Superconductivity in Fluorine-Arsenide Sr_{1-x}La_xFeAsF

Abstract

Since the discovery of superconductivity 1 at 26 K in oxy-pnictide LaF eAsO 1−x F x , enormous interests have been stimulated in the fields of condensed matter physics and material sciences.Among the five different structures in this broad type of superconductors 2,3,4,5,6 , the ZrCuSiAs structure has received special attention since the T c has been quickly promoted to 55-56 K 7,8,9,10,11 in fluorine doped oxy-pnictides REFeAsO (RE = rare earth elements). The superconductivity can also be induced by applying a high pressure to the undoped samples 12,13 . The mechanism of superconductivity in the FeAs-based system remains unclear yet, but it turns out to be clear that any change to the structure or the building blocks will lead to a change of the superconducting transition temperatures. In this Letter, we report the fabrication of the new family of compounds, namely fluorine-arsenides DvFeAsF (Dv = divalent metals) with the ZrCuSiAs structure and with the new building block DvF instead of the REO (both the layers DvF and REO have the combined cation state of "+1"). The undoped parent phase has a Spin-Density-Wave like transition at about 173 K for SrFeAsF, 118 K for CaFeAsF and 153 K for EuFeAsF. By doping electrons into the system the resistivity anomaly associated with this SDW transition is suppressed and superconductivity appears at 32 K in the fluorine-arsenide Sr 1−x La x FeAsF (x = 0.4). Our discovery here initiates a new method to obtain superconductors in the FeAs-based system.

Band structure of SrFeAsF and CaFeAsF—the base phases of a new group of oxygen-free FeAs superconductors

“…As the first step, the total energy (E tot ) versus cell volume calculations were carried out to determine the equilibrium structural parameters for T and O phases of SrFeAsF; the calculated values are presented in Table 1 and are in reasonable agreement with the available experiments [8][9][10][11].…”

“…Quite recently, the new isostructural oxygen-free quaternary phases -AFeAsF (where Ln = Sr, Ca and Eu) have been synthesized [8][9][10][11][12] and some of their properties have been determined. In particular, for these compounds the strong temperature-dependent anomalies in the specific heat, electrical resistance and magnetic susceptibility (which were interpreted as spin density wave (SDW) anomalies) were observed [8][9][10][11][12]. Similar SDW anomalies are known to be an important prerequisite for high-T C superconductivity in oxygencontaining FeAs materials.…”

“…Moreover, critical transitions with T C ~4K in the Co-substituted SrFeAsF (SrFe 0.875 Co 0.125 AsF) [9], T C ~22K in the Co-substituted CaFeAsF (CaFe 0.9 Co 0.1 AsF) [12], and T C ~ 31K in the La-substituted SrFeAsF (Sr 0.8 La 0.2 FeAsF) were reported [11]. These results clearly show that AFeAsF phases can serve as prototype materials for a new group of oxygen -free FeAs SCs.…”

Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors

“…Finally for the system Sr 1−x La x FFeAs superconductivity with T c =36 K was obtained by Zhu et al [18]. Also Zhu et al [18] provide some crystallographic and resistivity data on CaFFeAs and EuFFeAs compounds.…”

Possible Kondo effect in the iron arsenides

“…This behavior has been recently suggested as due to the strong exchange interaction among the so-called "preformed SDW moments" [8] or to the peculiarity of a two dimensional Fermi liquid [9]. Another silent feature, perhaps of equal significance, is the resistivity upturn at low temperatures in some 1111 series with [1,2,3] or without oxygens [10]. This feature is hindered by the emergent SDW instability or superconductivity and is hidden in some ternary compounds (the 122 series) [11,12].…”