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Search Results: 1 - 10 of 189902 matches for " G. Burnell "
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Characteristics of strong ferromagnetic Josephson junctions with epitaxial barriers
C. Bell,R. Loloee,G. Burnell,M. G. Blamire
Physics , 2005, DOI: 10.1103/PhysRevB.71.180501
Abstract: We present the measurement of superconductor / ferromagnetic Josephson junctions, based on an epitaxial Nb bottom electrode and epitaxial Fe20Ni80 barrier. Uniform junctions have been fabricated with a barrier thicknesses in the range 2-12 nm. The maximum critical current density ~ 2.4 \pm 0.2 * 10^9 Am^-2 was found for a devices with a 3 nm thick barrier at 4.2 K, corresponding to an average characteristic voltage I_C R_N ~ 16 \muV. The I_C R_N showed a non-monotonic behavior with Fe20Ni80 thickness. The variation of the resistance of a unit area AR_N, of the junctions with barrier thickness gave a Nb/Py specific interface resistance of 6.0 \pm 0.5 f\Omega m^2 and Fe20Ni80 resistivity of 174 \pm 50 n\Omega m, consistent with other studies in polycrystalline samples.
Promoting Democracy and Promoting Autocracy: Towards A Comparative Evaluation
Peter Burnell
Journal of Politics and Law , 2010, DOI: 10.5539/jpl.v3n2p3
Abstract: Canada may be about to create a publicly-funded Centre for Advancing Democracy. However, at the present time there is international speculation that autocracy promotion is increasing, now that the global wave of democratisation has stalled or gone into retreat. Some prominent authoritarian and semi-authoritarian regimes have gained in confidence; their potential to influence politics in other countries attracts growing attention. If international democracy promotion is to measure up to the challenge that this could pose then comparative evaluations of the performance of international support for democracy and for autocratic rule should be undertaken. Evaluation endeavours across this political divide face shared similarities and major complicating differences too. These make meaningful comparisons problematic. Nevertheless, new democracy promotion initiatives should employ whatever can be learned from studying autocracy promotion, so as to maximise their chances of success. Canada’s proposals for advancing democracy abroad offer a timely opportunity to address the challenge of comparing democracy and autocracy support, and in doing so become a world leader in the field.
The Relationship of Accountable Governance and Constitutional Implementation, with Reference to Africa
Peter Burnell
Journal of Politics and Law , 2009, DOI: 10.5539/jpl.v1n3p10
Abstract: This article offers a framework for analysing the interrelationships between democratic accountability and constitutional implementation with specific reference to sub-Saharan Africa. It opens by noting the subject’s importance and the contested meaning of key terms, before proceeding to elaborate the significance that constitutional implementation and accountability have for one another. The main purpose is to suggest an agenda lying at the interface between constitutional law and politics that is worthy of further research. The article argues there is considerable scope for analysts of law and politics to collaborate for the purpose of shedding light on many questions that cut across issues of both democratic accountability and constitutional implementation in Africa.
0-pi oscillations in nanostructured Nb/Fe/Nb Josephson junctions
Samanta Piano,J. W. A. Robinson,G. Burnell,M. G. Blamire
Physics , 2006, DOI: 10.1140/epjb/e2007-00210-8
Abstract: The physics of the $\pi$ phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from ``0'' to ``$\pi$'' states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of $1.98 \times 10^5$ m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the $I_CR_N$ product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.
Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions
J. W. A. Robinson,S. Piano,G. Burnell,C. Bell,M. G. Blamire
Physics , 2006, DOI: 10.1103/PhysRevLett.97.177003
Abstract: We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with barrier thickness implying repeated 0-pi phase-transitions in the superconducting order parameter. We show in particular that the Co barrier devices can be accurately modelled using existing clean limit theories and so that, despite the high exchange energy (309 meV), the large IcRN value in the pi-state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.
Proximity and Josephson effects in superconductor - antiferromagnetic Nb / γ-Fe50Mn50 heterostructures
C. Bell,E. J. Tarte,G. Burnell,C. W. Leung,D. -J. Kang,M. G. Blamire
Physics , 2003, DOI: 10.1103/PhysRevB.68.144517
Abstract: We study the proximity effect in superconductor (S), antiferromagnetic (AF) bilayers, and report the fabrication and measurement of the first trilayer S/AF/S Josephson junctions. The disordered f.c.c. alloy \gamma-Fe50Mn50 was used as the AF, and the S is Nb. Micron and sub-micron scale junctions were measured, and the scaling of $J_C (d_AF)$ gives a coherence length in the AF of 2.4 nm, which correlates with the coherence length due to suppression of $T_C$ in the bilayer samples. The diffusion constant for FeMn was found to be 1.7 \times 10$^{-4}$ m$^2$ s$^-1$, and the density of states at the Fermi level was also obtained. An exchange biased FeMn/Co bilayer confirms the AF nature of the FeMn in this thickness regime.
Controllable Josephson current through a pseudo-spin-valve structure
C. Bell,G. Burnell,C. W. Leung,E. J. Tarte,D. -J. Kang,M. G. Blamire
Physics , 2003, DOI: 10.1063/1.1646217
Abstract: A thin Co/Cu/Permalloy (Ni$_{80}$Fe$_{20}$) pseudo-spin-valve structure is sandwiched between superconducting Nb contacts. When the current is passed perpendicular to the plane of the film a Josephson critical current ($I_C$) is observed at 4.2 K, in addition to a magnetoresistance (MR) of $\sim$ 0.5 % at high bias. The hysteresis loop of the spin-valve structure can be cycled to modulate the zero field $I_C$ of the junction in line with the MR measurements. These modulations of resistance and $I_C$ occur both smoothly and sharply with the applied field. For each type of behaviour there is a strong correlation between shape of the MR loops and the $I_C$ modulation.
Decoupling of a Current-Biased Intrinsic Josephson Junction from its Environment
P. A. Warburton,A. R. Kuzhakhmetov,G. Burnell,M. G. Blamire,H. Schneidewind
Physics , 2003, DOI: 10.1103/PhysRevB.67.184513
Abstract: We have observed a dissipative phase diffusion branch in arrays of hysteretic high-Tc intrinsic Josephson junctions. By comparing the data with a thermal activation model we extract the impedance seen by the junction in which phase diffusion is occurring. At the plasma frequency this junction is isolated from its environment and it sees its own large (~ kilo Ohm) impedance. Our results suggest that stacks of Josephson junctions may be used for isolation purposes in the development of a solid state quantum computer.
Excess sub-millimetre emission from GRS 1915+105
R. N. Ogley,S. J. Bell Burnell,R. P. Fender,G. G. Pooley,E. B. Waltman
Physics , 2000, DOI: 10.1046/j.1365-8711.2000.03609.x
Abstract: We present the first detections of the black hole X-ray binary GRS 1915+105 at sub-millimetre wavelengths. We clearly detect the source at 350 GHz on two epochs, with significant variability over the 24 hr between epochs. Quasi-simultaneous radio monitoring indicates an approximately flat spectrum from 2 - 350 GHz, although there is marginal evidence for a minimum in the spectrum between 15 - 350 GHz. The flat spectrum and correlated variability imply that the sub-mm emission arises from the same synchrotron source as the radio emission. This source is likely to be a quasi-steady partially self-absorbed jet, in which case these sub-mm observations probe significantly closer to the base of the jet than do radio observations and may be used in future as a valuable diagnostic of the disc:jet connection in this source.
Planar Superconductor-Normal-Superconductor Josephson Junctions in MgB2
G. Burnell,D. -J. Kang,H. N. Lee,S. H. Moon,B. Oh,M. G. Blamire
Physics , 2001, DOI: 10.1063/1.1419041
Abstract: Since the discovery of superconductivity in MgB2 considerable progress has been made in determining the physical properties of the material, which are promising for bulk conductors. Tunneling studies show that the material is reasonably isotropic and has a well-developed s-wave energy gap (?), implying that electronic devices based on MgB2 could operate close to 30K. Although a number of groups have reported the formation of thin films by post-reaction of precursors, heterostructure growth is likely to require considerable technological development, making single-layer device structures of most immediate interest. MgB2 is unlike the cuprate superconductors in that grain boundaries do not form good Josephson junctions, and although a SQUID based on MgB2 nanobridges has been fabricated, the nanobridges themselves do not show junction-like properties. Here we report the successful creation of planar MgB2 junctions by localised ion damage in thin films. The critical current (IC) of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very high frequency applications.
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