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In this seminar I will review theoretical results and experimental observations of charge and heat transport in strongly-interacting monolayer graphene. Conducting materials typically exhibit either diffusive or ballistic charge transport. However, when electron-electron interactions dominate, a “hydrodynamic” regime emerges. Under these conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic phenomena similar to classical liquids. What is ever more striking, is that the Wiedemann-Franz law, which connects heat and charge transport in metals, is intrinsically violated in this transport regime. This fact can be used to increase thermoelectric conversion efficiencies of graphene-based devices. More stringent conditions yield, in undoped graphene, to a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge. The transition to this regime has been recently revealed, at room temperature, using spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution. There, the heat spreading observed corresponds to a giant diffusivity indicative of a Dirac fluid. Finally, I will comment on the violation of the Wiedemann-Franz law in conventional semimetals, discussing how graphene can help shed light on this long-standing problem.
Dr. Principi graduated in 2009 from the University of Pisa (Italy) and obtained his PhD in 2013 from Scuola Normale Superiore (Pisa, Italy). After two postdocs in the USA (University of Missouri, Columbia, MO) and in the Netherlands (Radboud University, Nijmegen), he joined the University of Manchester in 2017 and has been promoted to Senior Lecturer in 2021. He is the author of 67 publications which have received more than 4600 combined citations, referee for articles in major international journals, and grant reviewer for the Leverhulme Trust and the US Department of Energy. Dr. principi leads a group composed by 2 PDRAs, funded by the Leverhulme Trust, and 6 PGR students (plus 3 in co-supervision). His works on the hydrodynamic regime of electron transport allowed him to become the local coordinator of the MSCA-RISE network “Hydrotronics” funded by the EU Commission, which encompasses 7 renown worldwide institutions (University of Manchester, KIT, Weizmann Institute, IIT, and Radboud University, University of Chile and Louisiana State University). He has also co-organized the advanced school and conference “Low-Dimensional Quantum Systems” (Chile 2023).