title: Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe2As2 single crystals.
abstract: Superconductivity has been reversibly induced/suppressed in undoped CaFe2As2(Ca122) single crystals through proper thermal treatments, with Tcat ∼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 Tcis 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.
authors: Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002; zhaokxy@mail.ustc.edu.cn cwchu@uh.edu.|Department of Physics, University of Houston, Houston, TX 77204-5005.
authors: Kui
authors: K
authors: Zhao
authors: Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002.|Department of Physics, University of Houston, Houston, TX 77204-5005.|Department of Physics, University of Texas at Dallas, Richardson, TX 75080.
authors: Bing
authors: B
authors: Lv
authors: Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002.|Department of Physics, University of Houston, Houston, TX 77204-5005.
authors: Liangzi
authors: L
authors: Deng
authors: Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002.|Department of Physics, University of Houston, Houston, TX 77204-5005.
authors: Shu-Yuan
authors: SY
authors: Huyan
authors: Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002.|Department of Physics, University of Houston, Houston, TX 77204-5005.
authors: Yu-Yi
authors: YY
authors: Xue
authors: Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002; zhaokxy@mail.ustc.edu.cn cwchu@uh.edu.|Department of Physics, University of Houston, Houston, TX 77204-5005.|Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors: Ching-Wu
authors: CW
authors: Chu
keywords: CaFe2As2; annealing; high-Tc superconductivity; interface; mixed phase