Showing papers by "Libera! published in 1977"

Nonradiative transitions in color centers by the WKB approximation

Abstract: Nonradiative transitions occurring in color centers are studied by the WKB approximation; in particular the temperature dependence of the ${A}_{T}$ and ${A}_{X}$ emission-intensities in ${\mathrm{Tl}}^{+}$ centers is fairly well accounted for within a nonphenomenological model.

An operational analysis of the space-time structure

Abstract: We discuss the concepts related to space-time in a quantum-relativistic theory by means of the analysis of the physical procedures used to construct a new frame of reference starting from a pre-existent frame (transformation procedures). The physical objects which form a frame of reference are allowed to interact with the other physical objects and follow the laws of quantum physics. We suggest that there are conceptual limitations which do not permit the exact realization of a transformation of the Poincare group by means of physical procedures. We remark also that the operations performed in order to construct a frame of reference perturb the surrounding physical objects and are influenced by them. We propose some general theoretical schemes which take these facts into account and permit the separation of the geometrical effects of a transformation procedure from the physical ones. Finally we find the conditions which permit the construction of a Poincare-invariant theory of the usual kind by means of the introduction of some ideal concepts which have no direct operational meaning.

Principio di massimo forte per le frontiere di misura minima

Abstract: I risultati di questo lavoro generalizzano un principio di massimo forte (M. Miranda). La difficolta risiede nei punti singolari delle superficie minime.

Space-charge-limited currents in GaSe at different temperatures

Abstract: Space-charge-limited current measurements in GaSe crystals are made at different temperatures and in samples with various doping characteristics. The experimental results suggest that the trapping level structure is highly dependent on chemical purity of the samples. Single deep trap levels are prominent in the purest samples, while a continuous distribution of traps is present in the heavily doped ones. Moreover, the temperature dependence of space-charge-limited current measurements is important because it helps to identify the pertinent physical model. A very accurate determination of trap level energies is obtained from the temperature dependence over a wide range of the energy gap. Des mesures de courant limitee par la charge d'espace ont ete executees sur des cristaux de GaSe a temperatures differentes et sur echantillons avec differentes characteristiques d'impurete. Les resultats experimentaux indiquent que la structure des niveaux des pieges presents depend fortement de la purete chimique des echantillons. Dans les echantillons tres impures dominent les profonds niveaux de piege tandis que ceux tres impures presentent une distribution continuee de pieges. De plus, il est important d'etudier la dependance de la temperature de la courant limitee par la charge d'espace parce que cela permet de reconnaitre le modele physique plus pertinent. La dependance de temperature permet en effet de determiner precisement les niveaux d'energie des pieges presentes dans un grand intervalle de la bande d'energie prohibee.

Spin–orbit-allowed tunneling splitting in the T × εg Jahn-Teller problem splitting of the zero-phonon line

Abstract: An orbital triplet state T in a lattice of cubic symmetry (Oh), which by the Jahn-Teller effect is coupled to tetragonal vibrational modes (eg), is considered. The tunneling splitting in this T × eg, Jahn-Teller problem with spin-orbit interaction is computed in Ham's framework and by the WKB approximation in several cases. The results are compared to zero-phonon line splitting of V2+ impurities in MgO and KMgF3. On a considere un etat orbital triplement degenere T dans un reseau de symetrie cubique (Oh) couple aux modes tetragonaux par l'effet Jahn-Teller. On a evalue la separation d'energie par effet tunnel dans ce probleme Jahn-Teller T × eg en presence de l'interaction spin-orbite avec la theorie de Ham et avec l'approximation WKB. Les resultats sont compares avec les donnees experimentaux pour l'impurete V2+ concernant la structure de la transition a zero-phonon, dans MgO et KMgF3.

E2 strength distribution in 1s nuclei

Abstract: We study the quadrupole contribution to the photodisintegration cross-section of 1s nuclei (2H,3H,3He,4He) by means of isospin sum rules. The sum rules are derived, discussed and compared with the previously obtained ones and with the available experimental data.