【作者】 陈红；

【导师】 陈仙辉；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2009， 硕士

【摘要】 过渡金属氧化物由于所表现出来的奇异物理性质,如超导电性、庞磁阻、反常霍尔效应、大的热电势效应,而被人们广泛的研究。这些体系中的电荷、自旋轨道和晶格之间相互耦合,各种相互作用之间相互竞争,使体系呈现出丰富的相图,给人们展现出如电荷有序、磁有序、玻璃态、相分离、金属绝缘体转变、自旋态转变等有趣的物理现象。这些不仅在科研上,也在实际应用上有相当的价值,因而成为现在凝聚态物理学界的研究热点。在本论文中,我们对新型铁基高温超导122体系的Ba1?xKxFe2As2的相图进行研究,以及三角格子层状钴氧化物K0.5CoO2单晶和非费米液体的Li0.9Mo6O17单晶的磁性和输运性质进行了一系列实验上的研究和探索。相关研究结果已在EurophysicsLetters.以及Journal of Physics: Condensed Matter上发表。本论文共分为以下四章:1. KxCoO2、Li0.9Mo6O17和铁基超导导的的研究究进进展本章对上面提到的几种材料的基本物理性质以进行简述。其中KxCoO2体系,首先我们对NaxCoO2体系的基本结构和相图进行了概述,然后介绍了KxCoO2研究进展。对于铁基高温超导体,主要介绍了其基本结构、输运性质和电子相图。对于Li0.9Mo6O17,主要介绍了它的晶体结构以及对低温下引起金属绝缘转变原因这一热点问题展开了讨论。2. K0.5CoO2单晶输运和磁性质的研究本章中研究了K0.5CoO2单晶的磁化率、随磁场角度变化的面内磁阻行为,给出了一个可能的磁结构。K0.5CoO2的磁结构与Na0.5CoO2的相似,都是G型反铁磁耦合(面间和面内都是反铁磁耦合)。在在20K附近,磁化率场冷和零场冷形成一个很小的类似铁磁的回滞。并且当磁场躺在面内时,20K磁阻的绝对值比其他温度观察到的值大很多。这可能是由于当磁场躺在面内时,导致小磁矩Co3.5?δ在20K发生转动(spin-?op),在面内形成面内铁磁,这与在Na0.52CoO2中相似。3. Ba1?xKxFe2As2中自旋密度波(SDW)和超导导的的共存在本章中,我们合成了高质量的Ba1?xKxFe2As2系列样品,其中x从0至1。通过输运、XRD和中子衍射的研究,我们给出了结构相变、SDW转变和超导转变温度随K含量x变化的相图。发现在x < 0.15的区域只存在结构相变和SDW,且发生的是一级相变;x > 0.4的样品只存在低温超导态;而在0.15 < x < 0.4区域SDW和超导共存,且高温发生的结构相变和SDW转变是二级相变。4. Li0.9Mo6O17单晶磁输运性质的研究在本章中,我们分别研究了Li0.9Mo6O17单晶a、b、c三个方向的电阻率。我们给出了各向异性比ρc/ρa和ρc/ρb,这两个各向异性比证实了Li0.9Mo6O17的准一维性质。在TM温度之上,各向异性比几乎不随温度变化,在TM温度时突然迅速减小,我们认为这是由于在TM温度发生了维度转变。我们还分别测量了当磁场平行三个方向时,三个方向的电阻率。当磁场沿着c方向时,可以观测到反常的巨磁阻效应。而当磁场沿着a和b方向时磁场相对都很小。这些磁阻结果在CDW准两维化合物A0.9Mo6O17 ( A = K, Na, Tl )被观测到过。这暗示着维度转变导致Luttinger液体失稳,从而导致电子-电子相互作用占主导地位的CDW态。更多还原

【Abstract】 The transition metal oxides have been widely studied due to their intriguing prop-erties, such as colossal magnetoresistance, abnormal Hall e?ect, large thermoelectricale?ect and multiferronics. In these systems charge, spin, orbital and lattice are cou-pled to each other, and complicated phase diagrams are introduced by the competitionamong these interactions, giving rise to some interesting phenomena such as chargeordering, magnetic ordering, glassy behavior, phase separation, metal-insulator tran-sition and spin-stat transition. These are important not only for scientific purpose,but also for the possibility of application.In this dissertation, we have studied the phase diagram of 122-systemBa1?xKxFe2As2, and transport and magnetic properties of triangular layered cobaltoxides K0.5CoO2 and Li0.9Mo6O17 single crystals.The whole dissertation is constructed as following:1. Brief overview of the physical properties of the high-Tc pnictide andtriangular cobalt oxidesIn this chapter, the author has reviewed the basic physical properties on KxCoO2system, and iron-based superconductors and Li0.9Mo6O17. For KxCoO2 system, wehave reviewed the crystalline structure and phase diagram in NaxCoO2 first, andthen the physical properties of KxCoO2. For iron-based superconductors, we haveintroduced the crystalline structure, transport properties and phase diagram. ForLi0.9Mo6O17, we have reviewed the crystalline structure and discussed the hot issueabout the origin of metal-insulator transition.2. Magnetotransport properties in K0.50CoO2 single crystalsIn this chapter, We have measured susceptibility and angle-dependent magnetore-sistance (AMR) on K0.50CoO2 single crystals. A possible magnetic structure similarto that of Na0.5CoO2, a G-type anti-ferromagnetic (AF) structure (both in-plane andinter-plane are AF), is proposed. At～20 K, a loop is observed between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility. The absolute value of magnetore-sistance (MR) at 20 K is much larger than other cases with H applied within the abplane. This could arise from the magnetic field being applied in the ab plane, leadingto spin–?op of the small magnetic moment Co3.5?δsites at 20 K to form in-planeferromagnetic ordering, as observed in Na0.52CoO2. 3. Coexistence of the spin-density-wave and superconductivity in theBa1?xKxFe2As2In this chapter, The relation between the spin-density-wave (SDW) and super-conducting order is a central topic in current research on the FeAs-based high TCsuperconductors. Con?icting results exist in the LaFeAs(O,F)-class of materials, forwhich whether the SDW and superconductivity are mutually exclusive or they cancoexist has not been settled. Here we show that for the (Ba,K)Fe2As2 system, theSDW and superconductivity can coexist in an extended range of compositions. Theavailability of single crystalline samples and high value of the energy gaps would makethe materials a model system to investigate the high TC ferropnictide superconduc-tivity..4. Magnetotransport properties in purple bronze Li0.9Mo6O17 SingleCrystalwe have studied resistivity along the a, b and c axes on the same piece of thesingle crystal, respectively. The anisotropics ofρc/ρa andρc/ρb were given, whichconfirm the quasi-one-dimensionality of the compound. The sharp decrease in theanisotropy below TM indicates dimensional crossover at this temperature. Unusuallyhuge positive MR for H c has been observed. We invoked a modified two-bandmodel to fit the MR data which had been used in CDW compounds such as quasi-two-dimensional purple bronzes A0.9Mo6O17 ( A = K, Na, Tl ) and quasi-one-dimensionalconductor NbSe3. The behavior of MR rather supports the existence of CDW or SDWinstability in Li0.9Mo6O17.更多还原