Abstract
Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature Tc acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the T = 0 penetration depth in the electron-doped iron-based superconductor Ba(Fe1−xCox)2As2. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe2(As1−xPx)2 compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic–magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in λ(x) near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.
Highlights
A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order or weakly first order
Of the NV technique and the precise determination of Co-doping levels via wavelength dispersive spectroscopy (WDS) allow us to clearly identify an anomalous peak in λ(x) inside the superconducting dome near optimal doping (x = 0.057). This point coincides with the extrapolated location of the AFM/SC boundary, and of the QPT, as determined by scattering experiments. This result demonstrates that the occurrence of a sharp peak in λ very close to the QPT inside the dome is not limited to clean isovalently-substituted pte compounds with nodal superconducting gaps, and occur in the more disordered chargedoped fully-gaped iron pnictides
The effective demagnetization factor depends on the geometry of the sample [57], it is important to use samples with well defined shapes as determined from screening through a scanning electron microscope (SEM) Fig. 1(b)
Summary
The unconventional superconducting (SC) state of heavy fermions [1,2,3,4,5,6,7,8,9], cuprates [10–. This point coincides with the extrapolated location of the AFM/SC boundary, and of the QPT, as determined by scattering experiments This result demonstrates that the occurrence of a sharp peak in λ very close to the QPT inside the dome is not limited to clean isovalently-substituted pte compounds with nodal superconducting gaps, and occur in the more disordered chargedoped fully-gaped iron pnictides. This suggests that such an anomaly in the penetration depth is a more universal property of iron pnictides despite the theoretical result that a QPT alone is not enough to guarantee such an anomaly, shedding new light on the ce interplay between AFM quantum fluctuations and superconductivity in these systems
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