[论文解读] Evolution of Magnetic Double Helix and Quantum Criticality near a Dome of Superconductivity in CrAs

Masaaki Matsuda, F. K. Lin+14 more|arXiv (Cornell University)|Jan 8, 2019

Iron-based superconductors research参考文献 4被引用 5

一句话总结

本研究探究了CrAs在压力和磷掺杂下的量子临界性与超导性，发现一级结构相变抑制了超导穹顶附近的双螺旋磁序。中子衍射与非弹性散射表明，量子临界性在约10 kbar压力或5% P掺杂时出现，表现为非费米液体电阻率和增强的比热，表明存在接近二级的螺旋反铁磁量子相变，与非传统超导性相关。

ABSTRACT

At ambient pressure CrAs undergoes a first-order transition into a double-helical magnetic state at TN = 265 K, which is accompanied by a structural transition. The recent discovery of pressure-induced superconductivity in CrAs makes it important to clarify the nature of quantum phase transitions out of its coupled structural/helimagnetic order. Here we show, via neutron diffraction on the single-crystal CrAs under hydrostatic pressure (P), that the combined order is suppressed at Pc ~ 10 kbar, near which bulk superconductivity develops with a maximal transition temperature Tc ~ 2 K. We further show that the coupled order is also completely suppressed by phosphorus doping in CrAs1-xPx at a critical xc ~ 0.05, above which inelastic neutron scattering evidenced persistent antiferromagnetic correlations, providing a possible link between magnetism and superconductivity. In line with the presence of antiferromagnetic fluctuations near Pc (xc), the A coefficient of the quadratic temperature dependence of resistivity exhibits a dramatic enhancement as P (x) approaches Pc (xc), around which Res(T) has a non-Fermi-liquid form. Accordingly, the electronic specific-heat coefficient of CrAs1-xPx peaks out around xc. These properties provide clear evidences for quantum criticality, which we interpret as originating from a nearly second-order helimagnetic quantum phase transition that is concomitant with a first-order structural transition. Our findings in CrAs highlight the distinct characteristics of quantum criticality in bad metals, thereby bringing out new insights into the physics of unconventional superconductivity such as occurring in the high-Tc iron pnictides.

研究动机与目标

• 理解CrAs在超导穹顶附近量子相变的本质。
• 研究压力和掺杂下结构、螺旋磁序与超导序之间的相互作用。
• 确定量子临界点附近的反铁磁涨落是否与超导性相关。
• 通过电阻率与比热测量表征量子临界点附近的电子行为。

提出的方法

• 在静水压力下对单晶CrAs进行中子衍射，以探测磁序与结构序的演化。
• 对CrAs1-xPx进行非弹性中子散射，检测临界掺杂附近持续存在的反铁磁自旋关联。
• 测量CrAs1-xPx的电阻率与电子比热，以识别量子临界点附近的非费米液体行为。
• 分析转变温度Tc随压力与掺杂的依赖关系，绘制超导穹顶图。
• 对比压力与掺杂调控，识别普遍的量子临界行为。
• 利用电阻率二次温度依赖关系中的A系数检测量子临界增强。

实验结果

研究问题

• RQ1 静水压力如何抑制CrAs中的双螺旋磁序与结构序？
• RQ2 临界压力Pc ~ 10 kbar处的量子相变本质是什么？
• RQ3 CrAs1-xPx中反铁磁自旋涨落在量子临界点附近如何演化？
• RQ4 非费米液体电阻率行为在多大程度上表征了CrAs中的量子临界性？
• RQ5 是否存在量子临界涨落与CrAs中非传统超导性出现之间的直接关联？

主要发现

• CrAs中的双螺旋磁序与一级结构序在Pc ~ 10 kbar时完全被抑制，同时伴随体超导的出现，Tc ~ 2 K。
• 在xc ~ 0.05的磷掺杂下，耦合序被抑制，非弹性中子散射揭示了临界掺杂附近持续存在的反铁磁自旋关联。
• 电阻率二次温度依赖关系中的A系数在Pc与xc附近显著增强，表明非费米液体行为。
• 电子比热系数在xc ~ 0.05处达到峰值，为量子临界性提供了直接证据。
• 观测到的量子临界性源于接近二级的螺旋反铁磁转变，该转变与一级结构转变同时发生。
• 这些结果表明，在类似CrAs的不良金属中，量子临界性可能驱动非传统超导性，与高温超导铁砷化物类似。