MARVEL Distinguished Lecture — Stefano Baroni
A recording of the event is available here:
Gauge invariance of heat and charge transport coefficients
Transport coefficients have been recently shown to be largely independent of the microscopic representation of the current density of the conserved quantity being transported (charge/mass/energy) . This remarkable gauge invariance has been leveraged to lay down a rigorous density-functional theory of heat transport , as well as a general approach to it in solids, that nicely bridges the Boltzmann-Peierls kinetic model, which applies to crystals, and the Allen-Feldman one, which applies to glasses . In the case of charge transport, a combination of gauge invariance and Thouless’ quantisation of particle transport  allows one to express the electrical conductivity of a stoichiometric ionic conductor in terms of integer-valued, scalar, and time-independent atomic oxidation numbers, instead of real-valued, tensor, and time-dependent Born charges . The departure of non stoichiometric systems from this picture, due to the existence of localised electron pairs, can be fathomed in terms of topological effects on charge transport . In this talk I will review these concepts and report on some key applications of them to liquids and glasses.
 A. Marcolongo, P. Umari, and S. Baroni, Nat. Phys. 12, 80 (2016);
 L. Isaeva, G. Barbalinardo, D. Donadio, and S. Baroni, Nat. Commun. 10, 3853 (2019);
 D.Thouless, Phys. Rev. B, 27, 6083 (1983);
 F. Grasselli and S. Baroni, Nat. Phys. 15, 967 (2019);
 P. Pegolo, F. Grasselli, and S. Baroni, Phys. Rev. X, in press.
About the speaker
Stefano Baroni received his Dottore in Fisica degree from the University of Pisa in 1978. He is currently a full professor of theoretical condensed matter physics at the Scuola Internazionale Superiore di Studi Avanzati (SISSA) in Trieste, Italy. Stefano Baroni's scientific interests are at the frontier between theory and simulation: he likes to invent methods to compute properties and simulate processes previously deemed inaccessible to scientific computation, and to apply them to problems that are scientifically and technologically important. He is largely credited for the introduction of density-functional perturbation theory (DFPT), a methodology that is considered the state of the art for the computation of lattice dynamical properties in solids, including phonon frequencies and lifetimes. He has pioneered O(N) methods in electronic-structure theory and he has also introduced important innovations in quantum stochastic simulations. Recently, he has successfully extended DFPT so as to encompass electronic excited states through time-dependent density-functional and many-body perturbation theories. He has thoroughly applied these methodological innovations to a number of problems in semiconductor physics, the chemical physics of metal surfaces, and, more recently, molecular and magnetic spectroscopies. Over the past 5 years he has given important contributions to the theory and numerical simulation of adiabatic heat and charge transport in liquid and disordered systems.
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