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The effect of pressure on the superconducting transition temperature (Tc) of the Ca-intercalated graphite compound CaC6 has been investigated up to ∼ 16 kbar. Tc is found to increase under pressure with a large relative ratio ∆Tc/Tc of ≈ +0.4%/kbar. Using first-principles calculations, we show that the positive effect of pressure on Tc can be explained within the scope of electron-phonon theory due to the presence of a soft phonon branch associated to in-plane vibrations of the Ca atoms. Implications of the present findings on the current debate about the superconducting mechanism in graphite intercalation compounds are discussed.

We report the discovery of superconductivity below 1.65(6) K in Sr-intercalated graphite SrC(6), by susceptibility and specific heat (C(p)) measurements. In comparison with CaC(6), we found that the anisotropy of the upper critical fields for SrC(6) is much reduced. The C(p) anomaly at T(c) is smaller than the BCS prediction, indicating an anisotropic superconducting gap for SrC6 similar to CaC6. The significantly lower T(c) of SrC(6) as compared to CaC(6) can be understood in terms of "negative" pressure effects, which decreases the electron-phonon coupling for both in-plane intercalant and the out-of-plane C phonon modes. We observed no superconductivity for BaC(6) down to 0.3 K.

We are reporting an unexpected metal insulator transition at the ferromagnetic phase-transition temperature for thin films of La0.9Sr0.1MnO3 (<50 nm), grown on a (100) face of SrTiO3 substrate. For the thicker films (>50 nm), similar to the single crystal, no such transition is observed below TC. Additionally, we observe the suppression of the features associated with charge or orbital ordering in intentionally La-deficient thin films of La0.88Sr0.1MnO3 (<75 nm). In thin films, transmission electron microscopy reveals a compressive strain due to the epitaxial growth, that is, lattice parameters adopt those of the cubic lattice of SrTiO3. As the film thickness increases, coherent microtwinning is observed in the films and the films relax to a orthorhombic structure.

The superconducting state of Ca-intercalated graphite CaC6 has been investigated by specific heat measurements. The characteristic anomaly at the superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of the superconductivity. The temperature and magnetic field dependence of the electronic specific heat are consistent with a fully-gapped superconducting order parameter. The estimated electron-phonon coupling constant is λ = 0.60 -0.74 suggesting that the relatively high Tc of CaC6 can be explained within the weak-coupling BCS approach. The recent discovery of superconductivity in Ca-and Yb-intercalated graphite has refocussed considerable interest onto graphite intercalated compounds (GICs) [1,2]. The superconducting transition temperature for Caand Yb-intercalated graphite is T c ≈ 11.5 K and 6.5 K, respectively, significantly higher than the alkali-metal intercalated graphite phases studied in the 1980's [3]. Similar to MgB 2 where the hexagonal B sheets are intercalated with Mg, in the GICs the Ca or Yb atoms are sandwiched by the honeycomb graphene layers. The intercalated metal ions act as donors and transfer charge into the host graphene layers, resulting in partially filled graphene π-bands. In contrast to MgB 2 with a strong electronphonon coupling of the B σ-bands, the coupling strength of the graphite π-bands to in-plane phonon modes is expected to be small. Also out-of-plane phonon modes will not couple to the π-band because of the antisymmetric character of the π-orbitals. Thus the superconducting mechanism for the GICs could be rather different than that of MgB 2 .