ORCID Profile
0000-0003-3320-5128
Current Organisations
University of Portsmouth
,
American Museum of Natural History
,
CSIRO
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Publisher: AIP Publishing
Date: 15-09-2011
DOI: 10.1063/1.3636111
Abstract: The transport properties of Nb and Nb/Au thin film microbridges have been investigated. The presence of an Au layer on top of a Nb thin film was found to reduce the critical temperature and change the vortex dynamics due to the proximity effect. The voltage-current characteristics of Nb/Au bilayer microbridges were studied at low applied magnetic fields in the range of 0.15–3 mT. Two different types of vortex motion were observed as the current increased. Linear resistive branches were first observed with resistance values consistent with the normal resistance of non-superconducting channels, a direct evidence of the motion for in idual vortices while the vortex lattice is still pinned. At higher driving currents, the whole vortex lattice was moved causing an abrupt voltage jump from the mixed state to the normal state.
Publisher: American Chemical Society (ACS)
Date: 15-05-2020
Publisher: IOP Publishing
Date: 26-04-2016
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2022
Publisher: Oxford University Press (OUP)
Date: 03-06-2022
Abstract: Cosmological analyses of s les of photometrically identified type Ia supernovae (SNe Ia) depend on understanding the effects of ‘contamination’ from core-collapse and peculiar SN Ia events. We employ a rigorous analysis using the photometric classifier SuperNNova on state-of-the-art simulations of SN s les to determine cosmological biases due to such ‘non-Ia’ contamination in the Dark Energy Survey (DES) 5-yr SN s le. Depending on the non-Ia SN models used in the SuperNNova training and testing s les, contamination ranges from 0.8 to 3.5 per cent, with a classification efficiency of 97.7–99.5 per cent. Using the Bayesian Estimation Applied to Multiple Species (BEAMS) framework and its extension BBC (‘BEAMS with Bias Correction’), we produce a redshift-binned Hubble diagram marginalized over contamination and corrected for selection effects, and use it to constrain the dark energy equation-of-state, w. Assuming a flat universe with Gaussian ΩM prior of 0.311 ± 0.010, we show that biases on w are & .008 when using SuperNNova, with systematic uncertainties associated with contamination around 10 per cent of the statistical uncertainty on w for the DES-SN s le. An alternative approach of discarding contaminants using outlier rejection techniques (e.g. Chauvenet’s criterion) in place of SuperNNova leads to biases on w that are larger but still modest (0.015–0.03). Finally, we measure biases due to contamination on w0 and wa (assuming a flat universe), and find these to be & .009 in w0 and & .108 in wa, 5 to 10 times smaller than the statistical uncertainties for the DES-SN s le.
Publisher: Oxford University Press (OUP)
Date: 11-01-2023
Abstract: We present the luminosity functions and host galaxy properties of the Dark Energy Survey (DES) core-collapse supernova (CCSN) s le, consisting of 69 Type II and 50 Type Ibc spectroscopically and photometrically confirmed supernovae over a redshift range 0.045 & z & 0.25. We fit the observed DES griz CCSN light curves and K-correct to produce rest-frame R-band light curves. We compare the s le with lower redshift CCSN s les from Zwicky Transient Facility (ZTF) and Lick Observatory Supernova Search (LOSS). Comparing luminosity functions, the DES and ZTF s les of SNe II are brighter than that of LOSS with significances of 3.0σ and 2.5σ, respectively. While this difference could be caused by redshift evolution in the luminosity function, simpler explanations such as differing levels of host extinction remain a possibility. We find that the host galaxies of SNe II in DES are on average bluer than in ZTF, despite having consistent stellar mass distributions. We consider a number of possibilities to explain this – including galaxy evolution with redshift, selection biases in either the DES or ZTF s les, and systematic differences due to the different photometric bands available – but find that none can easily reconcile the differences in host colour between the two s les and thus its cause remains uncertain.
Publisher: IOP Publishing
Date: 13-10-2011
DOI: 10.1088/0957-4484/22/45/455501
Abstract: A washer-free Nb nanoSQUID has been developed for measuring magnetization changes from nanoscale objects. The SQUID loop is etched into a 250 nm wide Au/Nb bilayer track and the diameter of the SQUID hole is ∼70 nm. In the presence of a magnetic field perpendicular to the plane of the SQUID, vortex penetration into the 250 nm wide track can be observed via the critical current-applied field characteristic and the value at which vortex first penetrates is consistent with the theoretical prediction. Upon removing the applied field, the penetrated vortices escape the track and the critical current at zero field is restored.
Publisher: American Astronomical Society
Date: 20-10-2022
Abstract: The Dark Energy Spectroscopic Instrument (DESI) embarked on an ambitious 5 yr survey in 2021 May to explore the nature of dark energy with spectroscopic measurements of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the baryon acoustic oscillation method to measure distances from the nearby universe to beyond redshift z 3.5, and employ redshift space distortions to measure the growth of structure and probe potential modifications to general relativity. We describe the significant instrumentation we developed to conduct the DESI survey. This includes: a wide-field, 3.°2 diameter prime-focus corrector a focal plane system with 5020 fiber positioners on the 0.812 m diameter, aspheric focal surface 10 continuous, high-efficiency fiber cable bundles that connect the focal plane to the spectrographs and 10 identical spectrographs. Each spectrograph employs a pair of dichroics to split the light into three channels that together record the light from 360–980 nm with a spectral resolution that ranges from 2000–5000. We describe the science requirements, their connection to the technical requirements, the management of the project, and interfaces between subsystems. DESI was installed at the 4 m Mayall Telescope at Kitt Peak National Observatory and has achieved all of its performance goals. Some performance highlights include an rms positioner accuracy of better than 0.″1 and a median signal-to-noise ratio of 7 of the [O ii ] doublet at 8 × 10 −17 erg s −1 cm −2 in 1000 s for galaxies at z = 1.4–1.6. We conclude with additional highlights from the on-sky validation and commissioning, key successes, and lessons learned.
Publisher: IOP Publishing
Date: 13-05-2019
Abstract: We report on the fabrication and evaluation of nanoscale YBa
Publisher: IOP Publishing
Date: 04-02-2020
Publisher: AIP Publishing
Date: 20-12-2018
DOI: 10.1063/1.5063261
Abstract: The critical current density, Jc, can be increased by introducing defects throughout YBa2Cu3O7 superconducting thin films. We propose a new approach of substrate nanoengineering to produce well-controlled defects. LaAlO3 substrates have been ion-etched with different 34 nm deep patterns prior to the deposition of the films. An annealing step at 1000 °C after substrate etching has been undertaken to negate the Ar-ion damage to the surface. The Jc of the so-prepared s les measured at different temperatures has been compared to thin films deposited on plain substrates at the same time. In general, an increase in Jc is observed across all temperatures and fields as a result of both patterning and annealing. In particular, at zero field and 85 K, the largest enhancement of & % has been recorded for circle and triangle patterns. This new substrate nanoengineering technique is very promising for flux trapping of superconducting devices, particularly because an increase to critical current (Ic) also occurs. Further optimization of depth, size, and shape of the patterns is expected to produce further improvements to Jc.
Publisher: Oxford University Press (OUP)
Date: 2023
Abstract: The Rubin Observatory’s 10-yr Legacy Survey of Space and Time will observe near to 20 billion galaxies. For each galaxy the properties can be inferred. Approximately 105 galaxies observed per year will contain Type Ia supernovae (SNe), allowing SN host-galaxy properties to be calculated on a large scale. Measuring the properties of SN host galaxies serves two main purposes. The first is that there are known correlations between host galaxy type and SN type, which can be used to aid in the classification of SNe. Secondly, Type Ia SNe exhibit correlations between host-galaxy properties and the peak luminosities of the SNe, which has implications for their use as standardizable candles in cosmology. We have used simulations to quantify the improvement in host-galaxy stellar mass (M*) measurements when supplementing photometry from Rubin with spectroscopy from the 4-metre Multi-Object Spectroscopic Telescope (4MOST) instrument. We provide results in the form of expected uncertainties in M* for galaxies with 0.1 & z & 0.9 and 18 & rAB & 25. We show that for galaxies mag 22 and brighter, combining Rubin and 4MOST data reduces the uncertainty measurements of galaxy M* by more than a factor of 2 compared with Rubin data alone. This applies for elliptical and Sc-type hosts. We demonstrate that the reduced uncertainties in M* lead to an improvement of 7 per cent in the precision of the ‘mass step’ correction. We expect our improved measurements of host-galaxy properties to aid in the photometric classification of SNe observed by Rubin.
Publisher: American Physical Society (APS)
Date: 14-06-2012
Publisher: Royal Society of Chemistry (RSC)
Date: 2014
DOI: 10.1039/C3CC45667F
Abstract: Boron nitride nanotubes were functionalized with microperoxidase-11 in aqueous media, showing improved catalytic performance due to a strong electron coupling between the active centre of microperoxidase-11 and boron nitride nanotubes.
Publisher: Springer Science and Business Media LLC
Date: 06-10-2017
Publisher: American Astronomical Society
Date: 03-04-2020
Publisher: IOP Publishing
Date: 11-03-2014
Publisher: Oxford University Press (OUP)
Date: 26-10-2022
Abstract: Cosmological analyses with type Ia supernovae (SNe Ia) often assume a single empirical relation between colour and luminosity (β) and do not account for varying host-galaxy dust properties. However, from studies of dust in large s les of galaxies, it is known that dust attenuation can vary significantly. Here, we take advantage of state-of-the-art modelling of galaxy properties to characterize dust parameters (dust attenuation AV, and a parameter describing the dust law slope RV) for 1100 Dark Energy Survey (DES) SN host galaxies. Utilizing optical and infrared data of the hosts alone, we find three key aspects of host dust that impact SN cosmology: (1) there exists a large range (∼1–6) of host RV (2) high-stellar mass hosts have RV on average ∼0.7 lower than that of low-mass hosts (3) for a subs le of 81 spectroscopically classified SNe there is a significant (& σ) correlation between the Hubble diagram residuals of red SNe Ia and the host RV that when corrected for reduces scatter by $\\sim 13{{\\ \\rm per\\ cent}}$ and the significance of the ‘mass step’ to ∼1σ. These represent independent confirmations of recent predictions based on dust that attempted to explain the puzzling ‘mass step’ and intrinsic scatter (σint) in SN Ia analyses.
Publisher: Elsevier BV
Date: 2012
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 06-2018
Publisher: Oxford University Press (OUP)
Date: 20-03-2023
Abstract: We present an in-depth study of the late-time near-infrared plateau in Type Ia supernovae (SNe Ia), which occurs between 70 and 500 d. We double the existing s le of SNe Ia observed during the late-time near-infrared plateau with new observations taken with the Hubble Space Telescope, Gemini, New Technology Telescope, the 3.5-m Calar Alto Telescope, and the Nordic Optical Telescope. Our s le consists of 24 nearby SNe Ia at redshift & 0.025. We are able to confirm that no plateau exists in the Ks band for most normal SNe Ia. SNe Ia with broader optical light curves at peak tend to have a higher average brightness on the plateau in J and H, most likely due to a shallower decline in the preceding 100 d. SNe Ia that are more luminous at peak also show a steeper decline during the plateau phase in H. We compare our data to state-of-the-art radiative transfer models of nebular SNe Ia in the near-infrared. We find good agreement with the sub-Mch model that has reduced non-thermal ionization rates, but no physical justification for reducing these rates has yet been proposed. An analysis of the spectral evolution during the plateau demonstrates that the ratio of [Fe ii] to [Fe iii] contribution in a near-infrared filter determines the light curve evolution in said filter. We find that overluminous SNe decline slower during the plateau than expected from the trend seen for normal SNe Ia.
Publisher: Oxford University Press (OUP)
Date: 19-12-2022
Abstract: Recent analyses have found intriguing correlations between the colour (c) of type Ia supernovae (SNe Ia) and the size of their ‘mass-step’, the relationship between SN Ia host galaxy stellar mass (Mstellar) and SN Ia Hubble residual, and suggest that the cause of this relationship is dust. Using 675 photometrically classified SNe Ia from the Dark Energy Survey 5-yr s le, we study the differences in Hubble residual for a variety of global host galaxy and local environmental properties for SN Ia subs les split by their colour. We find a 3σ difference in the mass-step when comparing blue (c & 0) and red (c & 0) SNe. We observe the lowest r.m.s. scatter (∼0.14 mag) in the Hubble residual for blue SNe in low mass/blue environments, suggesting that this is the most homogeneous s le for cosmological analyses. By fitting for c-dependent relationships between Hubble residuals and Mstellar, approximating existing dust models, we remove the mass-step from the data and find tentative ∼2σ residual steps in rest-frame galaxy U − R colour. This indicates that dust modelling based on Mstellar may not fully explain the remaining dispersion in SN Ia luminosity. Instead, accounting for a c-dependent relationship between Hubble residuals and global U − R, results in ≤1σ residual steps in Mstellar and local U − R, suggesting that U − R provides different information about the environment of SNe Ia compared to Mstellar, and motivating the inclusion of galaxy U − R colour in SN Ia distance bias correction.
Publisher: AIP Publishing
Date: 26-03-2013
DOI: 10.1063/1.4798345
Abstract: We report the performance of twenty-four high temperature superconducting dc superconducting quantum interference devices (SQUIDs) whose Josephson junctions were formed from a YBa2Cu3O7-x (YBCO) grain boundary on an engineered MgO step-edge substrate. Each device has the same SQUID design with an inductance of 65 pH, and consisted of two SQUIDs with different junction widths, connected in series, and directly coupled to a large pick-up loop of 7.5 mm × 8.8 mm on a 1 × 1 cm2 chip. 58% of the devices had a peak-to-peak modulation voltage, ΔV, larger than 30 μV. The typical values of the white flux and field noise were 4.7 μΦo and 56 fT/Hz, respectively, in a magnetically shielded environment with 1/f corner frequency at 15–20 Hz. In an unshielded environment, there was no degradation of white noise.
Publisher: Oxford University Press (OUP)
Date: 26-05-2021
Abstract: The analysis of current and future cosmological surveys of Type Ia supernovae (SNe Ia) at high redshift depends on the accurate photometric classification of the SN events detected. Generating realistic simulations of photometric SN surveys constitutes an essential step for training and testing photometric classification algorithms, and for correcting biases introduced by selection effects and contamination arising from core-collapse SNe in the photometric SN Ia s les. We use published SN time-series spectrophotometric templates, rates, luminosity functions, and empirical relationships between SNe and their host galaxies to construct a framework for simulating photometric SN surveys. We present this framework in the context of the Dark Energy Survey (DES) 5-yr photometric SN s le, comparing our simulations of DES with the observed DES transient populations. We demonstrate excellent agreement in many distributions, including Hubble residuals, between our simulations and data. We estimate the core collapse fraction expected in the DES SN s le after selection requirements are applied and before photometric classification. After testing different modelling choices and astrophysical assumptions underlying our simulation, we find that the predicted contamination varies from 7.2 to 11.7 per cent, with an average of 8.8 per cent and an r.m.s. of 1.1 per cent. Our simulations are the first to reproduce the observed photometric SN and host galaxy properties in high-redshift surveys without fine-tuning the input parameters. The simulation methods presented here will be a critical component of the cosmology analysis of the DES photometric SN Ia s le: correcting for biases arising from contamination, and evaluating the associated systematic uncertainty.
Publisher: Springer Science and Business Media LLC
Date: 20-08-2022
DOI: 10.1038/S43246-022-00279-7
Abstract: Reconfigurable terahertz electronics devices with high tuneability are pivotal for next-generation high speed wireless communication and sensing technologies. Significant challenges exist for realizing these devices, particularly on the design of smart metastructures that can manipulate electromagnetic radiation at the terahertz frequencies and the fabrication of devices with effective tuneability and reconfigurability. Here, we incorporate graphene into a graphene/gold bilayer superimposed metamaterial structure, which enables efficient electrical tuning of terahertz waves. A 0.2 THz frequency-selective absorber is designed and experimentally developed using this graphene/gold bilayer metamaterial approach. The device demonstrates 16 dB litude tuning at 0.2 THz resonance and over 95% broadband modulation at just 6 V bias voltage while maintaining a benchmark high-quality factor resonance performance. The design and fabrication methods presented can be readily applied to produce a myriad of tuneable terahertz devices required for high-speed, reconfigurable THz wireless communication and sensing technologies.
Publisher: Springer Science and Business Media LLC
Date: 07-10-2019
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 08-2019
Publisher: Elsevier BV
Date: 09-2017
Publisher: IOP Publishing
Date: 12-08-2014
Publisher: Institute of Electronics, Information and Communications Engineers (IEICE)
Date: 2013
Publisher: Elsevier BV
Date: 09-2017
Location: Australia
Location: United Kingdom of Great Britain and Northern Ireland
Location: United States of America
Location: United States of America
No related grants have been discovered for Simon Lam.