Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe ₂ As ₂ single crystals

Abstract

Superconductivity has been reversibly induced/suppressed in undoped CaFe 2 As 2 (Ca122) single crystals through proper thermal treatments, with T c 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 c 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.

Low-temperature microstructural studies on superconducting CaFe2As2

“…It was later found 23–28 that the undoped single-crystalline Ca122 can stabilize in two slightly different non-superconducting tetragonal phases (PI and PII) after rapid quenching from 850 °C and prolonged annealing at 350 °C, respectively. The PI phase corresponds to a tetragonal (T) structure at room temperature [lattice parameter c = 11.547(1) Å], and transforms to a collapsed-tetragonal (cT) phase with a 10% shorter c -lattice [ c = 10.720(1) Å] below the T-cT structural transition temperature (T cT ) around 100 K. On the other hand, the PII phase also exhibits a tetragonal structure at room temperature but with a slightly longer c [ c = 11.763(1) Å].…”

“…It undergoes a tetragonal-to-orthorhombic (T-O) transition [ c = 11.653(1) Å] at the T-O structural transition temperature (T O ) around 170 K. The orthorhombic phase is ordered antiferromagnetically, and the T-O transition is closely related to the spin-density-wave transition. It was found that the PI and PII phases could be reversibly transformed by heat treatment 28 . Microscopic techniques were also utilized to reveal the homogeneity and strain effect in the Ca122 annealed at different temperatures 23,24 .…”

“…350 °C, over increasing time periods of 0–30 hours. It was found that as PI is gradually converted to PII, superconductivity at ~25 K emerges over a limited annealing time window 28,29 . The simultaneous appearance of the PI + PII-mixing phase and superconductivity at ambient pressure enables us to examine the microscopic relationship between the coexistence of the mixing phase and superconductivity.…”

Manipulating superconducting phases via current-driven magnetic states in rare-earth-doped CaFe2As2

“…The mechanism of the superconductivity in the Ca122 compounds is still unclear, but numerous results have suggested that an adequate Fe magnetic moment is crucial to mediating Cooper pairs in the compounds 1,2,4 . The interface superconductivity that emerges in undoped Ca122 has been reported to originate from the interface between the collapsed tetragonal (cT) and orthorhombic (O) phases or between the AFM antiphase domain boundaries 32,33 . For undoped and Co-doped Ca122, nuclear magnetic resonance (NMR) results suggest that FM fluctuations compete with superconductivity but can coexist with a stripe-type AFM phase 34 .…”

The Search for Higher Tc in Houston

“…This motivates our studies. We found [10] that Ca122 quenched from 850 °C or above has a tetragonal structure at room temperature (P 1 phase), but transforms to a collapsed-tetragonal phase with a 10% shorter c lattice parameter below T cT , the T-cT transition temperature [11]. No magnetic ordering is detected over the whole temperature range.…”