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Abstract:
We report measurements of the low-temperature thermal conductivity of
YBa2Cu3O7-delta (delta congruent-to 0.1) single crystals (T(c) = 84 K)
both parallel (kappa(a,b)) and perpendicular (kappa(c)) to the CuO2
planes. Whereas kappa(c) (T) is found to be identical, within
experimental resolution, with the phonon contribution kappa(ph) (T),
kappa(a,b) (T) contains an additional term linear in temperature, AT. We
ascribe AT to the contribution of unpaired electronic carriers residing
in the "chain layers". Measurements performed in external magnetic
fields B less-than-or-equal-to 8 T support this interpretation. Our
observations can be explained by an "internal multilayer" (IML) model in
which it is assumed that strong super-conductivity is generated within
the CuO2 layers and weak superconductivity is induced in the "chain
layers" by the proximity effect. The fit of the experimental results to
the IML model reveals that approximately 15% of the electronic carriers
remain unpaired in YBa2Cu3O7 below T = 1 K.
