Speaker
Description
Topological quantum phase transitions (TQPTs) describe a change in the electronic band structure (as in topological insulators), or in the shape of the Fermi surface (Lifshitz). It is commonly assumed that TQPTs don’t follow the conventional Ginzburg-Landau scheme, nonetheless this is not at odds with the possibility of a description based on the relevant observables of the system, beside the change in the global topological invariants.
Here, we introduce a functional integral fluctuations method to calculate the free energy for a paradigmatic model of the interacting quantum spin Hall insulators, where the strong electronic interactions are expected to drive a first order jump in the orbital polarization at the TQPT. In particular, we demonstrate that the mean-field approximation fails to capture this change of character, therefore emphasizing the fundamental role of the dynamical local fluctuations. Within our theory we address directly the response functions and identify a quantum critical endpoint along the transition line separating distinct insulating phases, corresponding to the divergence of the orbital compressibility. We establish that the discontinuous TQPT is determined by a synergetic coupling between the charge and the orbital polarization fluctuations.
Finally, we discuss the universality of our results with respect to the specific choice of the microscopic Hamiltonian.
