Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies

Abstract

The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

Scanning tunnelling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets

“…The main question is whether the external field H is capable of rotating the impurity moment, which is also coupled to the itinerant electrons via J K . To answer this question, we rely on electron spin resonance experiments that measured J K for Mn-doped iron pnictides37. The reported estimated value J K ≈1 meV is very small, suggesting that magnetic fields of the order of 10 T (achievable in STS set-ups) can unlock the impurity moment from the itinerant magnetic configuration.…”

“…On the one hand, the orientation of the itinerant ordered magnetic moments of the host system is fixed by the residual spin–orbit and magneto-elastic couplings. On the other hand, the coupling of the magnetic impurity to the host electronic system is determined by the Kondo-like interaction between the impurity and the conduction electrons J K (refs 35, 36), which in the case of Mn in BaFe 2 As 2 was found to be small by recent electron spin resonance studies37. This property, allied to the insensitivity of the itinerant magnetism to external magnetic fields3839, implies that magnetic fields are able to ‘unlock' the magnetic moment of the impurity from the magnetic order in the lattice, thus allowing for a change of its orientation with respect to the rigid magnetic structure of the host system.…”

Towards a quantitative description of tunneling conductance of superconductors: Application to LiFeAs

“…From the experimental point of view, there are no indications that Co and Ni carry a significant magnetic moment. As for Mn impurities, on the other hand, they are expected to be magnetic [33][34][35] but only weakly coupled to the itinerant electron spins and hence one expects their magnetic potential to not strongly modify the LDOS 36 . Finally, we discuss consequences of our basis transformation for the spatial modulations of the tunneling current, spectra and topographs close to impurities, e.g., using Eq.…”

Pressure effects on magnetic pair-breaking in Mn- and Eu-substituted BaFe2As2

“…Within the Abrikosov-Gor'kov (AG) formalism, hJ 2 ðqÞi plays the role of a magnetic impurity potential responsible for the pairbreaking effects that reduce T c . 19,20 In the ESR experiments, hJ 2 ðqÞi 1=2 is the microscopic parameter responsible for the linear increase of ESR linewidth of local moments diluted in metallic hosts (Korringa behavior) 16,21,22,24,25 (see Table I). To estimate whether the extracted value of hJ 2 ðqÞi ESR for the studied samples can account for the observed suppression of T c , we consider the conventional AG pairbreaking formalism 19,20 for the case where the gap function has the same amplitude and sign across the entire Brillouin zone…”

“…calculated for one mole for all compounds. 2,21 Therefore, by using the experimental DT exp c , we thus estimate the magnetic pair-breaking impurity potential hJ 2 ðqÞi 1=2 from the AG equation that would be necessary to cause the observed suppression of T c and compare it with the experimental value from the ESR data.…”