Single crystals of LnFeAsO1−xFx (Ln = La, Pr, Nd, Sm, Gd) and Ba1−xRbxFe2As2: Growth, structure and superconducting properties

A review of our investigations on single crystals of LnFeAsO1−xFx (Ln = La, Pr, Nd, Sm, Gd) and Ba1−xRbxFe2As2 is presented. A high-pressure technique has been applied for the growth of LnFeAsO1−xFx crystals, while Ba1−xRbxFe2As2 crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O, F) position in superconducting LnFeAsO1−xFx crystals. Resistivity measurements on LnFeAsO1−xFx crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba1−xRbxFe2As2 simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 109 A/m2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO1−xFx crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba1−xRbxFe2As2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO1−xFx. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (∼20 meV).