Quantum Phase Transition Induced by a Preformed Pair in a Boson-Fermion Model of Fulleride Superconductivity - Condensed Matter > SuperconductivityReport as inadecuate




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Abstract: There continues to be enormous interest in the BCS to BEC transition as therestill is no exact theory. We recently reported a revealing reinterpretation ofthe condensed phase Boson-Fermion Model BFM by comparing it to a cold atomformulation 1. While the ground and singly excited states appear to remaincontinuous in all models we have examined, the collective modes contain asingularity due to a Feshbach resonance tuned by doping causing a breakdownof the Migdal theorem. As a result of vertex corrections, there is afundamental change in the nature of the superconductivity due to the formationof preformed pairs as the previously suggested location 1 of a quantumcritical point in the fulleride phase diagram is passed. The result is aquantum phase transition QPT between BCS and BEC-like or Feshbach resonancesuperconductivity SC.We discuss features of the resonance and the role of the experimentallyobserved preformed pair formation in fullerides, essential to the Boson-FermionModel BFM, and often speculated since the work of Nozieres and Schmitt-Rink17. Here, we present arguments to establish a model of the preformed pairwhich can be favorably compared to a circular charge density wave CDWisolated on a fulleride molecule. The binding is much larger than a Cooperpair. The CDW seems to be stabalized by splitting of the Jahn-Teller activevibrational modes to reduce Coulomb repulsions. Our conclusions are: 1 thedoping of two electrons into triply degenerate orbitals results in theexperimentally observed singlet state CDW; and 2 this CDW preformed pairhas a dual role as doping is varied: suppression of BCS SC and enabling aFeshbach resonance form of SC.



Author: Richard H. Squire, Norman H. March

Source: https://arxiv.org/



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