Observation of the non-linear Meissner effect

Area of Science:

• Condensed matter physics
• Superconductivity research
• Materials science

Background:

• Nodal unconventional superconductors are predicted to exhibit a non-linear Meissner effect at zero temperature, where magnetic penetration depth (λ) varies linearly with magnetic field.
• This effect is a crucial indicator of the nodal state, alongside the linear-in-temperature dependence of λ, but has lacked convincing experimental verification.
• Distinguishing between true gap nodes and deep gap minima in superconductors is essential for understanding their fundamental properties.

Purpose of the Study:

• To experimentally confirm the existence of the non-linear Meissner effect in nodal superconductors.
• To utilize the effect of magnetic fields on λ(T) as a method to differentiate between gap nodes and non-nodal deep gap minima.
• To investigate the superconducting state of KFe2As2.

Main Methods:

• Measurements of magnetic penetration depth (λ) in CeCoIn5 and LaFePO superconductors.
• Application of varying dc magnetic fields to observe the temperature dependence of λ.
• Comparative analysis of experimental results with theoretical predictions for nodal and non-nodal superconductors.

Main Results:

• Clear experimental evidence of the non-linear Meissner effect was observed in CeCoIn5 and LaFePO.
• A method was developed using the influence of a small dc magnetic field on λ(T) to distinguish gap nodes from deep gap minima.
• Measurements on KFe2As2 suggest it possesses a non-nodal superconducting state.

Conclusions:

• The non-linear Meissner effect is experimentally confirmed in nodal superconductors, validating theoretical predictions.
• The developed method provides a reliable way to identify the presence or absence of gap nodes.
• KFe2As2 is identified as a material with a non-nodal superconducting state, distinct from CeCoIn5 and LaFePO.