Multiband superconductivity

A particular case of anisotropic superconductivity is multiband superconductivity, where the order parameter and the excitation gap are mainly different in different bands. 

The multiband superconductivity shows up only in the clean limit , where the single electron mean free path for the interband impurity scattering satisfies the condition / > hvF/Aav where vF is the Fermi velocity and Aav is the average superconducting gap [24,28,30,35],... 

The interchannel pairing or interband pairing that transfers a pair from the a -band to the b -band and viceversa in the multiband superconductivity theory is expressed by... 

Another charateristic feature of multiband superconductivity is that the order parameter shows the sign reversal in the case of a repulsive interband pairing interaction [26]. 

Fig. 11. Schematic diagram for (a) the dynamic Jahn-Teller effect and (b) the Moskalenko-Suhl-Kondo mechanism. In the former, the distorted Cg molecule undergoes the tunneling motion between three equivalent configurations. This results in the formation of the orbital-singlet state. In the latter, as shown by the black arrows, the Cooper pairs are transferred from one Fermi sphere to another, which is the pair-transfer process, a remarkable feature of multiband superconductors, and stabilizes the superconductivity. Also, as shown by the white arrows, the Cooper pairs are scattered coherently within each Fermi sphere, which is the pair-scattering process in usual superconductors.

To summarize Section 4.9, it can be concluded that a multiband scenario is indispensable for understanding the rich variety of superconducting and magnetic properties in HoNi2B2C. Many of these phenomena, in particular those occurring at temperatures between Tn and Tm, are not yet well understood. 

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