Study On Fermi Surface Structure Of High-T_c Superconductors La_2-xCe_xCuO₄ And FeSe By Electrical And Thermal Transport

The superconductivity of high-T_c superconductors can be tuned by a variety of parameters,such as doping,magnetic field,pressure and so on.During this process,both electronic states and Fermi surface(FS)structures of high-T_c superconductors undergo complex changes.To understand the mechanism of electron pairing,it is essential to elucidate the nature of the evolutions of both.Transport measurement is an easy but effectively method to study the ground state of the material.For examples,one can measure the Hall coefficient to obtain carrier concentration,measure the Seebeck to achieve the Fermi energy,and measure the angular magnetoresistance to study the symmetry of electronic states.Therefore,in this thesis,we study the FS of high-T_csuperconductors La_2-xCe_xCuO₄(LCCO)and FeSe by developing the electrical and thermal transport measurements.Firstly,we perform systematic magnetotransport measurements up to 60 T on optimally doped LCCO thin films(x=0.10)to study the antiferromagnetic(AF)phase transition.Due to the band folding caused by the AF order,the FS of LCCO is reconstructed,accordingly the resistivity and Hall coefficient curves show"upturn"and"kink"behaviors,respectively.With increasing magnetic field,both behaviors are gradually inhibited.At about 60 T,a quantum phase transition from AF state to polarized ferromagnetic state emerges.At this time,the Hall number changes from-0.1to 0.9,indicating the FS structure has been restored to a hole pocket from an electron pocket.The AF phase transition emerging in such strongly correlated system can be well described by the holographic model.In weak field limits,the Néel temperature(T_N)depends quadratically on the magnetic field,which agrees with the expectation of the mean-field theory.However,near the critical field B_c,the function of T_N has a logarithmic behavior,similar to the Beresinkii-Kosterlitz-Thouless transition.In AF region,there exists a crossover from positive(p-)to negative(n-)magnetoresistance(MR)with increasing magnetic field.The starting field of n-MR is consistent with the magnetic field where Hall number deviates from-0.1,indicating that the n-MR can be attributed to the spin modulation.The spin component induced by strong magnetic field can provide a polarized FM channel for carriers,thus improving the conductivity.The p-MR cannot be properly explained by the Drude model.This abnormal p-MR exists only within the AF region,and is also closely related to superconductivity.Our collaborators propose a reasonable configuration of topological order,by which the p-MR can be explained consistently.Although the p-MR effect needs further study,the relationship among the short-range AF order,p-MR,and superconductivity has been emerged from our experiments.Secondly,two thermal transport devices for Montana and PPMS equipment have been designed and built.After calibrated by standard sample,the Seebeck measurements on superconductors LCCO,FeSe_xTe_1-x,Li Ti₂O₄ and Mg Ti₂O₄ have been carried out.Besides that,we integrate ionic liquid gating(ILG)technique into thermal transport measurement device,by which the FS structure of superconductors can be obtained while its superconductivity is tuned by ILG.Finally,the evolution of FS structure has been studied during ILG on FeSe films.The S/T and R_H curves of FeSe film display a complex temperature dependence,indicating the intense competition between electron and hole carriers.By ILG,the T_cof FeSe is enhanced from 11 K to 40 K.At final state,the electron carriers dominant the transport and both S/T and R_H curves are independent of temperature,which indicate that the FS consists of only one electron pocket.We refine the transport parameters based on the Drude model and found that the carrier density of electron increases from9.5×10¹⁹cm^-3to 2.1×10¹⁹cm^-3,while the Fermi energy decreased to 84.4 me V during gating,with an effective mass of about 3.4m_e.These behaviors provide support for an electron-correlation scenario.Therefore,a clear picture of FS structure evolution can be drawn that the hole band gradually disappears and the electron band is gradually flattened as T_c increases.