Hasil untuk "physics.atom-ph"

C.S Adams, M Sigel, J Mlynek

Félix Therrien, Peter Graf, Vladan Stevanović

Finding an optimal match between two different crystal structures underpins many important materials science problems, including describing solid-solid phase transitions and developing models for interface and grain boundary structures. In this work, we formulate the matching of crystals as an optimization problem where the goal is to find the alignment and the atom-to-atom map that minimize a given cost function such as the Euclidean distance between the atoms. We construct an algorithm that directly solves this problem for large finite portions of the crystals and retrieves the periodicity of the match subsequently. We demonstrate its capacity to describe transformation pathways between known polymorphs and to reproduce experimentally realized structures of semi-coherent interfaces. Additionally, from our findings, we define a rigorous metric for measuring distances between crystal structures that can be used to properly quantify their geometric (Euclidean) closeness.

Ph Béquin, V Joly, Ph Herzog

R. E. Walkup, Ph. Avouris, A. P. Ghosh

L. Szasz

F. Myhrer

Pascal Szriftgiser

Pauli first noticed the hidden SO(4) symmetry for the hydrogen atom in the early stages of quantum mechanics [1]. Starting from that symmetry, one can recover the spectrum of a spinless hydrogen atom and the degeneracy of its states without explicitly solving Schrödinger's equation [2], [3]. In this paper, we derive that SO(4) symmetry and spectrum using a computer algebra system (CAS). While this problem is well known [4], [5], its solution involves several steps of manipulating expressions with tensorial quantum operators, including simplifying them by taking into account a combination of commutator rules and Einstein's sum rule for repeated indices. Therefore, it is an excellent model to test the current status of CAS concerning this kind of quantum-and-tensor-algebra computations and to showcase the CAS technique. Generally speaking, when capable, CAS can significantly help with manipulations that, like non-commutative tensor calculus subject to algebra rules, are tedious, time-consuming and error-prone. The presentation also shows two alternative patterns of computer algebra steps that can be used for systematically tackling more complicated symbolic problems of this kind.

Allen D. Allen

R. E. Olson, C. O. Reinhold, D. R. Schultz

J Bacri, A M Gomes

Daniel Kleppner

David A. Dows, Lina Hsu

Atom-atom intermolecular potentials derived from studies of crystal properties are used to calculate the second virial coefficients of a number of polyatomic gases: carbon dioxide, methane, ethane, cyclopropane, benzene, and ethylene. Calculated virial coefficients are in moderate agreement with observed data and could be brought into excellent agreement with changes in attractive potential parameters of a few percent. Effective intermolecular central potentials—averages including rotational correlation—for methane, ethane, and benzene are compared with previous averaged potentials. It is possible to obtain good fits to the data with potentials of greatly differing well depths and ``molecular volumes.'' The effective potential is temperature dependent and in the case of benzene clearly reflects the anisotropy of the molecule at lower temperatures.

Mark G. Sceats

The atom–atom encounter model for collisional energy transfer by monatomic colliders previously developed for diatomics [M.G. Sceats, Chem. Phys. Lett. 151, 281 (1988)] is extended to polyatomic molecules. For small molecules, such as triatomics, the density of states is sufficiently low at high energies that the result can be enumerated for each state. For large polyatomics a statistical approach based on either microcanonical or effective canonical distributions can be used when independent degrees of freedom are assumed. An effective anharmonicity at high energy is incorporated by inclusion of diagonal terms for these independent degrees of freedom. The primary feature of the model is that it accounts for kinematic effects, such as momentum transfer efficiency and the adiabatic nature of collisions, which are not accounted for in ergodic theories of energy transfer.

Henry Blumenfeld

Erna Karule

Melvin Lax