A Matrix Kato-Bloch Perturbation Method for Hamiltonian Systems

A generalized version of the Kato-Bloch perturbation expansion is presented. It consists of replacing simple numbers appearing in the perturbative series by matrices. This leads to the fact that the dependence of the eigenvalues of the perturbed system on the strength of the perturbation is not necessarily polynomial. The efficiency of the matrix expansion is illustrated in three cases: the Mathieu equation, the anharmonic oscillator and weakly coupled Heisenberg chains. It is shown that the matrix expansion converges for a suitably chosen subspace and, for weakly coupled Heisenberg chains, it can lead to an ordered state starting from a disordered single chain. This test is usually failed by conventional perturbative approaches.