Electron-vibron Coupling Research Articles

Memory effect in a molecular quantum dot with strong electron-vibron interaction

Polaron theory of tunneling through a molecular quantum dot (MQD) with strong electron-vibron interactions and attractive electron-electron correlations is developed. The dot is modeled as a d-fold-degenerate energy level weakly coupled to the leads. The effective attractive interaction between polarons in the dot results in a ``switching'' phenomenon in the current-voltage characteristics when $d>2,$ in agreement with the results for the phenomenological negative-$U$ model. The degenerate MQD with strong electron-vibron coupling has two stable current states in a certain interval of the bias voltage below some critical temperature.

A comparison of the chemisorption behaviour of PTCDA on different Ag surfaces

We have studied the adsorption of the molecular semiconductor 3,4,9,10-perylene-tetracarboxylic-acid-dianhydride (PTCDA) on a range of silver substrates, including Ag(1 0 0), Ag(1 1 0), Ag(1 1 1), and Ag(7 7 5). In all of these cases, special attention was placed upon the chemical bonding between the organic molecule and the metal surface, as this is of fundamental importance for the understanding of the organic–inorganic interface. We demonstrate that high resolution electron energy loss spectroscopy (HREELS) in the vibrational energy range is able to provide detailed insight into the intricacies of molecular chemisorption, including electron–vibron coupling effects. Specifically, by analogy with potassium-doped thin films of PTCDA we prove that the Ag(1 1 1) surface imparts metallicity to PTCDA monolayers.

Electron-vibron-breather interaction

We study the interaction of breathers in the context of a coupled electron-vibron lattice system. Starting with single-site excitations, it is demonstrated that constellations exist for which the coexistence of electronic and vibronic breathers is assured. The energy exchange between the vibrational and electronic subsystems and its impact on the breather formation are discussed in detail. The coupled electron-vibron dynamics shows a tendency toward energy redistribution into the vibronic degrees of freedom at the expense of the electronic energy content. Attention is paid to the relaxation dynamics in the energy exchange and we discuss the attainment of a steady regime for the coupled electron-vibron dynamics starting from a nonequilibrium state. It is demonstrated that the presence of breathers has a strong impact on the relaxation dynamics. Breathers can assist the relaxation process. With the help of a linear stability analysis, we show why the electronic subsystem acts as an energy donor while the vibron system serves as the energy acceptor. To this end we investigate the existence and stability of localized breathing eigenmodes capable of energy trapping. A frequency analysis reveals that strong exchange also occurs due to a temporal transition from single-frequency breathers to those oscillating with two frequencies and their temporal resonance interaction. Finally, the self-stabilized electron-vibron system relaxes to a combined electron-vibron breather. On increasing the electron-vibron coupling strength, only a vibronic phonobreather of large amplitude survives, whereas the electronic subsystem tends to energy equipartition.

Spin-Liquid Ground State in a Two-Dimensional Nonfrustrated Spin Model

We consider an exchange model describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on the square lattice. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit, and is known to show a spin-Peierls-like dimerization in one dimension. In two dimensions we calculate energy gaps, susceptibilities, and correlation functions with a Green's Function Monte Carlo. We find a finite spin gap and no evidence of any kind of order. We conclude that the ground state is, most likely, a spin liquid of resonating valence bonds.

Spin gap in low-dimensional Mott insulators with orbital degeneracy

We consider the exchanged Hamiltonian HST=−J∑〈rr′〉(2Sr⋅Sr′−12)(2Tr⋅Tr′−12), describing two isotropic spin-1/2 Heisenberg antiferromagnets coupled by a quartic term on equivalent bonds. The model is relevant for systems with orbital degeneracy and strong electron-vibron coupling in the large Hubbard repulsion limit. To investigate the ground state properties we use a Green’s Function Monte Carlo, calculating energy gaps and correlation functions, the latter through the forward walking technique. In one dimension we find that the ground state is a “crystal” of valence bond dimers. In two dimensions, the spin gap appears to remain finite in the thermodynamic limit, and, consistently, the staggered magnetization—signal of Néel long range order—seems to vanish. From the analysis of dimer–dimer correlation functions, however, we find no sign of a valance bond crystal. A spin liquid appears as a plausible scenario compatible with our findings.

The isotope effect in the doped-fullerene superconductors

Measurements [C.C. Chen and C.M. Lieber, Science 259, 655 (1993)] of the isotope effect in Rb 3C 60 superconductors exhibit a larger shift in the T c of a Rb 3( 12C 60) 0.5( 13C 60) 0.5 superconductor than expected on the basis of existing theories. It is proposed that the transfer of intermolecular H g vibrations (vibron) from one C 60 cluster to another in a single cluster type compound Rb 3( 13C 0.5 12C 0.5) 60, is different from that in the two cluster type compound, Rb 3( 12C 60) 0.5( 13C 60) 0.5. This difference leads to a reduction of the electron-vibron coupling constant for the latter superconductor. The T c s of mixed Rb 3( 12C 60) 1− c ( 13C 60) c superconductors are calculated for c < 0.5.

Charge transfer and electron-vibron coupling in dense solid hydrogen

Abstract We examine the role of charge transfer (CT) interactions in dense molecular hydrogen in relation to recently observed spectroscopic properties at megabar pressures. Specifically, we consider virtual H 2 + H 2 − states in which an electron is transferred to a neighbor. The Mulliken CT integral t admixes H 2 + H 2 − fluctuations of overlapping molecules. The amplitude of the charge fluctuations is the ionicity γ, which is found in H 2 dimers and lattices with nonoverlapping valence and conduction bands for t small compared to the CT excitation energy ω CT . Vibronic coupling within a dimer is found using a Herzberg-Teller expansion and gives explicit expressions for vibron shifts in Raman and IR spectra and for IR oscillator strength due to electron-vibron coupling. We estimate linear electron-vibron coupling constants and other parameters required to interpret the vibrational data at and above the 150-GPa transition of dense hydrogen within a CT model. We discuss important structural implications of equal IR and Raman shifts at the transition and relate anisotropic CT processes to the orientational state of the dense molecular solid.

Infrared intensity and charge transfer in hydrogen dimers

Direct evaluation of the dipole derivative μ′ for overlapping H 2 dimers is consistent with the vibron oscillator strength of ≈ 10 −5 at the 150 GPa transition of dense hydrogen. The angular anisotropy of μ′ follows closely the σ−σ ∗ overlap for charge transfer (CT) between closed-shell H 2 molecules. We relate electron-vibron coupling above 150 GPa to CT models for ion-radical solids and conjugated polymers.

Insulator-metal transition in solid hydrogen: Implication of electronic-structure calculations for recent experiments.

Electronic structure calculations for compressed molecular hydrogen are performed to provide more insight into the diversity of phenomena recently observed experimentally. We perform full-potential LAPW calculations and analyze them in terms of a molecular tight-binding model. We show that $\sigma _g$ and $\sigma _u$ bands overlap occurs at rather low pressure, while an insulating state persists at specific orientations up to 200 GPa, in accord with previous work, and is due to opening of hybridization gaps at the Fermi level. We also present calculations of electronic properties such as plasma frequencies and electron-vibron coupling.

Spectroscopic properties of the solvated electron in water, alcohols, amines, ethers and alkanes

The near-IR and visible absorption spectra of solvated electrons in water, alcohols, amines, ethers and alkanes were calculated in terms of the electron-vibron coupling and compared with experiment. The role of the H-bond is discussed.

Infrared absorption in hydrogen-bonded complexes and absorption of an excess electron in hydrogen-bonded solvents

A comparison was made between the theories of IR absorption in hydrogen-bonded complexes and near-IR and visible absorption of an excess electron in hydrogen-bonded solvents. The near-IR and visible absorption spectra of excess electrons in water, alcohols, amines and alkanes were calculated in terms of electron-vibron coupling. The role of the hydrogen bond on electron stabilization and solvation is discussed.

Ultrafast relaxation and modulation in the oxazine dye nile blue.

Time-delayed four-wave mixing, using incoherent laser pulses 5 ns long and 350 \AA{} wide, has been used to measure the optical phase relaxation of a large molecule (nile blue) in solution at 300 and 4.5 K. In the latter case an asymmetric response as well as a quantum beat (signaling a vibronic mode at 583 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) is observed. Its magnitude and shape gives its electron-vibron coupling factor S=0.79 together with the homogeneous relaxation time ${\mathit{T}}_{2}$=43.7 fs and the inhomogeneous relaxation time ${\mathit{T}}_{2}^{\mathrm{*}}$=13.3 fs (full width of 940 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Supporting evidence for the large value of S is presented.

Enhanced isotope effect in 13C-rich superconducting M xC 60 ( M= K, Rb): support for vibronic pairing

By means of non-resonant microwave absorption in low magnetic fields it is found that the superconducting T c significantly decreases in 60% 13C isotope substituted K 3C 60 and Rb 3C 60. The exponent α of the isotope effect, T crmc 1/m α, is estimated roughly to be α=1.2–2.25, much higher than predicted by the BCS theory. The intrinsic disorder is observed both in the wide-mass spectrum and the dramatically changed IR spectrum reflecting the inhomogeneous distribution of 13C isotopes in 13C 60− n 12 C 60 molecules. The large positive α supports the intramolecular vibronic pairing, and the decrease of the electron-vibron coupling λ due to the disturbed symmetry of H g modes in mixed 13C 60− n 12 C n molecules is proposed to contribute to the enhanced α, in addition to the conventional isotope frequency shift.