On the Groenewold-Van Hove problem for R^{2n}

We discuss the Groenewold-Van Hove problem for R^{2n}, and completely solve it when n = 1. We rigorously show that there exists an obstruction to quantizing the Poisson algebra of polynomials on R^{2n}, thereby filling a gap in Groenewold's original proof without introducing extra hypotheses. Moreover, when n = 1 we determine the largest Lie subalgebras of polynomials which can be unambiguously quantized, and explicitly construct all their possible quantizations.Comment: 15 pages, Latex. Error in the proof of Prop. 3 corrected; minor rewritin

Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots

The time evolution of optically excited carriers in semiconductor quantum wells and quantum dots is analyzed for their interaction with LO-phonons. Both the full two-time Green's function formalism and the one-time approximation provided by the generalized Kadanoff-Baym ansatz are considered, in order to compare their description of relaxation processes. It is shown that the two-time quantum kinetics leads to thermalization in all the examined cases, which is not the case for the one-time approach in the intermediate-coupling regime, even though it provides convergence to a steady state. The thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.

Raman, infrared and optical spectra of the spin-Peierls compound NaV_2O_5

We have measured polarized spectra of Raman scattering, infrared and optical transmission of NaV_2O_5 single crystals above the temperature of the spin-Peierls transition Tsp=35 K. Some of the far-infrared (FIR) phonon lines are strongly asymmetric, due to the spin-phonon interaction. In addition to the phonon lines, a broad band was observed in the c(aa)c Raman spectrum and in the E||a FIR transmission spectrum. A possible origin of these bands is discussed. The absorption band at 10000 cm-1 1.25 eV is attributed to vanadium d-d electronic transitions while the absorption edge above 3 eV is supposed to correspond to the onset of charge-transfer transitions.Comment: 7 figures, 8 page

Measurement of the Michel Parameter xi" in Polarized Muon Decay and Implications on Exotic Couplings of the Leptonic Weak Interaction

The Michel parameter xi" has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.Comment: 15 pages, 16 figures, 3 tables; submitted to Phys. Rev.

Potentially Diagnostic Electron Paramagnetic Resonance Spectra Elucidate the Underlying Mechanism of Mitochondrial Dysfunction in the Deoxyguanosine Kinase Deficient Rat Model of a Genetic Mitochondrial DNA Depletion Syndrome

A novel rat model for a well-characterized human mitochondrial disease, mitochondrial DNA depletion syndrome with associated deoxyguanosine kinase (DGUOK) deficiency, is described. The rat model recapitulates the pathologic and biochemical signatures of the human disease. The application of electron paramagnetic (spin) resonance (EPR) spectroscopy to the identification and characterization of respiratory chain abnormalities in the mitochondria from freshly frozen tissue of the mitochondrial disease model rat is introduced. EPR is shown to be a sensitive technique for detecting mitochondrial functional abnormalities in situ and, here, is particularly useful in characterizing the redox state changes and oxidative stress that can result from depressed expression and/or diminished specific activity of the distinct respiratory chain complexes. As EPR requires no sample preparation or non-physiological reagents, it provides information on the status of the mitochondrion as it was in the functioning state. On its own, this information is of use in identifying respiratory chain dysfunction; in conjunction with other techniques, the information from EPR shows how the respiratory chain is affected at the molecular level by the dysfunction. It is proposed that EPR has a role in mechanistic pathophysiological studies of mitochondrial disease and could be used to study the impact of new treatment modalities or as an additional diagnostic tool

Manifestation of quantum chaos on scattering techniques: application to low-energy and photo-electron diffraction intensities

Intensities of LEED and PED are analyzed from a statistical point of view. The probability distribution is compared with a Porter-Thomas law, characteristic of a chaotic quantum system. The agreement obtained is understood in terms of analogies between simple models and Berry's conjecture for a typical wavefunction of a chaotic system. The consequences of this behaviour on surface structural analysis are qualitatively discussed by looking at the behaviour of standard correlation factors.Comment: 5 pages, 4 postscript figures, Latex, APS, http://www.icmm.csic.es/Pandres/pedro.ht

Conductivity and hall-effect in highly resistive GaN layers

Highly resistive GaN layers grown by molecular beam epitaxy are characterized by temperature dependent conductivity and Hall effect measurements. Samples with rho(300) congruent to 3 x 10(3) Ohm cm show room temperature Hall mobility of 22 and 35 cm(2) V-1 s(-1) and have a temperature dependence mu(H) similar to T-x with x = 0.9 and 0.5. This is in contradiction to a sample with rho(300) congruent to 32 Ohm cm which has a room temperature mobility of 310 cm(2) V-1 s(-1) and a mu(H) similar to T-x with x = -1.4. The same activation energy of 0.23 eV, attributed to donor-like defects, is found for all three samples investigated. Temperature dependent conductivity data can be reasonably fitted considering band conduction. The presence of various hopping mechanisms is discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)02825-4]

Clan structure analysis and new physics signals in pp collisions at LHC

The study of possible new physics signals in global event properties in pp collisions in full phase space and in rapidity intervals accessible at LHC is presented. The main characteristic is the presence of an elbow structure in final charged particle MD's in addition to the shoulder observed at lower c.m. energies.Comment: 9 pages, talk given at Focus on Multiplicity (Bari, Italy, June 2004

The fractality of the relaxation modes in deterministic reaction-diffusion systems

In chaotic reaction-diffusion systems with two degrees of freedom, the modes governing the exponential relaxation to the thermodynamic equilibrium present a fractal structure which can be characterized by a Hausdorff dimension. For long wavelength modes, this dimension is related to the Lyapunov exponent and to a reactive diffusion coefficient. This relationship is tested numerically on a reactive multibaker model and on a two-dimensional periodic reactive Lorentz gas. The agreement with the theory is excellent