We study the electronic properties of disordered GaAs-AlxGa1−xAs semiconductor superlattices with structural long-range correlations. The system consists of quantum barriers and wells with different thicknesses and heights which fluctuate around their mean values randomly, following a long-range correlated pattern of fractal type characterized by a power spectrum of the type S(k) ∝ 1/k (2α−1) , where the exponent α quantifies the strength of the long-range correlations. For a given system size, we find a critical value of the exponent α (αc) for which a metal-insulator transition appears: for α < αc all the states are localized, and for α > αc, we find a continuous band of extended states. We also show that the existence of extended states causes a strong enhancement of the DC conductance of the superlattice at finite temperature, which increases in many orders of magnitude when crossing from the localized to the extended regime. Finally, we perform finite size scaling and we obtain the value of the critical exponent αc in the thermodynamic limit, showing that the transition is not a finite-size effect.

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Using transfer-matrix and stationary phase methods, we study the tunneling time (group delay time) in a ferromagnetic monolayer graphene superlattice. The system we peruse consists of a sequence of rectangular barriers and wells, which can be realized by putting a series of electronic gates on the top of ferromagnetic graphene. The magnetization in the two ferromagnetic layers is aligned parallel. We find out that the tunneling time for normal incident is independent of spin state of electron as well as the barrier height and electron Fermi energy while for the oblique incident angles the tunneling time depends on the spin state of electron and has an oscillatory behavior. Also the effect of barrier width on tunneling time is also investigated and shown that, for normal incident, the Hartman effect disappears in a ferromagnetic graphene superlattice but it appears for oblique incident angles when the x component of the electron wave vector in the barrier is imaginary

Solar energy is one of the cleanest energies that are abundant among renewable energy sources. To design a very-low-cost solar device, chlorophyll extracted from spinach was employed. Photosystem I (PSI) was extracted by homogenization of spinach leaves and centrifugation and fractionation by column chromatography. The dipole structure of such natural photodiodes is one of the special features of the PSI protein complex that we tried to align using pulsed external electric fields to improve solar cell efficiency. To align PSI dipoles, several approaches have exploited linkers such as amino acids or polymers. Because of the difficulties of nonpolar or polar biomolecules, here, our approach was to apply a uniform pulsed external electric field (PEEF). With respect to other existing methods, our procedure is a simple, low-cost, and efficient design of solar cells, which leads to improved efficiency of biophotovoltaic cells up to a four-fold increase.

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