Experimental studies of the effect of rapid afterburn on shock development of near-field explosions

Many conventional high explosives do not contain sufficient internal oxygen to fully combust the gaseous products which result from detonation of the explosive material. Because of this, under-oxygenated explosives continue to burn after detonation. This process, called afterburn, is known to influence the late-time pressure and energy released by the explosive, which has particular significance for confined explosives. Recent experimental work at the University of Sheffield, along with a small number of previous studies, has shown that some afterburn occurs at timescales commensurate with the development of the shock wave. This article presents the results from a series of tests measuring the reflected pressure acting on a rigid target following the detonation of small explosive charges. High-speed video is used to capture the emerging structure of the detonation products and air shock, while the spatial and temporal distributions of the reflected pressure are recorded using an array of 17 Hopkinson pressure bars set flush with an effectively rigid target. Tests are conducted in inert atmospheres and oxygen-rich atmospheres in order to assess the contribution of rapid afterburn on the development of the shock front and interaction with a rigid target situated close to the explosive charge. The results show that early-stage afterburn has a significant influence on the reflected shock parameters in the near-field

Magnetism, Critical Fluctuations and Susceptibility Renormalization in Pd

Some of the most popular ways to treat quantum critical materials, that is, materials close to a magnetic instability, are based on the Landau functional. The central quantity of such approaches is the average magnitude of spin fluctuations, which is very difficult to measure experimentally or compute directly from the first principles. We calculate the parameters of the Landau functional for Pd and use these to connect the critical fluctuations beyond the local-density approximation and the band structure.Comment: Replaced with the revised version accepted for publication. References updated, errors corrected, other change

Finite element simulation of plates under non-uniform blast loads using a point-load method: Buried explosives

There are two primary challenges associated with assessing the adequacy of a protective structure to resist explosive events: firstly the spatial variation of load acting on a target must be predicted to a sufficient level of accuracy; secondly, the response of the target to this load must also be quantified. When a high explosive is shallowly buried in soil, the added confinement given by the geotechnical material results in a blast which is predominantly directed vertically. This imparts an extremely high magnitude, spatially non-uniform load on the target structure. A recently commissioned experimental rig designed by the authors has enabled direct measurements of the blast load resulting from buried explosive events. These direct measurements have been processed using an in-house interpolation routine which evaluates the load acting over a regular grid of points. These loads can then be applied as the nodal-point loads in a finite element model. This paper presents results from a series of experiments where a free-flying plate was suspended above a shallow buried explosive. Dynamic and residual deformations are compared with finite element simulations of plates using the experimentally recorded, and interpolated, nodal point-loads. The results show very good agreement and highlight the use of this method for evaluating the efficacy of targets subjected to non-uniform blast loads

Quantum Critical Behavior and Possible Triplet Superconductivity in Electron Doped CoO2 Sheets

Density functional calculations are used to investigate the doping dependence of the electronic structure and magnetic properties in hexagonal Na$_x$CoO$_2$. The electronic structure is found to be highly two dimensional, even without accounting for the structural changes associated with hydration. At the local spin density approximation level, a weak itinerant ferromagnetic state is predicted for all doping levels in the range $x=0.3$ to $x=0.7$, with competing but weaker itinerant antiferromagnetic solutions. The Fermi surface, as expected, consists of simple rounded hexagonal cylinders, with additional small pockets depending on the $c$ lattice parameter. Comparison with experiment implies substantial magnetic quantum fluctuations. Based on the Fermi surface size and the ferromagnetic tendency of this material,it is speculated that a triplet superconducting state analogous to that in Sr$_2$RuO$_4$ may exist here.Comment: 4 pages, 1 figur

Electronic Structure, Magnetism and Superconductivity of Layered Iron Compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe(Se,Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.Comment: Proceedings ISS200

Toward mid-infrared, subdiffraction, spectral-mapping of human cells and tissue: SNIM (scanning near-field infrared microscopy) tip fabrication

Scanning near-field infrared microscopy (SNIM) potentially enables subdiffraction, broadband mid-infrared (MIR:3–25-μm wavelength range) spectral-mapping of human cells and tissue for real-time molecular sensing, with prospective use in disease diagnosis. SNIM requires an MIR-transmitting tip of small aperture for photon collection. Here, chalcogenide-glass optical fibers are reproducibly tapered at one end to form a MIR transmitting tip for SNIM. A wet-etching method is used to form the tip. The tapering sides of the tip are Al-coated. These Al-coated tapered-tips exhibit near-field power-confinement when acting either as the launch-end or exit-end of the MIR optical fiber. We report first time optimal cleaving of the end of the tapered tip using focused ion beam milling. A flat aperture is produced at the end of the tip, which is orthogonal to the fiber-axis and of controlled diameter. A FIB-cleaved aperture is used to collect MIR spectra of cells mounted on a transflection plate, under illumination of a synchrotron- generated wideband MIR beam

Order parameter symmetry in ferromagnetic superconductors

We analyze the symmetry and the nodal structure of the superconducting order parameter in a cubic ferromagnet, such as ZrZn$_2$. We demonstrate how the order parameter symmetry evolves when the electromagnetic interaction of the conduction electrons with the internal magnetic induction and the spin-orbit coupling are taken into account. These interactions break the cubic symmetry and lift the degeneracy of the order parameter. It is shown that the order parameter which appears immediately below the critical temperature has two components, and its symmetry is described by {\em co-representations} of the magnetic point groups. This allows us to make predictions about the location of the gap nodes.Comment: 12 pages, ReVTeX, submitted to PR

Cosmological scaling solutions in generalised Gauss-Bonnet gravity theories

The conditions for the existence and stability of cosmological power-law scaling solutions are established when the Einstein-Hilbert action is modified by the inclusion of a function of the Gauss-Bonnet curvature invariant. The general form of the action that leads to such solutions is determined for the case where the universe is sourced by a barotropic perfect fluid. It is shown by employing an equivalence between the Gauss-Bonnet action and a scalar-tensor theory of gravity that the cosmological field equations can be written as a plane autonomous system. It is found that stable scaling solutions exist when the parameters of the model take appropriate values.Comment: 10 pages and 5 figure

Electronic Structure of Fe-Based Superconductors

The electronic structure of the Fe-based superconductors is discussed, mainly from the point of view of first principles calculations in relation to experimental data. Comparisons and contrasts with cuprates are made. The problem of reconciling experiments indicating an $s$ symmetry gap with experiments indicating line nodes is discussed and a possible resolution is given.Comment: Updated references. Additional discussio

Making things happen : a model of proactive motivation

Being proactive is about making things happen, anticipating and preventing problems, and seizing opportunities. It involves self-initiated efforts to bring about change in the work environment and/or oneself to achieve a different future. The authors develop existing perspectives on this topic by identifying proactivity as a goal-driven process involving both the setting of a proactive goal (proactive goal generation) and striving to achieve that proactive goal (proactive goal striving). The authors identify a range of proactive goals that individuals can pursue in organizations. These vary on two dimensions: the future they aim to bring about (achieving a better personal fit within one’s work environment, improving the organization’s internal functioning, or enhancing the organization’s strategic fit with its environment) and whether the self or situation is being changed. The authors then identify “can do,” “reason to,” and “energized to” motivational states that prompt proactive goal generation and sustain goal striving. Can do motivation arises from perceptions of self-efficacy, control, and (low) cost. Reason to motivation relates to why someone is proactive, including reasons flowing from intrinsic, integrated, and identified motivation. Energized to motivation refers to activated positive affective states that prompt proactive goal processes. The authors suggest more distal antecedents, including individual differences (e.g., personality, values, knowledge and ability) as well as contextual variations in leadership, work design, and interpersonal climate, that influence the proactive motivational states and thereby boost or inhibit proactive goal processes. Finally, the authors summarize priorities for future researc