Resistivity in Ba(FeCo)As: Comparison of thin films and single crystals

Abstract

The temperature dependence of the resistivity of epitaxial Ba(Fe1−xCox)2As2 thin films (with nominal doping x=0.08,0.1 and 0.15) has been analysed and compared with analogous measurements on single crystals taken from literature. The ρfalse(Tfalse) of thin films looks different from that of single crystals, even when the cobalt content is the same. All ρfalse(Tfalse) curves can be fitted by considering an effective two‐band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a “hardening” of the electron–boson spectral function in thin films, associated with the strain induced by the substrate.

Optical and hidden transport properties of BaFe1.91Ni0.09As2 film

“…A hidden Fermi-liquid behavior found earlier for optimally doped Ba(Fe,Ni) 2 As 2 PLD film [10] and single crystal [32] is confirmed. From the resistivity measurement as a function of temperature we realize that the electron-boson coupling constant is strongly reduced and the representative boson energy is strongly increased as it happens with the other iron-based materials [39,40] and HTSC cuprates [45]. The similar behavior probably results from the fact that the mechanism responsible for superconductivity is, perhaps, similar for HTSC cuprates and ironbased materials.…”

“…As in the superconducting state, we set all spectral functions to be equal and they differ just for a scaling factor, the coupling constant (this is a very good approximation, especially if the coupling is mediated mainly by spin fluctuations), then α 2 tr,i F tr (Ω) = λ tr,i α 2 tr F n tr (Ω) and λ tr,i = N j=1 λ tr,ij . The specific shape of the spectral function for the antiferromagnetic spin fluctuation in the normal state is standard [40]. The shape and position of the peak are the same for the spectral functions in all bands.…”

Eliashberg Theory of a Multiband Non-Phononic Spin Glass Superconductor

“…In the iron pnictides, usually, the impurities are almost all concentrated in one band, i.e., in the hole band for the electron-doped materials as this case and in the electron band [52][53][54] for the hole-doped materials [55]. This means that, in the electrons-doped materials, k 22 >> k 11 , k 12 .…”

“…Indeed, the pure interband Eliashberg theory, also in the limit of weak coupling, is different from BCS theory [62]. I choose k 11 = k 12 = 0.2k 22 as happen in the Ba(Fe 1−x Co x ) 2 As 2 [52]. By using the typical parameters of iron pnictides and spin-glass systems, I find that k 22 3.1 meV (N(0) = 5.6 states/eV, S = 7/2, J = 0.12 meV and T SG = 15 K) [5,63].…”

“…The low-temperature value of the penetration depth λ L (0) should, in principle, be related to the plasma frequency by ω p = c/λ L (0) [67] and appears as a multiplicative factor of the summation. Here, w 1 = 0.72 and w 2 = 0.28 as in the Co-doped iron compounds [52]. In principle, the calculation of superfluid density (penetration depth) is important in order to compare theoretical predictions with the experiment because it is easier to find these measurements in the literature [61].…”