12,732 research outputs found
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules
Dust grains in cold circumstellar regions and dark-cloud interiors at 10-20 K
are covered by ice mantles. A nonthermal desorption mechanism is invoked to
explain the presence of gas-phase molecules in these environments, such as the
photodesorption induced by irradiation of ice due to secondary ultraviolet
photons. To quantify the effects of ice photoprocessing, an estimate of the
photon absorption in ice mantles is required. In a recent work, we reported the
vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the
solid phase. The aim was to estimate the VUV-absorption cross sections of
nonpolar molecular ice components, including CH4, CO2, N2, and O2. The column
densities of the ice samples deposited at 8 K were measured in situ by infrared
spectroscopy in transmittance. VUV spectra of the ice samples were collected in
the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged
hydrogen flow lamp. We found that, as expected, solid N2 has the lowest
VUV-absorption cross section, which about three orders of magnitude lower than
that of other species such as O2, which is also homonuclear. Methane (CH4) ice
presents a high absorption near Ly-alpha (121.6 nm) and does not absorb below
148 nm. Estimating the ice absorption cross sections is essential for models of
ice photoprocessing and allows estimating the ice photodesorption rates as the
number of photodesorbed molecules per absorbed photon in the ice.Comment: 9 pages, 6 figures, 7 table
Non perturbative renormalization group approach to surface growth
We present a recently introduced real space renormalization group (RG)
approach to the study of surface growth.
The method permits us to obtain the properties of the KPZ strong coupling
fixed point, which is not accessible to standard perturbative field theory
approaches. Using this method, and with the aid of small Monte Carlo
calculations for systems of linear size 2 and 4, we calculate the roughness
exponent in dimensions up to d=8. The results agree with the known numerical
values with good accuracy. Furthermore, the method permits us to predict the
absence of an upper critical dimension for KPZ contrarily to recent claims. The
RG scheme is applied to other growth models in different universality classes
and reproduces very well all the observed phenomenology and numerical results.
Intended as a sort of finite size scaling method, the new scheme may simplify
in some cases from a computational point of view the calculation of scaling
exponents of growth processes.Comment: Invited talk presented at the CCP1998 (Granada
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