Condensation, excitation, pairing, and superfluid density inhigh-Tc superconductors: the magnetic resonance mode as a roton analogue and a possiblespin-mediated pairing

Abstract

To find a primary factor determiningTc and a pairingmechanism in high-Tc cuprates, we combine the muon spin relaxation results onns/m* (superconductingcarrier density/effective mass), accumulated over the last 17 years, with the results from neutron and Ramanscattering, scanning tunnelling microscopy, specific heat, Nernst effect, andangle-resolved photoemission spectroscopy measurements. We identify the neutronmagnetic resonance mode as an analogue of the roton minimum in the superfluid4He, andargue that ns/m* and the resonance mode energy play a primary role in determiningTc in the underdoped region. We propose a picture wherein roton-like excitations in the cupratesappear as a coupled mode, which has resonance modes for spin and charge responses at differentmomentum transfers but the same energy transfer, as detected respectively by means of the neutronS = 1 mode andthe Raman S = 0 A1g mode. We shall call this the ‘hybrid spin/charge roton’. After discussing the role ofdimensionality in condensation, we propose a generic phase diagram for the cuprates withspatial phase separation in the overdoped region as a special case of the Bose–Einstein toBardeen–Cooper–Schrieffer crossover conjecture where the superconducting coupling is lostrapidly in the overdoped region. Using a microscopic model of charge motion resonatingwith antiferromagnetic spin fluctuations, we propose the possibility that the hybridspin/charge roton and higher-energy spin fluctuations mediate the superconducting pairing.In this model, the resonance modes can be viewed as a meson analogue and the‘dome’ shape of the phase diagram can be understood as a natural consequence ofdeparture from the competing Mott insulator ground state via carrier doping.