From the Hubbard to the SO(5) Ladder: A Numerical Study
  The Hubbard Hamiltonian on a two-leg ladder is studied numerically using quantum Monte Carlo and Exact Diagonalization techniques. A rung interaction, $V$, is turned on such that the resulting model has an exact SO(5) symmetry when $V=-U$. The evolution of the low energy excitation spectrum is presented from the pure Hubbard ladder to the SO(5) ladder. It is shown that the low energy excitations in the pure Hubbard ladder have an approximate SO(5) symmetry. 
Daniel Duffy (UCSB), Stephan Haas (ETH) and Eugene Kim (UCSB)