Browsing by Subject "Symmetry"

Effective computational materials search, categorization, and design necessitates a high-throughput (HT) approach. System by system analyses lack the scope and speed needed to uncover large portions of the materials landscape. By performing broad searches over structural or chemical classes of materials and guided by fundamental physical principles, materials with specific desired properties can be systematically found. Furthermore, the HT approach is an effective general tool for materials classification. Depending on the application, various properties can be computed leading to powerful classification schemes. To implement HT materials studies, however, a versatile and robust framework must first be developed. In this paper, the HT framework AFLOW that has been developed and used successfully over the last decade is presented. Specifically, attention is given to an origin-specific symmetry algorithm. The algorithm is designed to determine the relevant symmetry properties of an arbitrary crystal structure (e.g., point group, space group, etc.).

The structure-property relationship is the foundation for materials modeling, predicting the behavior of compounds based on structural characteristics. With the advancement of ab initio methods and high performance computing, atomic configurations are being explored at an unprecedented rate. To effectively navigate the vast search space, procedures are presented for analyzing and prototyping crystalline compounds for high-throughput simulation. Integrated into the Automatic Flow (AFLOW) framework for computational materials discovery, these tools are the underlying workhorse for symmetry classification and materials generation. In particular, algorithms are detailed for determining the set of isometries for crystals, featuring a comprehensive collection of symmetry descriptions along with routines to handle ill-conditioned structural data. A library of crystallographic structures is also introduced — showcasing nearly 600 prototypes with representatives from each space group — and is complemented with functionality for rapidly creating materials via prototype decoration. Lastly, a module for comparing crystalline compounds is described to identify duplicate entries within large data sets and detect novel structure-types, independent of representation. Mechanisms are featured for converting geometries into a standard prototype convention, providing a direct pathway for incorporation into the crystallographic library. With these autonomous computational approaches, compounds are automatically classified and generated, enabling the design of new and structurally distinct materials.

The high-throughput framework AFLOW that has been developed and used successfully over the last decade is improved to include fully-integrated software for crystallographic symmetry characterization. The standards used in the symmetry algorithms conform with the conventions and prescriptions given in the International Tables of Crystallography (ITC). A standard cell choice with standard origin is selected, and the space group, point group, Bravais lattice, crystal system, lattice system, and representative symmetry operations are determined. Following the conventions of the ITC, the Wyckoff sites are also determined and their labels and site symmetry are provided. The symmetry code makes no assumptions on the input cell orientation, origin, or reduction and has been integrated in the AFLOW high-throughput framework for materials discovery by adding to the existing code base and making use of existing classes and functions. The software is written in object-oriented C++ for flexibility and reuse. A performance analysis and examination of the algorithms scaling with cell size and symmetry is also reported.

It is of great importance to identify new sources of discrete symmetry violations because it can explain the baryon number asymmetry of our universe and also test the validity of various models beyond the standard model. Neutron Electric Dipole Moment (nEDM) and short-range force are such candidates for the new sources of P&T violations. A new generation nEDM experiment was proposed in USA in 2002, aiming at improving the current nEDM upperlimit by two orders of magnitude. Polarized 3He is crucial in this experiment and Duke is responsible for the 3He injection, measurements of 3He nuclear magnetic resonance (NMR) signal and some physics properties related to polarized 3He.

A Monte-Carlo simulation is used to simulate the entire 3He injection process in order to study whether polarized 3He can be successfully delivered to the measurement cell. Our simulation result shows that it is achievable to maintain more than 95% polarization after 3He atoms travel through very complicated paths in the presence of non-uniform magnetic fiels.

We also built an apparatus to demonstrate that the 3He precession signal can be measured under the nEDM experimental conditions using the Superconducting Quantum Interference Device (SQUID). Based on the measurement result in our lab, we project that the signal-to-noise ratio in the nEDM experiment will be at least 10.

During this SQUID test, two interesting phenomena were discovered. One is the pressure dependence of the T₁ of the polarized 3He which has never been reported before. The other is the discrepancy between the theoretically predicted T₂ and the experimentally measured T₂ of the 3He precession signal. To investigate these two interesting phenomena, two dedicated experiments were built, and two papers have been published in Physical Review A.

In addition to the nEDM experiment, polarized 3He is also used in the search for the exotic short-range force. The high pressure 3He cell used in this experiment has a very thin window (~250 μm) to maximize the effect from the force. We demonstrate that our new method could improve the current best experimental limit by two orders of magnitude. A rapid communication demonstrating the technique and the result was published in Physical Review D.