We report a new strategy to induce superconductivity in iron-based oxyarsenide. Instead of F − substitution for O 2− , we employed Th 4+ doping in GdFeAsO with the consideration of "lattice match" between Gd2O2 layers and Fe2As2 ones. As a result, superconductivity with T onset c as high as 56 K was realized in a Gd0.8Th0.2FeAsO polycrystalline sample. This Tc value is among the highest ever discovered in the iron-based oxypnictides.

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Diverging longitudinal resistivity with decreasing temperature is the most apparent transport property of a simple band insulator in distinction to any metals. However, such a distinct feature ceases to stand in various topological insulators due to the existing metallic surface states 1, 2 . Nevertheless there are other transport phenomena, such as resistivity plateau and negative MR, which may distinguish at least ideal TIs and ideal TSMs from topologically trivial materials. In an ideal 3D TI where the bulk states are completely gapped out near the Fermi level, a resistivity plateau can be clearly established because the only participating surface states are TRS protected and thus robust to disorders, leading to a saturation of resistivity in the low temperature regime [3][4][5][6] . In TSMs without coexisting bulk Fermi surfaces such as an ideal WSM, on the other hand, the bulk exci-2 tations come from the two separated Weyl nodes in momentum space which are chiral in nature owing to the lack of TRS or inversion symmetry [30][31][32] . When the applied magnetic field is parallel to the electric field direction, the density of the right/left chiral excitations increases/decreases accordingly as a consequence of the chiral anomaly [33][34][35] , resulting in a non-dissipative current from the left to right nodes along the field direction, hence an unconventional negative MR appears.Because the resistivity plateau and negative MR are opposite consequences for systems with or without TRS in the ideal situations mentioned above, coexistence of the both in a single material is unlikely or very difficult. Of course, these features should be more intriguing but much involved in realistic topological materials, and, TRS itself is not the only origin relevant to the resistivity plateau on a general ground. In two-dimensional electron gases or semiconducting films like graphite 36, 37 , for instance, the resistivity seemingly saturates after a field-induced metal-insulator transition while a clear resistivity plateau at lower temperatures was not reported. More recently, a field-induced plateau has been observed in LaSb, a potentially new candidate of TIs 38 . While the interpretation of all these and related behaviors in topological materials remains a theoretical challenge, materials realizations of these effects are highly desirable not only in confronting this challenge but also in technique applications.Here we report the discovery of a new TSM TaSb 2 , which crystallizes in a monoclinic structure with centrosymmetric space group C 12/m1 (SI- Fig. S1). The longitudinal resistivity and Hall effect were measured for various magnetic fields applied along different directions. The semicon-3 ducting behavior is associated with a low carrier density and a field-induced metal-to-insulator-like transition. High quality of the measured samples is indicated by ultrahigh mobility and XMR. The topological nature of the compound is evidenced by the Shubnikov de Haas (SdH) oscillation measurement as well as band structure calcu...

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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.

We have synthesized a novel europium bismuth sulfofluoride, Eu3Bi2S4F4, by solid-state reactions in sealed evacuated quartz ampules. The compound crystallizes in a tetragonal lattice (space group I4/mmm, a = 4.0771(1) Å, c = 32.4330(6) Å, 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, while Eu(2) has an average valence of ∼ +2.64(5). This anomalous Eu valence state is further confirmed and supported, respectively, by Mössbauer and magnetization measurements. The Eu(3+) 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.

Abstract. -We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe2As2 layers are squeezed while the La2O2 layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at ∼10 K for the optimally-doped LaFeAs1−xPxO (x = 0.25 ∼ 0.3). The upper critical fields at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms of chemical pressures and bond covalency.

This study puts forth two new members of fully ortho-benzannulated [n]­circulenes, heptabenzo[7]­circulene and octabenzo[8]­circulene, which are new negatively curved nanographenes and also represent unprecedented structures of septuple [4]­helicene and octuple [4]­helicene, respectively. The successful synthesis of them through Scholl reaction in good to excellent yields takes advantage of the reactivity of naphthalene. Quantum chemistry calculations reveal that heptabenzo[7]­circulene and octabenzo[8]­circulene are both flexible π-molecules and adopt saddle-shaped geometry of C 2 and D 2d symmetry, respectively, at the global energy minimum in agreement with the single-crystal structures. A serendipitous discovery from this study is that tetra­(tert-butyl) octabenzo[8]­circulene in the single crystals self-assemble into a supramolecular nanosheet with an unprecedented motif of π–π stacking. Such a new molecular packing mode, in combination with the demonstrated semiconducting property of octabenzo[8]­circulene, suggests a new supramolecular two-dimensional material.