Investigating the Anomalous Thermal and Electrical Transport Phenomena in YbMnBi2 and Indium-Doped (Pb,Sn)Te Alloys.

다국어 초록 (Multilingual Abstract)

This dissertation will center around the discussion of the investigation into the anomalous thermal and electrical transport phenomena in magnetic Weyl semimetal, YbMnBi2, as well as the characterization of its magnetization behavior. A theory-based experimental search for a new type of chiral anomaly in promising materials will also be covered.1. Thermoelectrics (TEs) are solid-state devices that can realize heat-electricity conversion. Transverse TEs require materials with a large Nernst effect, which typically requires a strong applied magnetic field. However, topological materials with magnetic order offer an alternative pathway for achieving large Nernst via the anomalous Hall effect and the accompanying anomalous Nernst effect (ANE) that arise from band topology. Here, we show that YbMnBi2 with a low Hall density and a chemical potential near the Weyl points has the highest ANE-dominated Nernst thermopower of any magnetic materials, Syx around 110 μV/K-1 (T = 254 K, 5 T ? |μ0H| ? 9 T applied along the spin canting direction), due to the synergism between classical contributions from filled electron bands, large Hall conductivity of topological origin, and large resistivity anisotropy. In addition, an appreciable thermal Hall angle of 0.02 < ∇yT/∇xT (-9 T) < 0.06 was observed (40 K < T < 310 K).2. How exactly the magnetization of YbMnBi2 changes with temperature and magnetic field remains indeterminate. Mysteries exist in the previous reports. Herein, through extensive magnetization characterization at various conditions, it was found that the magnetization behavior of YbMnBi2 showcases shared features in many aspects among multiple crystals in spite of a few sample-dependent details. The findings here hint at a more complex picture of the magnetic structure than what is currently known. This project hopefully can provide a foundation for future studies on thoroughly characterizing the magnetization behavior of YbMnBi2.3. Chiral anomaly, a signature of Weyl semimetal (WSM) phase, shows potential to efficiently modulate thermal or electrical transport in the device level, which normally requires an external magnetic field. Recently, indium-doped (Pb,Sn)Te alloys have been demonstrated to host giant Berry curvature dipoles in the WSM phase, giving rise to nonlinear Hall effect without the presence of magnetic field and magnetization. In this project, we present theory-based experimental search for a new type of chiral anomaly that is based on non-zero Berry curvature dipole. One signature of this new chiral anomaly is that in the absence of magnetic field and magnetization thermal conductivity exhibits anomalous changes with external electric field such that these variations are odd functions of E-field and proportional to it at a given temperature. In (Pb0.59Sn0.41)0.97In0.03Te single crystal, we observed a linear relationship between imposed electric field Ez along the polar axis and antisymmetric components of thermal conductivity κxx in the plane normal to z. The documented thermal conductivity behaviors with E-field in our experiments approximate theoretical predictions. This new type of chiral anomaly manifested in indium-doped (Pb,Sn)Te alloys unveils its potential for engineering a voltage-driven solid-state heat switch independent from magnetic field.