Electroreflectance spectroscopy in self-assembled quantum dots: lens symmetry

Modulated electroreflectance spectroscopy $\Delta R/R$ of semiconductor self-assembled quantum dots is investigated. The structure is modeled as dots with lens shape geometry and circular cross section. A microscopic description of the electroreflectance spectrum and optical response in terms of an external electric field (${\bf F}$) and lens geometry have been considered. The field and lens symmetry dependence of all experimental parameters involved in the $\Delta R/R$ spectrum have been considered. Using the effective mass formalism the energies and the electronic states as a function of ${\bf F}$ and dot parameters are calculated. Also, in the framework of the strongly confined regime general expressions for the excitonic binding energies are reported. Optical selection rules are derived in the cases of the light wave vector perpendicular and parallel to $$. Detailed calculation of the Seraphin coefficients and electroreflectance spectrum are performed for the InAs and CdSe nanostructures. Calculations show good agreement with measurements recently performed on CdSe/ZnSe when statistical distribution on size is considered, explaining the main observed characteristic in the electroreflectance spectra

On the structure of the energy distribution function in the hopping regime

The impact of the dispersion of the transport coefficients on the structure of the energy distribution function for charge carriers far from equilibrium has been investigated in effective-medium approximation for model densities of states. The investigations show that two regimes can be observed in energy relaxation processes. Below a characteristic temperature the structure of the energy distribution function is determined by the dispersion of the transport coefficients. Thermal energy diffusion is irrelevant in this regime. Above the characteristic temperature the structure of the energy distribution function is determined by energy diffusion. The characteristic temperature depends on the degree of disorder and increases with increasing disorder. Explicit expressions for the energy distribution function in both regimes are derived for a constant and an exponential density of states.Comment: 16 page

Dissipative Quantum Systems with Potential Barrier. General Theory and Parabolic Barrier

We study the real time dynamics of a quantum system with potential barrier coupled to a heat-bath environment. Employing the path integral approach an evolution equation for the time dependent density matrix is derived. The time evolution is evaluated explicitly near the barrier top in the temperature region where quantum effects become important. It is shown that there exists a quasi-stationary state with a constant flux across the potential barrier. This state generalizes the Kramers flux solution of the classical Fokker-Planck equation to the quantum regime. In the temperature range explored the quantum flux state depends only on the parabolic approximation of the anharmonic barrier potential near the top. The parameter range within which the solution is valid is investigated in detail. In particular, by matching the flux state onto the equilibrium state on one side of the barrier we gain a condition on the minimal damping strength. For very high temperatures this condition reduces to a known result from classical rate theory. Within the specified parameter range the decay rate out of a metastable state is calculated from the flux solution. The rate is shown to coincide with the result of purely thermodynamic methods. The real time approach presented can be extended to lower temperatures and smaller damping.Comment: 29 pages + 1 figure as compressed ps-file (uufiles) to appear in Phys. Rev.

On dispersive energy transport and relaxation in the hopping regime

A new method for investigating relaxation phenomena for charge carriers hopping between localized tail states has been developed. It allows us to consider both charge and energy {\it dispersive} transport. The method is based on the idea of quasi-elasticity: the typical energy loss during a hop is much less than all other characteristic energies. We have investigated two models with different density of states energy dependencies with our method. In general, we have found that the motion of a packet in energy space is affected by two competing tendencies. First, there is a packet broadening, i.e. the dispersive energy transport. Second, there is a narrowing of the packet, if the density of states is depleting with decreasing energy. It is the interplay of these two tendencies that determines the overall evolution. If the density of states is constant, only broadening exists. In this case a packet in energy space evolves into Gaussian one, moving with constant drift velocity and mean square deviation increasing linearly in time. If the density of states depletes exponentially with decreasing energy, the motion of the packet tremendously slows down with time. For large times the mean square deviation of the packet becomes constant, so that the motion of the packet is ``soliton-like''.Comment: 26 pages, RevTeX, 10 EPS figures, submitted to Phys. Rev.

Universal Crossover between Efros-Shklovskii and Mott Variable-Range-Hopping Regimes

A universal scaling function, describing the crossover between the Mott and the Efros-Shklovskii hopping regimes, is derived, using the percolation picture of transport in strongly localized systems. This function is agrees very well with experimental data. Quantitative comparison with experiment allows for the possible determination of the role played by polarons in the transport.Comment: 7 pages + 1 figure, Revte

Panel Discussion On Lipid Metabolism In Cardiovascular Disease†

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111126/1/jgs00741.pd

Dirac Equation Studies in the Tunnelling Energy Zone

We investigate the tunnelling zone V0 < E < V0+m for a one-dimensional potential within the Dirac equation. We find the appearance of superluminal transit times akin to the Hartman effect.Comment: 12 pages, 4 figure

Heterotic responses of tropical Elite maize accessions from Latin America with Brazilian testers.

Pouca enfase tem sido dada na identificacao de novas fontes de germoplasma tropical que podem ser usadas em programas de melhoramento de milho. Sao necessarias informacoes adicionais sobre a performance e classificacao heterotica de germoplasmas tropicais. O objetivo deste estudo foi identificar acessos elite originados do Programa Latino Americano de Milho (LAMP) que poderiam contribuir para aumentar o nivel de heterose com padroes heteroticos do Brasil. Setenta e dois acessos elites do LAMP foram cruzados com os testadores BR 105 (padrao heterotico flint) e BR 106 (padrao heterotico dentado). Os setenta e dois testcrosses, resultantes de cada cruzamento, e nove testemunhas foram avaliados em quatro locais. Utilizou-se o delineamento em latice simples 9 x 9. A analise combinada mostrou diferenca significativa entre tratamentos para o carater peso de espigas sendo tambem significativa a interacao tratamentos x locais. Foram identificados, com ambos os testadores, cruzamentos mais produtivos que o hibrido duplo BR 201 (testemunha comercial)> Com a variedade BR 105, a heterose em relacao ao pai superior variou de -28% a 26% e os acessos SE 032 e PE 001 foram selecionados para melhoramento. Com a variedade BR 106, a heterose em relacao ao pai superior variou de -35% a 17% e os acesses PE 011 e Pasco 14 foram selecionados para posteriores trabalhos de melhoramento. Os acessos selecionados serao melhorados atraves de esquemas de selecao recorrente para producao e caracteristicas agronomicas. Estes acessos podem ser explorados, dentro e entre os grupos heteroticos, para produzir combinacoes hibridas ou para serem usados como variedades melhoradas de milho

Disorder effects in electronic structure of substituted transition metal compounds

Investigating LaNi(1-x)M(x)O3 (M = Mn and Fe), we identify a characteristic evolution of the spectral function with increasing disorder in presence of strong interaction effects across the metal-insulator transition. We discuss these results vis-a-vis existing theories of electronic structure in simultaneous presence of disorder and interaction.Comment: Revtex, 4 pages, 3 postscript figures (To appear in Phys. Rev. Lett

Mechanical Relaxation in Glasses and at the Glass Transition

The Gilroy-Phillips model of relaxational jumps in asymmetric double-well potentials, developed for the Arrhenius-type secondary relaxations of the glass phase, is extended to a formal description of the breakdown of the shear modulus at the glass transition, the flow process.Comment: 13 pages, 11 figures, 49 ref