Entanglement Driven Phase Transitions in Spin-Orbital Models

To demonstrate the role played by the von Neumann entropy spectra in quantum phase transitions we investigate the one-dimensional anisotropic SU(2)$\otimes XXZ$ spin-orbital model with negative exchange parameter. In the case of classical Ising orbital interactions we discover an unexpected novel phase with Majumdar-Ghosh-like spin-singlet dimer correlations triggered by spin-orbital entanglement and having $k=\pi/2$ orbital correlations, while all the other phases are disentangled. For anisotropic $XXZ$ orbital interactions both spin-orbital entanglement and spin-dimer correlations extend to the antiferro-spin/alternating-orbital phase. This quantum phase provides a unique example of two coupled order parameters which change the character of the phase transition from first-order to continuous. Hereby we have established the von Neumann entropy spectral function as a valuable tool to identify the change of ground state degeneracies and of the spin-orbital entanglement of elementary excitations in quantum phase transitions.