Starts 22 Apr 2016 14:00
Ends 22 Apr 2016 15:30
Central European Time
SISSA, room 128
Graphene sheets encapsulated between crystals of boron nitride host a unique electron system that due to weak electron-phonon scattering allows micrometer-scale ballistic transport even at room temperature [1,2,3,4]. Above liquid nitrogen temperatures, these electron liquids are expected to display local equilibrium, enabled by strong electron-electron interactions [5]. Under these conditions, electrons in doped samples are expected to behave as a viscous liquid and may exhibit hydrodynamic phenomena akin to those observed in classical and quantum liquids. In this talk I will report on results of combined theoretical and experimental work [6,7] showing unambiguous evidence for this long-sought transport regime. In particular, I will discuss how high-quality graphene sheets in the Fermi liquid regime (exhibit an anomalous (negative) voltage drop near current injection points, which is attributed to the formation of whirlpools in the electron flow. Measurements of these quasi-local electrical signals enable to extract the value of the kinematic viscosity of the two-dimensional massless Dirac fermion liquid in graphene, which is found to be an order of magnitude larger than that of honey, in quantitative agreement with many-body theory [8]. Finally, I will discuss how our results near the charge neutrality point ( are compatible with the AdS/CFT viscosity bound [9,10].
Our work represents the first step towards the observation of nearly perfect fluidity and quantum turbulence in solid-state devices.

References
[1]    A.S. Mayorov et al., Nano Lett. 11, 2396 (2011).
[2]    L. Wang et al., Science 342, 614 (2013).
[3]    T. Taychatanapat et al., Nature Phys. 9, 225 (2013).
[4]    A. Woessner et al., Nature Mater. 14, 421 (2015).
[5]    M. Polini and G. Vignale, The quasiparticle lifetime in a doped graphene sheet.  In No-nonsense physicist: an overview of Gabriele Giuliani's work and life (eds. M. Polini, G. Vignale, V. Pellegrini, and J.K. Jain) (Edizioni della Normale, Pisa, 2016).
[6]    D. Bandurin, I. Torre, R.K. Kumar, M. Ben Shalom, A. Tomadin, A. Principi, G.H. Auton, E. Khestanova, K.S. NovoseIov, I.V. Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini, Science 351, 1055 (2016). [7]    I. Torre, A. Tomadin, A.K. Geim, and M. Polini, Phys. Rev. B 92, 165433 (2015). [8]    A. Principi, G. Vignale, M. Carrega, and M. Polini, Phys. Rev. B 93, 125410 (2016). [9]    P.K. Kovtun, D.T. Son, and A.O. Starinets, Phys. Rev. Lett. 94, 111601 (2005). [10]    M. Müller, J. Schmalian, and L. Fritz, Phys. Rev. Lett. 103, 025301 (2009).