Lower bound for ground state energy of BEC in a rotating optical lattice
  We use the frustrated XY model approximation of BEC in a rotating optical lattice and formulate the problem of the ground state in terms of eigenvectors and eigenvalues of frustrated adjacency matrix (coupling matrix). By using this formulation, we show that there is a lower bound for ground state energy in terms of maximum eigenvalue of this matrix. 
Y. Azizi
cond-mat.quant-gas