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arXiv:cond-mat/9501035 · Journal of Physics A Mathematical and General

Numerical investigation of correlation functions for the UqSU(2) invariant spin-1/2 Heisenberg chain

P F Arndt, T Heinzel

Published Jul 7, 1995• 3 citations
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Abstract

We consider the U q SU(2) invariant spin-1/2 XXZ quantum spin chain at the roots of unity q=exp(i pi /(m+1)), corresponding to different minimal models of conformal field theory. We conduct a numerical investigation of the correlation functions of U q SU(2) scalar two-point operators in order to find which operators in the minimal models they correspond to. Using graphical representations of the Temperley-Lieb algebra we are able to deal with chains of up to 28 sites. Depending on q, the correlation functions show different characteristics and finite-size behaviour. For m=2/3, which corresponds to the Lee-Yang edge singularity, we find the surface and bulk critical exponent -1/5. Together with the known result in the case m=3 (Ising model) this indicates that in the continuum limit the two-point operators involve conformal fields of spin-m-1/m+1. For other roots of unity q the chains are too short to determine the surface and bulk critical exponents.

DOI
10.1088/0305-4470/28/13/004

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We consider the Uq[SU(2)]U_q[SU(2)]Uq​[SU(2)] symmetric Heisenberg chain when q=eiπ/(m+1)q=e^{iπ/(m+1)}q=eiπ/(m+1) and mmm is integer. We consider the cases m=3m=3m=3 and m=5m=5m=5 which correspond to the Ising and 3-state Potts models. We study the finite size scaling (FSS) of the ground states in different quantum spin sectors and restricting to highest weights of type-II representations. We compute the levels by a diagrammatic technique which needs only the commutation relations of the underlying Temperley-Lieb algebra. The results match the FSS predictions which hold for the Bethe levels. (2 PostScript figures (or the corresponding tables) available from the author)
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The authors are concerned with the critical properties of the anisotropic Heisenberg chain, or XXZ model, with arbitrary integer or half-integer spin. The eigenspectra of these Hamiltonians, with periodic boundaries, are calculated for finite chains by solving numerically their associated Bethe ansatz equations. The resulting spectra are found to be in accord with the predictions of conformal invariance and the operator content is identified, for lattices with an even and odd number of sites. The results for spin 1 and spin 3/2 indicate that the conformal anomaly c for these models, in the gapless regime, has the value c=3S/(1+S), independent of the anisotropy, and the exponents vary continuously with the anisotropy as in the eight-vertex model. The operator content of these models indicate that the underlying field theory governing these critical spin-S models is described by composite fields formed by the product of Gaussian and Z(N) fields, with N=2S. Finally some of the irrelevant operators which produce the leading finite-size corrections of the eigenenergies are identified.
The antiferromagnetic Heisenberg chain is expected to have an extended symmetry, $[\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)]/{\mathrm{Z}}_{2}$, in the infrared limit whose physical interpretation is that the spin and dimer order parameters form the components of a common four-dimensional pseudovector. Here we numerically investigate this emergent symmetry using quantum Monte Carlo simulations of a modified Heisenberg chain (the $J\text{\ensuremath{-}}Q$ model) in which the logarithmic scaling corrections of the conventional Heisenberg chain can be avoided. We show how the two- and three-point spin and dimer correlation functions approach their forms constrained by conformal field theory as the system size increases and numerically confirm the expected effects of the extended symmetry on various correlation functions. We stress that sometimes the leading power laws of three-point (and higher) correlations are not given simply by the scaling dimensions of the lattice operators involved but can be faster decaying because of exact cancellations of contributions from the fields and currents under conformal symmetry.