Undergraduate Summer 2020

Supervisor:
Dr David Ward

Nature of project:
computational

Requirements:
Part 1B level physics or above

Efficient pixel acquisition for imaging with helium atoms

The project is intended to have more independent and group work than some projects to address a real experimental challenge. A chat application will be used to share material and thoughts about the project. The nature of the project is that it has no right or wrong answers and the project will benefit from multiple people working on different solutions. 

Imaging with helium involves moving a sample in a beam of helium atoms while measuring the number of helium atoms scattered to a fixed detector; moving the sample allows a real space scattering map of the sample to be formed. It’s been shown that the data contains information related to the topography of the sample and scattering dependant features such as diffraction, inelastic scattering etc. While the benefits of the technique are significant, it typically takes in excess of 300ms per pixel to collect enough data and therefore images of typically thousands of pixels square take many hours to acquire. The project involves making a sampling method to reconstruct images, acquiring as much information as possible and can consider a wide range of issues such as low resolution pre-imaging to guide higher resolution imaging, multiple-pass techniques, prior knowledge of the sample, accuracy vs speed, existing methods such as compressed sensing, in-painting etc. used either in compression algorithms or in other acquisition techniques.

The intention for the project is to provide a selection of experimental or simulated sample images and for candidates to take some time to independently find methods to gather the maximum information from the images with the fewest measured pixels. The group will then come together to apply their methods to previously unseen data and the different methods or techniques evaluated against each other.

Supervisor:
Dr Anton Tamtögl

Nature of project:
computational

Requirements:
Part 1B level physics or above

The role of adsorption geometry and interactions in surface diffusion

Triphenylphosphine (PPh₃ ) is an important ligand for organic and nanoparticle synthesis and shows a complex self-assembly behaviour on metal surfaces. The diffusion of PPh₃ on graphite has been studied experimentally with neutron scattering experiments. Unlike previous studies in the group, considering flat hydrocarbons on graphite, the PPh₃ molecule exhibits a completely different geometry: PPh₃ is pyramidal and adsorbs in an upright fashion in contrast to flat hydrocarbons which typically adsorb in a planar configuration.

The aim of the project is an analysis of the experimental scattering data and depending on the progress, some simple molecular dynamics simulations. It should thus allow to conclude whether the different adsorption geometry has an influence on the diffusive motion.

Supervisors:
Prof. Paul Dastoor
and Dr. Matthew Bergin

Nature of project:
computational

Requirements:
Part 1B level physics or above

Resolution in a scanning helium microscope

The resolution of a microscope is one of the most important parameters for describing its performance and is often used to compare different instruments with each other. However, it is relatively difficult to define a quantitate method for determining the resolution of an instrument and while some methods have been developed for other techniques, little discussion of the topic has occurred for a scanning helium microscope. Initial attempts have been made at applying established methods of determining the resolution of an image to both experimental data and crudely simulated images. The values obtained from the simulating images match our expectations, however the experimental result seems to be larger than expected. The aim of this project will be to apply more sophisticated ray tracing methods to create realistic images that can be used to test a range of resolution algorithms. Through studying the simulated images, we can better understand what controls the resolution in a scanning helium microscope and determine a standard definition of resolution for the field.

Supervisors:
Prof. Gil Alexandrowicz and Dr. Helen Chadwick.

Nature of project:
computational

Requirements:
Part 1B level physics or above

Simulations of magnetically manipulated atomic and molecular beams (two connected projects)

This summer project is related to a major European research project aimed at controlling the rotational quantum states of molecules during a collision with a solid surface ("Rotational Waves" - https://cordis.europa.eu/project/rcn/214696/factsheet/en ). The computational work carried out by the student will use classical and quantum mechanics based simulations to calculate the trajectories and spin states of molecules through the experimental beam line. The student will learn the physical principles of the new experimental technique ( https://www.nature.com/articles/ncomms15357 ) and gain experience in programming, executing and analysing numerical simulations. The results of this project will be used for the interpretation of experimental results from past and future molecular beam experiments. Due to Covid-19 restrictions the project will be supervised remotely, although once restrictions are lifted, students will be invited to visit the laboratory and the see the experimental setup they simulated.

Supervisor:
Dr Holly Hedgeland

Nature of project:
computational

Requirements:
Part 1B level physics or above

Semiconductor surface dynamics: rocking dimers and ad-atom tram tracks

 The rocking dimers of the Si(100) surface have been well-characterised by scanning tunnelling microscopy and spectroscopy, with the dynamic oscillation of the up and down atoms in the dimer pairs persisting on cooling until buckling of the rows is locked in at liquid helium temperatures. Helium spin echo has been used to observe the motion of the analogous Ge(100) surface and this project would involve analysis of that data set. In addition to the rocking dimer behaviour, the Ge(100) surface has been noted to contain adatoms at elevated temperatures. The project would seek to resolve the contributions to the data associated with these surface phenomena and elucidate the story of the motion of this semiconductor surface.

Supervisor:
Dr Nadav Avidor

Nature of project:
computational/theoretical

Requirements:
Part 1B level physics or above *

Stochastic representation of inter-adsorbate interactions in molecular dynamics simulations

In molecular dynamics simulations of surface diffusion, the interactions between adsorbates are normally represented by pair-wise inter-adsorbate interaction potentials. At each time step, the force between each pair of particles is calculated. Such representation, while crucial for accurate representation of the surface-system, is computationally time consuming, and challenging in cases where the interaction is anything beyond a simple dipole-dipole repulsion. It would be of interest to model the interactions using a more computationally economical approach, which provide an intuitive insight to the interactions. For example, using random force drawn out of coloured noise, with a noise spectrum which represents the time and strength characteristics of the interactions.

The project is to demonstrate that computationally such representation is possible, via two steps: (a) Simulate a known system such as the diffusion of sodium on hexagonal surface, using explicit inter-adsorbate interactions (b) simulate the system using Generalised Langevin MD simulator, without explicit interactions, but with a random force as explained above. By that, show that non-interacting Generalised Langevin simulations (with "memory friction") can be used to represent inter-adsorbate interactions.

Extensions to the project can be (a) look for mathematical relations between the two representations of the inter-adsorbate interactions, and (b) model a more challenging system such as water diffusion on a topological insulator, effectively extending a recent published work ( https://www.nature.com/articles/s41467-019-14064-7 ).

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above *

Long jumps in surface diffusion

When a particle in the adsorption well is 'jumping' out of the well, it will travel on the surface and will be re-settled in a different well. The length of travel is governed by factors such as the rate of energy dissipation, however, it is normally expected that the probability to travel to a far adsorption well, will be smaller than travelling to a nearer well. For some systems, this is not the case, and we would like to find out why.

This project is concerned partly with exciting experimental data, however, not via direct analysis of it. It is to explore a relation between long jumps and the power spectrum of the surface fluctuations (using simulations based on Generalised Langevin Equation).

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above

Machine learning for Langevin MD analysis
Progression towards schemes of a more accurate analysis (of surface dynamics) involves inclusion of more dimensions in our simulations, including 'physical dimensions' such as rotations, but also more detailed parametrisation of the potential energy surface (PES), or the inter-adsorbate interaction potentials etc. However, sweeping through a multi-dimensional problem can become computationally too expensive. The project would be to establish the framework for machine learning assisted MD simulations, to enable explorations of multi-variable problems. Application of the tool could be in analysis the diffusion of Anthraquinone on copper surfaces (DOI: 10.1126/science.1129309 and DOI: 10.1126/science.1135302).

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above

Position dependent friction

In analysing measurements of surface diffusion, we normally assume that nanoscale friction is not position dependent (can be averaged over the unit-cell). The project would be to explore and characterise when this assumption breaks.

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above

Water diffusion in an oxygen matrix

We have an exciting data-set which awaits more detailed analysis. The project would be to analyse the data using known analytical models and molecular dynamics simulations.

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above *

Resolving multi-species signal in helium spin echo measurements

In order to develop an application of helium spin-echo spectroscopy in studying chemical reactions, we would like to develop the analysis framework for resolving signals from multiple species diffusing at a surface . The project would be to explore this application by considering co-diffusion and how form-factors of different species affect the line-shape of our measured signal.

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above

Nanoscale mechanisms in growth of 2D materials

2D materials are becoming industrially important, and understand associated growth mechanisms can improve our approach to controlling the growth process. Recent experiments in the group, of growing hexagonal boron nitrid (hBN), or 2D oxygen matrices (see above the project for water in oxygen matrix), highlighted the need to understand phase transitions during growth.

The project is to simulate surface diffusion during phase transitions, in the presence of attractive interadsorbate forces, and to explore the kinetics of island formation during growth of 2D mateirals.

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above *

Quantum mechanical scattering calculations of molecular form factors

The helium-surface interaction potential is a central problem in helium atom scattering. Ability to resolve scattering potentials will enable us to include form factors in analysis of helium spin-echo (HeSE) experiments, which will be a game changer in developing novel applications for the HeSE technique.

The project is to perform exact quantum mechanical calculation using close-coupled equations, of helium scattering from a realistic surface. The aim would be to find the momentum dependent form-factor of a benzene molecule on hexagonal surface. A second step could be to compare the form-factor to previously measured data.

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above

Analysis of helium diffraction from water/Ni(111) using quantum mechanical scattering calculations

Supervisor:
Dr Nadav Avidor

Nature of project:
computational

Requirements:
Part 1B level physics or above

Supervisor:
Dr John Ellis

Nature of project: computational

Requirements: Part 1B level physics or above

A new compact high efficiency He detector for He microscopy and spin echo experiments
The surface physics group specializes in using helium atom scattering to study surfaces. It has pioneered the development of scanning helium atom microscopy (SHeM) for real space imaging and helium spin echo spectroscopy (HSE) for studies of ultrafast atomic scale motion. Both rely on efficient detectors of helium atoms and the group has engaged in a program of detector development. The current detectors rely on a high current, water cooled, solenoid that is cumbersome to construct/mount and consumes a lot of power and cooling water. Prelimnary work has been done in the group on replacing this with a permanent magnet driven alternative.

This project aims to convert these initial ideas to a workable design for a detector. This requires extensive use of finite element analysis techniques, and for a number of conceptual and technical problems to be overcome. Recent developments on other detectors in the group, however, suggests that this project is now feasible and has great potential for impact on the efficacy of helium scattering techniques.

Supervisor:
Dr John Ellis

Nature of project:
computational/theoretical

Requirements:
Good understanding of material Part II TP2

Simulations of a quantum particle connected to a heat bath: H diffusion on close packed surfaces
The quantum propagation of a particle connected to a heat bath as the subject of considerable current interest, relating, for example, to the dephasing of qbits in quantum computing. The surface physics group has a long standing interest in hydrogen atom diffusion, a key step in many of the processes related to the proposed ‘hydrogen economy’, and as a considerable body of high quality atomic length and time scale measurements of hydrogen diffusion across close packed metal surfaces. This data is controlled by inelastic tunneling – i.e. tunneling propagation connected to the heat bath of the phonons and electrons of the surface and in principle provides an excellent testing ground for ‘particle connected to a heat bath’ theories and methods.

This summer project is part of a wider, speculative and ambitious project, and the difficulty of the project should not be underestimated. However, the group already has well characterized wavefunctions for the hydrogen atoms in the atom-surface potential which have been benchmarked against experimental data – and these provide an excellent starting point for this project, which will provides an excellent chance to try and actually use theoretical and computational approaches to model and interpret real data.

Supervisor:
Dr John Ellis

Nature of project: computational

Requirements:
Part 1B level physics or above

Reduced coordinate representation of complex molecule diffusion at surfaces
The surface physics group has developed the technique of helium spin echo spectroscopy as a probe of motion of atoms and molecules across surfaces on atomic length (Angstrom) and time (ps to ns) scales, which provides a wealth of detailed information of surface diffusion. This provides an excellent testing ground of rate theory. The diffusion of molecules is particularly challenging- if they are composed of N atoms, then there are in principle 3N coordinates involved in the diffusion problem, making realistic modelling all but impossible. An alternative approach is to work with a reduce number of coordinates, and to this end a series of measurements have been made on the diffusion of rigid ‘ice puck’ like molecules , where one would start with the lateral (x-y) position of the molecule centre of mass on the surface and the angular orientation of the ice puck and see what extra degrees of freedom (coordinates) of the molecule are needed to explain the observed motion.

This project will focus on the diffusion of cobalt phthalocyanine, where measurements show an ‘anomalous’ diffusion jump length/orientation distribution indicative of the failure of simple centre of mass approaches to model and interpret the motion. This makes it an ideal candidate for modelling work to establish which extra degrees of freedom must be included in order to sufficiently represent the motion of this complex species.

Supervisor:
Dr Andrew Jardine

Nature of project:
computational

Requirements: Part 1B level physics or above

Dynamics on Quasicrystal Surfaces
Quasicrystals are an unusual form of matter that are ordered, but lack long range translational symmetry. The aim of this project will to review existing research on the dynamics in and on quasicrystal surfaces, and to develop simulations that will reveal how dynamics on quasi-crystals would show up in helium spin-echo measurements.

Supervisor:
Dr Andrew Jardine

Nature of project:
computational

Requirements:
Part 1B level physics or above *

Scattering from Disordered Surfaces
Understanding how atoms scatter from atomically disordered surfaces is of fundamental importance to the emerging technique of scanning helium atom microscopy (SHeM), as it determines how images are formed of many surfaces. The aim of this project will be to first develop computational models of randomly rough surfaces with different forms of atom-surface interaction potential. The second step will be to model scattering from those surfaces using a suitable approximate scattering method, such as the ‘sudden approximation’. The results from the project will be able to be supplied to a separate programme, modelling scattering from real surfaces.

Supervisor:
Dr Andrew Jardine

Nature of project:
computational

Requirements:
Part 1B level physics or above

Supervisor:
Dr David Ward

Nature of project:
computational

Requirements:
Part 1B level physics or above

A better helium source for Helium Microscopy (SHeM)
Supersonic helium sources have been a key technology enabling scattering experiments in surface science experiments for decades. Now the same types of sources are being used in Scanning Helium microscopy (SHeM) however the requirements differ from those required for traditional atom scattering. Recently data on helium sources has been acquired and the project will consider modelling sources to understand the data, particularly focussing on optimising designs for small helium sources and the optimisation of them for brightness for a new generation of microscopes. The nature of the project allows some flexibility in direction and may include fluid dynamic simulation of skimmer geometries using an existing software package to understand data or it may focus on design optimisation for the instrument.

Supervisor:
Dr David Ward

Nature of project:
computational

Requirements:
Part 1B level physics or above

Fast motion, the initial drop and the onset of friction
The project considers motion line-shapes in the intermediate scattering function and particularly the timescale which is faster than the characteristic presented through the co-efficient of friction for diffusive dynamics. At those timescales there are various competing processes including substrate phonons, wide multi-phonon background, the onset of diffusive dynamics and interactions. Traditional analysis methods rely on the central limit theorem and as such the detailed experimental noise spectrum does not need to be consistent with that of the simulation which is applied to particles. In the project a coloured noise approach will be used to consider existing experimental measurements of lithium dynamics on copper and attempt to reproduce experimental data using the generalised Langevin equation and will then consider the physical information that may be derived from the analysis. 

Supervisor:
Dr David Ward

Nature of project:
computational

Requirements:
Part 1B level physics or above *

Inelastic scattering in helium microscopy
Helium microscopy is a technique that enables the surfaces of delicate samples to be probed without the damage that can occur with some other techniques. Most of the experimental data collected so far has demonstrated contrast that can be attributed to surface topography. In the project contrast that is derived from the detailed nature of the scattering distribution is considered. The project will be conducted using a ray tracing framework that has already been developed in the research group and will consider the physics in the scattering distribution and how it may exhibit itself in a variety of samples. To start the project Debye-Waller scattering will be considered and particularly the effect of surface temperature on potential contrast formation; the project will develop to consider interesting technological materials such as graphene or other 2D materials and composite materials that are challenging to investigate with other techniques.