Ferromagnetism and nonmetallic transport of thin-film α-FeSi(2): a stabilized metastable material

A metastable phase α-FeSi_{2} was epitaxially stabilized on a silicon substrate using pulsed laser deposition. Nonmetallic and ferromagnetic behaviors are tailored on α-FeSi_{2} (111) thin films, while the bulk material of α-FeSi_{2} is metallic and nonmagnetic. The transport property of the films renders two different conducting states with a strong crossover at 50 K, which is accompanied by the onset of a ferromagnetic transition as well as a substantial magnetoresistance. These experimental results are discussed in terms of the unusual electronic structure of α-FeSi_{2} obtained within density functional calculations and Boltzmann transport calculations with and without strain. Our finding sheds light on achieving ferromagnetic semiconductors through both their structure and doping tailoring, and provides an example of a tailored material with rich functionalities for both basic research and practical applications.

Phase transitions and thermal-stress-induced structural changes in a ferroelectric Pb(Zr0.80Ti0.20)O3 single crystal

A single crystal of lead-zirconate-titanate, composition Pb(Zr0.80Ti0.20)O3, was studied by polarized-Raman scattering as a function of temperature. Raman spectra reveal that the local structure deviates from the average structure in both ferroelectric and paraelectric phases. We show that the crystal possesses several, inequivalent complex domain boundaries which show no sign of instability even 200 K above the ferroelectric-to-paraelectric phase transition temperature TC. Two types of boundaries are addressed. The first boundary was formed between ferroelectric domains below TC. This boundary remained stable up to the highest measurement temperatures, and stabilized the domains so that they had the same orientation after repeated heating and cooling cycles. These domains transformed normally to the cubic paraelectric phase. Another type of boundary was formed at 673 K and exhibited no signs of instability up to 923 K. The boundary formation was reversible: it formed and vanished between 573 and 673 K during heating and cooling, respectively. A model in which the crystal is divided into thin slices with different Zr/Ti ratios is proposed. The physical mechanism behind the thermal-stress-induced structural changes is related to the different thermal expansion of the slices, which forces the domain to grow similarly after each heating and cooling cycle. The results are interesting for non-volatile memory development, as it implies that the original ferroelectric state can be restored after the material has been transformed to the paraelectric phase. It also suggests that a low-symmetry structure, stable up to high temperatures, can be prepared through controlled deposition of layers with desired compositions.

Characterisation of the anticoagulant properties of a range of structurally diverse sulfated oligosaccharides

In this study, 17 sulfated oligosaccharides were assessed by the activated partial thromboplastin time (APTT) test for their anticoagulant activity and nine were found to exhibit significant activity. Chain length, monosaccharide makeup, and linkage all appear to be critical factors in determining anticoagulant activity, with the most active compounds being five- to sixfold less potent than unfractionated heparin (UFH). Phosphomannopentaose sulfate (PI-88), one of the most active sulfated oligosaccharides and a promising anticancer drug, was selected for further study. PI-88 gave a more linear APTT dose-response curve and displayed less patient-to-patient variation than UFH, with its activity being neutralised by protamine sulfate. However, PI-88 showed considerable species-to-species variation in its anticoagulant effect. It was found that PI-88 acted as an anticoagulant by enhancing the ability of heparin cofactor II (HCII) to inhibit thrombin, and did not act via antithrombin III (AT-III) in either inhibiting Factor Xa or thrombin. PI-88 also mildly prolonged the prothrombin time (PT), whilst it had no platelet pro-aggregatory activity, nor did it demonstrate direct fibrinolytic activity. Thus, PI-88 represents a potential antithrombotic agent deserving further study.