Van der Waals colloquium - Philip Moll (MPI Dresden): 'Hydrodynamic flow of electrons'



16:00 hrs


Sitterzaal, Oort building, Leiden


How similar is the flow of electrons in a solid to the flow of water in a pipe? Early on physicists envisioned these processes to be related and this picture prevails in our language (“an electric current flows”). However, this simple fluid analogy fails to describe the transport of electrons in a wire. The defining property of liquids is the conservation of momentum, and the resulting non-linear the Navier-Stokes equations lead to instabilities such as turbulence and vorticity. The analogous situation of static electron flow is typically much less exciting, as electrons relax their momentum by frequent collisions with lattice defects and phonons. Recent experimental and theoretical progress has shown hydrodynamic flow to exist in exceptional materials, where momentum relaxing collision processes are unusually rare while electron-electron scattering occurs at a fast rate. Here, the electron momentum can be quasi-conserved on a short timescale and hydrodynamic corrections tocharge transport can be observed. In our work, we focus on PdCoO2, a quasi two-dimensional electronic material with a low temperature mean free path of 20mm The technological key to this experiment is the fabrication of micron-sized structures of highest single crystal quality via Focused Ion Beam etching. This technique enables new types of experiments in crystalline solids, and some examples will be showcased ranging from challenging experiments under extreme conditions such as pulsed magnetic field or high pressure in diamond anvil cells to thermodynamic measurements and “crystals by design” in three dimensions. In the particular case of hydrodynamic transport, we fabricate crystal channels of different width (80…0.7mm) and observe an unusual scaling of the device resistivity on its cross-section that quantitatively agrees with the hydrodynamic predictions of viscous electron flow. Other groups have identified key signatures of hydrodynamic conduction in graphene, based on current flow vorticity and breaking of the Wiedemann-Franz law. Hence electrons in solids can at times indeed resemble this early picture of liquids flowing in a pipe.