A comparative study of the superconductivity in the Holstein and optical Su-Schrieffer-Heeger models

Theoretical studies suggest that Su-Schrieffer-Heeger-like electron-phonon ($e$-ph) interactions can mediate high-temperature bipolaronic superconductivity that is robust against repulsive electron-electron interactions. Here we present a comparative analysis of the pairing and competing charge/bond correlations in the two-dimensional Holstein and optical Su-Schrieffer-Heeger (SSH) models using numerically exact determinant quantum Monte Carlo. We find that the SSH interactions support light bipolarons and strong superconducting correlations out to relatively large values of the $e$-ph coupling $\lambda$ and densities near half-filling, while the Holstein interaction does not due to the formation of heavy bipolarons and competing charge-density-wave order. We further find that the Holstein and SSH models have comparable pairing correlations in the weak coupling limit for carrier concentrations $\langle n \rangle \ll 1$, where competing orders and polaronic effects are absent. These results support the proposal that SSH (bi)polarons can support superconductivity to larger values of $\lambda$ in comparison to the Holstein polaron, but that the resulting $T_\mathrm{c}$ gains are small in the weak coupling limit. We also find that the SSH model's pairing correlations are suppressed after including a weak on-site Hubbard repulsion. These results have important implications for identifying and engineering bipolaronic superconductivity.Comment: 10 pages including appendice

Rotational spectra and molecular structures of organometallic and organic molecules

Understanding the nature of chemical bonds constitutes a major theme of this thesis. This thesis investigates the gas phase rotational spectra, electronic charge distributions and molecular structures of organometallic and organic molecules, using high-resolution pulsed beam Fourier transform microwave spectroscopy (PBFTMS) and computational methods. High-resolution rotational spectra and structural parameters were obtained for the following organometallic molecules in the singlet electronic state, including three symmetric and five asymmetric top complexes: C₅H₅Nb(CO)₄, CH₃Mn(CO)₅, MnRe(CO)₁₀, C₅H₅Mo(CO)₃H, C₅H₅W(CO)₃H, C₅H₅NiC₃H₅, C5H₄(CH₃)FeC₅H₅ and (C₅H₄(CH₃))₂Fe. High-resolution rotational spectra and structural parameters were obtained for three organic molecules in the singlet electronic state: ortho-benzyne (C₆H₄) and the keto-enol tautomers, 2-hydroxypyridine and 2-pyridone (C₅H₅NO). In addition to the tautomeric forms, pure rotational spectra of the H-bonded dimer, 2-hydroxypyridine:2-pyridinone, were also obtained. These detailed spectral investigations yielded novel and useful information about the molecular properties of these molecules. Primarily, these results provided information regarding chemical bonding, vibrational ground state structures, structural isomers, conformational behavior, metal-hydrogen bonding and electronic charge distributions. Density functional theory (DFT) and ab-initio calculations were carried out in conjunction with the experiments, providing additional insights into further understanding the equilibrium structures, structural isomers and the electric field gradient distributions for these molecules

π\pi -π\pi and CH-π\pi DISPERSION INTERACTIONS IN THE N2_2-C5_5H5_5N AND CH4_4-C5_5H5_5N DIMERS

Author Institution: Department of Chemistry, University of Alberta, Edmonton, T6G 2G2, CanadaWe will discuss the results of our high resolution spectroscopic studies of the dinitrogen-pyridine (N$_2$-C$_5$H$_5$N) and methane-pyridine (CH$_4$-C$_5$H$_5$N) dimers. The two dimers represent simple binary van der Waals systems that are good prototypes for studying the weak $\pi$-$\pi$ and CH-$\pi$ bonds involving polar and non-polar aromatic molcules. The pyridine molecule is of great interest because many of its derivatives are prevalently found in plants and microorganisms. Our preliminary results show that the dimers adopt T-shaped configurations. The N$_2$ lies perpendicularly to the aromatic plane with its center of mass closes to the $c$-inertial axis of the free pyridine molecule; similarly, the CH$_4$ molecule in CH$_4$-C$_5$H$_5$N sits above the aromatic plane. These are the only configurations presently observed for both dimers in our experiments. The talk will focus on the rotational spectra and structural properties of these dimers

THE ROTATION SPECTRA OF METHYL MANGANESE PENTACARBONYL COMPLEX.aCOMPLEX.^{a}

$^{a}$Supported by THE NATIONAL SCIENCE FOUNDATIONAuthor Institution: Department of Chemistry, University of ArizonaMethyl manganese pentacarbonyl is an important organometallics complex used for modeling migration/insertion reactions. We have obtained the microwave spectrum of methyl manganese pentacarbonyl complex over a range of 5-10 GHz using a Fourier-transformed microwave spectrometer. The observed hyperfine splittings is well resolved for the transitions in 7 GHz range. We are analyzing quadrupole and rotational constants to obtain information on the electronic structure and molecular structure

Structures of solvated metal ion clusters by infrared multiphoton dissociation (IRMPD) spectroscopy and ab initio calculations

Molecular complexation and microsolvation processes play a key role in the transport of metals in aqueous fluids. Understanding the structure of molecular metal ions in aqueous media has therefore become a topic of intense research, with important implications for the transport of metals by vapor and isotopic fractionation processes at gas/liquid interfaces. In order to probe metal speciation and, in particular, the solvation environment around larger ion clusters, we have begun a systematic survey of representative metal-perchlorate clusters [Mn(ClO4)2n-1]+(H2O)m, M=Mn, Ni, Cu, Co, Zn) using a combination of electrospray ionization (ESI), ion resonance mass spectrometry and tunable IR spectroscopy. Briefly, ion cluster experiments were conducted on a modified ESI FT-ICR mass spectrometer mated to a Nd:YAG pumped table-top OPO/POA laser system. The OPO/OPA produces 10-15 mJ/pulse IR radiation over the 2500-4500 cm-1 range and is coupled to a CW-CO2 IR laser that is employed to preheat more strongly bound ion clusters. Metal perchlorate clusters were generated by ESI of dilute (0.1-1mM) solutions of metal perchlorate salts, and IR spectra, in the OH-stretching range (3400-3750 cm-1), were recorded on mass-selected ion clusters of the type [Mn(ClO4)2n-1]+(H2O)m. For example, in ESI mass spectra of aqueous Mn(ClO4)2 we identified clusters of the general form [Mnn(ClO4)2n-1]+(H2O)m with n≤3 and m≤5. Upon mass isolation of Mn2(ClO4)3]+(H2O)3, we observed slow dissociation to more stable [Mn2(ClO4)3]+(H2O)2, primarily due to background black-body radiation, and a shift in the IR spectra of the dihydrate to vibrations of O-H bonds not involved in hydrogen bonding. Measured IRMPD spectra of [Mn2(ClO4)3]+(H2O)m have also been compared against those predicted using MP2 theory using cc-pVTZ basis sets for Mn, cc-pVTZ for O and H and cc-pV(T + d)Z for Cl. Trends in the measured OH-stretching bands in [Mn2(ClO4)3]+(H2O)2 are qualitatively consistent with theory, which predicts a global minimum in which each H2O molecule attaches to one Mn site in [Mn2(ClO4)3]+ and, a higher energy (20.5 kJ/mol) isomer in which both water molecules are bound to one Mn site of [Mn2(ClO4)3]+ and H-bond with perchlorate oxygens giving rise to red-shifted OH stretching vibrations