Physics – Condensed Matter – Superconductivity
Scientific paper
2004-06-13
J. Phys.: Condens. Matter 16 (2004) S4515-S4540
Physics
Condensed Matter
Superconductivity
23 pages, 10 figures, to be published in Journal of Physics: Condensed Matter
Scientific paper
10.1088/0953-8984/16/40/007
To find out a primary determing factor of $T_{c}$ and a pairing mechanism in high-$T_{c}$ cuprates, we combine the muon spin relaxation results on $n_{s}/m^{*}$ (superconducting carrier density / effective mass), accumulated over the last 15 years, with the results from neutron and Raman scattering, STM, specific heat, Nernst effect and ARPES measurements. We identify the neutron magnetic resonance mode as an analogue of roton minimum in the superfluid $^{4}$He, and argue that $n_{s}/m^{*}$ and the resonance mode energy $\hbar\omega_{res}$ play a primary role in determining $T_{c}$ in the underdoped region. We propose a picture that roton-like excitations in the cuprates appear as a coupled mode, which has the resonance mode for spin and charge responses at different momentum transfers but the same energy transfers, as detected respectively, by the neutron S=1 mode and the Raman S=0 A1$_{g}$ mode. We shall call this as the ``hybrid spin/charge roton''. After discussing the role of dimensionality in condensation, we propose a generic phase diagram of the cuprates with spatial phase separation in the overdoped region as a special case of the BE-BCS crossover conjecture where the superconducting coupling is lost rapidly in the overdoped region. Using a microscopic model of charge motion resonating with antiferomagnetic spin fluctuations, we propose a possibility that the hybrid spin/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 of departure from the competing Mott insulator ground state via carrier doping.
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