Dynamical properties of spin-orbital chains in a magnetic field - art. no.024423

The excitation spectrum of the one-dimensional spin-orbital model in a magn etic field is studied, using a recently developed dynamical density-matrix renormalization-group technique. The method is applied to chains with up to 80 sites, and examined for test cases such as the one-dimensional spin-1/2 Heisenberg antiferromagnet whose excitation spectrum is known exactly from the Bethe ansatz. In the spin-orbital chain, the characteristic dynamical response depends strongly on the model parameters and the applied magnetic field. The coupling between the spin and orbital degrees of freedom is foun d to influence the incommensuration at finite magnetizations. In the region s of the phase diagram with only massive spin and orbital excitations, a fi nite field is required to overcome the spin gap. An orbital mode is found t o become massless in this partially spin-polarized regime, indicating a str ong coupling between the two degrees of freedom. In the critical region wit h three elementary gapless excitations, a prominent particle-hole excitatio n is observed at higher energies, promoted by the biquadratic term in the m odel Hamiltonian of the spin-orbital chain.