By: José M. P. Carmelo

From: Univ. Minho

At: Instituto de Investigação Interdisciplinar, Anfiteatro

[2011-06-09] 11:30

A trivial result is that at onsite repulsion U = 0 the global symmetry of the half-filled Hubbard model on a bipartite lattice is O(4) = SO(4) Ã— Z_{2}. Here the factor Z_{2} refers to the particle-hole transformation on a single spin under which the model Hamiltonian is not invariant for U /= 0. C. N. Yang and S. C. Zhang considered the most natural possibility that the SO(4) symmetry inherited from the U /= 0 Hamiltonian O(4) = SO(4) Ã— Z_{2 } symmetry was the model global symmetry for U > 0.[1] However, a recent study of the problem by the author and collaborators [2] revealed an exact extra hidden global U(1) symmetry emerging for U /= 0 in addition to SO(4), so that the model global symmetry is [SO(4) Ã— U(1)]/Z_{2} = SO(3) Ã— SO(3) Ã— U(1) = [SU(2) Ã— SU(2) Ã— U(1)]/Z_{2}^{2} . The factor 1/Z_{2}^{2} in SO(3) Ã— SO(3) Ã— U(1) = [SU(2) Ã— SU(2) Ã— U(1)]/Z_{2}^{2} imposes that both [S_{c} + S_{c}] and [S_{e} + S_{c}] are integer numbers. Here S_{e}, S_{s}, and S_{c} are the e-spin, the spin, and the eigenvalue of the generator of the new global U(1) symmetry, respectively. The latter is found in [2] to be one half the number of rotated-electron singly occupied sites. The extra hidden global U(1) symmetry is related to the U /= 0 local SU(2) Ã— SU(2) Ã— U(1) gauge symmetry of the Hubbard model on a bipartite lattice with transfer integral t = 0.[3] Such a local SU(2) Ã— SU(2) Ã— U(1) gauge symmetry becomes for finite U and t a group of permissible unitary transformations. Rather than the ordinary U(1) gauge subgroup of electromagnetism, for finite U/t here U(1) refers to a â€œnonlinearâ€ transformation.[3] Since the chemical-potential and magnetic-field operator terms commute with the Hamiltonian, for all densities its energy eigenstates refer to representations of the new found global SO(3)Ã—SO(3)Ã—U(1) = [SO(4)Ã—U(1)]/Z_{2} symmetry, which is expected to have important physical consequences. In addition to introducing the new-found extended global symmetry, in this talk some preliminary physical consequences are reported for the Hubbard model on the bipartite square lattice.[4]

1. C. N. Yang and S. C. Zhang, Mod. Phys. Lett. B 4, 758 (1990); S. C. Zhang, Phys. Rev. Lett. 65, 120 (1990).

2. J. M. P. Carmelo, Stellan Â¨Ostlund, and M. J. Sampaio, Ann. Phys. 325, 1550 (2010).

3. Stellan Â¨Ostlund and Eugene Mele, Phys. Rev. B 44, 12413 (1991).

4. J. M. P. Carmelo, Nucl. Phys. B 824, 452 (2010); Nucl. Phys. B 840, 553 (2010).