[论文解读] Emergence of a Novel Frozen Magnetic State in a Heavily Overdoped Non-Superconducting Copper Oxide

J. E. Sonier, C. V. Kaiser+6 more|arXiv (Cornell University)|Nov 2, 2009

Physics of Superconductivity and Magnetism被引用 1

一句话总结

本研究利用μ子自旋松弛技术，研究了重掺杂La2-xSrxCuO4中的铁磁涨落，发现在非超导区，温度低于10 K时出现静态、局域化的电子磁矩。结果支持了在高掺杂下弱局域铁磁性的理论预测，表明存在一种与长程有序不同的新型冻结磁态。

ABSTRACT

The doping of charge carriers into the CuO2 planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La2-xSrxCuO4 using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping.

研究动机与目标

• 研究在超导被抑制的非超导铜氧化物过掺杂区是否存在铁磁序。
• 确定在非超导、重掺杂的La2-xSrxCuO4相中，铁磁涨落或长程有序是否出现。
• 区分观察到的磁矩的起源是本征电子磁性还是外在杂质贡献。
• 检验铜氧化物体系在高掺杂水平下弱局域铁磁性的理论预测。

提出的方法

• 采用μ子自旋松弛（μSR）技术，在原子尺度探测重掺杂La2-xSrxCuO4中的局部磁场。
• 测量μ子自旋的去极化率，以检测静态磁矩的存在及其动力学行为。
• 分析温度依赖的μSR数据，识别在非超导区低于10 K时静态磁性的出现。
• 将观测到的磁行为与过掺杂铜氧化物中弱局域铁磁性的理论模型进行比较。
• 通过分析磁矩的空间分布和浓度，排除长程调和有序的可能性。
• 通过与已知磁性杂质的对比，评估外在杂质作为磁性来源的可能性。

实验结果

研究问题

• RQ1 La2-xSrxCuO4的非超导、重掺杂区是否会出现铁磁态？
• RQ2 观测到的磁矩是静态且局域的，还是表明长程调和有序？
• RQ3 观测到的磁性是否可归因于外在杂质，还是与本征电子效应一致？
• RQ4 过掺杂区的磁行为与弱局域铁磁性的理论预测相比如何？
• RQ5 铜氧化物中在超导穹顶之外形成的磁态具有何种性质？

主要发现

• 在非超导、重掺杂的La2-xSrxCuO4中，低于10 K时检测到明确的、源于电子的静态磁矩。
• 磁矩是局域化的，未形成调和长程有序态，表明其为无序、稀释的分布。
• 观测到的磁性与长程铁磁有序不一致，反而指向一种具有局域特征的新型冻结磁态。
• 结果与外在磁性杂质为主要来源的假设不一致，支持本征电子机制。
• 研究结果与电子能带结构计算的预测一致，即在高掺杂水平下存在弱局域铁磁性。
• 在无超导性的情况下出现静态磁矩，支持了过掺杂区存在竞争磁涨落的存在。