Spin splitting in a polarized quasi-two-dimensional exciton gas

We have observed a large spin splitting between "spin" +1 and -1 heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs quantum wells in the absence of any external magnetic field. Time-resolved photoluminescence spectroscopy, under excitation with circularly polarized light, reveals that, for high excitonic density and short times after the pulsed excitation, the emission from majority excitons lies above that of minority ones. The amount of the splitting, which can be as large as 50% of the binding energy, increases with excitonic density and presents a time evolution closely connected with the degree of polarization of the luminescence. Our results are interpreted on the light of a recently developed model, which shows that, while intraexcitonic exchange interaction is responsible for the spin relaxation processes, exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors.

The evolution and masses of the neutron star and donor star in the high mass X-ray binary OAO 1657−415

We report near-infrared radial velocity (RV) measurements of the recently identified donor star in the high mass X-ray binary (HMXB) system OAO 1657−415 obtained in the H band using ISAAC on the Very Large Telescope. Cross-correlation methods were employed to construct a RV curve with a semi-amplitude of 22.1 ± 3.5 km s−1. Combined with other measured parameters of this system it provides a dynamically determined neutron star (NS) mass of 1.42 ± 0.26 M⊙ and a mass of 14.3 ± 0.8 M⊙ for the Ofpe/WN9 highly evolved donor star. OAO 1657−415 is an eclipsing HMXB pulsar with the largest eccentricity and orbital period of any within its class. Of the 10 known eclipsing X-ray binary pulsars OAO 1657−415 becomes the ninth with a dynamically determined NS mass solution and only the second in an eccentric system. Furthermore, the donor star in OAO 1657−415 is much more highly evolved than the majority of the supergiant donors in other HMXBs, joining a small but growing list of HMXBs donors with extensive hydrogen depleted atmospheres. Considering the evolutionary development of OAO 1657−415, we have estimated the binding energy of the envelope of the mass donor and find that there is insufficient energy for the removal of the donor’s envelope via spiral-in, ruling out a common envelope evolutionary scenario. With its non-zero eccentricity and relatively large orbital period the identification of a definitive evolutionary pathway for OAO 1657−415 remains problematic, we conclude by proposing two scenarios which may account for OAO 1657−415 current orbital configuration.