Confinement of two-dimensional excitons in a non-homogeneous magnetic field

The effective Hamiltonian describing the motion of an exciton in an external non-homogeneous magnetic field is derived. The magnetic field plays the role of an effective potential for the exciton motion, results into an increment of the exciton mass and modifies the exciton kinetic energy operator. In contrast to the homogeneous field case, the exciton in a non-homogeneous magnetic field can also be trapped in the low field region and the field gradient increases the exciton confinement. The trapping energy and wave function of the exciton in a GaAs two-dimensional electron gas for specific circular magnetic field configurations are calculated. The results show than excitons can be trapped by non-homogeneous magnetic fields, and that the trapping energy is strongly correlated with the shape and strength of the non-homogeneous magnetic field profile.Comment: 9 pages, 12 figure

Exciton trapping in magnetic wire structures

The lateral magnetic confinement of quasi two-dimensional excitons into wire like structures is studied. Spin effects are take into account and two different magnetic field profiles are considered, which experimentally can be created by the deposition of a ferromagnetic stripe on a semiconductor quantum well with magnetization parallel or perpendicular to the grown direction of the well. We find that it is possible to confine excitons into one-dimensional (1D) traps. We show that the dependence of the confinement energy on the exciton wave vector, which is related to its free direction of motion along the wire direction, is very small. Through the application of a background magnetic field it is possible to move the position of the trapping region towards the edge of the ferromagnetic stripe or even underneath the stripe. The exact position of this 1D exciton channel depends on the strength of the background magnetic field and on the magnetic polarisation direction of the ferromagnetic film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte

The Specific Heat of a Ferromagnetic Film.

We analyze the specific heat for the $O(N)$ vector model on a $d$-dimensional film geometry of thickness $L$ using ``environmentally friendly'' renormalization. We consider periodic, Dirichlet and antiperiodic boundary conditions, deriving expressions for the specific heat and an effective specific heat exponent, \alpha\ef. In the case of $d=3$, for $N=1$, by matching to the exact exponent of the two dimensional Ising model we capture the crossover for \xi_L\ra\infty between power law behaviour in the limit {L\over\xi_L}\ra\infty and logarithmic behaviour in the limit {L\over\xi_L}\ra0 for fixed $L$, where $\xi_L$ is the correlation length in the transverse dimensions.Comment: 21 pages of Plain TeX. Postscript figures available upon request from [email protected]

Completeness and consistency of renormalisation group flows

We study different renormalisation group flows for scale dependent effective actions, including exact and proper-time renormalisation group flows. These flows have a simple one loop structure. They differ in their dependence on the full field-dependent propagator, which is linear for exact flows. We investigate the inherent approximations of flows with a non-linear dependence on the propagator. We check explicitly that standard perturbation theory is not reproduced. We explain the origin of the discrepancy by providing links to exact flows both in closed expressions and in given approximations. We show that proper-time flows are approximations to Callan-Symanzik flows. Within a background field formalism, we provide a generalised proper-time flow, which is exact. Implications of these findings are discussed.Comment: 33 pages, 15 figures, revtex, typos corrected, to be published in Phys.Rev.

Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry

To understand the nature of supernovae and neutron star (NS) formation, as well as binary stellar evolution and their interactions, it is important to probe the distribution of NS masses. Until now, all double NS (DNS) systems have been measured to have a mass ratio close to unity (q $\geq$ 0.91). Here we report the measurement of the individual masses of the 4.07-day binary pulsar J0453+1559 from measurements of the rate of advance of periastron and Shapiro delay: The mass of the pulsar is 1.559(5) $M_{\odot}$ and that of its companion is 1.174(4) $M_{\odot}$; q = 0.75. If this companion is also a neutron star (NS), as indicated by the orbital eccentricity of the system (e=0.11), then its mass is the smallest precisely measured for any such object. The pulsar has a spin period of 45.7 ms and a spin derivative of 1.8616(7) x$10^-19$; from these we derive a characteristic age of ~ 4.1 x $10^9$ years and a magnetic field of ~ 2.9 x $10^9$ G,i.e, this pulsar was mildly recycled by accretion of matter from the progenitor of the companion star. This suggests that it was formed with (very approximately) its current mass. Thus NSs form with a wide range of masses, which is important for understanding their formation in supernovae. It is also important for the search for gravitational waves released during a NS-NS merger: it is now evident that we should not assume all DNS systems are symmetric

Pulsar J1411+2551: A Low Mass New Double Neutron Star System

In this work, we report the discovery and characterization of PSR J1411+2551, a new binary pulsar discovered in the Arecibo 327 MHz Drift Pulsar Survey. Our timing observations of the radio pulsar in the system span a period of about 2.5 years. This timing campaign allowed a precise measurement of its spin period (62.4 ms) and its derivative (9.6 $\pm$ 0.7) $\times 10^{-20}\, \rm s\, s^{-1}$; from these, we derive a characteristic age of $\sim 10\,$Gyr and a surface magnetic field strength of 2.5 $\times 10^{9}$ G. These numbers indicate that this pulsar was mildly recycled by accretion of matter from the progenitor of the companion star. The system has an eccentric ($e\, = \, 0.17$) 2.61 day orbit. This eccentricity allows a highly significant measurement of the rate of advance of periastron, $\dot{\omega} = 0.07686 \pm 0.00046 ^{\circ}~{\rm yr}^{-1}$. Assuming general relativity accurately models the orbital motion, this implies a total system mass M = $2.538 \pm 0.022 M_{\odot}$. The minimum companion mass is $0.92\, M_{\odot}$ and the maximum pulsar mass is $1.62\, M_{\odot}$. The large companion mass and the orbital eccentricity suggest that PSR J1411+2551 is a double neutron star system; the lightest known to date including the DNS merger GW 170817. Furthermore, the relatively low orbital eccentricity and small proper motion limits suggest that the second supernova had a relatively small associated kick; this and the low system mass suggest that it was an ultra-stripped supernova.Comment: Accepted for publication in APJ letter

Quark-hadron phase transition in a neutron star under strong magnetic fields

We study the effect of a strong magnetic field on the properties of neutron stars with a quark-hadron phase transition. It is shown that the magnetic field prevents the appearance of a quark phase, enhances the leptonic fraction, decreases the baryonic density extension of the mixed phase and stiffens the total equation of state, including both the stellar matter and the magnetic field contributions. Two parametrisations of a density dependent static magnetic field, increasing, respectively, fast and slowly with the density and reaching $2-4\times 10^{18}$G in the center of the star, are considered. The compact stars with strong magnetic fields have maximum mass configurations with larger masses and radius and smaller quark fractions. The parametrisation of the magnetic field with density has a strong influence on the star properties.Comment: 15 pages, 6 figures, 8 tables, accepted for publication in J. Phys.

Avaliação de genótipos de feijão-caupi no Estado da Bahia no biênio 2010/11-2011/12.

O objetivo deste trabalho foi avaliar genótipos de feijão-caupi de portes ereto, semiereto e semiprostrado no Estado da Bahia. Foram avaliados 40 genótipos, sendo 20 de portes ereto e semiereto e 20 de porte semiprostrado, em cinco ambientes do estado da Bahia, no biênio 2010/11 e 2011/12. Todos os ensaios foram conduzidos em delineamento de blocos ao acaso com quatro repetições. As médias dos genótipos foram comparadas, dentro de grupo de porte, pelo teste de Scott-Knott (P<0,05). A cultivar BRS Guariba e a linhagem MNC03-737F-5-1 apresentaram médias de rendimento de grãos semelhantes á testemunha e outros oito genótipos. Os genótipos MNC02-675F-5, MNC03-761F-1, MNC03-736F-2, BRS Xiquexique e Pingo de Ouro 1-2 foram superiores à testemunha em rendimento de grãos.CONAC 2012. Disponível em: http://www.conac2012.org/resumos/pdf/124q.pdf. Acesso em: 19 jul. 2013