Transport and magnetic properties in topological Weyl materials

In condensed matter physics, a topological insulator (TI) is a kind of quantum matter, protected by symmetries, with an insulating gapped state in the bulk and a gapless state at the surface. In topological Dirac semimetals, there is a linearly dispersing Dirac cone represented in the metallic boundary state. While in topological Weyl semimetal, there are even numbers of nondegenerate nodes with a definite chirality. The unique electronic structures determine the transport and magnetic properties of topological insulators, which are key factors in their potential applications. In this thesis, we investigate the transport and magnetic properties of WTe2 and Mn3Ge by microfabrication and magnetic torque. In WTe2, a topological Weyl semimetal with broken parity symmetry, an extremely large magnetoresistance (XMR) is observed and the electrical properties of monolayer can be tuned by electrostatic field. In this work, we checked the XMR and quantum oscillations in the planar configuration reported in the literature [1], and further investigated the MR in a unique vertical device. In this configuration, we discovered a negative MR and quantum oscillations. The XMR are compared with two-band model, the electron and hole densities are found to be compensated. The observed negative MR is considered a signature of the chiral anomaly. In Mn3Ge, a topological Weyl semimetal with broken time reversal symmetry, a very large anomalous Hall effect is observed [2], comparable to that of ferromagnetic materials with much larger spontaneous magnetization. We characterized the magnetic anisotropy in both the in-plane and out-of-plane directions by a sensitive magnetic torque measurement. The easy axis is along and the magnetic ground state exhibits a six-fold symmetry, in agreement to the observations of NMR [3, 4]. Other than these methods, angle resolved photoemission spectroscopy (ARPES), field effect transistor (FET), and thermoelectric Seebeck measurement are also studied. The Mn3Ge samples were prepared in polished surface and cleaved surface to observe their electronic structures. Besides, electrostatic tuning methods were investigated in tuning the electrical properties of the 2D semiconductors. Moreover, thermoelectric measurements of both bulk and 2D materials were investigated. These techniques are promising in investigating the properties of topological Weyl semimetals in future research.