Direct evidence for the emergence of a pressure induced nodal superconducting gap in the iron-based superconductor Ba_0.65Rb_0.35Fe_2As_2

TL;DR

This study provides direct evidence that applying pressure to the iron-based superconductor Ba_0.65Rb_0.35Fe_2As_2 induces a transition from a nodeless to a nodal superconducting gap, revealing pressure as a tuning parameter for pairing symmetry.

Contribution

The paper demonstrates that hydrostatic pressure can induce a transition from nodeless s-wave to nodal d-wave superconductivity in Ba_0.65Rb_0.35Fe_2As_2, providing new insights into pairing mechanisms.

Findings

Pressure decreases magnetic penetration depth without changing Tc.

Low-temperature behavior shifts from exponential to power-law with pressure.

Data favors d-wave pairing as the origin of nodes.

Abstract

Identifying the superconducting (SC) gap structure of the iron-based high-temperature superconductors (Fe-HTS's) remains a key issue for the understanding of superconductivity in these materials. In contrast to other unconventional superconductors, in the Fe-HTS's both $d$-wave and extended s-wave pairing symmetries are close in energy, with the latter believed to be generally favored over the former. Probing the proximity between these very different SC states and identifying experimental parameters that can tune them, are of central interest. Here we report high-pressure muon spin rotation experiments on the temperature-dependent magnetic penetration depth (lambda) in the optimally doped Fe-HTS Ba_0.65Rb_0.35Fe_2As_2. At ambient pressure this material is known to be a nodeless s-wave superconductor. Upon pressure a strong decrease of (lambda) is observed, while the SC transition temperature remains nearly constant. More importantly, the low-temperature behavior of (1/lambda^{2}) changes from exponential saturation at zero pressure to a power-law with increasing pressure, providing unambiguous evidence that hydrostatic pressure promotes nodal SC gaps. Comparison to microscopic models favors a d-wave over a nodal s^{+-}-wave pairing as the origin of the nodes. Our results provide a new route of understanding the complex topology of the SC gap in Fe-HTS's.