Photoemission spectra of C60-: Electron-phonon coupling, Jahn-Teller effect, and superconductivity in the fullerides

Electron-vibrational renormalization in fullerenes through ab initio and machine learning methods

The effect of nuclear vibrations on the electronic eigenvalues and the HOMO-LUMO gap is known for several kinds of carbon-based materials, like diamond, diamondoids, carbon nanoclusters, carbon nanotubes and others, like hydrogen-terminated oligoynes and polyyne. However, it has not been widely analysed in another remarkable kind which presents both theoretical and technological interest: fullerenes. In this article we present the study of the HOMO, LUMO and gap renormalizations due to zero-point motion of a relatively large number (163) of fullerenes and fullerene derivatives. We have calculated this renormalization using density-functional theory with the frozen-phonon method, finding that it is non-negligible (above 0.1 eV) for systems with relevant technological applications in photovoltaics and that the strength of the renormalization increases with the size of the gap. In addition, we have applied machine learning methods for classification and regression of the renormalizations, finding that they can be approximately predicted using the output of a computationally cheap ground state calculation. Our conclusions are supported by recent research in other systems.

Few-femtosecond electronic and structural rearrangements of CH4+ driven by the Jahn-Teller effect

The Jahn-Teller effect (JTE) is central to the understanding of the physical and chemical properties of a broad variety of molecules and materials. Whereas the manifestations of the JTE in stationary properties of matter are relatively well studied, the study of JTE-induced dynamics is still in its infancy, largely owing to its ultrafast and non-adiabatic nature. For example, the time scales reported for the distortion of CH 4 + from the initial T d geometry to a nominal C 2 v relaxed structure range from 1.85 fs over 10 ± 2 fs to 20 ± 7 fs. Here, by combining element-specific attosecond transient-absorption spectroscopy and quantum-dynamics simulations, we show that the initial electronic relaxation occurs within 5 fs and that the subsequent nuclear dynamics are dominated by the Q2 scissoring and Q1 symmetric stretching modes, which dephase in 41 ± 10 fs and 13 ± 3 fs, respectively. Significant structural relaxation is found to take place only along the e-symmetry Q2 mode. These results demonstrate that CH 4 + created by ionization of CH 4 is best thought of as a highly fluxional species that possesses a long-time-averaged vibrational distribution centered around a D 2 d structure. The methods demonstrated in our work provide guidelines for the understanding of Jahn-Teller driven non-adiabatic dynamics in other more complex systems.

Evidence for Band Renormalizations in Strong-Coupling Superconducting Alkali-Fulleride Films

There has been a long-standing debate about the mechanism of the unusual superconductivity in alkali-intercalated fullerides. In this Letter, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of superconducting K_{3}C_{60} thin films. We observe a dispersive energy band crossing the Fermi level with the occupied bandwidth of about 130 meV. The measured band structure shows prominent quasiparticle kinks and a replica band involving the Jahn-Teller active phonon modes, which reflects strong electron-phonon coupling in the system. The electron-phonon coupling constant is estimated to be about 1.2, which dominates the quasiparticle mass renormalization. Moreover, we observe an isotropic nodeless superconducting gap beyond the mean-field estimation (2Δ/k_{B}T_{c}≈5). Both the large electron-phonon coupling constant and large reduced superconducting gap suggest a strong-coupling superconductivity in K_{3}C_{60}, while the electronic correlation effect is suggested by the observation of a waterfall-like band dispersion and the small bandwidth compared with the effective Coulomb interaction. Our results not only directly visualize the crucial band structure but also provide important insights into the mechanism of the unusual superconductivity of fulleride compounds.

Caged-electron states and split-electron states in the endohedral alkali C60

The low-lying electronic states of neutral X@C60 (X = Li, Na, K, Rb) have been computed and analyzed by employing state-of-the-art high level many-electron methods. Apart from the common charge-separated states, well known to be present in endohedral fullerenes, one non-charge-separated state has been found in each of the investigated systems. In Li@C60 and Na@C60, the non-charge-separated state is a caged-electron state already discussed before for Li@C60. This indicates that the application of this low-lying state of Li@C60 discussed before is also applicable for Na@C60. In K@C60 and Rb@C60, the electronic radial distribution analysis shows that this hitherto unknown non-charge-separated state possesses a different nature from that of a caged-electron state.

Toward Efficient GW Calculations Using Numerical Atomic Orbitals: Benchmarking and Application to Molecular Dynamics Simulations

The use of atomic orbitals in Hedin's GW approximation provides, in principle, an inexpensive alternative to plane-wave basis sets, especially when modeling large molecules. However, benchmarking of the algorithms and basis sets is essential for a careful balance between cost and accuracy. In this paper, we present an implementation of the GW approximation using numerical atomic orbitals and a pseudopotential treatment of core electrons. The combination of a contour deformation technique with a one-shot extraction of quasiparticle energies provides an efficient scheme for many applications. The performance of the implementation with respect to the basis set convergence and the effect of the use of pseudopotentials has been tested for the 117 closed-shell molecules from the G2/97 test set and 24 larger acceptor molecules from another recently proposed test set. Moreover, to demonstrate the potential of our method, we compute the thermally averaged GW density of states of a large photochromic compound by sampling ab initio molecular dynamics trajectories at different temperatures. The computed thermal line widths indicate approximately twice as large electron-phonon couplings with GW than with standard DFT-GGA calculations. This is further confirmed using frozen-phonon calculations.

Exploring the effect of hole localization on the charge-phonon dynamics of hole doped delafossite

For weak or moderate doping, electrical measurement is not suitable for detecting changes in the charge localization inside a semiconductor. Here, to investigate the nature of charge-phonon coupling in the presence of gradually delocalized holes within a weak doping regime (~10¹⁶ cm^-3), we examine the temperature dependent Raman spectra (303-817 K) of prototype hole doped delafossite [Formula: see text] (x  =  0/0.03, y  =  0/0.01). For both [Formula: see text] and [Formula: see text] phonons, negative lineshape asymmetry and relative thermal hardening are distinctly observed upon [Formula: see text] and [Formula: see text] doping. Using Allen formalism, charge density of states at the Fermi level per spin and molecule, and charge delocalization associated to [Formula: see text] plane, are estimated to increase appreciably upon codoping compared to the [Formula: see text]-axis. We delineate the interdependence between charge-phonon coupling constant ([Formula: see text]) and anharmonic phonon lifetime ([Formula: see text]), and deduce that excitation of delocalized holes weakly coupled with phonons of larger [Formula: see text] is the governing feature of observed Fano asymmetry ([Formula: see text]) reversal.

Nonempirical Calculation of Superconducting Transition Temperatures in Light-Element Superconductors

Recent progress in the fully nonempirical calculation of the superconducting transition temperature (T_c ) is reviewed. Especially, this study focuses on three representative light-element high-T_c superconductors, i.e., elemental Li, sulfur hydrides, and alkali-doped fullerides. Here, it is discussed how crucial it is to develop the beyond Migdal-Eliashberg (ME) methods. For Li, a scheme of superconducting density functional theory for the plasmon mechanism is formulated and it is found that T_c is dramatically enhanced by considering the frequency dependence of the screened Coulomb interaction. For sulfur hydrides, it is essential to go beyond not only the static approximation for the screened Coulomb interaction, but also the constant density-of-states approximation for electrons, the harmonic approximation for phonons, and the Migdal approximation for the electron-phonon vertex, all of which have been employed in the standard ME calculation. It is also shown that the feedback effect in the self-consistent calculation of the self-energy and the zero point motion considerably affect the calculation of T_c . For alkali-doped fullerides, the interplay between electron-phonon coupling and electron correlations becomes more nontrivial. It has been demonstrated that the combination of density functional theory and dynamical mean field theory with the ab initio downfolding scheme for electron-phonon coupled systems works successfully. This study not only reproduces the experimental phase diagram but also obtains a unified view of the high-T_c superconductivity and the Mott-Hubbard transition in the fullerides. The results for these high-T_c superconductors will provide a firm ground for future materials design of new superconductors.

Exotic s-wave superconductivity in alkali-doped fullerides

Alkali-doped fullerides (A3C60 with A = K, Rb, Cs) show a surprising phase diagram, in which a high transition-temperature (Tc) s-wave superconducting state emerges next to a Mott insulating phase as a function of the lattice spacing. This is in contrast with the common belief that Mott physics and phonon-driven s-wave superconductivity are incompatible, raising a fundamental question on the mechanism of the high-Tc superconductivity. This article reviews recent ab initio calculations, which have succeeded in reproducing comprehensively the experimental phase diagram with high accuracy and elucidated an unusual cooperation between the electron-phonon coupling and the electron-electron interactions leading to Mott localization to realize an unconventional s-wave superconductivity in the alkali-doped fullerides. A driving force behind the exotic physics is unusual intramolecular interactions, characterized by the coexistence of a strongly repulsive Coulomb interaction and a small effectively negative exchange interaction. This is realized by a subtle energy balance between the coupling with the Jahn-Teller phonons and Hund's coupling within the C60 molecule. The unusual form of the interaction leads to a formation of pairs of up- and down-spin electrons on the molecules, which enables the s-wave pairing. The emergent superconductivity crucially relies on the presence of the Jahn-Teller phonons, but surprisingly benefits from the strong correlations because the correlations suppress the kinetic energy of the electrons and help the formation of the electron pairs, in agreement with previous model calculations. This confirms that the alkali-doped fullerides are a new type of unconventional superconductors, where the unusual synergy between the phonons and Coulomb interactions drives the high-Tc superconductivity.

Electron-Vibration Coupling in Molecular Materials: Assignment of Vibronic Modes from Photoelectron Momentum Mapping

Electron-phonon coupling is one of the most fundamental effects in condensed matter physics. We here demonstrate that photoelectron momentum mapping can reveal and visualize the coupling between specific vibrational modes and electronic excitations. When imaging molecular orbitals with high energy resolution, the intensity patterns of photoelectrons of the vibronic sidebands of molecular states show characteristic changes due to the distortion of the molecular frame in the vibronically excited state. By comparison to simulations, an assignment of specific vibronic modes is possible, thus providing unique information on the coupling between electronic and vibronic excitation.

High-resolution photoelectron imaging of cold C₆₀⁻ anions and accurate determination of the electron affinity of C₆₀

High-resolution photoelectron imaging and spectroscopy of cold C₆₀⁻ anions are reported using a newly built photoelectron imaging apparatus coupled with an electrospray ionization source and a temperature-controlled cryogenic ion trap. Vibrationally resolved photoelectron spectra are obtained for the detachment transition from the ground state of C₆₀⁻ to that of C60 at various detachment wavelengths from 354.84 nm to 461.35 nm. The electron affinity of C60 is accurately measured to be 2.6835 ± 0.0006 eV. Numerous unexpected vibrational excitations are observed in the photoelectron spectra due to the Jahn-Teller effect in C₆₀⁻ and Hertzberg-Teller vibronic coupling in both C₆₀⁻ and C60. Both the relative intensities of vibrational peaks and their photoelectron angular distributions provide evidence for the vibronic couplings. The observed p-wave-like behavior in the angular distribution of the 0₀⁰ transition suggests that the electron is detached from an s-type orbital.

Dynamical Jahn-Teller effect and antiferromagnetism in Cs3C60

The dynamical Jahn-Teller effect on fullerene sites in insulating Cs(3)C(60) is investigated fully ab initio. The vibronic excitations of rotational type are at ≥ 65 cm(-1), while the net kinetic contribution to the Jahn-Teller stabilization energy constitutes approximately 90 meV. This means that no localization of distortions by intermolecular interactions is possible in these fullerides; therefore, free rotations of deformations take place independently on each C(60). The latter destroy the orbital ordering and establish a conventional exchange interaction between S = 1/2 on fullerene sites. The corresponding exchange model is derived and predicts the Néel temperature for A15 Cs(3)C(60) close to experiment.

Effects of electron-vibration coupling in transport through single molecules

Using scanning tunneling spectroscopy, we study the transport of electrons through C(60) molecules on different metal surfaces. When electrons tunnel through a molecule, they may excite molecular vibrations. A fingerprint of these processes is a characteristic sub-structure in the differential conductance spectra of the molecular junction reflecting the onset of vibrational excitation. Although the intensity of these processes is generally weak, they become more important as the resonant character of the transport mechanism increases. The detection of single vibrational levels crucially depends on the energy level alignment and lifetimes of excited states. In the limit of large current densities, resonant electron-vibration coupling leads to an energy accumulation in the molecule, which eventually leads to its decomposition. With our experiments on C(60) we are able to depict a molecular scale picture of how electrons interact with the vibrational degrees of freedom of single molecules in different transport regimes. This understanding helps in the development of stable molecular devices, which may also carry a switchable functionality.

Near-infrared photoabsorption by C60 dianions in a storage ring

We present a detailed study of the electronic structure and the stability of C(60) dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least approximately 1.5 eV. For C(60)(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C(60)(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C(60) to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10,723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11,500 cm(-1) to 13,500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C(60)(-) may also be a transition to a cone state.

Diamagnetic currents in the neutral He atoms

The mechanism of the occurrence of intraatomic diamagnetic currents in the neutral He atoms with microscopic sizes is investigated. It is found that most of all electrons can form electron pairs originating from attractive Coulomb interactions between two electrons with opposite spins occupying the 1s atomic orbital in the neutral He atom at 298 K. Intraatomic diamagnetic currents in the neutral He atoms with microscopic sizes can be explained by such electron pairing. The transition temperature Tc(He),(1s) value at which intraatomic diamagnetic currents can disappear in each He atom is estimated. The Tc(He),(1s) values for the neutral He atoms with microscopic sizes are estimated to be much larger than the superconducting transition temperatures Tc,BCS values for the conventional superconductors with macroscopic sizes. This result can be understood from continuous energy levels of electronic states in conventional superconductivity with macroscopic sizes, and from discrete energy levels of electronic states in the neutral He atoms with microscopic sizes. The energy difference between the occupied and unoccupied orbitals decreases with an increase in material size and thus the second-order perturbation effect becomes more important with an increase in material size. Therefore, the mechanism of the occurrence of intraatomic diamagnetic current in the neutral He atoms suggested in this research would not be true for materials with large sizes. The dependence of electronic properties on temperature in the diamagnetic currents in the neutral He atoms with microscopic sizes is studied and compared with that in the conventional superconductivity with macroscopic sizes.

Lifetimes of C60(2-) and C70(2-) dianions in a storage ring

C60(2-) and C70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200+/-30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s.

Vibrational cooling in a cold ion trap: vibrationally resolved photoelectron spectroscopy of cold C60(-) anions

We demonstrate vibrational cooling of anions via collisions with a background gas in an ion trap attached to a cryogenically controlled cold head (10-400 K). Photoelectron spectra of vibrationally cold C60(-) anions, produced by electrospray ionization and cooled in the cold ion trap, have been obtained. Relative to spectra taken at room temperature, vibrational hot bands are completely eliminated, yielding well-resolved vibrational structures and a more accurate electron affinity for neutral C60. The electron affinity of C60 is measured to be 2.683+/-0.008 eV. The cold spectra reveal complicated vibrational structures for the transition to the C60 ground state due to the Jahn-Teller effect in the ground state of C60(-). Vibrational excitations in the two A(g) modes and eight H(g) modes are observed, providing ideal data to assess the vibronic couplings in C60(-).

Electron−Phonon Interactions and Intra- and Intermolecular Charge Mobility in the Monocations of Annulenes

Possible electron pairing in pi-conjugated positively charged annulenes such as (CH)(18) (18an) and (CH)(30) (30an) is studied and compared with that in the positively charged acenes. The total electron-phonon coupling constants in the monocations (l(HOMO)) for 18an and 30an are estimated. The E(2g) modes of 1611 and 1201 cm(-1) most strongly couple to the highest occupied molecular orbitals (HOMO) in 18an and 30an, respectively. The l(HOMO) values for annulenes are larger than those for acenes. The phase pattern difference between the HOMO of acenes localized on the edge part of carbon atoms and the delocalized HOMO of annulenes is the main reason for the calculated results. In view of the calculated results of the l(HOMO) values, intramolecular electron mobility (sigma(intra,HOMO)), and the reorganization energies (RE(HOMO)) in the positively charged molecules, the monocations of annulenes cannot easily become good conductors compared with the monocations of acenes, but the condition of the attractive electron-electron interactions is realized more easily in the monocations of annulenes than in the monocations of acenes. The hypothetical intramolecular supercurrent originating from both intramolecular and intermolecular vibrations in the monocations of annulenes and acenes in a case where the distance between two adjacent molecules is too large for the molecular crystal to become normal metallic state, is also discussed.

Femtosecond pump/probe photoelectron spectroscopy of isolated C60 negative ions

We have measured pump/probe photoelectron spectra of mass-selected, near room temperature C60- in the gas phase. The lifetime of the vibrationally excited B- (2Eg) state at a calculated energy of 1.26 eV was found to be tau = 2.2+/-0.2 ps. The dominant decay process corresponds to intramolecular radiationless transitions into ground state C60-. This is in contrast to C60 for which pumping at the absorption onset (1.95 eV) leads to predominantly intersystem crossing.

Electron-phonon interactions in the monocations of polyacetylenes

Electron-phonon interactions in the monocations of trans-polyacetylenes such as C2H4 (2tpa), C4H6 (2tpa), C6H8 (6tpa), and C8H10 (8tpa) are studied. The C-C stretching Ag modes around 1700 cm(-1) afford the largest electron-phonon coupling constants in the monocations of polyacetylenes. However, the C-C bending Ag modes around 1200 cm(-1) afford much smaller electron-phonon coupling constants than the C-C stretching Ag modes around 1700 cm(-1) in the monocations of polyacetylenes. The total electron-phonon coupling constants for the monocations (l HOMO) are estimated to be 0.357, 0.285, 0.281, and 0.279 eV for 2tpa, 4tpa, 6tpa, and 8tpa, respectively. The l HOMO values for polyacetylenes with C 2h geometry hardly change with an increase in molecular size while those for polyacenes with D 2h geometry significantly decrease with an increase in molecular size. The l HOMO values for polyacetylenes are larger than those for polyacenes. The calculated results are rationalized in terms of the phase patterns of the molecular orbitals in detail. The electron transfer in the positively charged polyacetylenes is also discussed. Intramolecular electron mobility (sigma(intra,monocation)) in the positively charged polyacetylenes is estimated to be smaller than those for the positively charged polyacenes. The reorganization energies for the positively charged polyacetylenes are estimated to be larger than those for the positively charged polyacenes. Thus, the larger overlap integrals between two neighboring molecules are needed for the positively charged polyacetylenes to become good conductor than those for positively charged polyacenes. On the other hand, the conditions under which the electron-electron interactions are attractive are more easily realized in the monocations of polyacetylenes than in the monocations of polyacenes. The quality as conducting materials would not significantly depend on the molecular size in the positively charged polyacetylenes, compared with that in the positively charged polyacenes. Multimode problem is also treated in order to investigate how consideration of multimode problem is closely related to the characteristics of the electron-phonon interactions.

Electron−Phonon Interactions and Jahn−Teller Effects in the Monocation of Corannulene

Electron-phonon interactions in the monocation of corannulene are studied by using the hybrid Hartree-Fock (HF)/density-functional-theory (DFT) method in the Gaussian 98 program package. The C-C stretching mode of 1498 cm(-1) most strongly couples to the e1 highest occupied molecular orbitals (HOMO) in corannulene. The total electron-phonon coupling constant for the monocation (l(HOMO)) of corannulene is estimated to be 0.165 eV. The l(HOMO) value for corannulene is much larger than those for coronene and acenes with similar numbers of carbon atoms. The delocalized electronic structures and the intermediate characteristics between the strong sigma-orbital interactions and weak pi-orbital interactions originating from a bowl-shaped C(5v) geometry are the main reason that the l(HOMO) value for corannulene is much larger than those for planar D(6h) symmetric pi-conjugated coronene and D(2h) symmetric pi-conjugated acenes with similar numbers of carbon atoms. The electron transfer in the positively charged corannulene is also discussed. Intramolecular electron mobility (sigma(intra,monocation)) in the positively charged corannulene is estimated to be smaller than those for the positively charged pi-conjugated acenes and coronene. The reorganization energy for the positively charged corannulene (0.060 eV) is estimated to be larger than those for the positively charged acenes and coronene. The strong orbital interactions between two neighboring carbon atoms in the HOMO of corannulene with the bowl-shaped structure are the main reasons for the calculated results. Thus, the larger overlap integral between two neighboring molecules is needed for the positively charged corannulene to become a better conductor than those for positively charged coronene and acenes. The smaller density of states at the Fermi level n(0) values are enough for the conditions of the attractive electron-electron interactions to be realized in the monocation of corannulene than in the monocations of coronene and acenes with similar numbers of carbon atoms. The multimode problem is also treated in order to investigate how consideration of the multimode problem is closely related to the characteristics of the electron-phonon interactions.

Photoelectron Spectroscopy and Electronic Structures of Fullerene Oxides: C60Ox- (x = 1−3)

We report a photoelectron spectroscopy (PES) study on a series of fullerene oxides, C60Ox- (x = 1-3). The PES spectra reveal one isomer for C60O-, two isomers for C60O2, and multiple isomers for C60O3-. Compared to C60, the electronic structures of C60Ox are only slightly perturbed, resulting in similar anion photoelectron spectra. The electron affinity of C60Ox was observed to increase only marginally with the number of oxygen atoms, x, from 2.683 eV for C60, to 2.745 eV for C60O, and 2.785 eV/2.820 eV for C60O2 (two isomers). We also carried out theoretical calculations, which confirmed the observed isomers and showed that all the fullerene oxides are in the form of epoxide. The PES and theoretical calculations, as well as molecular orbital analysis, indicate that addition of oxygen atoms to the C60 cage only modifies the local carbon network and leave the rest of the fullerene cage largely intact geometrically and electronically.

Vibronic interactions and possible electron pairing in positively charged cyanodienes

The conditions under which the attractive electron-electron interactions are realized in the monocations of sigma-conjugated cyanodienes such as C(6)N(4)H(4), C(8)N(6)H(4), and C(10)N(8)H(4) and of pi-conjugated acenes are discussed. The total electron-phonon coupling constants for the monocations l(HOMO) of cyanodienes are much larger than those for the monocations of acenes. The strong sigma orbital interactions between two neighboring atoms in the highest occupied molecular orbitals (HOMO) of sigma-conjugated cyanodienes are the main reason for the calculated results. Furthermore, we discuss how the conditions under which the monocation crystals become good conductor are related to the molecular size. Both the l(HOMO) values and the reorganization energies between the neutral molecules and the monocations decrease with an increase in molecular size in cyanodienes. The calculated results for the sigma-conjugated cyanodienes are compared with those for the pi-conjugated acenes in order to investigate how the CH-N substitutions in cyanodienes are closely related to the l(HOMO) values and the reorganization energies. Both the l(HOMO) and the reorganization energies in the positively charged sigma-conjugated cyanodienes are much larger than those in the positively charged pi-conjugated acenes. This means that in order to become good conductors, the positively charged sigma-conjugated cyanodienes need larger overlap integral between two adjacent molecules than the positively charged pi-conjugated acenes. On the other hand, since the l(HOMO) values for cyanodienes are much larger than those for acenes, the condition of attractive electron-electron interactions is more easily to be realized in the monocations of cyanodienes than in the monocations of acenes. It is suggested that the positively charged sigma-conjugated cyanodienes cannot easily become good conductors, but the conditions under which the electron-electron interactions become attractive are realized more easily in the positively charged sigma-conjugated cyanodienes than in the positively charged pi-conjugated acenes.

Electron-phonon interactions in photoinduced excited electronic states in fluoroacenes

The electron-phonon coupling constants [l(B1u(HOMO-->LUMO))] in the photoinduced excited electronic states in fluoroacenes are estimated and compared with those in the monoanions (l(LUMO)) and cations (l(HOMO)). The l(B1u(HOMO-->LUMO)) values are much larger than the l(LUMO) and l(HOMO) values in fluoroacenes. Furthermore, the Coulomb pseudopotential mu* values for the excited electronic states are estimated to be smaller than those for the monoanions and cations. The complete phase patterns difference between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) is the main reason why the electron-phonon coupling constants and the mu* values are larger and smaller, respectively, in the photoinduced excited electronic states than in the monoanions and cations. The possible electron pairing and Bose-Einstein condensation in the excited electronic states of fluoroacenes are discussed. Because of larger electron-phonon coupling constants and smaller mu* values in the excited electronic states than in the charged states, the conditions under which the electron-electron interactions become attractive can be more easily realized, in principle, in the excited electronic states than in the charged states in fluoroacenes. The l(B1u(HOMO-->LUMO)) values hardly change by H-F substitution, even though the l(LUMO) and l(HOMO) values significantly increase by H-F substitution in acenes. Antibonding interactions between carbon and fluorine atoms in the HOMO and LUMO are the main reason why the l(B1u(HOMO-->LUMO)) values hardly change by H-F substitution in acenes.

Vibronic Interactions and Possible Electron Pairing in the Photoinduced Excited Electronic States in Molecular Systems: A Theoretical Study

Electron-phonon interactions in the photoinduced excited electronic states in molecular systems such as phenanthrene-edge-type hydrocarbons are discussed and compared with those in the monoanions and cations. The complete phase patterns difference between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) (the atomic orbitals between two neighboring carbon atoms combined in phase (out of phase) in the HOMO are combined out of phase (in phase) in the LUMO) are the main reason that the C-C stretching modes around 1500 cm(-1) afford much larger electron-phonon coupling constants in the excited electronic states than in the charged electronic states. The frequencies of the vibrational modes that play an essential role in the electron-phonon interactions for the excited electronic states are similar to those for the monoanions and cations in phenanthrene-edge-type hydrocarbons. Possible electron pairing and Bose-Einstein condensation in the photoinduced excited electronic states as well as those in the monoanions and cations in molecular systems such as phenanthrene-edge-type hydrocarbons are also discussed.

Vibrational spectroscopy of individual doping centers in a monolayer organic crystal

A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C60 molecules supported on a thin Al2O3 film grown on the NiAl(110) surface. Vibronic states of the doping centers are observed with differential conductance (dIdV) spectroscopy. The double-barrier nature of the junction results in bipolar transport: same states participate in charge transport at both bias voltage polarities. Identification of the dIdV features corresponding to bipolar conduction enables a new mode of vibrational spectroscopy with STM.

Vibronic Interactions in Negatively Charged Polyacetylene

Electron-phonon interactions in the monoanions of polyacetylenes such as C2H4 (2tpa), C4H6 (4tpa), C6H8 (6tpa), and C8H10 (8tpa) are studied and compared with those in the monoanions of polyacenes. The C-C stretching A(g) modes around 1500 cm(-1) the most strongly couple to the lowest unoccupied molecular orbitals (LUMO) in polyacetylenes. The estimated total electron-phonon coupling constants for the monoanions (l(LUMO)) are 0.579, 0.555, 0.463, and 0.401 eV for 2tpa, 4tpa, 6tpa, and 8tpa, respectively. The l(LUMO) values for polyacetylenes are much larger than those for polyacenes. Furthermore, the l(LUMO) value for polyacetylene with C(2h) geometry is estimated to be 0.254 eV, and is larger than that (0.024 eV) for polyacene with D(2h) geometry. The phase patterns difference between the LUMO of polyacenes localized on the edge part of carbon atoms, and the delocalized LUMO of polyacetylenes is the main reason for the calculated results. The single charge transfer through the molecule in polyacetylenes are also discussed. The reorganization energies between the neutral molecule and the corresponding monoanion are estimated to be 0.164, 0.144, 0.125, and 0.113 eV for 2tpa, 4tpa, 6tpa, and 8tpa, respectively. Such reorganization energy decreases with an increase in molecular size. The conditions under which the attractive electron-electron interactions are realized in the monoanions of polyacetylenes and polyacenes are discussed. In terms of the electron-phonon interactions and the reorganization energies, the relationships between the normal and possible superconducting states are briefly discussed. We find that the monoanions with smaller molecular size cannot easily become good conductors, however, the conditions under which the interactions between two electrons are attractive are more easily realized in the monoanions with smaller molecular size than in the monoanions with larger molecular size.

Dynamic Jahn-Teller effects in isolated C60- studied by near-infrared spectroscopy in a storage ring

We have measured the near-infrared absorption spectrum for isolated C60- ions at room temperature. Two bands, at 9145 cm(-1) and 10460 cm(-1), have been identified in addition to the main absorption band at 9382 cm(-1), seen also at low temperature in a matrix. An interpretation based on the theory of dynamic Jahn-Teller effects is proposed.

The effect of atomic substitution on electron–phonon interactions in negatively charged B, N-substituted acenes

Electron-phonon interactions in the monoanions of B, N-substituted acenes such as B(3)N(3)F(6) (1f) and B(5)N(5)F(8) (2f) are studied, and compared with those in the monoanions of B(3)N(3)H(6) (1h) and B(5)N(5)H(8) (2h), and B(3)N(3)D(6) (1d) and B(5)N(5)D(8) (2d). The low frequency modes around 500 cm(-1) as well as the frequency modes higher than 1000 cm(-1) strongly couple to the lowest unoccupied molecular orbitals (LUMO) in 1f and 2f. The total electron-phonon coupling constants (l(LUMO)) are estimated to be 2.710 and 2.054 eV for 1f and 2f, respectively, and those are estimated to be 0.342 and 0.235 eV for 1d and 2d, respectively, while those were estimated to be 0.340 and 0.237 eV for 1h and 2h, respectively. That is, the l(LUMO) value increases much more significantly by H-F substitution than by H-D substitution in B, N-substituted acenes. The larger displacements of B and N atoms in the vibronic active modes in 1f and 2f than those in 1d and 2d due to larger atomic mass of fluorine than that of deuterium, and the phase patterns difference between the LUMO in 1f and 2f, in which the atomic orbitals between N and its neighboring F atoms form strong sigma-antibonding interactions, and that in 1d and 2d, in which the atomic orbitals between two neighboring B and N atoms form weak pi-bonding and pi-antibonding interactions, are the main reason why the l(LUMO) value increases much more significantly by H-F substitution than by H-D substitution. The reorganization energies between the neutral molecules and the corresponding monoanions are estimated to be 0.122, 0.063, 0.733, and 0.830 eV for 1h, 2h, 1f, and 2f, respectively. Therefore, the estimated reorganization energies between the neutral molecules and the corresponding monoanions for 1f and 2f are much larger than those for 1h and 2h.

Jahn-Teller spectral fingerprint in molecular photoemission: c60

The h(u) hole spectral intensity for C60-->C+60 molecular photoemission is calculated at finite temperature by a parameter-free Lanczos diagonalization of the electron-vibration Hamiltonian, including the full 8 H(g), 6 G(g), and 2 A(g) mode couplings. The computed spectrum at 800 K is in striking agreement with gas-phase data. The energy separation of the first main shoulder from the main photoemission peak, 230 meV in C60, is shown to measure directly and rather generally the strength of the final-state Jahn-Teller coupling.

Band structure and Fermi surface of electron-doped C60 monolayers

C60 fullerides are challenging systems because both the electron-phonon and electron-electron interactions are large on the energy scale of the expected narrow band width. We report angle-resolved photoemission data on the band dispersion for an alkali-doped C60 monolayer and a detailed comparison with theory. Compared to the maximum bare theoretical band width of 170 meV, the observed 100-meV dispersion is within the range of renormalization by electron-phonon coupling. This dispersion is only a fraction of the integrated peak width, revealing the importance of many-body effects. Additionally, measurements on the Fermi surface indicate the robustness of the Luttinger theorem even for materials with strong interactions.