Low-Temperature Physical Properties of Topological Semimetals

Abstract

In recent years, the study of topological materials has been a hot topic in condensed matter physics. These materials have attracted considerable attention due to their intriguing physical properties and potential functional applications. This thesis first reviews the background knowledge of topological semimetals and then presents investigations of the low-temperature physical properties of three different topological semimetals.

The first discussed is shandite Co3Sn2S2, which is a Weyl semimetal. The study focuses on the effect of Fe-doping on the magnetic and transport properties of polycrystalline Co3Sn2S2. Fe-doping suppresses the sample’s magnetic order and lowers the Curie temperature. Moreover, Fe-doping results in the Kondo effect in the temperature-dependent resistivity and the skew-scattering contribution to the anomalous Hall effect. In addition, a hysteresis behavior of the low-magnetic field magnetoresistance in both pure and Fe-doped Co3Sn2S2 is observed. At last, the Seebeck coefficient and thermal conductivity are discussed.

The next material discussed is the single crystalline Ni3In2S2, which is also a semimetal from the shandite family like Co3Sn2S2. Ni3In2S2 displays an extremely large transverse magnetoresistance and a magnetic field-induced resistivity upturn behavior at low temperatures. Below 50 K, the magnetoresistance curves show a linear magnetic field dependence under high magnetic fields. Along with magnetotransport measurements, quantum oscillations and computation results indicate that it is the linearly dispersive bands that cause the high mobility and quantum linear magnetoresistance, thus leading to the extreme magnetoresistance in Ni3In2S2.

The last presented is the study on the type-II Dirac semimetal NiTe2 single crystal. NiTe2 exhibits a large magnetoresistance at low temperatures, of which the value and the magnetic-field dependence are sensitive to the field strength and the angle between the applied magnetic field and the current, showing the anisotropic feature of its magnetoresistance. The quantum oscillation results reveal the small effective mass and the nontrivial Berry phase originating from the Dirac point. The temperature-dependent Lorenz number deduced from the thermal conductivity significantly deviates from the Sommerfeld value, indicating the breakdown of the Wiedemann-Franz law.

Overall, this thesis focuses on the low-temperature properties of topological semimetals, especially the analyses of transport properties, which may serve as a reference for future research on topological materials.