Monte Carlo Simulation of the Semimetal-Insulator Phase Transition in Monolayer Graphene

A 2+1 dimensional fermion field theory is proposed as a model for the low-energy electronic excitations in monolayer graphene. The model consists of N=2 four-component Dirac fermions moving in the plane and interacting via a contact interaction between charge densities. For strong couplings there is a continuous transition to a Mott insulting phase. We present results of an extensive numerical study of the model's critical region, including the order parameter, its associated susceptibility, and for the first time the quasiparticle propagator. The data enables an extraction of the critical exponents at the transition, including the dynamical critical exponent, which are hypothesised to be universal features of a quantum critical point. The relation of our model with others in the literature is discussed, along with the implications for physical graphene following from our value of the critical coupling.Comment: 19 page

Science Pipelines for the Square Kilometre Array

The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of 5 zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources, and perform Faraday Tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe.Comment: Published in Galaxies, as part of a Special Issue on The Power of Faraday Tomograph

Strong interaction effects at a Fermi surface in a model for voltage-biased bilayer graphene

Monte Carlo simulation of a 2+1 dimensional model of voltage-biased bilayer graphene, consisting of relativistic fermions with chemical potential mu coupled to charged excitations with opposite sign on each layer, has exposed non-canonical scaling of bulk observables near a quantum critical point found at strong coupling. We present a calculation of the quasiparticle dispersion relation E(k) as a function of exciton source j in the same system, employing partially twisted boundary conditions to boost the number of available momentum modes. The Fermi momentum k_F and superfluid gap Delta are extracted in the limit j tends to zero for three different values of mu, and support a strongly interacting scenario at the Fermi surface with Delta of order O(mu). We propose an explanation for the observation mu < k_F in terms of a dynamical critical exponent z < 1.Comment: 12 pages, 5 figure

Cutting the cost of pulsar astronomy: Saving time and energy when searching for binary pulsars using NVIDIA GPUs

Using the Fourier Domain Acceleration Search (FDAS) method to search for binary pulsars is a computationally costly process. Next generation radio telescopes will have to perform FDAS in real time, as data volumes are too large to store. FDAS is a matched filtering approach for searching time-domain radio astronomy datasets for the signatures of binary pulsars with approximately linear acceleration. In this paper we will explore how we have reduced the energy cost of an SKA-like implementation of FDAS in AstroAccelerate, utilising a combination of mixed-precision computing and dynamic frequency scaling on NVIDIA GPUs. Combining the two approaches, we have managed to save 58% of the overall energy cost of FDAS with a (<3%) sacrifice in numerical sensitivity

The Implementation of a Real-Time Polyphase Filter

In this article we study the suitability of dierent computational accelerators for the task of real-time data processing. The algorithm used for comparison is the polyphase filter, a standard tool in signal processing and a well established algorithm. We measure performance in FLOPs and execution time, which is a critical factor for real-time systems. For our real-time studies we have chosen a data rate of 6.5GB/s, which is the estimated data rate for a single channel on the SKAs Low Frequency Aperture Array. Our findings how that GPUs are the most likely candidate for real-time data processing. GPUs are better in both performance and power consumption.Comment: Proceedings of WDS 2014, Charles University in Prague, Faculty of Mathematics and Physics Troja, Pragu

Setiburst: A Robotic, Commensal, Realtime Multi-Science Backend For The Arecibo Telescope

Radio astronomy has traditionally depended on observatories allocating time to observers for exclusive use of their telescopes. The disadvantage of this scheme is that the data thus collected is rarely used for other astronomy applications, and in many cases, is unsuitable. For example, properly calibrated pulsar search data can, with some reduction, be used for spectral line surveys. A backend that supports plugging in multiple applications to a telescope to perform commensal data analysis will vastly increase the science throughput of the facility. In this paper, we presen