Topological ground-state excitations and symmetry in the many-electron   one-dimensional problem
  We consider the Hubbard chain in a magnetic field and chemical potential. We introduce a pseudohole basis where all states are generated from a single reference vacuum. This allows the evaluation for all sectors of Hamiltonian symmetry of the model of the expression of the $\sigma $ electron and hole operators at Fermi momentum $\pm k_{F\sigma }$ and vanishing excitation energy in terms of pseudohole operators. In all sectors and to leading order in the excitation energy the electron and hole are constituted by one $c$ pseudohole, one $s$ pseudohole, and one {\it topological momenton}. These three quantum objects are confined in the electron or hole and cannot be separated. We find that the set of different pseudohole types which in pairs constitute the two electrons and two holes associated with the transitions from the $(N_{\uparrow }, N_{\downarrow })$ ground state to the $(N_{\uparrow }+1,N_{\downarrow })$, $(N_{\uparrow },N_{\downarrow }+1)$ and $(N_{\uparrow }-1, N_{\downarrow })$, $(N_{\uparrow },N_{\downarrow }-1)$ ground states, respectively, transform in the representation of the symmetry group of the Hamiltonian in the initial-ground-state sector of parameter space. We also find the pseudohole generators for the half-filling holon and zero-magnetic-field spinon. The pseudohole basis introduced in this paper is the only suitable for the extension of the present type of operator description to the whole Hilbert space. 
J.M.P. Carmelo and N.M.R. Peres