Path integral Monte Carlo simulations of solid molecular hydrogen surfaces and thin helium-4 films on molecular hydrogen substrates
Wagner, Marcus
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https://hdl.handle.net/2142/20316
Description
Title
Path integral Monte Carlo simulations of solid molecular hydrogen surfaces and thin helium-4 films on molecular hydrogen substrates
Author(s)
Wagner, Marcus
Issue Date
1994
Doctoral Committee Chair(s)
Ceperley, David M.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Engineering, Materials Science
Computer Science
Language
eng
Abstract
Based on Richard P. Feynman's formulation of quantum mechanics, Path Integral Monte Carlo is a computational ab-initio method to calculate finite temperature equilibrium properties of quantum many-body systems. As input, only fundamental physical constants and pair-potentials are required. We carry out the first ab-initio particle simulations of three related physical systems. First, the bare H$\sb2$ substrate is simulated between 0.5 and 1.3K, because a liquid H$\sb2$ film is a candidate for a new superfluid. We find evidence of quantum exchange in surface terraces for up to 1K. Second, the melting of the H$\sb2$ surface between 3 and 15K is examined since this is the cleanest example of quantum surface melting. Third, atomically thin superfluid $\sp4$He films on H$\sb2$ surfaces are simulated, calculating binding energies per $\sp4$He atom and third sound, an important experimental probe for superfuid $\sp4$He films. For all systems we compute density profiles perpendicular and parallel to the surface and compare to experiment. We treat both H$\sb2$ molecules and $\sp4$He atoms on the same footing, as spherical particles. For simulations of bulk/vapor interfaces and surface adsorption, a realistic representation of the macroscopic surface is crucial. Therefore, we introduce an external potential to account for arbitrarily layered substrates and long-range corrections. Two algorithms for parallel computers with independent processors are introduced, one to manage concurrent simulations of entire phase-diagrams, and one to improve input/output speed for files shared by all processors.
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