Using the framework of the quantum separation of variables (SoV) for
higher rank quantum integrable lattice models , we introduce some
foundations to go beyond the obtained complete transfer matrix spectrum
description, and open the way to the computation of matrix elements of
local operators. This first amounts to obtain simple expressions for
scalar products of the so-called separate states, that are transfer
matrix eigenstates or some simple generalization of them. In the higher
rank case, left and right SoV bases are expected to be
pseudo-orthogonal, that is for a given SoV co-vector
\langle\underline{\mathbf{h}}\rangle⟨𝐡̲⟩,
there could be more than one non-vanishing overlap
\langle{\underline{\mathbf{h}}}|{\underline{\mathbf{k}}}\rangle⟨𝐡̲|𝐤̲⟩
with the vectors |{\underline{\mathbf{k}}}\rangle|𝐤̲⟩
of the chosen right SoV basis. For simplicity, we describe our method to
get these pseudo-orthogonality overlaps in the fundamental
representations of the \mathcal{Y}(gl_3)𝒴(gl3)
lattice model with NN
sites, a case of rank 2. The non-zero couplings between the co-vector
and vector SoV bases are exactly characterized. While the corresponding
SoV-measure stays reasonably simple and of possible
practical use, we address the problem of constructing left and right SoV
bases which do satisfy standard orthogonality (by standard we mean
\langle{\underline{\mathbf{h}}}|{\underline{\mathbf{k}}}\rangle \propto \delta_{\underline{\mathbf{h}}, \underline{\mathbf{k}}}⟨𝐡̲|𝐤̲⟩∝δ𝐡̲,𝐤̲).
In our approach, the SoV bases are constructed by using families of
conserved charges. This gives us a large freedom in the SoV bases
construction, and allows us to look for the choice of a family of
conserved charges which leads to orthogonal co-vector/vector SoV bases.
We first define such a choice in the case of twist matrices having
simple spectrum and zero determinant. Then, we generalize the associated
family of conserved charges and orthogonal SoV bases to generic simple
spectrum and invertible twist matrices. Under this choice of conserved
charges, and of the associated orthogonal SoV bases, the scalar products
of separate states simplify considerably and take a form similar to the
\mathcal{Y}(gl_2)𝒴(gl2)
rank one case.