Nonlinear Anomalous Nernst Effect In Graphene Induced By Trigonal Warping

The longitudinal thermoelectric effects are often applied to convert the waste heat into electric energy in conventional thermoelectric devices.However,the thermoelectric con-version effect of longitudinal-effect-based devices is limited by the difficulty of regulating heat and electricity simultaneously.If the lateral thermoelectric effect is exploited,it is ex-pected to achieve synergistic regulation of heat and electricity and increase the efficiency of thermoelectric conversion since the electron and heat current are perpendicular.As a result,transverse thermoelectric effects,such as the anomalous Nernst effect（ANE）,are widely used in the study of the performance of thermoelectric devices.On the other hand,it has been shown that the performance of thermoelectric devices can be potentially improved in nonlinear effect based thermoelectric devices,leading to practical applications and provid-ing a new platform for the reuse of waste heat from devices.In 2015,Inti Sodemann and Liang Fu discovered the nonlinear anomalous Hall effect in time-reversal invariant but inversion symmetry broken materials,triggering broad inter-ests in nonlinear anomalous transport phenomena such as the magnus Hall effect and the nonlinear anomalous Nernst effect（NANE）.The nonlinear Nernst effect,a nonlinear trans-verse thermoelectric effect,is originated from the Berry curvature near the Fermi energy.The study of the nonlinear Nernst effect is currently restricted to materials with spin-orbit coupling and tilted Dirac cones.This thesis have discovered a new energy band structure which could give rise to a nonzero nonlinear Nernst effect,namely,the trigonal warping energy band structure on Fermi surfaces.This thesis demonstrate that a nonzero nonlinear Nernst current can still be generated in materials that are completely absence of spin-orbit coupling and tilted Dirac cones.This thesis theoretically investigate the NANE in trigonal-warping monolayer and bi-layer graphene in presence of strain and found that both strain and trigonal warping are necessary to generate NANE in graphene.In monolayer graphene,the nonlinear anomalous Nernst coefficientΛ_x^Tmanifests itself a linear dependence on the strain strength.Whereas,Λ_x^Tare nonlinearly dependent on the strain strength in trigonal-warping bilayer graphene due to its rich fermiology.When the temperature approaches zero temperature and the Fer-mi energy is located in either the conduction or valence bands,Λ_x^Tin monolayer and bilayer graphene tends to be a constant.Furthermore,theΛ_x^Tin bilayer graphene is four orders of magnitude larger than that of monolayer graphene and comparable to those reported in materials with spin-orbit coupling and tilted Dirac,such as MoS₂and bilayer WTe₂.The research results could provide new ideas for the application of nonlinear quantum transport and thermoelectric devices.