Stars and brown dwarfs in the sigma Orionis cluster. III. OSIRIS/GTC low-resolution spectroscopy of variable sources

Context. Although many studies have been performed so far, there are still dozens of low-mass stars and brown dwarfs in the young sigma Orionis open cluster without detailed spectroscopic characterisation. Aims. We look for unknown strong accretors and disc hosts that were undetected in previous surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran Telescopio Canarias under very poor weather conditions. These objects display variability in the optical, infrared, Halpha, and/or X-rays on time scales of hours to years. We complemented our spectra with optical and near-/mid-infrared photometry. Results. For seven targets, we detected lithium in absorption, identified Halpha, the calcium doublet, and forbidden lines in emission, and/or determined spectral types for the first time. We characterise in detail a faint, T Tauri-like brown dwarf with an 18 h-period variability in the optical and a large Halpha equivalent width of -125+/-15 AA, as well as two M1-type, X-ray-flaring, low-mass stars, one with a warm disc and forbidden emission lines, the other with a previously unknown cold disc with a large inner hole. Conclusions. New unrevealed strong accretors and disc hosts, even below the substellar limit, await discovery among the list of known sigma Orionis stars and brown dwarfs that are variable in the optical and have no detailed spectroscopic characterisation yet.Comment: A&A, in press (accepted for publication in section 14. Catalogs and data of Astronomy and Astrophysics

Nonlinear spin-polarized transport through a ferromagnetic domain wall

A domain wall separating two oppositely magnetized regions in a ferromagnetic semiconductor exhibits, under appropriate conditions, strongly nonlinear I-V characteristics similar to those of a p-n diode. We study these characteristics as functions of wall width and temperature. As the width increases or the temperature decreases, direct tunneling between the majority spin bands decreases the effectiveness of the diode. This has important implications for the zero-field quenched resistance of magnetic semiconductors and for the design of a recently proposed spin transistor.Comment: 5 pages, 3 figure

The landscape of quantum transitions driven by single-qubit unitary transformations with implications for entanglement

This paper considers the control landscape of quantum transitions in multi-qubit systems driven by unitary transformations with single-qubit interaction terms. The two-qubit case is fully analyzed to reveal the features of the landscape including the nature of the absolute maximum and minimum, the saddle points and the absence of traps. The results permit calculating the Schmidt state starting from an arbitrary two-qubit state following the local gradient flow. The analysis of multi-qubit systems is more challenging, but the generalized Schmidt states may also be located by following the local gradient flow. Finally, we show the relation between the generalized Schmidt states and the entanglement measure based on the Bures distance