25 research outputs found
Vibrations and Berry Phases of Charged Buckminsterfullerene
A simple model of electron-vibron interactions in buckminsterfullerene ions
is solved semiclassically. Electronic degeneracies of C induce
dynamical Jahn-Teller distortions, which are unimodal for and
bimodal for . The quantization of motion along the Jahn-Teller
manifold leads to a symmetric-top rotator Hamiltonian. I find Molecular
Aharonov-Bohm effects where electronic Berry phases determine the vibrational
spectra, zero point fluctuations, and electrons' pair binding energies. The
latter are relevant to superconductivity in alkali-fullerenes.Comment: Latex 11 pages. IIT-00
Van Hove Singularities in disordered multichannel quantum wires and nanotubes
We present a theory for the van Hove singularity (VHS) in the tunneling
density of states (TDOS) of disordered multichannel quantum wires, in
particular multi-wall carbon nanotubes. We assume close-by gates which screen
off electron-electron interactions. Diagrammatic perturbation theory within a
non-crossing approximation yields analytical expressions governing the
disorder-induced broadening and shift of VHS's as new subbands are opened. This
problem is nontrivial because the (lowest-order) Born approximation breaks down
close to the VHS. Interestingly, compared to the bulk case, the boundary TDOS
shows drastically altered VHS, even in the clean limit.Comment: 4 pages, 2 figures, accepted with revisions in PR
Superconductivity in carbon nanotube ropes
We investigate the conditions in which superconductivity may develop in ropes
of carbon nanotubes. It is shown that the interaction among a large number of
metallic nanotubes favors the appearance of a metallic phase in the ropes,
intermediate between respective phases with spin-density-wave and
superconducting correlations. These arise in samples with about 100 metallic
nanotubes or more, where the long-range Coulomb interaction is very effectively
reduced and it may be overcome by the attractive interaction from the exchange
of optical phonons within each nanotube. We estimate that the probability for
the tunneling of Cooper pairs between neighboring nanotubes is much higher than
that for single electrons in a disordered rope. The effect of pair hopping is
therefore what establishes the intertube coherence, and the tunneling amplitude
of the Cooper pairs dictates the scale of the transition to the superconducting
state.Comment: 12 page
Renormalization Group Approach to the Coulomb Pseudopotential for C_{60}
A numerical renormalization group technique recently developed by one of us
is used to analyse the Coulomb pseudopotential () in
for a variety of bare potentials. We find a large reduction in due to
intraball screening alone, leading to an interesting non-monotonic dependence
of on the bare interaction strength.
We find that is positive for physically reasonable bare parameters,
but small enough to make the electron-phonon coupling a viable mechanism for
superconductivity in alkali-doped fullerides. We end with some open problems.Comment: 12 pages, latex, 7 figures available from [email protected]
Aharonov-Bohm spectral features and coherence lengths in carbon nanotubes
The electronic properties of carbon nanotubes are investigated in the
presence of disorder and a magnetic field parallel or perpendicular to the
nanotube axis. In the parallel field geometry, the -periodic
metal-insulator transition (MIT) induced in metallic or semiconducting
nanotubes is shown to be related to a chirality-dependent shifting of the
energy of the van Hove singularities (VHSs). The effect of disorder on this
magnetic field-related mechanism is considered with a discussion of mean free
paths, localization lengths and magnetic dephasing rate in the context of
recent experiments.Comment: 22 pages, 6 Postscript figures. submitted to Phys. Rev.
W=0 Pairing in Carbon Nanotubes away from Half Filling
We use the Hubbard Hamiltonian on the honeycomb lattice to represent the
valence bands of carbon single-wall nanotubes. A detailed symmetry
analysis shows that the model allows W=0 pairs which we define as two-body
singlet eigenstates of with vanishing on-site repulsion. By means of a
non-perturbative canonical transformation we calculate the effective
interaction between the electrons of a W=0 pair added to the interacting ground
state. We show that the dressed W=0 pair is a bound state for resonable
parameter values away from half filling. Exact diagonalization results for the
(1,1) nanotube confirm the expectations. For nanotubes of length ,
the binding energy of the pair depends strongly on the filling and decreases
towards a small but nonzero value as . We observe the existence
of an optimal doping when the number of electrons per C atom is in the range
1.21.3, and the binding energy is of the order of 0.1 1 meV.Comment: 16 pages, 6 figure
Observation and Assignment of Silent and Higher Order Vibrations in the Infrared Transmission of C60 Crystals
We report the measurement of infrared transmission of large C60 single
crystals. The spectra exhibit a very rich structure with over 180 vibrational
absorptions visible in the 100 - 4000 cm-1 range. Many silent modes are
observed to have become weakly IR-active. We also observe a large number of
higher order combination modes. The temperature (77K - 300K) and pressure (0 -
25KBar) dependencies of these modes were measured and are presented. Careful
analysis of the IR spectra in conjunction with Raman scattering data showing
second order modes and neutron scattering data, allow the selection of the 46
vibrational modes C60. We are able to fit *all* of the first and second order
data seen in the present IR spectra and the previously published Raman data
(~300 lines total), using these 46 modes and their group theory allowed second
order combinations.Comment: REVTEX v3.0 in LaTeX. 12 pages. 8 Figures by request. c60lon
An ARPES view on the high-Tc problem: phonons vs spin-fluctuations
We review the search for a mediator of high-Tc superconductivity focusing on
ARPES experiment. In case of HTSC cuprates, we summarize and discuss a
consistent view of electronic interactions that provides natural explanation of
both the origin of the pseudogap state and the mechanism for high temperature
superconductivity. Within this scenario, the spin-fluctuations play a decisive
role in formation of the fermionic excitation spectrum in the normal state and
are sufficient to explain the high transition temperatures to the
superconducting state while the pseudogap phenomenon is a consequence of a
Peierls-type intrinsic instability of electronic system to formation of an
incommensurate density wave. On the other hand, a similar analysis being
applied to the iron pnictides reveals especially strong electron-phonon
coupling that suggests important role of phonons for high-Tc superconductivity
in pnictides.Comment: A summary of the ARPES part of the Research Unit FOR538,
http://for538.wmi.badw.d