Abstract
The thermodynamic parameters of the superconducting state in CaC6 have been determined in the framework of the isotropic and anisotropic Eliashberg formalism. The obtained results determine the anisotropy effect on the values of the thermodynamic functions. In particular, it has been found that the anisotropy of the electron-phonon coupling constant and the Coulomb pseudopotential significantly affects the order parameter and the wave function renormalization factor, which determines the electron effective mass. These results mean that anisotropy should be also visible in the total normalized density of states. In the case of the thermodynamic critical field, anisotropy lowers its value in the low-temperature area. On the other hand, it does not affect the specific heat jump at the critical temperature.
Highlights
The classical mechanism of the formation of the superconducting state is related to the interaction occurring between the electron gas and the crystal lattice vibrations [1,2].In the case when the value of the electron-phonon coupling constant (λ) is not higher than 0.3, the thermodynamic properties of the superconducting phase can be successfully described by the BCS theory [3,4]
Such effects are included in the Eliashberg formalism, which is the natural generalization of the BCS theory [5]
The result is the non-linear system of equations for the order parameter and the wave function renormalization factor, which structure allows the quantitative description of the strong-coupling and the retardation effects
Summary
The classical mechanism of the formation of the superconducting state is related to the interaction occurring between the electron gas and the crystal lattice vibrations (phonons) [1,2]. In which λ > 0.3, the thermodynamics of the superconducting state can be relevantly dependent on the strong-coupling and the retardation effects Such effects are included in the Eliashberg formalism, which is the natural generalization of the BCS theory [5]. The authors have stated that the predicted phase III (space group Pmmn) with the formation of the carbon nanofoam is found to be stable at a wide pressure range with the critical temperature up to 14.7 K at 78 GPa. The proof that the pairing mechanism in CaC6 results from the electron-phonon interaction is the observation of the Ca isotope effect [25]. We have analyzed the CaC6 superconducting state, at the ambient pressure, in order to determine the effect of anisotropy on the values of the thermodynamic functions. The results obtained in the one-band approach have been presented
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