s-wave symmetry along the c-axis and s+d in-plane superconductivity in bulk YBa_2Cu_4O_8

Abstract

To clarify the order parameter symmetry of cuprates, the magnetic penetration depth $\lambda$ was measured along the crystallographic directions $a$, $b$, and $c$ in single crystals of YBa${}_2$Cu${}_4$O${}_8$ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of ${\lambda }_a^{-2}$ and ${\lambda }_b^{-2}$ exhibit an inflection point at low temperatures as is typical for two-gap superconductivity (TGS) with $s+d-$wave character in the planes. Perpendicular to the planes a pure s-wave gap is observed thereby highlighting the important role of c-axis effects. We conclude that these are generic and universal features in the bulk of cuprates.

5 pages, 2 figures