120 research outputs found

    Reconciling Conductance Fluctuations and the Scaling Theory of Localization

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    We reconcile the phenomenon of mesoscopic conductance fluctuations with the single parameter scaling theory of the Anderson transition. We calculate three averages of the conductance distribution: exp()\exp(), and 1/1/ where gg is the conductance in units of e2/he^2/h and R=1/gR=1/g is the resistance and demonstrate that these quantities obey single parameter scaling laws. We obtain consistent estimates of the critical exponent from the scaling of all these quantities

    Scaling of the conductance distribution near the Anderson transition

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    The single parameter scaling hypothesis is the foundation of our understanding of the Anderson transition. However, the conductance of a disordered system is a fluctuating quantity which does not obey a one parameter scaling law. It is essential to investigate the scaling of the full conductance distribution to establish the scaling hypothesis. We present a clear cut numerical demonstration that the conductance distribution indeed obeys one parameter scaling near the Anderson transition

    Symmetry, dimension and the distribution of the conductance at the mobility edge

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    The probability distribution of the conductance at the mobility edge, pc(g)p_c(g), in different universality classes and dimensions is investigated numerically for a variety of random systems. It is shown that pc(g)p_c(g) is universal for systems of given symmetry, dimensionality, and boundary conditions. An analytical form of pc(g)p_c(g) for small values of gg is discussed and agreement with numerical data is observed. For g>1g > 1, lnpc(g)\ln p_c(g) is proportional to (g1)(g-1) rather than (g1)2(g-1)^2.Comment: 4 pages REVTeX, 5 figures and 2 tables include

    Probability distribution of the conductance at the mobility edge

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    Distribution of the conductance P(g) at the critical point of the metal-insulator transition is presented for three and four dimensional orthogonal systems. The form of the distribution is discussed. Dimension dependence of P(g) is proven. The limiting cases gg\to\infty and g0g\to 0 are discussed in detail and relation P(g)0P(g)\to 0 in the limit g0g\to 0 is proven.Comment: 4 pages, 3 .eps figure

    Wavelength dependent collective effects in the multiphoton ionization of atomic deuterium

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    This paper presents the results of an experimental investigation into collective effects in the transient plasma formed by multiphoton ionization of atomic deuterium with a pulsed laser. The laser wavelength is varied in a narrow range around 243 nm, so that the photoionization is resonant with the metastable 2S1/2 state. The ion yield, the ion time-of-flight spectra, and the yield of Lyman-a photons have been measured as a function of laser intensity ~from 1 to 340 MW/cm2! and laser detuning around the 1S1/2-2S1/2 two-photon resonance. During and shortly after the laser pulse, collective effects resulting from the mutual interaction of the photoelectrons and the ions affect the spatial and temporal distribution of the ions. Because of the near-degeneracy of the 2S1/2, 2P1/2 , and 2P3/2 states, the resonant multiphoton ionization is affected by the Stark mixing of these states in the collective field. As a result, the time-dependent yields of ions and of Lyman-a photons are modulated by the interplay of the multiphoton ionization of the atoms and the collective effects in the plasma. From the measurements it is deduced that collective effects are important above a critical charge density of 33108 ions/cm3. An asymmetry is observed in the line profile of the total ion yield as a function of laser detuning. This asymmetry is interpreted to be due to the effect of the collective field upon the intermediate resonant 2S1/2 state of the photoionization process

    Wavelength dependent collective effects in the multiphoton ionization of atomic deuterium

    Get PDF
    This paper presents the results of an experimental investigation into collective effects in the transient plasma formed by multiphoton ionization of atomic deuterium with a pulsed laser. The laser wavelength is varied in a narrow range around 243 nm, so that the photoionization is resonant with the metastable 2S1/2 state. The ion yield, the ion time-of-flight spectra, and the yield of Lyman-a photons have been measured as a function of laser intensity ~from 1 to 340 MW/cm2! and laser detuning around the 1S1/2-2S1/2 two-photon resonance. During and shortly after the laser pulse, collective effects resulting from the mutual interaction of the photoelectrons and the ions affect the spatial and temporal distribution of the ions. Because of the near-degeneracy of the 2S1/2, 2P1/2 , and 2P3/2 states, the resonant multiphoton ionization is affected by the Stark mixing of these states in the collective field. As a result, the time-dependent yields of ions and of Lyman-a photons are modulated by the interplay of the multiphoton ionization of the atoms and the collective effects in the plasma. From the measurements it is deduced that collective effects are important above a critical charge density of 33108 ions/cm3. An asymmetry is observed in the line profile of the total ion yield as a function of laser detuning. This asymmetry is interpreted to be due to the effect of the collective field upon the intermediate resonant 2S1/2 state of the photoionization process

    Conductance fluctuations and boundary conditions

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    The conductance fluctuations for various types for two-- and three--dimensional disordered systems with hard wall and periodic boundary conditions are studied, all the way from the ballistic (metallic) regime to the localized regime. It is shown that the universal conductance fluctuations (UCF) depend on the boundary conditions. The same holds for the metal to insulator transition. The conditions for observing the UCF are also given.Comment: 4 pages RevTeX, 5 figures include

    Polarization Correlation Measurements of Electron Impact Excitation of Hs2pd at 54.4 eV

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    First direct measurements are reported of the linear reduced Stokes parameters P1,P2 for H(2p) excited by electron impact at the benchmark energy of 54.4 eV. The results differ significantly from previous values deduced from angular correlation measurements which are in serious conflict with all sophisticated theoretical approaches. Our results support the trend of theoretical predictions for P2 and confirm that its value is negative at electron scattering angles above 100±, as predicted by theory

    Polarization Correlation Measurements of Electron Impact Excitation of Hs2pd at 54.4 eV

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    First direct measurements are reported of the linear reduced Stokes parameters P1,P2 for H(2p) excited by electron impact at the benchmark energy of 54.4 eV. The results differ significantly from previous values deduced from angular correlation measurements which are in serious conflict with all sophisticated theoretical approaches. Our results support the trend of theoretical predictions for P2 and confirm that its value is negative at electron scattering angles above 100±, as predicted by theory

    Comparing the effects of sun exposure and vitamin D supplementation on vitamin D insufficiency, and immune and cardio-metabolic function: the Sun Exposure and Vitamin D Supplementation (SEDS) Study

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    BACKGROUND Adults living in the sunny Australian climate are at high risk of skin cancer, but vitamin D deficiency (defined here as a serum 25-hydroxyvitamin D (25(OH)D) concentration of less than 50 nmol/L) is also common. Vitamin D deficiency may be a risk factor for a range of diseases. However, the optimal strategies to achieve and maintain vitamin D adequacy (sun exposure, vitamin D supplementation or both), and whether sun exposure itself has benefits over and above initiating synthesis of vitamin D, remain unclear. The Sun Exposure and Vitamin D Supplementation (SEDS) Study aims to compare the effectiveness of sun exposure and vitamin D supplementation for the management of vitamin D insufficiency, and to test whether these management strategies differentially affect markers of immune and cardio-metabolic function. METHODS/DESIGN The SEDS Study is a multi-centre, randomised controlled trial of two different daily doses of vitamin D supplementation, and placebo, in conjunction with guidance on two different patterns of sun exposure. Participants recruited from across Australia are aged 18-64 years and have a recent vitamin D test result showing a serum 25(OH)D level of 40-60 nmol/L. DISCUSSION This paper discusses the rationale behind the study design, and considers the challenges but necessity of data collection within a non-institutionalised adult population, in order to address the study aims. We also discuss the challenges of participant recruitment and retention, ongoing engagement of referring medical practitioners and address issues of compliance and participant retention. TRIAL REGISTRATION Australia New Zealand Clinical Trials Registry: ACTRN12613000290796 Registered 14 March 2013
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