arXiv subjects

Paul Harrison

Publications and source records attributed to Paul Harrison.

17 recordsLinked to original sources

arXiv2025

quollr: An R Package for Visualizing 2-D Models from Nonlinear Dimension Reductions in High-Dimensional Space

Nonlinear dimension reduction methods provide a low-dimensional representation of high-dimensional data by applying a Nonlinear transformation. However, the complexity of the transformations and data structures can create wildly different representations depending on the method and hyper-parameter choices. It is difficult to determine whether any of these representations are accurate, which one is the best, or whether they have missed important structures. The R package quollr has been developed as a new visual tool to determine which method and which hyper-parameter choices provide the most accurate representation of high-dimensional data. The scurve data from the package is used to illustrate the algorithm. Single-cell RNA sequencing (scRNA-seq) data from mouse limb muscles are used to demonstrate the usability of the package.

stat.ME↗

arXiv2025

cardinalR: Generating Interesting High-Dimensional Data Structures

Simulated high-dimensional data is useful for testing, validating, and improving algorithms used in dimension reduction, supervised and unsupervised learning. High-dimensional data is characterized by multiple variables that are dependent or associated in some way, such as linear, nonlinear, clustering or anomalies. Here we provide new methods for generating a variety of high-dimensional structures using mathematical functions and statistical distributions organized into the R package cardinalR. Several example data sets are also provided. These will be useful for researchers to better understand how different analytical methods work and can be improved, with a special focus on nonlinear dimension reduction methods. This package enriches the existing toolset of benchmark datasets for evaluating algorithms.

stat.ME↗

arXiv2025

Choosing Better NLDR Layouts by Evaluating the Model in the High-dimensional Data Space

Nonlinear dimension reduction (NLDR) techniques such as tSNE, and UMAP provide a low-dimensional representation of high-dimensional data ($p\text{-}D$) by applying a nonlinear transformation. NLDR often exaggerates random patterns. But NLDR views have an important role in data analysis because, if done well, they provide a concise visual (and conceptual) summary of $p\text{-}D$ distributions. The NLDR methods and hyper-parameter choices can create wildly different representations, making it difficult to decide which is best, or whether any or all are accurate or misleading. To help assess the NLDR and decide on which, if any, is the most reasonable representation of the structure(s) present in the $p\text{-}D$ data, we have developed an algorithm to show the $2\text{-}D$ NLDR model in the $p\text{-}D$ space, viewed with a tour, a movie of linear projections. From this, one can see if the model fits everywhere, or better in some subspaces, or completely mismatches the data. Also, we can see how different methods may have similar summaries or quirks.

stat.ME↗

arXiv2018

Laura++ : a Dalitz plot fitter

The Dalitz plot analysis technique has become an increasingly important method in heavy flavour physics. The Laura++ fitter has been developed as a flexible tool that can be used for Dalitz plot analyses in different experimental environments. Explicitly designed for three-body decays of heavy-flavoured mesons to spinless final state particles, it is optimised in order to describe all possible resonant or nonresonant contributions, and to accommodate possible CP violation effects.

hep-ex↗

arXiv2015

IVOA Recommendation: Registry Relational Schema Version 1.0

Registries provide a mechanism with which VO applications can discover and select resources - first and foremost data and services - that are relevant for a particular scientific problem. This specification defines an interface for searching this resource metadata based on the IVOA's TAP protocol. It specifies a set of tables that comprise a useful subset of the information contained in the registry records, as well as the table's data content in terms of the XML VOResource data model. The general design of the system is geared towards allowing easy authoring of queries.

astro-ph.IM↗

arXiv2015

IVOA recommendation: Parameter Description Language Version 1.0

This document discusses the definition of the Parameter Description Language (PDL). In this language parameters are described in a rigorous data model. With no loss of generality, we will represent this data model using XML. It intends to be a expressive language for self-descriptive web services exposing the semantic nature of input and output parameters, as well as all necessary complex constraints. PDL is a step forward towards true web services interoperability.

astro-ph.IM↗

arXiv2015

Client Interfaces to the Virtual Observatory Registry

The Virtual Observatory Registry is a distributed directory of information systems and other resources relevant to astronomy. To make it useful, facilities to query that directory must be provided to humans and machines alike. This article reviews the development and status of such facilities, also considering the lessons learnt from about a decade of experience with Registry interfaces. After a brief outline of the history of the standards development, it describes the use of Registry interfaces in some popular clients as well as dedicated UIs for interrogating the Registry. It continues with a thorough discussion of the design of the two most recent Registry interface standards, RegTAP on the one hand and a full-text-based interface on the other hand. The article finally lays out some of the less obvious conventions that emerged in the interaction between providers of registry records and Registry users as well as remaining challenges and current developments.

astro-ph.IM↗

arXiv2014

IVOA Recommendation: StandardsRegExt: a VOResource Schema Extension for Describing IVOA Standards

This document describes an XML encoding standard for metadata about IVOA standards themselves, referred to as StandardsRegExt. It is intended to allow for the discovery of a standard via an IVOA identifier that refers to the standard. It also allows one to define concepts that are defined by the standard which can themselves be referred to via an IVOA identifier (augmented with a URL fragment identifier). Finally, it can also provide a machine interpretable description of a standard service interface. We describe the general model for the schema and explain its intended use by interoperable registries for discovering resources.

astro-ph.IM↗

arXiv2014

IVOA Recommendation: SimpleDALRegExt: Describing Simple Data Access Services

An application that queries or consumes descriptions of VO resources must be able to recognize a resource's support for standard IVOA protocols. This specification describes how to describe a service that supports any of the four fundamental data access protocols -- Simple Cone Search (SCS), Simple Image Access (SIA), Simple Spectral Access (SSA), Simple Line Access (SLA) -- using the VOResource XML encoding standard. A key part of this specification is the set of VOResource XML extension schemas that define new metadata that are specific to those protocols. This document describes in particular rules for describing such services within the context of IVOA Registries and data discovery as well as the VO Standard Interface (VOSI) and service self-description. In particular, this document spells out the essential markup needed to identify support for a standard protocol and the base URL required to access the interface that supports that protocol.

astro-ph.IM↗

arXiv2011

IVOA Recommendation: VOResource: an XML Encoding Schema for Resource Metadata Version 1.03

This document describes an XML encoding standard for IVOA Resource Metadata, referred to as VOResource. This schema is primarily intended to support interoperable registries used for discovering resources; however, any application that needs to describe resources may use this schema. In this document, we define the types and elements that make up the schema as representations of metadata terms defined in the IVOA standard, Resource Metadata for the Virtual Observatory [Hanicsh et al. 2004]. We also describe the general model for the schema and explain how it may be extended to add new metadata terms and describe more specific types of resources.

astro-ph.IM↗

arXiv2011

IVOA Recommendation: Simple Image Access Specification Version 1.0

This specification defines a protocol for retrieving image data from a variety of astronomical image repositories through a uniform interface. The interface is meant to be reasonably simple to implement by service providers. A query defining a rectangular region on the sky is used to query for candidate images. The service returns a list of candidate images formatted as a VOTable. For each candidate image an access reference URL may be used to retrieve the image. Images may be returned in a variety of formats including FITS and various graphics formats. Referenced images are often computed on the fly, e.g., as cutouts from larger images.

astro-ph.IM↗

arXiv2011

IVOA Recommendation: Universal Worker Service Pattern Version 1.0

The Universal Worker Service (UWS) pattern defines how to manage asynchronous execution of jobs on a service. Any application of the pattern defines a family of related services with a common service contract. Possible uses of the pattern are also described.

astro-ph.IM↗

arXiv2011

IVOA Recommendation: VODataService: a VOResource Schema Extension for Describing Collections and Services Version 1.1

VODataService refers to an XML encoding standard for a specialized extension of the IVOA Resource Metadata that is useful for describing data collections and the services that access them. It is defined as an extension of the core resource metadata encoding standard known as VOResource [Plante et al. 2008] using XML Schema. The specialized resource types defined by the VODataService schema allow one to describe how the data underlying the resource cover the sky as well as cover frequency and time. This coverage description leverages heavily the Space-Time Coordinates (STC) standard schema [Rots 2007]. VODataService also enables detailed descriptions of tables that includes information useful to the discovery of tabular data. It is intended that the VODataService data types will be particularly useful in describing services that support standard IVOA service protocols.

astro-ph.IM↗

arXiv2011

Magnetotunnelling in resonant tunnelling structures with spin-orbit interaction

Magnetotunnelling spectroscopy of resonant tunnelling structures provides information on the nature of the two-dimensional electron gas in the well. We describe a model based on nonequilibrium Green's functions that allows for a comprehensive study of the density of states, tunnelling currents and current spin polarization. The investigated effects include the electron-phonon interaction, interface roughness scattering, Zeeman effect and the Rashba spin-orbit interaction. A qualitative agreement with experimental data is found regarding the satellite peaks. The spin polarization is predicted to be larger than ten percent for magnetic fields above 2 Tesla and having a structure even at the satellite peaks. The Rashba effect is confirmed to be observable as a beating pattern in the density of states but found to be too small to affect the tunnelling current.

cond-mat.mes-hall↗

arXiv2008

Quantum transport in semiconductor quantum dot superlattices: electron-phonon resonances and polaron effects

Electron transport in periodic quantum dot arrays in the presence of interactions with phonons was investigated using the formalism of nonequilibrium Green's functions. The self-consistent Born approximation was used to model the self-energies. Its validity was checked by comparison with the results obtained by direct diagonalization of the Hamiltonian of interacting electrons and longitudinal optical phonons. The nature of charge transport at electron -- phonon resonances was investigated in detail and contributions from scattering and coherent tunnelling to the current were identified. It was found that at larger values of the structure period the main peak in the current -- field characteristics exhibits a doublet structure which was shown to be a transport signature of polaron effects. At smaller values of the period, electron -- phonon resonances cause multiple peaks in the characteristics. A phenomenological model for treatment of nonuniformities of a realistic quantum dot ensemble was also introduced to estimate the influence of nonuniformities on current -- field characteristics.

cond-mat.mes-hall↗

arXiv2007

Symmetry of $k\cdot p$ Hamiltonian in pyramidal InAs/GaAs quantum dots: Application to the calculation of electronic structure

A method for the calculation of the electronic structure of pyramidal self-assembled InAs/GaAs quantum dots is presented. The method is based on exploiting the exact $\bar{C}_{4}$ symmetry of the 8-band $k\cdot p$ Hamiltonian with the strain taken into account via the continuum mechanical model. The operators representing symmetry group elements were represented in the plane wave basis and the group projectors were used to find the symmetry adapted basis in which the corresponding Hamiltonian matrix is block diagonal with four blocks of approximately equal size. The quantum number of total quasi-angular momentum is introduced and the states are classified according to its value. Selection rules for interaction with electromagnetic field in the dipole approximation are derived. The method was applied to calculate electron and hole quasibound states in a periodic array of vertically stacked pyramidal self-assembled InAs/GaAs quantum dots for different values of the distance between the dots and external axial magnetic field. As the distance between the dots in an array is varied, an interesting effect of simultaneous change of ground hole state symmetry, type and the sign of miniband effective mass is predicted. This effect is explained in terms of the change of biaxial strain. It is also found that the magnetic field splitting of Kramer's double degenerate states is most prominent for the first and second excited state in the conduction band and that the magnetic field can both separate otherwise overlapping minibands and concatenate otherwise nonoverlapping minibands.

cond-mat.other↗

arXiv2007

Density matrix theory of transport and gain in quantum cascade lasers in a magnetic field

A density matrix theory of electron transport and optical gain in quantum cascade lasers in an external magnetic field is formulated. Starting from the general quantum kinetic treatment, we describe the intra- and inter-period electron dynamics at the non-Markovian, Markovian and Boltzmann approximation levels. Interactions of electrons with longitudinal optical phonons and classical light field are included in the present description. The non-Markovian calculation for a prototype structure reveals significantly different gain spectra in terms of linewidth and additional polaronic features in comparison to the Markovian and Boltzmann ones. Despite strongly opposed interpretations of the origin of the transport processes in the non-Markovian or Markovian and the Boltzmann approaches, they yield comparable values of the current densities.

cond-mat.mes-hall↗