Pui-Wai Ma T.-H. Hubert Chan
We have devised a new type of feedforward neural network. It is equivariant with respect to unitary operators $U(n)$. The input and output can be vectors in $\\mathbb{C}^n$ with arbitrary dimension~$n$. No convolution layer is required in our implementation, and we also avoid errors due to truncated higher order terms in Fourier-like transformat…
Preprint PublishedPui-Wai Ma Sergei L. Dudarev
The concept of elastic dipole tensor of a defect is generalised to enable the treatment of lattice distortions, produced by defects at elevated temperatures. Thermodynamic and statistical mechanics derivations show the feasibility of computing the formation free energy and finite-temperature elastic dipole tensor of a $frac{1}{2}langle 111 rangle$ …
Preprint PublishedJacob Chapman Pui-Wai Ma Sergei Dudarev
Fundamental flaws in the Heisenberg Hamiltonian are highlighted in the context of its application to BCC Fe, including the particular issues arising when modelling lattice defects. Exchange integrals are evaluated using the magnetic force theorem. The bilinear exchange coupling constants are calculated for all the interacting pairs of atomic ma…
PublishedPui-Wai Ma S L Dudarev
Using ab initio density function theory calculations, we have determined the structure of self-interstitial atom (SIA) defects in the most commonly occurring face-centred cubic (FCC) metals. The most stable SIA defects in Al, Ca, Ni, Cu, Pd and Ag are the {100} dumbbells whereas octahedral SIA configurations have the lowest energy in Pt, R…
Preprint PublishedPui-Wai Ma D. R. Mason S. L. Dudarev
We performed ab initio density functional theory simulations of 1/2{111} interstitial dislocation loops, closed and open vacancy loops, {100} interstitial loops and voids in tungsten, using simulation cells involving from 2000 to 2700 atoms. The size of the loops transcends the microscopic scale and reaches the mesoscopic scale where as…
Preprint PublishedFederico Baraglia Pui-Wai Ma
We develop a dynamic model for the evolution of an ensemble of hundreds of interacting irradiation-induced mobile nanoscale defects in a micrometre size sample. The model uses a Langevin defect dynamics approach coupled to a finite element model, treated using the superposition method. The elastic field of each defect is described by its elastic…
Preprint PublishedPui-Wai Ma S. L. Dudarev
Vacancy formation and migration control self-diffusion in pure crystalline materials, whereas irradiation produces high concentrations of vacancy and self-interstitial atom defects, exceeding by many orders of magnitude the thermal equilibrium concentrations. The defects themselves, and the extended dislocation microstructure formed under irradi…
Preprint PublishedPui-Wai Ma S. L. Dudarev
For several decades, the striking contradiction between the Huang diffuse scattering experiments, resistivity recovery data, and predictions derived from density functional theory (DFT) remained one of the mysteries of defect physics in molybdenum. Since the nineteen seventies, observations of Huang X-ray diffuse scattering appeared to indicate …
Preprint PublishedPui-Wai Ma S. L. Dudarev
CALANIE (CALculation of ANIsotropic Elastic energy) program evaluates an elastic interaction correction to the total energy of a localized object, for example a defect in a solid material simulated using an {it ab initio} or molecular statics approach, resulting from the use of periodic boundary conditions. The correction, computed using a fully el…
Preprint PublishedJ. Chapman Pui-Wai Ma S. L. Dudarev
Magnetic plasma confinement is a key element of fusion tokamak power plant design, yet changes in magnetic properties of alloys and steels occurring under neutron irradiation are often overlooked. We perform a quantitative study exploring how irradiation-induced precipitation affects magnetic properties of Fe-Cr alloys. Magnetic properties are simu…
Preprint Published