Recently superconductivity was discovered in a new compound composed mostly of graphite layers and a small amount of calcium, CaC6. Its transition temperature of 11.5 Kelvins is nearly twice the transition temperature of the previously known layered graphite compounds. The immediate question raised by this discovery is the mechanism for superconductivity in this compound.
Materials Science Division scientists have now found the answer. They found a downward shift of the transition temperature on substituting a heavier isotope of Ca for the naturally occurring lighter one, showing not only that superconductivity is due to the conventional electron-phonon interaction, but also that the low-energy Ca phonons and not the high-energy layered C phonons are responsible.
This is opposite to the behavior of another famous superconductor, MgB2, where the high-energy layered B phonons drive the superconductivity, producing a record-setting transition temperature of 40 K for the electron-phonon mechanism. This work demonstrates the remarkable scope and variety of superconductivity that, despite scientists' considerable knowledge, remains a tantalizing challenge for modern science to explain.
The result was published in PRB:
"Large Ca isotope effect in the CaC6 superconductor", D. G. Hinks et al., Phys. Rev. B 75, 014509 (2007).
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