Phonons, electronic charge response and electron-phonon interaction in the high-temperature superconductors

We investigate in the framework of linear response theory the complete phonon dispersion, phonon induced electronic charge response, electron-phonon interaction and dielectric and infrared properties of the high-temperature superconductors (HTSC's). In particular the experimentally observed strong renormalization of the in-plane oxygen bond-stretching modes (OBSM) which appear upon doping in the HTSC's is discussed. It is shown that the characteristic softening, indicating a strong EPI, is most likely a generic effect of the CuO plane and is driven by a nonlocal coupling of the displaced ions to the localized charge-fluctuations (CF's) at the Cu and O ions. The different behaviour of the OBSM during the insulator-metal transition via the underdoped phase is calculated and from a comparison of these modes conclusions about the electronic state in the HTSC's are drawn. The underdoped state is modelled in terms of a charge response which is insulator-like at the Cu and is competing with a metallic charge response at the O-network in the CuO plane. For the non-cuprate HTSC Ba-Bi-O also a strong renormalization of the OBSM is predicted. C-axis polarized infrared and Raman-active modes of the HTSC's are calculated in terms of CF's and anisotropic dipole-fluctuations and the problem of a metallic character of the BiO planes is studied.Interlayer phonons and their accompanying charge response are investigated. Depending on the interlayer coupling calculations are performed from the static, adiabatic- to the non-adiabatic regime.It is shown that phonon-plasmon mixing and a strong long-ranged non-adiabatic EPI becomes evident within a certain region around the c-axis. Both the OBSM and the non-adiabatic coupled c-axis phonon-plasmon modes are found to be important for pairing in the HTSC's.Comment: 65 pages,20 figures. Extended version to appear in Physica Status Solidi (b) 2004; figure 20 has been corrected; references have been adde

Theoretical study on the electronic, structural, properties and reactivity of a series of mono-, di-, tri- and tetrachlorothiophenes as well as corresponding radical cation forms as monomers for conducting polymers

In this paper, electrical and structural properties of mono-, di-, tri- and tetrachlorothiophenes and their radical cations have been studied using the density functional theory and B3LYP method with 6-311++G** basis set. The effects of the number and position of the substituent of chlorine atoms on the properties of the thiophene ring for all chlorothiophenes and their radical cations have been studied. Vibrational frequencies, nuclear chemical shielding constants, spin-density distribution, size and direction of dipole moment vector, ionization potential, electric polarizabilities and NICS values of these compounds have been calculated as well. The analysis of these data showed that double bonds in 3-chlorothiophene are more delocalized and it is the best possible candidate monomer among all chlorothiophenes for the synthesis of corresponding conducting polymers with modified characteristics

The role of charge localisation in mass spectrometry.

Ab-Initio molecular orbital calculations have been used to investigate the degree of charge localisation associated with the formation of ground state molecular radical cations upon electron impact for some simple amides, thio-amides, urea, thiourea, and their N-methyl substituted analogues, and guanidine. Some of the thio-amides which were not commercially available, were synthesised for the mass spectrometric studies. Ionisation energies have been calculated from the molecular orbital energies, using Koopmans' Theorem, and related to the predicted site of charge location in the molecular radical cations. The molecular orbital data was also used to study the effect of N-methylation on ionisation energy. The results obtained were found to be in close agreement with results obtained from photoelectron spectroscopy and electron impact mass spectrometry. Mulliken population analysis was used to obtain total atomic charges from the molecular orbital calculations so that the charge distributions in the neutral molecules and the radical cations could be compared. The molecular orbital calculations predict the major change in charge distribution to be equally shared between each nitrogen atom of urea, upon ionisation, with very little charge located on the oxygen; however for thiourea the change in charge distribution is largely located on the sulphur atom with very little change in the charge on either of the nitrogen atoms. These results are in agreement with previous predictions based on observed experimental data. The amides show more delocalisation with the charge more evenly distributed between nitrogen and oxygen, although N-methylation causes the charge to be preferentially located on nitrogen. The thio amides show the charge preferentially located on sulphur throughout. Molecular orbital calculations have also been used to investigate the energetics of the major fragmentation reaction in some of these compounds in relation to the predicted site of charge location in both the ground state molecular radical cations, and the fragment ions. This study has enabled an attempt to be made at rationalising the observed electron impact mass spectra on the basis of the calculated change in charge distribution upon ionisation of the compounds studied

Modeling ionic liquids, diffusion on surfaces and catalysis: a graduate student looks high and low

A challenging area of computational chemistry is the accurate treatment of reactions taking place in solvents or on surfaces as in the fields of surface chemistry and heterogeneous catalysis. This thesis focuses on electronic structures studies of tetrazolium based ionic liquids, diffusion of Aluminium on the Si(100)-2x1 reconstructed surface, and the nitroaldol reaction. In addition a method for the interaction of the universal force field with the effective fragment potential method has been developed. Example calculations on small clusters of silica and water have been carrier out using this method

Solvation Structures and Dynamics of Small Molecules: Experimental and Computational Studies Using Carbonyl Vibrational Modes as Probe

Solutions are ubiquitous in both the global environment and the human body, and play a significant role in scientific research and industrial production. The structures and dynamics of solutions have been studied for centuries. However, conventional experimental methods, whose intrinsic measuring time is on the order of nanoseconds to microseconds, could not detect the fast dynamics taking place in the solution on the timescale of femto- and pico-second. In this dissertation, the ultrafast two-dimensional infrared (2DIR) spectroscopy was applied to characterize the structure and dynamics in three different types of solutions on the sub-picosecond timescale. Linear Fourier transform infrared spectroscopy (FTIR) and computational calculations were performed to confirm the interpretation of the microscopic framework. Firstly, the ion effect on water structural dynamic properties at high concentration was studied by looking into the solvation shell of acetate ion in D2O and in 6M NaCl solution. The FTIR lineshape of the carboxylate asymmetric mode and a dynamics component extracted from 2DIR is not affected by the highly concentrated salts, which is proved to be a particularity of the acetate ion by a comparative study on the azide ion. Ab initio molecular dynamics (AIMD) simulations confirmed the experimental observations and linked the observed vibrational phenomenon to the thermal rotation of acetate methyl group (—CH3). Secondly, ion solvation structure and dynamics in organic solvents were investigated. By AIMD and ab initio umbrella sampling (AIUS), the structure and dynamics of the lithium solvation shell in cyclic and in linear carbonates were compared. The intercalation of entire molecules into the first solvation shell of lithium is found in cyclic carbonate, which leads to a more rigid and more organized solvation structure in cyclic carbonate. Finally, a conductive neutral molecular mixture of acetic acid (HAc) and N- vi methylimidazole (C1Im) was characterized by both experimental and theoretical methods. The broadband peaks in FTIR was assigned as delocalized proton stretching mode in HAc-C1Im complexes. AIMD simulation, DFT calculation, and NMR measurements confirmed the existence of proton delocalization in the hydrogen bond between HAc and C1Im

Modeling Chemical Reactivity in Aqueous and Organic Systems: From Electronic Structure Methods to Force Field Development

Modeling reactivity in chemical systems has evolved dramatically in line with the capabilities of modern computing. Despite the advances in computational ability, the level in which one can model a system depends on a number of factors including the region of reactivity, size of the system, level of sophistication required in the molecular description, and so on. Electronic structure methods allow for a detailed description of the potential energy surface and inherently include all essential physics required for reactivity to occur, however these methods are limited by their computational expense. On the other hand, force fields allow for an atomistic description of the interactions and drastically reduce the simulation time, yet typical force fields are dependent on a fixed bond topology, and as such, cannot model bond cleavage and formation. This dissertation addresses modeling reactivity from electronic structure methods to force field development for reactive systems. The first section of the dissertation will focus on the hydrated HCl system. Accurately modeling covalent HCl, as well as ionization and subsequent proton shuttling, is essential in systems such as gas-liquid nucleation in the atmosphere, concentrated acid solutions, and HCl at the air-water interface. The amount of sampling required for gas-liquid nucleation pathways, or simulation time for large system sizes in the case of concentrated acid simulations necessitates an expedient description of the potential energy surface. To this end, a reactive force field has been developed. In order to determine the solvent environment factors required for an accurate force field description, ab initio molecular dynamics and metadynamics have been performed on HCl(H2O)n(n=1-22). These simulations will be discussed in chapter two, while the development and performance of a reactive force field based on the multi-state empirical bond formalism will be described in chapter three. The second section of the dissertation will focus on modeling reactivity with electronic structure methods for two organic systems. The systems range from determining the factors guiding the regioselectivity of silyloxyallyl cations by analyzing reaction profiles, SAPT energy decomposition, and molecular orbital analysis (chapter four), to the formation of an EDA complex and the corresponding charge transfer (chapter five)

Electronic structure of conducting organic polymers: insights from time-dependent density functional theory

Cataloged from PDF version of article.Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e. g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e. g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs. (C) 2014 John Wiley & Sons, Ltd

Charge Transport in DNA-Based Devices

Charge migration along DNA molecules has attracted scientific interest for over half a century. Reports on possible high rates of charge transfer between donor and acceptor through the DNA, obtained in the last decade from solution chemistry experiments on large numbers of molecules, triggered a series of direct electrical transport measurements through DNA single molecules, bundles and networks. These measurements are reviewed and presented here. From these experiments we conclude that electrical transport is feasible in short DNA molecules, in bundles and networks, but blocked in long single molecules that are attached to surfaces. The experimental background is complemented by an account of the theoretical/computational schemes that are applied to study the electronic and transport properties of DNA-based nanowires. Examples of selected applications are given, to show the capabilities and limits of current theoretical approaches to accurately describe the wires, interpret the transport measurements, and predict suitable strategies to enhance the conductivity of DNA nanostructures.Comment: A single pdf file of 52 pages, containing the text and 23 figures. Review about direct measurements of DNA conductivity and related theoretical studies. For higher-resolution figures contact the authors or retrieve the original publications cited in the caption