Structural Changes and the Nature of Superconductivity in Rare-earth Doped CaFe2As2

Abstract

Chemical substitution into iron-pnictide parent compounds (e.g. AFe<sub>2<\sub>As<sub>2<\sub> where A=Ba, Sr, or Ca) has proven to be an effective means to induce bulk high-temperature superconductivity in these systems. By doping CaFe<sub>2<\sub>As<sub>2<\sub> with rare-earth lanthanides (La, Ce, Pr, and Nd), we have observed a 47 K superconducting phase coexisting with a lattice distorting &ldquo;collapse&rdquo; transition. Both of these effects have important ramifications: the collapse transition occurs when interlayer As atoms form a bond, shrinking the <italic>c-axis<\italic> lattice constant and simultaneously quenching the iron magnetic moment. This transition is further explored in context of a similar system, Sr-doped BaNi<sub>2<\sub>As<sub>2<\sub>. The superconducting phase, given the right combination of conditions, appears with a critical temperature as high as 49 K, but always in a very small volume of the sample (as determined by shielding effects). This has led to interesting theories about the nature of this superconductivity. A recently posited idea of &ldquo;interfacial superconductivity&rdquo; has been ruled out by our tests. Additionally, increasing the concentration of rare-earth atoms does not increase the superconducting volume fraction, but, in fact lowers the transition temperature, excluding the hypothesis that rare-earth defects are responsible for the minority superconducting phase. New pressure measurements have shown that the superconducting phase is stabilized when antiferromagnetic order is fully suppressed.