Variation of the critical temperature with the lattice parameter in K3C60

Abstract

Alkali-doped fullerides are a fascinating system exhibiting the convergence of s-wave superconductivity and Mott physics. A direct relationship between the critical temperature Tc and lattice constant established by applying physical or chemical pressure in these compounds is generally believed to be the foundation for the identification of the microscopic electron pairing(s). Here the evolution of Tc with lattice compression for the first fulleride K3C60 is systematically studied via the high-pressure electrical transport and synchrotron x-ray diffraction measurements on the same well-characterized sample. The combination of the theoretically calculated density of states at the Fermi level, the electron-phonon coupling and mean phonon-frequency determined from high-pressure Raman spectra yields the Coulomb pseudopotential ranging from 0.15 to 0.32 with increasing pressure in terms of the McMillan formula. This indicates that the conventional phonon-mediated mechanism cannot account for the high-pressure evolution of Tc solely. However, the obtained pressure derivative of Tc and bulk modulus are used to reproduce well the experimentally observed isotope exponent in this superconductor within the theoretical framework of the non-phononic theory. These results and findings together demonstrate the important role of the study of the lattice effect on Tc played in determining the mechanisms of superconductivity in fullerides.