8,306 research outputs found

    Bound states of edge dislocations: The quantum dipole problem in two dimensions

    Full text link
    We investigate bound state solutions of the 2D Schr\"odinger equation with a dipole potential originating from the elastic effects of a single edge dislocation. The knowledge of these states could be useful for understanding a wide variety of physical systems, including superfluid behavior along dislocations in solid 4^4He. We present a review of the results obtained by previous workers together with an improved variational estimate of the ground state energy. We then numerically solve the eigenvalue problem and calculate the energy spectrum. In our dimensionless units, we find a ground state energy of -0.139, which is lower than any previous estimate. We also make successful contact with the behavior of the energy spectrum as derived from semiclassical considerations.Comment: 6 pages, 3 figures, submitted to PR

    Coupled-channels analysis of the 16^{{\bf 16}}O+208^{{\bf 208}}Pb fusion barrier distribution

    Get PDF
    Analyses using simplified coupled-channels models have been unable to describe the shape of the previously measured fusion barrier distribution for the doubly magic 16^{16}O+208^{208}Pb system. This problem was investigated by re-measuring the fission excitation function for 16^{16}O+208^{208}Pb with improved accuracy and performing more exact coupled-channels calculations, avoiding the constant-coupling and first-order coupling approximations often used in simplified analyses. Couplings to the single- and 2-phonon states of 208^{208}Pb, correctly taking into account the excitation energy and the phonon character of these states, particle transfers, and the effects of varying the diffuseness of the nuclear potential, were all explored. However, in contrast to other recent analyses of precise fusion data, no satisfactory simultaneous description of the shape of the experimental barrier distribution and the fusion cross-sections for 16^{16}O+208^{208}Pb was obtained.Comment: RevTex, 29 pages, 7 postscript figures, to appear in PR

    High-Resolution Observations of Interstellar Ca I Absorption -- Implications for Depletions and Electron Densities in Diffuse Clouds

    Get PDF
    We present high-resolution (FWHM ~ 0.3-1.5 km/s) spectra of interstellar Ca I absorption toward 30 Galactic stars. Comparisons of the column densities of Ca I, Ca II, K I, and other species -- for individual components identified in the line profiles and also when integrated over entire lines of sight -- yield information on relative electron densities and depletions. There is no obvious relationship between the ratio N(Ca I)/N(Ca II) [equal to n_e/(Gamma/alpha_r) for photoionization equilibrium] and the fraction of hydrogen in molecular form f(H2) (often taken to be indicative of the local density n_H). For a smaller sample of sightlines for which the thermal pressure (n_H T) and local density can be estimated via analysis of the C I fine-structure excitation, the average electron density inferred from C, Na, and K (assuming photoionization equilibrium) seems to be independent of n_H and n_H T. While the n_e obtained from the ratio N(Ca I)/N(Ca II) is often significantly higher than the values derived from other elements, the patterns of relative n_e derived from different elements show both similarities and differences for different lines of sight -- suggesting that additional processes besides photoionization and radiative recombination commonly and significantly affect the ionization balance of heavy elements in diffuse IS clouds. Such additional processes may also contribute to the (apparently) larger than expected fractional ionizations (n_e/n_H) found for some lines of sight with independent determinations of n_H. In general, inclusion of ``grain-assisted'' recombination does reduce the inferred n_e, but it does not reconcile the n_e estimated from different elements. The depletion of calcium may have a much weaker dependence on density than was suggested by earlier comparisons with CH and CN.Comment: aastex, 70 pages, accepted to ApJ

    Space-Time Variation of Physical Constants and Relativistic Corrections in Atoms

    Get PDF
    Detection of high-redshift absorption in the optical spectra of quasars have provided a powerful tool to measure spatial and temporal variations of physical ``constants'' in the Universe. It is demonstrated that high sensitivity to the variation of the fine structure constant alpha can be obtained from a comparison of the spectra of heavy and light atoms (or molecules). We have performed calculations for the pair FeII and MgII for which accurate quasar and laboratory spectra are available. A possibility of 10510^5 times enhanced effects of the fundamental constants variation suitable for laboratory measurements is also discussed.Comment: 8 pages; LaTeX; Submitted to Phys. Rev. Let

    Dinosaur tracks from the Kilmaluag Formation (Bathonian, Middle Jurassic) of Score Bay, Isle of Skye, Scotland, UK

    Get PDF
    Tracks of a juvenile theropod dinosaur with footprint lengths of between 2 and 9 cm as well as adults of the same ichnospecies with footprints of about 15–25 cm in length were found in the Bathonian (Middle Jurassic) Kilmaluag Formation of Score Bay, northwestern Trotternish Peninsula, Isle of Skye, Scotland, UK. Two footprint sizes occur together on the same bedding plane in the central portion of Score Bay, both in situ and on loose blocks. Another horizon containing footprints above this was also identified. The footprints from the lowest horizon were produced in a desiccated silty mud that was covered with sand. A close association of both adults and juveniles with similar travel direction indicated by the footprints may suggest post-hatching care in theropod dinosaurs. Other footprints, produced on a rippled sandy substrate, have been found on the slightly higher bedding plane at this locality. Loose blocks found 130 m to the northeast in the central part of Score Bay have not been correlated with any in situ sediments, but were preserved in a similar manner to those from the higher bedding plane. These tracks represent the youngest dinosaur remains yet found in Scotland

    Wigner Functions and Separability for Finite Systems

    Full text link
    A discussion of discrete Wigner functions in phase space related to mutually unbiased bases is presented. This approach requires mathematical assumptions which limits it to systems with density matrices defined on complex Hilbert spaces of dimension p^n where p is a prime number. With this limitation it is possible to define a phase space and Wigner functions in close analogy to the continuous case. That is, we use a phase space that is a direct sum of n two-dimensional vector spaces each containing p^2 points. This is in contrast to the more usual choice of a two-dimensional phase space containing p^(2n) points. A useful aspect of this approach is that we can relate complete separability of density matrices and their Wigner functions in a natural way. We discuss this in detail for bipartite systems and present the generalization to arbitrary numbers of subsystems when p is odd. Special attention is required for two qubits (p=2) and our technique fails to establish the separability property for more than two qubits.Comment: Some misprints have been corrected and a proof of the separability of the A matrices has been adde

    Separability and Fourier representations of density matrices

    Get PDF
    Using the finite Fourier transform, we introduce a generalization of Pauli-spin matrices for dd-dimensional spaces, and the resulting set of unitary matrices S(d)S(d) is a basis for d×dd\times d matrices. If N=d1×d2×...×dbN=d_{1}\times d_{2}\times...\times d_{b} and H^{[ N]}=\bigotimes H^{% [ d_{k}]}, we give a sufficient condition for separability of a density matrix ρ\rho relative to the H[dk]H^{[ d_{k}]} in terms of the L1L_{1} norm of the spin coefficients of ρ>.\rho >. Since the spin representation depends on the form of the tensor product, the theory applies to both full and partial separability on a given space H[N]H^{[ N]}% . It follows from this result that for a prescribed form of separability, there is always a neighborhood of the normalized identity in which every density matrix is separable. We also show that for every prime pp and n>1n>1 the generalized Werner density matrix W[pn](s)W^{[ p^{n}]}(s) is fully separable if and only if s≀(1+pn−1)−1s\leq (1+p^{n-1}) ^{-1}

    Quantum Knitting

    Get PDF
    We analyze the connections between the mathematical theory of knots and quantum physics by addressing a number of algorithmic questions related to both knots and braid groups. Knots can be distinguished by means of `knot invariants', among which the Jones polynomial plays a prominent role, since it can be associated with observables in topological quantum field theory. Although the problem of computing the Jones polynomial is intractable in the framework of classical complexity theory, it has been recently recognized that a quantum computer is capable of approximating it in an efficient way. The quantum algorithms discussed here represent a breakthrough for quantum computation, since approximating the Jones polynomial is actually a `universal problem', namely the hardest problem that a quantum computer can efficiently handle.Comment: 29 pages, 5 figures; to appear in Laser Journa

    An efficient Fredholm method for calculation of highly excited states of billiards

    Full text link
    A numerically efficient Fredholm formulation of the billiard problem is presented. The standard solution in the framework of the boundary integral method in terms of a search for roots of a secular determinant is reviewed first. We next reformulate the singularity condition in terms of a flow in the space of an auxiliary one-parameter family of eigenproblems and argue that the eigenvalues and eigenfunctions are analytic functions within a certain domain. Based on this analytic behavior we present a numerical algorithm to compute a range of billiard eigenvalues and associated eigenvectors by only two diagonalizations.Comment: 15 pages, 10 figures; included systematic study of accuracy with 2 new figures, movie to Fig. 4, http://www.quantumchaos.de/Media/0703030media.av

    Quasi-fission reactions as a probe of nuclear viscosity

    Full text link
    Fission fragment mass and angular distributions were measured from the ^{64}Ni+^{197}Au reaction at 418 MeV and 383 MeV incident energy. A detailed data analysis was performed, using the one-body dissipation theory implemented in the code HICOL. The effect of the window and the wall friction on the experimental observables was investigated. Friction stronger than one-body was also considered. The mass and angular distributions were consistent with one-body dissipation. An evaporation code DIFHEAT coupled to HICOL was developed in order to predict reaction time scales required to describe available data on pre-scission neutron multiplicities. The multiplicity data were again consistent with one-body dissipation. The cross-sections for touch, capture and quasi-fission were also obtained.Comment: 25 pages REVTeX, 3 tables, 13 figures, submitted to Phys. Rev
    • 

    corecore