Philip J. W. Moll

High magnetic-field scales and critical currents in SmFeAs(O, F) crystals

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.

High-pressure flux growth, structural, and superconducting properties ofLnFeAsO (Ln=Pr, Nd, Sm) single crystals

Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high pressure. The crystals are of good structural quality and become superconducting when O is partially substituted by F (PrFeAsO1−xFx and NdFeAsO1−xFx) or when Fe is substituted by Co (SmFe1−xCoxAsO). From magnetization measurements, we estimate the temperature dependence and anisotropy of the upper critical field and the critical current density of underdoped PrFeAsO0.7F0.3 crystal with Tc ≈ 25 K. Single crystals of SmFe1−xCoxAsO with maximal Tc up to 16.3 K for x ≈ 0.08 were grown. From transport and magnetic measurements, we estimate the critical fields and their anisotropy and find these superconducting properties to be quite comparable to the ones in SmFeAsO1−xFx with a much higher Tc ≈ 50 K. The magnetically measured critical current densities are as high as 109 A/m2 at 2 K up to 7 T, with indication of the usual fishtail effect. The upper critical field estimated from resistivity measurements is anisotropic with slopes of ∼−8.7 T/K (H||ab plane) and ∼−1.7 T/K (H||c axis). This anisotropy (∼5) is similar to that in other Ln1111 crystals with various higher Tcs.

Th-substituted SmFeAsO: Structural details and superconductivity with Tc above 50 K

We report structural, magnetic, and transport properties of polycrystalline samples and single crystals of superconducting Sm1−xThxFeAsO with maximal Tc above 50 K, prepared under high pressure. Bulk superconducting samples do not undergo a structural phase transition from tetragonal to orthorhombic symmetry at low temperatures. The unit-cell parameters a and c shrink with Th substitution and the fractional atomic coordinate of the As site zAs remains almost unchanged while that of Sm/Th zSm/Th increases. Upon warming from 5 to 295 K the increase in the FeAs layer thickness is dominant, while the changes in the other structural building blocks are minor, and they compensate each other, since the As-Sm/Th distance contracts by about the same amount as the O-Sm/Th expands. The polycrystalline and single-crystalline samples are characterized by a full diamagnetic response in low magnetic field, by a high intergrain critical current density for polycrystalline samples, and by a critical current density on the order of 8×105 A/cm2 for single crystals at 2 K in fields up to 7 T. The magnetic penetration depth anisotropy γλ increases with decreasing temperature, in a similar way to that of SmFeAsO1−xFy single crystals. The upper critical field estimated from resistance measurements is anisotropic with slopes of ∼5.4 T/K (H∥ab plane) and ∼2.7 T/K (H∥c axis), at temperatures sufficiently far below Tc. The low-temperature upper critical field anisotropy γH is in the range of ∼2, consistent with the tendency of a decreasing γH with decreasing temperature, previously reported for SmFeAsO1−xFy single crystals. © 2010 The American Physical Society

Superconductivity at 23 K and low anisotropy in Rb-substituted BaFe2As2 single crystals

Single crystals of

Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors

{\text{Ba}}_{1\ensuremath{-}x}{\text{Rb}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}

Scaling between magnetic field and temperature in the high-temperature superconductor BaFe2(As1-xPx)2

with

Interplay between magnetism and superconductivity in EuFe2−xCoxAs2studied by57Fe and151Eu Mössbauer spectroscopy

x=0.05\ensuremath{-}0.1

Intrinsic Josephson junctions in the iron-based multi-band superconductor (V2Sr4O6)Fe2As2

have been grown from Sn flux and are bulk superconductors with

Field-induced density wave in the heavy-fermion compound CeRhIn5

{T}_{c}

Tetrathiofulvalene and tetracyanoquinodimethane crystals: Conducting surface versus interface

up to 23 K. The crystal structure was determined by x-ray diffraction analysis, and Sn is found to be incorporated for