Session X05: Hydrodynamic electron flow in topological materials

Experiments on electron hydrodynamics with and without applied magnetic fields

I will describe recent experiments studying electrical transport both in bulk and width-restricted channels of the delafossite metals PdCoO₂ and PtCoO₂. Data taken both in magnetic fields and in zero applied magnetic fields will be discussed, as will recent theoretical progress in understanding the pronounced deviations that are seen from standard ohmic transport.   

Electron hydrodynamics beyond momentum and viscosity

The key feature of recent experimental realizations of hydrodynamic transport in metals has been the presence of a velocity field in the long wavelength description of the system. This field arises due to a long-lived momentum density and a signature consequence are non-Ohmic viscous effects in transport. In this talk I will discuss various scenarios in which additional collective modes, beyond momentum, can influence the long-wavelength description of the material. These modes can arise due to (in general fluctuating) order parameters such as superfluidity, as well as additional conserved quantities such as spin, transverse momentum or imbalance modes, and lead to a different classes of unconventional, hydrodynamic transport. I will give an overview of experimental signatures of these regimes.   

Transport without quasiparticles in graphene and Weyl semi-metals

Graphene and Weyl semi-metals have electronic excitations with a massless Dirac spectrum. At charge neutrality, the long-range Coulomb interaction remains unscreened, and electronic quasiparticles are not well defined. In sufficiently clean samples, thermal and electrical transport enters a hydrodynamic regime: insights on the nature of transport in this regime have emerged from holographic connections to the dynamics of black holes. A large enhancement of the Wiedemann-Franz ratio of the thermal and electrical conductivities was predicted, and is in good agreement with observations on graphene. In Weyl semi-metals, an axial-gravitational anomaly was employed to predict magnetotransport properties, and is consistent with a positive longitudinal magnetothermoelectric conductance observed in NbP.   

Hydrodynamic flow of electrons in topological semimetals

Materials with strongly-correlated electrons exhibit interesting phenomena such as metal-insulator transitions and high-temperature superconductivity. In stark contrast to ordinary metals, electron transport in these materials is thought to resemble the flow of viscous fluids. Despite their differences, it is predicted that transport in both, conventional and correlated materials, is fundamentally limited by the uncertainty principle applied to energy dissipation. Here we present recent experiments on the hydrodynamic electron flow in the Weyl-semimetal WP₂ as well as in the Dirac semimetal PtSn₄. Using thermal and magneto-electric transport experiments to explore WP₂, we observe the transition from a conventional metallic state, at higher temperatures, to a hydrodynamic electron fluid below 20 K. The hydrodynamic regime is characterized by a viscosity-induced dependence of the electrical resistivity on the square of the channel width, and by the observation of a strong violation of the Wiedemann-Franz law. From magneto-hydrodynamic experiments and complementary Hall measurements, the relaxation times for momentum and energy dissipating processes are extracted. Following the uncertainty principle, both are limited by the Planckian bound of dissipation, independent of the underlying transport regime. Moreover, exprimental signatures of a hydrodynamic electron-phonon liquid in PtSn₄ will be discussed.
  

Viscous electron flow in graphene

I will review our experiments on measuring electron viscosity in graphene and discuss its non-trivial consequences.