Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap

The superfluid phase transition of bosons in a two-dimensional (2D) system with disorder and an external parabolic potential is studied. The theory is applied to experiments on indirect excitons in coupled quantum wells. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The slope of the external parabolic trap is assumed to change slowly enough to apply the local density approximation (LDA) for the superfluid density, which allows us to calculate the Kosterlitz-Thouless temperature $T_{c}(n(r))$ at each local point $r$ of the trap. The superfluid phase occurs around the center of the trap ($\mathbf{r}=0$) with the normal phase outside this area. As temperature increases, the superfluid area shrinks and disappears at temperature $T_{c}(n(r=0))$. Disorder acts to deplete the condensate; the minimal total number of excitons for which superfluidity exists increases with disorder at fixed temperature. If the disorder is large enough, it can destroy the superfluid entirely. The effect of magnetic field is also calculated for the case of indirect excitons. In a strong magnetic field $H$, the superfluid component decreases, primarily due to the change of the exciton effective mass.Comment: 13 pages, 3 figure

El sistema de Poole: un estudio formal

Con el auge de la Inteligencia Artificial comenzaron a proliferar, a partir de la década de los 80, sistemas cuyo objetivo es reconstruir formalmente el razonamiento revocable. Los sistemas de Poole y Reiter se inscriben en esta vertiente. El estudio metateórico de los sistemas para razonamiento revocable condujo a Gabbay, Makinson y otros a definir propiedades debilitadas como la monotonía cauta, la monotonía racional, etc.: propiedades positivas mínimas esperables en sistemas no-monótonos. En el presente trabajo proponemos algunas definiciones alternativas y ofrecemos pruebas que evidencian ventajas comparativas del sistema de Poole