81 research outputs found
Efficient simulations with electronic open boundaries
We present a reformulation of the Hairy Probe method for introducing electronic open boundaries that is appropriate for steady state calculations involving non-orthogonal atomic basis sets. As a check on the correctness of the method we investigate a perfect atomic wire of Cu atoms, and a perfect non-orthogonal chain of H atoms. For both atom chains we find that the conductance has a value of exactly one quantum unit, and that this is rather insensitive to the strength of coupling of the probes to the system, provided values of the coupling are of the same order as the mean inter-level spacing of the system without probes. For the Cu atom chain we find in addition that away from the regions with probes attached, the potential in the wire is uniform, while within them it follows a predicted exponential variation with position. We then apply the method to an initial investigation of the suitability of graphene as a contact material for molecular electronics. We perform calculations on a carbon nanoribbon to determine the correct coupling strength of the probes to the graphene, and obtain a conductance of about two quantum units corresponding to two bands crossing the Fermi surface. We then compute the current through a benzene molecule attached to two graphene contacts and find only a very weak current because of the disruption of the π-conjugation by the covalent bond between the benzene and the graphene. In all cases we find that very strong or weak probe couplings suppress the current
Quench Risk Increase With Radiation Damage
Superconducting magnets are often proposed to confine plasma in fusion
reactors. Superconducting material enables the magnets to carry current
densities that would melt materials with non-zero resistance. Quench occurs
when superconductivity is lost and the current starts to generate heat. Unless
prevented with a fast enough control system, the heat generated during a quench
can cause catastrophic damage to the coils. This work describes a less-studied
heating mechanism that increases the likelihood and aggressiveness of fusion
magnet quenches. Defects accumulate in the magnet structural material under
irradiation by the fusion process. The defects store energy in the material and
change thermal and normal state electrical properties. Wigner energy is
released when defects anneal. After a 0.9 mDPA neutron irradiation, a 10 K
disturbance from 20 K is predicted to release enough energy to result in a
final temperature of 40 K. Irradiation damage also reduces the quench time
constant by increasing normal state resistivity and thus Ohmic heating. The
continuous operation of a fusion reactor produces an increasingly unstable
thermodynamic system in superconducting magnets by changing electrical and
thermal properties with irradiation damage. The temperature margin between
operation and quench runaway reduces with irradiation. The next steps are to
include these observations in quench models and validate the predictions
experimentally. Implications of this work is felt by all fusion powerplant
projects planning to leverage superconducting magnets. Designs will recognize
this risk with more stringent specifications on quench control systems and
maximum duration of coil operation at cryogenic temperature between periodic
releases of Wigner energy to avoid catastrophic quench failures.Comment: 6 pages, 9 figures, 1 table. Work presented at Symposium on Fusion
Engineering 2023 in Oxford on July 13th. The work is a hot topic and has been
shared publicly. The work has significant implications for the design of some
private machines. After publishing on Arxiv the work will be shared for peer
review. The work will be submitted for a IEEE Transactions on plasma science
early Septembe
A comprehensive experimental study of whispering gallery modes in a cylindrical microresonator excited by a tilted fiber taper
Whispering gallery modes (WGMs) excitation in a cylindrical microresonator formed by a section of silica optical fiber has been studied. Evanescent light coupling into the microresonator is realized using a tapered optical fiber, fabricated by a microheater brushing technique. Several types of silica fibers with different diameters are studied as microresonators, and the influence of the resonator's diameter on the excitation of WGMs is investigated. The excitation of WGMs in a cylindrical fiber resonator were studied with changes to the tilt angle between the microcylinder and the fiber taper in the range of angles from a perpendicular position (0°) to large tilt angles (24°). The evolution of the fiber taper transmission spectrum with the change of the tilt angle results in changes in the intensity, broadening of and a blue shift in the WGM resonance spectra. Overall losses in the taper transmission spectrum decrease with the increase of the taper tilt angle from its perpendicular position, followed by a complete disappearance of the WGM resonances at large tilt angles greater than 20°
Nearly Monodispersion CoSm Alloy Nanoparticles Formed by an In-situ Rapid Cooling and Passivating Microfluidic Process
An in siturapid cooling and passivating microfluidic processhas been developed for the synthesis of nearly monodispersed cobalt samarium nanoparticles (NPs) with tunable crystal structures and surface properties. This process involves promoting the nucleation and growth of NPs at an elevated temperature and rapidly quenching the NP colloids in a solution containing a passivating reagent at a reduced temperature. We have shown that Cobalt samarium NPs having amorphous crystal structures and a thin passivating layer can be synthesized with uniform nonspherical shapes and size of about 4.8 nm. The amorphous CoSm NPs in our study have blocking temperature near 40 K and average coercivity of 225 Oe at 10 K. The NPs also exhibit high anisotropic magnetic properties with a wasp-waist hysteresis loop and a bias shift of coercivity due to the shape anisotropy and the exchange coupling between the core and the thin oxidized surface layer
Whispering Gallery Modes in Standard Optical Fibres for Fibre Profiling Measurements and Sensing of Unlabelled Chemical Species
Whispering gallery mode resonances in liquid droplets and microspheres have attracted considerable attention due to their potential uses in a range of sensing and technological applications. We describe a whispering gallery mode sensor in which standard optical fibre is used as the whispering gallery mode resonator. The sensor is characterised in terms of the response of the whispering gallery mode spectrum to changes in resonator size, refractive index of the surrounding medium, and temperature, and its measurement capabilities are demonstrated through application to high-precision fibre geometry profiling and the detection of unlabelled biochemical species. The prototype sensor is capable of detecting unlabelled biomolecular species in attomole quantities
Multiscale modelling for fusion and fission materials: the M4F project
The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)
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