engleski

Slow fracton modes in antiferromagnetic correlations of high-Tc superconductors

In cuprate superconductors superconductivity develops as a unique crossover between the two extremes characterizing these compounds, an antiferromagnetic (AF) Mott insulator phase, on one side, and a non-superconducting itinerant metal on the other. The hole doping tunes a cuprate inside such a broad spectrum of phases. Numerous inelastic neutron scattering studies on underdoped samples shows most directly that the static AF order of the parent phase does not disappear by doping but transforms in such a way that at least fluctuating and local AF order persists, coexisting with superconductivity. Structurally, the model of stripe-type correlations accounts for the coexistence while their genuine dynamics might hide the most interesting physics of cuprates, including the pairing mechanism. Indeed, there is an impressive accumulation of knowledge in favour of AF correlations underlying the pairing. Some crucial properties of these correlations have recently been pointed out: they should involve small velocity collective modes of the wave vector identical to that of the static AF order while their energy scale (<50 meV) should be rather low compared with any other involved. Here we show on phenomenological ground that the latter dynamics originates, irrespective of structural details of stripe order, from general geometrical constraints known to characterize the problem of disordered interpenetrated phases. In particular, we show that dynamics of the Euclidean-to-fractal crossover introduces a new 10-50 meV energy scale, associated with slow (or almost localized) and well-defined fracton modes, and provides a consistent and natural interpretation for redistribution of spectral weight and other inelastic neutron scattering observations. We conclude therefore that the fracton excitations substantially contribute to pairing interactions in high-Tc cuprates.

superconductivity; antiferromagnetism; pairing interactions; fractal; fracton

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