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The modified XXZ Heisenberg chain, conformal invariance, surface exponents of c<1 systems, and hidden symmetries of the finite chains

Uwe Grimm, Vladimir Rittenberg

hep-tharXiv:hep-th/0311085

Abstract

The spin-1/2 XXZ Heisenberg chain with two types of boundary terms is considered. For the first type, the Hamiltonian is hermitian but not for the second type which includes the Uq[SU(2)] symmetric case. It is shown that for a certain `tuning' between the anisotropy angle and the boundary terms the spectra present unexpected degeneracies. These degeneracies are related to the structure of the irreducible representations of the Virasoro algebras for c<1.

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