Strong-coupling superconductivity due to soft boson modes in MgB2, cuprates, borocarbides and some heavy fermions
Abstract
We report micro-Raman measurements that reveal a low-energy soft mode at 16-19 meV in MgB2 which becomes sharper in the superconducting state. With a boson spectrum based on this mode we reproduce, within Eliashberg theory, the Tc, the T-shape of the NMR relaxation rate as well as unusual dip-hump features in available tunneling data. We compare MgB2 to cuprates, borocarbides and UPd2Al3, where we identify soft bosonic modes at energies which scale with their Tc suggesting that all these materials are very strong coupling superconductors to soft bosonic modes, that are possibly precursors of quantum instabilities.
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