Transport, Relaxation and Line shape in Quantum Dissipative Systems with Applications to Photosynthesis and Conjugated Polymers
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The thesis entitled as “Transport, Relaxation and Line shape in Quantum Dissipative Systems with Applications to Photosynthesis and Conjugated Polymers” has been divided into five major parts and ten chapters. In Part I, excitation energy transfer process is investigated in a photosynthetic Fenna-Matthews-Olson complex. It is observed that fluctuations can not only destroy coherence, but they can also facilitate it under appropriate circumstances. Pronounced temperature effect has been found in intermediate coupling regime and long time limit. The measure of quantum entanglement concurrence and coherence length is used to study the dynamic localization and delocalization. Interetingly, complete delocalization is detected at short time limit whereas presence of finite delocalization is observed in the long time limit. In Part II, quantum diffusion in a one-dimensional lattice is characterized by dynamic disorder for the spatially correlated and uncorrelated bath models. Coherent migration of exciton is observed in the same bath case even at long times while the dynamics becomes incoherent in the independent bath case in Markovian limit. Interestingly, diffusive behavior is found even in the non-Markovian limit where the diffusion coefficient is quite lower than that in the Markovian limit because of the underestimation of coherence. In Part III, line shape of an exciton in a one-dimensional lattice consisting of regularly placed and equally separated optical two-level systems is explored for both linear array and cyclic ring systems of different sizes. In the slow modulation limit, it is observed that the line shape is broadened and the number of peaks is increased as compared to the ones obtained from the diagonalization of the Toeplitz matrix (TM). However, the line shape shows motional narrowing with peaks at the values predicted by TM in the fast modulation limit. A connection is established between the line shape and the population transfer dynamics. The unique role of the rate of fluctuation has been revealed in the sustenance and propagation of coherence. In Part IV, role of quantum coherence in establishing non-canonical population distribution is investigated in extended molecular systems in terms of the transition in bath states. The interplay between fluctuation and temperature is also explained in determining equilibrium distribution. In Part V, the derivation of the temperature dependent and temperature independent quantum stochastic Liouville equation (QSLE) is presented. The temperature independent QSLE is derived using quantum Liouville equation and joint probability distribution over system and bath variable. For the derivation of temperature dependent QLSE the Feynman-Vernon approach is used.