10 research outputs found
Steady State Behavior of Mechanically Perturbed Spin Glasses and Ferromagnets
A zero temperature dynamics of Ising spin glasses and ferromagnets on random
graphs of finite connectivity is considered, like granular media these systems
have an extensive entropy of metastable states. We consider the problem of what
energy a randomly prepared spin system falls to before becoming stuck in a
metastable state. We then introduce a tapping mechanism, analogous to that of
real experiments on granular media, this tapping, corresponding to flipping
simultaneously any spin with probability , leads to stationary regime with a
steady state energy . We explicitly solve this problem for the one
dimensional ferromagnet and spin glass and carry out extensive
numerical simulations for spin systems of higher connectivity. The link with
the density of metastable states at fixed energy and the idea of Edwards that
one may construct a thermodynamics with a flat measure over metastable states
is discussed. In addition our simulations on the ferromagnetic systems reveal a
novel first order transition, whereas the usual thermodynamic transition on
these graphs is second order.Comment: 11 pages, 7 figure
Scaling of the distribution of fluctuations of financial market indices
We study the distribution of fluctuations over a time scale (i.e.,
the returns) of the S&P 500 index by analyzing three distinct databases.
Database (i) contains approximately 1 million records sampled at 1 min
intervals for the 13-year period 1984-1996, database (ii) contains 8686 daily
records for the 35-year period 1962-1996, and database (iii) contains 852
monthly records for the 71-year period 1926-1996. We compute the probability
distributions of returns over a time scale , where varies
approximately over a factor of 10^4 - from 1 min up to more than 1 month. We
find that the distributions for 4 days (1560 mins) are
consistent with a power-law asymptotic behavior, characterized by an exponent
, well outside the stable L\'evy regime . To
test the robustness of the S&P result, we perform a parallel analysis on two
other financial market indices. Database (iv) contains 3560 daily records of
the NIKKEI index for the 14-year period 1984-97, and database (v) contains 4649
daily records of the Hang-Seng index for the 18-year period 1980-97. We find
estimates of consistent with those describing the distribution of S&P
500 daily-returns. One possible reason for the scaling of these distributions
is the long persistence of the autocorrelation function of the volatility. For
time scales longer than days, our results are
consistent with slow convergence to Gaussian behavior.Comment: 12 pages in multicol LaTeX format with 27 postscript figures
(Submitted to PRE May 20, 1999). See
http://polymer.bu.edu/~amaral/Professional.html for more of our work on this
are
A study on the interfacial properties of electrodeposited single carbon fiber/epoxy composites using tensile and compressive fragmentation tests
Sphingolipid Analogues Inhibit Development of Malaria Parasites
<i>Plasmodium</i>-infected erythrocytes have
been shown
to employ sphingolipids from both endogenous metabolism as well as
existing host pools. Therapeutic agents that limit these supplies
have thus emerged as intriguing, mechanistically distinct putative
targets for the treatment of malaria infections. In an initial screen
of our library of sphingolipid pathway modulators for efficacy against
two strains of the predominant human malaria species <i>Plasmodium
falciparum</i> and <i>Plasmodium knowlesi</i>, a series
of orally available, 1-deoxysphingoid bases were found to possess
promising in vitro antimalarial activity. To better understand the
structural requirements that are necessary for this observed activity,
a second series of modified analogues were prepared and evaluated.
Initial pharmacokinetic assessments of key analogues were investigated
to evaluate plasma and red blood cell concentrations in vivo