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Guttridge A.,Durham University | Hopkins S.A.,Durham University | Kemp S.L.,Durham University | Boddy D.,Durham University | And 5 more authors.
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2016

We report a robust technique for laser frequency stabilisation that enables the reproducible loading of in excess of 109 Yb atoms from a Zeeman slower directly into a magneto-optical trap (MOT) operating on the 1S0→3P1 transition, without the need for a first stage MOT on the 1S0→3P1 transition. We use a simple atomic beam apparatus to generate narrow fluorescence signals on both the 399 nm 1S0→3P1 transition used for the Zeeman slower and the 556 nm 1S0→3P1 transition. We present in detail the methods for obtaining spectra with a high signal-to-noise ratio and demonstrate error signals suitable for robust frequency stabilisation. Finally we demonstrate the stability and precision of our technique through sensitive measurements of the gravitational sag of the Yb MOT as a function of the intensity of the laser cooling beams, which are in good agreement with theory. These results will be important for efficient loading of the atoms into an optical dipole trap. © 2016 IOP Publishing Ltd.

Morrison K.,Blackett Laboratory | Berenov A.,Prince Consort Rd | Cohen L.F.,Blackett Laboratory
Materials Research Society Symposium Proceedings | Year: 2011

Hysteresis is unattractive for magnetocaloric applications because it introduces loss in the cooling cycle. It is however usually associated with a first order transition and large entropy change. In this paper we review the sources of hysteresis in magnetocaloric materials and in particular in manganite systems where the nature of the transition in terms of whether it is indeed a first order transition remains elusive. © 2011 Materials Research Society.

Morrison K.,Blackett Laboratory | Pecharsky V.K.,U.S. Department of Energy | Gschneidner Jr. K.A.,U.S. Department of Energy | Cohen L.F.,Blackett Laboratory
Materials Research Society Symposium Proceedings | Year: 2011

A 100 micron fragment of a b-axis oriented single crystal Gd 5Si 2Ge 2 has been studied using microcalorimetry, enabling the separate measurement of the heat capacity and the latent heat. The sample was taken from the same crystal previously studied with Hall probe imaging, which showed that the phase transition is seeded by a second phase of Gd 5Si 1.5Ge 1.5 nanoplatelets on the increasing field sweep direction only. The multiple transition features observed in the latent heat signature suggests a nucleation size of approximately 20 μm, consistent with the lengthscale suggested by Hall imaging. The difference in nucleation and growth process with field sweep direction is clearly identified in the latent heat. We show that the latent heat contribution to the entropy change is of the order of 50% of the total entropy change and unlike other systems studied, the transition does not broaden (and the latent heat contribution does not diminish significantly) as magnetic field and temperature are increased within the parameter range explored in these experiments. © 2011 Materials Research Society.

Banks P.,Blackett Laboratory | Franks N.P.,Blackett Laboratory | Dickinson R.,Imperial College London
Anesthesiology | Year: 2010

Background: The general anesthetic gas xenon is neuroprotective and is undergoing clinical trials as a treatment for ischemic brain injury. A small number of molecular targets for xenon have been identified, the N-methyl-d-aspartate (NMDA) receptor, the two-pore-domain potassium channel TREK-1, and the adenosine triphosphate-sensitive potassium channel (KATP). However, which of these targets are relevant to acute xenon neuroprotection is not known. Xenon inhibits NMDA receptors by competing with glycine at the glycine-binding site. We test the hypothesis that inhibition of the NMDA receptor at the glycine site underlies xenon neuroprotection against hypoxia-ischemia. Methods: We use an in vitro model of hypoxia-ischemia to investigate the mechanism of xenon neuroprotection. Organotypic hippocampal brain slices from mice are subjected to oxygen-glucose deprivation, and injury is quantified by propidium iodide fluorescence. Results: We show that 50% atm xenon is neuroprotective against hypoxia-ischemia when applied immediately after injury or after a delay of 3 h after injury. To validate our method, we show that neuroprotection by gavestinel is abolished when glycine is added, confirming that NMDA receptor glycine site antagonism underlies gavestinel neuroprotection. We then show that adding glycine abolishes the neuroprotective effect of xenon, consistent with competitive inhibition at the NMDA receptor glycine site mediating xenon neuroprotection. Conclusions: We show that xenon neuroprotection against hypoxia-ischemia can be reversed by increasing the glycine concentration. This is consistent with competitive inhibition by xenon at the NMDA receptor glycine site, playing a significant role in xenon neuroprotection. This finding may have important implications for xenon's clinical use as an anesthetic and neuroprotectant. Copyright © 2010, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins.

Expert P.,Imperial College London | Expert P.,Blackett Laboratory | Lambiotte R.,Imperial College London | Chialvo D.R.,Northwestern University | And 5 more authors.
Journal of the Royal Society Interface | Year: 2011

Adaptive behaviour, cognition and emotion are the result of a bewildering variety of brain spatio-temporal activity patterns. An important problem in neuroscience is to understand the mechanism by which the human brain's 100 billion neurons and 100 trillion synapses manage to produce this large repertoire of cortical configurations in a flexible manner. In addition, it is recognized that temporal correlations across such configurations cannot be arbitrary, but they need to meet two conflicting demands: while diverse cortical areas should remain functionally segregated from each other, they must still perform as a collective, i.e. they are functionally integrated. Here, we investigate these large-scale dynamical properties by inspecting the character of the spatio-temporal correlations of brain resting-state activity. In physical systems, these correlations in space and time are captured by measuring the correlation coefficient between a signal recorded at two different points in space at two different times. We show that this two-point correlation function extracted from resting-state functional magnetic resonance imaging data exhibits self-similarity in space and time. In space, self-similarity is revealed by considering three successive spatial coarse-graining steps while in time it is revealed by the 1/f frequency behaviour of the power spectrum. The uncovered dynamical self-similarity implies that the brain is spontaneously at a continuously changing (in space and time) intermediate state between two extremes, one of excessive cortical integration and the other of complete segregation. This dynamical property may be seen as an important marker of brain well-being in both health and disease. © 2010 The Royal Society.

Sweeney T.R.,Blackett Laboratory | Cisnetto V.,Blackett Laboratory | Bose D.,Imperial College London | Bailey M.,Institute of Cancer Research | And 4 more authors.
Journal of Biological Chemistry | Year: 2010

Foot-and-mouth disease virus (FMDV), a positive sense, single-stranded RNA virus, causes a highly contagious disease in cloven-hoofed livestock. Like other picornaviruses,FMDVhas a conserved 2C protein assigned to the superfamily 3 helicases, a group of AAA+ ATPases that has a predicted N-terminal membrane-binding amphipathic helix attached to the main ATPase domain. In infected cells, 2C is involved in the formation of membrane vesicles, where it co-localizes with viral RNA replication complexes, but its precise role in virus replication has not been elucidated. We show here that deletion of the predicted N-terminal amphipathic helix enables overexpression in Escherichia coli of a highly soluble truncated protein, 2C(34-318), that has ATPase and RNA binding activity. ATPase activity was abrogated by point mutations in the Walker A (K116A) and B (D160A) motifs and Motif C (N207A) in the active site. Unliganded 2C(34-318) exhibits concentration-dependent self-association to yield oligomeric forms, the largest of which is tetrameric. Strikingly, in the presence of ATP and RNA, FMDV 2C(34-318) containing the N207A mutation, which binds but does not hydrolyze ATP, was found to oligomerize specifically into hexamers. Visualization of FMDV 2C-ATP-RNA complexes by negative stain electron microscopy revealed hexameric ring structures with 6-fold symmetry that are characteristic of AAA+ ATPases. ATPase assays performed by mixing purified active and inactive 2C(34-318) subunits revealed a coordinated mechanism of ATP hydrolysis. Our results provide new insights into the structure and mechanism of picornavirus 2C proteins that will facilitate new investigations of their roles in infection. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.

Whitehead T.M.,University of Cambridge | Schonenberg L.M.,University of Cambridge | Kongsuwan N.,Blackett Laboratory | Needs R.J.,University of Cambridge | Conduit G.J.,University of Cambridge
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2016

We propose a smooth pseudopotential for the contact interaction acting between ultracold atoms confined to two dimensions. The pseudopotential reproduces the scattering properties of the repulsive contact interaction up to 200 times more accurately than a hard disk potential, and in the attractive branch gives a tenfold improvement in accuracy over the square well potential. Furthermore, the potential enables diffusion Monte Carlo simulations of the ultracold gas to be run 15 times quicker than was previously possible. © 2016 American Physical Society.

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