EuFe 2 As 2 shows a spin-density wave ͑SDW͒ type transition at 190 K and antiferromagnetic ͑AF͒ order below 20 K. Here, we have studied the effect of K substitution on the SDW transition at high temperature and AF Eu order at low temperature. 50% K substitution suppresses the SDW transition and in turn gives rise to high-temperature superconductivity below T c = 32 K, as observed in the electrical resistivity, AC susceptibility, as well as magnetization. A well defined anomaly in the specific heat provides additional evidence for bulk superconductivity. Below 10 K, short-range magnetic order of the Eu moments is suggested by a broad feature in the specific-heat data. Electronic structure calculations reveal very close similarity with the nonmagnetic superconductor Sr 0.5 K 0.5 Fe 2 As 2 , but yield localized 4f magnetic moments for the remaining Eu atoms.
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 report a systematic study of the influence of antiferromagnetic and ferromagnetic phases of Eu 2+ moments on the superconducting phase upon doping the As site by isovalent P, which essentially acts like chemical pressure on EuFe 2 As 2 . Bulk superconductivity with transition temperatures of 22 and 28 K are observed for x = 0.16 and 0.20 samples, respectively. The Eu ions order antiferromagnetically for x 0.13, while bulk superconductivity coexists with Eu-antiferromagnetism for 0.13 < x < 0.22. In contrast, a crossover is observed for x 0.22 whereupon the Eu ions order ferromagnetically and superconductivity is fully suppressed. Densityfunctional-theory-based calculations reproduce the observed experimental findings consistently. We discuss in detail the coexistence of superconductivity and magnetism in a tiny region of the phase space and comment on the competition of ferromagnetism and superconductivity in the title compound.
We studied the temperature-pressure phase diagram of EuFe 2 As 2 by electrical resistivity measurements. The spin-density-wave transition at T 0 associated with the FeAs-layers is continuously suppressed with increasing pressure, while the antiferromagnetic ordering temperature of the Eu 2+ moments seems to be nearly pressure independent up to 2.6 GPa. Above 2 GPa a sharp drop of the resistivity, ͑T͒, indicates the onset of superconductivity at T c Ϸ 29.5 K. Surprisingly, on further reducing the temperature, ͑T͒ is increasing again and exhibiting a maximum caused by the ordering of the Eu 2+ moments, a behavior which is reminiscent of reentrant superconductivity as it is observed in the ternary Chevrel phases or in the rare-earth nickel borocarbides.
Electron spin resonance (ESR) measurements of the ferromagnetic (FM) Kondo lattice system CeRuPO show a well defined ESR signal which is related to the Ce3+ magnetism. In contrast, no ESR could be observed in the antiferromagnetic (AFM) homologue CeOsPO. Additionally, we detect an ESR signal in ferromagnetic YbRh while it was absent in a number of Ce or Yb intermetallic compounds with dominant AFM exchange. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity to a quantum critical point, but seems to be connected to the presence of FM fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature (as observed in YbRh2Si2) but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.
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