电子型高温超导体Nd_2-xCe_xCuO₄单晶及热电材料Bi_2-xPb_xSr₂Co₂O_y和Bi_xTiS₂多晶的热电性质研究

【作者】 刘剑；

【导师】 杨宏顺； 曹烈兆；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2007， 博士

【摘要】 过渡金属化合物是凝聚态物理中的研究热点之一，它包含了丰富而奇异的物理性质，如铜氧化物的高温超导电性，钴氧化物的高效热电效率等等。这些奇异的性质激发了人们极大的兴趣和研究热情。到现在为止，还没有成熟的理论能够解释这些奇异的性质，因而对过渡金属化合物的研究一直受到广泛的关注。在本论文中，我们对电子型超导体Nd_2-xCe_xCuO₄单晶和高效热电材料Bi_2-xPb_xSr₂Co₂O_y及Bi_xTiS₂多晶的热电性质（电阻率，热电势，热导率）进行了一系列的研究和探索。部分研究结果已在Phys. Lett. A，低温物理学报上发表，还有部分成果正在整理发表中。本论文分为以下六章：第一章热电效应及高温超导体综述在本章中，我们首先在第一部分中简述了电阻率，热电势和热导率的理论研究进展，讨论了产生这些输运现象的物理机制，为后续的研究打下基础。同时还简要介绍了热电效率在实际中的应用。在第二部分中，我们简述了高温超导体的结构和相图，并综述了铜氧化物高温超导体的反常行为和这方面的实验和理论研究进展。第二章快速测量热电势的新方法本章中首先简要介绍了传统的测量热电势和热导率的稳态法。为了提高热电势的测量速度，我们设计了一种新型的测量方法—锯齿波法，可以在不降低精度的情况下大幅度提高测量速度，测量时间减小到传统方法的1／5。第三章电子型超导体Nd_1.85Ce_0.15CuO₄的低温物性研究本章中对Nd_1.85Ce_0.15CuO₄的输运性质及不同退火条件的影响进行了系统的研究。Nd_1.85Ce_0.15CuO₄单晶的输运性质，如各向异性电阻率，面内热电势和热导率等，都可以很好地在两能带模型（空穴的窄能带和电子的宽能带）下解释。超导起始温度和载流子密度关系不大，而只由Ce掺杂量决定。900℃氮气退火的主要效果是去除了过量的氧，这时费米能增大，电子型宽能带的结构基本不变，而空穴型窄能带变得更窄，而且中心值的能量变大。而1080℃氮气退火引起的分解，使费米能降低，空穴型窄能带中心值的能量也变小。过量氧的去除增强了体系的各向异性；而分解减弱了体系的各向异性。第四章Nd₂CuO₄单晶及其掺Zn样品的输运性质研究本章中研究了Nd₂Cu_1-xZn_xO₄（x=0，0.01）单晶样品退火前后ab面内的电阻和热电势。所有样品的热电势和电阻都在高温区（＞75K）满足半导体行为，而在中间温区（35 K～75 K），都满足变程跳跃行为，这表明Nd₂Cu_1-xZn_xO₄是存在无序的Mott绝缘体。在高温区，热电势得到的能隙要比电阻中的小得多，这表明体系中存在极化子。体系磁结构的变化会显著影响热电势的行为，对电阻行为也有影响。在235K附近，反铁磁转变使热电势行为表现出热回滞现象。而在75K附近的磁结构转变，使电阻和热电势从半导体行为转变成变程跳跃行为。35K的磁转变发生后，热电势也发生了变化。退火前样品的热电势逐渐饱和，而退火后样品的热电势快速地增大。氮气退火对35K以下的热电势行为有显著的影响，其内涵的物理性质还需要深入的研究。第五章Bi_2-xPb_xSr₂Co₂O_y多晶样品的热电性质研究本章中研究了不同Pb掺杂的Bi_2-xPb_xSr₂Co₂O_y多晶样品的电阻率和热电势行为。高温区热电势的常数行为以及金属型的电阻率表明体系中导电的能带很窄。体系中，无序导致了Anderson局域化。在中间温区电阻率表现出热激活行为，而在热电势上没有观察到明显的变化。在更低的温区，所有热激活出的电子和空穴都处于定域态，电阻率和热电势都表现为变程跳跃行为。重金属元素Pb的掺杂有助于提高体系的热电效率。第六章Bi_xTiS₂多晶的热电性质研究本章中研究了不同Bi掺杂组分的Bi_xTiS₂多晶样品的热电性质。Bi掺杂的主要效果是转移其价电子到母体层中Ti的导带3d带中，每个Bi原子提供5个电子，增加了载流子浓度，而没有引入新的散射机制影响电子的输运，因此热电势的行为可以很好地归一化。TiS₂体系中特殊的椭球形的费米口袋对输运性质有非常重要的影响，其特征波矢的电声子曵引引起了热电势的低温峰。随着掺杂的增多，费米口袋逐渐变大。Bi的掺入减小了声子热导，从而提高了体系的热电效应。但是综合考虑电阻率，热电势和热导率的贡献，体系热电效率的优值系数ZT随着掺杂量先增大后减小，在x=0.15附近存在一个最佳的热电效率。更多还原

【Abstract】 Transition metal compounds is a main topic in condensed matter physics for their interesting and abnormal properties, such as high-T_c superconductivity, high thermoelectric efficiency, et al, which have attracted much attention of researching. In this dissertation, we have studied the electron-type superconductor Nd_2-xCe_xCuO₄ single crystal and thermoelectric materials Bi_2-xPb_xSr₂Co₂O_y and Bi_xTiS₂ polycrystalline in detail. The dissertation was arranged as six parts:Chapter 1 Summaries for thermoelectric effects and the properties of High-T_c superconductorsIn part I, the theories of resistivity, thermopower and thermal conductivity, are firstly summarized, and the physical insight of these transport properties is also discussed. And then, the applications of thermoelectric effects are recommended. In part II, the author reviews the structure and phase diagram of the high-T_C superconductors. Their anomalous properties are briefly discussed and the recent development in theoretical and experimental physics about this topic is also mentioned.Chapter 2 A new method of fast thermopower measurementIn this chapter, the traditional steady-method of the measurement on thermopower and thermal conductivity is introduced at first. To quicken the measurement of thermopower, the new method of fast thermopower measurement was developed. In the same request of resolution, this method reduces the measurement time considerably, with a factor of five. Chapter 3 Studies on the transport properties of Nd_1.85Ce_0.15CuO₄ single crystalIn this chapter, the transport properties of Nd_1.85Ce_0.15CuO₄ under different annealed conditions are investigated systematically. The transport properties, such as resistivity, thermopower and thermal conductivity, can be well explained in a two-band model, with a hole-type narrow band and an electron-type broad band. The transition temperature of superconducting is not affected by carrier concentration, but by the Ce doping level. The main effect of annealing processes at 900℃is to remove excess oxygen. As a result, Fermi level increases. The electron-type broad band is not changed, while the hole-type narrow band becomes narrower, and its central energy is enhanced. Furthermore, the decomposition, produced when annealed at 1080℃, decreases Fermi level and the central energy of narrow band. The removal of oxygen enhances the anisotropy while the decomposition does opposite.Chapter 4 Resistivity and Thermopower of Nd_xCu_1-xZn_xO₄ single crystalsIn this chapter, resistivity and thermopower of Nd₂Cu_1-xZn_xO₄（x=0, 0.01） single crystal are studied in detail, before and after annealed in N₂ at 900℃. At high temperatures （>75 K）, both resistivity and thermopower represent a semi-conducting behavior, while at intermediate temperatures （35 K～75 K）, both of them behave as variable range hopping （VRH）, indicating that Nd₂Cu_1-xZn_xO₄ is a Mott insulator with disorder. The gaps, calculated from thermopower are much smaller than that from resistivity, indicating the presence of polarons. The transition of magnetic structure would influence the behavior of thermopower remarkably, and affect the behavior of resistivity slightly. The antiferromagnetic transition around 235 K brings in the special thermal hysteresis in thermopower, and around 75 K, resistivity and thermopower transfer from semi-conducting behavior to VRH behavior, caused by magnetic transition. Furthermore, the magnetic transition around 35 K also influences the behavior of thermopower. Below 35 K, the thermopower tends to saturate before annealed, while decreases rapidly after annealed, whose origin still calls for more investigations.Chapter 5 Study on the thermoelectric properties of Bi_2-xPb_xSr₂Co₂O_yIn this chapter, the thermopower and resistivity of Bi_2-xPb_xSr₂Co₂O_y polycrystalline samples with different Pb doping level are investigated in detail. The large constant behavior of thermopower at high temperatures, together with metallic resistivity, indicates a narrower band contribution. As a result of Anderson localization, brought in by disorder, the resistivity represents thermal-activated behavior at intermediate temperatures, while no change can be seen in the thermopower. At low temperatures, all the thermal-activated electrons and holes are in the localized states, thus both the resistivity and thermopower represent variable range hopping behavior. The doping of heavy metal element Pb enhances the thermoelectric efficiency.Chapter 6 Study on the thermoelectric properties of Bi_xTiS₂In this chapter, the thermoelectric properties of Bi_xTiS₂ polycrystalline samples, with different Bi intercalating level, are investigated in detail. The main effect of Bi intercalation is to transfer five electrons to 3d band of Ti atoms in the host layers, and increases the carrier concentration. While Bi intercalation does not bring in any new scattering mechanism to influence electron transport, thus the thermopower can be well normalized. The thermopower are related to the special lens-shaped Fermi pockets of TiS₂ system. With more carriers are doped, the Fermi pockets expand, and the characteristic energy hω_c and corresponding temperature of the phonon peak increase. Bi intercalation reduces the thermal conductivity of phonons, and thus enhances the thermoelectric efficiency. However, if resistivity, thermopower and thermal conductivity are taken into account together, the thermoelectric figure of merit ZT increases with increasing intercalation and then decreases. There is a best ZT when the Bi intercalation is about 0.15.更多还原