Fundamental Physical Constants: Looking from Different Angles

We consider fundamental physical constants which are among a few of the most important pieces of information we have learned about Nature after its intensive centuries-long studies. We discuss their multifunctional role in modern physics including problems related to the art of measurement, natural and practical units, origin of the constants, their possible calculability and variability etc

An accurate determination of the Avogadro constant by counting the atoms in a 28Si crystal

The Avogadro constant links the atomic and the macroscopic properties of matter. Since the molar Planck constant is well known via the measurement of the Rydberg constant, it is also closely related to the Planck constant. In addition, its accurate determination is of paramount importance for a definition of the kilogram in terms of a fundamental constant. We describe a new approach for its determination by "counting" the atoms in 1 kg single-crystal spheres, which are highly enriched with the 28Si isotope. It enabled isotope dilution mass spectroscopy to determine the molar mass of the silicon crystal with unprecedented accuracy. The value obtained, 6.02214084(18) x 10^23 mol^-1, is the most accurate input datum for a new definition of the kilogram.Comment: 4 pages, 5 figures, 3 table

Temperature-dependent Optoelectronic Properties of Quasi-2D Colloidal Cadmium Selenide Nanoplatelets

Colloidal Cadmium Selenide (CdSe) nanoplatelets (NPLs) are a recently developed class of efficient luminescent nanomaterial suitable for optoelectronic device applications. A change in temperature greatly affects their electronic bandstructure and luminescence properties. It is important to understand how-and-why the characteristics of NPLs are influenced, particularly at elevated temperature, where both reversible and irreversible quenching processes come into picture. Here we present a study on the effect of elevated temperature on the characteristics of colloidal CdSe NPLs. We used an effective-mass envelope function theory based 8-band k$\cdot$p model and density-matrix theory considering exciton-phonon interaction. We observed the photoluminescence (PL) spectra at various temperatures for their photon emission energy, PL linewidth and intensity by considering the exciton-phonon interaction with both acoustic and optical phonons using Bose-Einstein statistical factors. With rise in temperature we observed a fall in the transition energy (emission redshift), matrix element, Fermi factor and quasi Fermi separation, with reduction in intraband state gaps and increased interband coupling. Also, there was a fall in the PL intensity, along with spectral broadening due to an intraband scattering effect. The predicted transition energy values and simulated PL spectra at varying temperatures exhibit appreciable consistency with experimental results. Our findings have important implications for application of NPLs in optoelectronic devices, such as NPL lasers and LEDs, operating much above room temperature.Comment: Published in Royal Society of Chemistry, Nanoscale (12 pages, 9 figures

Quantum statistical approach to optical properties in dense plasmas

In this dissertation the transition rates, the ionization potential depression, and the optical spectra in dense plasmas are investigated within a quantum-statistical approach. The dynamical structure factors are introduced to account for the detailed spatial and temporal correlations and fluctuations of the plasma environment. The transition rates between Rydberg states are found to be more reasonably explained by a coherent quasi-classical wave packet description. As another important result, a general expression for the ionization potential depression in plasmas is obtained.In der Arbeit werden die Übergangsraten, die Absenkung der Kontinuumskante und die optischen Spektren in dichten Plasmen mit Hilfe eines quantenstatistischen Zugangs untersucht. Dynamische Strukturfaktoren werden eingeführt, um die räumlichen und zeitlichen Korrelationen und Schwankungen der Plasmaumgebung zu berücksichtigen. Die Übergangsraten zwischen den Rydbergzuständen lassen sich durch eine kohärente quasi-klassische Wellenpaket-Darstellung besser erklären. Ein weiteres zentrales Ergebnis ist die Herleitung eines allgemeinen Ausdrucks für die Absenkung der Kontinuumskante in Plasmen