Topological phase transitions from ab initio simulations
CIC nanoGUNE Seminars
 Speaker

Pablo Aguado, Research Fellow, Theory, CIC nanoGUNE
 When

2022/11/07
12:00  Place
 nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia  San Sebastian
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**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 pressureinduced topological
phase transition in PbTe. Conventional local or semilocal exchangecorrelation
approximations in density functional theory (DFT) calculation are well known
to give the wrong band ordering in PbTe, predicting a nontrivial 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
singleparticle states. We show that an inexpensive correction from the off
diagonal elements of the G0W0 selfenergy 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 gapclosing 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.