Search Results (14 total)

In SOLEIL, 5 solid state amplifiers provide the required rf power at 352  MHz: 1×35  kW in the booster and 4×190  kW in the storage ring. They consist in a combination of a large number of 330 W elementary modules (1×147 in the booster and 4×724 in the storage ring), based on a design developed in-house, with MOSFETs (metal-oxide-semiconductor field-effect transistors), integrated circulators, and individual power supplies. Although quite innovative and challenging for the required power range, this technology is very attractive and presents significant advantages as compared to the more conventional vacuum tubes, klystrons, or inductive output tubes (IOTs). The booster and two of the storage ring power plants have been successfully commissioned and the first operational experience is quite satisfactory. The amplifiers proved to be very reliable as well as easy and flexible in operation; they have not been responsible for any beam time loss.

In this work we performed ab initio pseudopotential, density functional calculations of the structure and electronic properties of two hypothetical carbon structures with cubic symmetry: C₆ bcc, body centered with 12 atoms per cubic unit cell, and C₂₀ sc, simple cubic with 20 atoms per unit cell. The former is semiconducting with an indirect gap of 2.5 eV—within the local density approximation to the exchange and correlation energy functional—while the latter is metallic. Both have similar zero-pressure densities of about one atom per 6.9 Å³, which is intermediate between graphite and diamond, and similar bulk moduli of about 350 GPa at ambient conditions. Both are metastable with respect to graphite and diamond, and no phase transition is expected in the range of pressures studied.

In this paper we report on ab initio pseudopotential density-functional calculations of some possible high-pressure phases of carbon. The total energies of several hybrid diamond-graphite structures were calculated as a function of volume using density-functional theory and the local density approximation. The lowest calculated transition pressures between hexagonal-graphite and hybrid structures were 17 and 20 GPa, which compare well with the experimental value of 14 GPa for the transition at low temperatures between graphite and a still unidentified hard transparent phase. The electronic densities of states for the different structures are presented. Also, the x-ray powder diffraction patterns for a few structures were simulated and qualitatively compared to published experimental diffraction patterns.

In this paper, results of ab initio pseudopotential density-functional calculations of indium adsorption on graphitelike surfaces are presented. The adsorption energy was calculated as a function of In coverage, and it is shown that, for low surface densities, In becomes positively charged by donating about one electron to the underlying nanotube surface. This is consistent with experimental observations of bias-assisted In flux in the direction opposite to that of electron flow. The effects of nanotube surface curvature on In adsorption are shown to be small. Based on the calculated energy barrier between two neighboring energy minima and the calculated vibrational frequencies, the hopping rate for In adsorbed on graphene is estimated. It is also shown that In adsorption is stronger on and around a Stone-Wales defect on a graphene sheet, which suggests that defects can work as nucleation centers for crystal growth. Adsorption of In on BN sheets and of Au on graphene is also discussed. No significant charge transfer is present in these two alternative systems and the adsorption energies are weaker.

As in the case of carbon nanotubes, also boron nitride nanotubes may host arrays of C₆₀ molecules and form a nanopeapod (NPP). The observed separation between C₆₀ molecules in BN NPP’s is consistently shorter than in carbon NPP’s, which influences their electronic properties. Here we report on total-energy pseudopotential density functional theory (DFT) calculations for polymerized and nonpolymerized C₆₀ chains, and optimize their atomic structures to provide a description of their energetic landscape. A fully polymerized C₆₀ chain and a C₆₀ dimer are found to be more stable than nonpolymerized C₆₀, respectively, by 0.89 and 0.38 eV∕C₆₀. The geometry and energetics of an encapsulated C₆₀ chain is not significantly different with respect to the isolated molecule. Encapsulation energies in BN and carbon NPP’s are, respectively, 1.56 and 1.67 eV∕C₆₀, which are significantly larger than the calculated activation energy for C₆₀ polymerization, supporting the hypothesis that encapsulated C₆₀’s in NPP’s are partially polymerized. Band structure analysis show that polymerization does not affect the gap width of the C₆₀ chain. BN NPP’s are semiconductors with a gap width determined by the C₆₀. The lowest unoccupied C₆₀ states lie just above the Fermi level in metallic carbon NPP’s and charge transfert could take place, affecting the C₆₀ geometry.

In this work the superconducting transition temperature of hole-doped BC₃ was studied. The total energy, phonon frequencies, and electron-phonon couplings were calculated for different hole doping levels using the ab initio pseudopotential method within the local density approximation. The harmonic and anharmonic phonon frequencies were calculated by using the frozen-phonon approximation. As in MgB₂, the electron-phonon coupling between the electronic states in the σ bands and phonon modes associated with bond stretching was found to be very strong. The calculation predicts that the superconducting temperature will increase as a function of doping level.

Different stacking arrangements of BC₃ layered crystals are studied with the use of the ab initio pseudopotential density-functional method. The total energies, lattice constants, electron energy band structures and density of states, as well as phonon frequencies are calculated for the possible bulk BC₃ structures obtained by full relaxations starting from different initial atomic configurations of ABAB (or ABCABC)⋅⋅⋅ layer stacking. Two stable BC₃ structures, one semiconductor and the other metal, are obtained, which have lower total energies comparing with those of the structures proposed previously. Our calculations show that except for these two BC₃ structures, all the structures we studied, including the BC₃ structures proposed previously, have imaginary phonon frequencies corresponding to the relative, parallel motion of the adjacent BC₃ layers, indicating the instability of the layer stacking in these structures.

Ab initio pseudopotential total-energy calculations on infinite monatomic chains of Au, Al, Ag, Pd, Rh, and Ru were performed within the local-density approximation. We used the frozen phonon approximation to study the stability of these chains as a function of strain. Within a window of strains the Au, Al, Ag, Pd, and Rh linear chains are stable with respect to q=π/a deformations. For large strains all the chains dimerize. All the chains exibit at least one zero-strain zigzag stable equilibrium configuration, and Au, Al, and Rh zigzag chains exibit two. The ideal strengths of the different chains were calculated. The stability of the chains is discussed in connection with the electronic structure.

Ab initio pseudopotential total energy calculations of MoSe nanowires were performed within the local density approximation. The Li₂Mo₆Se₆ crystal is composed of molecular chains, which can be separated from one another to form individual nanowires approximately 3 Å in diameter. In this study we consider three systems: the quasi-one-dimensional bulk crystal Li₂Mo₆Se₆, one isolated MoSe nanowire, and one isolated MoSe nanowire with Li adsorbates. The equilibrium structures and the electronic structures of the three systems were calculated and compared to each other. The calculated density of states of an isolated MoSe wire is compared with experimental tunneling spectroscopy measurements of the local density of states. The binding energy of a Li atom to an isolated wire was calculated and the effects of Li adsorption are discussed. In addition, the calculated value for the ideal tensile strength of a single MoSe nanowire is presented and compared with estimated values for carbon nanotubes.

We show that for specially designed linear dispersive media with one absorption line and one gain line the Sommerfeld precursors of a pulse can be amplified leading to an earlier detection of the signal. Also, we show that in some systems with one strong absorption line, a carefully placed gain resonance must induce a discontinuity in the imaginary part of the frequency dependent index of refraction and in the first derivative of its real part.

Ab initio pseudopotential total energy density-functional theory–local-density approximation calculations were performed to study the crystalline structures of Ge under pressure. Following the well established sequence of structural phases (diamond→β-Sn→Imma→sh) under increasing pressure, we predict a transition into a new phase, with Cmca space-group symmetry, at 90±2 GPa. We estimate the superconducting transition temperature T_c for this phase to be in the range 2 to 7 K, the same range obtained previously by detailed calculations for the Ge-sh phase. The Cmca phase should remain stable up to 137±10 GPa where a transition to the hcp structure is predicted to occur. The same path is followed by Si although at lower pressures.

A systematic study of the longitudinal effective mass associated with either shallow impurity states or conduction Landau levels under in-plane magnetic fields is presented for Fibonacci superlattices. We analyze the relation between the mean longitudinal effective masses and the localization length of the corresponding electronic or impurity states. Under appropriate conditions we show that the mean longitudinal effective mass weakly depends on the nature of the electron state and localization length, and is essentially related to the Cantor-like energy spectra or “band structure.”

A detailed study of the ground-state binding energies of donor states on semiconducting superlattices composed by different layer arrangements is presented. The effects of magnetic fields, applied parallel to the superlattice growth direction on the binding energies and on energies associated to transition from electron-conduction subband and acceptor states, are analyzed and compared with available experimental data. The energies are obtained via variational calculations and adopting an effective one-dimensional potential describing both the impurity Coulomb and magnetic interactions. A discussion of the effects of the layers arrangement (periodic, quasiperiodic, or random) on the impurity binding energy is presented.

The donor binding energies and density of impurity states of a hydrogenic impurity in a quantum dot are presented within the effective-mass approximation following a variational procedure. The emphasis is placed on the dependence of the binding energy on the volume of the dot and on the impurity position. We show that the results for the donor binding energy in the quantum dot goes to the exact limits of a square-cross-sectional quantum-well wire and a quantum well when appropriate limits are considered. Comparing the donor binding energies for cubic and spherical quantum dots, we found that the values are very close provided the dots have similar volumes.