Imaging phonon-mediated hydrodynamic flow in WTe2 with cryogenic quantum magnetometry

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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