Nature © Macmillan Publishers Ltd 19988 letters to nature 456 NATURE | VOL 394 | 30 JULY 1998 amount to Ϯ10%, this would result in a total T c variation of 7 K in the bulk phase diagram. In strained films a total variation of ϳ40 K is observed, together with a drastic change in T ‫ء‬ . Hence, the lattice deformations associated with the strain fundamentally modify the energy scales, leading to the formation and condensation of the superconducting pairs. As the pair formation strongly influences the spin and charge excitation spectrum of the normal state, r(T) and the metal-insulator transition are also considerably modified. Further work to identify and quantify the subtle lattice deformations responsible for these changes needs to be done. In any case, the data reported here emphasize that T c increases as the distance between consecutive CuO 2 planes increases, in contradiction of a recent theoretical prediction 30 . Our data also show that adjustment of the epitaxial strain offers a unique way to increase T c and to reveal the fundamental role played by the interlayer coupling. Ⅺ

In this paper is reported an extensive NMR characterization of N-methyl-N-propyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI) room-temperature ionic liquid and its mixtures with LiTFSI. NMR was used to investigate the interactions between the ionic liquid and lithium salt and the diffusion coefficients of all ionic species present in these mixtures. The results are compared with previous DSC, Raman, and electrochemical investigations.

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Here we report on the complex nature of the phase diagram of N-alkyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide ionic liquids using several complementary techniques and on their structural order in the molten state using small-wide angle x-ray scattering. The latter study indicates that the piperidinium aliphatic alkyl chains tend to aggregate, forming alkyl domains embedded into polar regions, similar to what we recently highlighted in the case of other ionic liquids.