1,915 research outputs found
Sharp lines in the absorption edge of EuTe and PbEuTe in high magnetic fields
The optical absorption spectra in the region of the \fd transition energies
of epitaxial layers of of EuTe and \PbEuTe, grown by molecular beam epitaxy,
were studied using circularly polarized light, in the Faraday configuration.
Under \sigmam polarization a sharp symmetric absorption line (full width at
half-maximum 0.041 eV) emerges at the low energy side of the band-edge
absorption, for magnetic fields intensities greater than 6 T. The absorption
line shows a huge red shift (35 meV/T) with increasing magnetic fields. The
peak position of the absorption line as a function of magnetic field is
dominated by the {\em d-f} exchange interaction of the excited electron and the
\Euion spins in the lattice. The {\em d-f} exchange interaction energy was
estimated to be eV. In \PbEuTe the same absorption line
is detected, but it is broader, due to alloy disorder, indicating that the
excitation is localized within a finite radius. From a comparison of the
absorption spectra in EuTe and \PbEuTe the characteristic radius of the
excitation is estimated to be \AA.Comment: Journal of Physics: Condensed Matter (2004, at press
Spin-polarized tunneling currents through a ferromagnetic insulator between two metallic or superconducting leads
Using the Keldysh formalism the tunneling current through a hybrid structure
where a confined magnetic insulator (I) is sandwiched between two non-magnetic
leads is calculated. The leads can be either normal metals (M) or
superconductors (S). Each region is modelled as a single band in tight-binding
approximation in order to understand the formation of the tunneling current as
clearly as possible. The tunneling process itself is simulated by a
hybridization between the lead and insulator conduction bands. The insulator is
assumed to have localized moments which can interact with the tunneling
electrons. This is described by the Kondo Lattice Model (KLM) and treated
within an interpolating self-energy approach. For the superconductor the
mean-field BCS theory is used. The spin polarization of the current shows a
strong dependence both on the applied voltage and the properties of the
materials. Even for this idealized three band model there is a qualitative
agreement with experiment.Comment: 15 pages, 23 figures, accepted for publication in PR
Cool Companions to White Dwarf Stars from the Two Micron All Sky Survey All Sky Data Release
We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook and Sion catalog, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The color selection technique, which identifies candidate binaries containing a white dwarf and a low-mass stellar (or substellar) companion via their distinctive locus in the near-infrared color-color diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 27 firm candidates (19 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L-type (i.e., potential brown dwarf) companion. Only one previously known white dwarf + brown dwarf binary is detected. This result is discussed in the context of the 2MASS detection limits, as well as other recent observational surveys that suggest a very low rate of formation (or survival) for binary stars with extreme mass ratios
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