Physics - Journal Articles

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    The return of the spin period in DW Cnc and evidence of new high state outbursts
    (Oxford University Press, 2021-11-26) Duffy, C.; Ramsay, G.; Steeghs, D.; Kennedy, Mark R.; West, R. G.; Wheatley, P. J.; Dhillon, V. S.; Ackley, K.; Dyer, M. J.; Galloway, D. K.; Gill, S.; Acton, J. S.; Burleigh, M. R.; Casewell, S. L.; Goad, M. R.; Henderson, B. A.; Tilbrook, R. H.; Strøm, P. A.; Anderson, D. R.; European Southern Observatory; University of Warwick; University of Leicester; Queen's University Belfast; Université de Genève; Deutsches Zentrum für Luft- und Raumfahrt; University of Cambridge; Science and Technology Facilities Council; University of Warwick; Monash University; University of Sheffield; Armagh Observatory and Planetarium; National Astronomical Research Institute of Thailand; Instituto de Astrofísica de Canarias; University of Portsmouth; Turun Yliopisto; Australian Research Council; Northern Ireland Executive; Irish Research Council
    DW Cnc is an intermediate polar which has previously been observed in both high and low states. Observations of the high state of DW Cnc have previously revealed a spin period at ∼38.6 min, however, observations from the 2018 to 2019 low state showed no evidence of the spin period. We present results from our analysis of 12 s cadence photometric data collected by Next Generation Transit Survey of DW Cnc during the high state which began in 2019. Following the previously reported suppression of the spin period signal, we identify the return of this signal during the high state, consistent with previous observations of it. We identify this as the restarting of accretion during the high state. We further identified three short outbursts lasting ∼1 d in DW Cnc with a mean recurrence time of ∼60 d and an amplitude of ∼1 mag. These are the first outbursts identified in DW Cnc since 2008. Due to the short nature of these events, we identify them not as a result of accretion instabilities but instead either from instabilities originating from the interaction of the magnetorotational instability in the accretion disc and the magnetic field generated by the white dwarf or the result of magnetic gating.
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    The peculiar chemical abundance of the transitional millisecond pulsar PSR J1023+0038-Li enhancement
    (Oxford University Press, 2022-03-02) Shahbaz, T.; González-Hernández, J. I.; Breton, R. P.; Kennedy, Mark R.; Sánchez, D. M.; Linares, M.; Ministerio de Ciencia e Innovación; European Research Council; Irish Research Council; European Regional Development Fund; Fundación Canaria de Investigación y Salud
    Using high-resolution optical spectroscopy we determine the chemical abundance of the secondary star in the binary millisecond pulsar PSR J1023+0038. We measure a metallicity of [Fe/H] = 0.48 ± 0.04 which is higher than the Solar value and in general find that the element abundances are different compared to the secondary stars in X-ray binaries and stars in the solar neighbourhood of similar Fe content. Our results suggest that the pulsar was formed in a supernova explosion. We find that supernova models, where matter that has been processed in the supernova is captured by the secondary star leading to abundance anomalies, qualitatively agree with the observations. We measure Li abundance of A(Li) = 3.66 ± 0.20, which is anomalously high compared to the Li abundance of stars with the same effective temperature, irrespective of the age of the system. Furthermore, the Li abundance in PSR J1023+0038 is higher than the Cosmic value and what is observed in young Population I stars and so provides unambiguous evidence for fresh Li production. The most likely explanation is the interaction of high-energy gamma-rays or relativistic protons from the pulsar wind or intrabinary shock with the CNO nuclei in the secondary star’s atmosphere via spallation which leads to substantial Li enrichment in the secondary star’s atmosphere.
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    Quantum control and quantum speed limits in supersymmetric potentials
    (IOP Publishing, 2022-09-06) Campbell, C.; Li, Jing; Busch, T.; Fogarty, T.; Okinawa Institute of Science and Technology Graduate University; Science Foundation Ireland
    Supersymmetry allows one to build a hierarchy of Hamiltonians that share the same spectral properties and which are pairwise connected through common super-potentials. The iso-spectral properties of these Hamiltonians imply that the dynamics and therefore control of different eigenstates are connected through supersymmetric intertwining relations. In this work we explore how this enables one to study general dynamics, shortcuts to adiabaticity and quantum speed limits for distinct states of different supersymmetric partner potentials by using the infinite box as an example.
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    A concertina-shaped vibration energy harvester-assisted NFC sensor with improved wireless communication range
    (Institute of Electrical and Electronics Engineers (IEEE), 2022-01) Paul, Kankana; Gawade, Dinesh R.; Simorangkir, Roy B. V. B.; O'Flynn, Brendan; Buckley, John L.; Amann, Andreas; Roy, Saibal; Science Foundation Ireland; Horizon 2020; European Regional Development Fund
    The explosive growth of wireless sensor platforms and their emerging wide range of application areas make the development of a sustainable and robust power source, an essential requirement to enable widespread deployment of these wireless devices. As a solution to this cardinal issue, this paper reports the design and fabrication of a resonant Vibration Energy Harvester (VEH) that comprises interleaved springs, manifesting a concertina shaped structure that can enable large mechanical amplitudes of oscillation. Within a relatively small footprint (9cm3), this concertina-VEH yields a large power density of 455.6μW/cm3g2 while operating at a resonant frequency of 75Hz. Additionally, the feasibility of the implemented VEH to support NFC based wireless sensor platform, that is yet uncharted, is also investigated in this work. A very low-power consumption Near Field Communication (NFC) wireless sensor node has been designed and developed for this purpose. The developed concertina VEH has been employed to power the electronics interface of this NFC sensor. Using mechanical energy derived from as low as 0.2g excitation, our study shows that the VEH can enhance the electromagnetic interaction between the transmitting antenna and the reader, resulting in a 120% increase in wireless communication range for the NFC sensor node. Such a high-performance energy harvester assisted NFC sensor node has the potential to be used in a wide range of Internet of Things (IoT) platforms as a reliable and sustainable power solution.
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    Atomistic analysis of Auger recombination in c-plane (In,Ga)N/GaN quantum wells: Temperature-dependent competition between radiative and nonradiative recombination
    (American Physical Society, 2022-05-12) McMahon, Joshua M.; Kioupakis, Emmanouil; Schulz, Stefan; Science Foundation Ireland; Sustainable Energy Authority of Ireland; University of Michigan
    We present an atomistic theoretical study of the temperature dependence of the competition between Auger and radiative recombination in c-plane (In,Ga)N/GaN quantum wells with indium (In) contents of 10%, 15%, and 25%. The model accounts for random alloy fluctuations and the connected fluctuations in strain and built-in field. Our investigations reveal that the total Auger recombination rate exhibits a weak temperature dependence; at a temperature of 300 K and a carrier density of n3D=3.8×1018cm−3, we find total Auger coefficients in the range of ≈6×10−30cm6/s(10% In) to ≈3×10−31cm6/s (25% In), thus large enough to significantly impact the efficiency in (In,Ga)N systems. Our calculations show that the hole-hole-electron Auger rate dominates the total rate for the three In contents studied; however, the relative difference between the hole-hole-electron and electron-electron-hole contributions decreases as the In content is increased to 25%. Our studies provide further insight into the origin of the “thermal droop” (i.e., the decrease in internal quantum efficiency with increasing temperature at a fixed carrier density) in (In,Ga)N-based light-emitting diodes. We find that the ratio of radiative to nonradiative (Auger) recombination increases in the temperature range relevant to the thermal droop (≥300 K), suggesting that the competition between these processes is not driving this droop effect in c-plane (In,Ga)N/GaN quantum wells. This finding is in line with recent experimental studies.