Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2

Abstract

A key aspect of unconventional pairing by the antiferromagnetic spin-fluctuation mechanism is that the superconducting energy gap must have opposite sign on different parts of the Fermi surface. Recent observations of non-nodal gap structure in the heavy-fermion superconductor CeCu${}_2$Si${}_2$ were then very surprising, given that this material has long been considered a prototypical example of a superconductor where the Cooper pairing is magnetically mediated. Here we present a study of the effect of controlled point defects, introduced by electron irradiation, on the temperature-dependent magnetic penetration depth $\lambda (T)$ in CeCu${}_2$Si${}_2$. We find that the fully-gapped state is robust against disorder, demonstrating that low-energy bound states, expected for sign-changing gap structures, are not induced by nonmagnetic impurities. This provides bulk evidence for $s_{++}$-wave superconductivity without sign reversal.

arXiv comment5 pages, 4 figures + Supplemental Material (1 page, 1 figure). Will appear in Phys. Rev. Lett

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• Nature of the spin resonance mode in CeCoIn${}_5$
• Multi-band $s_{++}$ superconductivity in ${\text{V}}_3\text{Si}$ determined from the response to a controlled disorder
• Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu${}_2$Si${}_2$ versus YbRh${}_2$Si${}_2$
• Possible multi-orbital ground state in CeCu${}_2$Si${}_2$
• High-energy magnetic excitations from heavy quasiparticles in CeCu${}_2$Si${}_2$
• Fulde-Ferrell-Larkin-Ovchinnikov state in strongly correlated d-wave superconductors
• An emerging global picture of heavy fermion physics
• Observation of two critical points linked to the high field phase B in CeCu2Si2
• Novel attractive pairing interaction in strongly correlated superconductors
• Determination of the magnetic penetration depth with measurements of the vortex-penetration field for type-II superconductors
• Interplay between charge, magnetic and superconducting order in a Kondo lattice with an attractive Hubbard interaction
• Revealing sign-reversal $s^{+-}$-wave pairing by quasiparticle interference in the heavy-fermion superconductor CeCu${}_2$Si${}_2$
• Topological Phase Transition in Superconductors with Mirror Symmetry
• Anomalous impurity effect in the heavy-fermion superconductor CeCu2Si2
• CeCu${}_2$Si${}_2$ and YbRh${}_2$Si${}_2$: Strange Cases of Heavy-Fermion Superconductivity
• Unconventional superconductivity in the nickel-chalcogenide superconductor, TlNi${}_2$Se${}_2$
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• Determination of the London penetration depth with the tunnel diode oscillator technique