Stefano Baroni
(SISSA)

Recent theoretical advances, based Thouless’ topological theory of adiabatic charge transport, have allowed for a rigorous quantum-mechanical definition of atomic oxidation states in condensed matter and provided a practical way to compute them from first principles. By the time this result was achieved, the microscopic theory of thermal conduction was laid on a much more solid ground than ever before, thanks to the introduction of the so-called gauge invariance of heat transport coefficients. Gauge invariance stipulates that, although the energy flux used to calculate heat conductivity via the the Green-Kubo formula is inherently ill-defined, the resulting transport coefficient remains unaffected by such an indeterminacy, as it ought to be, being a measurable quantity. By combining the topological definition of oxidation states with the gauge invariance of transport coefficients, it was demonstrated that, under rather general topological conditions, the electric conductivity of ionic conductors can be expressed in terms of integer-valued, scalar, and time-independent atomic oxidation numbers, instead of real-valued, tensor, and time-dependent Born charges. The breakdown of some of these conditions determines a nontrivial conduction regime—intermediate between ionic and electronic transport—whereby charge can flow without any mass convection, even in electronic insulators. In this talk, I will review these advances, present the results of a few numerical experiments demonstrating them, and discuss their connection with the well-established Marcus theory of electron transfer.

 
Coffee and tea refreshments will be served after the Seminar in the Leonardo Bar

 

Zoom registration link:
https://zoom.us/meeting/register/tJYrdOqurzwvG9fjdUXuwOP02J5L3xTOSK4z