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首页> 外文学位 >Geometric magnetic frustration, frustrated ferroelectricity, and superconductivity in transition metal compounds.
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Geometric magnetic frustration, frustrated ferroelectricity, and superconductivity in transition metal compounds.

机译：过渡金属化合物的几何磁性受挫，铁电受挫和超导。

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Understanding the relationships between the structures and properties of materials has been a long-standing goal in solid state chemistry. Systems that display correlated electron behavior -- that is, when the interactions between electrons within a single atomic site or between adjacent sites are non-negligible -- are particularly rich in properties but challenging to understand. The first part describes a study of the properties of two layered triangular lattice materials. NaVO2 contains triangular layers of V3+ (d2) ions. The octahedral coordination of the vanadium ions splits the five d-orbital-derived bands into a triply degenerate t2g and a doubly degenerate eg set. A Jahn-Teller distortion, along with a form of geometric magnetic frustration, give rise to two successive orbital ordering transitions. NaFeO2 contains triangular layers of Fe3+ (d5) with the electrons spread out across the d-orbital-derived bands. In this case, no structural distortion is observed down to 1 K. Instead, geometric magnetic frustration prevents magnetic ordering, giving rise to a series of intricate magnetic states. The second part describes preliminary results on a series of reduced niobium pyrochlores in which electric dipoles do not adopt long range order. This is analogous to geometric magnetic frustration, but little is known about these systems displaying frustrated ferroelectricity. The third part describes a series of detailed chemical characterizations of members of the new "iron-based" superconductors. Fe1+deltaSe is shown to be extremely sensitive to defects and composition, with the best superconductors found when it is nearly stoichiometric, with formula Fe 1.01Se. 77Se NMR provides evidence of the importance of spin fluctuations in the observed superconductivity. LaNiPO is shown to be similarly sensitive to composition. The low temperature specific heat of LaNiPO is consistent with the presence of spin fluctuations. Taken together, these results suggest that spin fluctuations may be important for superconductivity in the entire family of materials.
机译：理解材料的结构与性能之间的关系一直是固态化学的长期目标。显示相关电子行为的系统-即当单个原子位点内的电子之间或相邻位点之间的电子相互作用不可忽略时-具有特别丰富的特性，但难以理解。第一部分描述了对两层三角晶格材料的性能的研究。 NaVO2包含V3 +（d2）离子的三角形层。钒离子的八面体配位将五个d轨道衍生的谱带分成三重简并的t2g和双简并的t2g。 Jahn-Teller畸变以及某种形式的几何磁挫折会引起两个连续的轨道有序过渡。 NaFeO2包含三角形的Fe3 +（d5）层，电子在d轨道衍生的能带上散布。在这种情况下，直到1 K都没有观察到结构变形。相反，几何磁性受阻阻止了磁性排序，从而产生了一系列复杂的磁性状态。第二部分介绍了一系列还原的烧焦铌的初步结果，其中电偶极子不采用长程有序。这类似于几何磁挫折，但对这些显示挫折的铁电的系统知之甚少。第三部分描述了新型“铁基”超导体成员的一系列详细化学特征。 Fe1 +δSe对缺陷和成分非常敏感，当接近化学计量时，发现最佳的超导体，其分子式为Fe 1.01Se。 77Se NMR提供了自旋波动在观察到的超导中的重要性的证据。 LaNiPO被证明对成分同样敏感。 LaNiPO的低温比热与自旋波动的存在相一致。综上所述，这些结果表明自旋涨落对于整个材料家族中的超导性可能很重要。

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