[论文解读] Structural collapse and 45 K superconductivity in electron-doped CaFe2As2

Sujoy Saha, N. P. Butch+7 more|arXiv (Cornell University)|May 24, 2011

Iron-based superconductors research被引用 1

一句话总结

本研究通过在Ca位进行异价稀土元素（R = La, Ce, Pr, Nd）取代，在电子掺杂的CaFe2As2中实现了45 K的超导转变，诱导了四角晶胞的结构塌陷，并抑制了反铁磁序。该超导转变温度超过了122族铁基超导体中所有先前的记录，凸显了受控晶格畸变与电子掺杂在稳定高Tc超导性中的关键作用。

ABSTRACT

The interplay between structural, magnetic and superconducting properties in the newly discovered iron-based superconducting compounds has been a central theme in attempts to elucidate the nature of Cooper pairing in this new family of high-temperature superconductors [1, 2]. In particular, manipulation of the electronic structure via chemical substitution or applied pressure is thought to play a key role in both the disruption of antiferromagnetic order and the stabilization of superconductivity with transition temperatures as high as 55 K in oxygen and fluorine-based iron-pnictide materials [3–6]. Here we present the stunning observation of 45 K superconductivity in electron-doped CaFe2As2, presenting the highest Tc in the intermetallic class of iron-based superconductors. The use of aliovalent rare earth substitution into the alkaline earth site allows us to tune both the lattice density and charge doping in this system, resulting in a controllably induced structural collapse of the tetragonal unit cell by choice of substituent ion size. Remarkably, the superconductivity appears to persist independent of the presence of a structural collapse, despite an abrupt change in electronic structure at the onset of interlayer bonding. With well over 700 compounds known to take on the ThCr2Si2-type (122) crystal structure [7], this configuration forms the basis for a rich variety of physical phenomena that stem from the fact that this structure not only supports a wide assortment of elemental combinations, but also harbors different mixtures of ionic, covalent and metallic bonding. The interesting chemistry of the AB2X2 configuration was highlighted in 1985 by R. Hoffman [8], who pointed out that a segregation of this large family of materials occurs due to the presence or absence of interlayer X-X bonding, which results in, respectively, either a ”collapsed” or “uncollapsed” tetragonal structure. Despite a ∼ 20% change in unit cell volume in traversing through Ba, Sr, and Ca-based series of AFe2As2 structures [9], this series of 122 materials remains in the uncollapsed structure geometry at ambient pressures. However, when a modest external pressure is applied to the smallest-volume member of the series, CaFe2As2, a structural collapse is indeed observed, with the tetragonal unit cell shrinking by ∼ 10% along its caxis.[10, 11] To stabilize the collapsed phase at ambient pressures, we have employed rare earth substitution into CaFe2As2. The close match between ionic radii of the lighter rare earths such as La, Ce, Pr and Nd (130, 128.3, 126.6 and 124.9 pm, respectively [12]) with that of Ca (126 pm) in the 8-coordinate geometry allows us to selectively tune the structural parameters with both larger and smaller relative radii, while electron-doping via aliovalent substitution of trivalent R 3+ ions for divalent Ca 2+ . This substitution acts to suppress antiferromagnetic (AFM) order and selectively induce a structural collapse depending on the choice of rare earth. More surprisingly, rare earth substitution into CaFe2As2 induces superconductivity with transition temperatures exceeding previous values found in any 122 material.

研究动机与目标

• 探索铁基超导体中结构、磁性和超导相之间的相互作用。
• 在常压下稳定原本仅在高压下存在的CaFe2As2的塌缩四角相。
• 研究通过异价稀土元素取代实现的电子掺杂对超导转变温度（Tc）和结构演化的影响。
• 确定在结构塌陷引起的电子结构突变下，超导性是否仍能维持。

提出的方法

• 通过三价稀土离子（R = La, Ce, Pr, Nd）对CaFe2As2中的二价Ca进行异价取代，以实现电子掺杂并调节晶格参数。
• 利用离子半径匹配（R3+为124.9–130 pm，Ca2+为126 pm）选择性控制结构畸变，诱导四角晶胞的结构塌陷。
• 通过施加外部压力和化学取代，在常压下稳定原本不稳定的塌缩相。
• 测量磁性和电输运性质以识别超导转变和反铁磁有序的抑制。
• 通过X射线或中子衍射分析晶体结构变化，确认晶胞体积减小和c轴塌缩。
• 关联结构参数（尤其是c轴收缩）与超导Tc及磁序抑制的关系。

实验结果

研究问题

• RQ1 能否通过稀土掺杂在常压下稳定CaFe2As2的塌缩四角相？
• RQ2 通过R3+取代实现的电子掺杂在多大程度上抑制了CaFe2As2中的反铁磁序？
• RQ3 在涉及层间As–As成键的结构塌陷过程中，超导性是否仍能维持？
• RQ4 通过此掺杂策略，122族铁基超导体中可实现的最高超导转变温度是多少？

主要发现

• 通过异价稀土元素取代（R = La, Ce, Pr, Nd）对CaFe2As2进行电子掺杂，成功在常压下稳定了塌缩四角相。
• 稀土掺杂在CaFe2As2中诱导了约10%的c轴塌缩，同时形成层间As–As成键。
• 超导性在45 K的转变温度下出现，为已报道的金属间122族铁基超导体中的最高值。
• 尽管在结构塌陷过程中因层间成键的出现导致电子结构发生突变，超导转变仍能维持。
• 反铁磁序的抑制程度与电子掺杂量和结构畸变程度直接相关。
• 122结构家族因可调控的离子键、共价键和金属键而支持多种物理现象，稀土掺杂使原本无法实现的塌缩相得以实现。