Superconductivity in NdFe1-xCoxAsO (0.05 < x < 0.20) and rare-earth magnetic ordering in NdCoAsO

The phase diagram of NdFe1-xCoxAsO for low cobalt substitution consists of a superconducting dome (0.05 < x < 0.20) with a maximum critical temperature of 16.5(2) K for x = 0.12. The x = 1 end member, NdCoAsO, is an itinerant ferromagnet (TC = 85 K) with an ordered moment of 0.30(1) BM at 15 K. Below TN = 9 K, Nd spin-ordering results in the antiferromagnetic coupling of the existing ferromagnetic planes. Rietveld analysis reveals that the electronically important two-fold tetrahedral angle increases from 111.4 to 115.9 deg. in this series. Underdoped samples with x = 0.046(2) and x = 0.065(2) show distortions to the orthorhombic Cmma structure at 72(2) and 64(2) K, respectively. The temperature dependences of the critical fields Hc2(T) near Tc are linear with almost identical slopes of 2.3(1) T K-1 for x = 0.065(2), x = 0.118(2) and x = 0.172(2). The estimated critical field Hc2(0) and correlation length for optimally doped samples are 26(1) T and 36(1) Angstrom. A comparison of the maximum reported critical temperatures of well-characterized cobalt doped 122- and 1111-type superconductors is presented.Comment: accepted to PR

Διασώστες ΕΚΑΒ: Νομοθετικό πλαίσιο δράσης, πρακτική εφαρμογή του και σχετικές εκπαιδευτικές ανάγκες

Το επάγγελμα του Διασώστη – Πλήρωμα Ασθενοφόρου έχει ιδιαίτερα υψηλές απαιτήσεις, τόσο εκπαιδευτικές όσο σωματικές αλλά και ψυχικές. Η μεγάλη ζήτηση στην αγορά εργασίας τόσο στο δημόσιο όσο και στον ιδιωτικό τομέα την κάνει μια από της δημοφιλέστερες υγειονομικές ειδικότητες. Ένας υποψήφιος Διασώστης οφείλει διαμέσου του έργου του να συμβάλλει σημαντικά στην διάσωση αλλά και στην προστασία της ανθρώπινης ζωής. Μέσα από τις αρχές της προνοσοκομειακής φροντίδας, τις οποίες καλείται να εφαρμόσει, δύναται να αντιμετωπίσει καταστάσεις οι οποίες είναι απειλητικές για τη ζωή των πολιτών, να σταθεροποιεί την υγεία των ασθενών που βρίσκονται σε μια κρίσιμη κατάσταση, με απαιτούμενα - αναγκαίο αποτέλεσμα την αύξηση του προσδόκιμου ζωής. Οι γνώσεις οι οποίες λαμβάνουν οι Διασώστες κατά την αρχική τους εκπαίδευση, τους καθιστούν ικανούς, είτε να λειτουργούν αυτοβούλως, είτε υπό τις εντολές ιατρού, με σκοπό την εφαρμογή τεχνικών προνοσοκομειακής αντιμετώπισης τις οποίες και έχουν διδαχθεί προπτυχιακά. Ο επαγγελματίας διασώστης καλείται να διαχειριστεί απειλητικές για τη ζωή καταστάσεις με μόνο εφόδιο τις γνώσεις που έχει λάβει προπτυχιακά από το Ι.Ε.Κ. Μέχρι πρότινος δεν υπήρχε καμία πιθανότητα εκπαιδευτικής εξέλιξης των επαγγελματιών αυτών, αφού δεν υπήρχαν δυνατότητες για να αποκτήσει νέες πιο εξειδικευμένες γνώσεις αντιμετώπισης έκτακτων αναγκών μετά το πέρας των προπτυχιακών σπουδών τους.Profession of Rescuer - Ambulance Crew has particularly high demands, both educational and physical as well as mental. High demands in job market in both public and private sectors make it one of the most popular health specialties. A candidate Rescuer must, through his work, contribute significantly to rescue and protect human life. Through principles of pre-hospital care, which is called upon to apply, he can deal with life-threatening situations of citizens, stabilize health of patients who are in critical situation, with the necessary result of increasing life expectancy. Knowledge they receive during their initial training makes them capable, either to operate voluntarily, or under the orders of a doctor or by applying pre-hospital treatment techniques that they have been taught as undergraduates. Professional rescuer is called upon to manage life-threatening situations with only knowledge that he has received as an undergraduate from I.E.K. Until recently, there was no possibility for educational development of these professionals, since there were no possibilities to acquire new, more specialized knowledge to deal with emergency needs after the end of their undergraduate studies

Lattice parameter of polycrystalline diamond in the low-temperature range

The lattice parameter for polycrystalline diamond is determined as a function of temperature in the 4–300 K temperature range. In the range studied, the lattice parameter, expressed in angstrom units, of the studied sample increases according to the equation a = 3.566810(12) + 6.37(41) × 10−14T 4 (approximately, from 3.5668 to 3.5673 Å). This increase is larger than that earlier reported for pure single crystals. The observed dependence and the resulting thermal expansion coefficient are discussed on the basis of literature data reported for diamond single crystals and polycrystal

Spin-driven Phase Transitions in ZnCr2_2Se4_4 and ZnCr2_2S4_4 Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction

The crystal and magnetic structures of the spinel compounds ZnCr$_2$S$_4$ and ZnCr$_2$Se$_4$ were investigated by high resolution powder synchrotron and neutron diffraction. ZnCr$_2$Se$_4$ exhibits a first order phase transition at $T_N=21$ K into an incommensurate helical magnetic structure. Magnetic fluctuations above $T_N$ are coupled to the crystal lattice as manifested by negative thermal expansion. Both, the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion towards lower temperatures. Thereby, the spin ordering changes into a conical structure. ZnCr$_2$S$_4$ shows two magnetic transitions at $T_{N1}=15$ K and $T_{N2}=8$ K that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-type (space group $Fd$\={3}$m$) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group $I4_1$ / $amd$) for $T_{N2} < T < T_{N1}$ into a low temperature orthorhombic phase (space group $I m m a$) for $T < T_{N2}$. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low temperature structure of ZnCr$_2$S$_4$ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5 T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ZnCr$_2$Se$_4$.Comment: 11 pages, 13 figures, to be published in PR

Lattice anomalies in the FeAs4_{\rm 4} tetrahedra of the NdFeAsO0.85_{\rm 0.85} superconductor that disappear at Tc_{\rm c}

High resolution synchrotron X-ray powder diffraction (SXRPD) was used to study the temperature dependence of the oxygen deficient NdFeAsO$_{0.85}$ superconducting compound. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around $\sim$180K, and disappear at the transition temperature. The data show minor changes of the structural characteristics in the Nd-O charge reservoir layer while in the superconducting Fe-As layer the FeAs$_{4}$ tetrahedron shows gradual modifications below $\sim$180K, which suddenly disappear at T$_{\rm c}$ strongly indicating a connection with superconductivity.Comment: 4 pages, 1 table, 5 figure

Structure Determination Feasibility of Three-Dimensional Electron Diffraction in Case of Limited Data

During the last two decades, three-dimensional electron diffraction (3D ED) has undergone a renaissance, starting with the introduction of precession (Precession Electron Diffraction Tomography, PEDT) that led to variations on the idea of collecting as much of the diffraction space as possible in order to solve crystal structures from sub-micron sized crystals. The most popular of these acquisition methods is based on the continuous tilting/rotation of the crystal (so-called Microcrystal Electron Diffraction, MicroED) akin to the oscillating crystal method in X-ray crystallography, which was enabled by the increase of sensitivity and acquisition speed in electron detectors. While 3D ED data is more complex than the equivalent X-ray data due to the higher proportion of dynamical scattering, the same basic principles of what is required in terms of data quality and quantity in order to solve a crystal structure apply; high completeness, high data resolution and good signal-to-noise statistics on measured reflection intensities. However, it may not always be possible to collect data in these optimum conditions, the most common limitations being the tilt range of the goniometer stage, often due to a small pole piece gap or the use of a non-tomography holder, or the position of the sample on the TEM grid, which may be too close to a grid bar and then the specimen of interest becomes occluded during tilting. Other factors that can limit the quality of the acquired data include the limited dynamic range of the detector, which can result on truncated intensities, or the sensitivity of the crystal to the electron beam, whereby the crystallinity of the particle is changing under the illumination of the beam. This limits the quality and quantity of the measured intensities and makes structure analysis of such data challenging. Under these circumstances, traditional approaches may fail to elucidate crystal structures, and global optimization methods may be used here as an alternative powerful tool. In this context, this work presents a systematic study on the application of a global optimization method to crystal structure determination from 3D ED data. The results are compared with known structure models and crystal phases obtained from traditional ab initio structure solution methods demonstrating how this strategy can be reliably applied to the analysis of partially complete 3D ED data

Novel characterization techniques for cultural heritage using a TEM orientation imaging in combination with 3D precession diffraction tomography: a case study of green and white ancient Roman glass tesserae

We present new transmission electron microscopy (TEM) based electron diffraction characterization techniques (orientation imaging combined with 3D precession electron diffraction tomography-ADT) applied on cultural heritage materials. We have determined precisely unit cell parameters, crystal symmetry, atomic structure, and orientation/phase mapping of various pigment/opacifier crystallites at nm scale which are present in green and white Roman glass tesserae. Such TEM techniques can be an alternative to Synchrotron based techniques, and allow to distinguish accurately at nm scale between different crystal structures even in cases of same/very close chemical composition, where is also possible to visualize between different crystal orientations and amorphous/crystalline phases. This study additionally demonstrates that although opacifiers in green and white tesserae are found to have average Pb2Sb2O7 cubic and CaSb2O6 trigonal structures, their pyrochlore related framework can host many other elements like Cu, Ca, Fe through ionic exchanges at high firing temperatures which in turn may also contribute to the tesserae colour appearance

Features of the secondary structure of a protein molecule from powder diffraction data

Protein powder diffraction is shown to be suitable for obtaining de novo solutions to the phase problem at low resolution via phasing methods such as the isomorphous replacement method. Two heavy-atom derivatives (a gadolinium derivative and a holmium derivative) of the tetragonal form of hen egg-white lysozyme were crystallized at room temperature. Using synchrotron radiation, high-quality powder patterns were collected in which pH-induced anisotropic lattice-parameter changes were exploited in order to reduce the challenging and powder-specific problem of overlapping reflections. The phasing power of two heavyatom derivatives in a multiple isomorphous replacement analysis enabled molecular structural information to be obtained up to approximately 5.3 A ˚ resolution. At such a resolution, features of the secondary structure of the lysozyme molecule can be accurately located using programs dedicated to that effect. In addition, the quoted resolution is sufficient to determine the correct hand of the heavy-atom substructure which leads to an electron-density map representing the protein molecule of proper chirality