Abstract
Superconductivity is studied in hybrids consisting of ultrathin superconducting film/few layer graphene. Two different superconductors are used for this purpose, Nb and NbN. An increase in 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 as 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 elastic coupling between nearest-neighbor 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, α 2(ω), in NbN could lead to the insensitivity of T c on the low-energy phonons generated by the graphene, the almost constant behavior of α 2(ω) in Nb favors the increase of the superconducting critical temperature.
Highlights
The problem of tuning of the critical temperature Tc by affecting the phonon spectrum of a superconducting material arose naturally after that Eliashberg developed the equations of the theory of superconductivity [1,2] without limiting the value of the electron-phonon interaction (EPhI)
While the strong oscillating nature of the electron-phonon interaction, 2( ), in NbN could lead to the insensitivity of Tc 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
Raman spectroscopy measurements showed that the number of graphene layers in the systems were 1-2 for Nb/G and 2-4 or 7-8 for NbN/G, depending, in this last case, on the deposition parameters and the precursor used during the graphene synthesis
Summary
The problem of tuning of the critical temperature Tc by affecting the phonon spectrum of a superconducting material arose naturally after that Eliashberg developed the equations of the theory of superconductivity [1,2] (see [3,4,5,6,7,8]) without limiting the value of the electron-phonon interaction (EPhI). The solution of the equations is expressed in terms of the Eliashberg spectral function 2( )F( ) where 2( ) determines the strength of the EPhI and F( ) represents the phonon density of states (PhDOS). By calculating the spectral function, or by extracting it from tunneling measurements [1, 9,10,11,12], and solving the Eliashberg equations, it is possible to fully describe the superconducting properties of the metal. The prefactor TD/1.45 and numbers in the exponent of
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