The Hubbard model in bipartite lattice has been rigorously proved to have $\ensuremath{\eta}$-paired states as eigenstates by C. N. Yang [Phys. Rev. Lett. 63, 2144 (1989)]. In this paper, this result is first generalized to a triangular lattice and then even to an arbitrary lattice structure. We also consider its generalization to higher-spin Hubbard models. The conditions under which these models have $\ensuremath{\eta}$-paired states as eigenstates were obtained.
The Hubbard model in bipartite lattice has been rigorously proved to have \ensuremathη-paired states as eigenstates by C. N. Yang [Phys. Rev. Lett. 63, 2144 (1989)]. In this paper, this result is first generalized to a triangular lattice and then even to an arbitrary lattice structure. We also consider its generalization to higher-spin Hubbard models. The conditions under which these models have \ensuremathη-paired states as eigenstates were obtained.
The $\ensuremath{\eta}$-pairing states are a set of exactly known eigenstates of the Hubbard model on hypercubic lattices, first discovered by Yang [C. N. Yang, Phys. Rev. Lett. 63, 2144 (1989)]. These states are not many-body scar states in the Hubbard model because they occupy unique symmetry sectors defined by the so-called $\ensuremath{\eta}$-pairing SU(2) symmetry. We study an extended Hubbard model with bond-charge interactions, popularized by Hirsch [J. E. Hirsch, Physica C 158, 326 (1989)], where the $\ensuremath{\eta}$-pairing states survive without the $\ensuremath{\eta}$-pairing symmetry and become true scar states. We also discuss similarities between the $\ensuremath{\eta}$-pairing states and exact scar towers in the spin-1 XY model found by Schecter and Iadecola [M. Schecter and T. Iadecola, Phys. Rev. Lett. 123, 147201 (2019)], and systematically arrive at all nearest-neighbor terms that preserve such scar towers in one dimension. We also generalize these terms to arbitrary bipartite lattices. Our study of the spin-1 XY model also leads us to several scarred models, including a spin-1/2 ${J}_{1}\ensuremath{-}{J}_{2}$ model with Dzyaloshinskii-Moriya interaction, in realistic quantum magnet settings in one and two dimensions.
Generalized η-pairing approach to interacting non-Hermitian systems in arbitrary dimensions
Developing a general and rigorous analytical approach to non-Hermitian many-body systems is a challenging task. Here, we generalize the eta-pairing theory to very general non-Hermitian Hubbard models and find many novel phenomena without Hermitian analogs. For instance, the Hermitian conjugate of an eta-pairing eigenoperator may not be an eigenoperator, eta-pairing eigenoperators can be spatially modulated, and the SU(2) pseudospin symmetry may not be possible even if H commutes with the eta-pairing operators. Remarkably, these novel non-Hermitian phenomena are closely related to each other by several theorems we establish and can lead to, for example, new types of eta-pairing operators (e.g., the notion of non-Hermitian angular-momentum operators) and the Fock space localization of many-body eta-pairing eigenstates. Some issues on the SO(4) and particle-hole symmetries are clarified. Our general eta-pairing theory also reveals a previously unnoticed unification of these symmetries of the Hubbard model. These general results can be illustrated with concrete examples such as the generalized Hatano-Nelson-Hubbard models and a general two-sublattice model. In particular, the general two-sublattice model can reveal the eta-pairing structure [e.g., the SO(4) symmetry] in systems with Hermitian hoppings, including the original eta-pairing theory for square lattice, the extension to triangular lattice, and some topological systems. Our results establish a new and rigorous theoretical framework for studying interacting non-Hermitian systems in arbitrary spatial dimensions, even without bulk translation symmetry.
We investigate Hubbard models with bond-charge interactions on general graphs. For a Hamiltonian H of such a model, we provide the condition on its parameters under which the η-pairing method can be employed to construct its exact eigenstates. We arrive at this condition by finding that the requirement for the η-pairing state (η†)N∣0⟩ to be an eigenstate of H is identical to the requirement for it to be an eigenstate of a Hubbard-type Hamiltonian Hm. When the condition for (η†)N∣0⟩ to be an eigenstate of the Hubbard-type Hamiltonian Hm is satisfied, we demonstrate that there are additional states, distinct from (η†)N∣0⟩, which are also exact eigenstates of Hm. Our results enhance the understanding of Hubbard models on general graphs, both with and without bond-charge interactions.
Abstract We investigate Hubbard models with bond–charge interactions on general graphs. For a Hamiltonian H of such a model, we provide the condition on its parameters under which the η -pairing method can be employed to construct its exact eigenstates. We arrive at this condition by finding that the requirement for the η -pairing state ( η † ) N ∣ 0 〉 to be an eigenstate of H is identical to the requirement for it to be an eigenstate of a Hubbard-type Hamiltonian H m without bond–charge interactions. When the condition for ( η † ) N ∣ 0 〉 to be an eigenstate of the Hubbard-type Hamiltonian H m is satisfied, we demonstrate that there are additional states, distinct from ( η † ) N ∣ 0 〉 , which are also exact eigenstates of H m . Our results enhance the understanding of Hubbard models on general graphs, both with and without bond–charge interactions.
We consider N-particle generalizations of $\ensuremath{\eta}$-pairing states in a chain of N-component fermions and show that these states are exact (high-energy) eigenstates of an extended SU(N) Hubbard model. We compute the singlet correlation function of the states and find that its behavior is qualitatively different for even and odd N. When N is even, these states exhibit off-diagonal long-range order in N-particle reduced density matrix. On the other hand, when N is odd, the correlations decay exponentially with distance in the bulk, but end-to-end correlations do not vanish in the thermodynamic limit. Finally, we prove that these states are the unique ground states of suitably tailored Hamiltonians.
We consider the Hubbard model and its extensions on bipartite lattices. We define a dynamical group based on the -pairing operators introduced by Yang, and define coherent pairing states, which are combinations of eigenfunctions of -operators. These states permit exact calculations of numerous physical properties of the system, including energy, various fluctuations and correlation functions, including pairing off-diagonal long-range order to all orders. This approach is complementary to BCS, in that these are superconducting coherent states associated with the exact model, although they are not eigenstates of the Hamiltonian.