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 ν≥1/5\nu \ge 1/5

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 $\nu$. Wigner crystal is found to be the ground state for filling factors between $\nu = 1/3$ and $\nu = 1/5$ provided electrons interact via the bare Coulomb potential. Even at $\nu =1/5$ 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 ($V_1$) and next near ($V_2$) neighbor repulsion. At half filling we find three phases: Superfluid (SF), checkerboard solid and striped solid depending on the relative values of $V_1$, $V_2$ 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 $V_2$ 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