Superconductivity is studied in hybrids consisting of ultrathin superconducting film/few layer graphene. Two different superconductors were used at this purpose, Nb and NbN. An increase of the superconducting critical temperature, T c , is observed when graphene is put into contact with Nb. The largest increase is obtained for the thinnest Nb layer, which has a T c 8% larger with respect to the single Nb film. In the case of NbN the effect is not so pronounced. Experimental data are discussed by considering the possible modification of the phonon spectrum in the superconductor due to the presence of the graphene. Within an elementary one-dimensional model based on an elastic coupling between nearestneighbor atoms, we demonstrate that the phonon spectrum in the superconductor is modified at low energies with the subsequent enhancement of the effective electron-phonon coupling constant. While the strong oscillating nature of the electron-phonon interaction, 2 (), in NbN could lead to the insensitivity of T c on the low-energy phonons generated by the graphene, the almost constant behavior of 2 () in Nb favors the increase of the superconducting critical temperature.
We demonstrate the growth features of III-nitrides on graphene buffer layers obtained by the CVD method on a copper catalyst with different dominant grain orientations. The reflection high-energy electron diffraction technique (RHEED) is used to map the 2D reciprocal space structures of graphene buffers and growing nitride layers. The RHEED reciprocal space pattern for the graphene layer grown on a (111) textured copper foil and transferred to a SiO2/Si substrate demonstrates the sixfold symmetry characteristic of a highly oriented material. In turn, graphene grown on a Cu (100) foil consists of two types of domains that are 30° rotated relative to each other. It has also been demonstrated that III-nitride films exactly repeat the texture of the 2D graphene buffers. The GaN sample grown over the highly textured substrate demonstrates a clear sixfold symmetry of the RHEED reciprocal space map as well as {101¯3} XRD pole figure, which is close to 2D surface morphology. In turn, the GaN film grown over the graphene buffer layer transferred from the Cu (100) textured foil has 12-fold axial symmetry, which is equivalent to the essentially two-domain in-plane orientation of the initial graphene.
We propose the evaluation of strain in Bi2Se3 films based on the correlation analysis of in-plane (E2g) and out-of-plane (A21g) Raman mode positions as well as the algorithm of phonon deformation potential calculation for biaxial in-plane strain.
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