Here we report synthesis and characterization of Mg1−xScxB2 (0.12Tc>6 K. We find that the Sc doping moves the chemical potential through the 2D/3D electronic topological transition (ETT) in the σ band where the "shape resonance" of interband pairing occurs. In the 3D regime beyond the ETT we observe a hardening of the E2g Raman mode with a significant line-width narrowing due to suppression of the Kohn anomaly over the range 0

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Local structure of REOFeAs (RE=La, Pr, Nd, Sm) system has been studied as a function of chemical pressure varied due to different rare-earth size. Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements in the fluorescence mode has permitted to compare systematically the inter-atomic distances and their mean square relative displacements (MSRD). We find that the Fe-As bond length and the corresponding MSRD hardly show any change, suggesting the strongly covalent nature of this bond, while the Fe-Fe and Fe-RE bond lengths decrease with decreasing rare earth size. The results provide important information on the atomic correlations that could have direct implication on the superconductivity and magnetism of REOFeAs system, with the chemical pressure being a key ingredient.

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We report boron isotope effect on the E 2g phonon mode by micro-Raman spectroscopy on the ternary Mg 1−x Al x B 2 system, synthesized with pure isotopes 10 B and 11 B. The isotope coefficient on the E 2g mode frequency is nearly 0.5 in the wide range of Al, with a tendency to decrease at MgB 2 ͑x =0͒. The intraband electron-phonon ͑e-ph͒ coupling relative to the sigma band has been extracted from the E 2g line-shape parameters. By tuning the Fermi energy near the electronic topological transition ͑ETT͒, where the sigma Fermi surface changes from two-dimensional to three-dimensional topology ͑in range 0 Ͻ x Ͻ 0.28͒, the E 2g mode shows the Kohn anomaly accompanied with a splitting into a hard and a soft component. The results suggest that the intraband hardly plays any role to control the high T c of Mg 1−x Al x B 2 The common physical features of diborides with the multigap FeAs-based superconductors and cuprates are discussed.

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