11,255 research outputs found

    Scaling up integrated photonic reservoirs towards low-power high-bandwidth computing

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    Dimension-8 SMEFT Analysis of Minimal Scalar Field Extensions of the Standard Model

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    We analyze the constraints obtainable from present data using the Standard Model Effective Field Theory (SMEFT) on extensions of the Standard Model with additional electroweak singlet or triplet scalar fields. We compare results obtained using only contributions that are linear in dimension-6 operator coefficients with those obtained including terms quadratic in these coefficients as well as contributions that are linear in dimension-8 operator coefficients. We also implement theoretical constraints arising from the stability of the electroweak vacuum and perturbative unitarity. Analyzing the models at the dimension-8 level constrains scalar couplings that are not bounded at the dimension-6 level. The strongest experimental constraints on the singlet model are provided by Higgs coupling measurements, whereas electroweak precision observables provide the strongest constraints on the triplet model. In the singlet model the present di-Higgs constraints already play a significant role. We find that the current constraints on model parameters are already competitive with those anticipated from future di- and tri-Higgs measurements. We compare our results with calculations in the full model, exhibiting the improvements when higher-order SMEFT terms are included. We also identify regions in parameter space where the SMEFT approximation appears to break down. We find that the combination of current constraints with the theoretical bounds still admits regions where the SMEFT approach is not valid, particularly for lower scalar boson masses.Comment: 66 Pages, 14 Figures, 4 Table

    Chiral active fluids: Odd viscosity, active turbulence, and directed flows of hydrodynamic microrotors

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    While the number of publications on rotating active matter has rapidly increased in recent years, studies on purely hydrodynamically interacting rotors on the microscale are still rare, especially from the perspective of particle based hydrodynamic simulations. The work presented here targets to fill this gap. By means of high-performance computer simulations, performed in a highly parallelised fashion on graphics processing units, the dynamics of ensembles of up to 70,000 rotating colloids immersed in an explicit mesoscopic solvent consisting out of up to 30 million fluid particles, are investigated. Some of the results presented in this thesis have been worked out in collaboration with experimentalists, such that the theoretical considerations developed in this thesis are supported by experiments, and vice versa. The studied system, modelled in order to resemble the essential physics of the experimentally realisable system, consists out of rotating magnetic colloidal particles, i.e., (micro-)rotors, rotating in sync to an externally applied magnetic field, where the rotors solely interact via hydrodynamic and steric interactions. Overall, the agreement between simulations and experiments is very good, proving that hydrodynamic interactions play a key role in this and related systems. While already an isolated rotating colloid is driven out of equilibrium, only collections of two or more rotors have experimentally shown to be able to convert the rotational energy input into translational dynamics in an orbital rotating fashion. The rotating colloids inject circular flows into the fluid, such that detailed balance is broken, and it is not a priori known whether equilibrium properties of colloids can be extended to isolated rotating colloids. A joint theoretical and experimental analysis of isolated, pairs, and small groups of hydrodynamically interacting rotors is given in chapter 2. While the translational dynamics of isolated rotors effectively resemble the dynamics of non-rotating colloids, the orbital rotation of pairs of rotors can be described with leading order hydrodynamics and a two-dimensional analogy of FaxĂ©n’s law is derived. In chapter 3, a homogeneously distributed ensemble of rotors (bulk) as a realisation of a chiral active fluid is studied and it is explicitly shown computationally and experimentally that it carries odd viscosity. The mutual orbital translation of rotors and an increase of the effective solvent viscosity with rotor density lead to a non-monotonous behaviour of the average translational velocity. Meanwhile, the rotor suspension bears a finite osmotic compressibility resulting from the long-ranged nature of hydrody- namic interactions such that rotational and odd stresses are transmitted through the solvent also at small and intermediate rotor densities. Consequently, density inhomogeneities predicted for chiral active fluids with odd viscosity can be found and allow for an explicit measurement of odd viscosity in simulations and experiments. At intermediate densities, the collective dynamics shows the emergence of multi-scale vortices and chaotic motion which is identified as active turbulence with a self-similar power-law decay in the energy spectrum, showing that the injected energy on the rotor scale is transported to larger scales, similar to the inverse energy cascade of clas- sical two-dimensional turbulence. While either odd viscosity or active turbulence have been reported in chiral active matter previously, the system studied here shows that the emergence of both simultaneously is possible resulting from the osmotic compressibility and hydrodynamic mediation of odd and active stresses. The collective dynamics of colloids rotating out of phase, i.e., where a constant torque instead of a constant angular velocity is applied, is shown to be qualitatively very similar. However, at smaller densities, local density inhomogeneities imply position dependent angular velocities of the rotors resulting from inter-rotor friction. While the friction of a quasi-2D layer of active colloids with the substrate is often not easily modifiable in experiments, the incorporation of substrate friction into the simulation models typically implies a considerable increase in computational effort. In chapter 4, a very efficient way of incorporating the friction with a substrate into a two-dimensional multiparticle collision dynamics solvent is introduced, allowing for an explicit investigation of the influences of substrate on active dynamics. For the rotor fluid, it is explicitly shown that the influence of the substrate friction results in a cutoff of the hydrodynamic interaction length, such that the maximum size of the formed vortices is controlled by the substrate friction, also resulting in a cutoff in the energy spectrum, because energy is taken out of the system at the respective length. These findings are in agreement with the experiments. Since active particles in confinement are known to organise in states of collective dynamics, ensembles of rotationally actuated colloids are studied in circular confinement and in the presence of periodic obstacle lattices in chapters 5 and 6, respectively. The results show that the chaotic active turbulent transport of rotors in suspension can be enhanced and guided resulting from edge flows generated at the boundaries, as has recently been reported for a related chiral active system. The consequent collective rotor dynamics can be regarded as a superposition of active turbulent and imposed flows, leading to on average stationary flows. In contrast to the bulk dynamics, the imposed flows inject additional energy into the system on the long length scales, and the same scaling behaviour of the energy spectrum as in bulk is only obtained if the energy injection scales, due to the mutual generation of rotor translational dynamics throughout the system and the edge flows, are well separated. The combination of edge flow and entropic layering at the boundaries leads to oscillating hydrodynamic stresses and consequently to an oscillating vorticity profile. In the presence of odd viscosity, this consequently leads to non-trivial steady-state density modulations at the boundary, resulting from a balance of osmotic pressure and odd stresses. Relevant for the efficient dispersion and mixing of inert particles on the mesoscale by means of active turbulent mixing powered by rotors, a study of the dynamics of a binary mixture consisting out of rotors and passive particles is presented in chapter 7. Because the rotors are not self-propelled, but the translational dynamics is induced by the surrounding rotors, the passive particles, which do not inject further energy into the system, are transported according to the same mechanism as the rotors. The collective dynamics thus resembles the pure rotor bulk dynamics at the respective density of only rotors. However, since no odd stresses act between the passive particles, only mutual rotor interactions lead to odd stresses leading to the accumulation of rotors in the regions of positive vorticity. This density increase is associated with a pressure increase, which balances the odd stresses acting on the rotors. However, the passive particles are only subject to the accumulation induced pressure increase such that these particles are transported into the areas of low rotor concentration, i.e., the regions of negative vorticity. Under conditions of sustained vortex flow, this results in segregation of both particle types. Since local symmetry breaking can convert injected rotational into translational energy, microswimmers can be constructed out of rotor materials when a suitable breaking of symmetry is kept in the vicinity of a rotor. One hypothetical realisation, i.e., a coupled rotor pair consisting out of two rotors of opposite angular velocity and of fixed distance, termed a birotor, are studied in chapter 8. The birotor pumps the fluid into one direction and consequently translates into the opposite direction, and creates a flow field reminiscent of a source doublet, or sliplet flow field. Fixed in space the birotor might be an interesting realisation of a microfluidic pump. The trans- lational dynamics of a birotor can be mapped onto the active Brownian particle model for single swimmers. However, due to the hydrodynamic interactions among the rotors, the birotor ensemble dynamics do not show the emergence of stable motility induced clustering. The reason for this is the flow created by birotor in small aggregates which effectively pushes further arriving birotors away from small aggregates, which eventually are all dispersed by thermal fluctuations

    Learning disentangled speech representations

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    A variety of informational factors are contained within the speech signal and a single short recording of speech reveals much more than the spoken words. The best method to extract and represent informational factors from the speech signal ultimately depends on which informational factors are desired and how they will be used. In addition, sometimes methods will capture more than one informational factor at the same time such as speaker identity, spoken content, and speaker prosody. The goal of this dissertation is to explore different ways to deconstruct the speech signal into abstract representations that can be learned and later reused in various speech technology tasks. This task of deconstructing, also known as disentanglement, is a form of distributed representation learning. As a general approach to disentanglement, there are some guiding principles that elaborate what a learned representation should contain as well as how it should function. In particular, learned representations should contain all of the requisite information in a more compact manner, be interpretable, remove nuisance factors of irrelevant information, be useful in downstream tasks, and independent of the task at hand. The learned representations should also be able to answer counter-factual questions. In some cases, learned speech representations can be re-assembled in different ways according to the requirements of downstream applications. For example, in a voice conversion task, the speech content is retained while the speaker identity is changed. And in a content-privacy task, some targeted content may be concealed without affecting how surrounding words sound. While there is no single-best method to disentangle all types of factors, some end-to-end approaches demonstrate a promising degree of generalization to diverse speech tasks. This thesis explores a variety of use-cases for disentangled representations including phone recognition, speaker diarization, linguistic code-switching, voice conversion, and content-based privacy masking. Speech representations can also be utilised for automatically assessing the quality and authenticity of speech, such as automatic MOS ratings or detecting deep fakes. The meaning of the term "disentanglement" is not well defined in previous work, and it has acquired several meanings depending on the domain (e.g. image vs. speech). Sometimes the term "disentanglement" is used interchangeably with the term "factorization". This thesis proposes that disentanglement of speech is distinct, and offers a viewpoint of disentanglement that can be considered both theoretically and practically

    Exploring the Structure of Scattering Amplitudes in Quantum Field Theory: Scattering Equations, On-Shell Diagrams and Ambitwistor String Models in Gauge Theory and Gravity

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    In this thesis I analyse the structure of scattering amplitudes in super-symmetric gauge and gravitational theories in four dimensional spacetime, starting with a detailed review of background material accessible to a non-expert. I then analyse the 4D scattering equations, developing the theory of how they can be used to express scattering amplitudes at tree level. I go on to explain how the equations can be solved numerically using a Monte Carlo algorithm, and introduce my Mathematica package treeamps4dJAF which performs these calculations. Next I analyse the relation between the 4D scattering equations and on-shell diagrams in N = 4 super Yang-Mills, which provides a new perspective on the tree level amplitudes of the theory. I apply a similar analysis to N = 8 supergravity, developing the theory of on-shell diagrams to derive new Grassmannian integral formulae for the amplitudes of the theory. In both theories I derive a new worldsheet expression for the 4 point one loop amplitude supported on 4D scattering equations. Finally I use 4D ambitwistor string theory to analyse scattering amplitudes in N = 4 conformal supergravity, deriving new worldsheet formulae for both plane wave and non-plane wave amplitudes supported on 4D scattering equations. I introduce a new prescription to calculate the derivatives of on-shell variables with respect to momenta, and I use this to show that certain non-plane wave amplitudes can be calculated as momentum derivatives of amplitudes with plane wave states

    Structure and adsorption properties of gas-ionic liquid interfaces

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    Supported ionic liquids are a diverse class of materials that have been considered as a promising approach to design new surface properties within solids for gas adsorption and separation applications. In these materials, the surface morphology and composition of a porous solid are modified by depositing ionic liquid. The resulting materials exhibit a unique combination of structural and gas adsorption properties arising from both components, the support, and the liquid. Naturally, theoretical and experimental studies devoted to understanding the underlying principles of exhibited interfacial properties have been an intense area of research. However, a complete understanding of the interplay between interfacial gas-liquid and liquid-solid interactions as well as molecular details of these processes remains elusive. The proposed problem is challenging and in this thesis, it is approached from two different perspectives applying computational and experimental techniques. In particular, molecular dynamics simulations are used to model gas adsorption in films of ionic liquids on a molecular level. A detailed description of the modeled systems is possible if the interfacial and bulk properties of ionic liquid films are separated. In this study, we use a unique method that recognizes the interfacial and bulk structures of ionic liquids and distinguishes gas adsorption from gas solubility. By combining classical nitrogen sorption experiments with a mean-field theory, we study how liquid-solid interactions influence the adsorption of ionic liquids on the surface of the porous support. The developed approach was applied to a range of ionic liquids that feature different interaction behavior with gas and porous support. Using molecular simulations with interfacial analysis, it was discovered that gas adsorption capacity can be directly related to gas solubility data, allowing the development of a predictive model for the gas adsorption performance of ionic liquid films. Furthermore, it was found that this CO2 adsorption on the surface of ionic liquid films is determined by the specific arrangement of cations and anions on the surface. A particularly important result is that, for the first time, a quantitative relation between these structural and adsorption properties of different ionic liquid films has been established. This link between two types of properties determines design principles for supported ionic liquids. However, the proposed predictive model and design principles rely on the assumption that the ionic liquid is uniformly distributed on the surface of the porous support. To test how ionic liquids behave under confinement, nitrogen physisorption experiments were conducted for micro‐ and mesopore analysis of supported ionic liquid materials. In conjunction with mean-field density functional theory applied to the lattice gas and pore models, we revealed different scenarios for the pore-filling mechanism depending on the strength of the liquid-solid interactions. In this thesis, a combination of computational and experimental studies provides a framework for the characterization of complex interfacial gas-liquid and liquid-solid processes. It is shown that interfacial analysis is a powerful tool for studying molecular-level interactions between different phases. Finally, nitrogen sorption experiments were effectively used to obtain information on the structure of supported ionic liquids

    New Techniques for High Orders in Scattering Amplitudes

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    This thesis uses four-dimensional unitarity and augmented recursion to calculate a selection of Yang-Mills amplitudes. This selection consists of the full-colour, two-loop, all-plus helicity amplitudes for ïŹve- and six-points; a conjecture for an n-point sub-subleading in colour two-loop amplitude; calculation of the cut-constructible piece of the full-colour, two-loop, all-plus helicity n-point amplitude. A new technique for calculating the cut constructible part of the leading in colour two-loop, ïŹve-point, single-minus helicity amplitude is presented. The correct infrared divergent piece of this single-minus amplitude was calculated, as well as the correct transcendental two pieces at ïŹnite order. Logarithms containing Mandelstam variables including only positive helicity legs were unable to be correctly calculated, but the calculation of this ïŹnal amplitude uncovered many new relations involving generalised hypergeometric functions such as the Appell functions

    Theory and simulation of moiré graphene multilayers

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    Graphene has been hailed as a material which is going to revolutionise myriad technologies due to its extraordinary stability, mechanical strength yet flexibility, and remarkable transport properties. Furthermore, it was recently discovered that if two graphene layers are stacked and twisted relative to one another, referred to as twisted bilayer graphene (tBLG), correlated insulating states and superconductivity are observed, even though graphene does not intrinsically exhibit these properties. These phases only emerge at twist angles close to the "magic angle" of 1.1 degrees, and by tuning the temperature and doping level, the system can undergo electronic phase transitions between these states. I studied electron interactions and electronic screening in tBLG and other moiré graphene multilayers. In the absence of external and internal electronic screening, I found the on-site Hubbard parameter of the flat bands of tBLG scales linearly with twist angle. Upon considering internal screening, this linear scaling breaks down, where the Hubbard interaction energy decreases more rapidly towards the magic angle owing to increased screening. Moreover, external screening, from proximity to metallic gates which dope tBLG, was found to substantially affect these Hubbard interactions, owing to the moiré length scale of the magic-angle being comparable to the distance to these metallic gates. For a sufficiently small separation to these gates, I predicted that the correlated insulating states should be screened-out and the superconducting phase should be stabilised. Long-ranged Hartree interactions were found to induced doping-dependent band-flattening in tBLG that I predicted to increase the magic-angle range of tBLG. For moiré graphene multilayers, the role of these Hartree interactions were found to sensitively depend on the stacking sequence of the structure: systems with alternating twist angles have similar interaction-driven band flattening, but systems where there are also adjacent layers that are aligned have no such interaction-driven band flattening.Open Acces

    Playing no solo imagination: synthesising the rhythmic emergence of sound and sign through embodied drum kit performance and writing

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    This practice-based PhD explores musical imagination by examining the relationships of embodied musical performance and writing. The submission comprises audio recordings of original musical material and accompanying literary output, which are contextualised through a written commentary. Through creative practice-led research based on the author’s experience as a performing musician, the thesis explores and details the generative relationship between imagination and intersubjectivity. In demonstrating musical performance as an ecologically-grounded activity animated by polyvalent real and imaginary elements, the thesis ultimately challenges the notion of an autonomous, solo subject in musical performance practice. The research context draws on music, creative writing and a range of artistic and theoretical scholarship on the subjective experience: of emotion and feeling; intersubjectivity and embodiment; semiotics and the musical imagination; histories of time and rhythm. By engaging performance and writing as situated, ecological activities, creative practice is used productively as a research methodology through the following devices: (1) The drum kit—the author’s primary performance vehicle—is treated to a broadly historical and theoretical examination of material practice. A ‘hybrid drum kit’—combining acoustic drums, cymbals, and synthetic sounds—is proposed, and used by the author as the basis for this project’s practical explorations; (2) Rhythm is conceptualised and deployed as a systematic and recursive method for musical play, in order to investigate the interrelationship of sonic, semantic and physical elements; (3) Creative writing, based on theories of embodied cognition, is used to explore and inscribe the imagination of musical play. This creative practice methodology is used to articulate and respond to the following questions: (a) What is the felt relationship between listening and inscription? (b) How do particular words, diagrams, real and imagined materials effect the sound of drum performance? (c) How do movements of the body relate to semantic and timbral conventions? The methodology is productive generating emergent structures which express embodied cognition, demonstrating the function of musical imagination. The approach serves simultaneously to expose the bias of perceptual filtering, and to challenge conventions of movement and quantification that condition musical subjectivity. The research is formally presented in a way that reflects the synthesis of real, imagined, poetic and analytic elements under scrutiny in this thesis, through a series of interconnected units: thesis, audio recordings, and attendant written outputs. Exercises generate scores, in turn performed and recorded live. Sonic and written outcomes are combined, resulting in two publications, and a speculative performance. Narrated by a number of fictional characters, through various imaginary spaces, these outputs constitute three ‘Rhythmic Figure’ studies—‘Ductus,’ ‘Nsular’ and ‘Gyri’—produced as independent documents, and presented in the central ‘Garden’ section of the thesis. ‘Anteroom’ and ‘Exits’ sections, framing the ‘Garden,’ introduce, and conclude the thesis, respectively. In its original, creative demonstration of the interconnected contribution of non-verbal, sensory, and intersubjective imagination to musical play, this creative practice research project contributes argument and evidence for the manifold ways of knowing music—listen, feel, move, write—which sit beyond discursive norms.This practice-based PhD explores musical imagination by examining the relationships of embodied musical performance and writing. The submission comprises audio recordings of original musical material and accompanying literary output, which are contextualised through a written commentary. Through creative practice-led research based on the author’s experience as a performing musician, the thesis explores and details the generative relationship between imagination and intersubjectivity. In demonstrating musical performance as an ecologically-grounded activity animated by polyvalent real and imaginary elements, the thesis ultimately challenges the notion of an autonomous, solo subject in musical performance practice. The research context draws on music, creative writing and a range of artistic and theoretical scholarship on the subjective experience: of emotion and feeling; intersubjectivity and embodiment; semiotics and the musical imagination; histories of time and rhythm. By engaging performance and writing as situated, ecological activities, creative practice is used productively as a research methodology through the following devices: (1) The drum kit—the author’s primary performance vehicle—is treated to a broadly historical and theoretical examination of material practice. A ‘hybrid drum kit’—combining acoustic drums, cymbals, and synthetic sounds—is proposed, and used by the author as the basis for this project’s practical explorations; (2) Rhythm is conceptualised and deployed as a systematic and recursive method for musical play, in order to investigate the interrelationship of sonic, semantic and physical elements; (3) Creative writing, based on theories of embodied cognition, is used to explore and inscribe the imagination of musical play. This creative practice methodology is used to articulate and respond to the following questions: (a) What is the felt relationship between listening and inscription? (b) How do particular words, diagrams, real and imagined materials effect the sound of drum performance? (c) How do movements of the body relate to semantic and timbral conventions? The methodology is productive generating emergent structures which express embodied cognition, demonstrating the function of musical imagination. The approach serves simultaneously to expose the bias of perceptual filtering, and to challenge conventions of movement and quantification that condition musical subjectivity. The research is formally presented in a way that reflects the synthesis of real, imagined, poetic and analytic elements under scrutiny in this thesis, through a series of interconnected units: thesis, audio recordings, and attendant written outputs. Exercises generate scores, in turn performed and recorded live. Sonic and written outcomes are combined, resulting in two publications, and a speculative performance. Narrated by a number of fictional characters, through various imaginary spaces, these outputs constitute three ‘Rhythmic Figure’ studies—‘Ductus,’ ‘Nsular’ and ‘Gyri’—produced as independent documents, and presented in the central ‘Garden’ section of the thesis. ‘Anteroom’ and ‘Exits’ sections, framing the ‘Garden,’ introduce, and conclude the thesis, respectively. In its original, creative demonstration of the interconnected contribution of non-verbal, sensory, and intersubjective imagination to musical play, this creative practice research project contributes argument and evidence for the manifold ways of knowing music—listen, feel, move, write—which sit beyond discursive norms
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