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Fermion Monte Carlo

Description: We review the fundamental challenge of fermion Monte Carlo for continuous systems, the "sign problem". We seek that eigenfunction of the many-body Schriodinger equation that is antisymmetric under interchange of the coordinates of pairs of particles. We describe methods that depend upon the use of correlated dynamics for pairs of correlated walkers that carry opposite signs. There is an algorithmic symmetry between such walkers that must be broken to create a method that is both exact and as effective as for symmetric functions, In our new method, it is broken by using different "guiding" functions for walkers of opposite signs, and a geometric correlation between steps of their walks, With a specific process of cancellation of the walkers, overlaps with antisymmetric test functions are preserved. Finally, we describe the progress in treating free-fermion systems and a fermion fluid with 14 <sup>3</sup>He atoms.
Date: December 1, 1998
Creator: Kalos, M. H. & Pederiva, F.
Partner: UNT Libraries Government Documents Department

Quantum imitations of physical phenomena.

Description: Quantum imitation is an attempt to exploit quantum laws to advantage, and thus accomplish efficient simulation of physical phenomena. We discuss the fundamental concepts behind this new paradigm of information processing, such as the connection between models of computation and physical systems, along with the first imitation of a toy quantum many-body problem.
Date: January 1, 2001
Creator: Ortiz, G. (Gerardo)
Partner: UNT Libraries Government Documents Department

Bose condensation in {sup 4}He and neutron scattering

Description: The discovery of superfluidity in liquid {sup 4}He below T{sub {lambda}} = 2.17 K, and its phenomenological characterization since then, has been one of the great success stories of condensed matter physics. The relation of superfluidity to the behavior of atoms was conjectured by F. London in 1938. Superfluidity is a manifestation of the Bose condensation of helium atoms, the extensive occupation of the zero momentum state. Ever since {sup 4}He has been the paradigm in the search for Bose condensates in other systems. At the Pune meeting scientists have heard exciting new evidence for Bose condensates of laser cooled alkali atoms in magnetic traps, of excitons in Cu{sub 2}O, and possibly pre-formed Cooper pairs of electrons in the high {Tc} perovskite superconductors. There remains the holy-grail of forming a Bose condensate in spin-polarized hydrogen. In the current excitement for new types of Bose condensates, and new phenomena such as atom lasers, it may be useful to recall the older story of the experimental verification of a relation between superfluidity and Bose condensation in {sup 4}He. This topic has been investigated over many years by neutron scattering experiments and quantum many-body theory. The authors goal is to illustrate the difficulties of establishing the existence of a Bose condensate in a strongly interacting system, even though its macroscopic effects are manifest. The author assumes readers have access to a review by Silver and Sokol which emphasizes the neutron scattering theory through 1990 and a review by Snow and Sokol of the deep inelastic neutron scattering (DINS) experiments through 1995.
Date: April 1, 1997
Creator: Silver, R.N.
Partner: UNT Libraries Government Documents Department

Investigations of the dynamics and electromagnetic interactions of few-body systems. Progress report, June 30, 1994--September 30, 1995

Description: This progress report summarizes the work of The George Washington University (GW) nuclear theory group during the period 1 July 1994 - 30 September 1995 under DOE Grant No. DE-FG02-95-ER40907 mainly dealing with photonuclear reactions and few-body problems of nuclei. This report contains: papers published or in press, submitted for publication, and in preparation; invited talks at conferences and meetings; invited talks at universities and laboratories; contributed papers or abstracts at conferences; visitors to the group; and research progress by topic.
Date: October 1, 1995
Creator: Lehman, D.R.
Partner: UNT Libraries Government Documents Department

Incorporating many-body effects into modeling of semiconductor lasers and amplifiers

Description: Major many-body effects that are important for semiconductor laser modeling are summarized. The authors adopt a bottom-up approach to incorporate these many-body effects into a model for semiconductor lasers and amplifiers. The optical susceptibility function ({Chi}) computed from the semiconductor Bloch equations (SBEs) is approximated by a single Lorentzian, or a superposition of a few Lorentzians in the frequency domain. Their approach leads to a set of effective Bloch equations (EBEs). The authors compare this approach with the full microscopic SBEs for the case of pulse propagation. Good agreement between the two is obtained for pulse widths longer than tens of picoseconds.
Date: June 1, 1997
Creator: Ning, C.Z.; Moloney, J.V. & Indik, R.A.
Partner: UNT Libraries Government Documents Department

Baryon-baryon mixing in hypernuclei

Description: Implications of few-body hypernuclei for the understanding of the baryon-baryon interaction are examined. Octet-octet coupling effects not present in conventional, non strange nuclei are the focus. The need to identify strangeness {minus}2 hypernuclei to test model predictions is emphasized.
Date: May 1998
Creator: Gibson, B. F.
Partner: UNT Libraries Government Documents Department

Nuclear Structure Near the Drip Lines

Description: Experiments with beams of unstable nuclei will make it possible to look closely into many aspects of the nuclear many-body problem. Theoretically, exotic nuclei represent a formidable challenge for the nuclear many-body theories and their power to predict nuclear properties in nuclear terra incognita.
Date: August 10, 1998
Creator: Nazarewicz, W.
Partner: UNT Libraries Government Documents Department

Two-particle picture and electronic structure calculations

Description: We derive exact formal expressions for the self-energy, (capital Sigma <sup>(<i>n</i>)</sup>, describing the in- teraction of <i>n</i> particles with one another and with the rest of the particles in an interacting quantum N-particle system In contrast to traditional treatments, in which the single-particle self-energy is built out of interactions of a particle with the rest of the system, here a general n-particle quantity, (capital sigma)<sup>(<i>n</i>)</sup>, is obtained in a straight- forward fashion by integrating the exact <i>N</i>-particle Green function, <i>G</i><sup>(N)</sup>, over the coordinates of N <sup>_</sup> <i>n</i> particles and inverting An alternative expression, based on the canonical many-body equation of motion for the Green function is also discussed and compared with that derived through the integration process. The methodology is developed with respect to two-particle states, with the two-particle Green function being the central quantity from which the single-particle self-energy and Green function are derived It is suggested that the two-particle Green function be calculated directly in six-dimensional space in a two-particle generalization of density functional theory and the corresponding local density approximation. Methods for the calculation of the single-particle, <i>n</i> = 1, self-energy and effective single-particle t-matrix are discussed, and the methodology is illustrated by means of calculations on a model system.
Date: June 24, 1998
Creator: Gonis, A; Schulthess, T C & Turchi, P E A
Partner: UNT Libraries Government Documents Department

Final state effects and correlation in atomic and molecular systems

Description: This is the final report of a three-year Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The project`s goals were to experimentally examine phenomena caused by final state interactions of the photo-excited electron with the remaining ionic system. This includes multiple ionization by a single photon, which cannot be described solely within a one-electron framework, but requires explicit incorporation of correlation effects between the second electron and the primary electron. Having only two electrons, helium is one of the simplest systems which exhibits electron correlation and thus is an ideal choice for testing electron correlation models in atoms. The authors measured the double to single ionization ratio in helium and the multiple ionization ratio in several other rare gases to test the recent theories. They found an unexpected ionization channel caused by Compton scattering.
Date: April 1, 1996
Creator: Bartlett, R.J.; Morgan, D.V.; Samson, J.A. & Sagurton, M.
Partner: UNT Libraries Government Documents Department

Quantum Debye-Hueckel theory and the possible plasma phase transition

Description: In this paper the author first sketches the calculation of the pressure of a neutral, ion-electron gas as an expansion in powers of the electron charge, e, by means of the Matsubara, finite-temperature, many-body, perturbation theory. He then goes on to derive the Debye-H{umlt u} term and other equations to support his contentions. His results support but do not prove the existence of a phase transition.
Date: September 3, 1998
Creator: Baker, G., Jr.
Partner: UNT Libraries Government Documents Department

Coupled spin and charge collective excitations in a spin polarized electron gas

Description: The charge and longitudinal spin responses induced in a spin polarized quantum well by a weak electromagnetic field are investigated within the framework of the linear response theory. The authors evaluate the excitation frequencies for the intra- and inter-subband transitions of the collective charge and longitudinal spin density oscillations including many-body corrections beyond the random phase approximation through the spin dependent local field factors, G{sub {sigma}}{sup {+-}} (q,{omega}). An equation-of-motion method was used to obtain these corrections in the limit of long wavelengths, and the results are given in terms of the equilibrium pair correlation function. The finite degree of spin polarization is shown to introduce coupling between the charge and spin density modes, in contrast with the result for an unpolarized system.
Date: August 12, 1997
Creator: Marinescu, D.C.; Quinn, J.J. & Yi, K.S.
Partner: UNT Libraries Government Documents Department

Estimate for the maximum compression of single shocks

Description: The authors derive that the maximum compression for any single-shock Hugoniot has an upper bound of 7. For the principal Hugoniot, they present a simple analytic estimate for the maximum compression as a function of {rho}{sub o} (initial density), A (atomic weight), Z (atomic number), and {Delta}E (the sum of cohesion, dissociation, and total ionization energies).
Date: February 1, 1998
Creator: Johnson, J.D.
Partner: UNT Libraries Government Documents Department

Proton Emission from Gamow Resonance

Description: We demonstrated that it is possible to calculate the complex energy of the deformed Gamow state with a precision that is high enough so that the half-life calculated from the imaginary part of the energy is meaningful. We also performed a comparison between non-adiabatic and adiabatic calculations. It can be concluded that, in many cases, the corrected adiabatic treatment cannot be used as a substitute for the full non-adiabatic calculations.
Date: October 19, 2001
Creator: Vertse, T.
Partner: UNT Libraries Government Documents Department

The Nuclear Collective Motion

Description: Current developments in nuclear structure are discussed from a theoretical perspective. First, the progress in theoretical modeling of nuclei is reviewed. This is followed by the discussion of nuclear time scales, nuclear collective modes, and nuclear deformations. Some perspectives on nuclear structure research far from stability are given. Finally, interdisciplinary aspects of the nuclear many-body problem are outlined.
Date: March 20, 2001
Creator: Nazarewicz, W.
Partner: UNT Libraries Government Documents Department

Strongly correlated electronic materials

Description: This is the final report of a 3-year project. Novel electronic materials characterized by strong electronic correlations display a number of unexpected, often extraordinary, properties. These are likely to play a major role in purpose-specific high-technology electronic materials of the future developed for electronic, magnetic, and optical applications. This project sought to develop predictive control of the novel properties by formulating, solving and applying many-body models for the underlying microscopic physics. This predictive control required the development of new analytical and numerical many-body techniques and strategies for materials of varying strengths of interactions, dimensionality and geometry. Results are compared with experiment on classes of novel materials, and the robust techniques are used to predict additional properties and motivate key additional experiments.
Date: April 1, 1996
Creator: Bedell, K.; Albers, R.; Balatsky, A.; Bishop, A.; Bonca, J.; Gubernatis, J. et al.
Partner: UNT Libraries Government Documents Department

Optimal prediction of stiff oscillatory systems

Description: We consider some large systems of differential equations that have been introduced as model many-body problems. These systems have solutions that oscillate on a wide range of time scales. We apply the formalism of optimal prediction to these systems, using conditional expectations of the equations of motion to construct effective equations for the most slowly-varying quantities. We verify the accuracy of the effective equations in examples, comparing solutions of the original and new systems, and we show that the new equations give accurate answers for slow variables with relatively little computational effort.
Date: May 1, 1999
Creator: Kast, Anton P.
Partner: UNT Libraries Government Documents Department

Final Report of Strongly Interacting Fermion Systems

Description: There has been significant progress in three broad areas: (A) Optical properties, (B) Large-scale computations, and (C) Many-body systems. In this summary the emphasis is primarily on those papers that point to the research plans. At the same time, some important analytic work is not neglected, some of it even appearing in the description of large-scale Computations. Indeed one of the aims of such computations is to give new insights which lead to development of models capable of simple analytic or nearly analytic analysis.
Date: June 25, 2001
Creator: Wilkins, J. W.
Partner: UNT Libraries Government Documents Department

Exotic nuclei from a theoretical perspective

Description: One of the main frontiers of nuclear structure today is the physics of radioactive nuclear beams. Experiments with radioactive beams will make it possible to look closely into many aspects of the nuclear many-body problem. What makes this subject both exciting and difficult is: (i) the weak binding and corresponding closeness of the particle continuum, implying a large diffuseness of the nuclear surface and extreme spatial dimensions characterizing the outermost nucleons, and (ii) access to the exotic combinations of proton and neutron numbers which offer prospects for completely new structural phenomena.
Date: November 1, 1998
Creator: Nazarewicz, W.
Partner: UNT Libraries Government Documents Department