Using applied field, pressure, and light to control magnetic states of materials

Due to their low energy scales, flexible architectures, and unique exchange pathways, molecule-based multiferroics host a number of unique properties and phase transitions under external stimuli. In this dissertation, we reveal the magnetic- and pressure-driven transitions in [(CH3)2NH2]Mn(HCOO)3 and (NH4)2[FeCl5(H2O)], present a detailed investigation of these materials away from standard equilibrium phases, and develop rich two- and three-dimensional phase diagrams. The first platform for exploring phase transitions is [(CH3)2NH2]Mn(HCOO)3. This type-I multiferroic contains Mn centers linked by formate ligands creating Mn-O-C-O-Mn superexchange pathways. Magnetization measurements reveal two transitions - a spin-flop and a transition to the fully polarized state - and the loss of long-range order above the Neel temperature. Extending to the high-pressure regime, we perform vibrational spectroscopy across the order-disorder transition and use a correlation group analysis to determine the high pressure space groups. The superexchange pathway plays a crucial role in triggering the structural crossover to lower symmetry. Despite having driving different space groups above/below the order-disorder temperature, compression lowers each symmetry to the polar space group P1. We develop the pressure - temperature - magnetic field phase diagram for [(CH3)2NH2]Mn(HCOO)3 and articulate the potential for enhanced polarization under compression. The type-II multiferrroic (NH4)2[FeCl5(H2O)] is different. It hosts a unique set of exchange pathways mediated by through-space hydrogen- and halogen-bonding. Magnetization displays a series of transitions, including the spin-flop, transition to the fully saturated state, and many associated reorientation transitions. Extending to high-pressure studies, we employ infrared absorption and Raman scattering under compression to reveal an increase in hydrogen bonding and changes in the FeCl5H2O polyhedron that are unique to this regime. A space group analysis uncovers a sequence of space group changes that suggests it is driven to a polar space group. We generate the complete three-dimensional phase diagram, which displays the many competing structural and magnetic interactions. Together, these findings uncover magnetically-driven quantum phase transitions and reduced symmetry under compression to likely polar space groups. This work motivates extended investigations of non-equilibrium phases under external stimuli in these and other molecule-based materials with low energy scales, flexible architectures and unique spin interactions

The Evolution of Neural Network-Based Chart Patterns: A Preliminary Study

A neural network-based chart pattern represents adaptive parametric features, including non-linear transformations, and a template that can be applied in the feature space. The search of neural network-based chart patterns has been unexplored despite its potential expressiveness. In this paper, we formulate a general chart pattern search problem to enable cross-representational quantitative comparison of various search schemes. We suggest a HyperNEAT framework applying state-of-the-art deep neural network techniques to find attractive neural network-based chart patterns; These techniques enable a fast evaluation and search of robust patterns, as well as bringing a performance gain. The proposed framework successfully found attractive patterns on the Korean stock market. We compared newly found patterns with those found by different search schemes, showing the proposed approach has potential.Comment: 8 pages, In proceedings of Genetic and Evolutionary Computation Conference (GECCO 2017), Berlin, German

Habitable Climate Scenarios for Proxima Centauri b With a Dynamic Ocean

The nearby exoplanet Proxima Centauri b will be a prime future target for characterization, despite questions about its retention of water. Climate models with static oceans suggest that an Earth-like Proxima b could harbor a small dayside region of surface liquid water at fairly warm temperatures despite its weak instellation. We present the first 3-dimensional climate simulations of Proxima b with a dynamic ocean. We find that an ocean-covered Proxima b could have a much broader area of surface liquid water but at much colder temperatures than previously suggested, due to ocean heat transport and depression of the freezing point by salinity. Elevated greenhouse gas concentrations do not necessarily produce more open ocean area because of possible dynamic regime transitions. For an evolutionary path leading to a highly saline present ocean, Proxima b could conceivably be an inhabited, mostly open ocean planet dominated by halophilic life. For an ocean planet in 3:2 spin-orbit resonance, a permanent tropical waterbelt exists for moderate eccentricity. Simulations of Proxima Centauri b may also be a model for the habitability of planets receiving similar instellation from slightly cooler or warmer stars, e.g., in the TRAPPIST-1, LHS 1140, GJ 273, and GJ 3293 systems.Comment: Submitted to Astrobiology; 38 pages, 12 figures, 5 table

SpF: Enabling Petascale Performance for Pseudospectral Dynamo Models

Pseudospectral (PS) methods possess a number of characteristics (e.g., efficiency, accuracy, natural boundary conditions) that are extremely desirable for dynamo models. Unfortunately, dynamo models based upon PS methods face a number of daunting challenges, which include exposing additional parallelism, leveraging hardware accelerators, exploiting hybrid parallelism, and improving the scalability of global memory transposes. Although these issues are a concern for most models, solutions for PS methods tend to require far more pervasive changes to underlying data and control structures. Further, improvements in performance in one model are difficult to transfer to other models, resulting in significant duplication of effort across the research community.We have developed an extensible software framework for pseudospectral methods called SpF that is intended to enable extreme scalability and optimal performance. High-level abstractions provided by SpF unburden applications of the responsibility of managing domain decomposition and load balance while reducing the changes in code required to adapt to new computing architectures. The key design concept in SpF is that each phase of the numerical calculation is partitioned into disjoint numerical kernels that can be performed entirely in-processor. The granularity of domain-decomposition provided by SpF is only constrained by the data-locality requirements of these kernels. SpF builds on top of optimized vendor libraries for common numerical operations such as transforms, matrix solvers, etc., but can also be configured to use open source alternatives for portability. SpF includes several alternative schemes for global data redistribution and is expected to serve as an ideal testbed for further research into optimal approaches for different network architectures.In this presentation, we will describe the basic architecture of SpF as well as preliminary performance data and experience with adapting legacy dynamo codes. We will conclude with a discussion of planned extensions to SpF that will provide pseudospectral applications with additional flexibility with regard to time integration, linear solvers, and discretization in the radial direction

GEOS-5 Chemistry Transport Model User's Guide

The Goddard Earth Observing System version 5 (GEOS-5) General Circulation Model (GCM) makes use of the Earth System Modeling Framework (ESMF) to enable model configurations with many functions. One of the options of the GEOS-5 GCM is the GEOS-5 Chemistry Transport Model (GEOS-5 CTM), which is an offline simulation of chemistry and constituent transport driven by a specified meteorology and other model output fields. This document describes the basic components of the GEOS-5 CTM, and is a user's guide on to how to obtain and run simulations on the NCCS Discover platform. In addition, we provide information on how to change the model configuration input files to meet users' needs

Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets

Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) is a 3-Dimensional General Circulation Model (GCM) developed at the NASA Goddard Institute for Space Studies for the modeling of atmospheres of Solar System and exoplanetary terrestrial planets. Its parent model, known as ModelE2 (Schmidt et al. 2014), is used to simulate modern and 21st Century Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing effort to expand the capabilities of ModelE2 to handle a broader range of atmospheric conditions including higher and lower atmospheric pressures, more diverse chemistries and compositions, larger and smaller planet radii and gravity, different rotation rates (slowly rotating to more rapidly rotating than modern Earth, including synchronous rotation), diverse ocean and land distributions and topographies, and potential basic biosphere functions. The first aim of ROCKE-3D is to model planetary atmospheres on terrestrial worlds within the Solar System such as paleo-Earth, modern and paleo-Mars, paleo-Venus, and Saturn's moon Titan. By validating the model for a broad range of temperatures, pressures, and atmospheric constituents we can then expand its capabilities further to those exoplanetary rocky worlds that have been discovered in the past and those to be discovered in the future. We discuss the current and near-future capabilities of ROCKE-3D as a community model for studying planetary and exoplanetary atmospheres.Comment: Revisions since previous draft. Now submitted to Astrophysical Journal Supplement Serie

Mean flow instabilities of two-dimensional convection in strong magnetic fields

The interaction of magnetic fields with convection is of great importance in astrophysics. Two well-known aspects of the interaction are the tendency of convection cells to become narrow in the perpendicular direction when the imposed field is strong, and the occurrence of streaming instabilities involving horizontal shears. Previous studies have found that the latter instability mechanism operates only when the cells are narrow, and so we investigate the occurrence of the streaming instability for large imposed fields, when the cells are naturally narrow near onset. The basic cellular solution can be treated in the asymptotic limit as a nonlinear eigenvalue problem. In the limit of large imposed field, the instability occurs for asymptotically small Prandtl number. The determination of the stability boundary turns out to be surprisingly complicated. At leading order, the linear stability problem is the linearisation of the same nonlinear eigenvalue problem, and as a result, it is necessary to go to higher order to obtain a stability criterion. We establish that the flow can only be unstable to a horizontal mean flow if the Prandtl number is smaller than order , where B0 is the imposed magnetic field, and that the mean flow is concentrated in a horizontal jet of width in the middle of the layer. The result applies to stress-free or no-slip boundary conditions at the top and bottom of the layer

Effects of Intermittent Caffeine Ingestion on Aerobic Power During a 16.1K Cycling Time Trial

Please view abstract in the attached PDF fil