Imaging electron hydrodynamics in graphene

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Non-Technical Abstract Our understanding of how electrons move in materials is based on simple, semiclassical, equations where all electron interactions - with the lattice, with disorder, and with each other - is done by changing the description of the electron into a different particle that accounts for the interactions. However, in real materials with small disorder or strong electron-electron interactions, a continuous hydrodynamic description is a better model where the electrons are said to act like a liquid. Only a handful of experiments have recently detected the presence of such a strange electron liquid with many new theories to explain it, which means this is the beginning of a whole new field of novel and useful technologies. This proposal is driven by a unique opportunity at UCSB to combine a new, ultra-clean materials system (graphite gated monolayer graphene) with a newly developed, high sensitivity scanned probe technique that can directly probe the novel electron flows down to the nanoscale. New electron phenomena will be directly observed such as viscosity-driven whirlpools and the onset of turbulence of electron flows. There is also an opportunity to match these tiny electron flow patterns with large scale electrical transport measurements. The hydrodynamic regime presents an opportunity to build novel devices based on manipulating electron fluid phenomena that have never before been imagined. This research is also tightly coupled with a strong educational plan that aims to imbue the next generation of scientists with an excitement for novel electronic materials, quantum sensors, and their applications. Technical Abstract Electron flow in low dimensional systems is typically described by semiclassical equations of motion, in which all electron interactions - with the lattice, with disorder, and with each other - leads only to a dressing of the bare electron into an electron-like quasiparticle whose dynamics determine current flow. However, in quantum critical systems, strong inter-electron collisions can wash away the individual electronic degrees of freedom - the electron-electron collision length is much shorter than all other dimensions. Macroscopic observables such as electrical and thermal conductivity are then described by the equations of fluid mechanics, and the macroscopic parameters-density, viscosity, and mean velocity-which emerge from the microscopic collisions between electrons. Recently, several groups have reported signatures of the hydrodynamic regime in graphene - a material whose band structure mimics the gapless dispersion of a quantum critical point. However, these experiments have all focused on macroscopic transport properties. As such they are unable to directly probe the emergence of hydrodynamic flow on all the relevant length scales. Here, the principal investigators propose to combine state-of-the-art graphene devices with an order of magnitude higher mobility than previously realized along with state-of-the art local magnetometry, utilizing nitrogen-vacancy center defects in diamond, to probe the emergence of hydrodynamic flow across the relevant length scales. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.