Since the first report on superconductivity at 26K in F-doped LaFeAsO at the end of February 2008 [1], the superconducting transition temperature has been quickly raised to about 55 K and several new superconductors of a general formula LnFeAsO_1-xF_x (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy) abbreviated as Ln1111 have been synthesized. Electrons carriers can be introduced by substituting F for O or by deficiency of O. By substituting Sr²⁺ for La³⁺ in La1111, holes are introduced. Later, superconductivity in AFe₂As₂ (A=Ca,Sr,Ba) (called A122) with maximum T_c = 38 K has been reported. They have a more simple structure in which (Fe₂As₂)-layers, identical to those in Ln1111 are separated by single elemental A layers.

There are some similarities between the new pnictades superconductors and the cuprate superconductors due to the layered structure and the fact that both Fe and Cu are 3d elements. However, there are important differences: doping on the Fe site in Sr122 or Ba122 by substitution of Fe by Co leads to the appearance of superconductivity in contrast to cuprates, where substitution of Cu suppresses superconductivity. For cuprates, doping with one oxygen atom is equivalent with two fluorine atoms doping. In FeAs-superconductors a O-deficiency is much less effective as a source of electrons than F-substitution.

Structural parameters play also an important role for obtaining highest T_c, which is achieved when As-Fe-As bond angle corresponds to ideal FeAs₄ tetrahedron.

Due to layered structure one can expect anisotropy of superconducting properties, which are, obviously, best investigated on single crystals. Single crystals are also required for spectroscopic investigations. We succeeded in the growth of the first free standing FeAs-pnictides crystals (SmFeAsO_1-xF_y) using a high-pressure cubic-anvil technique [2,3]. They have been grown using NaCl/KCl flux. Such flux has very low solubility, therefore for more efficient crystal growth very high temperature should be used, but Ln1111 becames unstable. The decomposition can be counteracted by applying high pressure, which stabilises Ln1111 at high temperature.

Critical current measurements on these crystals show significantly large J_c = 2x10⁵A/cm² at T=15 K and 7 T field. Having single crystals we were able to study magnetic penetration depth and upper critical field anisotropy, which in cuprates and low temperature superconductors are equal and constant. Torque magnetometer [4] and resistivity measurements of Ln1111 single crystals show, that these anisotropies are distinctly different and temperature dependent similar to MgB₂ single crystals.