High-resolution measurements of the magnetic penetrationdepth on Yba2Cu4O8 single crystals

In order to unveil the intrinsic electrodynamic response of the CuO2 layers of cuprate superconductors and to differentiate this response from that of the charge reservoir layers, we measured the in-plane (ab) magnetic penetration depth on Yba2Cu4O8 (Y124) single crystals by using a high-resolution mutual inductance technique. Among the various high-Tc families of cuprate superconductors, Y124 is a stoichiometric compound characterized by a double CuO chain per unit cell acting as charge reservoir and by the absence of any disorder or structural inhomogeneities arising from chemical substitutions and/or oxygen vacancies. This feature makes Y124 an ideal system for our study. The experimental data show, in the low temperature region down to 4.2 K, a marked linear dependence of the variation Δλab≡λab(T)−λab (4.2 K) with a small downturn at 9 K. This behavior confirms the existence of zero-energy thermally activated excitations, that are also observed in the Yba2Cu3O7−δ (Y123) counterpart possessing only one CuO chain per unit cell. Whilst such linear behavior is consistent with the d-wave symmetry picture previously established for cuprates, the measured thermal activation rate, ∝∂λab/∂T, is however, more than a factor 4 larger in comparison with Y123. This result is consistent with a proximity model of superconducting planes and metallic chains and suggests that the reservoir layers strongly affect the low temperature behavior of the magnetic penetration depth in cuprates