhomogeneous within the Brillouin zone, and its shape depends on the decomposition of N into prime factors. It proves that non all elementary excitations in a Heisenberg magnet can be adequately described in terms of quasiparticles, and in particular, they not necessarily exhibit a band structure.

DOI
10.1051/jphys:0198400450102900

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•Jan 1, 2006•31 citations
A general formula for the number of linearly independent states from (2 s + 1)N-dimensional Hilbert space for a linear chain of N spins s corresponding to a wave number k has been derived. As shown, this density is inhomogeneous within the Brillouin zone, and its shape depends on the decomposition of N into prime factors. It proves that non all elementary excitations in a Heisenberg magnet can be adequately described in terms of quasiparticles, and in particular, they not necessarily exhibit a band structure.
The kinematics of the one-dimensional finite Heisenberg magnetic ring with impurities is discussed in the light of the general recipe of Weyl (1952). The cyclic group generating the ring, and the group of all its automorphisms, play the roles of the obvious and hidden symmetry respectively. The hidden symmetry group imposes a property of the distribution of quantum states of a translationally invariant ensemble of magnets over the finite Brillouin zone. Inhomogeneities of the distribution arise from irregular orbits of the action of the cyclic group on the set of all magnetic configurations of the ensemble. Size-dependent effects, like rarefied bands, are classified using the arithmetic structure of integers, (prime numbers, socles and arithmetic exponents), which is more appropriate than the linear order in the ring of integers.
We compare the excitation spectra in the presence of a magnetic field of a number of integrable (exactly solvable) and nonintegrable quantum spin chains of various spin value s. The archetypal Bethe-ansatz integrable model is the s= 1/2 Heisenberg antiferromagnet (HB AFM). The excitation spectra are characterized by a soft mode which tracks across the Brillouin zone as the field increases to its saturation value. A class of Bethe-ansatz integrable models with SU(2) symmetry and the general spin s display excitation spectra qualitatively similar to the spin- 1/2 model above, for all s. A second class of Bethe-ansatz integrable models has SU(n) symmetry, where n=2s+1. Like the SU(2) integrable chains, these models have gapless excitation spectra, but the basic Brillouin zone changes from k=±2π/(2s+1)a. Studies show that periodicity of the SU(3) member of the class changes (increases) as the field increases to saturation. For both classes of integrable models, there is a single type of excitation pattern which is generically similar for all s. In the case of the other models, on the other hand, numerical studies show that the excitations divide into at least two distinct classes. In the case of the s=1 HB AFM, at high fields (corresponding to SzT=N,N−1, . . .,N/2) the excitations map approximately onto the complete set of excitations for s= 1/2 , whereas at low fields (SzT=N/2,N/2−1,. . .,0) the excitations have notable classical character. In the case of the s=1 model with pure biquadratic exchange, one set of excitations, corresponding to SzT even (SzT=N,N−2,. . .,2,0), again shows an approximate mapping to the complete excitation set for s= 1/2 . The second class of excitations, corresponding to SzT odd, are very different. They are symmetric about k=±π/2a for all SzT, i.e., correspond to a basic Brillouin zone of ±π/2a.
Considering the adjacent spin operators SjzSj+1zS^z_jS^z_{j+1}Sjz​Sj+1z​ and Sj−Sj+1−S^-_jS^-_{j+1}Sj−​Sj+1−​ in the S=1/2 Heisenberg chain, we give a determinant representation of their form factors. The dynamical structure factors of the respective operators are computed over the whole Brillouin zone in several magnetic fields and the resulting signal is analyzed in terms of excitation types. Among other results, we find that the SjzSj+1zS^z_jS^z_{j+1}Sjz​Sj+1z​ dynamical structure factor carries a large weight of the 4-spinon excitations which are distinguishable from the 2-spinon signal because they are located outside the 2-spinon spectrum.
The T=0T=0T=0 dynamical properties of the one-dimensional (1D) s=12s=\frac{1}{2}s=21​ Heisenberg antiferromagnet in a uniform magnetic field are studied via the Bethe ansatz for cyclic chains of N sites. The ground state at magnetization 0<Mz<N/2,0<{M}_{z}<N/2,0<Mz​<N/2, which can be interpreted as a state with 2Mz{2M}_{z}2Mz​ spinons or as a state of $N/2\ensuremath{-}{M}_{z}$ magnons, is reconfigured here as the vacuum for a different species of quasiparticles, the psinons and antipsinons. We investigate three kinds of quantum fluctuations, namely the spin fluctuations parallel and perpendicular to the direction of the applied magnetic field and the dimer fluctuations. The dynamically dominant excitation spectra are found to be sets of collective excitations composed of two quasiparticles excited from the psinon vacuum in different configurations. The Bethe ansatz provides a framework for (i) the characterization of the new quasiparticles in relation to the more familiar spinons and magnons, (ii) the calculation of spectral boundaries and densities of states for each continuum, (iii) the calculation of transition rates between the ground state and the dynamically dominant collective excitations, (iv) the prediction of line shapes for dynamic structure factors relevant for experiments performed on a variety of quasi-1D antiferromagnetic compounds, including KCuF3,{\mathrm{KCuF}}_{3},KCuF3​, Cu(C4H4N2)(NO3)2,\mathrm{Cu}({\mathrm{C}}_{4}{\mathrm{H}}_{4}{\mathrm{N}}_{2})({\mathrm{NO}}_{3}{)}_{2},Cu(C4​H4​N2​)(NO3​)2​, and CuGeO3.{\mathrm{CuGeO}}_{3}.CuGeO3​.