DSpace Collection:
https://dspace.lboro.ac.uk/2134/6623
2017-06-26T17:17:03ZHigh-frequency acoustoelectronic phenomena in miniband superlattices
https://dspace.lboro.ac.uk/2134/25277
Title: High-frequency acoustoelectronic phenomena in miniband superlattices
Authors: Apostolakis, Apostolos
Abstract: The motion of a quantum particle in a periodic potential can generate rich dynamics in the presence of a driving field. Such systems include, but are not limited to, semiconductor superlattices which exhibit a very anisotropic band structure that results into pronounced nonlinearities and high carrier mobility. In this thesis, we investigate the semiclassical dynamics and electron transport in a spatially periodic potential driven by a propagating wave.
Firstly, we examine the transport features of an electron in a single miniband superlattice driven by a high-frequency acoustic plane wave. In this system, the nonlinear electron dynamics crucially depends on the amplitude of the acoustic wave. The transport characteristics are studied by means of a non-linearised kinetic model. In particular, to provide a realistic description of the directed transport, we employ the exact path-integral solutions of the Boltzmann transport equation. The calculated electron drift velocity and the time-averaged velocity show a nonmonotonic dependence upon the amplitude of the acoustic wave with multiple pronounced extrema. We found out that the changes in the velocity-amplitude characteristics are directly associated with a series of global bifurcations due to topological rearrangements of the phase space of the system. These dramatic transformations are connected with superlattice intraminiband transitions, and accompanied by inelastic emission (absorption) of the quantum particle. The bifurcations also signify the transitions between different dynamical regimes, involving unconfined electron motion, wave-dragging and phonon-assisted Bloch oscillations. Each regime has a characteristic spectral fingerprint, which manifests itself in appearance of specific high-frequency components in the spectra of the corresponding averaging trajectory.
Secondly, we consider to use the acoustically pumped superlattices for an amplification of THz electromagnetic waves, involving the mechanisms similar to the Bloch gain in electrically biased superlattices. In particular, we predict the tunable THz gain due to nonlinear oscillations which are associated with the localised motion of electrons confined by a propagating potential wave. Traditionally, one of the key issues which emerges from considering different schemes for achieving small signal gain in superlattices, is the control of electric stability. Here, it is shown that for our case of the fast miniband electrons driven by an acoustic wave, terahertz gain can occur without the electric instability. Additionally, we find that the characteristic changes in the averaged velocities are connected to the shape of gain profiles.
Consequently, the analytic findings, which determine the transitions between different dynamical regimes at the bifurcations, hold up for the behaviour of amplification of high-frequency electromagnetic waves. The increase of the miniband width, results in an enhancement of the effect of phase space restructuring on the drift velocity and high-frequency gain.
Finally, we analyse the case for a superlattice device utilising acoustic waves with a very slow propagation speed. Benefiting from a simple solution of the Boltzmann equation, here we clarify the role of spatial nonlinearity both in miniband electron dynamics and in amplification of an electromagnetic wave. We show that nonlinear Bloch oscillations occur at a single critical value of the wave amplitude, inducing high negative differential drift velocity. Within this model, we also explain how the amplification of a high-frequency signal can arise below the threshold for an excitation of Bloch oscillations.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2017-01-01T00:00:00ZAspects of neutron residual stress analysis
https://dspace.lboro.ac.uk/2134/25162
Title: Aspects of neutron residual stress analysis
Authors: Wimpory, Robert C.
Abstract: This thesis is concerned with the physical principles, methodology and applications of
neutron diffraction in the measurement of residual stress. Work on three main areas is
presented. 1) Carbon steels 2) Data and Peak Broadening analysis and 3) Single lap
glue shear joints. The Carbon steels section shows the drastic effect of the content of
carbon on the measured stress. This is an aspect which has been somewhat neglected
in the past. The carbon is in the form of cementite, which is a hard compound and
causes the carbon steel to act like a composite material, the ferrite acting as a soft
matrix and the cementite as a reinforcement. The consequence of this is that the two
components develop high microstresses with plastic deformation. This is clearly
illustrated in the work of [Bon 97] where values of approx. 460 MPa in the residual
stress in the ferrite are balanced by negative residual stresses of 2300 MPa in
cementite yielding an overall macro residual stress of zero. In this work it has been
shown that even knowledge of the cementite and ferrite residual stresses and fractions
may not be sufficient to accurately calculate the macro stress since the ferrite
unloading curve is non linear. The use of a single valued constant modulus to convert
from strain to stress is hence not valid.
Peak shape analysis enables dislocation density and cell size estimates to be made.
The thesis examines several methods of data weighting and deconvolution in order to
asses the best means of extracting this information from standard residual stress data.
Care should be taken for the peaks with very low backgrounds when finding the
Gaussian and Lorentzian components. A weighting that avoids the strong bias of zero
and I counts in the detector channels should be used e.g. W = I / ( 10 + Y). Lorentzian
and Gaussian components can be successfully extracted from asymmetrical peaks (of
peaks that broaden symmetrically), using deconvolution method 1, although the data
should be of good quality. Reproducibility has been shown in the Gaussian,
Lorentzian and FWHM for different instruments at different institutes. This is
extremely important for the use of these values for peak broadening analysis and for
estimation of the plastic deformation within a sample.
The neutron diffraction technique has been used to investigate the longitudinal
stresses in the adherend produced as a result of cure and due to the application of a
tensile load in a single lap shear joint. The results throw doubt on widely used finite
element predictions.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.1999-01-01T00:00:00ZPath integral calculation of the Wigner function
https://dspace.lboro.ac.uk/2134/24509
Title: Path integral calculation of the Wigner function
Authors: Lindsey, Neil
Abstract: Elementary Wigner function calculations of the infinite square well and Schroedinger cat
states are presented as an introduction to the quasi-probability function. An entangled
cat state is calculated and the Wigner function of the state is found. Properties
of the entanglement of the state and the nature of its entanglement are found to be
distinguishable by this distribution.
This work is mostly concerned with obtaining the Wigner function via a path integral
method, following a previously published technique. The method approximates the
ground state Wigner function by finding the classical path associated with each point
in phase space, assuming the P-function of the Hamiltonian of the system is able to
be found. The imaginary part of action determines the phase of the path integral
and depends on the geometry of the path; specifically the area which it encloses. An
investigation into two systems, the Morse potential and the double well potential, was
performed to try and find classical paths enclosing area and thus recreating the negative
features of the exact Wigner function. The minimisation of the action found the classical
path for each phase space point. This was performed numerically using tools created in
Excel and Mathematica. In general, it was discovered that the classical paths did not
enclose any area and therefore the Wigner function approximations were everywhere
positive. The majority of those paths which were found to enclose some area produce a
phase which is not large enough to change the sign of the path integral.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2008-01-01T00:00:00ZSecondary electron yield measurements of anti-multipacting surfaces for accelerators
https://dspace.lboro.ac.uk/2134/23255
Title: Secondary electron yield measurements of anti-multipacting surfaces for accelerators
Authors: Wang, Sihui
Abstract: Electron cloud is an unwanted effect limiting the performance of particle accelerators with positively charged particle beams of high-intensity and short bunch spacing. However, electron cloud caused by beam induced multipacting can be sufficiently suppressed if the secondary electron yield (SEY) of accelerator chamber surface is lower than unity. Usually, the SEY is reduced by two ways: modification of surface chemistry and engineering the surface roughness. The objective of this PhD project is a systematic study of SEY as a function of various surface related parameters such as surface chemistry and surface morphology, as well as an effect of such common treatments for particle accelerators as beam pipe bakeout and surface conditioning with a beam, ultimately aiming to engineer the surfaces with low SEY for the electron cloud mitigation. In this work, transition metals and their coatings and laser treated surface were studied as a function of annealing treatment and electron bombardment. The transition metal thin films have been prepared by DC magnetron sputtering for further test.
In the first two Chapter of this thesis, the literature review on electron emission effect is introduced, which includes the process of the electron emission, the influence factor and examples of low SEY materials. In the third Chapter, the experimental methods for SEY measurements and surface investigation used in this work are described. In Chapter 4, the SEY measurement setup which is built by myself are introduced in detail. In Chapter 5 transition metals and their coatings and non-evaporable getter (NEG) coatings have been studied. All the samples have been characterized by SEY measurements, their surface morphology was analysed with Scanning Electron Microscopy (SEM) and their chemistry was studied with X-ray Photoelectron Spectroscopy (XPS). Different surface treatments such as conditioning by electron beam, thermal treatment under vacuum on the sample surfaces have been investigated. For example, the maximum SEY (δmax) of as-received Ti, Zr, V and Hf were 2.30, 2.31, 1.72 and 2.45, respectively. After a dose of 7.9×10-3 C mm-2, δmax of Ti drops to 1.19. δmax for Zr, V and Hf drop to 1.27, 1.48 and 1.40 after doses of 6.4×10-3 Cmm-2, 1.3×10-3 and 5.2×10-3 Cmm-2, respectively. After heating to 350 ⁰C for 2.5 hours, the SEY of bulk Ti has dropped to 1.21 and 1.40, respectively. As the all bulk samples have a flat surface, there are no difference of morphology. So this reduction of SEY is believed to be a consequence of the growth of a thin graphitic film on the surface after electron bombardment and the removal of the contaminations on the surface after annealing.
Chapter 6 of this thesis is about the laser treated surface. Laser irradiation can transform highly reflective metals to black or dark coloured metal. From SEM results, metal surfaces modified by a nanosecond pulsed laser irradiation form a highly organised pyramid surface microstructures, which increase the surface roughness. Due to this reason, δmax of as-received laser treated surface could be lower than 1, which can avoid the electron cloud phenomenon. In this Chapter, the influence of different laser treatment parameters, such as power, hatch distance, different atmospheres on SEY has been investigated. Meanwhile, different surface treatments such as electron conditioning and thermal treatments are studied on the laser treated surface with the investigation of XPS. For example, the δmax of as-received type I with hatch distance 50, 60 and 80 μm in Air are 0.75, 0.75 and 0.80, respectively. After heating to 250 ºC for 2 hours, in all case the δmax drop to 0.59, 0.60, 0.62, respectively. The SEYs of all as-received samples are lower than 1 due to the increasing the roughness on the surface by the special pyramid structure. After thermal treatment, the SEY reduces even further. This is caused by removing the contaminations on the surfaces.
In conclusion, the present study has largely improved the knowledge of the electron cloud mitigation techniques by surface engineering of vacuum chambers. On the one hand, the surface treatments can modify the surface chemistry, such as the produce the graphic carbon layer on the surface by electron condition and the removal the contamination layer on the top of the surface by thermal treatment. On the other hand, the SEY could be critically low by engineering the surface roughness. Both methods allow reaching δmax less than unity. The efficiency of laser treated surface for e-cloud was demonstrated for a first time leading to a great interest to this new technology application for existing and future particle accelerators.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2016-01-01T00:00:00Z