http://www.dspace.cam.ac.uk:80/handle/1810/198332 Wed, 22 May 2013 14:11:40 GMT 2013-05-22T14:11:40Z http://www.dspace.cam.ac.uk:80/retrieve/372557/croc.gif http://www.dspace.cam.ac.uk:80/handle/1810/198332 http://www.dspace.cam.ac.uk:80/handle/1810/244598 Title: Study of thin metal films and oxide materials for nanoelectronics applications Authors: De Los Santos Valladares, Luis Abstract: Different types of thin metal films and oxide materials are studied for their potential application in nanoelectronics: gold and copper films, nickel nanoelectrodes, oxide nanograin superconductors, carboxyl ferromagnetic microspheres and graphene oxide flakes. The crystallization and surface morphology of gold and copper films on SiO2/Si substrates is investigated as a function of annealing temperature. Annealing arranges the Au crystallites in the [111] direction and changes the morphology of the surface. Relaxation of the Au layer at high temperatures is responsible for the initial stages of cluster formation. These may form at disordered points on the surface and become islands when the temperature is increased. In the case of Cu/SiO2/Si films, oxides are formed after thermal oxidation at different temperatures up to 1000 °C. The phase evolution Cu -> Cu + Cu2O -> Cu2O -> Cu2O + CuO -> CuO is detected. Pure Cu2O films are obtained at 200 °C, whereas uniform CuO films without structural surface defects are obtained in the temperature range 300 - 550 °C. A resistivity phase diagram, which is obtained from the current-voltage response of the copper oxides, is presented. In the case of thin nickel films, the necessary reagents, conditions and processes required to obtain nano and atomic gaps between soft and clean nickel electrodes are described by using a conventional electrochemical cell. Current-voltage characteristics are also presented to evaluate possible applications of the nanogap electrodes in electronic nanodevices. In addition to the metal surfaces, oxides materials such as the superconductor LaCaBaCu3O7 (La1113), carboxyl ferromagnetic microparticles and graphene oxide flakes are studied. La1113 is a high critical temperature superconductor with TC(onset) = 80 K and its structure is similar to the tetragonal YBCO. This thesis explores the attachment of La1113 nanograins on Au(111) surfaces through selfassembled monolayers of HS-C8H16-HS [octane (di)thiol] for their potential application in nanotransistors. It is found that La1113 particles (100 nm mean diameter) can be functionalized by octane (di)thiol without affecting their superconducting critical temperature (TC = 80 K). A design for a superconducting transistor fabricated by immobilized La1113 nanograins in between two gold electrodes which could be controlled by an external magnetic field gate is suggested. Furthermore, the mechanical reorientation of thiolated ferromagnetic microspheres bridging a pair of gold electrodes under an external magnetic field is studied. Finally, a flexible film made of graphene oxide flakes is prepared and characterized by X ray diffraction. It is achieved by the chemical oxidation of commercial graphite and the subsequent reaction with NaOH. It is found that the interlayer distance between graphene increases upon oxidation due to the formation of chemical groups and results in the delamination and flexibility of the flakes. Description: Appendix A Pages 132-134 have been removed from this online version of the thesis for publisher copyright reasons. These had contained page images from the cover of Nanotechnology, Vol. 21, Nov 2010 and its corresponding web alert Tue, 10 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244598 2012-01-10T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244593 Title: Cooling an electron gas using quantum dot based electronic refrigeration Authors: Prance, Jonathan Robert Abstract: Studies of two-dimensional electron gases (2DEGs) in semiconductors form an active and productive field of condensed matter physics research. As well as having interesting inherent properties, they are used as the foundation for constructing various nano-scale electronic devices, such as quantum wires and quantum dots. Conventionally, low temperature measurements of 2DEGs are made by cooling the sample to 1.5 K with liquid Helium-4, to 300 mK with liquid Helium-3, or even down to a few mK using a dilution refrigerator. However, at lower temperatures the electron gas becomes increasingly decoupled from the lattice in which it resides. Below ~ 1 K the coupling can be weak enough for the electron gas to be significantly elevated in temperature due to parasitic heating. In this thesis we present the experimental and theoretical investigation of a refrigeration scheme that has the potential to cool 2DEGs below the temperatures currently available. Cooling to ever lower temperatures would be beneficial for studying fragile fractional quantum Hall states, non-Fermi-liquid behaviour in bilayer 2DEGs, or interactions like the Kondo effect that occur between quantum dots and 2DEGs. The scheme we investigate is called the Quantum Dot Refrigerator (or QDR) and is based upon the energy selective transport of electrons through the singleparticle states of quantum dots. By using a pair of dots, both hot electrons and hot holes can be selectively removed from an otherwise electrically isolated 2DEG. The result is a net current that continuously removes heat. This type of refrigerator is best suited to be used in conjunction with a dilution fridge or Helium-3 system to provide a final stage of cooling. The scheme was first investigated theoretically in 1993 by Edwards et al. but, to our knowledge, has never before been demonstrated experimentally. We detail the fabrication and measurement of a QDR device that is designed to cool an isolated 6 µm2 2DEG. In order to interpret the behaviour of the device, a model was developed to take account of electrostatic interactions between the components of the system (the quantum dots and the isolated 2DEG). Electrostatic interactions were found to be significant for our design, but were neglected in previous work. Our model predicts that their presence complicates, but does not invalidate, the principle of operation of a QDR. By comparing measurements of the current through the QDR with predictions of the model, we show that the observed behaviour is consistent with cooling of the isolated 2DEG by up to 100 mK at ambient temperatures around 250 mK. Although these temperatures are well within the reach of conventional refrigeration techniques, the results are a compelling proof-of-concept demonstration that the QDR principle is sound and can achieve significant refrigeration in the right conditions. Finally, we discuss future directions for improving QDR performance and characterisation, and for lowering the achievable base temperature. We also suggest how QDRs could be used to provide cold reservoirs for a nano-scale electronic device, and explore the limitations that would apply to such an experiment. Mon, 12 Oct 2009 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244593 2009-10-12T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244393 Title: On-chip single photon sources based on quantum dots in photonic crystal structures Authors: Schwagmann, Andre Abstract: In order to harness the enormous potential of schemes in optical quantum information processing, readily scalable photonic circuits will be required. A major obstacle for this scalability is the monolithic integration of quantum light sources with the photonic circuit on a single chip. This dissertation presents the experimental demonstration of different in-plane single photon sources that allow for this integration with planar light circuits. To this end, the spontaneous recombination of excitons in single indium arsenide quantum dots was exploited to generate single photons. The emission into on-chip waveguides was achieved by the use of advanced two-dimensional photonic crystal structures. First, slow-light effects in a unidirectional photonic crystal waveguide were exploited to achieve on-demand single photon emission with a rate of up to 18.7 MHz, corresponding to a remarkable estimated internal device efficiency of up to 47%. Waveguide-coupled L3 defect cavities with record Q-factors of up to 5150 were then studied for improved Purcell enhancement of the emission, and in-plane single photon generation from such a device was demonstrated. Finally, an electrically tunable, integrable quantum light source with a total tuning range of 1.9 nm was demonstrated by exploiting the quantum-confined Stark effect in an electrical PIN diode. These results are the first demonstrations of in-plane single photon emission at optical wavelengths and mark an important cornerstone for the realisation of fully integrated quantum photonic circuits in optical quantum information science. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244393 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244347 Title: Exciton localization mechanisms in wurtzite/zinc-blende GaAs nanowires Authors: Graham, Alexandra; Corfdir, Pierre; Heiss, Martin; Conesa-Boj, Sonia; Uccelli, Emanuele; Fontcuberta i Morral, Anna; Phillips, Richard Abstract: We investigate the emission properties of excitons in GaAs nanowires containing quantum disks formed by structural alternation between the zinc-blende and wurtzite phases, by means of temperature-dependent photoluminescence. At 10 K the emission from an ensemble of disks is distributed in a band of full width at half maximum ∼30 meV, whereas the emission linewidth for a single disk is 700 μeV. While the disk ensemble emission exhibits an S-shaped temperature dependence, the emission from single quantum disks follows the temperature dependence of the band gap over the whole temperature range. This indicates that intradisk exciton localization on impurities is negligible and that increasing the temperature induces a transfer of excitons from narrow to thick disks along the length of the wires. Our observations of the emission linewidth for single crystal-phase quantum disks show a scattering rate of excitons with acoustic phonons eight times larger than the values usually reported for (Al,Ga)As/GaAs quantum wells. This large scattering rate demonstrates that the electron effective mass in wurtzite GaAs is much heavier than in zinc-blende GaAs and is evidence of coupling between the Γ7 and Γ8 conduction bands of wurtzite GaAs. Fri, 01 Mar 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244347 2013-03-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244333 Title: Improved wave functions for quantum Monte Carlo Authors: Seth, Priyanka Abstract: Quantum Monte Carlo (QMC) methods can yield highly accurate energies for correlated quantum systems. QMC calculations based on many-body wave functions are considerably more accurate than density functional theory methods, and their accuracy rivals that of the most sophisticated quantum chemistry methods. This thesis is concerned with the development of improved wave function forms and their use in performing highly-accurate quantum Monte Carlo calculations. All-electron variational and diffusion Monte Carlo (VMC and DMC) calculations are performed for the first-row atoms and singly-positive ions. Over 98% of the correlation energy is retrieved at the VMC level and over 99% at the DMC level for all the atoms and ions. Their first ionization potentials are calculated within chemical accuracy. Scalar relativistic corrections to the energies, mass-polarization terms, and one- and two-electron expectation values are also evaluated. A form for the electron and intracule densities is presented and fits to this form are performed. Typical Jastrow factors used in quantum Monte Carlo calculations comprise electron-electron, electron-nucleus and electron-electron-nucleus terms. A general Jastrow factor capable of correlating an arbitrary of number of electrons and nuclei, and including anisotropy is outlined. Terms that depend on the relative orientation of electrons are also introduced and applied. This Jastrow factor is applied to electron gases, atoms and molecules and is found to give significant improvement at both VMC and DMC levels. Similar generalizations to backflow transformations will allow useful additional variational freedom in the wave function. In particular, the use of different backflow functions for different orbitals is expected to be important in systems where the orbitals are qualitatively different. The modifications to the code necessary to accommodate orbital-dependent backflow functions are described and some systems in which they are expected to be important are suggested. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244333 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244270 Title: Bayesian methods for gravitational waves and neural networks Authors: Graff, Philip B. Abstract: Einstein’s general theory of relativity has withstood 100 years of testing and will soon be facing one of its toughest challenges. In a few years we expect to be entering the era of the first direct observations of gravitational waves. These are tiny perturbations of space-time that are generated by accelerating matter and affect the measured distances between two points. Observations of these using the laser interferometers, which are the most sensitive length-measuring devices in the world, will allow us to test models of interactions in the strong field regime of gravity and eventually general relativity itself. I apply the tools of Bayesian inference for the examination of gravitational wave data from the LIGO and Virgo detectors. This is used for signal detection and estimation of the source parameters. I quantify the ability of a network of ground-based detectors to localise a source position on the sky for electromagnetic follow-up. Bayesian criteria are also applied to separating real signals from glitches in the detectors. These same tools and lessons can also be applied to the type of data expected from planned space-based detectors. Using simulations from the Mock LISA Data Challenges, I analyse our ability to detect and characterise both burst and continuous signals. The two seemingly different signal types will be overlapping and confused with one another for a space-based detector; my analysis shows that we will be able to separate and identify many signals present. Data sets and astrophysical models are continuously increasing in complexity. This will create an additional computational burden for performing Bayesian inference and other types of data analysis. I investigate the application of the MOPED algorithm for faster parameter estimation and data compression. I find that its shortcomings make it a less favourable candidate for further implementation. The framework of an artificial neural network is a simple model for the structure of a brain which can “learn” functional relationships between sets of inputs and outputs. I describe an algorithm developed for the training of feed-forward networks on pre-calculated data sets. The trained networks can then be used for fast prediction of outputs for new sets of inputs. After demonstrating capabilities on toy data sets, I apply the ability of the network to classifying handwritten digits from the MNIST database and measuring ellipticities of galaxies in the Mapping Dark Matter challenge. The power of neural networks for learning and rapid prediction is also useful in Bayesian inference where the likelihood function is computationally expensive. The new BAMBI algorithm is detailed, in which our network training algorithm is combined with the nested sampling algorithm MULTINEST to provide rapid Bayesian inference. Using samples from the normal inference, a network is trained on the likelihood function and eventually used in its place. This is able to provide significant increase in the speed of Bayesian inference while returning identical results. The trained networks can then be used for extremely rapid follow-up analyses with different priors, obtaining orders of magnitude of speed increase. Learning how to apply the tools of Bayesian inference for the optimal recovery of gravitational wave signals will provide the most scientific information when the first detections are made. Complementary to this, the improvement of our analysis algorithms to provide the best results in less time will make analysis of larger and more complicated models and data sets practical. Mon, 08 Oct 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244270 2012-10-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244264 Title: Estimation of the Z->vv background to New Physics searches in ATLAS Authors: Sandoval, Tanya Abstract: This thesis describes a series of studies related to searches for new phenomena, beyond the Standard Model of particle physics, in high energy hadron collisions. In such searches, it becomes crucial to identify the Standard Model backgrounds in order to resolve a potential new signal. The thesis presents a method that uses photon events to determine one of such backgrounds, caused by the production of Z boson events. The studies performed to validate the method, both theoretically and experimentally, are presented and the method was shown to be successful as well as to provide reliable results. Theoretically, the method is found to be robust up to a ~10% uncertainty. Experimentally, the method is implemented to estimate the Z(vv) + jets background for the SUSY 0l + E_T^miss + jets search in the ATLAS experiment at the Large Hadron Collider, where this background is one of the most important components for the final sensitivity and is impossible to measure directly. The main experimental results presented are the latest from ATLAS at the time of writing, corresponding to the full dataset of proton-proton collisions delivered by the LHC in 2011 (4.7 fb^-1) at a centre of mass energy of 7 TeV. Given that this method has been mainstream since 2010, brief comparisons to the results from previous analyses that used smaller datasets with the same centre of mass energy are also given, as well as additional cross-checks that support the robustness and validity of the method. The results presented here have contributed to the determination of the world's best limits with respect to SUSY models, which currently exclude equal mass squarks and gluinos below 1.4 TeV. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244264 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244254 Title: Imaging simulations of selected science with the Magdalena Ridge Observatory Interferometer Authors: Creech-Eakman, Michelle; Young, John; Haniff, Christopher; Buscher, David; Elvis, Martin; Chiavassa, Andrea; Schartmann, Marc Abstract: We present simulated observations of surface features on Red Supergiant (RSG) stars and clumpy dust structures surrounding Active Galactic Nuclei (AGN) with the Magdalena Ridge Observatory Interferometer (MROI). These represent two of the classes of astrophysical targets enumerated in the MROI Key Science Mission that are typical of the types of complex astrophysical phenomena that the MROI has been designed to image. The simulations are based on source structures derived from recent theoretical models and include both random and systematic noise on the measured Fourier data (visibility amplitudes and closure phases) consistent with our expectations for typical such targets observed with the MROI. Image reconstructions, obtained using the BSMEM imaging package, are presented for 4-, 6- and 8- telescope implementations of the array. Although a rudimentary imaging capability is demonstrated with only 4 telescopes, the detailed features of targets are only reliably determined when at least 6 telescopes are present. By the tine 8 telescope are used, the reconstructed images are sufficiently faithful to allow the discrimination between competing models, confirming the design goal of the MROI, i.e. to offer model-independent near-infrared imaging on sub-milliarcsecond scales. Wed, 30 Jun 2010 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244254 2010-06-30T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244105 Title: Plasmonic interactions in the quantum tunnelling regime Authors: Savage, Kevin John Abstract: Driven by exciting new research and applications, top-down and bottom-up fabrication techniques are producing ever more intricate, reproducible, plasmonic nano-architectures with gaps and junctions approaching the single nanometre and atomic scales. Such atomic-sized features promote the intersection of physics, chemistry and biology in plasmonics. Consequently, understanding light-matter interactions in such closely spaced, electromagnetically coupled, metallic nanosystems is of vital importance to a tremendous variety of current and future nanophotonic technologies. This thesis describes the first dynamically controlled, optically broadband, experimental investigations of light-driven plasmonic coupling between two metal nanostructures with sub-nanometre separation. A new experimental apparatus and nanosystem alignment technique was developed to enable the required sub-nanometre inter-nanoparticle geometry to be created and probed. Two conducting atomic force microscopy tips with nanoparticle functionalised apices are brought into nanoscale `tip-to-tip' axial alignment with dynamically-controlled spacing and ultra-wide optical access. Resonant electrical parametric mixing, created by oscillating the electromechanically coupled tips, is utilised to extract an electronic signal due to nanoscale changes in inter-tip position. Experimental results match theory confirming the viability of the technique. By functionalising the tip apices, this unique multi-functional observation platform allows the plasmonic response of nanoparticle dimers with sub-nanometre separations to be characterised. By simultaneously capturing both the electrical and optical properties of tip-mounted gold nanoparticles with controllable sub-nanometre separation, the first evidence for the quantum regime of optically driven tunnelling plasmonics is revealed in unprecedented detail. It is demonstrated that quantum mechanical effects are critically important at approximately the 0.3 nm scale where spatially non-local tunnelling plasmonics controls the optical response. All observed phenomena are in good agreement with a recently developed quantum-corrected model of plasmonic systems. The findings imply that tunnelling establishes a quantum limit for plasmonic field enhancement and confinement. Additionally, the work suggests the highly enhanced local density of photonic states in nanoscale cavities could enable coherent plasmon-exciton coupling. This thesis prompts new experimental and theoretical investigations into quantum-domain plasmonic systems, and impacts the future of nanoplasmonic device engineering, nanoscale photochemistry and plasmon-mediated electron tunnelling. Tue, 13 Nov 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244105 2012-11-13T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244037 Title: Role of the dielectric mismatch on the properties of donors in semiconductor nanostructures bounded by air Authors: Corfdir, Pierre; Lefebvre, Pierre Abstract: We compute by envelope function calculations the binding energy EB of donor atoms in thin slabs of semiconductor bounded by air, accounting for the dielectric mismatch between air and the semiconductor. We detail how EB depends on the donor-site and on the thickness of the slab. We show that due to the competition between surface and dielectric mismatch effects, EB does not monotonically decrease from the center to the surface of the nanostructures. Finally, we discuss our results in regard to recent photoluminescence experiments performed on ensemble and single GaN nanowires. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244037 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243945 Title: Double parton scattering in proton-proton collisions Authors: Gaunt, Jonathan Abstract: Double hard parton-parton interactions are expected to occur frequently in proton-proton (p-p) collisions at the LHC. They can give rise to significant backgrounds to certain rare single scattering (SPS) signals, and are an interesting signal process in their own right. In this thesis, we discuss the theoretical description of the double parton scattering (DPS) cross section in the context of Quantum ChromoDynamics (QCD). After an overview of QCD and an introduction to DPS in Chapter 1, we describe in Chapter 2 a framework for calculating the p-p DPS cross section introduced by Snigirev et al., in which this cross section is expressed in terms of double PDFs D_p^{ij}(x_1,x_2,Q_A^2,Q_B^2) (dPDFs). We show that the equal-scale dPDFs are subject to momentum and number sum rule constraints, and use these in the construction of an explicit set of leading order (LO) equal-scale dPDFs (the 'GS09' dPDFs). The leptonic same-sign WW DPS signal obtained using GS09 dPDFs is compared with that obtained using simple factorised forms, and the prospects of observing this signal taking into account SPS backgrounds are analysed. We discuss two ways in which the dPDF framework for describing p-p DPS is deficient in Chapter 3. We discuss interference and correlated parton effects in flavour, spin, colour, and parton type, which are ignored by the dPDF framework. We then study DPS-type graphs in which the parton pairs from both protons have arisen from a perturbative 1->2 branching, derive an expression for the part of such graphs associated with the particles arising from the 1->2 branchings being almost on-shell, and use this to demonstrate that the treatment of these graphs by the the dPDF framework is unsatisfactory. In Chapter 4, we study DPS-type graphs in which the parton pair from only one proton has arisen from a perturbative 1->2 branching. We discover that such graphs contribute to the LO p-p DPS cross section, and that crosstalk between partons in the 'nonperturbatively generated' pair is allowed provided that it occurs at a lower scale than that of the perturbative 1->2 branching in the other proton. The result of this analysis is combined with that of the previous chapter to propose a formula for the LO total DPS cross section, and our proposal is compared with those from other authors. We finish in Chapter 5 with some conclusions and suggestions for further work. Mon, 08 Oct 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243945 2012-10-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243916 Title: The Relativistic Shift in the Mössbauer Effect AND Coupled Superconductors Authors: Josephson, Brian Abstract: Part I of this dissertation predicts a temperature dependence, with relativistic causes, of the frequency of Mössbauer effect gamma rays. Part II contains the original analysis of the behaviour of two superconductors linked in a way that permits electron exchange. Description: Dissertation submitted for the 1962 fellowship election, Trinity College, Cambridge. © B D Josephson 1962. Thu, 25 Oct 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243916 2012-10-25T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243864 Title: Enhancing solar cells with plasmonic nanovoids Authors: Lal, Niraj Narsey Abstract: This thesis explores the use of plasmonic nanovoids for enhancing the efficiency of thin-film solar cells. Devices are fabricated inside plasmonically resonant nanostructures, demonstrating a new class of plasmonic photovoltaics. Novel cell geometries are developed for both organic and amorphous silicon solar cell materials. An external-quantum efficiency rig was set up to allow simultaneous microscope access and micrometer-precision probe-tip control for optoelectronic characterisation of photovoltaic devices. An experimental setup for angle-resolved reflectance was extended to allow broadband illumination from 380 - 1500nm across incident angles 0 - 70 degrees giving detailed access to the energy-momentum dispersion of optical modes within nanostructured materials. A four-fold enhancement of overall power conversion efficiency is observed in organic nanovoid solar cells compared to flat solar cells. The efficiency enhancement is shown to be primarily due to strong localised plasmon resonances of the nanovoid geometry, with close agreement observed between experiment and theoretical simulations. Ultrathin amorphous silicon solar cells are fabricated on both nanovoids and randomly textured silver substrates. Angle-resolved reflectance and computational simulations highlight the importance of the spacer layer separating the absorbing and plasmonic materials. A 20% enhancement of cell efficiency is observed for nanovoid solar cells compared to flat, but with careful optimisation of the spacer layer, randomly textured silver allows for an even greater enhancement of up to 50% by controlling the coupling to optical modes within the device. The differences between plasmonic enhancement for organic and amorphous silicon solar cells are discussed and the balance of surface plasmon absorption between a semiconductor and a metal is analytically derived for a broad range of solar cell materials, yielding clear design principles for plasmonic enhancement. These principles are used to outline future directions of research for plasmonic photovoltaics. Mon, 02 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243864 2012-07-02T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243700 Title: Importance of excitonic effects and the question of internal electric fields in stacking faults and crystal phase quantum discs: The model-case of GaN Authors: Corfdir, Pierre; Lefebvre, Pierre Abstract: We compute using envelope function calculations the energy and the oscillator strength of excitons in zinc blende/wurtzite quantum wells (QWs), such as those that appear in many examples of semiconductor nanowires, and in basal plane stacking faults (BSFs). We address specifically the model-case of GaN. In addition to the electron-hole Coulomb interaction, we account for the quantum-confined Stark effect. We demonstrate that despite the type-II band alignment at the zinc blende/wurtzite interfaces, a significant binding and a rather strong oscillator strength are preserved by excitonic effects. When adjacent crystal phase QWs are coupled together, we compute increased as well as decreased exciton oscillator strength with respect to the single QW case, depending on the QW-QW coupling scheme. Comparing the results of our calculations with available data, we finally conclude in favor of the absence of built-in electric fields perpendicular to the BSF planes. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243700 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243621 Title: Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities Authors: Corfdir, Pierre; Levrat, Jacques; Rossbach, Georg; Butté, Raphaël; Feltin, Eric; Carlin, Jean-François; Christmann, Gabriel; Lefebvre, Pierre; Ganière, Jean-Daniel; Grandjean, Nicolas; Deveaud-Plédran, Benoît Abstract: We report on the direct observation of biexcitons in a III nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243621 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243618 Title: Sub-10-nanometre metallic gaps for use in molecular electronics Authors: Curtis, Kellye Suzanne Abstract: This thesis presents the development of a selective-etch fabrication process to create sub-10 nanometre metallic gaps and the subsequent use of the gaps to study the electronics of nanocrystals and molecules. A complete picture of the success of the process required both examination by scanning electron microscopy as well as probing the current response to an applied bias at low temperature. The empty gaps were fully characterised before self-assembling 7 nm CdSe nanocrystals onto the metal with the help of linker molecules. The I-V characteristics of the empty gaps showed a reduction of the tunnelling barrier height from the expected value (~5.1 eV, the work function of Au) when the results were fitted to the Simmons tunnelling model for a metal-insulator-metal system. Results indicate that after the barrier height is surpassed, a transition from direct to field-effect (Fowler-Nordheim) tunnelling occurs. After CdSe assembly, the collected I-V characteristics of the system at 77 K showed varied results. Many devices displayed conductance peaks at low voltages comparable to the results of the shadow evaporation process for 4.2 nm nanocrystals (also documented in this thesis). Several devices revealed switching between multiples of fundamental curves, suggesting conduction through multiples of nanocrystals. Mon, 02 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243618 2012-07-02T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243505 Title: Presentations of bdj50 conference lectures Authors: Speakers Description: June 2012 was the 50th anniversary of the publication of Brian Josephson's famous paper: "Possible new effects in superconductive tunnelling", Physics Letters 1, 251 (1962). The Cavendish Laboratory marked this anniversary with a one-day meeting held in Cambridge on Saturday 23rd June 2012, with invited speakers covering the past, present and future of Josephson physics. This dspace item contains the presentations that accompanied the talks, listed by speaker (two of the speakers, Anderson and Waldram, did not show any slides, and so their contributions do not feature in this list). The audio recordings are available at sms.cam.ac.uk/collection/1275043, and the programme can be seen at http://www.phy.cam.ac.uk/conferences/Josephson/programme.php. Fri, 22 Jun 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243505 2012-06-22T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242281 Title: Transmission-mode imaging in the environmental scanning electron microscope (ESEM) Authors: Staniewicz, Lech Thomas Leif Abstract: Electron microscopy was first conducted in the 1930s with the advent of the TEM and later the STEM. In 1969, the first commercial SEM was released, with the possibility of retrofitting it to behave like a STEM following soon afterwards. In 1979, Danilatos and Robinson advanced electron microscopy by creating a new type of SEM which allowed a controlled quantity of gas into the sample chamber, termed ESEM. The most recent evolution in this line was the combination of ESEM and STEM in 2005, a procedure termed Wet STEM. The focus of this work is on investigating applications of this new technique, along with the contrast mechanisms involved in forming an image. To that end, a wide variety of samples will be imaged. Clay and paint suspensions (colloids) are used to test Wet STEM’s capacity to image submerged objects, as well as thin objects which are stacked together. Diblock copolymer films are used to test Wet STEM’s ability to distinguish chemically similar materials without staining, the physical effects of heavy metal staining and to demonstrate the necessity of gas for the purpose of charge neutralisation. Single cell biological samples are also investigated. Internal contrast in mammalian cells is visible without recourse to staining, but chemical fixation is required despite maintaining a high relative humidity. Bacteria are more resilient and as such are easier to image than animal cells, requiring no prior treatment. When exposed to low relative humidity, bacteria are found to collapse. The collapse pattern is observed to differ between wild-type and cytoskeletal-deficient bacteria of the same species and strain, so it is likely that dehydration-induced collapse offers information about the position and shape of the bacterial cytoskeleton. Tue, 07 Feb 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242281 2012-02-07T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241920 Title: DnaA and the timing of chromosome replication in Escherichia coli as a function of growth rate Authors: Grant, Matthew AA; Saggioro, Chiara; Ferrari, Ulisse; Bassetti, Bruno; Sclavi, Bianca; Cosentino Lagomarsino, Marco Abstract: Abstract Background In Escherichia coli, overlapping rounds of DNA replication allow the bacteria to double in faster times than the time required to copy the genome. The precise timing of initiation of DNA replication is determined by a regulatory circuit that depends on the binding of a critical number of ATP-bound DnaA proteins at the origin of replication, resulting in the melting of the DNA and the assembly of the replication complex. The synthesis of DnaA in the cell is controlled by a growth-rate dependent, negatively autoregulated gene found near the origin of replication. Both the regulatory and initiation activity of DnaA depend on its nucleotide bound state and its availability. Results In order to investigate the contributions of the different regulatory processes to the timing of initiation of DNA replication at varying growth rates, we formulate a minimal quantitative model of the initiator circuit that includes the key ingredients known to regulate the activity of the DnaA protein. This model describes the average-cell oscillations in DnaA-ATP/DNA during the cell cycle, for varying growth rates. We evaluate the conditions under which this ratio attains the same threshold value at the time of initiation, independently of the growth rate. Conclusions We find that a quantitative description of replication initiation by DnaA must rely on the dependency of the basic parameters on growth rate, in order to account for the timing of initiation of DNA replication at different cell doubling times. We isolate two main possible scenarios for this, depending on the roles of DnaA autoregulation and DnaA ATP-hydrolysis regulatory process. One possibility is that the basal rate of regulatory inactivation by ATP hydrolysis must vary with growth rate. Alternatively, some parameters defining promoter activity need to be a function of the growth rate. In either case, the basal rate of gene expression needs to increase with the growth rate, in accordance with the known characteristics of the dnaA promoter. Furthermore, both inactivation and autorepression reduce the amplitude of the cell-cycle oscillations of DnaA-ATP/DNA. Description: RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are. Wed, 21 Dec 2011 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241920 2011-12-21T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241720 Title: Solid-state photonic interfaces using semiconductor quantum dots Authors: Boyer de la Giroday, Antoine Abstract: New technologies based on the properties of quantum mechanics promise to revolutionise the way information is processed by outperforming what is possible using classical devices. Examples include massively parallel processing using quantum computers, verifiably secure communication using quantum cryptography, and measurement with sensitivity beyond classical limitation with quantum metrology. Realising the full potential of these technologies necessitates the ability to communicate quantum information over large distances, a key requirement for future quantum networks. However, developing practical implementations of long-distance quantum communication is challenging as it necessitates three major ingredients: light-matter interfaces, elementary quantum operations, and quantum memories. This thesis describes work that has been undertaken to address these requirements using semiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded inside conventional diode structures constitute high-fidelity controllable interfaces between optical qubits and solid-state qubits. Indeed, the polarisation state of a photon was transferred into the spin state of an electron-hole pair and eventually restored through radiative recombination of the electron and the hole with a fidelity up to 95%. Moreover, spins were manipulated using subnanosecond modulation of a vertical electric field applied to the quantum dots. By controlling this electrical modulation, we demonstrated elementary phase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiative lifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control the spatial separation of the electron and the hole and therefore prevent their recombination. Comprehensive studies of electric field eff ects upon the photoluminescence of quantum dot molecules lead to a clear understanding and a good control over their physical properties. Single photons were stored in individual quantum dot molecules up to 1μs and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons was transferred into the spin state of electron-hole pairs with a fidelity above 90%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment. Our work on single quantum dots could be extended in the near future to allow for two-qubits quantum operations by con fining a second electron-hole pair to be electrically manipulated. Storage of a superposition of spin states in a quantum dot molecule should also be possible if the spin states are made degenerate, which is feasible using the electric fi eld dependence of the energy splitting between the spin states discussed in this thesis. We believe that combining both approaches will lead to the development of a controllable multi-qubit quantum memory for polarised light, a building block for long distance quantum communication based on semiconductor nanotechnology. Tue, 07 Feb 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241720 2012-02-07T00:00:00Z