The Exploration Of New Thermoelectric Materials And The Study Of Transport Properties In Topological And Correlated Electronic Systems

Facing the challenge of energy shortage and environmental pollution,thermoelec-tric materials,as a kind of clean energy that can realize mutual conversion between heat and electric,have always attracted widespread attention.Since the concept of ther-moelectric materials was put forward,thermoelectric materials have been studied for nearly a hundred years.However,it still cannot be applied on a large scale due to the relatively low thermoelectric figure of merit,that is,the relatively low thermoelectric conversion efficiency.The development of thermoelectric materials has gone through two main stages.The first stage is after the energy band theory was put forward,peo-ple have the concept of semiconductors.With the rise of research in the semiconductor field,thermoelectric materials have also developed by leaps and bounds.In the second stage,scientists further proposed the concepts of"quantum confinement" and"electron crystal phonon glass" based on the traditional narrow band gap semiconductors that may be good thermoelectric materials.At present,the main thermoelectric materials are still concentrated in narrow-band gap semiconductors,and the thermoelectric figure of merit has grown slowly in recent years.Therefore,to further increase the thermoelectric fig-ure of merit of thermoelectric materials,new material systems or new physical driving forces may be required.In this paper,we mainly studied the thermoelectric properties and the transport properties of topological materials and correlated electronic materials.The main mate-rial systems are Dirac semimetals Cd3 As2 and correlated electronic materials EuMnSb2.In Cd3As2,we found that it has a strong anisotropic magnetothermoelectric figure of merit.When the magnetic field is parallel to the[112]and[100]crystal orientations,the maximum of magnetothermoelectric figure of merit differs by nearly 2.5 times.The main reason is the strong anisotropic magnetoresistance,that is,the magnetic field re-quired to reach the quantum limit is different in different crystal orientations.Com-pared with the thermoelectric figure of merit with and without a magnetic field parallel to the[112]crystal orientation,at the temperature corresponding to the maximum,the magnetothermoelectric figure of merit increases by nearly 6 times from 0.2 to 1.19.Subsequently,in order to further improve the magnetothermoelectric performance of Cd3As2 and study its physical mechanism,we have grown samples of Cd3As2 with dif-ferent carrier concentrations.Based on the quantum oscillation data,we have obtained the position of Cd3As2 Fermi level with different carrier concentrations relative to the Dirac point.Through the measurement of resistivity,thermopower,and thermal con-ductivity,We found that as the Fermi surface moves away from the Dirac point,the magnetothermoelectric performance of Cd3As2 gradually increases,but the magnetic field and temperature corresponding to the maximum of magnetothermoelectric figure of merit also increase.We also found that the bipolar effect/Dirac liquid behavior may be the origin of reaching the peak value of the magnetothermoelectric figure of merit.At the same time,we also found that,in this system,due to the small angle scattering,the system has a very small Lorentz constant.Our work illustrates the importance of magnetic field for improving the thermoelectric performance of topological quantum materials and opens up a new way for studying the thermoelectric properties of topo-logical quantum materials.In EuMnSb2,we have found a large thermopower enhanced by spin entropy.The large thermopower can only be achieved above the antiferromag-netic transition temperature(TN)of Eu2+sublattice and completely quenched in the antiferromagnetic order state.In addition,an obvious magnetic field-dependent ther-mopower is only observed above TN,which further supports the spin entropy enhanced thermopower above TN.This work will provide a new paradigm for improving the performance of thermoelectric materials by introducing spin degrees of freedom.In addition,in chapter 5 of the article,we also use the methods of heat transport to study the abnormal transport behavior of Mn(Bi1-xSbx)2Te4,expounding the influence of the bipolar effect on the transport properties of topological insulators.In the last chapter of the article,we studied the electrical transport behavior of FeSe under a strong mag-netic field,and explained the importance of strong magnetic field to the study of FeSe superconducting mechanism.The dissertation is divided into six chapters as follows:1.IntroductionIn this chapter,we first introduce the principles of the basic thermoelectric effect and magnetothermoelectric effect(thermomagnetic effect),as well as the definition of the thermoelectric figure of merit that measures the performance of thermoelectric ma-terials and the energy conversion efficiency corresponding to the thermoelectric figure of merit.In addition,we discuss the basic method of improving the thermoelectric fig-ure of merit based on the coupling relationship among the thermoelectric parameters,and the current development,prospects and significance of thermoelectric materials.Finally,we will introduce the significance of exploring new thermoelectric materials(including topological thermoelectric materials and correlated electronic thermoelec-tric materials)to further improve the thermoelectric performance of materials compared with traditional semiconductor thermoelectric materials.2.Anisotropy magnetothermoelectric properties of topological semimetal Cd3As2Thermoelectric materials can be used to convert between heat and electricity.In this chapter,we study the anisotropy magnetothermoelectric properties of the topologi-cal semimetal Cd3 As2.We found that the increase of the power factor and the decrease of the thermal conductivity occur simultaneously under the magnetic field.As a result,the thermoelectric figure of merit has been greatly improved.Compared with the mag-netic field parallel to the[100]crystal orientation,when the magnetic field is parallel to the[112]crystal orientation,the thermoelectric figure of merit increases more signifi-cantly.Under a temperature of 350K and a magnetic field of 7 Tesla,the magnetother-moelectric figure of merit of the magnetic field parallel to the[112]crystal orientation increases to 1.1(0 Tesla magnetic field=0.17),which is approximately 6 times higher.According to the experimental data,we have also carried out theoretical calculations.The theoretical calculation results suggest that the nature of the dramatic increase in the thermoelectric figure of merit comes from the linear Dirac band characteristics and the large proportion of electronic thermal conductivity in the total thermal conductivity.Our experimental results also open up a new way to greatly improve the thermoelectric performance of quantum topology materials.3.Magnetothermoelectric properties of Dirac semimetal Cd3As2 with different carrier concentrationsBased on the experimental results in Chapter 2,the magnetothermoelectric prop-erties of Cd3As2 with different carrier concentrations in this chapter are studied under a magnetic field parallel to the[112]crystal direction.The magnetothermoelectric prop-erties of all the carrier concentration crystals increase with the increase of temperature at low temperature and reach the maximum value at high temperature due to the drastic increase of thermal conductivity.With the increase of carrier concentration,the maxi-mum of the magnetothermoelectric figure of merit and the corresponding temperature and magnetic field of maximum also increase gradually.For the abnormal increase in thermal conductivity at high temperature,it may be due to the bipolar effect or the Dirac liquid behavior.The maximum of thermoelectric figure of merit is 1.24 at a tempera-ture 450 K,under a 9 Tesla magnetic field.In addition to the linear dispersion band,small angle scattering and dominent electron thermal conductivity in the total thermal conductivity also play an important role.The experimental results in this chapter will be helpful to optimize the thermoelectric properties of quantum topological materials.4.Large thermopower enhanced by spin entropy in antiferromagnet EuMnSb2For traditional semiconductor thermoelectric materials,only charge and lattice de-grees of freedom are considered as the key factors to determine the thermoelectric prop-erties of materials.In strongly correlated electric materials,by introducing the interac-tion between electrons,the spin/orbit degrees of freedom are predicted to significantly increase the thermopower of the materials itself.However,a direct proof for large ther-mopower enhanced by spin entropy is still elusive in experiment.Here,we successfully observe a large thermopower enhanced by spin entropy in antiferromagnet EuMnSb2.The large thermopower is only observed above the antiferromagnetic transition tem-perature(TN)of Eu2+sublattice and totally quenched in the antiferromagnetically or-dered state of Eu2+ sublattice,which unambiguously demonstrates the spin entropy of Eu2+ sublattice as the origin for large thermopower above TN.Moreover,a signifi-cant magnetic-field dependence of thermopower was observed only above TN,which further supports the spin-entropy-enhanced thermopower above TN.By measuring the spin entropy from Eu2+ sublattice,a tight correlation between the field-dependent ther-mopower and the spin entropy of Eu2+sublattice is also revealed.The present work would provide a new paradigm to enhance the performance of thermoelectric materials by involving spin degree of freedom.5.Possible bipolar effect induced anomalous transport behaviors in magnetic topo-logical insulator Mn(Bi1-xSbx)2Te4MnBi2Te4 has attracted tremendous interest as the first intrinsic magnetic topolog-ical insulator.Due to its heavily n-doped nature,it is difficult to observe the nature of its surface state and for further electronic applications,so we must tuning its carrier concentration.In this chapter,we have systematically measure the resistivity,Seebeck coefficient,and thermal conductivity of Mn(Bi1-xSbx)2Te4(0?x?0.51)single crys-tals.We find that the carrier concentrations at room temperature can be continuously tuned from-9.47×1019 to 5.21×1019 cm-3 as varying x from 0 to 0.51.In the crys-tals with the Fermi level locating close to the charge neutral point in the bulk band gap,drastic changes in resistivity,Seebeck coefficient,and thermal conductivity are observed around a certain temperature T*.Our results suggest that the bipolar effect possibly palys an important role in determining the transport properties in narrow bulk-band topological insulators when the Fermi level is located near the charge neutral point inside the bulk gap.6.Electrical transport properties of FeSe single crystal under high magnetic fieldUnderstanding the normal electronic state is crucial for unveiling the mechanism of unconventional superconductivity.Here,by applying the magnetic field up to 37 T on FeSe single crystal,we are able to reveal the normal-state transport properties after superconductivity is completely suppressed.The normal-state resistivity exhibits Fermi liquid behavior at low temperature.Large orbital magnetoresistance is observed in the nematic state with H//c and magnetoresistance is negligible with H//ab.The magnitude of orbital magnetoresistance shows an unusual reduction and Kohler's rule is severely violated below 10?25 K which is possibly related to the spin fluctuations.Our results indicate that spin fluctuations play an important role in the normal-state transport properties in FeSe albeit the Fermi liquid nature at low temperature.