The Jovian electron spectrum: 1978-1984

Observations of Jovian electrons through six consecutive 13-month Jovian synodic periods from 1978 to 1984 have been made by the University of Chicago electron spectrometer onboard the ISEE-3 (ICE) spacecraft. The Jovian electron spectrum was determined from 5 to 30 Mev and was found to have a shape which is not a power law in kinetic energy, but cuts off at approximately 30 MeV. The average shape of the spectrum over each of the six intervals of best magnetic connection remains the same for all intervals within uncertainties

Further observations of protons resulting from the decay of neutrons ejected by solar flares

The solar flare of 1984 April 24 produced a large gamma ray fluence with energy 2MeV. The time profile of the interplanetary flux from this flare indicates the presence of decaying solar neutrons. This makes a total of three neutron flares so far observed by this method. The three flares are used to place constraints on the fluence and spectra of neutrons emitted by the Sun

Extension of the spin-1/2 frustrated square lattice model: the case of layered vanadium phosphates

We study the influence of the spin lattice distortion on the properties of frustrated magnetic systems and consider the applicability of the spin-1/2 frustrated square lattice model to materials lacking tetragonal symmetry. We focus on the case of layered vanadium phosphates AA'VO(PO4)2 (AA' = Pb2, SrZn, BaZn, and BaCd). To provide a proper microscopic description of these compounds, we use extensive band structure calculations for real materials and model structures and supplement this analysis with simulations of thermodynamic properties, thus facilitating a direct comparison with the experimental data. Due to the reduced symmetry, the realistic spin model of layered vanadium phosphates AA'VO(PO4)2 includes four inequivalent exchange couplings: J1 and J1' between nearest-neighbors and J2 and J2' between next-nearest-neighbors. The estimates of individual exchange couplings suggest different regimes, from J1'/J1 and J2'/J2 close to 1 in BaCdVO(PO4)2, a nearly regular frustrated square lattice, to J1'/J1 ~ 0.7 and J2'/J2 ~ 0.4 in SrZnVO(PO4)2, a frustrated square lattice with sizable distortion. The underlying structural differences are analyzed, and the key factors causing the distortion of the spin lattice in layered vanadium compounds are discussed. We propose possible routes for finding new frustrated square lattice materials among complex vanadium oxides. Full diagonalization simulations of thermodynamic properties indicate the similarity of the extended model to the regular one with averaged couplings. In case of moderate frustration and moderate distortion, valid for all the AA'VO(PO4)2 compounds reported so far, the distorted spin lattice can be considered as a regular square lattice with the couplings (J1+J1')/2 between nearest-neighbors and (J2+J2')/2 between next-nearest-neighbors.Comment: 14 pages, 9 figures, 4 table

Dynamic response functions for the Holstein-Hubbard model

We present results on the dynamical correlation functions of the particle-hole symmetric Holstein-Hubbard model at zero temperature, calculated using the dynamical mean field theory which is solved by the numerical renormalization group method. We clarify the competing influences of the electron-electron and electron-phonon interactions particularity at the different metal to insulator transitions. The Coulomb repulsion is found to dominate the behaviour in large parts of the metallic regime. By suppressing charge fluctuations, it effectively decouples electrons from phonons. The phonon propagator shows a characteristic softening near the metal to bipolaronic transition but there is very little softening on the approach to the Mott transition.Comment: 13 pages, 19 figure

On the t-Term Rank of a Matrix

For t a positive integer, the t-term rank of a (0,1)-matrix A is defined to be the largest number of 1s in A with at most one 1 in each column and at most t 1s in each row. Thus the 1-term rank is the ordinary term rank. We generalize some basic results for the term rank to the t-term rank, including a formula for the maximum term rank over a nonempty class of (0,1)-matrices with the the same row sum and column sum vectors. We also show the surprising result that in such a class there exists a matrix which realizes all of the maximum terms ranks between 1 and t.Comment: 18 page

Interchain monomer contact probability in two-dimensional polymer solutions

Using molecular dynamics simulation of a standard bead-spring model we investigate the density crossover scaling of strictly two-dimensional self-avoiding polymer chains focusing on properties related to the contact exponent set by the intrachain subchain size distribution. Irrespective of the density sufficiently long chains are found to consist of compact packings of blobs of fractal perimeter dimension dp = 5/4

Diffusion and Interdiffusion in Binary Metallic Melts

We discuss the dependence of self- and interdiffusion coefficients on temperature and composition for two prototypical binary metallic melts, Al-Ni and Zr-Ni, in molecular-dynamics (MD) computer simulations and the mode-coupling theory of the glass transition (MCT). Dynamical processes that are mainly entropic in origin slow down mass transport (as expressed through self diffusion) in the mixture as compared to the ideal-mixing contribution. Interdiffusion of chemical species is a competition of slow kinetic modes with a strong thermodynamic driving force that is caused by non-entropic interactions. The combination of both dynamic and thermodynamic effects causes qualitative differences in the concentration dependence of self-diffusion and interdiffusion coefficients. At high temperatures, the thermodynamic enhancement of interdiffusion prevails, while at low temperatures, kinetic effects dominate the concentration dependence, rationalized within MCT as the approach to its ideal-glass transition temperature $T_c$. The Darken equation relating self- and interdiffusion qualitatively reproduces the concentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, the kinetic contributions to interdiffusion can be slower than the lower bound suggested by the Darken equation. As temperature is decreased, the agreement with Darken's equation improves, due to a strong coupling of all kinetic modes that is a generic feature predicted by MCT.Comment: 16 pages, 12 figure

Paleolakes and life on early Mars

Two distinct directions have begun to elucidate key parameters in the search for extinct life on Mars. Carbonate sediments, deposited about 10,000 years ago in association with biological activity, have been sampled from the paleolake beds of Lake Vanda and Meirs in the McMurdo Dry Valleys in Antarctica. These samples are being analyzed for simple biological signatures that remain in cold and dry paleolake sediments, namely microfossils, percent carbonate, and total organic carbon. Our second initiative is the study of Colour Lake, in the Canadian Arctic, that periodically maintains a perennial ice cover. Physical measurements started this year will be used to determine one end point for ice covered lake environments and will be compared to continuous measurements from Antarctic lakes started in November 1985. Interestingly, Colour Lake also supports benthic mat communities, but the low pH precludes carbonate deposition. This research will broaden our knowledge base for what conditions are necessary for ice covered lake formation and what biological signatures will remain in paleolake deposits