PACS numbers: * Electronic address: stockert@cpfs.mpg.de 1The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu 2 Si 2 . A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the major driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point.While conventional superconductivity (SC) is generally incompatible with magnetism, magnetic excitations seem to play an important role in the Cooper pair formation of unconventional superconductors such as the high-T c cuprates or the low-T c organic and heavyfermion (HF) superconductors. Since the discovery of SC in CeCu 2 Si 2 1 , antiferromagnetic (AF) spin excitations have been proposed as a viable mechanism for SC 2-4 . The discovery of SC at the boundary of AF order in CePd 2 Si 2 5 has pushed this notion into the framework of AF quantum criticality 6 . Unfortunately, such quantum critical points (QCPs) proximate to HF superconductors typically arise under pressure, which makes it difficult to probe their magnetic excitation spectrum.Here, we report a detailed study of the magnetic excitations in CeCu 2 Si 2 , which exhibits SC below T c ≈ 0.6 K. This prototypical HF compound is ideally suited for our purpose, since SC here is in proximity to an AF QCP already at ambient pressure (cf. Fig. 1(a)).As displayed in Fig. 1(b) CeCu 2 Si 2 crystallises in a structure with body-centred tetragonal symmetry and is one of the best studied HF superconductors and well characterised by low-temperature transport and thermodynamic measurements 7 . Moreover, those measurements in the field-induced normal state have already provided evidence that the QCP in this compound is of the three-dimensional (3D) spin-density-wave (SDW) type 8 . The spatial anisotropy of the spin fluctuations in superconducting CeCu 2 Si 2 was measured at T = 0.06 K and at an energy transfer ω = 0.2 meV and is shown in Fig. 1(c). These magnetic correlations display only a small anisotropy (a factor of 1.5) in the correlation lengths 2 between the [110] and the [001] direction. Therefore, these quite isotropic spin fluctuations are in line with thermodynamic and transport measurements exhibiting C/T = γ 0 − a √ T or ρ − ρ 0 = AT α , α = 1 − 1.5 8,9 , and strongly support a three-dimensional quantum critical SDW scenario 10 . We are able to identify the magnetic excitations in the normal state of paramagnetic, ...

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the threedimensional (3D) 4 2 /m screw symmetry around the ( 1 2 1 2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal-and SC-state spin dynamics and in the out-ofplane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe-sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe-sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ω res , as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ħ hω res ≈ 4.3 k B T c holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.