The growth of carbon-substituted magnesium diboride Mg͑B 1−x C x ͒ 2 single crystals with 0 ഛ x ഛ 0.15 is reported, and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, T c of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field H c2 is observed. Comparing with the nonsubstituted compound, H c2 at 15 K for x = 0.05 is enhanced by more than a factor of 2 for H oriented both perpendicular and parallel to the ab plane. This enhancement is accompanied by a reduction of the H c2 -anisotropy coefficient ␥ from 4.5 ͑for the nonsubstituted compound͒ to 3.4 and 2.8 for the crystals with x = 0.05 and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows H c2 ͑defined at zero resistance͒ higher than 7 and 24 T for H oriented perpendicular and parallel to the ab plane, respectively. Observed increase of H c2 cannot be explained by the change in the coherence length due to the disorder-induced decrease of the mean free path only.

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

With the discovery of new superconducting materials, such as the iron pnictides, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature T(c) is high, intrinsic electronic properties might render applications difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates. Although many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed and estimates point to a very high upper critical field, their application potential is less clear. Thus, we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65 T. Our detailed study of the transport properties of SmFeAsO(0.7)F(0.25) single crystals reveals a promising combination of high (>2 x 10(6) A cm(-2)) and nearly isotropic critical current densities along all crystal directions. This favourable intragrain current transport in SmFeAs(O, F), which shows the highest T(c) of 54 K at ambient pressure, is a crucial requirement for possible applications. Essential in these experiments are four-probe measurements on focused-ion-beam-cut single crystals with a sub-square-micrometre cross-section, with current along and perpendicular to the crystallographic c axis.

Single crystals of Mg 1-x Al x B 2 have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with x < 0.1 were obtained as a result of optimization of already developed MgB 2 crystal growth procedure. Systematic decrease of the c-axis lattice constant with increasing Al content, when the a-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in Mg 1-x Al x B 2 crystals was found to be slightly different than that one observed for polycrystalline samples since, even a very small substitution of 1-2% of Al leads to the decrease of T c by about 2-3 K. X-ray and high resolution transmission electron microscopy investigations indicate on the appearance of second precipitation phase in the crystals with x > 0.1. This is in a form of non-superconducting MgAlB 4 domains in the structure of superconducting Mg 1-x Al x B 2 matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, H c2 , for H//c for the samples with small x, significant reduction of the H c2 anisotropy at lower temperatures, and decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with T c in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing T c down to 20 K. It may be related to an appearance of the precipitation phase in the Mg 1-x Al x B 2 structure. 74.70.Ad, 74.62.Dh, 81.10.-h, 74.25.Ha

Parent compounds AFe 2 As 2 (A = Ca, Ba, Eu) of the 122 family of the iron-based superconductors have been studied by 57 Fe Mössbauer spectroscopy in the temperature range 4.2-∼300 K. Spin density waves (SDW) have been found with some confidence. They are either incommensurate with the lattice period or the ratio of the respective periods is far away from the ratio of small integers. SDW shape is very unconventional (i.e., differs from the sinusoidal shape). Magnetic order starts with lowered temperature as narrow sheets of the significant electron spin density separated by areas with very small spin density. Magnetic sheets are likely to be ordered in the alternate antiferromagnetic fashion as the material as a whole behaves similarly to the collinear antiferromagnet. A further lowering of temperature simply expands sheet thickness leading to the near triangular SDW. Finally, sheets fill the whole available space and the almost rectangular shape of the SDW is reached. The substantial maximum amplitude of SDW appears at the temperature just below the magnetic onset temperature, and this maximum amplitude increases slightly with lowering temperature. The square root from the mean squared hyperfine field behaves versus temperature according to the universality class (1,2) (i.e., with the electronic spin space having dimensionality equal to unity and the real space having dimensionality equal to 2). The more or less pronounced tail above transition temperature due to the development of incoherent SDW is seen.