Electrical transport and fermiology in microstructured topological semimetals

Over the past decade, topological semimetals have attracted considerable interest due to their distinctive band structure, characterized by gapless, linearly dispersing bulk bands and robust topological surface states. These features are anticipated to result in unique transport properties. Among others, previous studies on topological conductors have reported high electron mobility, extreme magnetoresistance, and low electrical resistivity, preserved in scaled samples having thickness below 100 nm. These properties are especially promising for addressing a critical challenge in information technology: power consumption.
By 2040, it is projected that global energy production will no longer suffice to meet the demands of increasingly power-hungry computing systems. Therefore, the development of new, advanced materials that enable energy-efficient
computing is urgently needed. In this thesis, electrical transport in topological semimetals is investigated,
focusing on properties that can be harnessed for next-generation low-power electronic devices. Properties such as electron mobility, magnetoresistance, and carriers’ scattering time are analyzed in relation to the Fermi surface characteristics, mainly measured through quantum oscillations experiments.
To ensure precise control over sample dimensions and geometry, and to enable device prototyping without the need for thin-film growth, experiments are conducted on microstructured samples fabricated using focused ion beam (FIB) techniques, starting from single crystals.
The thesis focuses on two materials: the topological Weyl semimetal NbP and the topological multifold semimetal CoSi. NbP exhibits anisotropic and high mobility and magnetoresistance, which, according to our quantum oscillations experiments, appear to originate from a linearly dispersive electron Fermi surface
pocket, among the many populating NbP. Transverse electron focusing experiments are used to precisely probe the peculiar shape of this Fermi surface pocket of electrons and estimate their mobility. The properties of NbP are leveraged to demonstrate a low-power magnetic field-effect transistor, as well as the advantages of directional electronic transport in scaled conductors. On the other hand, the chiral crystal CoSi, which hosts multifold fermions,
features topological surface states extended in the reciprocal space, about two orders of magnitude longer than NbP. This feature is expected to result in enhanced topological transport properties compared to Weyl semimetals, including a longer lifetime of chiral anomaly-induced currents. Our data confirm this hypothesis, revealing a longer scattering time for the chiral current that exceeds previously reported values. CoSi is discussed for its potential as topological scaled conductor.
Finally, the techniques used for studying topological semimetals are applied to measure hot electron temperatures and investigate scattering mechanisms in a high electron mobility transistor (HEMT) during operation. Remarkably, our
data suggest the emergence of hydrodynamic electron flow in the transistor’s channel at low temperatures and finite current.