Superballistic flow of viscous electron fluid through graphene constrictions

Abstract

Electron-electron (e-e) collisions can impact transport in a variety of surprising and sometimes counterintuitive ways 1-6 . Despite strong interest, experiments on the subject proved challenging because of the simultaneous presence of di erent scattering mechanisms that suppress or obscure consequences of e-e scattering 7-11 . Only recently, su ciently clean electron systems with transport dominated by e-e collisions have become available, showing behaviour characteristic of highly viscous fluids 12-14 . Here we study electron transport through graphene constrictions and show that their conductance below 150 K increases with increasing temperature, in stark contrast to the metallic character of doped graphene 15 . Notably, the measured conductance exceeds the maximum conductance possible for free electrons 16,17 . This anomalous behaviour is attributed to collective movement of interacting electrons, which 'shields' individual carriers from momentum loss at sample boundaries 18,19 . The measurements allow us to identify the conductance contribution arising due to electron viscosity and determine its temperature dependence. Besides fundamental interest, our work shows that viscous e ects can facilitate high-mobility transport at elevated temperatures, a potentially useful behaviour for designing graphene-based devices.Graphene hosts a high-quality electron system with weak phonon coupling 20,21 such that e-e collisions can become the dominant scattering process at elevated temperatures, T . In addition, the electronic structure of graphene inhibits Umklapp processes 15 , which ensures that e-e scattering is momentum conserving. These features lead to a fluid-like behaviour of charge carriers, with the momentum taking on the role of a collective variable that governs local equilibrium. Previous studies of the electron hydrodynamics in graphene were carried out using the vicinity geometry and Hall bar devices of a uniform width. Anomalous (negative) voltages were observed, indicating a highly viscous flow, more viscous than that of honey 12,22,23 . In this report, we employ a narrow constriction geometry (Fig. 1a) which offers unique insight into the behaviour of viscous electron fluids. In particular, the hydrodynamic conductance through such constrictions becomes

Quantum aspects of hydrodynamic transport from weak electron-impurity scattering

“…Introduction -Recent experimental reports of peculiar transport phenomena in ultraclean graphene [1][2][3][4][5] and other materials [6][7][8] have generated much excitement regarding the role of hydrodynamic transport in these experiments. Nevertheless, because of the lack of microscopic understanding on the hydrodynamic transport of electrons, experiments have been interpreted largely through analogy with classical fluids.…”

Quantized Electron Transport Through Graphene Nanoconstrictions

“…Recently, another type of GNC in encapsulated graphene was studied for sizes less than 200 nm, yet edge roughness was not negligible . Previous studies by Tombros et al considered small GNCs in suspended graphene with very high mobility, although the channel was not well defined due to the fabrication technique used (high DC current during annealing to create a constriction without a control on the final width).…”

Towards properties on demand in quantum materials

“…Electrostatic gating is readily attainable in vdW heterostructures and offers an additional degree of control over these materials. For example, electrostatic tuning of a high-mobility electron liquid in graphene allows one to select the regime of carrier density when the flow of electrons becomes viscous, whereas electric conductance can exceed the limits of ballistic transport 39 . Oxide heterostructures have also enabled the stabilization of interesting electronic phases at the interfaces of dissimilar oxides 40 or via interactions with the substrate.…”