Coexistence of superconductivity and complex 4 f magnetism in Eu0.5Ce0.5BiS2F

Two-Dimensional Pb Square Nets from Bulk (RO)nPb (R = Rare Earth Metals, n = 1,2)

All two-dimensional (2D) materials of group IV elements from Si to Pb are stabilized by carrier doping and interface bonding from substrates except graphene which can be free-standing. The involvement of strong hybrid of bonds, adsorption of exotic atomic species, and the high concentration of crystalline defects are often unavoidable, complicating the measurement of the intrinsic properties. In this work, we report the discovery of seven kinds of hitherto unreported bulk compounds (RO)nPb (R = rare earth metals, n = 1,2), which consist of quasi-2D Pb square nets that are spatially and electronically detached from the [RO]δ+ blocking layers. The band structures of these compounds near Fermi levels are relatively clean and dominantly contributed by Pb, resembling the electron-doped free-standing Pb monolayer. The R2O2Pb compounds are metallic at ambient pressure and become superconductors under high pressures with much enhanced critical fields. In particular, Gd2O2Pb (9.1 μB/Gd) exhibits an interesting bulk response of lattice distortion in conjunction with the emergence of superconductivity and magnetic anomalies at a critical pressure of 10 GPa. Our findings reveal the unexpected facets of 2D Pb sheets that are considerably different from their bulk counterparts and provide an alternative route for exploring 2D properties in bulk materials.

Valence State of Eu and Superconductivity in Se-Substituted EuSr2Bi2S4F4 and Eu2SrBi2S4F4

Recently, we reported the synthesis and investigations of EuSr₂Bi₂S₄F₄ and Eu₂SrBi₂S₄F₄. We have now been able to induce superconductivity in EuSr₂Bi₂S₄F₄ by Se substitution at the S site (isovalent substitution) with T_c = 2.9 K in EuSr₂Bi₂S₂Se₂F₄. The other compound, Eu₂SrBi₂S₄F₄, shows a significant enhancement of T_c. In Se-substituted Eu₂SrBi₂S_4-xSe_xF₄, we find T_c = 2.6 K for x = 1.5 and T_c = 2.8 K for x = 2, whereas T_c = 0.4 K in the Se-free sample. In addition to superconductivity, an important effect associated with Se substitution is that it gives rise to remarkable changes in the Eu valence. Our ¹⁵¹Eu Mössbauer and X-ray photoemission spectroscopic measurements show that Se substitution in both of the compounds Eu₂SrBi₂S₄F₄ and EuSr₂Bi₂S₄F₄ gives rise to an increase in the Eu²⁺ component in the mixed-valence state of Eu.

Coexistence of superconductivity and ferromagnetism in Sr0.5Ce0.5FBiS2-xSex (x = 0.5 and 1.0), a non-U material with Tc < TFM

We have carried out detailed magnetic and transport studies of the new Sr_0.5Ce_0.5FBiS_2-xSe_x (0.0 ≤ x ≤ 1.0) superconductors derived by doping Se in Sr_0.5Ce_0.5FBiS₂. Se–doping produces several effects: it suppresses semiconducting–like behavior observed in the undoped Sr_0.5Ce_0.5FBiS₂, the ferromagnetic ordering temperature, T_FM, decreases considerably from 7.5 K (in Sr_0.5Ce_0.5FBiS₂) to 3.5 K and the superconducting transition temperature, T_c, gets enhanced slightly to 2.9–3.3 K. Thus in these Se–doped materials, T_FM is marginally higher than T_c. Magnetization studies provide evidence of bulk superconductivity in Sr_0.5Ce_0.5FBiS_2-xSe_x at x ≥ 0.5 in contrast to the undoped Sr_0.5Ce_0.5FBiS₂ (x = 0) where magnetization measurements indicate a small superconducting volume fraction. Quite remarkably, as compared with the effective paramagnetic Ce–moment (~2.2 μ_B), the ferromagnetically ordered Ce–moment in the superconducting state is rather small (~0.1 μ_B) suggesting itinerant ferromagnetism. To the best of our knowledge, Sr_0.5Ce_0.5FBiS_2-x Se_x (_x = 0.5 and 1.0) are distinctive Ce–based bulk superconducting itinerant ferromagnetic materials with T_c < T_FM. Furthermore, a novel feature of these materials is that they exhibit a dual and quite unusual hysteresis loop corresponding to both the ferromagnetism and the coexisting bulk superconductivity.

Role of valence changes and nanoscale atomic displacements in BiS2-based superconductors

Superconductivity within layered crystal structures has attracted sustained interest among condensed matter community, primarily due to their exotic superconducting properties. EuBiS₂F is a newly discovered member in the BiS₂-based superconducting family, which shows superconductivity at 0.3 K without extrinsic doping. With 50 at.% Ce substitution for Eu, superconductivity is enhanced with Tc increased up to 2.2 K. However, the mechanisms for the T_c enhancement have not yet been elucidated. In this study, the Ce-doping effect on the self-electron-doped superconductor EuBiS₂F was investigated by X-ray absorption spectroscopy (XAS). We have established a relationship between Ce-doping and the T_c enhancement in terms of Eu valence changes and nanoscale atomic displacements. The new finding sheds light on the interplay among superconductivity, charge and local structure in BiS₂-based superconductors.