A Collaborative Project of theoretical/computational research on strongly coupled plasmas will be continued by Boston College (BC; Dr. Gabor J. Kalman, Principal Investigator) and the University of Vermont (UVM; Dr. Kenneth I. Golden, Principal Investigator).

The coupling strength of a plasma is characterized by the ratio of the average Coulomb interaction energy to the average kinetic energy. The plasma is considered to be strongly coupled when this ratio is exceeds unity to the extent that the collection of charged particles is in the liquid or solid phase, exotic states of matter that are in marked contrast to the traditional gaseous plasmas studied in Tokamak fusion devices and space physics. Strong Coulomb interactions are exhibited by a variety of fascinating classical and quantum plasma systems, e.g., layered charged-particle systems in semiconductor quantum wells, layered charged particles confined in cryogenic traps, astrophysical plasmas (giant planetary interiors, white dwarf interiors, the outer crusts of neutron stars, etc.), and laboratory dusty plasmas.

The collaborative UVM/BC Project addresses issues central to the physics of strongly coupled plasma dynamics. Two general objectives for continued progress in this field are set forth. The first objective is to continue building up the theoretical framework for the description of the dynamics of strongly coupled plasma liquids. The second objective is the application of this understanding, gained in the study of strongly coupled plasmas, to novel physical systems that are in the forefront of condensed matter physics research. It is now recognized that the various guises of the quantum Coulomb liquid in semiconductors exhibit behavior that is to be understood within the framework of strongly coupled plasma dynamics while also presenting some unique ramifications of the original issues (formation of dipoles, ultra-relativistic-like behavior of graphene electrons, etc). The analysis of the collective modes (natural frequencies) and response functions pertaining to these novel semiconductor plasmas is the second main objective of the Project.

The research, with its first-principles microscopic and simulation approaches to the determination of the dynamic properties of a wide variety of strongly coupled Coulomb systems, will substantially add to the fundamental knowledge base of plasma and condensed matter physics. This research will continue to provide new insights into the borderline area between these two disciplines by highlighting the similarity of the underlying physics that governs the behavior of plasma and condensed matter systems. The experiments that can be carried out along the predictions of this research can come from either discipline and are expected to be relevant to both.

The proposed research program will continue to promote the teaching and training of participating undergraduate and graduate students at both academic institutions. It will also continue to provide cutting edge research and learning opportunities to scientists (including postdoctoral and graduate students) at foreign institutions (the Research Institute of Solid State Physics and Optics of the Hungarian Academy of Sciences; Polytechnic University of Valencia). The results of this research will continue to be presented at international conferences (such as the 2002, 2005, 2008 Strongly Coupled Coulomb Systems Conferences) and disseminated to the public through the major outreach efforts made by the conference organizers

Project Report

Plasmas are many particle systems of charged particles, occurring in Nature as well as in laboratory settings. A macroscopic body formed by a plasma can be in the gaseous, liquid or solid state. The interaction between the charged particles is via the electrostatic (Coulomb) force. As a result of this interaction the particles acquire a potential energy. At the same time, the thermal motion of the particles creates a kinetic energy for the system. It is the value of the ratio of the two, the potential energy/kinetic energy, usually referred to as the coupling parameter that governs the behavior of the system. In particular, plasmas with low values of this parameter are in the gaseous state, while those with higher values (usually referred to as strongly coupled plasmas) form a liquid or solid state. Examples for such systems are electrons in low dimensional semiconductor structures, stellar interiors, ionic liquids, dusty plasmas in laboratory discharges, etc). Our collaborative research program with the University of Vermont addressed theoretical issues relating to strongly coupled plasmas. In our earlier NSF supported work we developed an approximation method, called the Quasi-Localized Charge Approximation (QLCA), which is based on a physical picture that characterizes strongly coupled plasmas: this is the phenomenon of localization, when particles in their attempt to avoid each other settle down in a small spatial domain (the extreme of this scenario is the formation of a crystal lattice). Using the resulting mathematical formalism we have been able to describe wave excitations and wave propagation in strongly coupled plasmas. The broad applicability of the approach allowed us to treat a number of diverse many particle systems. Our main intellectual efforts have concentrated on broadening the scope of the QLCA in different directions. (i) While the originally developed QLCA was able to describe systems without dissipation, we now have developed the framework for treating more realistic dissipative systems. (ii) We have examined the consequences of strong coupling when rather than one single species of particles two or more species of particles interact strongly in the plasma. (iii) We have revealed that the so-called Fano-effect, which was discovered in the 1930-s as a peculiar eature in atomic spectra and since then has gained applicability for a variety of atomic systems, also appears in an unexpected guise in strongly coupled plasmas. (iv) We have extended our formalism to treat particles endowed with an electric or magnetic dipole moment and therefore interacingt in a more complex fashion than particles possessing an electric charge only. (v) We have revealed that a feature that had been identified in the sound waves propagating in liquid Helium, and which had been subject to a variety of theoretical investigations, the so-called roton minimum is, in fact, common with what occurs in strongly coupled plasmas and thus can be explained within the same theoretical framework. (vi) We have reformulated the QLCA so as to preserve its applicability to inhomogeneous systems: this is of importance, since many experiments take place under conditions that lead to inhomogeneity. (vii) A many body system akin to plasmas is system of charged particles interacting through a screened short range electrostatic (Yukawa) potential, rather than a bare long range electrostatic(Coulomb) potential. We have used the QLCA as well to explore the remarkable differences and similarities between the two systems. Our analytic work was supported by a close collaboration with a research group at the Wigner Centre for Physics of the Hungarian Academy of Sciences in Budapest, Hungary. This research team is recognized for its expertise in computer simulations of strongly coupled plasmas. As a result of our collaboration our theoretical models were thoroughly checked and corroborated. Mutual visits by the Budapest group to Boston College and members of our group to the Wigner Centre took place regularly. We also had an ongoing collaboration with the research group of the Institute of Theoretical Physics and Astrophysics in Kiel, Germany: two members of the group had an extended stay at Boston College. Intellectual merit of the outcome. Creating a novel approximation scheme, which has become widely used in the field in relation to problems, which had been difficult to treat theoretically, is an intellectually remarkable success of the project, with genuine transformative aspect in the field of strongly coupled plasmas and neutral liquids. Broader impact or the outcome. The broad impact of the project is reflected by the application to many areas of physics, such as plasma physics (dusty laboratory plasmas), condensed matter physics, (electron-electron and electron-hole bilayers in semiconductors, bosonic superfluids, liquid helium), astrophysics (ionic mixtures in white dwarf and giant planetary interiors,) and even high energy physics (recently discovered quark-gluon plasmas), either because these systems consist of strongly interacting charged particles or because, in their dynamical behavior, they emulate such systems.

National Science Foundation (NSF)
Division of Physics (PHY)
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Steven J Gitomer
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Boston College
Chestnut Hill
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