Study On The Unconventional Superconducting Pairing Symmetry In Heavy Fermion CeCu₂Si₂

Since the first discovery of superconductivity in CeCu2Si2 in 1979 by German scientists,revealing its unconventional superconducting pairing mechanism has aroused great concern.The rotational specific heat measurement down to much lower temperature on CeCu2Si2 in 2014 from Japanese group provides clear evidence of multiband superconductivity with nodeless gap,which has been further convinced by subsequent scanning tunneling microscopy(STM/STS)and London penetration depth measurements.Several possible superconducting pairing states,such as s±-wave,s++-wave and loop-nodal s wave,have been proposed by theoretical calculations,which severely challenge the widely accepted d-wave pairing symmetry based on single-band structure for more than 30 years.Further theoretical and experimental studies are urgently needed to clarify them.However,because of the particular topological structure of Fermi surface(FS)of heavy Fermion superconductors,conventional phase-sensitive measurements cannot be directly applied to detect the pairing symmetry,so the pairing order parameters of superconductors cannot be confirmed directly and effectively.Fortunately,recent high-resolution STM technique has been developed and could measure the quasiparticle band dispersions at much lower temperature and the detailed k-space gap functions within μeV energy scale.The method of measuring the quasiparticle coherence spectral and the local electron spectral induced by the disruption of superconducting pairs by impurity in real space has been successfully applied to identify the d-wave pairing symmetry in the heavy Fermion superconductor CeCoIns.Therefore,we extend this technique to the study of unconventional superconductivity of CeCu2Si2 in this article.First,we calculated the multiband structure of CeCu2Si2 by first-principles method,and obtained the topological structure of Fermi surface which is consistent with the earlier theoretical calculation.It is confirmed that CeCu2Si2 is a multiband superconductor.Based on the above first-principles calculations,we established an effective tight-binding model with two band.Secondly,we use the T-matrix approach method calculated the local electronic structures induced by impurity scattering.The results show that:when only considering the intra-band impurity scattering,there exists no intra-gap impurity resonance states for s-wave pairing near the Fermi energy;however,for the d-wave and loop-nodal s-wave pairing,the impurity-induced resonance states are very significant and can be distinguished by their locations near or far from the Fermi energy.Finally,we effectively confirmed the pairing of s-waves with or without sign changes by introducing the inter-band impurity scattering.The study reveals that only the sign change s±-wave pairing can produce the impurity resonance states.In conclusion,we propose a competitive method to determine the superconducting pair symmetry of CeCu2Si2 at atmospheric pressure:The measurement of impurity-induced local electron spectral by high resolution STM combined with the theoretical criterion will effectively clarify the existing differences and determine the superconducting pairing symmetry.