Pressure-Induced Superconductivity in Eu_0.5Ca_0.5Fe₂As₂: Wide Zero-Resistivity Region Due to Suppression of Eu Magnetic Order and Chemical Pressure

Abstract

To clarify competition between FeAs-based superconductivity and antiferromagnetism of Eu 2+ , and the superconducting properties in EuFe 2 As 2 , we investigated a Ca-substituted sample, Eu 0.5 Ca 0.5 Fe 2 As 2 , under high pressure. Under ambient pressure, the sample exhibits a spin-density-wave (SDW) transition at T SDW = 191 K and antiferromagnetic order at T N = 4 K, but no evidence of superconductivity down to 2 K. The Ca-substitution certainly weakens the antiferromagnetism. With increasing pressure, T SDW shifts to lower temperature and becomes more unclear. Above 1.27 GPa, pressure-induced superconductivity with zero resistivity is observed at around T c = 20 K. At 2.14 GPa, T c reaches a maximum value of 24 K and the superconducting transition becomes sharpest. The chemical pressure and the suppression of Eu magnetic order induced by Ca substitution enlarge the zero resistivity region compared to pure EuFe 2 As 2 .

Magnetic phase diagram of Ca-substituted EuFe2As2

“…The T SDW ∼ 190 K is nearly the same for all compounds, except for CaFe 2 As 2 (T SDW ∼ 170 K), in good agreement with the literature data. 23,24,28 A much weaker anomaly, corresponding to the antiferromagnetic ordering of Eu 2+ magnetic moments, is observed below 19 K. As expected, the dilution of the Eu magnetic sublattice (via nonmagnetic Ca ions) leads to a decreased Néel temperature T N . The observed T N for Eu 1-x Ca x Fe 2 As 2 are in good agreement with those reported by Mitsuda et al 24 The evolution of both the T SDW and T N values with Caconcentration is summarized in Fig.…”

High-Pressure Electrical Resistivity Measurements of EuFe2As2Single Crystals

“…The measurement was performed in zero field at ambient pressure outside a pressure device with current direction I ab. are qualitatively similar to each other, and are consistent with previous results [9,20,21,29]. It is worthwhile to mention that, as shown in the upper left inset, a magnetic/structural transition temperature T 0 = 194 K for the sample with RRR = 15 is higher than T 0 = 190 K for the sample with RRR = 7.…”

Effect of Substitution of Ca2+for Eu2+on Pressure-Induced Superconductivity in EuFe2As2

“…13) We obtained many pieces of plate-like single crystals with typical dimensions of ∼ 3 × 3 × 0.1 mm 3 . The powder X-ray diffraction (XRD) was carried out at room temperature.…”

“…13) Therefore, the difference in P c between x = 0 and x = 0.15 is likely to be explained semi-quantitatively by the chemical pressure. In that case, the sample with x = 0.15 deserves to exhibits the superconductivity with zero resistivity since EuFe 2 As 2 performs the superconductivity with zero resistivity at around 2.8 GPa.…”

“…In the previous paper, we reported that Casubstitution by 50 % weakens the AFM of Eu 2+ and results in the pressure-induced bulk superconductivity with zero * E-mail address: 3da@phys.kyushu-u.ac.jp resistivity in a wide pressure range of 1.2 ∼ 2.4 GPa. 13,14) In the present paper, to clarify more detailed effects of Ca substitution for Eu on the Fe-based superconductivity, the behavior of Eu 0.85 Ca 0.15 Fe 2 As 2 under high pressure is mainly presented.…”