Quantum phase transitions in exactly solvable one-dimensional compass   models
  We present an exact solution for a class of one-dimensional compass models which stand for interacting orbital degrees of freedom in a Mott insulator. By employing the Jordan-Wigner transformation we map these models on noninteracting fermions and discuss how spin correlations, high degeneracy of the ground state, and $Z_2$ symmetry in the quantum compass model are visible in the fermionic language. Considering a zigzag chain of ions with singly occupied $e_g$ orbitals ($e_g$ orbital model) we demonstrate that the orbital excitations change qualitatively with increasing transverse field, and that the excitation gap closes at the quantum phase transition to a polarized state. This phase transition disappears in the quantum compass model with maximally frustrated orbital interactions which resembles the Kitaev model. Here we find that finite transverse field destabilizes the orbital-liquid ground state with macroscopic degeneracy, and leads to peculiar behavior of the specific heat and orbital susceptibility at finite temperature. We show that the entropy and the cooling rate at finite temperature exhibit quite different behavior near the critical point for these two models. 
Wen-Long You, Peter Horsch and Andrzej M. Ole\'s
cond-mat.str-el quant-ph