## "Topological phase transitions from ab initio simulations"
Pablo Aguado Research Fellow, Theory, CIC nanoGUNE In recent years the issue of topological phase transitions has been actively investigated, including by means of ab initio simulations. Here we will present two examples that showcase not only the predictive power of ab initio methods but also some of the potential pitfalls in the study of topological phase transitions. First we will discuss the pressure-induced topological phase transition in PbTe. Conventional local or semilocal exchange-correlation approximations in density functional theory (DFT) calculation are well known to give the wrong band ordering in PbTe, predicting a non-trivial topology of its electronic structure at the equilibrium volume. The correct trivial topology at zero pressure is recovered adding G0W0 quasiparticle corrections. However, we find that the conventional diagonal G0W0 approximation produces artifacts in the band structure due to the wrong orbital character of the DFT single-particle states. We show that an inexpensive correction from the off-diagonal elements of the G0W0 self-energy is enough to correct this artifacts and, for example, recover the characteristic linear dispersion of electrons at the topological transition. Another, more fundamental question in the physics of topological insulators is whether the topologically nontrivial properties survive at finite temperatures and, if so, whether they disappear only at the temperature of topological gap closing. We study this problem, using quantum fidelity as a measure, by means of ab initio methods supplemented by an effective dissipative theory built on the top of the ab initio electron and phonon band structures. In the case of SnTe, the prototypical crystal topological insulator, we reveal the presence of a characteristic temperature, much lower than the gap-closing one, that marks a loss of coherence of the topological state. This result suggests that the conventional criterion for the phase transition, i.e. the closing of the gap, might not always be valid. |