Anomalous Transverse Transport Properties And Its Modulation In A Magnetic Weyl Semimetal Co₃Sn₂S₂

Anomalous Nernst effect and anomalous Hall effect have the same physical mechanism,they are connected by the Mott relation.Compared with anomalous Hall effect,anomalous Nernst effect is more sensitive to the information near the Fermi level.By modulating the anomalous Nernst effect,we can investigate the essence of the anomalous transport effect deeply.However,little is known about the modulation of the anomalous Nernst effect for now.Recently,Co₃Sn₂S₂has been proved as a magnetic Weyl semimetal experimentally.The gaint anomalous Hall effect in Co₃Sn₂S₂is related to the the large Berry curvature from the nodal lines and Weyl points.Co₃Sn₂S₂provides an ideal platform for the study of the anomalous Nernst effect and its modulation.In this thesis,we investigated the anomalous Nernst response to the carriers mobility and chemical doping in Co₃Sn₂S₂samples.We studied the anomalous transport properties and the structure of Fermi surface in Co₃Sn₂S₂by transport measurements.This thesis is organized as follows:1.We investigated the anomalous Nernst effect in Co₃Sn₂S₂single crystals with diffferent mo-bility.It was found that the chemical composition,Fermi energy and carriers density in these samples are almost the same.The difference lies in the carriers mobility.The ordinary Nernst effect correlates with mobility,but anomalous Nernst effect anti-correlates with mobility.We found that the intrinsic anomalous Nernst effect can be modulated by the mean-free-path of carriers.2.We modulated the anomalous tranverse properties of Co₃Sn₂S₂through chemical doping.These impurities enhance the contribution of extrinsic mechanism to the anomalous transverse trans-port properties,and the Fermi level can be shifted by chemical doping.The electron doping will change the sign of the anomalous Nernst coefficient,while the hole doping will increase the anoma-lous Nernst coefficient at low temperature,the modulation in Nernst effect is related to the change of the band structure caused by the chemical doping.In the doped-samples,the relationship between the anomalous transverse thermoelectric conductivity and the doping ratio verifies the broadening effect of chemical doping on the band structure.3.We investigated the anomalous thermal Hall effect in Co₃Sn₂S₂samples with different mobil-ity.The anomalous thermal Hall conductivity and the anomalous Hall conductivity obey the Wiedemann-Franz law below 100 K,which confirms that the anomalous transport effects originate from the intrinsic mechanism in Co₃Sn₂S₂and it is independent with the mobility.The anomalous Ettingshausen coef-ficient and the anomalous Nernst coefficient satisfy the Bridgman relation,which proves the reliability of our measurements.4.We investigated the quantum oscillation in Co₃Sn₂S₂single crystals obtained by self-flux method.Based on the temperature-dependent and angle-dependent quantum oscillation measurements and theoretical calculations,we studied the Fermi geometry and magnetic breakdown of Co₃Sn₂S₂.We note that the intrinsic anomalous Nernst effect can be modulated by the mean-free-path of carriers,which provides new insights into the response of electrons with nontrivial topology to a ther-mal gradient.The modulation of the anomalous Nernst effect by chemical doping is based on the shift of the Fermi level,which verifies the relative position of the Weyl point and the Fermi level has a significant influence on the anomalous transport effects.These studies on the modulation of anoma-lous Nernst effect provide reference for exploring the thermoelectric application.We confirmed that the anomalous transport effect of Co₃Sn₂S₂originates from the intrinsic mechanism.The intrinsic Berry curvature is related to the topological band structure near the Fermi surface.The Fermi surface obtained from our research results provide reference for further research.