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).

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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.

The in-plane magnetic penetration depth ab of the iron selenide superconductor with the nominal composition FeSe 0.85 was studied by means of muon-spin rotation. The measurements of ab −2 ͑T͒ are inconsistent with a simple isotropic s-wave type of the order parameter but are rather in favor of two-gap ͑s + s͒ and anisotropic s-wave order parameter symmetries, thus implying that the superconducting energy gap in FeSe 0.85 contains no nodes.

The superconducting and magnetic properties of Fe y Se 0.25 Te 0.75 single crystals ͑0.9Յ y Յ 1.1͒ were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y Ͻ 1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y Ն 1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe y Se 1−x Te x can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.

We use femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in a nearly optimally doped pnictide superconductor with T{c}=49.5 K. Multiple relaxation processes are evident, with distinct superconducting state quasiparticle recombination dynamics exhibiting a T-dependent superconducting gap, and a clear "pseudogaplike" feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude Delta{PG}=61+/-9 meV above T{c}. Both the superconducting and pseudogap components show saturation as a function of fluence with distinct saturation fluences 4 and 40 microJ/cm{2}, respectively.

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.