Researches On The Thermoelectric Transport Properties Of Quasi-one-dimensional Materials And Magnetic Topological Materials

Thermoelectric technology can realize the mutual conversion between electric energy and heat energy through thermoelectric power generation and energized cooling,and has the characteristics of simplicity,high reliability and environmental friendliness,so it is widely used in space power generation,automobile exhaust gas and factory waste heat power generation and various refrigeration fields.The performance of thermoelectric materials is usually characterized by the dimensionless thermoelectric figure of merit（z T）,which is comprehensively controlled by three basic physical parameters,namely Seebeck coefficient,electric conductivity and thermal conductivity.The mutual restriction among the three parameters makes it impossible to limitlessly increase the thermoelectric performance,which hinders the development and application of thermoelectric devices.An important research direction in the field of thermoelectricity is to explore new thermoelectric materials with high performance.Previous studies have shown that one can modulate the electronic density of states and effective mass in the vicinity of the Fermi energy level of thermoelectric material via quantum confinement effect of low dimensions to improve its thermoelectric performance.Under the guidance of this method,in this dissertation,we have found a high-performance quasi-one-dimensional thermoelectric material Tl₂Cu_6+xTe₄ with great application value,the figure of merit z T of Tl₂Cu_6.01Te₄ reaches 1.3 near room temperature.Next,large thermoelectric power factors and transverse Nernst coefficients caused by phonon drag effect at low temperature have been found in quasi one-dimensional systems AMn₆Bi₅（A=alkali metals）,revealing high potential thermoelectric applications value in such quasi-one-dimensional systems.In addition,due to the topologically nontrivial band structure,topological materials provide an unprecedented opportunity to explore new thermoelectric phenomena and regulate new physical states.In this dissertation,the thermoelectric transport properties of the magnetic topological material NdAlSi has also been reported and a series of innovative progress have been achieved.This dissertation mainly consists of the following six parts:In the first part,we introduce several traditional（magneto-）thermoelectric effects,including the Seebeck effect,the Peltier effect,the Thomson effect,and the Nernst effect,as well as the important index to weigh the performance of thermoelectric materials.Then the microscopic mechanism of thermoelectric transport,several typical low dimensional and topological thermoelectric materials and their performance optimization are introduced.Finally,the significance of the topic selection and research contents of this paper are briefly summarized.In the second part,the experimental methods involved in this thesis are briefly described,mainly including the crystal growth,characterization,the introduction to measurement methods and the origins and correction of errors.In the third part,the thermoelectric transport properties of the quasi-one-dimensional material Tl₂Cu₆Te₄ are systematically illustrated.High quality Tl₂Cu_6+xTe₄single crystals were synthesized for the first time,and its intrinsic high thermoelectric coefficient and low thermal conductivity are revealed.The thermoelectric figure of merit z T reaches 1.3（1.5）at 300 K（400 K）,which is comparable with the traditional thermoelectric material Bi₂Te₃ and has potential application value.Tl₂Cu₆Te₄ not only has a quasi-one-dimensional crystal structure,but also possesses（hollow）pancake-like Fermi pockets and own relatively high carrier concentration.These characteristics guarantee its large Seebeck coefficient and high conductivity.In addition,the weakly bonded Cu atoms inside the Cu Te₄ tetrahedron and unbonded lone pair of electrons from Tl⁺result in the rattler-like effect in Tl₂Cu₆Te₄,resulting in the formation of low-frequency optical branch phonons with very strong anharmonicity and extremely low lattice thermal conductivity.Tl₂Cu₆Te₄provides an excellent research platform for the study of low dimensional high performance thermoelectric materials.In the fourth part,the thermoelectric transport properties of the quasi-one-dimensional antiferromagnetic AMn₆Bi₅（A=alkali metals）materials family are systematically introduced and some significant differences between them are found expectedly and unexpectedly.All the materials show considerable thermopower caused by phonon drag effect in the low temperature region,among which the sign of thermopower for Na Mn₆Bi₅ changes from negative to positive with decreasing temperature.In addition,a large transverse Nernst effect at low temperature has been detected in AMn₆Bi₅ except Na Mn₆Bi₅,which mainly results from the sharp increase of carrier mobility and the increase of phonon mean free path at low temperature.We hypothesize that the various behaviors are ascribed to the large differences in the size of alkali metal cations.This system provides a unique platform to further study and regulate the competition and coupling of spin,lattice,charge and orbital degrees of freedom as well as the synergy of dimensional effect in quasi-one-dimensional antiferromagnetic materials.In the fifth part,we systematically describe the thermoelectric transport properties of the magnetic topological material NdAlSi.In addition to the conventional periodic quantum oscillations of Seebeck and Nernst coefficients depending on the change of magnetic field at constant temperature,we also find that Seebeck coefficient and Nernst coefficient both oscillate obviously as a function of temperature at constant magnetic field,which is a new type of quantum oscillation.Further analysis shows that this new type of oscillation is due to the destructive interference of quantum oscillation from one split Fermi surface caused by the strong exchange of Weyl electrons and f electrons.Our researches open up a new way to study topological systems based on f-electrons,and the subsequent exploration of these materials can offer greater possibilities for realizing new and exotic quantum phenomena.In the sixth part,we briefly summarize the main research contents of this paper and provide an outlook on the future researches.