12 research outputs found
Clusters of interstitial carbon atoms near the graphite surface as a possible origin of dome-like features observed by STM
Formation of clusters of interstitial carbon atoms between the surface and
second atomic layers of graphite is demonstrated by means of molecular dynamics
simulations. It is shown that interstitial clusters result in the dome-like
surface features that may be associated with some of the hillocks observed by
STM on the irradiated graphite surface.Comment: 7 pages, 7 eps figures, submitted to Surface Scienc
The effect of interstitial clusters and vacancies on the STM image of graphite
Making use of the tight-binding Green's function technique, we have
calculated the STM images of graphite with surface and sub-surface defects,
while taking into account the relaxation of the lattice due to defects. We have
demonstrated that two different physical mechanisms may result in the formation
of hillocks in the STM images: buckling of the graphite surface due to
interstitials between the uppermost graphite layers and the enhancement of the
electron density of states close to the Fermi energy on the carbon atoms in the
vicinity of vacancies. Our results indicate that small hillocks may originate
both from the interstitial clusters and from the vacancies. By contrast,
however, large hillocks in excess of 10 \AA~ in diameter can be caused only by
interstitial clusters.Comment: Submitted to Surface Scienc
Wigner Crystalization in the Lowest Landau Level for
By means of exact diagonalization we study the low-energy states of seven
electrons in the lowest Landau level which are confined by a cylindric external
potential modelling the rest of a macroscopic system and thus controlling the
filling factor . Wigner crystal is found to be the ground state for
filling factors between and provided electrons
interact via the bare Coulomb potential. Even at the solid state has
lower energy than the Laughlin's one, although the two energies are rather
close. We also discuss the role of pseudopotential parameters in the lowest
Landau level and demonstrate that the earlier reported gapless state, appearing
when the short-range part of the interaction is suppressed, has nothing in
common with the Wigner crystalization in pure Coulomb case.Comment: 9 pages, LaTex, 8 figure
The one-dimensional Bose-Hubbard Model with nearest-neighbor interaction
We study the one-dimensional Bose-Hubbard model using the Density-Matrix
Renormalization Group (DMRG).For the cases of on-site interactions and
additional nearest-neighbor interactions the phase boundaries of the
Mott-insulators and charge density wave phases are determined. We find a direct
phase transition between the charge density wave phase and the superfluid
phase, and no supersolid or normal phases. In the presence of nearest-neighbor
interaction the charge density wave phase is completely surrounded by a region
in which the effective interactions in the superfluid phase are repulsive. It
is known from Luttinger liquid theory that a single impurity causes the system
to be insulating if the effective interactions are repulsive, and that an even
bigger region of the superfluid phase is driven into a Bose-glass phase by any
finite quenched disorder. We determine the boundaries of both regions in the
phase diagram. The ac-conductivity in the superfluid phase in the attractive
and the repulsive region is calculated, and a big superfluid stiffness is found
in the attractive as well as the repulsive region.Comment: 19 pages, 30 figure
Nature of the quantum phase transitions in the two-dimensional hardcore boson model
We use two Quantum Monte Carlo algorithms to map out the phase diagram of the
two-dimensional hardcore boson Hubbard model with near () and next near
() neighbor repulsion. At half filling we find three phases: Superfluid
(SF), checkerboard solid and striped solid depending on the relative values of
, and the kinetic energy. Doping away from half filling, the
checkerboard solid undergoes phase separation: The superfluid and solid phases
co-exist but not as a single thermodynamic phase. As a function of doping, the
transition from the checkerboard solid is therefore first order. In contrast,
doping the striped solid away from half filling instead produces a striped
supersolid phase: Co-existence of density order with superfluidity as a single
phase. One surprising result is that the entire line of transitions between the
SF and checkerboard solid phases at half filling appears to exhibit dynamical
O(3) symmetry restoration. The transitions appear to be in the same
universality class as the special Heisenberg point even though this symmetry is
explicitly broken by the interaction.Comment: 10 pages, 14 eps figures, include
RADIATION RESISTANCE THEORY OF PERSPECTIVE HIGH TEMPERATURE SUPERCONDUCTORS
Une méthode universelle de calcul de l'effet de radiation sur la température critique des intermétalliques ayant la structure A-15 est proposée.A universal technique of calculating the radiation effect on the A-15 intermetallic compounds critical temperature is proposed