$U_q\mathfrak{sl}_2$-invariant non-compact boundary conditions for the XXZ spin chain

Abstract

We introduce new Uqsl2-invariant boundary conditions for the open XXZ spin chain. For generic values of q we couple the bulk Hamiltonian to an infinite-dimensional Verma module on one or both boundaries of the spin chain, and for q = e iπ p a 2p-th root of unity -to its p-dimensional analogue. Both cases are parametrised by a continuous "spin" α ∈ C.To motivate our construction, we first specialise to q = i, where we obtain a modified XX Hamiltonian with unrolled quantum group symmetry, whose spectrum and scaling limit is computed explicitly using free fermions. In the continuum, this model is identified with the (η, ξ) ghost CFT on the upper-half plane with a continuum of conformally invariant boundary conditions on the real axis. The different sectors of the Hamiltonian are identified with irreducible Virasoro representations.Going back to generic q we investigate the algebraic properties of the underlying lattice algebras. We show that if q α / ∈ ±q Z , the new boundary coupling provides a faithful representation of the blob algebra which is Schur-Weyl dual to Uqsl2. Then, modifying the boundary conditions on both the left and the right, we obtain a representation of the universal two-boundary Temperley-Lieb algebra. The generators and parameters of these representations are computed explicitly in terms of q and α. Finally, we conjecture the general form of the Schur-Weyl duality in this case.This paper is the first in a series where we will study, at all values of the parameters, the spectrum and its continuum limit, the representation content of the relevant lattice algebras and the fusion properties of these new spin chains.

Fused K-operators and the $q$-Onsager algebra

“…• Finally, it is very desirable to construct K-operators of arbitrary complex spins, i.e., associated to U q sl 2 Verma modules of complex weights, as it would give essentially the corresponding universal K-matrix. Such integrable quantum spin-chains with integrable boundary conditions based on the Verma modules were recently introduced [CGS22], and it was shown [CGJS22] a deep connection to the q-Onsager algebra via the common XXZ spin chain spectrum with the integrable non-diagonal boundary conditions.…”