Single crystals of LnFeAsO 1−xF x (Ln = La, Pr, Nd, Sm, Gd) and Ba 1−xRb xFe 2As 2: Growth, structure and superconducting properties

Abstract

A review of our investigations on single crystals of LnFeAsO 1− x F x (Ln = La, Pr, Nd, Sm, Gd) and Ba 1− x Rb x Fe 2As 2 is presented. A high-pressure technique has been applied for the growth of LnFeAsO 1− x F x crystals, while Ba 1− x Rb x Fe 2As 2 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 LnFeAsO 1− x F x crystals. Resistivity measurements on LnFeAsO 1− x F x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba 1− x Rb x Fe 2As 2 simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2 × 10 9 A/m 2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO 1− x F x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba 1− x Rb x Fe 2As 2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO 1− x F x . Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (∼20 meV).