Breakup of superconducting order parameter into 2 subbands of Cooper pairs increases temperature range of fluctuations in the pseudogap phase

The opening of the pseudogap in underdoped cuprates breaks up the Fermi surface, into 4 disconnected arcs centered on the nodal directions. In the superconducting phase, the d-wave order parameter breaks up into 2 subbands of Cooper pairs along (1,1) and (1,-1), with strong intra-subband and weak inter-subband couplings. This multiple-band superconductivity allows a low-energy Leggett mode to emerge due to phase fluctuations between the subbands. We propose that the overdamped Leggett mode is responsible for the highly unusual wide temperature range of superconducting fluctuations observed in the c-axis infrared conductivity in the pseudogap phase.