Thermodynamic study of gap structure and pair-breaking effect by magnetic field in the heavy-fermion superconductorCeCu2Si2

Abstract

This paper presents the results of specific-heat and magnetization measurements, in particular their field-orientation dependence, on the first discovered heavy-fermion superconductor CeCu2Si2 (Tc ∼ 0.6 K). We discuss the superconducting gap structure and the origin of the anomalous pairbreaking phenomena, leading e.g., to the suppression of the upper critical field Hc2, found in the high-field region. The data show that the anomalous pair breaking becomes prominent below about 0.15 K in any field direction, but occurs closer to Hc2 for H c. The presence of this anomaly is confirmed by the fact that the specific-heat and magnetization data satisfy standard thermodynamic relations. Concerning the gap structure, field-angle dependences of the low-temperature specific heat within the ab and ac planes do not show any evidence for gap nodes. From microscopic calculations in the framework of a two-band full-gap model, the power-law-like temperature dependences of C and 1/T1, reminiscent of nodal superconductivity, have been reproduced reasonably. These facts further support multiband full-gap superconductivity in CeCu2Si2.

Fully gapped superconductivity with no sign change in the prototypical heavy-fermion CeCu 2 Si 2

“…However, these results would also be explained by the multigap nature of the superconductivity shown here by our C ( T ), λ( T ), and κ( H ) measurements. The absence of the coherence (Hebel-Slichter) peak in 1/ T 1 ( T ) below T c may be explained by the quasi-particle damping especially for anisotropic gap and thus does not give conclusive evidence for the sign-changing gap ( 21 ). Inelastic neutron scattering shows an enhancement of magnetic spectral weight at about E ~ 2Δ ( 43 ), which could be interpreted in terms of a spin resonance expected in superconductors with a sign-changing gap.…”

Plain s-Wave Superconductivity near Magnetic Criticality: Enhancement of Attractive Electron–Boson Coupling Vertex Corrections

“…Even so, a fundamental question is why the high-T c plain s-wave state appears against the repulsive interaction by spinfluctuations. More surprisingly, the plain s-wave state is reported in heavy-fermion superconductor CeCu 2 Si 2 near the magnetic phase, according to the measurements of the specific heat, penetration depth, thermal conductivity, and electron irradiation effect on T c [11,12].Therefore, it is a significant problem for theorists to establish a general mechanism of the plain s-wave superconductivity in strongly correlated electron systems. One important feature of these s-wave superconductors would be the orbital degrees of freedom.…”

The dominant role of critical valence fluctuations on high Tc superconductivity in heavy fermions

“…On the other hand, the low-pressure SC pocket in CeCu 2 Si 2 is the best candidate for spin fluctuation mediated SC [59], because the magnetic collapse at p c ≈ 0 and the VCO at p cr ≈ 4.2 GPa are exceptionally well separated. Though, the scenario of single-band nodal-d-wave SC at p = 0 in CeCu 2 Si 2 is now strongly challenged [60][61][62][63].…”