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Phase-flip bifurcation plays an important role in the transition to synchronization state in unidirectionally coupled parametrically excited pendula. In coupled identical system it is the cause of complete synchronization whereas in case of coupled non-identical system it causes desynchronization. In coupled identical systems negativity of conditional Lyapunov exponent is not always sufficient for complete synchronization. In complete synchronization state the largest conditional Lyapunov exponent and the second largest Lyapunov exponent are equal in magnitude and slope., Comment: 7 pages

Auxiliary system approach and various nearest neighbor methods are widely used to detect generalized synchronization in non-identical coupled systems. These methods generally give contradictory results. Therefore one method alone is not sufficient to predict correct result. We show in this report that it is necessary to apply multiple methods together to come to a conclusion. These methods show a signature of generalized synchronization in diffusively coupled non-identical chaotic parametric excited pendula. But we finally find it to be the almost synchronization. It is achieved when the second Lyapunov exponent and both the system's transverse Lyapunov exponents are almost equal. The transition from asynchronous state to almost synchronization is through frequency entrainment as coupling constant is increased. Non-identity of the pendula are realized by mismatch in amplitude of parametric forcing. The frequency entrainment regime does not depend on amplitude mismatch whereas onset of almost synchronization increases with increase in mismatch. The systems, Comment: 5 pages, 10 figures

We have studied the structural and electronic properties of defect chalcopyrite semiconductors AAl2Se4 (A = Ag, Cu, Cd, Zn) using Density Functional Theory (DFT) based first principle technique within Tight binding Linear Muffin Tin orbital (TB-LMTO) method. Our calculated structural parameters such as lattice constants, anion displacement parameter (u), tetragonal distortion ({\eta} = c/2a), bond lengths and bulk modulus are in good agreement with other work. Our band structure calculation suggests that these compounds are direct band gap semiconductors having band gaps 2.40, 2.50, 2.46 and 2.82 eV for AAl2 Se4 (A = Ag, Cu, Cd, Zn) respectively. Calculated band gaps are in good agreement with other experimental and theoretical works within LDA limitation. We have made a quantitative estimation of the effect of p-d hybridization and structural distortion on the electronic properties. The reduction in band gap due to p-d hybridization are 19.47%, 21.29%, 0% and 0.7% for AAl2 Se4 (A = Ag, Cu, Cd, Zn) respectively. Increment of the band gap due to structural distortion is 11.62%, 2.45%, 2.92% and 9.30% in case of AgAl2 Se4, CuAl2 Se4, CdAl2 Se4 and ZnAl2 Se4 respectively . We have also discussed the bond nature of all four compounds., Comment: 17 pages, 11 figures, submitted to Solid state Sciences

We have carried out ab-initio calculation and study of structural and electronic properties of AgAlM2 (M = S, Se, Te) chalcopyrite semiconductors using Density Functional Theory (DFT) based self consistent Tight binding Linear Muffin Tin orbital (TB-LMTO) method. Calculated equlibrium values of lattice constants, anion displacement parameter (u), tetragonal distortion ({\eta} = c/2a) and bond lengths have good agreement with experimental values. Our study suggests these semiconductors to be direct band gap semiconductors with band gaps 1.98 eV, 1.59 eV and 1.36 eV respectively. These are in good agreement with experimental value within the limitation of local density approximation (LDA). Our explicit study of the effects of anion displacement and p-d hybridization show that band gap increases by 9.8%, 8.2% and 5.1% respectively for AgAlM2 (M = S, Se, Te) due to former effect and decreases by 51%, 47% and 42% respectively due to later effect., Comment: 15 pages, 17 figures, This article has been communicated to Solid State Communications