Search Results - (Author, Cooperation:Franceschetti)

1077-3118

AIP Digital Archive

Physics

We have calculated the pressure coefficients α of a few optical transitions in (001), (111), (110), and (201) GaP/InP ordered superlattices using ab initio methods. The equilibrium atomic geometries under hydrostatic pressure are obtained by direct minimization of the elastic enthalpy. We find that (i) the pressure coefficient of the lowest energy transition is uniformly high, due to the Γ1c character of the conduction-band minimum; (ii) the pressure coefficient of the transition to the second lowest conduction state at Γ¯ distinguishes the (111)-oriented (CuPt) superlattice (α=4.0 meV/kbar) from the remaining structures (α(approximately-equal-to)−2 meV/kbar). This is so because in CuPt we have L folding, while in the other structures we have X folding; (iii) the calculated pressures for the Γ→X crossover are 45, 43, 12, and 16 kbar for the (001), (111), (110), and (201) superlattices, respectively. These trends reflect the zero-pressure Γ1c–X1c energy separation and the Γ1c pressure coefficient of these structures. © 1994 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using a plane-wave pseudopotential method we investigate the electronic structure of free-standing and of AlAs-embedded GaAs quantum dots, wires, and films. We predict that (i) the confinement energy of the valence-band maximum (VBM) is larger in AlAs-embedded than in free-standing quantum structures, because of the zero-confinement character of the VBM wave function in the latter case; (ii) small GaAs quantum structures have an indirect band gap, whereas large GaAs quantum structures have a direct band gap; (iii) the conduction-band minimum of small free-standing quantum structures originates from the GaAs X1c valley, while it derives from the AlAs X1c state in AlAs-embedded quantum structures; (iv) the critical size for the direct/indirect crossover is larger in embedded quantum structures than in free-standing quantum structures. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

In a quantum dot made of an indirect gap material such as Si, the electron–hole Coulomb interaction alone can give rise to "dark" excitons even in the absence of exchange interaction. We present the predicted excitonic spectra for hydrogen-passivated Si dots and find very good agreement with the recent experiment of Wolkin, Jorne, Fauchet, Allan, and Delerue [Phys. Rev. Lett. 82, 197 (1999)]. The calculated splitting between dark and bright excitons, arising from Coulomb and exchange interactions, agrees very well with the optical data of Calcott, Nash, Canham, Kane, and Brumhead [J. Phys Condens. Matter 5, L91 (1993)]. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using atomistic pseudopotential wave functions we calculate the quasiparticle gap, the optical gap and the electron and hole addition energies of CdSe nanocrystals. We find that the quasiparticle gap and the addition energies depend strongly on the dielectric constant of the surrounding material, while the optical gap is rather insensitive to the environment. We provide scaling lows for these quantities as a function of the quantum dot size, and compare our results with recent scanning tunneling spectroscopy experiments. © 2000 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A single-band approach for semiconductor clusters which accounts for the nonparabolicity of the energy bands was recently used by Rama Krishna and Friesner [M.V. Rama Krishna and R.A. Friesner, Phys. Rev. Lett. 67, 629 (1991)]. We compare the results of this method (denoted here as single-band truncated-crystal, or SBTC, approximation) with a direct pseudopotential band-structure calculation for free-standing hydrogen-passivated GaAs quantum films, wires, and dots. The direct pseudopotential calculation, which includes coupling between all bands, shows that isolated GaAs quantum films, wires, and dots have an indirect band gap for thicknesses below 16, 28, and at least 30 A(ring) (8, 14, and at least 15 ML), respectively; beyond these critical dimensions the transition becomes direct. A comparison of the SBTC approximation with the direct pseudopotential calculation shows that (i) the confinement energy of the valence-band maximum is overestimated by the SBTC method, because the zero-confinement character of this state is neglected; (ii) the confinement energy of the Γ-derived conduction state (direct band gap) is slightly overestimated by the SBTC approximation, mainly because of the assumption of infinite potential barriers at the boundaries; (iii) the confinement energy of the X-derived conduction state (indirect band gap) is severely underestimated by the SBTC method; (iv) while the SBTC approximation predicts "quantum deconfinement'' (i.e., reduction of gap as size is reduced) for the direct gap of thin GaAs quantum wires, such effect is not present in the direct pseudopotential calculation. © 1996 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Exact impedance expressions are derived for thin layers of supported electrolyte between either identical redox electrodes or nonidentical electrodes of the parent metal type. While the symmetry of the former allows one to treat the system as two identical half-cells in series, the asymmetric concentration perturbation set up in the latter precludes such a separation. The impedance result obtained in the asymmetric case can be represented by an equivalent linear electrical circuit consisting of capacitances, resistances, an inductance, and several lengths of uniform transmission line, one of which exhibits an inductance per unit length. The general impedance result and the corresponding circuit reduce to the expected forms if the electrodes are identical or one electrode is kinetically reversible or ideally polarizable. Representative impedance results are displayed in the complex plane.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A discrete time correlated random walk in one dimension is investigated. Combinatorial arguments are used to calculate the exact probability distribution PN(L), the probability that the correlated random walker arrives at a distance L steps to the right of its starting point after exactly N steps. PN(L) is calculated using arbitrary initial conditions which permit the influence of end effects and boundary conditions to be calculated and several special cases are considered in detail. PN(L) with arbitrary initial conditions is calculated both with and without a bias for motion in one direction yielding a useful model for the combined diffusion and drift of charged particles undergoing a correlated random walk in an applied field. The relation of the correlated random walk to the Ising model is also discussed. © 1998 American Institute of Physics.

Electronic Resource

1460-9568

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Knockout Otx1 mice present a microcephalic phenotype mainly due to reduced deep neocortical layers and spontaneous recurrent seizures. We investigated the excitable properties of layer V pyramidal neurons in neocortical slices prepared from Otx1–/– mice and age-matched controls. The qualitative firing properties of the neurons of Otx1–/– mice were identical to those found in wild-type controls, but the proportion of intrinsically bursting (IB) neurons was significantly smaller. This is in line with the lack of the Otx1 gene contribution to the generation and differentiation of neurons destined for the deep neocortical layers, in which IB neurons are located selectively in wild-type rodents. The pyramidal neurons recorded in Otx1–/– mice responded to near-threshold electrical stimulation of the underlying white matter, with aberrant polysynaptic excitatory potentials often leading to late action potential generation. When the strength of the stimulus was increased, the great majority of the Otx1–/– neurons (78%) responded with a prominent biphasic inhibitory postsynaptic potential that was significantly larger than that observed in the wild-type mice, and was often followed by complex postinhibitory depolarizing events. Both late excitatory postsynaptic potentials and postinhibitory excitation were selectively suppressed by NMDA receptor antagonists, but not by AMPA antagonists. We conclude that the cortical abnormalities of Otx1–/– neocortex due to a selective loss of large projecting neurons lead to a complex rearrangement of local circuitry, which is characterized by an excess of N-methyl-d-aspartate-mediated polysynaptic excitation that is counteracted by GABA-mediated inhibition in only a limited range of stimulus intensity. Prominent postsynaptic inhibitory potentials may also act as a further pro-epileptogenic event by synchronizing abnormal excitatory potentials.

Electronic Resource

0021-3020

Romance Studies

REVIEWS

0021-3020

Romance Studies

REVIEWS

0021-3020

Romance Studies

REVIEWS

0001-9593

Linguistics and Literary Studies

History

MISCELLANEA

0001-9593

Linguistics and Literary Studies

History

0001-9593

Linguistics and Literary Studies

History

MISCELLANEA

0001-9593

Linguistics and Literary Studies

History

RECENSIONI

0001-9593

Linguistics and Literary Studies

History

RECENSIONI

0001-9593

Linguistics and Literary Studies

History

RECENSIONI

0001-9593

Linguistics and Literary Studies

History

RECENSIONI

0001-9593

Linguistics and Literary Studies

History

Miscellanea

0001-9593

Linguistics and Literary Studies

History

Miscellanea