Time-Dependent Density Matrix Renormalization Group Method for Quantum Transport with Phonon Coupling in Molecular Junction.

Quantum transport in molecular junctions has attracted great attention. The charge motion in a molecular junction can cause geometric deformation, leading to strong electron phonon coupling, which was often overlooked. We have formulated a nearly exact method to assess the time-dependent current and occupation number in the molecular junction modeled by the electron-phonon coupled bridge state using the time-dependent density matrix renormalization group (TD-DMRG) method. The oscillation period and amplitude of the current are found to be dependent on the electron phonon coupling strength and energy level alignment with the electrodes. In an attempt to better understand these phenomena, we have devised a new approximation that explains the bistability phenomenon and the behavior of steady currents in the strong electron-phonon coupling regime. Comparisons have been made with the multilayer-multiconfiguration time-dependent Hartree (ML-MCTDH) method and the analytical result in the purely electronic limit. Furthermore, we explore the entropy of different orderings, extending to the electron phonon model problems. Regarding finite temperature, the thermal Bogoliubov transformation of both fermions and bosons is used and compared with imaginary time evolution results.

On the fly swapping algorithm for ordering of degrees of freedom in density matrix renormalization group.

Density matrix renormalization group (DMRG) and its time-dependent variants have found widespread applications in quantum chemistry, includingab initioelectronic structure of complex bio-molecules, spectroscopy for molecular aggregates, and charge transport in bulk organic semiconductors. The underlying wavefunction ansatz for DMRG, matrix product state (MPS), requires mapping degrees of freedom (DOF) into a one-dimensional topology. DOF ordering becomes a crucial factor for DMRG accuracy. In this work, we propose swapping neighboring DOFs during the DMRG sweeps for DOF ordering, which we term 'on the fly swapping' (OFS) algorithm. We show that OFS is universal for both static and time-dependent DMRG with minimum computational overhead. Examples are given for one dimensional antiferromagnetic Heisenberg model,ab initioelectronic structure of N2molecule, and the S1/S2internal conversion dynamics of pyrazine molecule. It is found that OFS can indeed improve accuracy by finding better DOF ordering in all cases.

Proton-Exchange-Induced Configuration Rearrangement in a Poly(ionic liquid) Solution: A NMR Study.

Polymeric ionic liquids have emerged recently as a promising alternative to traditional polymers as the polymer electrolyte membrane materials of choice because of their strongly decoupled dynamics between the polymer backbone and the counterions. Knowledge of proton exchange and transport mechanism in such materials is critical to the design and development of new poly(ionic liquid) materials with improved electrochemical properties. Our NMR results show that the proton exchange between the labile proton of the diethylmethylammonium (NH122) cation and H2O molecules is accompanied by a concerted configuration rearrangement of the ammonium. Through a combination of PFG-NMR and proton relaxation (line width) analysis, we demonstrate that at lower temperatures the labile proton diffuses along with the NH122 ammonium cation as an integral unit, whereas at higher temperatures the NH/H2O proton exchange sets in gradually, and the PFG-NMR measured diffusion coefficient is a population-averaged value between the two exchanging sites.

Proton Transport in Hierarchical-Structured Nafion Membranes: A NMR Study.

It is known that hierarchical structure plays a key role in many unique material properties such as self-cleaning effect of lotus leaves and the antifogging property of the compound eyes of mosquitoes. This study reports a series of highly ordered mesoporous Nafion membranes with unique hierarchical structural features at the nanometer scale. Using NMR, we show for the first time that, at low RH conditions, the proton in the ionic domains migrates via a surface diffusion mechanism and exhibits approximately 2 orders of magnitude faster transport than that in the nanopores, whereas the nanopores play a role of reservoir and maintain water and thereby conductivity at higher temperature and lower humidities. Thereby creating hierarchical nanoscale structures is a feasible and promising strategy to develop PEMs that would enable efficient electrochemical performance in devices such as fuel cells, even in the absence of high humidity and at elevated temperatures.

Properties of amorphous calcium carbonate and the template action of vaterite spheres.

The fast mixing of aqueous solutions of calcium chloride and sodium carbonate could immediately result in amorphous calcium carbonate (ACC). Under vigorous stirring, the formed ACC in the precipitation system will dissolve first and, then, transform within minutes to produce crystalline forms of vaterite and calcite. After that, the solution-mediated mechanism dominates the transformation of the thermodynamically unstable vaterite into the thermodynamically stable calcite. Although ACC is the least stable form of the six anhydrous phases of calcium carbonate (CaCO(3)), it could be, however, produced and stabilized by a variety of organisms. To better understand the formation-transformation mechanism of ACC and vaterite into calcite, ex-situ methods (i.e., scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectroscopy) were used to characterize the formation-transformation process of ACC and vaterite in aqueous systems without organic additives, showing that ACC sampled at different conditions has different properties (i.e., lifetime, morphology, and spectrum characterization). It is also very interesting to capture the obviously polycrystalline particles of CaCO(3) during the transformation process from vaterite to calcite, which suggests the formation mechanism for the calcite superstructure with multidimensional morphology.

[The influence of oxy-organics on the emission intensity of flame atomic emission spectrometry (FAES) of alkali metals].

Flame atomic emission spectrometry (FAES) is frequently used to analyze various alkaline elements. The effects of concentrated oxy-organics in water solution on the radiation intensity of alkaline elements FAES were studied. The species and concentration of oxy-organics may affect the radiation intensity variation amplitude of alkaline elements and also the tendency of the mutation can be different dramatically. Similarly, the emission intensity of alkaline elements can be affected by isomers or variation of the quantity of functional group, and the trend is different either.