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# A neutron scattering study of the under-doped Ba$$_{1-x}$$K$$_{x}$$Fe$$_{2}$$As$$_{2}$$, x=0.09 and 0.17 self-flux grown single crystals and the universality of the tricritical point

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### Abstract

We present a combination of elastic neutron scattering measurements in zero and 14.5 T and magnetization measurements in zero and 14 T on under-doped superconducting Ba$$_{1-x}$$K$$_{x}$$Fe$$_{2}$$As$$_{2}$$ x=0.17, and the same measurements in zero field on a non-superconducting crystal with x=0.09. The data suggest that the under-doped materials may not be electronic phase separated but rather have slightly inhomogeneous potassium doping. The temperature dependence of the magnetic order parameter (OP) below the transition of the sample with x=0.09 is more gradual than that for the case of the un-doped BaFe$$_{2}$$As$$_{2}$$, suggesting that this doping may be in the vicinity of a tricritical point. We advance therefore the hypothesis that the tricritical point is a common feature of all superconducting 122s. For the x=0.17 sample, while T$$_{c}$$ is suppressed from $$\approx$$17 K to $$\approx$$8 K by a magnetic field of 14 T, the intensity of the magnetic Bragg peaks (1 0 3) at 1.2 K is enhanced by 10$$%$$ showing competition of superconductivity (SC) and antiferromagnetism (AFM). The intensity of the magnetic Bragg peaks (1 0 3) in the (T$$_{c}$$, T$$_{N}$$) temperature interval remain practically unchanged in 14.5 T within a 10$$%$$ statistical error. The present results are discussed in the context of the existing literature.

### Most cited references25

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### Superconductivity at 38 K in the Iron Arsenide$$({\mathrm{Ba}}_{1-x}{\mathrm{K}}_{x}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$$

(2008)
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### Superconductivity at 22 K in Co-doped BaFe2As2 Crystals

(2008)
Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals below Tc = 22 K, as demonstrated by resistivity, magnetic susceptibility, and specific heat data. Hall data indicate that the dominant carriers are electrons, as expected from simple chemical reasoning. This is the first example of superconductivity induced by electron doping in this family of materials. In contrast to the cuprates, the BaFe2As2 system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for BaFe1.8Co0.2As2 indicate the inter-band scattering due to Co is weak.
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### Magnetic order in BaFe2As2, the parent compound of the FeAs based superconductors in a new structural family

(2008)
In addition to higher Tc compared with the ubiquitous cuprates for a material composed of a single electronically active layer, the newly discovered LnFeAsO superconductors offer additional compositional variation. In a similar fashion to the CuO2 layers in cuprates, the FeAs layers now dominate the electronic states that produce superconductivity. Cuprate superconductors distinguish themselves structurally by adopting different stacking of the Cu-O and electronically inactive "spacer" layers. Using the same structural philosophy, materials with the formula (A,K)Fe2As2,A=Ba or Sr have been reported and possess a Tc~38 K. Here, we report the neutron diffraction studies of BaFe2As2 that shows, in contrast to previous studies on the LnFeAsO materials, an antiferromagnetic transition which concurs with first-order structural transition. Although the magnetic and structural transitions occur differently in the AFe2As2 and LnFeAsO-type materials, this work clearly demonstrates that the complete evolution to a low symmetry structure is a pre-requirement for the magnetic order.
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### Author and article information

###### Journal
14 November 2011
2012-02-23
1111.3329
10.1103/PhysRevB.85.144506