School of Physics - Research Publications
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ItemNo Preview AvailableBelle II observation prospects for axionlike particle production from B meson annihilation decay(American Physical Society (APS), 2024-01-01)
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ItemPersistent gravitational radiation from glitching pulsars - II. Updated scaling with vortex number(OXFORD UNIV PRESS, 2024-01-23)ABSTRACT Superfluid vortices pinned to nuclear lattice sites or magnetic flux tubes in a neutron star evolve abruptly through a sequence of metastable spatial configurations, punctuated by unpinning avalanches associated with rotational glitches, as the stellar crust spins down electromagnetically. The metastable configurations are approximately but not exactly axisymmetric, causing the emission of persistent, quasimonochromatic, current quadrupole gravitational radiation. The characteristic gravitational wave strain h0 as a function of the spin frequency f and distance D from the Earth is bounded above by $h_0 = 1.2\substack{+1.3 \\ -0.9} \times 10^{-32} (f/30\,\,{\rm Hz})^{2.5} (D/1\,\,{\rm kpc})^{-1}$, corresponding to a Poissonian spatial configuration (equal probability per unit area, i.e. zero inter-vortex repulsion), and bounded below by $h_0 = 1.8\substack{+2.0 \\ -1.5} \times 10^{-50} (f/30\,\,{\rm Hz})^{1.5} (D/1\,\,{\rm kpc})^{-1}$, corresponding to a regular array (periodic separation, i.e. maximum inter-vortex repulsion). N-body point vortex simulations predict an intermediate scaling, $h_0 = 7.3\substack{+7.9 \\ -5.4} \times 10^{-42} (f/30\,\,{\rm Hz})^{1.9} (D/1\,\,{\rm kpc})^{-1}$, which reflects a balance between the randomizing but spatially correlated action of superfluid vortex avalanches and the regularizing action of inter-vortex repulsion. The scaling is calibrated by conducting simulations with Nv ≤ 5 × 103 vortices and extrapolated to the astrophysical regime Nv ∼ 1017(f/30 Hz). The scaling is provisional, pending future computational advances to raise Nv and include three-dimensional effects such as vortex tension and turbulence.
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ItemOn the creation of near-surface nitrogen-vacancy centre ensembles by implantation of type Ib diamond(SPRINGER HEIDELBERG, 2023-11-28)Abstract Dense, near-surface (within $$\sim 10$$ ∼ 10 nm) ensembles of nitrogen-vacancy (NV) centres in diamond are moving into prominence as the workhorse of many envisaged applications, from the imaging of fast-fluctuating magnetic signals to enacting nuclear hyperpolarisation. Unlike their bulk counterparts, near-surface ensembles suffer from charge stability issues and reduced formation efficiency due to proximity to the diamond surface. Here we examine the prospects for creating such ensembles by implanting nitrogen-rich type Ib diamond, aiming to exploit the high bulk nitrogen density to combat surface-induced band bending. This approach has previously been successful at creating deeper ensembles, however we find that in the near-surface regime there are fewer benefits over nitrogen implantation into pure diamond substrates. Our results suggest that control over diamond surface termination during annealing is key to successfully creating high-yield near-surface NV ensembles generally and implantation into type Ib diamond may be worth revisiting once that has been accomplished. Graphical Abstract
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ItemHistone FRET reports the spatial heterogeneity in nanoscale chromatin architecture that is imparted by the epigenetic landscape at the level of single foci in an intact cell nucleus.(Springer Science and Business Media LLC, 2024-01)Genome sequencing has identified hundreds of histone post-translational modifications (PTMs) that define an open or compact chromatin nanostructure at the level of nucleosome proximity, and therefore serve as activators or repressors of gene expression. Direct observation of this epigenetic mode of transcriptional regulation in an intact single nucleus, is however, a complex task. This is because despite the development of fluorescent probes that enable observation of specific histone PTMs and chromatin density, the changes in nucleosome proximity regulating gene expression occur on a spatial scale well below the diffraction limit of optical microscopy. In recent work, to address this research gap, we demonstrated that the phasor approach to fluorescence lifetime imaging microscopy (FLIM) of Förster resonance energy transfer (FRET) between fluorescently labelled histones core to the nucleosome, is a readout of chromatin nanostructure that can be multiplexed with immunofluorescence (IF) against specific histone PTMs. Here from application of this methodology to gold standard gene activators (H3K4Me3 and H3K9Ac) versus repressors (e.g., H3K9Me3 and H3K27Me), we find that while on average these histone marks do impart an open versus compact chromatin nanostructure, at the level of single chromatin foci, there is significant spatial heterogeneity. Collectively this study illustrates the importance of studying the epigenetic landscape as a function of space within intact nuclear architecture and opens the door for the study of chromatin foci sub-populations defined by combinations of histone marks, as is seen in the context of bivalent chromatin.
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ItemNo Preview AvailableJADES: Probing interstellar medium conditions at z ∼ 5.5-9.5 with ultra-deep JWST/NIRSpec spectroscopy(EDP SCIENCES S A, 2023-09-12)We present emission-line ratios from a sample of 27 Lyman-break galaxies from z ∼ 5.5 − 9.5 with −17.0 < M1500 < −20.4, measured from ultra-deep JWST/NIRSpec multi-object spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES). We used a combination of 28 h deep PRISM/CLEAR and 7 h deep G395M/F290LP observations to measure, or place strong constraints on, ratios of widely studied rest-frame optical emission lines including Hα, Hβ, [O II] λλ3726, 3729, [Ne III] λ3869, [O III] λ4959, [O III] λ5007, [O I] λ6300, [N II] λ6583, and [S II] λλ6716, 6731 in individual z > 5.5 spectra. We find that the emission-line ratios exhibited by these z ∼ 5.5 − 9.5 galaxies occupy clearly distinct regions of line-ratio space compared to typical z ∼ 0 − 3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [O III]/[O II] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [O III]/[O II] > 10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [O II]/Hβ and [O III]/[O II], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [N II] λ6583 in our sample, either in individual galaxies, or a stack of all G395M/F290LP spectra. The emission-line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log U ∼ −1.5), and a range of metallicities spanning from ∼0.1 × Z⊙ to higher than ∼0.3 × Z⊙, suggesting we are probing low-metallicity systems undergoing periods of rapid star formation, driving strong radiation fields. These results highlight the value of deep observations in constraining the properties of individual galaxies, and hence probing diversity within galaxy population.
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ItemNo Preview AvailableReal-time, label-free detection and identification of bacteria through non-invasive optical imaging.(Elsevier BV, 2024)Currently, traditional and newer molecular and mass spectrometry techniques of identifying bacteria from biological samples requires lengthy sample preparation, growth and labelling/staining assays. Thus, there is a pressing clinical need for an adjunct method that accurately identifies bacteria in real time. Here we report on the evaluation of confocal microscopy for the identification of clinically important and multi-drug resistant (MDR) bacteria in real time, using their intrinsic fluorescence features, i.e., emission spectra and fluorescence lifetime. The results demonstrate that difference in emission spectra and fluorescence lifetimes can be used as a fingerprint for identification of 12 bacterial species and MDR strains in real-time. Photostability or time-traces of bacteria demonstrated that these parameters could be used for tracking and recording without a need for labelling. Further, dilution experiments demonstrated that using intrinsic fluorescence S. aureus, Klebsiella pneumoniae and Escherichia coli bacteria can be detected and identified at clinically relevant concentrations as low as 2 × 102 CFU/mL. This non-invasive, non-labelling optical methodology may serve as the basis for development of a device that would quickly and accurately identify bacteria in biological samples. Thus, this intrinsic fluorescence technique would provide clinicians information, within minutes from sampling, to base accurate and specific treatments for patients.
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ItemNo Preview AvailableSpread of activation and interaction between channels with multi-channel optogenetic stimulation in the mouse cochlea(ELSEVIER, 2023-12)For individuals with severe to profound hearing loss resulting from irreversibly damaged hair cells, cochlear implants can be used to restore hearing by delivering electrical stimulation directly to the spiral ganglion neurons. However, current spread lowers the spatial resolution of neural activation. Since light can be easily confined, optogenetics is a technique that has the potential to improve the precision of neural activation, whereby visible light is used to stimulate neurons that are modified with light-sensitive opsins. This study compares the spread of neural activity across the inferior colliculus of the auditory midbrain during electrical and optical stimulation in the cochlea of acutely deafened mice with opsin-modified spiral ganglion neurons (H134R variant of the channelrhodopsin-2). Monopolar electrical stimulation was delivered via each of four 0.2 mm wide platinum electrode rings at 0.6 mm centre-to-centre spacing, whereas 453 nm wavelength light was delivered via each of five 0.22 × 0.27 mm micro-light emitting diodes (LEDs) at 0.52 mm centre-to-centre spacing. Channel interactions were also quantified by threshold changes during simultaneous stimulation by pairs of electrodes or micro-LEDs at different distances between the electrodes (0.6, 1.2 and 1.8 mm) or micro-LEDs (0.52, 1.04, 1.56 and 2.08 mm). The spread of activation resulting from single channel optical stimulation was approximately half that of monopolar electrical stimulation as measured at two levels of discrimination above threshold (p<0.001), whereas there was no significant difference between optical stimulation in opsin-modified deafened mice and pure tone acoustic stimulation in normal-hearing mice. During simultaneous micro-LED stimulation, there were minimal channel interactions for all micro-LED spacings tested. For neighbouring micro-LEDs/electrodes, the relative influence on threshold was 13-fold less for optical stimulation compared electrical stimulation (p<0.05). The outcomes of this study show that the higher spatial precision of optogenetic stimulation results in reduced channel interaction compared to electrical stimulation, which could increase the number of independent channels in a cochlear implant. Increased spatial resolution and the ability to activate more than one channel simultaneously could lead to better speech perception in cochlear implant recipients.
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ItemNo Preview AvailableTiming is Everything: Stochastic Optogenetic Stimulation Reduces Adaptation in Retinal Ganglion Cells.(IEEE, 2023-07)Optogenetics gives us unprecedented power to investigate brain connectivity. The ability to activate neural circuits with single cell resolution and its ease of application has provided a wealth of knowledge in brain function. More recently, optogenetics has shown tremendous utility in prosthetics applications, including vision restoration for patients with retinitis pigmentosa. One of the disadvantages of optogenetics, however, is its poor temporal bandwidth, i.e. the cell's inability to fire at a rate that matches the optical stimulation rate at high frequencies (>30 Hz). This research proposes a new strategy to overcome the temporal limits of optogenetic stimulation. Using whole-cell current clamp recordings in mouse retinal ganglion cells expressing channelrhodopsin-2 (H134R variant), we observed that randomizing inter-pulse intervals can significantly increase a retinal ganglion cell's temporal response to high frequency stimulation.Clinical Relevance- A significant disadvantage of optogenetic stimulation is its poor temporal dynamics which prohibit its widespread use in retinal prosthetics. We have shown that randomizing the interval between stimulation pulses reduces adaptation in retinal ganglion cells. This stimulation strategy may contribute to new levels of functional restoration in therapeutics which incorporate optogenetics.
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ItemNo Preview AvailableMid-Infrared Gas Classification Using a Bound State in the Continuum Metasurface and Machine Learning(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2023-10-01)
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ItemNo Preview AvailableScandium Kα and Kβ x-ray spectra with ab initio satellite intensities and energy eigenvalues(American Physical Society (APS), 2024-02-01)