Third Cumulant of the total Transmission of diffuse Waves

The probability distribution of the total transmission is studied for waves multiple scattered from a random, static configuration of scatterers. A theoretical study of the second and third cumulant of this distribution is presented. Within a diagrammatic approach a theory is developed which relates the third cumulant normalized to the average, $\langle \langle T_a^3 \rangle \rangle$, to the normalized second cumulant $\langle \langle T_a^2 \rangle \rangle$. For a broad Gaussian beam profile it is found that $\langle \langle T_a^3 \rangle \rangle= \frac{16}{5} \langle \langle T_a^2 \rangle \rangle^2$. This is in good agreement with data of optical experiments.Comment: 16 pages revtex, 8 separate postscript figure

GABA shunt enzymes and the relationship with morphine abstinence

Selective inhibition of tbe rate-limiting step in tbe degradation of tbe inhibitory neurotransmitter Y·aminobutyric acid (GABA) might be of potential use in the treatment of many neurological or psychiatric disorders since it might correct a central GABA deficiency. Alternatively, as such diseases have not been correlated convincingly witb changes in tbe GABA-ergic system, inhibition of this rate-limiting step by specific drugs in clinical trials could be useful in demonstrating tbe existence of a GABA deficiency in tbese disturbances. The study of effects of drugs exerting a tberapeutic action in specific neurological or psychiatric disorders on the rate-limiting enzyme in GABA degradation might also be useful. Therefore, tbe primary object of tbis tbesis has been to find the rate-limiting step in GABA degradation. GABA is degraded by tbe consecutive action of two enzymes: GABA-transaminase and SSA-dehydrogenase. It is almost generally believed tbat GABA-transaminase is rate-limiting in GABA degradation. As a consequence, SSA-dehydrogenase has been almost completely ignored as a likely candidate for a regulatory function in tbe GABA ·shunt. Based on in vitro experiments it is suggested in this tbesis that SSA-dehydrogenase may have a regulatory function in the GABA shunt. To test this hypothesis in vivo it became of crucial importance to fmd a behavioural correlate of increased GABA-ergic activity in tbe rat. Such a behavioural correlate might be the increased locomotor activity and quasi-morphine abstinence behaviour observed after administration of di·n-propylacetate (DPA) to tbe rat. This drug is used in the treatment of petit mal epilepsy (SIMON & PENRY, 1975) and probably acts via inhibition of SSA-dehydrogenase (HARVEY et al, 1975; ANLEZARK et al., 1976). The special character of tbe behaviour observed after administration of DPA suggests tbat an increased GABA-ergic activity might be related to the well known morphine abstinence syndrome. Therefore, some studies have been conducted witb morphine to demonstrate this relationship. In addition, this DPA-induced abstinence behaviour has been studied pharmacologically to demonstrate its relationship witb an overactive GABA-ergic system in vivo

Thick Co-based coating on cast iron by side laser cladding:Analysis of processing conditions and coating properties

The objective of this work was to create Co-based coatings (compositionally close to Stellite 6) on compacted graphite and gray cast iron,substrates with a high power laser (2 kW continuous Nd:YAG) cladding process. The relationships between the relevant laser cladding parameters (i.e. laser beam scanning speed, laser power and powder feeding rate) and the main geometrical characteristics of a single laser track (height, width, dilution, etc.) were examined. A gradual variation of a single processing parameter was used for an appropriate experimental analysis and statistical correlations study between main processing parameters and geometrical characteristics of an individual laser track. These relations lead to the design of a laser cladding processing map that can be used as a guideline for the selection and further tuning of proper processing parameters for laser cladding of extensive layer. The coatings with thickness from 1.0 to 3.3 mm were created on flat substrates without cracks and other major defects. The nucrostructural features of these coatings were studied using optical microscopy, scanning electron microscopy (Philips XL30 FEG), EDS (EDAX) and XRD. Mechanical properties were determined using microhardness measurement, scratch test (CSM Revetest) analysis at room temperature and using the tribotesting (CSM HT Tribometer) at room and elevated (up to 525 degrees C) temperatures. (c) 2006 Elsevier B.V. All rights reserved

Intensity Distribution of Waves Transmitted Through a Multiple Scattering Medium

The distributions of the angular transmission coefficient and of the total transmission are calculated for multiple scattered waves. The calculation is based on a mapping to the distribution of eigenvalues of the transmission matrix. The distributions depend on the profile of the incoming beam. The distribution function of the angular transmission has a stretched exponential decay. The total-transmission distribution grows log-normally whereas it decays exponentially.Comment: 8 pages, revtex3.0, 3 postscript figures, NvR0

"Optical conductance fluctuations: diagrammatic analysis in Landauer approach and non-universal effects"

The optical conductance of a multiple scattering medium is the total transmitted light of a diffuse incoming beam. This quantity, very analogous to the electronic conductance, exhibits universal conductance fluctuations. We perform a detailed diagrammatic analysis of these fluctuations. With a Kadanoff-Baym technique all the leading diagrams are systematically generated. A cancellation of the short distance divergencies occurs, that yields a well behaved theory. The analytical form of the fluctuations is calculated and applied to optical systems. Absorption and internal reflections reduce the fluctuations significantly.Comment: 25 pages Revtex 3.0, 18 seperate postscript figure

Deviations from the Gaussian distribution of mesoscopic conductance fluctuations

The conductance distribution of metallic mesoscopic systems is considered. The variance of this distribution describes the universal conductance fluctuations, yielding a Gaussian distribution of the conductance. We calculate diagrammatically the third cumulant of this distribution, the leading deviation from the Gaussian. We confirm random matrix theory calculations that the leading contribution in quasi-one dimension vanishes. However, in quasi two dimensions the third cumulant is negative, whereas in three dimensions it is positive.Comment: 9 pages, Revtex, with eps figures,to appear in Phys Rev

The Initial Mass Function toward the low-mass end in the Large Magellanic Cloud with HST/WFPC2 Observations

We present V and I equivalent HST/WFPC2 photometry of two areas in the Large Magellanic Cloud: The southern part of the stellar association LH 52, located on the western edge of the super-shell LMC 4, and a field between two associations, which is located on the southwestern edge of the shell, and which accounts for the general background field of the galaxy. The HST/WFPC2 observations reach magnitudes as faint as V=25 mag, much deeper than have been observed earlier in stellar associations in the LMC. We determine the MF for main-sequence stars in the areas. Its slope (Gamma) in both areas is steeper for stars with masses M < 2 M_solar (-4 < Gamma < -6), compared with stars of M > 2 M_solar (-1 < Gamma < -2). Thus, as far as the field of the LMC concerns the MF does not have a uniform slope throughout its observed mass range. The MF of the general field of the LMC was found previously to be steeper than the MF of a stellar association for massive stars with M > 5 M_solar. We conclude that this seems to be also the case toward lower masses down to M ~ 1 M_solar. Our data allow to construct the field-subtracted, incompleteness-corrected, main-sequence MF of the southwestern part of the young stellar association LH 52, which accounts for the Initial Mass Function (IMF) of the system. Its mean slope is found to be comparable, but still more shallow than a typical Salpeter IMF (Gamma = -1.12 +/- 0.24) for masses down to ~ 1 M_solar. We found indications that the IMF of the association probably is ``top-heavy'', due to the large number of intermediate-mass stars in the field of the system, while the general LMC field is found to be responsible for the low-mass population, with M < 2 M_solar, observed in both fields.Comment: 15 pages, 9 figures, accepted for publication in Ap

Measurement of the Probability Distribution of Total Transmission in Random Waveguides

Measurements have been made of the probability distribution of total transmission of microwave radiation in waveguides filled with randomly positioned scatterers which would have values of the dimensionless conductance g near unity. The distributions are markedly non-Gaussian and have exponential tails. The measured distributions are accurately described by diagrammatic and random matrix calculations carried out for nonabsorbing samples in the limit g >> 1 when g is expressed in terms of the variance of the distribution, which equals the degree of long-range intensity correlation across the output face of the sample.Comment: 5 pages, 5 post script figures, RevTe