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Register a new userQuantum imaging of current flow in graphene
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Added by
Jean-Philippe Tetienne
Publication date
2016
fields
Physics
and research's language is
English
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Since its first isolation in 2004, graphene has been found to host a plethora of unusual electronic transport phenomena, making it a fascinating system for fundamental studies in condensed-matter physics as well as offering tremendous opportunities for future electronic and sensing devices. However, to fully realise these goals a major challenge is the ability to non-invasively image charge currents in monolayer graphene structures and devices. Typically, electronic transport in graphene has been investigated via resistivity measurements, however, such measurements are generally blind to spatial information critical to observing and studying landmark transport phenomena such as electron guiding and focusing, topological currents and viscous electron backflow in real space, and in realistic imperfect devices. Here we bring quantum imaging to bear on the problem and demonstrate high-resolution imaging of current flow in graphene structures. Our method utilises an engineered array of near-surface, atomic-sized quantum sensors in diamond, to map the vector magnetic field and reconstruct the vector current density over graphene geometries of varying complexity, from mono-ribbons to junctions, with spatial resolution at the diffraction limit and a projected sensitivity to currents as small as 1 {mu}A. The measured current maps reveal strong spatial variations corresponding to physical defects at the sub-{mu}m scale. The demonstrated method opens up an important new avenue to investigate fundamental electronic and spin transport in graphene structures and devices, and more generally in emerging two-dimensional materials and thin film systems.
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In the presence of strong interactions, electrons in condensed matter systems can behave hydrodynamically thereby exhibiting classical fluid phenomena such as vortices and Poiseuille flow. While in most conductors large screening effects minimize electron-electron interactions, hindering the search for possible hydrodynamic candidate materials, a new class of semimetals has recently been reported to exhibit strong interactions. In this work, we study the current flow in the layered semimetal tungsten ditelluride (WTe2) by imaging the local magnetic field above it using a nitrogen-vacancy (NV) defect in diamond. Our cryogenic scanning magnetometry system allows for temperature-resolved measurement with high sensitivity enabled by the long defect spin coherence. We directly measure the spatial current profile within WTe2 and find it differs substantially from the uniform profile of a Fermi liquid, indicating hydrodynamic flow. Furthermore, our temperature-resolved current profile measurements reveal an unexpected non-monotonic temperature dependence, with hydrodynamic effects strongest at ~20 K. We further elucidate this behavior via ab initio calculations of electron scattering mechanisms, which are used to extract a current profile using the electronic Boltzmann transport equation. These calculations show quantitative agreement with our measurements, capturing the non-monotonic temperature dependence. The combination of experimental and theoretical observations allows us to quantitatively infer the strength of electron-electron interactions in WTe2. We show these strong electron interactions cannot be explained by Coulomb repulsion alone and are predominantly phonon-mediated. This provides a promising avenue in the search for hydrodynamic flow and strong interactions in high carrier density materials.
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