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Superconductivity below 0.3 K and a charge-density-wave-like (CDW-like) anomaly at 280 K were observed in EuBiS2F recently. Here we report a systematic study of structural and transport properties in Eu0.5Ln0.5BiS2F (Ln = La, Ce, Pr, Nd, Sm) by electrical resistivity, magnetization, and specific heat measurements. The lattice constants have a significant change upon rare earth substitution for Eu, suggesting an effective doping. As Ln is changed from Sm to La, the superconducting transition temperature T c increases from 1.55 K to 2.8 K. In contrast to the metallic parent compound, the temperature dependence of electrical resistivity displays semiconducting-like behavior for all the Eu0.5 Ln 0.5BiS2F samples. Meanwhile, the CDW-like anomaly observed in EuBiS2F is completely suppressed. Unlike the mixed valence state in the undoped compound, Eu ions in these rare-earth-doped samples are mainly divalent. A specific anomaly at 1.3 K resembling that in EuBiS2F suggests the coexistence of superconductivity and spin glass state for Eu0.5La0.5BiS2F. Coexistence of ferromagnetic order and superconductivity is found below 2.2 K in Eu0.5Ce0.5BiS2F samples. Our results supplies a rich diagram showing that many interesting properties can be induced in BiS2-based compounds.

We report the effect of Sr-doping in the BiS2-based superconductor Sr x Bi2S4F4. Eu3Bi2S4F4 is a self-doped compound with a mixed Eu valence state. By the partial substitution of Sr for Eu, T c gradually decreases and superconductivity disappears above 0.3 K when . Magnetic-susceptibility and specific-heat measurements reveal that Sr substitution leads to a decrease in both Eu2+ and Eu3+ populations. The decreased Eu3+ population, and the corresponding lower charge carrier density, may be the main origin for the suppression of superconductivity. In addition, we find a significant increase in the Sommerfeld coefficient upon Sr doping, which may be due to the Kondo effect between the magnetic moments (associated to Eu2+ ions) and the conducting electrons. This work implies that the Kondo effect could compete with superconductivity in Eu3Bi2S4F4.

We investigated the negative-chemical-pressure effect of Eu3Bi2S4−x Se x F4 (0 ≤ x ≤ 2.0) by the partial substitution of S with Se. The crystalline lattice substantially expands as Se is doped, suggesting an effective negative chemical pressure. With Se/S doping, the charge-density-wave–like anomaly is suppressed, and meanwhile the superconducting transition temperature is enhanced. For , T c reaches 3.35 K and bulk superconductivity is confirmed by the strong diamagnetic signal, with shielding volume fraction over 90%. Magnetic-susceptibility, specific-heat and Hall-effect measurements reveal that the Se/S doping increases the carrier density, corresponding to the increase of the average Eu valence. Our work provides a rare paradigm of negative-chemical-pressure effect.

We report electrical and magnetic measurements of an optimally doped Eu(Fe0.88Ir0.12)2As2 crystal which shows superconductivity at Tsc?=?22?K and magnetic ordering of the Eu2+ spins below 20?K. The results suggest that the Eu2+ spins lie flat ferromagnetically in the ab plane at Tabm?=?20?K, and then tilt toward the c-axis at Ttiltm?=?17.4?K. The isothermal magnetization loop at low temperatures shows both ferromagnetic behavior and superconducting characteristics, unambiguously demonstrating the coexistence of ferromagnetism and superconductivity. The upper critical field measured is remarkably reduced, as compared to other Fe-based superconductors with the same Tsc, and it exhibits abnormal temperature dependence featured by the existence of an inflection point around Tabm, where the anisotropy ratio ? (??H?abc2/H?cc2) shows a minimum value smaller than 1.0. These observations can be explained by a ferromagnetic exchange field of ?30?T which tilts its direction toward the c-axis below Ttiltm. The strong internal field, much higher than the intrinsic lower critical field expected, leads to the absence of Meissner state, which is confirmed by the magnetic measurements under ultra-low fields.