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高轨道过渡金属氢化物中金属-绝缘体转变的调控研究
中文摘要

金属-绝缘体相变(MIT)是强关联电子物理中非常重要的宏观量子现象。以3d过渡金属氧化物为代表的强关联电子体系中,通过适当掺杂载流子或者施加化学或物理压力展宽能带,可以实现Mott绝缘体的金属化。在MIT转变区域,电子兼具局域和巡游特性,电荷、自旋、轨道和晶格等量子自由度存在强烈的耦合与竞争,且各种相互作用能量尺度接近,对外部激励的响应异常敏感,很容易调控各种相互作用能量之间的微妙平衡,实现不同量子态之间的转变,进而诱发诸如高温超导和庞磁电阻等奇特的物理现象。因此,MIT及其综合外场调控一直是强关联电子领域的重点研究内容。 相对于3d过渡金属氧化物,4d和5d轨道电子更接近局域-巡游过渡区域,而且自旋-轨道耦合作用显著增强,几乎与在位库伦能U和能带宽度W的能量尺度相当,造成了许多新颖的电、磁学性质。因此,本论文针对多个具有金属-绝缘体转变的4d/5d过渡金属氧化物体系进行了系统的调控研究,试图揭示MIT相变机制以及相关现象的物理现象。取得的主要结果包括: 1.针对烧绿石Cd₂Ru₂O₇在反铁磁有序温度以下出现的反常金属态,我们利用高压合成了Cd₂Ru₂O₇和系列Cd位掺杂的Cd〓A〓Ru〓O〓(A=Ca,Pb)多晶样品,并进行了详细的高压和物性研究。我们发现无论是对Cd₂Ru₂O₇施加~1 GPa的静水压还是用~10%的Ca²⁺或者Pb²⁺取代Cd²⁺,都会破坏Cd₂Ru₂O₇的反铁磁金属基态,取而代之的是反铁磁绝缘基态,这表明Cd₂Ru₂O₇的反铁磁金属基态是非常不稳定的。我们提出,Cd₂Ru₂O₇中的Ru⁵⁺-4d³电子态恰好处于巡游到局域电子过渡的区域,施加较小静水压可以增强Cd-O杂化,增加Ru-4d电子的局域性,而少量Ca²⁺或Pb²⁺掺杂造成的晶格无序也可以增强Ru-4d电子的局域性,这都使得Cd₂Ru₂O₇的金属态被抑制,转变为反铁磁绝缘态。 2.具有烧绿石结构的T1₂Ru₂O₇在120 K附近发生伴随结构相变的MIT,对其晶体结构的分析表明,其低温下可能形成准一维自旋为1的Haldane自旋链。我们利用高压合成了T1₂Ru₂O₇多晶样品,并对其MIT进行了高压和化学掺杂的调控研究。我们发现,随着压力的增加,MIT温度首先向低温移动,然后在1 GPa以上迅速向高温移动,同时电阻曲线上的MIT也变得不明显。这可能是由于压力会稳定T1₂Ru₂O₇中的轨道序,增加准一维Haldane自旋链的自旋能隙,从而逐渐稳定绝缘基态。而在(T1〓Bi〓)〓Ru〓O〓体系中,随着Bi³⁺掺杂量的增加,MIT逐渐被抑制,稳定了金属基态。这应该归因于Bi³⁺-6s孤对电子的引入,通过与Ru-4d轨道电子的杂化破坏了其轨道序并展宽了能带,从而逐渐抑制了T1₂Ru₂O₇的MIT,并稳定了金属基态。 3.对于具有反铁磁绝缘体基态的Y₂Ru₂O₇烧绿石,我们系统研究了Y³⁺位掺杂Bi³⁺和Pb²⁺离子对基态的调控规律,发现两者都会逐渐抑制反铁磁绝缘基态,转变为顺磁金属。对于等价态的Bi³⁺掺杂,Bi³⁺-6s²孤对电子的引入对于增加电子载流子浓度、展宽能带进而诱导金属化起到关键作用;而Pb²⁺掺杂造成的空穴掺杂应该是造成金属化的主要驱动力。 4.利用高温高压方法合成了两种具有金红石型结构的化合物Cr₂ReO₄和CrReO₄,并详细表征了其晶体结构、电输运、磁性和磁结构等性质。二者均是具有3d-5d阳离子有序排列的化合物,随着温度减低会出现二级反铁磁相变,通过粉末中子衍射确定了二者的磁结构。电输运测量表明二者均是半导体,并符合Mott的变程跳跃导电机制,并且电阻率导数在反铁磁转变温度附近出现了较小的反常,表明在这种3d-5d电子耦合材料体系中存在自旋与电荷自由度之间的耦合。 5.CrSb₂是窄带半导体,我们利用高温高压合成了CrSb₂的高压相,并对其物理性质进行了详细的研究。我们发现CrSb₂高压相转变为金属,而且在降温时会出现两个连续的磁转变,首先在~160 K发生铁磁转变,然后在~86 K出现反铁磁转变。在常压下施加外磁场,会逐渐抑制低温的反铁磁转变,在3T以上获得铁磁金属基态。而施加高压时,反铁磁转变迅速往高温移动,铁磁转变往低温移动,二者相遇后系统基态转变为反铁磁金属,进一步增加压力会逐渐抑制反铁磁转变,在~9 GPa时被完全压制,实现反铁磁量子临界点。 关键词:4 d/5d过渡金属氧化物;金属-绝缘体转变;高温高压合成;巡游铁磁体

英文摘要

Metal-insulator transition (MIT) is one of the most important manifiestation of quantum phenomena in strongly correlated electron systems. The Mott insulating state frequently observed in the 3d transition-metal oxides can be convereted to metallic via doping charge carriers or applying chemical/physical pressure. In the transition region, the 3d electrons acquire both localized and itinerant characters, and the strong interplay of charge, spin, orbital, and lattice degrees of freedom can induce many intriguing quantum states of matter, which are prone to external stimuli and can give rise to some exotic phenomena such as high-T〓 superconductivity or colossal magnetoresistance. Therefore, MIT and its manipulation have attracted considerable interest in the strongly correlated electron systems. In comparison with the well-studied 3d transition-metal oxides, the electronic states of 4d/5d conterparts are closer to the localized-to-itinerant transition due to the spatially more extended 4d and 5d orbitals. In addition, the enhanced spin-orbit interaction, comparable with the onsite Coulomb interaction U and electronic bandwidth W, gives rise to many novel phenomena that are the subject of extensive investigations in recent years. In this diseration, we investigated the MIT in several 4d/5d transition-metal oxides with an aim to uncover the mechanism and associated unusual phenomena. The main results include: 1.The cubic pyrochlore Cd₂Ru₂O₇ develops a peculiar metallic state below the antiferromagnetic (AF) order around T〓 = 100 K. We have studied the effects of hydrostatic pressure and the isovalent chemical substitutions on the electrical transport properties of Cd₂Ru₂O₇ synthesized under high pressure. We found that the metallic state below T〓 is very fragile and can be immediately convereted to an insulating state by either application of hydrostatic pressure ~ 1 GPa or replacing ~ 10% Ca²⁺ or Pb²⁺ for Cd²⁺. In comparison with other Ru pyrochlores, we proposed that Ru⁵⁺-4d³ electrons are located near the crossover from localized to itinerant state and thoese external stimuli can enhance the localized character by strengthening the Cd-0 valvency under pressure or introducing chemical disorders via doping. 2.Another cubic pyrochlore T1₂Ru₂O₇ exhibits a sharp MIT around 120 K accompanied with a structural transition. Previous studies have shown that the development of orbital ordering below the structural transition might facilitate the formation of quasi-1D S = 1 Haldane chain with a spin gap. We have synthesized the T1₂Ru₂O₇ pyrochlore under high pressure and invistiaged the effect of hydrostatic pressure and the isovalent chemical substitution. We found that the application of pressure first reduces slightly the MIT and then shifts it to higher temperatures gradaully, with the MIT less pronounaced. It is likely that the pressure can stabilize the orbital order and enlarge the spin gap of Q1D Haldane chains. On the other hand, substitutions of Bi³⁺ for Tl³⁺ can suppress the MIT and stbalize a metallic ground state gradually, presemubaly due to the obital hybridization between the Bi³⁺-6p orbital with the Ru⁴⁺-t〓 orbitals as well as the introduction of lone pair Bi³⁺-6s² electrons. 3.We also studided the doping-induced MIT in the series of cubic pyrochlores Y〓R〓Ru〓O〓 (R = Bi or Pb) synthesized at ambient pressure. In both cases, the AF insulating state of Y₂Ru₂O₇ can be continuous transformed to the paramagnetic state upon doping. For the isovalent Bi³⁺ doping, the introduction of lone-pair electrons and the orbital hybridizations are important for the MIT, while the hole doping in the Pb²⁺-doped samples should also contribute to the MIT. 4.We also synthesized cation-ordered inverse trirutile Cr₂ReO₆ and monoclinic birutile CrReO₄ under moderate high-pressure and high-temperaure conditions, and performed detailed characterizations on their structural, transport, and magnetic properties. A second-order AF transition is found to take place at T〓 = 67 K for C₂ReO₆ and T〓 = 98 K for CrReO₄, respectively. Their magnetic structures were determined from the analysis of the NPD patterns. Both compounds are confirmed to display a semiconducting behavior following the Mott’s VRH conduction mechanism A weak anomaly can be discerned around T〓 from the temperature derivative of resistivity for both compounds, signaling a coupling between spin and charge degrees of freedom for these 3d-5d coupled electron systems. 5.CrSb₂ is a narrow-band semiconductor. We obtained its high-pressure form and investigated its physical properties in detail. The high-temperature phase of CrSb₂ is metallic, and undergoes two successive magnetic transition upon cooling, i.e. a ferromagnetic (FM) transition at ~ 160 K and an AF-like transition at 86 K. The AF transiton can be suppressed gradually by applying external magneic fields at ambient pressure, resulting in a ferromagnetic metal ground state above 3 T. On the other hand, the AF transition can be enhanced quickly by pressure and meet with FM transition, forming a new AF state, which can be eventually suppressed by pressure of~ 9 GPa, forming an AF quantum critical point. Key Words : 4d/5d transition metal oxide; Metal-insulator transition; High-temperature and high-pressure synthesis; itinerant ferromagnet

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