Anisotropic and nonlinear transport in microstructured 3D topological semimetals

Topological semimetals are frequently discussed as materials platforms for future electronics that exploit the remarkable properties of their quasiparticles. These ideas are mostly based on dispersion relations that mimic relativistic particles, such as Weyl Fermions, and the nontrivial quantum phase evolution in a Berry curvature landscape. These ideas are fascinating, yet so-far entirely theoretical. This thesis is concerned with experimentally investigating these prospects using Focused Ion Beam prototyping. Three science cases are pursued in its course:The first project involves searching for the chiral zero sound in microstructuredWeyl semimetals. The chiral zero sound mode is a collective excitation of the Fermi surface, that is predicted to lead to giant magnetic quantum oscillations in the thermal conductivity. We developed a technique based on the idea of the 3w method and successfully measured the thermal conductivity of a TaAs microbar at cryogenic temperatures. We found that the third harmonic voltage of the material within the magnetic field showed large quantum oscillations without the magnetoresistance background as in the regular Shubnikov-de Haas (SdH) effect. We demonstrated this effect comprehensively in topological semimetal CoSi, due to its simpler oscillation frequencies. Third harmonic voltage is related to the derivative of the resistance due to the nonlinear current-voltage relation of a semimetal under self-heating. The enhanced quantum oscillations appear because the derivative of the resistance reacts more sensitive to Landau quantization than the resistance. This alternative way of measuring quantum oscillations may improve the sensitivity with which we probe topological semimetals.The aim of the second experiment is the search for nonlocal voltages in microstructured Cd3As2 devices. Theory predicts non-local voltages to appear due to a quantum process combining Fermi arc states with bulk chiral Landau levels, known as the Weyl orbit. We demonstrate that due to the field-induced giant conductivity anisotropy in semimetals, the current path is strongly altered, forming two long-ranged current beams propagating along the magnetic field. This phenomenon sometimes referred to as "current jetting", is a general property of high mobility semimetals. It can also cause nonlocal signals, leaving it impossible to unequivocally identify the nonlocal voltages from topological effects. Finite element simulations of the current jetting effect in Cd3As2 microstructures quantitatively capture the current path in the devices. This experiment presents a significant step towards understanding the microscopic current patterns in 3D Dirac semimetal microdevices.In the third experiment, we report a thorough study on the transport properties of the recently discovered Kagome superconductor CsV3Sb5, which is considered an ideal platform to study the interplay between topology and correlations. By designing a unique membrane-based low-strain microstructure, we are able to detect the intrinsic properties of this material. This allows us to settle the ongoing debate about the existence of 3D ellipsoidal Fermi surfaces and to uncover the unexpected absence of magnetoresistance at moderately high temperatures, which violates Kohler scaling. Our results indicate that the electrical properties of CsV3Sb5 have mixed dimensionality, and the 3D nature has to be considered in the physical description of this material.