Full-Gap Superconductivity Robust against Disorder in Heavy-Fermion CeCu_{2}Si_{2}.

Area of Science:

• Condensed matter physics
• Superconductivity research
• Materials science

Background:

• Unconventional superconductors often exhibit sign-changing energy gaps due to magnetic pairing mechanisms.
• The heavy-fermion superconductor CeCu_{2}Si_{2} was thought to be a prime example of such a material.
• Recent findings suggested a non-nodal gap structure in CeCu_{2}Si_{2}, contradicting expectations.

Purpose of the Study:

• To investigate the impact of controlled point defects on the superconducting properties of CeCu_{2}Si_{2}.
• To determine if disorder induces expected low-energy bound states in the presence of a non-nodal gap.
• To provide bulk evidence clarifying the superconducting pairing symmetry in CeCu_{2}Si_{2}.

Main Methods:

• Electron irradiation was used to introduce controlled point defects into CeCu_{2}Si_{2} samples.
• The temperature-dependent magnetic penetration depth (λ(T)) was measured to probe the superconducting gap structure.
• Analysis focused on the robustness of the fully gapped state against nonmagnetic impurities.

Main Results:

• The superconducting energy gap in CeCu_{2}Si_{2} was found to be robust against disorder introduced by electron irradiation.
• Nonmagnetic impurities did not induce the expected low-energy bound states associated with sign-changing gap structures.
• Measurements of the magnetic penetration depth confirmed the absence of significant gap nodes.

Conclusions:

• CeCu_{2}Si_{2} exhibits a fully gapped superconducting state that is resilient to non-magnetic disorder.
• The findings provide strong bulk evidence for an s_{++}-wave superconducting pairing symmetry without sign reversal.
• This challenges the prevailing notion that antiferromagnetic spin-fluctuations necessitate a sign-changing gap in this heavy-fermion superconductor.