Propagation of Exchange Bias in CoFe/FeMn/CoFe Trilayers

CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{eb}$ depends strongly on the order of depositions and is much higher at CoFe/FeMn than at FeMn/CoFe interfaces. By combining the two bilayer structures into symmetric CoFe/FeMn($t_\mathrm{FeMn}$)/CoFe trilayers, $H_{eb}^t$ and $H_{eb}^b$ of the top and bottom CoFe layers, respectively, are both enhanced. Reducing $t_\mathrm{FeMn}$ of the trilayers also results in enhancements of both $H_{eb}^b$ and $H_{eb}^t$. These results evidence the propagation of exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by the FeMn antiferromagnetic order

Automatic analysis of Pole Mounted Auto-Recloser data for fault diagnosis and prognosis

Fault diagnosis is a key part of a control and protection engineer’s role to ensure the effective and stable performance of electrical power networks. One challenge is to support the analysis and application of expert judgement to the, often, large data sets generated. To assist engineers with this task and improve network reliability, this research focuses on analysing previous fault activity in order to obtain an early-warning report to assist fault diagnosis and fault prognosis. This paper details the design of an integrated system with a fault diagnosis algorithm utilising available Supervisory Control And Data Acquisition (SCADA) alarm data and 11kV distribution network data captured from Pole Mounted Auto-Reclosers (PMARs) (provided by a leading UK network operator). The developed system will be capable of diagnosing the nature of a circuit’s previous fault activity, underlying circuit activity and evolving fault activity and the risk of future fault activity. This will provide prognostic decision support for network operators and maintenance staff

Field theoretic description of charge regulation interaction

In order to find the exact form of the electrostatic interaction between two proteins with dissociable charge groups in aqueous solution, we have studied a model system composed of two macroscopic surfaces with charge dissociation sites immersed in a counterion-only ionic solution. Field-theoretic representation of the grand canonical partition function is derived and evaluated within the mean-field approximation, giving the Poisson-Boltzmann theory with the Ninham-Parsegian boundary condition. Gaussian fluctuations around the mean-field are then analyzed in the lowest order correction that we calculate analytically and exactly, using the path integral representation for the partition function of a harmonic oscillator with time-dependent frequency. The first order (one loop) free energy correction gives the interaction free energy that reduces to the zero-frequency van der Waals form in the appropriate limit but in general gives rise to a mono-polar fluctuation term due to charge fluctuation at the dissociation sites. Our formulation opens up the possibility to investigate the Kirkwood-Shumaker interaction in more general contexts where their original derivation fails.Comment: 12 pages, 9 figures, submitted to EPJ

Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory

We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive-definite pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio L = 0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion.Comment: 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor changes in structure of pape

Aging and ultra-slow equilibration in concentrated colloidal hard spheres

We study the dynamic behaviour of concentrated colloidal hard spheres using Time Resolved Correlation, a light scattering technique that can detect the slow evolution of the dynamics in out-of-equilibrium systems. Surprisingly, equilibrium is reached a very long time after sample initialization, the non-stationary regime lasting up to three orders of magnitude more than the relaxation time of the system. Before reaching equilibrium, the system displays unusual aging behaviour. The intermediate scattering function decays faster than exponentially and its relaxation time evolves non-monotonically with sample age.Comment: Submitted to the proceedings of the 6th EPS Liquid Matter Conference, Utrecht 2-6 July 200

Dynamics of Diblock Copolymers in Dilute Solutions

We consider the dynamics of freely translating and rotating diblock (A-B), Gaussian copolymers, in dilute solutions. Using the multiple scattering technique, we have computed the diffusion and the friction coefficients D_AB and Zeta_AB, and the change Eta_AB in the viscosity of the solution as functions of x = N_A/N and t = l_B/l_A, where N_A, N are the number of segments of the A block and of the whole copolymer, respectively, and l_A, l_B are the Kuhn lengths of the A and B blocks. Specific regimes that maximize the efficiency of separation of copolymers with distinct "t" values, have been identified.Comment: 20 pages Revtex, 7 eps figures, needs epsf.tex and amssymb.sty, submitted to Macromolecule

Long-range forces between two excited mercury atoms and associative ionization

The long-range quadrupole-quadrupole ($\sim R^{-5}$) and leading dispersion ($\sim R^{-6}$) interactions between all pairs of excited Hg($6s6p$) $^3P_0$, $^3P_1$, $^3P_2$, and $^1P_1$ atoms are determined. The quadrupole moments are calculated using the {\it ab initio} relativistic configuration-interaction method coupled with many-body perturbation theory. The van der Waals coefficients are approximated using previously calculated static polarizabilities and expressions for the dispersion energy that are validated with similar systems. The long-range interactions are critical for associative ionization in thermal and cold collisions, and are found to be quite different for different pairs of interacting states. Based on this knowledge and the short-range parts of previously calculated potential curves, improved estimates of the chemi-ionization cross sections are obtained.Comment: accepted in Phys Rev