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We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampoules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) A, c = 32.4330(6) A, and Z = 2), in which CaF2-type Eu3F4 layers and NaCl-like BiS2 bilayers stack alternately along the crystallographic c axis. There are two crystallographically distinct Eu sites, Eu(1) and Eu(2) at the Wyckoff positions 4e and 2a, respectively. Our bond-valence-sum calculation, based on the refined structural data, indicates that Eu(1) is essentially divalent, whilst Eu(2) has an average valence of +2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mossbauer and magnetization measurements. The Eu3+ components donate electrons into the conduction bands that are mainly composed of Bi- 6px and 6py states. Consequently, the material itself shows metallic conduction, and superconducts at 1.5 K without extrinsic chemical doping.
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Various samples of sodium cobalt oxyhydrate with relatively large amounts of Na$^{+}$ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of a cobalt valence of $sim$+3.48, which was compared with a previously obtained one of $sim$+3.40. The superconductivity was significantly affected by the isovalent exchanger of Na$^{+}$ and H$_{3}$O$^{+}$, rather than by variation of Co valence, suggesting the presence of multiple kinds of Fermi surface. Furthermore, the high-field magnetic susceptibility measurements for one sample up to 30 T indicated an upper critical field much higher than the Pauli limit supporting the validity of the spin-triplet pairing mechanism.
Materials with exceptional magnetism and superconductivity usually conceive emergent physical phenomena. Here, we investigate the physical properties of the (Eu,La)FeAs2 system with double magnetic sublattices. The parent EuFeAs2 shows anisotropy-associated magnetic behaviors, such as Eu-related moment canting and exchange bias. Through La doping, the magnetic anisotropy is enhanced with ferromagnetism of Eu2+ realized in the overdoped region, and a special exchange bias of the superposed ferromagnetic/superconducting loop revealed in Eu0.8La0.2FeAs2. Meanwhile, the Fe-related antiferromagnetism shows unusual robustness against La doping. Theoretical calculation and 57Fe Mossbauer spectroscopy investigation reveal a doping-tunable dual itinerant/localized nature of the Fe-related antiferromagnetism. Coexistence of the Eu-related ferromagnetism, Fe-related robust antiferromagnetism, and superconductivity is further revealed in Eu0.8La0.2FeAs2, providing a platform for further exploration of potential applications and emergent physics. Finally, an electronic phase diagram is established for (Eu,La)FeAs2 with the whole superconducting dome adjacent to the Fe-related antiferromagnetic phase, which is of benefit for seeking underlying clues to high-temperature superconductivity.
Resonating valence bond (RVB) theory of high Tc superconductivity, an electron correlation based mechanism, began as an insightful response by Anderson, to Bednorz and Mullers discovery of high Tc superconductivity in cuprates in late 1986. Shortly a theoretical framework for quantum spin liquids and superconductivity was developed. This theory adresses a formidable strong coupling quantum manybody problem, in modern times. It is built on certain key experimental facts: i) survival of a dynamical Mott localization in a metallic state, ii) proliferation of bond singlets and iii) absence of fermi liquid quasi particles. After summarising RVB theory I will provide an aerial view of, mostly, new superconductors where I believe that, to a large degree RVB mechanism is at work and indicate prospects for even higher Tcs.
Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS$_2$F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6$p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ~ 50 times, owing to the significant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verified by band-structure calculations. Thus, EuBiS$_2$F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and $f$-electron valence instability.
We report Eu-local-spin magnetism and Ni-doping-induced superconductivity (SC) in a 112-type ferroarsenide system Eu(Fe$_{1-x}$Ni$_{x}$)As$_2$. The non-doped EuFeAs$_2$ exhibits two primary magnetic transitions at $sim$100 and $sim$ 40 K, probably associated with a spin-density-wave (SDW) transition and an antiferromagnetic ordering in the Fe and Eu sublattices, respectively. Two additional successive transitions possibly related to Eu-spin modulations appear at 15.5 and 6.5 K. For the Ni-doped sample with $x$ = 0.04, the SDW transition disappears, and SC emerges at $T_mathrm{c}$ = 17.5 K. The Eu-spin ordering remains at around 40 K, followed by the possible reentrant magnetic modulations with enhanced spin canting. Consequently, SC coexists with a weak spontaneous magnetization below 6.2 K in Eu(Fe$_{0.96}$Ni$_{0.04}$)As$_2$, which provides a complementary playground for the study of the interplay between SC and magnetism.
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