Density-Matrix Algorithm for Phonon Hilbert Space Reduction in the Numerical Diagonalization of Quantum Many-Body Systems

Combining density-matrix and Lanczos algorithms we propose a new optimized phonon approach for finite-cluster diagonalizations of interacting electron-phonon systems. To illustrate the efficiency and reliability of our method, we investigate the problem of bipolaron band formation in the extended Holstein Hubbard model.Comment: 14 pages, 6 figures, Workshop on High Performance Computing in Science and Engineering, Stuttgart 200

Auger recombination suppression and band alignment in GaAsBi/GaAs heterostructures

Using a combination of experimental and theoretical techniques we present the dependence of the bandgap Eg and the spin orbit splitting energy so, with Bi concentration in GaAsBi/GaAs samples. We find that the concentration at which so,> Eg occurs at 9%. Both spectroscopic as well as first device results indicate a type I alignment

Variational Gaussian approximation for Poisson data

The Poisson model is frequently employed to describe count data, but in a Bayesian context it leads to
 an analytically intractable posterior probability distribution. In this work, we analyze a variational Gaussian
 approximation to the posterior distribution arising from the Poisson model with a Gaussian prior. This is
 achieved by seeking an optimal Gaussian distribution minimizing the Kullback-Leibler divergence from
 the posterior distribution to the approximation, or
 equivalently maximizing the lower bound for the model evidence. We derive an explicit expression for
 the lower bound, and show the existence and uniqueness of the optimal Gaussian approximation. The lower
 bound functional can be viewed as a variant of classical Tikhonov regularization that penalizes also the
 covariance. Then we develop an efficient alternating direction maximization algorithm for solving
 the optimization problem, and analyze its convergence. We discuss strategies for reducing the computational
 complexity via low rank structure of the forward operator and the sparsity of the covariance. Further, as an
 application of the lower bound, we discuss hierarchical Bayesian modeling for selecting the
 hyperparameter in the prior distribution, and propose a monotonically convergent algorithm for determining
 the hyperparameter. We present extensive numerical experiments to illustrate the Gaussian approximation and the algorithms

Policing the community together: the impact of technology on citizen engagement

Despite broad and often varied underlying definitions, a common theme throughout community and neighbourhood policing strategies establishes the need to target improvements in the relationship and level of engagement between the police and the communities they serve. Community policing approaches have long underpinned a desire to move away from reactive policing models towards those which establish a more proactive philosophy, responsive to the wants and needs of the community. The near ubiquitous proliferation of smartphones and other ICTs (Information and Communication Technologies) means they are often seen as a vector through which initiatives of all kinds can instil a culture of proactive engagement with their respective stakeholder communities. This paper builds upon existing research which suggests that technologies for crime prevention should be designed to support communications and problem-solving rather than used simply as a means to disseminate information, unpacking a number of the core concepts that are considered central to participation and effective engagement; social capital, public participation and social and digital inclusion. Moreover, examples of wider initiatives are comparatively discussed, not just those associated with community policing, which target the engagement of communities through the use of technology, and more specifically mobile applications, before reflecting on the empirical evidence and experiences gleaned through the EU H2020 funded ‘UNITY’ project, a project that sought to establish effective strategies for engagement between police and citizen communities

The role of temperature and frequency on fretting wear of a like-on-like stainless steel contact

The influences of environmental temperature and fretting frequency on the mechanisms and rates of wear in a like-on-like 304 stainless steel contact were examined, and mainly attributed to changes in the mechanical response of the bulk material and to changes in the behaviour of the oxide debris formed in the fretting process. At low temperatures, wear proceeds by continual oxide formation and egress from the contact, whilst at high temperatures, the rate of wear is much reduced, associated with the development of oxide formed into a protective bed within the contact. The temperature at which the change between these two behaviours took place was dependent upon the fretting frequency, with evidence that, at this transition temperature, changes in behaviour can occur as the fretting test proceeds under a fixed set of conditions. An interaction diagram has been developed which provides a coherent framework by which the complex effects of these two parameters can be rationalised in terms of widely accepted physical principles

Formalization of Transform Methods using HOL Light

Transform methods, like Laplace and Fourier, are frequently used for analyzing the dynamical behaviour of engineering and physical systems, based on their transfer function, and frequency response or the solutions of their corresponding differential equations. In this paper, we present an ongoing project, which focuses on the higher-order logic formalization of transform methods using HOL Light theorem prover. In particular, we present the motivation of the formalization, which is followed by the related work. Next, we present the task completed so far while highlighting some of the challenges faced during the formalization. Finally, we present a roadmap to achieve our objectives, the current status and the future goals for this project.Comment: 15 Pages, CICM 201

RaKUn: Rank-based Keyword extraction via Unsupervised learning and Meta vertex aggregation

Keyword extraction is used for summarizing the content of a document and supports efficient document retrieval, and is as such an indispensable part of modern text-based systems. We explore how load centrality, a graph-theoretic measure applied to graphs derived from a given text can be used to efficiently identify and rank keywords. Introducing meta vertices (aggregates of existing vertices) and systematic redundancy filters, the proposed method performs on par with state-of-the-art for the keyword extraction task on 14 diverse datasets. The proposed method is unsupervised, interpretable and can also be used for document visualization.Comment: The final authenticated publication is available online at https://doi.org/10.1007/978-3-030-31372-2_2

Out-of-equilibrium physics in driven dissipative coupled resonator arrays

Coupled resonator arrays have been shown to exhibit interesting many- body physics including Mott and Fractional Hall states of photons. One of the main differences between these photonic quantum simulators and their cold atoms coun- terparts is in the dissipative nature of their photonic excitations. The natural equi- librium state is where there are no photons left in the cavity. Pumping the system with external drives is therefore necessary to compensate for the losses and realise non-trivial states. The external driving here can easily be tuned to be incoherent, coherent or fully quantum, opening the road for exploration of many body regimes beyond the reach of other approaches. In this chapter, we review some of the physics arising in driven dissipative coupled resonator arrays including photon fermionisa- tion, crystallisation, as well as photonic quantum Hall physics out of equilibrium. We start by briefly describing possible experimental candidates to realise coupled resonator arrays along with the two theoretical models that capture their physics, the Jaynes-Cummings-Hubbard and Bose-Hubbard Hamiltonians. A brief review of the analytical and sophisticated numerical methods required to tackle these systems is included.Comment: Chapter that appeared in "Quantum Simulations with Photons and Polaritons: Merging Quantum Optics with Condensed Matter Physics" edited by D.G.Angelakis, Quantum Science and Technology Series, Springer 201