The unusually high T_c in rare-earth-doped single crystalline CaFe₂As₂

Abstract

In rare-earth-doped single crystalline CaFe₂As₂, the mysterious small volume fraction which superconducts up to 49 K, much higher than the bulk T_c ~ 30 s K, has prompted a long search for a hidden variable that could enhance the T_c by more than 30% in iron-based superconductors of the same structure. Here we report a chemical, structural and magnetic study of CaFe₂As₂ systematically doped with La, Ce, Pr and Nd. Coincident with the high T_c phase, we find extreme magnetic anisotropy, accompanied by an unexpected doping-independent T_c and equally unexpected superparamagnetic clusters associated with As vacancies. These observations lead us to conjecture that the tantalizing T_c enhancement may be associated with naturally occurring chemical interfaces and may thus provide a new paradigm in the search for superconductors with higher T_c.

Keywords:

• Fe-pnictides
• interfacial superconductivity
• magnetic anisotropy
• superparamagnetic clusters

Acknowledgement

The helpful discussion with J.E. Hoffman of Harvard University during the later part of this work is greatly appreciated.

Funding

The work in Houston is supported in part by US Air Force Office of Scientific Research [grant number FA9550-09-1-0656]; US Air Force Research Laboratory [Subcontract R15901 (CONTACT)] through Rice University; the T. L. L. Temple Foundation; the John J. and Rebecca Moores Endowment and the State of Texas through the Texas Center for Superconductivity at the University of Houston.