We discover a robust coexistence of superconductivity and ferromagnetism in an iron arsenide RbEuFe4As4. The new material crystallizes in an intergrowth structure of RbFe2As2 and EuFe2As2, such that the Eu sublattice turns out to be primitive instead of being body-centered in EuFe2As2. The FeAs layers, featured by asymmetric As coordinations, are hole doped due to charge homogenization. Our combined measurements of electrical transport, magnetization and heat capacity unambiguously and consistently indicate bulk superconductivity at 36.5 K in the FeAs layers and ferromagnetism at 15 K in the Eu sublattice. Interestingly, the Eu-spin ferromagnetic ordering belongs to a rare third-order transition, according to the Ehrenfest classification of phase transition. We also identify an additional anomaly at ∼ 5 K, which is possibly associated with the interplay between superconductivity and ferromagnetism.further revised the electronic phase diagram because of the discovery of a reentrant spin glass state. Recent x-ray resonant magnetic scattering[3] and neutron scattering[4] experiments however indicated long-range ferromagnetic orderings for Eu spins in superconducting EuFe 2 (As 1−x P x ) 2 with x = 0.19 and 0.15, respectively. It was demonstrated that the Eu spins align exactly along the c axis, in contradiction to the spin-canting scenario. So far, this discrepancy remains unresolved. Note that the spin-tilting angle (∼20 • from the c axis, as detected by Mössbauer measurements[2]) coincides with the direction that connects the interlayer next-nearest (NN) Eu atoms because of the body-centered Eu sublattice. To clarify whether the Eu-sublattice type is relevant to Eu spin orientations, it is desirable to study a related material system in which Eu atoms form a primitive tetragonal lattice.Local-moment FM and spin-singlet SC are known to be mutually incompatible [20][21][22], which makes their coexistence (hereafter abbreviated as FM+SC) very rare [23]. The FM+SC phenomenon observed in FeSCs has been ascribed to the multi-orbital character as well as the robustness of superconductivity against magnetic fields [10,24]. On the one hand, the zero-temperature upper critical magnetic field, H c2 (0), of FeSCs is typically higher than 50 T [25,26], which is large enough to fight the internal exchange field that is comparable to the hyperfine field on the Eu nucleus (∼ 25 T) [2]. On the other hand, the Eu-spin FM can be satisfied even in the presence of SC, because the Fe-3d multi-orbitals enable both superconducting pairing (dominated by the d yz and d zx electrons [27]) and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between Eu local moments. The RKKY interaction can be mediated arXiv:1605.04396v3 [cond-mat.supr-con]

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe 2 -class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe 2 , PdTe 2 and PtTe 2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here we report the synthesis of a new type-II Dirac semimetal NiTe 2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry's phase associated with these Dirac fermions. Our first principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe 2 may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers.

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, EuBiS2F, 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-6p 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-4f and Bi-6p electrons, as verified by band-structure calculations. Thus, EuBiS2F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and f -electron valence instability.

Superconductivity in low-dimensional compounds has long attracted much interest. Here we report superconductivity in a low-dimensional ternary telluride Ta4Pd3Te16 in which the repeating layers contain edge-sharing octahedrally-coordinated PdTe2 chains along the crystallographic b axis. Measurements of electrical resistivity, magnetic susceptibility and specific heat on the Ta4Pd3Te16 crystals, grown via a self-flux method, consistently demonstrate bulk superconductivity at 4.6 K. Further analyses of the data indicate significant electron-electron interaction, which allows electronic Cooper pairing in the present system.Comment: 3 pages, 3 figure

We report an unprecedented anisotropic superconductivity in Eu(Fe0.75Ru0.25)2As2 single crystals. The in-plane resistivity (ρ ab ) and vertical magnetization (Mc) show a superconducting transition at TSC = 23 K with zero resistance and magnetic shielding, respectively. In comparison, the out-of-plane resistivity (ρc) does not go to zero, neither does the horizontal magnetization (M ab ) show the shielding against external fields. Magnetization and Mössbauer data indicate that the Eu 2+ spins order ferromagnetically below 19.5 K with the moments tilted 20 • from the c-axis. Our result suggests spontaneous-vortex phase and/or Fulde-Ferrell-Larkin-Ovchinnikov state in the ferromagnetic superconductor.