Attività del Dipartimento

Colloqui di Fisica

Frequent collisions between constituents in a classical or quantum liquid (like ³He) manifest through a transport coefficient called the shear viscosity [1]. The flow of these liquids, and also that of exotic quantum many-particle systems like ultracold ⁶Li atoms near a Feshbach resonance [2] and quark-gluon plasmas at relativistic heavy-ion colliders [3], is often described by three equations expressing the conservation of mass, momentum (the Navier-Stokes equation), and energy.
Realizing hydrodynamic transport in a solid has proven challenging, because of ever present processes that lead to momentum dissipation in the electron subsystem [4]. Even when suitable conditions are met, key questions have remained largely unexplored: how do you diagnose the emergence of hydrodynamic electron flow in a conventional field-effect transistor? How do you measure the viscosity of an electron liquid in such a setup? What is the impact of viscosity on electron transport?
In this Colloquium I will try and answer these questions. I will first report on results of combined theoretical and experimental work [5,6,7] showing unambiguous evidence for the long-sought hydrodynamic solid-state transport regime. In particular, I will discuss how high-quality doped graphene sheets above liquid nitrogen temperatures exhibit negative non-local resistance near current injection points and whirlpools in the spatial current pattern [5,6,7]. Measurements of these non-local electrical signals allow to extract the value of the kinematic viscosity of the two-dimensional electron liquid in graphene, which is found to be an order of magnitude larger than that of honey and to compare well with many-body theoretical predictions [8]. I will then discuss viscous electron transport across a point contact [9] and ideas on how to probe hydrodynamic behavior via the use of engineered short-wavelength plasmon-phonon polaritons in hybrid stacks containing graphene, Boron Nitride, and metal gates [10].