High temperature behavior of Sr-doped layered cobaltites Y(Ba1-xSrx)Co2O5.5: phase stability and structural properties

In this article we present a neutron diffraction in-situ study of the thermal evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2 O5+{\delta}. Neutron thermodiffractograms and magnetic susceptibility measurements are reported in the temperature range 20 K <= T <= 570 K, as well as high resolution neutron diffraction experiments at selected temperatures. Starting from the as-synthesized samples with {\delta} ~ 0.5, we show that the room temperature phases remain stable up to 550 K, where they start loosing oxygen and transform to a vacancy-disordered "112" structure with tetragonal symmetry. Our results also show how the so-called "122" structure can be stabilized at high temperature (around 450 K) in a sample in which the addition of Sr at the Ba site had suppressed its formation. In addition, we present the structural and magnetic properties of the resulting samples with a new oxygen content {\delta} ~ 0.25 in the temperature range 20 K <= T <= 300 K

Probing phase coexistence and stabilization of the spin-ordered ferrimagnetic state by Calcium addition in the YBa_{1-x}Ca_{x}Co_{2}O_{5.5} layered cobaltites using neutron diffraction

In this article we study the effects of a partial substitution of Ba with the smaller cation Ca in the layered cobaltites YBaCo_2O_{5+\delta} for \delta \approx 0.5. Neutron thermodiffractograms are reported for the compounds YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5} (x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high resolution neutron diffraction experiments at selected temperatures for the samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have found the magnetic properties to be strongly affected by the cationic substitution. Although the "122" perovskite structure seems unaffected by Ca addition, the magnetic arrangements of Co ions are drastically modified: the antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic phase with spin state order is stabilized below T \sim 290 K. For the sample with x_{Ca}=0.05 a fraction of AFM phase coexists with the ferrimagnetic one below T \sim 190 K, whereas for x_{Ca}=0.10 the AFM order is completely lost. The systematic refinement of the whole series has allowed for a better understanding of the observed low-temperature diffraction patterns of the parent compound, YBaCo_2O_{5.5}, which had not yet been clarified. A two-phase scenario is proposed for the x_{Ca}=0 compound which is compatible with the phase coexistence observed in the x_{Ca}=0.05 sample

New four-dimensional integrals by Mellin-Barnes transform

This paper is devoted to the calculation by Mellin-Barnes transform of a especial class of integrals. It contains double integrals in the position space in d = 4-2e dimensions, where e is parameter of dimensional regularization. These integrals contribute to the effective action of the N = 4 supersymmetric Yang-Mills theory. The integrand is a fraction in which the numerator is a logarithm of ratio of spacetime intervals, and the denominator is the product of powers of spacetime intervals. According to the method developed in the previous papers, in order to make use of the uniqueness technique for one of two integrations, we shift exponents in powers in the denominator of integrands by some multiples of e. As the next step, the second integration in the position space is done by Mellin-Barnes transform. For normalizing procedure, we reproduce first the known result obtained earlier by Gegenbauer polynomial technique. Then, we make another shift of exponents in powers in the denominator to create the logarithm in the numerator as the derivative with respect to the shift parameter delta. We show that the technique of work with the contour of the integral modified in this way by using Mellin-Barnes transform repeats the technique of work with the contour of the integral without such a modification. In particular, all the operations with a shift of contour of integration over complex variables of two-fold Mellin-Barnes transform are the same as before the delta modification of indices, and even the poles of residues coincide. This confirms the observation made in the previous papers that in the position space all the Green function of N = 4 supersymmetric Yang-Mills theory can be expressed in terms of UD functions.Comment: Talk at El Congreso de Matematica Capricornio, COMCA 2009, Antofagasta, Chile and at DMFA seminar, UCSC, Concepcion, Chile, 24 pages; revised version, Introduction is modified, Conclusion is added, five Appendices are added, Appendix E is ne

Neutron scattering at high temperature and levitation techniques

Studies of the liquid state present an obvious fundamental interest and are also important for technological applications since the molten state is an essential stage in various industrial processes (e.g. glass making, single crystal growing, iron and steel making). Most of the physical properties of a high-temperature liquid are related to its atomic structure. Thus it is important to develop devices to probe the local environment of the atoms in the sample. At very high temperature, it is difficult to use conventional furnaces, which present several problems. In particular, physical contact with the container can contaminate the sample and/or modify its structural properties. Such problems encouraged the development of containerless techniques, which are powerful tools to study high-temperature melts. By eliminating completely any contact between sample and container, it is possible to study the sample with a very high degree of control and to access very high temperatures. An additional advantage of levitation methods is that it is possible to supercool hot liquids down to several hundred of degrees below their equilibrium freezing point, since heterogeneous nucleation processes are suppressed