Competition of magnetism and superconductivity in underdoped (Ba_1-xK_x)Fe₂As₂

Hyperfine fields in the BaFe2As2 family and their relation to the magnetic moment

“…4 (b). Experimental data for B exp [10], referring again to the upper axis, are in good agreement with the enhanced theoretical field B hf . In particular, the experimental B exp is also nearly constant over a large range of concentration; in variance to the electron doped systems considered above.…”

“…It is quite common to assume that the ratio A [25]. These values give for BaFe 2 As 2 with an experimental hyperfine field B exp = −5.47 T a magnetic moment µ avg ∼ 0.4 -0.5 µ B [9,10]. Later on it was questioned whether these ratios A avg hf are applicable to the iron pnictides [24,25].…”

“…Nowadays, a lot of 57 Fe Mössbauer spectroscopy data are available for the BaFe 2 As 2 family with different types of substitution and doping [10,[24][25][26]. The previously mentioned discrepancy between neutron diffraction and 57 Fe Mössbauer spectroscopy is often ascribed to possible * gerald.derondeau@cup.uni-muenchen.de non-zero contributions of d-orbitals to the hyperfine field with an opposite sign to that of the Fermi contact field [24].…”

Specific Heat of Ba0.59K0.41Fe2As2, an Fe-Pnictide Superconductor with Tc = 36.9 K, and a New Method for Identifying the Electron Contribution

“…In the 122 series of Fe-pnictide superconductors, doped BaFe 2 As 2 , a high-T structural/magnetic transition, which is shifted to lower temperatures by the doping that produces the superconductivity, poses an additional obstacle to obtaining C lat : In BaFe 2 As 2 the transition is first-order and occurs near 140 K, 1 but in the K-doped superconductors, Ba 1-x K x Fe 2 As 2 , it is completely suppressed 2,3 at x ~ 0.35. Superconductivity occurs in the high-T tetragonal phase for optimally doped (x ~ 0.4) and overdoped samples, [2][3][4][5] but in the low-T orthorhombic phase for underdoped (x ~ 0.1 -0.2) samples. 2,3,4 Essentially all specific-heat data for high-T c Fe-pnictide superconductors have been analyzed by the same two-step procedure: In the first step, which is typical of that taken for any high-T c superconductor, an approximation for C lat was obtained in a fit to high-T normal-state data, extrapolated to low temperatures, and subtracted from C to obtain C es .…”

“…Superconductivity occurs in the high-T tetragonal phase for optimally doped (x ~ 0.4) and overdoped samples, [2][3][4][5] but in the low-T orthorhombic phase for underdoped (x ~ 0.1 -0.2) samples. 2,3,4 Essentially all specific-heat data for high-T c Fe-pnictide superconductors have been analyzed by the same two-step procedure: In the first step, which is typical of that taken for any high-T c superconductor, an approximation for C lat was obtained in a fit to high-T normal-state data, extrapolated to low temperatures, and subtracted from C to obtain C es . In the second step, the derived C es was analyzed with expressions based on the α model 6 as extended to two-gap superconductors 7 to derive the parameters characteristic of the electron bands.…”