Uncovering Hydrodynamic Transport in Topological Semimetals

Topological semimetals featuring Dirac/Weyl band crossings have been shown to exhibit unconventional responses under external fields and extraordinary transport signatures such as ultrahigh carrier mobility, violation of Wiedemann-Franz law, collective flow, etc. The advances in materials synthesis have facilitated the fabrication of micro- and nano-scale devices, with high-quality single crystals of low impurity levels. There the charge carriers suffer less momentum relaxation and flow collectively resembling a fluid flow in a pipe, termed as <i>hydrodynamic</i> electron transport [1], which is shown to be more efficient in a narrow conducting channel than the conventional ballistic regime and has garnered much research attention.<br/>This talk will uncover the underlying mechanism to realize the hydrodynamic regime in bulk topological semimetals, i.e. via electron-electron interaction mediated by a virtual phonon. We start with explicitly establishing the microscopic length scales for electrons and phonons in two prototypical topological semimetals WP₂ and WTe₂ from first principles calculations, and show how different combinations of them lead to various transport regimes – diffusive, ballistic, and hydrodynamic – in the numerical solution of the Boltzman transport equation [2,3]. We discuss how to distinguish them in light of recent experimental measurements with temperature and space resolution [2,4,5]. Finally, we present generalized principles for discovering and optimizing hydrodynamic effect in much broader anisotropic metallic materials and <i>ab initio</i> predictions on anisotropic semimetals ZrSiS and TaAs₂, which we hope to call for experimental verifications and inspire new device concepts [6].<br/><br/><br/>Ref:<br/><br/>1 R. N. Gurzhi, Soviet Physics Uspekhi 11, 255 (1968)<br/>2 U. Vool, et al., Nat. Phys. (2021)<br/>3 van Delft, et al. Nat. Commun. 12(1) 1-9 (2021)<br/>4 J. A. Sulpizio, et al., Nature 576, 75 (2019)<br/>5 G. Varnavides, et al., arXiv:2106.00697 (2021)<br/>6 Y. Wang, et al., arXiv:2109.00550 (2021)