Cooperative Effects in CeCu₂Si₂

Unconventional superconductivity in heavy-fermion compounds

“…Given these estimates, it appears that superconductivity in CeCu 2 Si 2 is paramagnetically-limited [72]; it is likely that H c2 (0) > H p0 due to strong spin-orbit scattering, which is not accounted for in our estimate of H p0 . The quadratic temperature dependence of the specific heat and thermal expansion coefficient [73] and nuclear quadrupole resonance measurements [74] in the superconducting state of CeCu 2 Si 2 are consistent with the presence of line nodes [60] in the superconducting energy gap ∆( k). However, a recent study demonstrated that the specific heat deviates from a quadratic temperature dependence at very low-temperature [75]; this behavior is inconsistent with the presence of line nodes, but the specific heat data within the superconducting state can be accounted for by a scenario involving nodeless multiband superconductivity [75].…”

Unconventional superconductivity

“…has recently revealed conventional behavior. In earlier studies on the specific heat of CeCu2Si2, Tc=0.63 K, Lang et al [49] found C ~ T 2 down to approximately 0.1 K. Now, in 2014 Kittaka et al [50], in a slightly less ideal crystal (Tc=0.6 K, C/Tc smaller by 20%) analyze specific heat down to 0.04 K. In this altered analysis using more modern theoretical insight, they fit their data over the whole temperature range of 0.04 to 0.6 K to a two band, fully gapped BCS model (with energy gaps 1/kBTc=1.76 and 2/kBTc=0.7) as has been done routinely to both IBS and MgB2. Thus, partly due to improved analysis, and partly due to data slightly lower in temperature than previously, the conclusion of Kittaka et al is that superconductivity in CeCu2Si2 is fully gapped, non-nodal.…”

“…When each class of superconductor is discussed more thoroughly in the following sections below, a more complete discussion will be offered. Table 1 (From Heffner and Norman [48]) Theoretical temperature dependencies for several low temperature measurements in clean, defect-free samples, assuming a spherical Fermi surface and either line ('polar') or point ('axial') nodes in the superconducting gap structure The specific heat in the superconducting state of CeCu2Si2 follows [49] C ~ T 2 -consistent with line nodes in the superconducting gap -down to 0.1 Tc. As already mentioned, this old result has been superseded [50] by specific heat data below 0.15 Tc that can be fit to a two band model, where both band gaps are fully gapped BCS-like.…”

Superconducting phases off-electron compounds

“…Under magnetic field the superconductivity exhibits strong Pauli limiting with H c2 ≈ 0.45 T (Rauchschwalbe et al, 1982). The leading order temperature dependence of the specific heat C(T ), thermal expansion α (Lang et al, 1991) and penetration depth λ (Gross et al, 1988) vary as T 2 characteristic of line nodes. NMR and NQR show the absence of a Hebel-Slichter peak and a power law dependence of the spin-lattice relaxation rate, also suggesting line nodes (Ishida et al, 1999;Kawasaki et al, 2004).…”