Single crystals of NdFe1-xCoxAsO with x = 0, 0.025, 0.05, 0.075, 0.10 and 0.15 were grown via high pressure synthesis. Structural and elemental analysis showed successful substitution of Co for Fe with no detectable change in the oxygen site occupation. Magnetic, electrical transport and tunnel-diode resonator measurements were used to characterize the NdFe1-xCoxAsO crystals. These measurements indicated that superconductivity was achieved with x >= 0.025 and optimal doping was established with x = 0.05 with the maximum superconducting critical temperature of 25 K. Measurement of temperature dependent resistance in applied magnetic fields as high as 14 T showed a broadening of the superconducting transition and an H-c2(T) curve that was consistent with those of other iron pnictide superconductors.

The in-plane magnetic penetration depth, λ(T), has been measured in single crystals of NdFe1−xCoxAsO, NdFeAsO1−xFx, and LaFeAsO1−xFx. In the Nd compounds we found an upturn in λ(T) below 4K and attribute it to the paramagnetic contribution from the Nd3+ ions. After the correction for this contribution, the in-plane temperature dependent London penetration depth, λL(T), in F-doped crystals was found to follow a power-law dependence, ΔλL(T)∝Tn, n≈2. Similar power-law behavior was found in direct measurements of non-magnetic LaFeAsO1−xFx with n≈2. In NdFe1−xCoxAsO, on the other hand, after the paramagnetic correction, we found n≈3. Our results indicate unconventional pairing in the pnictides, incompatible with a single isotropic gap or a gap with line nodes. The power-law behavior is observed down to T/Tc=0.02 indicating significant gap anisotropy.

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free-path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations in the dopant atom density on the scale of a few dozen to 100 nm. These variations do not go beyond 5%-we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-tesla fields is characterized by the presence of a peak effect, the location of which in the (B,T) plane is consistent with an order-disorder transition of the vortex lattice.

In- and out-of-plane magnetic penetration depths were measured in three iron-based pnictide superconducting systems. All studied samples of both 122 systems show a robust power-law behavior, $\lambda (T) T^n$, with the sample-dependent exponent n=2-2.5, which is indicative of unconventional pairing. This scenario could be possible either through scattering in a $s_{\pm }$ state or due to nodes in the superconducting gap. In the Nd-1111 system, the interpretation of data may be obscured by the magnetism of rare-earth ions. The overall anisotropy of the pnictide superconductors is small. The 1111 system is about two times more anisotropic than the 122 system. Our data and analysis suggest that the iron-based pnictides are complex superconductors in which a multiband three-dimensional electronic structure and strong magnetic fluctuations play important roles.

We present an overview of the electronic properties of iron arsenic high temperature superconductors with emphasis on low energy band dispersion, Fermi surface and superconducting gap. ARPES data is compared with full-potential linearized plane wave (FLAPW) calculations. We focus on single layer NdFeAsO0.9F0.1 (R1111) and two layer Ba1−xKxFe2As2 (B122) compounds. We find general similarities between experimental data and calculations in terms of character of Fermi surface pockets, and overall band dispersion. We also find a number of differences in details of the shape and size of the Fermi surfaces as well as the exact energy location of the bands, which indicate that magnetic interaction and ordering significantly affects the electronic properties of these materials. The Fermi surface consists of several hole pockets centered at Γ and electron pockets located in zone corners. The size and shape of the Fermi surface changes significantly with doping. Emergence of a coherent peak below the critical temperature Tc and diminished spectral weight at the chemical potential above Tc closely resembles the spectral characteristics of the cuprates, however the nodeless superconducting gap clearly excludes the possibility of d-wave order parameter. Instead it points to s-wave or extended s-wave symmetry of the order parameter.

Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO0.9F0.1 (NFAOF). Individual crystallites up to similar to 200 x 100 x 30 mu m(3) in size could be mapped at various temperatures. The in-grain, persistent current density is about j similar to 10(5) A cm(-2) and the magnetic relaxation rate in a remanent state peaks at about T-m similar to 38 K. By comparison with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO0.9F0.1 is similar to high-T-c cuprates, in between Bi2Sr2CaCu2O8+x and YBa2Cu3O7-x in terms of the vortex response in the mixed state. We find an apparent crossover in the pinning strength at similar to 38 K that could be associated either with vortex decoupling or melting. Below this temperature the static and dynamic vortex behavior is consistent with the collective pinning and creep.

The superconducting penetration depth lambda(T) has been measured in RFeAsO(0.9)F(0.1) (R = La, Nd) single crystals (R-1111). In Nd-1111, we find an upturn in lambda(T) upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, Deltalambda_{L}(T) proportional, variantT;{2} at low temperatures. The same T2 variation of lambda(T) was found in nonmagnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe(2)As(2) (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

Systematic studies of the NdFeAsOF superconducting energy gap using point-contact Andreev-reflection (PCAR) spectroscopy are presented. At low temperatures the PCAR conductance spectra show a pair of gap-like peaks at about ± (4–7) mV and in most cases also a pair of humps at around ± 10 mV. Fits to the s-wave two-gap model of the PCAR conductance allowed to determine two superconducting energy gaps in the system. However, the energy-gap features disappear at T* = 15–20 K, much below the particular Tc of the junction under study. At T* a zero-bias conductance (ZBC) peak emerges, which at higher temperatures usually overwhelms the spectrum with an intensity significantly higher than the conductance signal at lower temperatures. Possible causes of this unexpected temperature effect are discussed. In some cases the conductance spectra show just a reduced conductance around the zero-bias voltage, the effect persisting well above the bulk transition temperature. This indicates the presence of a pseudogap in the system.

Systematic studies of the NdFeAsOF superconducting energy gap via the point-contact Andreev-reflection (PCAR) spectroscopy are presented. The PCAR conductance spectra show at low temperatures a pair of gap-like peaks at about 4 - 7 mV indicating the superconducting energy gap and in most cases also a pair of humps at around 10 mV. Fits to the s-wave two-gap model of the PCAR conductance allowed to determine two superconducting energy gaps in the system. The energy-gap features however disappear already at T* = 15 to 20 K, much below the particular Tc of the junction under study. At T* a zero-bias conductance (ZBC) peak emerges, which at higher temperatures usually overwhelms the spectrum with intensity significantly higher than the conductance signal at lower temperatures. Possible causes of this unexpected temperature effect are discussed. In some cases the conductance spectra show just a reduced conductance around the zero-bias voltage, the effect persisting well above the bulk transition temperature. This indicates a presence of the pseudogap in the system.

at bands located just below the chemical potential. We observe a superconducting gap of 20 meV, which indicates that this system is in the strong coupling regime. The emergence of a coherent peak below the critical temperature T c and diminished spectral weight at the chemical potential above T c closely resembles the spectral characteristics of the cuprates.

Single crystalline samples of Ba(Fe1-xCox)(2)As-2 with x < 0.12 have been grown and characterized via microscopic, thermodynamic, and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high-temperature tetragonal to low-temperature orthorhombic) and magnetic (high-temperature nonmagnetic to low-temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K/% Co. In addition, for x >= 0.038 superconductivity is stabilized, rising to a maximum T-c of approximately 23 K for x approximate to 0.07 and decreasing for higher x values. The T-x phase diagram for Ba(Fe1-xCox)(2)As-2 indicates that either superconductivity can exist in both low-temperature crystallographic phases or that there is a structural phase separation. Anisotropic superconducting upper critical-field data [H-c2(T)] show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.