Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session GD: Mini-symposium on Nuclei as Mesoscopic Systems I |
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Sponsoring Units: DNP Chair: Ani Aprahamian, University of Notre Dame Room: Gaylord Opryland Hermitage A |
Saturday, October 28, 2006 9:00AM - 9:36AM |
GD.00001: Emergent phenomena in mesoscopic nuclear systems Invited Speaker: For the lightest nuclei numerical solutions of few-body problem are constructed with increasing accuracy. The theory focuses on finding the correct nucleon-nucleon interaction and an accurate description of the individual nucleonic motion. Such approach becomes quickly intractable with increasing particle number. On the other hand, certain simple features begin to emerge, which reflect collective properties of many nucleons. Some of them become familiar macroscopic properties as compressibility, surface tension, viscosity, heat capacity, entropy when the particle number becomes very large. Phase transitions or hydrodynamic behavior may develop in this limit. Other features, as shell structure or rotational bands, remain mesoscopic, i.e. they are only important up to not too large a number of particles. Since these emergent phenomena are not sensitive to the details of the interactions between the constituent particles, they may appear in non-nuclear mesoscopic systems quite analogous to nuclei. Fluctuations of the average quantities become very important in the mesoscopic regime, which may lead to qualitatively new behavior. The nuclei that can be studied experimentally are essentially mesoscopic in nature. Strong fluctuations combined with a pronounced shell structure restrict the accuracy with which the macroscopic properties of neutron matter can be determined from finite nuclei. However macroscopic characteristics as compressibility, binding energy, and symmetry energy of neutron matter determine the properties of neutron stars. [Preview Abstract] |
Saturday, October 28, 2006 9:36AM - 9:48AM |
GD.00002: Exotic phenomena in nuclei Thomas Neff, Hans Feldmeier, Robert Roth In the Fermionic Molecular Dynamics (FMD) model the nuclear many-body system is described using Slater determinants with Gaussian wave-packets as single-particle states. The flexibility of the FMD wave functions allows for a consistent description of shell model like structures, deformed states, cluster structures as well as halos. An effective interaction derived from the realistic Argonne V18 interaction using the Unitary Correlation Operator Method is used for all nuclei. Results for nuclei in the $p$-shell will be presented. Halo features are present in the Helium isotopes, cluster structures are studied in Beryllium and Carbon isotopes. The interplay between shell structure and cluster structures in the ground and the Hoyle state in ${}^{12}$C will be discussed. [Preview Abstract] |
Saturday, October 28, 2006 9:48AM - 10:00AM |
GD.00003: Quantum Chaos and Thermodynamics of Self-Bound Mesoscopic Systems Vladimir Zelevinsky There are different languages for description of excited states in small self-bound systems, like complex nuclei: in terms of thermodynamical concepts (temperature and entropy) or in terms of properties of individual quantum levels at given excitation energy. Are such descriptions complementary, mutually exclusive or equivalent? We give arguments in favor of equivalence of these approaches under an appropriate choice of a ``thermometer.'' Many-body quantum chaos serves as a stirring instrument that mixes close eigenfunctions and introduces a smoothly evolving degree of complexity as a necessary feature of thermal equilibrium. With a consistent choice of the mean field, a quasiparticle thermometer can do the job extending the region of validity of Fermi-liquid theory. The incoherent parts of residual interaction play the role of a heat bath. [Preview Abstract] |
Saturday, October 28, 2006 10:00AM - 10:12AM |
GD.00004: Nucleonic Shells: a Paradigm Shift? Witold Nazarewicz Shell structure is a fundamental property of leptodermous finite Fermi systems. It results from a one-body motion of weakly interacting quasi-particles in an average mean-field potential. The concept of single-particle motion in nuclei, developed in the late forties, is a cornerstone of nuclear structure. But how robust is this concept? A significant new theme concerns shell structure near the particle drip lines and in the superheavy nuclei. Theoretical predictions and experimental discoveries in the last decade indicate that nucleonic shell structure is being recognized now as a more local concept. It is already known that the magic numbers in light neutron-rich nuclei are not the immutable benchmarks they were once thought to be. The existence of loosely bound nuclei near the drip lines crucially depends on many-body correlations that are impacted by the presence of the low-lying continuum of unbound nuclear states that can decay by particle emission. In the superheavy elements that owe existence to quantum stabilization, the familiar localization of shell effects at magic numbers is basically gone. The study of very exotic nuclei at the limits of isospin and mass will provide the missing links in our present understanding. [Preview Abstract] |
Saturday, October 28, 2006 10:12AM - 10:24AM |
GD.00005: Chaos in quantum many-body systems Gary Mitchell To describe the fluctuation properties of neutron resonance spacings, Wigner made the first application of Random Matrix Theory (RMT) to nuclear physics. This approach has proven remarkably successful, but the development was both lengthy and sporadic. Data of sufficiently high quality (both pure and complete) were limited in sample size. Bohigas and colleagues combined the best available neutron and proton resonance data to provide conclusive evidence for agreement with the predictions of the relevant version of RMT -- the Gaussian Orthogonal Ensemble (GOE). Subsequent tests of spectra (level statistics) in different mass and energy regions are reviewed. The transition strength distribution for one channel follows a chi squared distribution of one degree of freedom (the Porter-Thomas distribution), which results from a Gaussian distribution for the transition amplitudes. The (relatively recent) precise experimental confirmation of the Gaussian distribution is described. In spite of the extremely successful application of RMT in many fields, development in nuclear physics has been relatively limited. The primary reason is that the the standard measures used to analyze the expermental data are too sensitive to mistakes. An alternative measure is suggested. [Preview Abstract] |
Saturday, October 28, 2006 10:24AM - 10:36AM |
GD.00006: Thermodynamics of Pairing in the Mesoscopic Nuclear System Tony Sumaryada, Alexander Volya We present a systematic study of the thermodynamic properties of pairing correlation in mesoscopic nuclear system. The realistic and model Hamiltonians are used in this study. Various thermodynamic quantities are calculated and analyzed using the exact solution of pairing. We conduct an in-depth comparison of microcanonical, canonical and grand canonical approaches. The nature of the pairing phase transition in small system is of a particular interest. We discuss the onset of discontinuity in thermodynamic variables, fluctuations, and evolution of zeros of the partition function in the complex temperature plane associated with the transition to a superconducting phase. [Preview Abstract] |
Saturday, October 28, 2006 10:36AM - 10:48AM |
GD.00007: Thermal signatures of pairing correlations in nuclei and nanoparticles L. Fang, S. Schmidt, Y. Alhassid Pairing correlations in nuclei at zero temperature are well documented but much less is known about their thermal signatures. Nuclei are in the crossover regime between the bulk BCS limit and the fluctuation-dominated regime. We have used the shell model Monte Carlo approach to study pairing correlations at finite temperature beyond the BCS limit. We identify signatures of pairing correlations in both the heat capacity and moment of inertia [1]. These signatures depend on the particle- number parity of protons and neutrons. Ultra-small metallic grains (nanoparticles) whose linear size is below a few nanometers are also close to the fluctuation-dominated regime. We use auxiliary-field Monte Carlo methods to study pairing correlations in such nanoparticles and find odd-even effects in their heat capacity and spin susceptibility, in analogy to the signatures found in nuclei. This work was supported in part by the U.S. DOE grant No. DE-FG-0291-ER-40608. [1] Y. Alhassid, G.F. Bertsch, L. Fang, and S. Liu, Phys. Rev. C {\bf 72}, 064326 (2005). [Preview Abstract] |
Saturday, October 28, 2006 10:48AM - 11:00AM |
GD.00008: Collectivity and random interactions Calvin Johnson Diverse quantum systems, from nuclei to buckyballs to superconducting metals, often show similar collective behaviors, such as pairing or collective bands. These universalities show up when one uses random two-body interactions. In this talk I will discuss some new signatures of collectivity found in fermion systems with random interactions. [Preview Abstract] |
Saturday, October 28, 2006 11:00AM - 11:12AM |
GD.00009: Exploring super-radiance phenomena in mesoscopic systems Alexander Volya, Vladimir Zelevinsky Mesoscopic physics is a term used to address many-body world between macro and micro. Quantum wires and dots, prototypes of quantum computers, molecular structures, atomic nuclei and even multi-quark hadrons all fall under this definition. From formation to decay, the life of a mesoscopic system is inseparable from outside perturbations; the coupling to the continuum of external states is a common element. The super-radiance to be discussed in this talk is a robust collective phenomenon appearing when this coupling becomes strong. Using schematic and realistic examples from different branches of science and from nuclear physics in particular we address the formation of a collective super-radiant mode in nuclei and condensed matter systems under conditions of regular and chaotic dynamics. Phase transition into super-radiant regime, competition of collective decay mode and other collective many-body features are to be discussed. Super-radiance saturates the entire continuum coupling in a few states making the remaining quantum many-body states quasi-stable. This counterintuitive enhancement of stability that appears in response to a strong continuum coupling is investigated. [Preview Abstract] |
Saturday, October 28, 2006 11:12AM - 11:24AM |
GD.00010: Density-functional theory for resonant Fermi gases Thomas Papenbrock Resonant Fermi gases interact via short-ranged forces that exhibit a two-body bound state at zero energy. Within the local density approximation, the form of the density functional is strongly constrained and contains only a small number of parameters. The parameters can be determined by exploiting the universality of the density functional, and by comparing results from Kohn-Sham density-functional theory with analytical solutions for the harmonically trapped two-body system. The results for the leading term and the correction due to a large scattering length agree rather well with Monte Carlo studies. The correction due to a small effective range is a prediction. [Preview Abstract] |
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