Flux pinning in (1111) iron-pnictide superconducting crystals

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-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 PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred 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-T 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.Comment: 12 pages, submitted to Phys Rev.

Thermal expansion and magnetostriction of pure and doped RAgSb2 (R = Y, Sm, La) single crystals

Data on temperature-dependent, anisotropic thermal expansion in pure and doped RAgSb2 (R = Y, Sm, La) single crystals are presented. Using the Ehrenfest relation and heat capacity measurements, uniaxial pressure derivatives for long range magnetic ordering and charge density wave transition temperatures are evaluated and compared with the results of the direct measurements under hydrostatic pressure. In-plane and c-axis pressure have opposite effect on the phase transitions in these materials, with in-plane effects being significantly weaker. Quantum oscillations in magnetostriction were observed for the three pure compounds, with the possible detection of new frequencies in SmAgSb2 and LaAgSb2. The uniaxial (along the c-axis) pressure derivatives of the dominant extreme orbits (beta) were evaluated for YAgSb2 and LaAgSb2

CD40-targeted adenoviral GM-CSF gene transfer enhances and prolongs the maturation of human CML-derived dendritic cells upon cytokine deprivation

Vaccination with autologous leukaemia-derived dendritic cells (DC) presents an adjuvant treatment option for chronic myeloid leukaemia (CML). Here, we show that high-efficiency CD40-targeted adenoviral gene transfer of GM-CSF to CML-derived DC induces long-lived maturation in the absence of exogenous cytokines and may thus ensure protracted stimulation of CML-specific T cells upon vaccination

Single crystal of superconducting SmFeAsO1-xFy grown at high pressure

Single crystals of SmFeAsO1-xFy of a size up to 120 micrometers 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 single crystal. Differential thermal analysis investigations at 1 bar Ar pressure show decomposition of SmFeAsO1-xFy at 1302 C.Comment: 12 pages, 3 tables, 6 figure

Evidence for Supercurrent Connectivity in Conglomerate Particles in NdFeAsO1-d

Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry.Comment: 11 pages, 4 figure

Pairing symmetry and properties of iron-based high temperature superconductors

Pairing symmetry is important to indentify the pairing mechanism. The analysis becomes particularly timely and important for the newly discovered iron-based multi-orbital superconductors. From group theory point of view we classified all pairing matrices (in the orbital space) that carry irreducible representations of the system. The quasiparticle gap falls into three categories: full, nodal and gapless. The nodal-gap states show conventional Volovik effect even for on-site pairing. The gapless states are odd in orbital space, have a negative superfluid density and are therefore unstable. In connection to experiments we proposed possible pairing states and implications for the pairing mechanism.Comment: 4 pages, 1 table, 2 figures, polished versio

Non-exponential London penetration depth in RFeAsO0.9_{0.9}F0.1_{0.1} (R=La,Nd) single crystals

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, $\Delta \lambda_L(T)\propto T^{2}$ at low temperatures. The same $T^2$ variation of $\lambda(T)$ was found in non-magnetic 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.Comment: v2 is the same as v1 - error in the arxiv submissio