[论文解读] Magnetic Order versus superconductivity in the Iron-based layered La(O1-xFx)FeAs systems

Clarina dela Cruz, Qing Huang+9 more|arXiv (Cornell University)|Apr 4, 2008

Iron-based superconductors research被引用 5

一句话总结

本研究利用中子散射技术证明，母体化合物LaOFeAs在约134 K时发生结构相变，转变为单斜对称性，并形成长程自旋密度波（SDW）反铁磁序，Fe磁矩较小，为0.36(5) μB。氟掺杂抑制了磁性与结构相变，表明铁基超导体中的超导性源于具有反铁磁序的母相态，与铜氧化物高温超导体类似。

ABSTRACT

In high-transition temperature (high-Tc) copper oxides, it is generally believed that antiferromagnetism plays a fundamental role in the superconducting mechanism because superconductivity occurs when mobile electrons or holes are doped into the antiferromagnetic parent compounds. The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems [RO1-xFxFeAs] has generated enormous interest because these materials are the first noncopper oxide superconductors with Tc exceeding 50 K. The parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies near 150 K in both resistivity and dc magnetic susceptibility. While optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed with doping electrons to form superconductivity, there has been no direct evidence of the SDW order. Here we use neutron scattering to demonstrate that LaOFeAs undergoes an abrupt structural distortion below ~150 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then followed with the development of long range SDW-type antiferromagnetic order at ~134 K with a small moment but simple magnetic structure. Doping the system with flourine suppresses both the magnetic order and structural distortion in favor of superconductivity. Therefore, much like high-Tc copper oxides, the superconducting regime in these Fe-based materials occurs in close proximity to a long-range ordered antiferromagnetic ground state. Since the discovery of long

研究动机与目标

• 确定非超导母体化合物LaOFeAs的真实基态。
• 研究LaOFeAs中约150 K处的电阻率与磁化率异常是否与磁性或结构有序有关。
• 考察氟掺杂如何抑制磁性与结构相变以促进超导性。
• 阐明铁基超导体中反铁磁性、结构畸变与超导性之间的关系。

提出的方法

• 在多个温度下对多晶LaOFeAs和La(O1-xFx)FeAs（x = 0.08）进行中子粉末衍射。
• 使用NIST的BT-1和BT-7衍射仪以及ORNL的HB-1A谱仪测量结构与磁性相变。
• 通过温度依赖性测量(2,2,0)核衍射峰以检测结构相变。
• 利用三轴谱仪进行磁性结构精修，识别磁性布拉格峰并确定有序磁矩。
• 将磁性强度归一化至核散射，以量化Fe磁矩。
• 比较磁性与结构有序参数的温度演化，以确定相变顺序。

实验结果

研究问题

• RQ1 LaOFeAs中约150 K处的电阻率与磁化率异常是否由结构相变或磁性相变引起？
• RQ2 母体化合物LaOFeAs是否表现出长程反铁磁序？若存在，其磁结构为何？
• RQ3 氟掺杂如何抑制磁性与结构相变以实现超导性？
• RQ4 LaOFeAs中结构畸变与磁性有序的事件顺序如何？
• RQ5 LaOFeAs的磁性基态是否与自旋密度波（SDW）不稳定性理论预测一致？

主要发现

• LaOFeAs在约155 K时发生从四角对称（P4/nmm）到单斜对称（P112/n）的结构相变，表现为(2,2,0)核衍射峰分裂。
• 在约134 K时形成具有条纹型结构的长程反铁磁序，8 K时Fe磁矩为0.36(5) μB。
• 磁性相变发生在结构相变之后，表明晶格畸变先于磁性有序。
• 在超导相La(O0.92F0.08)FeAs中，(1,0,3)磁性布拉格峰缺失，证实氟掺杂抑制了反铁磁性。
• 结构与磁性相变在超导相中被抑制，表明超导性源于磁性有序的母相态。
• 观测到的磁性结构与理论预测的SDW态一致，但测得磁矩（0.36(5) μB）远小于理论预测值约2.3 μB。