Jianlan Wu

Efficient energy transfer in light-harvesting systems, I: optimal temperature, reorganization energy and spatial–temporal correlations

Understanding the mechanisms of efficient and robust energy transfer in light-harvesting systems provides new insights for the optimal design of artificial systems. In this paper, we use the Fenna-Matthews-Olson (FMO) protein complex and phycocyanin 645 (PC 645) to explore the general dependence on physical parameters that help maximize the efficiency and maintain its stability. With the Haken-Strobl model, the maximal energy transfer efficiency (ETE) is achieved under an intermediate optimal value of dephasing rate. To avoid the infinite temperature assumption in the Haken-Strobl model and the failure of the Redfield equation in predicting the Forster rate behavior, we use the generalized Bloch-Redfield (GBR) equation approach to correctly describe dissipative exciton dynamics, and we find that maximal ETE can be achieved under various physical conditions, including temperature, reorganization energy and spatial-temporal correlations in noise. We also identify regimes of reorganization energy where the ETE changes monotonically with temperature or spatial correlation and therefore cannot be optimized with respect to these two variables.

Linear and nonlinear response functions of the Morse oscillator: Classical divergence and the uncertainty principle

The algebraic structure of the quantum Morse oscillator is explored to formulate the coherent state, the phase-space representations of the annihilation and creation operators, and their classical limits. The formulation allows us to calculate the linear and nonlinear quantum response functions for microcanonical Morse systems and to demonstrate the linear divergence in the corresponding classical response function. On the basis of the uncertainty principle, the classical divergence is removed by phase-space averaging around the microcanonical energy surface. For the Morse oscillator, the classical response function averaged over quantized phase space agrees exactly with the quantum response function for a given eigenstate. Thus, phase-space averaging and quantization provide a useful way to establish the classical-quantum correspondence of anharmonic systems.

Calculations of nonlinear spectra of liquid Xe. II. Fifth-order Raman response

The polarization dependence and temporal profile of the fifth-order Raman response function and corresponding correlation function in liquid Xe are studied both analytically and numerically. Based on the symmetry of an isotropic sample, the fifth-order Raman response function has twelve distinct tensor elements, ten of which are independent, and the corresponding correlation function has twelve distinct tensor elements, seven of which are independent. The coefficients for decomposition into independent components are calculated explicitly based on the tensor property of an isotropic sample and are used to identify different coupling mechanisms in liquid Xe. The two-dimensional profile of the fifth-order Raman response function is evaluated by a simple hydrodynamic expression derived using the Gaussian factorization scheme. An alternative approach reduces the fifth-order Raman response function to time correlation functions that are easy to compute.

Extended hierarchy equation of motion for the spin-boson model

An extended hierarchy equation of motion (HEOM) is proposed and applied to study the dynamics of the spin-boson model. In this approach, a complete set of orthonormal functions are used to expand an arbitrary bath correlation function. As a result, a complete dynamic basis set is constructed by including the system reduced density matrix and auxiliary fields composed of these expansion functions, where the extended HEOM is derived for the time derivative of each element. The reliability of the extended HEOM is demonstrated by comparison with the stochastic Hamiltonian approach under room-temperature classical ohmic and sub-ohmic noises and the multilayer multiconfiguration time-dependent Hartree theory under zero-temperature quantum ohmic noise. Upon increasing the order in the hierarchical expansion, the result obtained from the extended HOEM systematically converges to the numerically exact answer.

Density Measurement of 1-D Confined Water by Small Angle Neutron Scattering Method : Pore Size and Hydration Level Dependences

Small angle neutron scattering (SANS) is used to measure the absolute density of water contained in 1-D cylindrical pores of a silica material MCM-41-S with pore diameters of 19 and 15 A. By being able to suppress the homogeneous nucleation process inside the narrow pore, one can keep water in the liquid state down to at least 160 K. From a combined analysis of SANS data from both H(2)O and D(2)O hydrated samples, we determined the absolute value of the density of 1-D confined water. We found that the average density of water inside the fully hydrated 19 A pore is 8% higher than that of the bulk water at room temperature. The temperature derivative of the density shows a pronounced peak at T(L) = 235 K signaling the crossing of the Widom line at ambient pressure and confirming the existence of a liquid-liquid phase transition at an elevated pressure. Pore size and hydration level dependences of the density are also studied.

THE ELECTROCHEMICAL PROPERTIES OF HYDROGEN STORAGE ZR-BASED LAVES PHASE ALLOYS

Abstract Zr-based Laves phase alloys Zr(Mn Cr V Ni) 2 were designed by means of the orthogonal method, and the electrochemical properties of the alloy electrodes were investigated. It was found that the maximum discharge capacity reached 343 mA h g −1 , the discharge capacity ratio at a discharge rate of 200-50 mA g −1 was more than 80%, the number of activation cycles is less than 13 and the self-discharge ratio after 2 weeks varied between 16.1 and 53.1%. X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis tests showed that the as-cast Zr-based Laves phase alloy consisted of at least eight phases. The major cubic C15-type Laves phase coexisted with the hexagonal C14-type Laves phase, orthorhombic Zr 7 Ni 10 phase, tetragonal, Zr 9 Ni 11 phase and a few other phases. X-ray photoelectron spectroscopy proved that Zr and Mn were richer in the outer layer of the Zr-based Laves phase hydride electrode; Ni and Cr were deficient on the surface of the electrode. After 540 charging-discharging cycles, the discharge capacities of the Zr-based Laves phase hydride electrodes decreased by 20–30%. This was mainly caused by the formation of oxides at the surface of the electrodes.

Higher-order kinetic expansion of quantum dissipative dynamics: mapping quantum networks to kinetic networks.

We apply a new formalism to derive the higher-order quantum kinetic expansion (QKE) for studying dissipative dynamics in a general quantum network coupled with an arbitrary thermal bath. The dynamics of system population is described by a time-convoluted kinetic equation, where the time-nonlocal rate kernel is systematically expanded of the order of off-diagonal elements of the system Hamiltonian. In the second order, the rate kernel recovers the expression of the noninteracting-blip approximation method. The higher-order corrections in the rate kernel account for the effects of the multi-site quantum coherence and the bath relaxation. In a quantum harmonic bath, the rate kernels of different orders are analytically derived. As demonstrated by four examples, the higher-order QKE can reliably predict quantum dissipative dynamics, comparing well with the hierarchic equation approach. More importantly, the higher-order rate kernels can distinguish and quantify distinct nontrivial quantum coherent effects, such as long-range energy transfer from quantum tunneling and quantum interference arising from the phase accumulation of interactions.

A continued fraction resummation form of bath relaxation effect in the spin-boson model.

In the spin-boson model, a continued fraction form is proposed to systematically resum high-order quantum kinetic expansion (QKE) rate kernels, accounting for the bath relaxation effect beyond the second-order perturbation. In particular, the analytical expression of the sixth-order QKE rate kernel is derived for resummation. With higher-order correction terms systematically extracted from higher-order rate kernels, the resummed quantum kinetic expansion approach in the continued fraction form extends the Pade approximation and can fully recover the exact quantum dynamics as the expansion order increases.

Ground-state shapes and structures of colloidal domains

In charged colloidal suspensions, the competition between square-well attraction and long-range Yukawa repulsion leads to various stable domains and Wigner supercrystals. Using a continuum model and symmetry arguments, a phase diagram of spheres, cylinders, and lamellae is obtained as a function of two control parameters, the volume fraction and the ratio between the surface tension and repulsion. Above a critical value of the ratio, the microphase cannot be supported by the Yukawa repulsion and macroscopic phase separation occurs. This finding quantitatively explains the lack of pattern formation in simple liquids because of the small hard sphere diameter in comparison with the size of macromolecules. The phase diagram also predicts microphase separation at zero value of the ratio, suggesting the possibility of self-assembly in repulsive systems.

Minimal Model of Quantum Kinetic Clusters for the Energy-Transfer Network of a Light-Harvesting Protein Complex

The energy absorbed in a light-harvesting protein complex is often transferred collectively through aggregated chromophore clusters. For population evolution of chromophores, the time-integrated effective rate matrix allows us to construct quantum kinetic clusters quantitatively and determine the reduced cluster-cluster transfer rates systematically, thus defining a minimal model of energy-transfer kinetics. For Fenna-Matthews-Olson (FMO) and light-havrvesting complex II (LCHII) monomers, quantum Markovian kinetics of clusters can accurately reproduce the overall energy-transfer process in the long-time scale. The dominant energy-transfer pathways are identified in the picture of aggregated clusters. The chromophores distributed extensively in various clusters can assist a fast and long-range energy transfer.