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

TL;DR

This study uses muon spin rotation to measure the magnetic penetration depth in YBa2Cu4O8, revealing s+d wave symmetry in the planes and s-wave symmetry along the c-axis, indicating universal features in cuprate superconductors.

Contribution

It provides direct bulk evidence of mixed s+d wave symmetry in the planes and pure s-wave along the c-axis in YBa2Cu4O8, clarifying the order parameter symmetry in cuprates.

Findings

Inflection point in λ_a^{-2} and λ_b^{-2} indicating two-gap superconductivity.

Pure s-wave gap observed along the c-axis.

Universal features in bulk cuprate superconductors.

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.