期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：46
页码：12968-12973
DOI：10.1073/pnas.1616264113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceOne of the major goals for scientists in the field of superconductivity and materials science has been to obtain superconductors with higher critical temperatures (Tc). One way that has long been proposed to achieve enhanced Tcs is to take advantage of artificially or naturally assembled interfaces. The present work clearly demonstrates that high-Tc superconductivity in the well-known nonsuperconducting compound CaFe2As2 can be induced by antiferromagnetic/metallic layer stacking and provides the most direct evidence to date for the interface-enhanced Tc in this compound. The observations offer an avenue to higher Tc. Superconductivity has been reversibly induced/suppressed in undoped CaFe2As2 (Ca122) single crystals through proper thermal treatments, with Tc 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 {degrees}C on the as-synthesized crystals, which were obtained by quenching the crystal ingot from 850 {degrees}C, reveals the emergence of superconductivity over a narrow time window. Whereas the onset Tc 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.
