Non-zero Berry Curvature-induced Anomalous Thermal Transport Behavior In Some Topological Materials

The directional movement of（quasi）particles carrying charge or energy driven by an electric field or temperature gradient is called electrical or thermal transport.Electrical and thermal transport properties are macroscopic manifestations of the basic information of condensed matter.In-depth study of electrical and thermal transport behaviors of materials can help people understand the origin and physical mechanism of topological properties in materials.In early studies,measuring Hall conductivity was the main means of study,but later it was found that the corresponding thermal effect,thermal Hall conductivity,as well as the anomalous Nernst and Ettingshausen thermoelectric coefficients,can also reflect basic information about materials.In addition to revealing the existence of topological states,such large thermal and thermoelectric transport in metal includes contributions from the Berry curvature in the underlying band structure.What is more fascinating is that since the carrier of heat flow is no longer limited to electric charge,the research of thermal transport has been extended to the field of insulator,which provides conditions for exploring topological excitation in insulator.In this paper,a high-precision thermal transport test system is built independently.Based on this device,topological excitation and scattering mechanisms of different topological materials under low temperature and magnetic field are studied through thermal and electrical transport tests.It is found that the anomalous thermal Hall effect and Nernst effect exist in the topological metal CsV₃Sb₅,and the thermal transport behavior deviates from Wiedemann-Franz law.The measured thermal Hall conductivity is greater than the theoretical value calculated by Wiedemann-Franz law,indicating that the low energy excitation of electric neutral in the system contributes to the thermal Hall conductivity.In the 9 T magnetic field,the multi-band effect dominates.With the decrease of the magnetic field,the maximum value of Nernst effect moves to high temperature,and the Hall resistivity compensation point moves to low temperature,indicating that the anomalous transverse transport of the low field is the contribution of non-zero Berry curvature.The thermal transport behavior of transition metal phosphide compounds CrP and WP₂ is also found to deviate from Wiedemann-Franz law due to inelastic electron-phonon scattering,but in contrast to CsV₃Sb₅,the Lorentz ratio of CrP and WP₂ is smaller than the theoretical value.This behavior in CsV₃Sb₅ is unusual and has other physical mechanisms.CsV₃Sb₅,CrP,and WP₂ all have topological band structures,so the difference is not due to topological excitation of electrons.In addition,anomalous transverse transport behavior is also found in WP₂.The origin of this anomalous behavior is related to the Berry curvature of the topological band structure.A large thermal Hall effect has been observed in a ferrimagnet FeCr₂S₄ with spin dependent topological excitation,and the magnetic field dependence of the thermal Hall conductivity of FeCr₂S₄ is saturated,indicating that the large thermal Hall effect is closely related to the magnon.Both FeCr₂S₄ and CsV₃Sb₅ have Kagome lattice structure,and there are magnetic atoms on the lattice site of such frustrate lattice.The difference is that the carrier in CsV₃Sb₅ contributes to the thermal conductivity,while the thermal conductivity in FeCr₂S₄ has almost no carrier contribution.However,a large thermal Hall effect is also found in FeCr₂S₄,suggesting that the spin-dependent topological excitation still contributes significantly to the thermal Hall conductivity in Kagome structures that can produce large Berry curvature,even if the carrier is removed.The results support the hypothesis that the additional increment of the thermal Hall conductivity of CsV₃Sb₅ is due to spin-dependent neutral excitation.