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We report on an exact diagonalization study of fractional quantum Hall states at filling factor $\nu=2/3$ in a system with a four-fold degenerate $n$=0 Landau level and SU(4) symmetric Coulomb interactions. Our investigation reveals previously unidentified SU(3) and SU(4) singlet ground states which appear at flux quantum shift 2 when a spherical geometry is employed, and lie outside the established composite-fermion or multicomponent Halperin state patterns. We evaluate the two-particle correlation functions of these states, and discuss quantum phase transitions in graphene between singlet states with different number of components as magnetic field strength is increased., Comment: 5+2 pages, 3 figures

Electron paramagnetic spectra of fast neutron irradiated GaAs, obtained in the 4.2-300 K temperature range, have been analysed in terms of a quadruplet, whose parameters are constrained to the As Ga 4+ model, and a singlet. The temperature variation of the quadruplet hyperfine constant and the intensity, which does not follow a Curie law, are ascribed to an exchange interaction between As Ga 4+ and an associated defect, like related to the singlet line. The generation rate corresponds to some 10 2 complex defects per absorbed fast neutron Utilisation des spectres de resonance paramagnetique obtenus entre 4,2 et 300 K sur GaAs apres irradiation aux neutrons rapides, en decomposant le spectre experimental en un quadruplet contraint aux parametres du modele As Ga 4+ et un singulet. La variation de la constante hyperfine du quadruplet et celle de l'intensite, anormale par rapport a une loi de Curie, sont attribuees a un effet d'echange entre As Ga 4+ et un defaut associe, responsable du singulet

We present a theoretical investigation of reactive and non-reactive collisions of Li and Rb atoms with C 2 − molecular anions at low temperatures in the context of sympathetic cooling in hybrid trap experiments. Based on accurate potential energy surfaces for the singlet and triplet states of the Li– C 2 − and Rb– C 2 − systems, we show that the associative electronic detachment reaction is inefficient but that it occurs with a large rate for collisions involving excited states of either Li/Rb or C 2 − . We also investigate rotationally inelastic collisions in order to explore the cooling of the translational and rotational degrees of freedom of C 2 − in hybrid ion-atom traps. The effect of micromotion is taken into account by considering Tsallis distributions of collision energies. We show that the translational cooling occurs much more rapidly than rotational cooling and that the presence of excited atoms leads to losses of anions on a timescale comparable to that of rotational cooling.