Colin A. Bates

Bio: Colin A. Bates is an academic researcher from University of Nottingham. The author has contributed to research in topics: Vibronic coupling & Jahn–Teller effect. The author has an hindex of 10, co-authored 46 publications receiving 331 citations.

Vibronic interactions in the visible and near-infrared spectra of C60− anions

Abstract: Electron-phonon coupling is an important factor in understanding many properties of the C60 fullerides. However, there has been little success in quantifying the strength of the vibronic coupling in C60 ions, with considerable disagreement between experimental and theoretical results. We will show that neglect of quadratic coupling in previous models for C60- ions results in a significant overestimate of the linear coupling constants. Including quadratic coupling allows a coherent interpretation to be made of earlier experimental and theoretical results which at first sight are incompatible.

Calculation of images of oriented C_60 molecules using molecular orbital theory

Abstract: Using Huckel molecular-orbital theory, images are created to represent the electron distributions expected for a C60 molecule adsorbed on a substrate. Three different orientations of the C60 molecule on the substrate are considered. The effect of the interaction of the molecule with the substrate is treated purely from the basis of symmetry using group theoretical methods. The resulting electron distributions are then used to generate idealized images which represent how the molecule may appear when observed in a scanning tunneling microscope (STM) experiment. Comparison is made with STM images appearing in the literature. It is found that the more complicated ab initio methods usually employed to simulate STM images are not required in order to match observed results. Furthermore, we find that an unequivocal identification of the orbitals responsible for a given STM image cannot be made from analysis of the STM image alone.

Jahn-Teller effects in the fullerene anion C 60 2 −

Abstract: The motion of the electrons in a fullerene molecule is strongly influenced by Jahn-Teller coupling to vibrations of the ${\mathrm{C}}_{60}$ molecular cage. Furthermore, cooperative vibronic coupling between molecules can result in different macroscopic phases. We investigate the linear Jahn-Teller (JT) effects applicable to isolated fullerene anions ${\mathrm{C}}_{60}^{2\ensuremath{-}}$ and ${\mathrm{C}}_{60}^{4\ensuremath{-}}.$ Analytical expressions for the energies of the quantum-mechanical states are derived as a function of the vibronic coupling constant. The results obtained will help the understanding of some of the wide range of structural, orientational, magnetic, and superconductive phase transitions seen in fullerene-based solids.

Visualization of static Jahn-Teller effects in the fullerene anion C 60 −

Abstract: Adsorption of ${\text{C}}_{60}$ on a metallic surface may be expected to result in electronic charge transfer to the organic adsorbant, even if a buffer layer is included between the ${\text{C}}_{60}$ molecules and substrate. Subsequently, intramolecular coupling between the molecule's electronic and vibrational degrees of freedom would be expected to result in species subject to Jahn-Teller (JT) distortions. In this work, we use H\"uckel molecular-orbital theory to visualize the effect that JT interactions may have on ${\text{C}}_{60}^{\ensuremath{-}}$ ions imaged using scanning tunneling microscopy (STM). Several distortion symmetries and adsorption orientations are considered and the resulting simulations are compared to STM images in the literature.

Optical spectra of transition-metal ions in corundum

Abstract: The polarized optical spectra of the ions Ti3+, V3+, Cr3+, Mn3+, Co3+, and Ni3+ in corundum single crystals have been studied at temperatures from 4.2° to 1200°K. A theory of the band strength based on the point‐charge model and p‐d mixing has been developed and applied to the data with results in fair agreement with experiment. The effects of temperature show that the vibrational‐electronic contribution to band strength is quite small at low temperature but may be appreciable at high temperatures. The crystal‐field parameters have been calculated as convergent lattice sums. The observed trigonal‐field parameter has the opposite sign from that calculated by the point‐charge model if the impurity ion is assumed to occupy an Al3+ ion position in the lattice, but has the same sign as calculated for an ion 0.1 A displaced along the c3 axis toward the empty octahedral site. Details of the spectra have been interpreted as showing that the surroundings of an ion are distorted in some electronic states.

Possible light-induced superconductivity in K3C60 at high temperature.

Abstract: The use of light to control the properties of condensed-matter materials is a promising area of research, with the long-term prospect that it might lead to the development of quantum devices driven by light. In particular, it was shown recently that nonlinear excitation of certain phonons in bilayer copper oxides induces superconducting-like optical properties at temperatures far above the material's superconducting transition temperature (Tc). This effect was accompanied by the disruption of competing charge-density-wave correlations, explaining some, but not all, of the experimental results. Andrea Cavalleri and colleagues now report that by exciting metallic K3C60 with mid-infrared optical pulses, they can induce a large increase in carrier mobility, accompanied by the opening of a gap in the optical conductivity. Strikingly, these same signatures are observed at equilibrium when K3C60 cools below its Tc of 20 K.

Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution

Abstract: Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha–Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochr...

An optically stimulated superconducting-like phase in K3C60 far above equilibrium Tc

Abstract: The control of non-equilibrium phenomena in complex solids is an important research frontier, encompassing new effects like light induced superconductivity. Here, we show that coherent optical excitation of molecular vibrations in the organic conductor K3C60 can induce a non-equilibrium state with the optical properties of a superconductor. A transient gap in the real part of the optical conductivity and a low-frequency divergence of the imaginary part are measured for base temperatures far above equilibrium Tc=20 K. These findings underscore the role of coherent light fields in inducing emergent order.