• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 6678
  • 939
  • 700
  • 678
  • 6
  • 4
  • 3
  • 2
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 12212
  • 6797
  • 3139
  • 3137
  • 3137
  • 2087
  • 2029
  • 1856
  • 1108
  • 632
  • 594
  • 587
  • 476
  • 365
  • 353
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Milli-Kelvin thermodynamic and transport measurements of low dimensional systems in high magnetic fields

Smith, Martin J. January 2009 (has links)
This thesis presents an investigation into aspects of the integer quantum Hall effect, specifically the near-dissipationless state of the longitudinal resistivity ρxx between Landau levels, and the associated broadening of the levels. Eddy currents induced by a time varying magnetic field B are considored in chapter 4. The temperature dependences of the eddy currents were measured over the range 100 mK to 1600 mK. The peak current at filling factor ν = 2 was shown to saturate at > 800 mK, more robust than previously observed, but was reduced by elevating the temperature to 1600 mK. The saturated regime is associated with a breakdown of the quantum Hall effect, and in this case, the most likely candidate for the saturation is an electron heating effect. Sweep-rate dependences were characterised for a range of filling factors and temperatures, and even for the lowest sweep rates, never entered a linear regime. Induced currents ν = 1, 2 and 4 all saturated at the same critical value at 100 mK, but ν = 4 was shown to reduce with slower sweep rates, consistent with the prediction that the ρxx minima is not as small as for lower Landau levels. Induced current decays were measured to be similar to previous work, a fast initial decay attributed to breakdown of the QHE followed by a much longer slow decay. The eddy decay of ν = 1 at low temperature, in the slow decay regime, is among the most persistent reported. It was shown that the assumptions of previous work had not evaluated the mutual inductance of the eddy current in the presence of the magnet sufficiently. By fitting a suitable function to the IV characteristic of ν = 1 the shape of the induced current was modeled. The model agreed with the data, producing a similar shape and a very long time constant for the slow decay.In chapter 5 the hysteresis in the magnetoresistance of a quantum point contact was investigated, through a simultaneous transport and magnetometry measurement. Induced currents corresponding to filling factors up to ν = 8 were measured. Three corresponding features were measured in the magnetoresistance of a QPC, one more than previously seen. The temperature dependence was measured simultaneously, and for Landau level filling factor ν = 1, the general shape of the curves was the same. The sweep rate IV characteristics of the the two experiments were similar. Sweeping the magnetic field B to a fixed field position and waiting, demonstrated that both phenomena decay with time, a fast decay of seconds and a slow decay taking more than 10, 000 seconds. An attempt was made to affect the induced eddy current by switching the QPC gate on/off. Experiments on a fast timescale, 10 ms, resolved structure in the induced currents that has previously been attributed to the noisy breakdown of the quantum Hall effect. By performing a simultaneous measurement, individual breakdown events were seen and correlated. After investigating the zero-resistance state in chapter 4 and chapter 5 with induced currents, exactly how the zero-resistance state varied between Landau levels was the topic of chapter 6. A method was presented for the fabrication of a novel device, to measure the magnetisation and the heat capacity of a 2DES at the same time. AuGe thin film resistors were grown in only 10 bilayers, reducing the heat capacity per unit area by approximately an order of magnitude on previous workers. The AuGe thermometers were shown to be ‘tunable’, i.e. the temperature dependence was dictated by the annealing conditions after growth, so thermometers with different gold concentrations due to growth conditions, could be tuned to have similar temperature dependences. Low temperature thermometers with small heat capacities were repeatably produced, and thermometer D5 is presented in this thesis with a sensitivity of S = 0.58. At an elevated refrigerator temperature of nearly 300mK, heat pulses of ∼ 26nJ were resolved on a device which had a 100% front processing success rate, but was not etched from the back. It was shown that a device to measure the broadening of the low temperature, high magnetic field 2DES density of states is possible.

Modification and monitoring of magnetic properties with ultrafast laser pulses

Al-Jarah, Uday Ali Sabeeh January 2013 (has links)
Investigations of the static and dynamic electronic, optical and acoustic properties of different nanostructures are presented. Magneto-optical Kerr effect (MOKE) magnetometry has been used to probe the magnetic properties of the magnetic nanostructures. A time-resolved all-optical pump-probe technique, using femtosecond laser pulses, has been employed to investigate the ultrafast magnetisation dynamics, and transient polarisation and reflectivity responses. The magnetic samples studied were permalloy (NiFe) nanowire arrays and multilayered CoNi/Pt films and nanodot arrays, while the non-magnetic samples were phase change GeSbTe thin films. These structures have attracted much attention because their properties can be advantageous in data storage applications. Static MOKE measurements of the NiFe nanowires revealed zero coercivity and remanence, regardless of the direction of the applied magnetic field, with the magnetic easy axis perpendicular to the axis of the nanowire. This is the result of antiferromagnetic alignment of the magnetization in adjacent nanowires at remanence. Time-resolved MOKE (TRMOKE) measurements performed upon the nanowires showed increasing demagnetisation with increasing pump fluence, with a larger response being observed when the magnetic field was applied perpendicular to the nanowire axis. This behaviour, together is believed to result from the formation of vortices at the end of the nanowires. Moreover, the TRMOKE response revealed oscillations due to modes of magnetic precession with frequencies that have minima at a field rather close to the saturation field of the samples. For lower fields, the frequencies decrease with increasing applied field, while for higher fields, they increase with increasing applied field. This behaviour is believed to result from the strong dipolar interactions that can overwhelm the shape anisotropy of an individual nanowire leading to a switching of the easy axis from parallel to perpendicular to the nanowire axis. The magnetisation may also break up into domains for field values less than the saturation value, which results in a decrease in the dipolar coupling with decreasing applied field. Static MOKE measurements of the CoNi/Pt multilayers showed that the saturation Kerr rotation increases with increasing packing density of the sample, while the coercive field decreases after patterning, but increases with decreasing diameter among the patterned samples. AC-MOKE measurements revealed that increasing pump fluence leads to decreasing coercivity and increasing demagnetisation, which is attributed to the increased heating of the surface of the dots and, thus, an increased temperature. Full demagnetisation and total loss of coercivity were achieved for all the nanodot arrays. The AC-MOKE results are in good agreement with the results of TRMOKE measurements. Transient polarisation measurements showed a clear specular-optical Kerr effect (SOKE) response for all the samples. This response appears as a peak at the zero delay position that has maximum (zero) effect when the pump and probe electric fields lie 450 (00 or 900) apart, and is accompanied by longer-lived damped oscillatory modes for the nanowires and nanodot arrays. A mechanism involving the optically induced electric polarisation of the nanodots and nanowires has been suggested to explain this response. Moreover, an epitaxial GeSbTe film revealed a robust dependence of the transient polarisation upon the sample orientation which suggests a strong influence of the crystallographic structure for this sample. The time-resolved reflectivity (TRR) measurements for the nanowire and nanodot arrays revealed a linear dependence of the amplitude of the transient reflectivity upon the pump fluence. A number of oscillatory modes with different GHz frequencies were observed to be superimposed upon an exponentially relaxing background, while a single mode was observed in the CoNi/Pt continuous film. These oscillations are believed to result from the excitation of surface acoustic waves (SAW). Two principal mechanisms have been suggested to explain the excitation of SAWs within the nanodot arrays. A discrepancy between the experimental and frequencies predicted by an existing model was found which is believed to be due to the neglect of the sample composition and the SAW velocity of the nanostructures within this model. The development of a model that overcomes these weaknesses is suggested for future work. An additional THz frequency mode was observed within the GeSbTe which is believed to arise from the excitation of optical and acoustic phonon modes. Further work is required to identify the observed phonon modes and to relate the associated optically induced linear birefringence to a specific structural distortion.

On the nanostructure of biogenic and bio-inspired calcium carbonate as studied by electron microscopy techniques

van de Locht, Renée January 2014 (has links)
Most biominerals in nature are formed from both organic and inorganic (mineral) compounds, and are thus by definition a composite material. They are hierarchically ordered from the nanoscale and often have superior mechanical properties compared to synthetic ceramics [1]. This study focusses on the structural characterisation of aragonite and calcite biominerals, combined with the investigation of formation mechanisms through the synthesis of bio-inspired or biomimetic crystals. To this end a multi-length scale study of aragonite and calcite based minerals is presented, based primarily on electron microscopy techniques and supported by Raman spectroscopy and chemical analysis of the organic compounds. Aragonite skeletal material from corals is studied in detail from the nano-to microscale. This is compared to calcium carbonate crystals precipitated in the presence of organic molecules with hydroxyl-groups, namely ethanol. Secondly, we look at the calcite based system of coccolithophores (marine algae) which precipitate their exoskeleton intracellulary. Such crystals formed in confinement are compared to the structure of synthetically produced calcite nanowires, grown in track-etch membranes. The coral’s spherulites (roughly 10-20 µm in size) were found to consist of three distinct crystalline phases. This microstructural sequence could for the first time be directly correlated to diurnal growth bands observed in optical transmission images and are linked to a light enhanced calcification process. The synthetic CaCO3 precipitation experiments showed that increasing ratios of ethanol resulted in a shift of crystal phase and morphology from single crystal rhombohedral calcite to branched polycrystalline aragonite, the latter being similar to the coral. The calcite coccoliths of Rhabdosphaera clavigera exhibit centrally positioned, several micrometre long five-fold symmetric spines. The spines are made up of spiral staircase arrangements of {104} single crystal calcite rhombohedra. However the rim of the coccolith has complex shaped, kinked, crystal elements. It was found that such unconventional crystal shapes can be promoted by external anisotropic surface stresses as was seen for the calcite nanowires investigated in this study.

Production and oscillations of a Bose Einstein condensate on an atom chip

Yuen, Benjamin January 2014 (has links)
This thesis describes production of and experiments with a Bose-Einstein condensate of approximately 2 × 10[superscript 4] [superscript 87]Rb atoms, trapped at the surface of an atom chip. In the first half of this thesis I describe the process of trapping and cooling the atomic vapour close to the surface of an atom chip. This process, which cools the vapour by over 9 orders of magnitude, involves a highly complex sequence of events which I implemented and optimised over the first two years of my PhD. In the early stages of this process, the atomic vapour is laser cooled and magneto-optically trapped. The vapour is then transferred to a highly elongated magnetic trap produced by high field gradients a few hundred microns from the surface of the atom chip. Here the vapour is evaporatively cooled to below the transition temperature where a Bose-Einstein condensate emerges. A simple existing analytic model of evaporative cooling is extended in this work to account for the shape of our highly elongated trap. Predictions of this model are presented here along with experimental observations with which it has good agreement. The second part of my thesis investigates some of the characteristics of the condensate, and dynamics of its low energy collective oscillations in the trap, based on experimental measurements taken in the final 18 months of my PhD. In particular, measurements taken of the centre of mass oscillations of the condensate along the long axis of the trap are presented. In the zero temperature limit the condensate is expected to behave as a perfect superfluid, and these low energy oscillations should go undamped. However, at finite temperature where not all atoms in the gas are condensed, damping is observed. In our experiment significant damping is found with an 1/e decay rate which varies between 2s[superscript -1] and 8s[superscript -1], depending on the fraction of non-condensed atoms in the gas. A finite temperature formalism is then used to describe the likely damping mechanism - Landau damping. We use a simple model of this formalism which estimates the temperature dependence of the damping rate γ(T), but find this gives a significant overestimation of the rates we measure. However, we argue that a straightforward adaptation to this model reduces the predicted damping rate significantly, and suggests a functional form of γ(T) that is in much better agreement with our experimental measurements.

Implementation of a multinucleon neutrino interaction simulation and comparison with T2K data

Sinclair, Peter January 2014 (has links)
A multinucleon neutrino interaction mode is implemented into the NEUT neutrino interaction generator. The model allows the simulation of fully differential event kinematics and uses the prediction of Nieves et al. for the total cross section and final-state charged lepton kinematics. The model is fit to T2K near-detector data using a Markov chain monte carlo method, and the goodness-of-fit is evaluated. The CCQE model in NEUT is fit to published data from the MiniBooNE experiment to inform the size of the prior systematic uncertainties associated with the charged-current quasielastic interaction mode. The priors were applied in the 2012 T2K analyses. The fit required development of monte carlo reweighting functions which allowed changes to the nuclear model to be simulated. Both the fits, and the development of the reweighting functions, are discussed in detail.

Ultrafast measurements in condensed matter

Okell, William January 2014 (has links)
In this thesis I describe the development of an apparatus for performing attosecond photoelectron measurements in condensed matter, and the completion of attosecond streaking measurements on metal films using the apparatus. A commercial Ti:sapphire chirped pulse amplification laser system was used to generate 28fs, 2.5mJ pulses with a central wavelength of 790nm and a 1kHz repetition rate. These pulses were post-compressed using a hollow fibre system. The resulting few-cycle pulses had sub-4fs duration, and 0.4mJ energy. Diagnostics performed on the hollow fibre system revealed ionisation induced carrier-envelope phase fluctuations at input pulse energies in excess of 1mJ. These fluctuations were avoided for attosecond experiments by careful choice of the experimental parameters. The few-cycle pulses were used to generate isolated attosecond pulses through high-harmonic generation, in an amplitude-gating scheme. An ultra-high vacuum compatible two-part extreme-ultraviolet multilayer mirror setup was developed for attosecond streaking measurements on surfaces. The base pressure in the experimental chamber is 3 x 10^-9 mbar. Attosecond streaking measurements were performed on amorphous WO3 and polycrystalline Au, without any prior surface cleaning. The results indicate the applicability of attosecond streaking to a very general class of solid state samples. In WO3 the 4f photoemission precedes the valence photoemission by 140 ± 190as. In gold, possible signatures of plasmon dynamics were detected.

Using rare decays to probe the standard model at LHCb

Sepp, Indrek January 2014 (has links)
The Large Hadron Collider beauty Experiment (LHCb) detector is one of the four main particle detectors on the Large Hadron Collider (LHC). It is dedicated to the study of physics processes involving b quarks. This thesis presents three analyses of data collected by LHCb. The first measures the photoelectron yield of the Ring Imaging Cherenkov Detector (RICH) detector subsystem, which distinguishes between pions, kaons and protons. The yield is seen to be 15%(19%) less than that in the simulation for the C4 F10 (CF4) radiator medium. The result is a Particle Identification (PID) performance which is sufficient for the physics goals of LHCb, albeit slightly less than expected from simulation. No evidence is found for the deterioration of the Hybrid Photon Detector (HPD) quantum efficiency, mirror reflectivity or RICH medium transparency over the course of 2011 and 2012 data collection. The second analysis measures the dependence of the b → Λb to b → Bd hadronisation ratio on the pT and η of the Λb and Bd. An exponential function with a plateau provides the best fit for the pT dependence. A linear dependence of the ratio to η is also observed. These observations are substantial improvements on previous measurements of the dependencies that can aid the development of QCD models and simulation frameworks that describe b quark hadronisation. The third analysis presents the world's first search for the decays Bs → μ+ μ- μ+μ- and Bd → μ+ μ- μ+ μ- . Upper limits are set on the branching fractions of both decays that are ~ 2 orders of magnitude above Standard Model (SM) expectations. These limits begin to exclude the phase-space of supersymmetric models where the decays are mediated by S and P sgoldstinos.

Characterization of particle dark matter via multiple probes

Strege, Charlotte January 2014 (has links)
The dark matter problem is one of the most striking puzzles in physics today. Cosmological and astrophysical observations have provided strong evidence that over 80% of the matter in the Universe is dark. However, direct proof for the existence of dark matter particles from laboratory experiments is still lacking, so that the physical nature of dark matter remains unknown. Possible solutions are found in theoretical models of new physics, which propose new particles that are excellent dark matter candidates, thus presenting a fundamental connection between elementary particle physics and the astrophysical dark matter. In this thesis, I adopt a multi-messenger approach towards the identification and characterisation of the dark matter particle. I apply advanced statistical and numerical techniques to probe theoretical models and derive robust constraints on the nature and properties of dark matter in light of the full range of existing experimental results. I present global fits analyses of three models of supersymmetry (the cMSSM, the NUHM and the MSSM-15), including data from collider searches for new physics, cosmology experiments, astro-particle dark matter searches, and the Higgs boson discovery. A strong complementarity between the LHC and astro-particle experiments is observed, highlighting the benefits of a combined analysis. I find that constrained models, such as the cMSSM and the NUHM, that were appropriate targets for global fits prior to the start of LHC operations, have been placed under strong pressure by recent data sets. I present the first statistically convergent profile likelihood maps of a 15-dimensional MSSM, which is only weakly constrained by the existing data, and is a much more suitable framework for phenomenological studies of supersymmetry. I derive robust and statistically meaningful constraints on the supersymmetric parameters and dark matter properties in this model. Detection prospects for the cMSSM and the NUHM are positive, while fully probing the rich phenomenology of the MSSM-15 is more difficult. I present the regions of the parameter spaces that are most promising to explore with future searches and pinpoint the signatures characteristic of supersymmetric dark matter in these models. A very effective experimental strategy is the direct detection of dark matter. I explore the statistical limitations of next-generation direct detection experiments in the case of a significant detection. I find that the uncertainty and bias in the reconstructed WIMP properties is particularly severe for heavy WIMPs, but can also be significant for intermediate-mass WIMPs leading to several hundreds of events. I demonstrate that the precision and accuracy of the WIMP characterisation can be considerably improved by exploiting the complementarity between different target materials, and by increasing the experimental exposure.

Laser cooling of CaF molecules

Zhelyazkova, Valentina January 2014 (has links)
Cold and ultracold molecules are highly desirable for a diverse range of applications in physics and chemistry such as precision measurements, tests of fundamental physics, quantum simulation and information processing, quantum chemistry, and the physics of strongly correlated quantum matter. Laser cooling is usually infeasible in molecules because their rotational and vibrational transitions make is difficult to come up with a closed scattering cycle. Recently, a narrow range of diatomic molecules, one of which is CaF, has been shown to possess a convenient electronic structure and a highly-diagonal Franck-Condon matrix and thus be amenable to laser cooling. This thesis describes experiments on laser cooling of CaF radicals produced in a supersonic source. We first investigate the increased fluorescence when multi-frequency resonant light excites the molecules from the four hyperfine levels of the ground X²Σ+(N = 1, v = 0) state to the first excited A²π½(J' = 1=2; v' = 0) state. The number of photons scattered per molecule increases significantly from one or two in the single frequency case to more than 50 before the molecules get pumped into the X²Σ+(N = 1; v = 1) state. We demonstrate laser cooling and slowing of CaF using counter-propagating laser light which causes the molecules to scatter more than a thousand photons on the X (N = 1, v = 0, 1) <->A (J' = 1=2; v' = 0) transition. The effect of the laser cooling is to slow a group of molecules from 600 ms-1 to about 580 ms-1 and to narrow their velocity distribution from an initial temperature of 3 K down to 300 mK. In addition, chirping the frequency of the cooling light to keep it on resonance with the decelerating molecules doubles the deceleration and further compresses the velocity distribution. The effect of the laser cooling is limited by the optical pumping of molecules in the X (N = 1, v = 2) state.

Coarse-grained molecular dynamics

Edmunds, David January 2014 (has links)
In this work, we investigate the application of coarse-graining (CG) methods to molecular dynamics (MD) simulations. These methods provide access to length and time scales previously inaccessible to traditional materials simulation techniques. However, care must be taken when applying any coarse-graining strategy to ensure that we preserve the material properties of the system we are interested in. We discuss common CG strategies, including their strengths, weaknesses and ease of application. The theory of coarse-graining is discussed within the framework of statistical mechanics, together with an analytic derivation of the CG partition function for a harmonic potential. We then apply this theory to a simple system of two interacting dimers, obtaining expressions for the CG free and internal energy. This example serves as a motivation for how to coarse-grain more realistic systems numerically. We introduce five different approaches to generating a CG potential, which we have termed the rigid and relaxed approximation, the constrained pair approach, the unconstrained box approach and the entropic approach. We apply each of these techniques to a system of C60 molecules, comparing our results against reference fully atomistic MD simulations of the same system. We find that the constrained pair approach provides an optimal balance between ease of generation and accuracy when compared to the reference model.

Page generated in 0.0238 seconds