Bulk Superconductivity at 14 K in Single Crystals of Fe1+yTexSe1-x

Resistivity, magnetic susceptibility and heat capacity measurements are reported for single crystals of Fe1+yTexSe1-x grown via a modified Bridgeman method with 0 < y < 0.15, and x= 1, 0.9, 0.75, 0. 67, 0.55 and 0.5. Although resistivity measurements show traces of superconductivity near 14 K for all x except x=1, only crystals grown with compositions near x=0.5 exhibit bulk superconductivity. The appearance of bulk superconductivity correlates with a reduction in the magnitude of the magnetic susceptibility at room temperature and smaller values of y, the concentration of Fe in the Fe(2) site.Comment: Submitted to Phys. Rev.

Phonon Density of States of LaFeAsO1-xFx

We have studied the phonon density of states (PDOS) in LaFeAsO1-xFx with inelastic neutron scattering methods. The PDOS of the parent compound(x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x~0.1). Good agreement is found between the experimental PDOS and first-principle calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity

Surface Geometric and Electronic Structure of BaFe2As2(001)

BaFe2As2 exhibits properties characteristic of the parent compounds of the newly discovered iron (Fe)-based high-TC superconductors. By combining the real space imaging of scanning tunneling microscopy/spectroscopy (STM/S) with momentum space quantitative Low Energy Electron Diffraction (LEED) we have identified the surface plane of cleaved BaFe2As2 crystals as the As terminated Fe-As layer - the plane where superconductivity occurs. LEED and STM/S data on the BaFe2As2(001) surface indicate an ordered arsenic (As) - terminated metallic surface without reconstruction or lattice distortion. It is surprising that the STM images the different Fe-As orbitals associated with the orthorhombic structure, not the As atoms in the surface plane.Comment: 12 pages, 4 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

Pressure effects on the electron-doped high Tc superconductor BaFe(2-x)Co(x)As(2)

Application of pressures or electron-doping through Co substitution into Fe sites transforms the itinerant antiferromagnet BaFe(2)As(2) into a superconductor with the Tc exceeding 20K. We carried out systematic transport measurements of BaFe(2-x)Co(x)As(2) superconductors in pressures up to 2.5GPa, and elucidate the interplay between the effects of electron-doping and pressures. For the underdoped sample with nominal composition x = 0.08, application of pressure strongly suppresses a magnetic instability while enhancing Tc by nearly a factor of two from 11K to 21K. In contrast, the optimally doped x=0.20 sample shows very little enhancement of Tc=22K under applied pressure. Our results strongly suggest that the proximity to a magnetic instability is the key to the mechanism of superconductivity in iron-pnictides.Comment: 5 figure

Superconductivity in Co-doped SmFeAsO

Here we report the synthesis and basic characterization of SmFe1-xCoxAsO (x=0.10, 0.15). The parent compound SmFeAsO itself is not superconducting but shows an antiferromagnetic order near 150 K, which must be suppressed by doping before superconductivity emerges. With Co-doping in the FeAs planes, antiferromagnetic order is destroyed and superconductivity occurs at 15 K. Similar to LaFe1-xCoxAsO, the SmFe1-xCoxAsO system appears to tolerate considerable disorder in the FeAs planes. This result is important, which indicates difference between cuprare superconductors and the iron-based arsenide ones.Comment: 11 pages, 3 figure

Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals

Point-contact Andreev reflection spectroscopy (PCARS) is applied to investigate the gap structure in iron pnictide single crystal superconductors of the AFe_2As_2 (A=Ba, Sr) family ("Fe-122"). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba_{0.6}K_{0.4})Fe_2As_2 and Ba(Fe_{0.9}Co_{0.1})_2As_2, and the other with a V^{2/3} background conductance universally observed extending even up to 100 meV for Sr_{0.6}Na_{0.4}Fe_2As_2 and Sr(Fe_{0.9}Co_{0.1})_2As_2. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe_2As_2. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba_{0.6}K_{0.4}Fe_2As_2, double peaks due to Andreev reflection with strongly-sloping background are frequently observed for point-contacts on freshly-cleaved c-axis surfaces. If normalized by a background baseline and analyzed by the Blonder-Tinkham-Klapwijk model, the data show a gap size ~3.0-4.0 meV with 2\Delta_0/k_BT_c ~ 2.0-2.6, consistent with the smaller gap size reported in the LnFeAsO family ("Fe-1111"). For the Ba(Fe_{0.9}Co_{0.1})_2As_2, G(V) curves typically display a zero-bias conductance peak.Comment: 12 pages, 6 figure

Crystal chemical simulation of superconductors on the basis of oxide and intermetallic layers

Simulation of 'hybrid' superconductors of 3d-, 4d- and 5d-transition elements consisting of two different superconducting fragments located between positively charged ions planes - B'O2 oxide planes and B2C2 intermetallic layers - has been performed on the basis of the structure of Sr2Mn3As2O2 (A2(B2C2)(B'O2)). The oxide planes are similar to those of CuO2 in high-temperature superconducting cuprates while the intermetallic layers - to those of Ni2B2 in low-temperature superconducting borocarbides RNi2B2C and Fe2As2 layers in high-temperature superconducting oxypnictides RFeAsO1-xFx.Comment: Title changed by the Editor of Supercond. Sci. Technol., published versio

Nernst effect of the new iron-based superconductor LaO1−x_{1-x}Fx_{x}FeAs

We report the first Nernst effect measurement on the new iron-based superconductor LaO$_{1-x}$F$_{x}$FeAs $(x=0.1)$. In the normal state, the Nernst signal is negative and very small. Below $T_{c}$ a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to conventional type-II superconductors. However, a clear deviation of the Nernst signal from normal state background and an anomalous depression of off-diagonal thermoelectric current in the normal state between $T_{c}$ and 50 K are observed. We propose that this anomaly in the normal state Nernst effect could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde