Zhao Kui, Lv Bing, Deng Liangzi, Huyan Shu-Yuan, Xue Yu-Yi, Chu Ching-Wu
Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002;
Department of Physics, University of Houston, Houston, TX 77204-5005.
Proc Natl Acad Sci U S A. 2016 Nov 15;113(46):12968-12973. doi: 10.1073/pnas.1616264113. Epub 2016 Oct 31.
Superconductivity has been reversibly induced/suppressed in undoped CaFeAs (Ca122) single crystals through proper thermal treatments, with T at ∼25 K at ambient pressure and up to 30 K at 1.7 GPa. We found that Ca122 can be stabilized in two distinct tetragonal (T) phases at room temperature and ambient pressure: PI with a nonmagnetic collapsed tetragonal (cT) phase at low temperature and PII with an antiferromagnetic orthorhombic (O) phase at low temperature, depending on the low-temperature annealing condition. Neither phase at ambient pressure is superconducting down to 2 K. However, systematic annealing for different time periods at 350 °C on the as-synthesized crystals, which were obtained by quenching the crystal ingot from 850 °C, reveals the emergence of superconductivity over a narrow time window. Whereas the onset T is insensitive to the anneal time, the superconductive volume fraction evolves with the time in a dome-shaped fashion. Detailed X-ray diffraction profile analyses further reveal mesoscopically stacked layers of the PI and the PII phases. The deduced interface density correlates well with the superconducting volume measured. The transport anomalies of the T-cT transition, which is sensitive to lattice strain, and the T-O transition, which is associated with the spin-density-wave (SDW) transition, are gradually suppressed over the superconductive region, presumably due to the interface interactions between the nonmagnetic metallic cT phase and the antiferromagnetic O phase. The results provide the most direct evidence to date for interface-enhanced superconductivity in undoped Ca122, consistent with the recent theoretical prediction.
通过适当的热处理，未掺杂的CaFeAs（Ca122）单晶中的超导性已被可逆地诱导/抑制，在常压下Tc约为25 K，在1.7 GPa下可达30 K。我们发现，在室温和常压下，Ca122可以稳定在两种不同的四方（T）相中：低温下具有非磁性塌缩四方（cT）相的PI相和低温下具有反铁磁正交（O）相的PII相，这取决于低温退火条件。在常压下，这两个相在2 K以下都不超导。然而，对通过将850 °C的晶锭淬火得到的合成晶体在350 °C下进行不同时间段的系统退火，发现在一个狭窄的时间窗口内出现了超导性。虽然超导起始温度对退火时间不敏感，但超导体积分数随时间呈圆顶状变化。详细的X射线衍射轮廓分析进一步揭示了PI相和PII相的介观堆叠层。推导的界面密度与测量的超导体积相关性良好。对晶格应变敏感的T-cT转变和与自旋密度波（SDW）转变相关的T-O转变的输运异常在超导区域逐渐受到抑制，这可能是由于非磁性金属cT相和反铁磁O相之间的界面相互作用。这些结果为未掺杂的Ca122中界面增强超导性提供了迄今为止最直接的证据，与最近的理论预测一致。
