C60 molecular configurations leading to ferromagnetic exchange interactions in TDAE*C60

The charge-transfer salt tetrabis(dimethylamino)ethylene-fullerene (C₆₀), or TDAE*C₆₀, is a rare exception among pure organic crystalline systems, because it shows a transition to a ferromagnetic (FM) state with fully saturated s=1/2 molecular spins at a respectable T _c=16 K. In spite of extensive experimental and theoretical work over the past ten years, the origin of the ferromagnetism in TDAE*C₆₀ has remained a mystery. To resolve this problem, we performed a comparative structural study of two different magnetic forms of TDAE*C₆₀ crystals, one of which is magnetic and the other is nonmagnetic at low temperatures, and fully correlated their structural properties (particularly, the intermolecular orientations) with their magnetic properties. We identified the relative orientation of C₆₀ molecules along the c axis as the primary variable that controls the ferromagnetic order parameter and showed that both FM and low-temperature spin-glass-like ordering are possible in this material, depending on the orientational state of the C₆₀ molecules. Thus, we resolved the apparent contradictions intrinsic to different macroscopic measurements and opened a path to a microscopic understanding of p-electron FM exchange interactions.     

Effect of exchange and Coulomb interactions on the π-electron assembly in polyacetylene

It is emphasized that electron loss experiments by Ritsko et al. on both polyacetylene and polydiacetylene demonstrate the existence of collective plasmon excitations of the π-electron assembly due to the long range of the Coulomb interaction. The effect of exchange is here argued to be of importance for the plasmon dispersion.     

Coulomb explosion patterns of fast C60 clusters in solids

The molecular dynamics method is used to simulate Coulomb explosion of fast C60 ion clusters in an Al target, taking into account dynamical screening of interionic interactions by the electron gas of the target. It is found that the wake forces in the medium are strong enough, depending on cluster speed, to stabilize the whole cluster against Coulomb explosion, or to compress the trailing part of the cluster, for prolonged times of penetration through the target. This is encouraging news for such cluster-ion beams applications where massive energy depositions in small volumes of targets are desired.     

STM observation of Coulomb staircase behavior through C60 clusters

Two samples of C60 clusters were prepared on highly oriented pyrolytic graphite (HOPG) substrate by thermal evaporation. One is ∼1 nm and the other is ∼15 nm in radius. The current flowing across C60 clusters was measured using scanning tunneling microscope in atmosphere at room temperature. Coulomb staircase behavior was successively observed in the current–voltage (I–V) characteristics. Interestingly, the capacitances between HOPG and C60 clusters, estimated from the I–V characteristics, were almost the same between these two samples. We discussed this result taking into account the relaxation and localization time of electrons injected into C60 clusters.     

Coulomb potentials in heavy-ion interactions

The usual point charge approximation for the Coulomb potential in heavy-ion interactions is compared with more realistic treatments. Elastic scattering and transfer reaction calculations appear to be insensitive to the form of the Coulomb potential used.     

Influence of inter-dot Coulomb repulsion and exchange interactions on conductance through double quantum dot

Conductance and other physical quantities are calculated in double quantum dots (DQD) connected in series in the limit of coherent tunnelling using a Green's function technique. The inter-dot Coulomb repulsion and the exchange interaction are studied by means of the Kotliar and Ruckenstein slave-boson mean-field approach. The crossover from the atomic to the molecular limit is analyzed in order to show how the conductance in the model depends on the competition between the level broadening (dot-lead coupling) and the dot-dot transmission. The double Kondo effect was found in the gate voltage characteristics of the conductance in the atomic limit. In the case, when each dot accommodates one electron, the Kondo resonant states are formed between dots and their adjacent leads and transport is dominated by hopping between these two resonances. In the molecular limit the conductance vanishes for sufficiently low gate voltages, which means the Kondo effect disappeared. For small dot-lead coupling the transport characteristics are very sensitive on the influence of the inter-dot Coulomb repulsion and the position of the local energy level. The resonance region is widened with increase of the inter-dot Coulomb interactions while the exchange interaction has opposite influence.     

13C Hyperfine interactions in CD3C60 and the distribution of unpaired spin on the C60 cage

¹³C hyperfine interactions obtained for CD₃C₆₀ are compared with previous data for MuC₆₀ and used to map the unpaired spin distribution on the C₆₀ surface of these radicals with the help of semi-empirical and density functional calculations. It is concluded that the unpaired spin is predominantly located on seven atoms near the point of attachment of the substituent.     

Coulomb deexcitation and quasi-resonant charge exchange revisited

The processes of Coulomb deexcitation and quasi-resonant charge exchange have been considered in the framework of the asymptotic theory of the non-adiabatic transitions. The approach used is scrutinized and improved in the light of recent critical statements. The unambiguous agreement between different calculations for energies ∈ > 0.5 eV proves the approximation used to be valid. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.