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Register a new userInsular superconductivity in Co-doped iron pnictide CaFe$_{1-x}$Co$_x$AsF
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The presence of macroscopic phase separation between the superconducting and magnetic phases in cfcaf is demonstrated by muon spin rotation (muSR) measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (T_m > T_c), while Co-doping induces strong randomness. The volumetric fraction of superconducting phase is nearly proportional to the Co content $x$ with constant superfluid density. These observations suggest the formation of superconducting islands (or domains) associated with Co ions in the Fe$_2$As$_2$ layers, indicating a very short coherence length.
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The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy (STM) to image the electronic impact of Co atoms on the ground state of the LiFe$_{1-x}$Co$_x$As system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strong-coupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Born-limit scattering effect in a s$pm$-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.
Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN ~ 145 K). Replacing Fe with Co is expected to both electron dope the system and introduce disorder in the FeAs layer. For x = 0.05 antiferromagnetic order is destroyed and superconductivity is observed at Tconset = 11.2 K. For x = 0.11 superconductivity is observed at Tc(onset) = 14.3 K, and for x = 0.15 Tc = 6.0 K. Superconductivity is not observed for x = 0.2 and 0.5, but for x = 1, the material appears to be ferromagnetic (Tc ~ 56 K) as judged by magnetization measurements. We conclude that Co is an effective dopant to induce superconductivity. Somewhat surprisingly, the system appears to tolerate considerable disorder in the FeAs planes.
Here we report the synthesis and basic characterization of SmFe1-xCoxAsO (x=0.10, 0.15). The parent compound SmFeAsO itself is not superconducting but shows an antiferromagnetic order near 150 K, which must be suppressed by doping before superconductivity emerges. With Co-doping in the FeAs planes, antiferromagnetic order is destroyed and superconductivity occurs at 15 K. Similar to LaFe1-xCoxAsO, the SmFe1-xCoxAsO system appears to tolerate considerable disorder in the FeAs planes. This result is important, which indicates difference between cuprare superconductors and the iron-based arsenide ones.
The Ru doping effect on the Dirac cone states is investigated in iron pnictide superconductors Ba(Fe$_{1-x}$Ru$_x$As)$_2$ using the transverse magnetoresistance (MR) measurements as a function of temperature. The linear development of MR against magnetic field $B$ is observed for $x$ = 0 - 0.244 at low temperatures below the antiferromagnetic transition. The $B$-linear MR is interpreted in terms of the quantum limit of the Dirac cone states by using the model proposed by Abrikosov. An intriguing evidence is shown that the Dirac cone state persists on the electronic phase diagram where the antiferromagnetism and the superconductivity coexist.
We report magnetotransport measurements and its scaling analysis for the optimally electron doped Sr(Fe${_{0.88}}$Co${_{0.12}}$)${_2}$As${_2}$ system. We pbserve that both the Kohlers and modified Kohlers scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence similar to many cuprates and heavy fermion systems. The fact that this temperature dependence is seen in spite of the violation of modified Kohlers scaling suggests that the Hall angle and the magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetoresistance in this system, which does not harbor a spin density wave ground state. Implcations of our observations are discussed in the context of spin fluctuations in strongly correlated electron systems.
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