The anisotropy gamma of the superconducting state of high quality single crystals of MgB2 was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from gamma approximately 6 at 15 K to 2.8 at 35 K. Reversible torque data near T(c) reveal a field dependent anisotropy, increasing nearly linearly from gamma approximately equal to 2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by nonlocal effects of anisotropic pairing and/or the k--> dependence of the effective mass tensor.

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.

Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of Tc14.6 K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal x-ray and neutron powder diffraction, superconducting quantum interference device and torque magnetometry, and muon-spin rotation (SR). Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe1.045Se0.406Te0.594. Additionally, a minor hexagonal Fe7Se8 impurity phase was identified. The magnetic penetration depth at zero temperature obtained by means of SR was found to be ab(0)=491 (8) PREPRINT (April 7, 2010) Anisotropic Iron-chalcogenide single crystals with the nominal composition FeSe0.5Te0.5 and a transition temperature of Tc 14.6 K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe1.045Se0.406Te0.594. Additionally, a minor hexagonal Fe7Se8 impurity phase was identified. The magnetic penetration depth λ at zero temperature was found to be λ ab (0) = 491(8) nm in the ab-plane and λc(0) = 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density ρs(0) ∝ λ −2 (0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and ironpnictides. The temperature dependences of both λ ab and λc are well described by a two-gap s+s-wave model with the zero-temperature gap values of ∆S(0) = 0.51(3) meV and ∆L(0) = 2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter γ λ (T ) = λc(T )/λ ab (T ) increases with decreasing temperature, in agreement with γ λ (T ) observed in the iron-pnictide superconductors.superconducting properties of single-crystalline FeSe 0.5 Te 0.

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).

Single crystals of SmFeAsO1−xFy of a size up to 120 × 100 µm2 have been grown from NaCl/KCl flux at a pressure of 30 kbar and temperature of 1350-1450 °C using the cubic anvil high-pressure technique. The superconducting transition temperature of the obtained single crystals varies between 45 and 53 K. Obtained crystals are characterized by a full diamagnetic response in low magnetic fields and by a high critical current density in high magnetic fields. Structural refinement has been performed on the single crystal. Differential thermal analysis investigations at 1 bar Ar pressure show decomposition of SmFeAsO1−xFy at 1302 °C.Abstract. Single crystals of SmFeAsO 1-x F y of a size up to 120x100 μm 2 have been grown from NaCl/KCl flux at pressure of 30 kbar and temperature of 1350-1450 °C using cubic anvil high-pressure technique. Superconducting transition temperature of the obtained single crystals varies between 45 and 53 K. Obtained crystals are characterized by full diamagnetic response in low magnetic field and by high critical current density in high magnetic field. Structure refinement has been performed on single crystal. Differential thermal analysis investigations at 1 bar Ar pressure show decomposition of SmFeAsO 1-x F y at 1302 °C.

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 the oxypnictide superconductor SmFeAsO 0.8 F 0.2 with T c ≃ 45(1) K were investigated by torque magnetometry. The crystals of mass ≤ 0.1 µg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ ≃ 8 at T T c to γ ≃ 23 at T ≃ 0.4T c . This unusual behavior of γ signals unconventional superconductivity.

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