Sondheimer oscillations as a probe of non-ohmic flow in type-II Weyl semimetal WP2
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As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm’s law. Depending on the length scales of momentum conserving (<em>l<sub>MC</sub></em>) and relaxing (<em>l<sub>MR</sub></em>) electron scattering, and the device size (<em>d</em>), current flows may shift from ohmic to ballistic to hydrodynamic regimes and more exotic mixtures thereof. So far, an <em>in situ</em>, in-operando methodology to obtain these parameters self-consistently within a micro/nanodevice, and thereby identify its conduction regime, is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain <em>l<sub>MR</sub></em> in micro-devices even when <em>l<sub>MR</sub>>>d</em>. This gives information on the bulk <em>l<sub>MR </sub></em>complementary to quantum oscillations, which are sensitive to all scattering processes. We extract <em>l<sub>MR</sub></em> from the Sondheimer amplitude in the topological semimetal WP<sub>2</sub>, at elevated temperatures up to T~40K, in a range most relevant for hydrodynamic transport phenomena. Our data on µm-sized devices are in excellent agreement with experimental reports of the large bulk <em>l<sub>MR</sub></em> and thus confirm that WP<sub>2 </sub>can be microfabricated without degradation. Indeed, the measured scattering rates match well with those of theoretically predicted electron-phonon scattering, thus supporting the notion of strong momentum exchange between electrons and phonons in WP<sub>2</sub> at these temperatures. These results conclusively establish Sondheimer oscillations as a quantitative probe of <em>l<sub>MR</sub></em> in micro-devices in studying non-ohmic electron flow.
