20 research outputs found
On the Ground State of Quantum Gravity
In order to gain insight into the possible Ground State of Quantized
Einstein's Gravity, we have devised a variational calculation of the energy of
the quantum gravitational field in an open space, as measured by an asymptotic
observer living in an asymptotically flat space-time. We find that for Quantum
Gravity (QG) it is energetically favourable to perform its quantum fluctuations
not upon flat space-time but around a ``gas'' of wormholes, whose size is the
Planck length (cm). As a result, assuming such
configuration to be a good approximation to the true Ground State of Quantum
Gravity, space-time, the arena of physical reality, turns out to be well
described by Wheeler's Quantum Foam and adequately modeled by a space-time
lattice with lattice constant , the Planck lattice.Comment: 10 pages, LaTex, the version to appear in Physics Letters
Electron-phonon interaction in the solid form of the smallest fullerene C
The electron-phonon coupling of a theoretically devised carbon phase made by
assembling the smallest fullerenes C is calculated from first
principles. The structure consists of C cages in an {\it fcc} lattice
interlinked by two bridging carbon atoms in the interstitial tetrahedral sites
({\it fcc}-C). The crystal is insulating but can be made metallic by
doping with interstitial alkali atoms. In the compound NaC the
calculated coupling constant is 0.28 eV, a value much larger
than in C, as expected from the larger curvature of C. On the
basis of the McMillan's formula, the calculated =1.12 and a
assumed in the range 0.3-0.1 a superconducting T in the range 15-55 K is
predicted.Comment: 7 page
Stability of C20 fullerene chains
The stability of (C20)N chains with N = 3 - 7 is analyzed by numerical
simulation using a tight-binding potential and molecular dynamics. Various
channels of losing the cluster-chain structure of the (C20)N complexes are
observed, including the decay of C20 clusters, their coalescence, and the
separation of one C20 fullerene from the chain.Comment: To appear in JETP Letter
Structure and Stability of Two-Dimensional Complexes of C_20 Fullerenes
Two-dimensional complexes of C_20 fullerenes connected to each other by
covalent bonds have been studied. Several isomers with different types of
intercluster bonds have been revealed. The lifetimes of the (C_20)_MxM systems
with M = 2 and 3 have been directly calculated at T = 1800 - 3300 K making use
of molecular dynamics. It has been shown that these complexes lose their
periodic cluster structure due to either coalescence of two fullerenes C_20 or
decay of C_20 fullerenes. The activation energies of these processes exceed 2
eV.Comment: 17 pages, 5 figure
Decay and fusion as two different mechanisms of stability loss for the (C_20)_2 cluster dimer
The thermal stability of the (C_20)_2 cluster dimer consisting of two C_20
fullerenes is examined using a tight-binding approach. Molecular dynamics
simulations of the (C_20)_2 dimer at temperatures T = 2000 - 3500 K show that
the finite lifetime \tau of this metastable system is determined by two
fundamentally different processes, the decay of one of the C_20 fullerenes and
the fusion of two C_20 fullerenes into the C_40 cluster. The activation
energies for these processes Ea = 3.4 and 2.7 eV, respectively, as well as
their frequency factors, have been determined by analyzing the dependence of
\tau on T.Comment: Slightly modified version of the paper to appear in JETP Let
Anomalous Thermal Stability of Metastable C_20 Fullerene
The results of computer simulation of the dynamics of fullerene C_20 at
different temperatures are presented. It is shown that, although it is
metastable, this isomer is very stable with respect to the transition to a
lower energy configuration and retains its chemical structure under heating to
very high temperatures, T ~ 3000 K. Its decay activation energy is found to be
E_a ~ 7 eV. Possible decay channels are studied, and the height of the minimum
potential barrier to decay is determined to be U = 5.0 eV. The results obtained
make it possible to understand the reasons for the anomalous stability of
fullerene C_20 under normal conditions.Comment: Slightly corrected version of the paper submitted to Phys. Solid
Stat
Ab initio phonon dispersion relations of -Ga
We present the ab initio phonon dispersion relations of -Ga. The calculations are carried out within density functional perturbation theory by using either norm-conserving pseudopotential and 4s and 4p electrons in the valence or ultrasoft pseudopotential and 3d electrons in the valence as well. The inclusion of 3d electrons in the valence turned out to be necessary to better reproduce the experimental frequencies of the stretching modes of the Ga 2 dimers present in the -Ga structure. Copyright Springer-Verlag Berlin/Heidelberg 2003