David G. Free

Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

The discovery of high-temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li(+), Na(+), K(+), Ba(2+)) or PbO- or perovskite-type oxide layers, and also in Fe(1.01)Se (ref. 8) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li(x)(NH(2))(y)(NH(3))(1-y)Fe(2)Se(2) (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.

Gradual destruction of magnetism in the superconducting family NaFe1−xCoxAs

The interplay and coexistence of superconducting, magnetic and structural order parameters in NaFe{1-x}Co{x}As has been studied using SQUID magnetometry, muon-spin rotation and synchrotron x-ray powder diffraction. Substituting Fe by Co weakens the ordered magnetic state through both a suppression of T_N and a reduction in the size of the ordered moment. Upon further substitution of Fe by Co the high sensitivity of the muon as a local magnetic probe reveals a magnetically disordered phase, in which the size of the moment continues to decrease and falls to zero around the same point at which the magnetically-driven structural distortion is no longer resolvable. Both magnetism and the structural distortion are weakened as the robust superconducting state is established.

Low-temperature nuclear and magnetic structures of La 2 O 2 Fe 2 O Se 2 from x-ray and neutron diffraction measurements

This paper describes the low temperature nuclear and magnetic structures of La2O2Se2.Fe2O by analysis of X-ray and neutron diffraction data. The material has been demonstrated to order antiferromagnetically at low temperatures, with TN \approx 90 K a propagation vector of k = (1/2 0 1/2), resulting in a spin arrangement similar to that in FeTe, despite there being no apparent lowering in symmetry of the nuclear structure.

Structural characterization and physical properties of the new transition metal oxyselenide La2O2ZnSe2.

The quaternary transition metal oxyselenide La(2)O(2)ZnSe(2) has been shown to adopt a ZrCuSiAs-related structure with Zn(2+) cations in a new ordered arrangement within the [ZnSe(2)](2-) layers. This cation-ordered structure can be derived and described using the symmetry-adapted distortion mode approach. La(2)O(2)ZnSe(2) is an direct gap semiconductor with an experimental optical band gap of 3.4(2) eV, consistent with electronic structure calculations.

Spin-reorientation transition in CeMnAsO.

High-resolution X-ray and neutron powder diffraction are used to reveal details of the spin-reorientation transition in the layered oxide pnictide CeMnAsO. Above 38 K, the localized moments on Mn(2+) are antiferromagnetically ordered in a checkerboard fashion within the antifluorite-type MnAs planes and are oriented perpendicular to the planes. Below 38 K, reorientation of these moments into the planes commences. This is complete by 34 K and is coincident with long-range ordering of the Ce(3+) moments. The Ce(3+) and Mn(2+) moments have an arrangement that is different in detail from that in the isostructural NdMnAsO and PrMnSbO. There is no evidence for structural distortion, as found for PrMnSbO and related Pr(3+)-containing compounds, although there is evidence for a very slight (0.025%) misfit between the magnetic and structural cells below the spin-reorientation transition. It is clarified that neutron powder diffraction methods are unable to distinguish between collinear and noncollinear arrangements of manganese and lanthanide moments when the moments have a component parallel to the MnAs planes. A proposal from computational analysis that NdMnAsO and CeMnAsO should adopt different magnetic structures on the basis of the different balances between biquadratic and antisymmetric exchange interactions should be tested using alternative methods.

Soft chemical control of the crystal and magnetic structure of a layered mixed valent manganite oxide sulfide

Oxidative deintercalation of copper ions from the sulfide layers of the layered mixed-valent manganite oxide sulfide Sr2MnO2 Cu 1.5S2 results in control of the copper-vacancy modulated superstructure and the ordered arrangement of magnetic moments carried by the manganese ions. This soft chemistry enables control of the structures and properties of these complex materials which complement mixed-valent perovskite and perovskite-related transition metal oxides.