Electronic transport of topological Weyl semimetal and Van der Waals ferromagnetic materials

Quantum materials with unique band structure and ferromagnetic materials are attracting intensive interest due to their exotic properties and broad potential applications in electronic, optoelectronic and spintronic devices. The development of several experimental techniques such as mechanical exfoliation and improved processes for transferring low dimensional materials have enabled electronic devices with novel properties to be constructed.

In this dissertation , we address problems from the certain spectrum of topology band materials such as topological Weyl semimetals (TWSs) in addition to ferromagnetic materials.

In 3D topological Weyl semimetal (TWS) ZrTe5, the angular dependence of the longitudinal magnetoresistance Rxx was studied at different magnetic fields. Two large negative magnetoresistance dips near 𝜃 = 55° and 235° at high magnetic field 9 T were observed. Surprisingly, in the tilted angles 𝜃 = 53°,55°and 57°, negative longitudinal magnetoresistance (LMR) due to the chiral anomaly was observed at temperatures less than 70 K while it was suppressed at temperatures above 70 K. The disappearance of the planar Hall effect (PHE) above 70 K indicated that the negative LMR and PHE originated from the same physical mechanism.

We also systematically studied the transport properties of newly emerged vdW magnet Fe5GeTe2 with various thicknesses down to 6.8 nm. The thickness-dependent coercivity and remanence of Fe5GeTe2 nanoflakes reveal a gradual evolution of domain structure from multi-domain to singledomain behavior. The angle-dependent anomalous Hall effect further confirms this evolution in Fe5GeTe2 nanoflakes. Furthermore, we found that the ferromagnetism of Fe5GeTe2 nanoflakes is dramatically suppressed by a protonic gate and eventually disappears at a higher gate voltage, which are attributed to the proton intercalation induced electron doping. Further DFT calculations indicates that F5GT is very sensitive to charge doping, which can potentially cause a ferromagnetism to antiferromagnetism transition in F5GT. Our work reveals F5GT as a promising candidate for developing novel spin-related applications and provides methods to engineer the magnetic properties of vdW magnet for specific purposes.

Finally, For van der Waals (vdW) magnet Fe0.33−δTaS2 (δ ≤ 0.05), Here we reported a very large topological Hall effect and anomalous Hall effect in gate-tuned itinerant vdW magnet Fe0.33−δTaS2 (δ ≤ 0.05) nanoflakes at low temperatures. Using a newly developed protonic gate technology, the maximum amplitude of topological Hall resistivity 𝜌𝑥𝑦𝑇 boosted by a protonic gate reaches 1.4 𝜇Ω ∙ 𝑐𝑚 (about 460% of the zero-bias value), larger than most of the known magnetic materials. Theoretical analysis indicates that such a large topological Hall effect originates from the two-dimensional Bloch-type chiral spin textures (skyrmions, chiral domain walls) due to the large Dzyaloshinskii-Moriya interaction, while the large intrinsic anomalous Hall effect comes from the gapped nodal lines due to the spin orbit coupling.