27 research outputs found
Influence of band structure effects on domain-wall resistance in diluted ferromagnetic semiconductors
Intrinsic domain-wall resistance (DWR) in (Ga,Mn)As is studied theoretically
and compared to experimental results. The recently developed model of spin
transport in diluted ferromagnetic semiconductors [Van Dorpe et al., Phys. Rev.
B 72, 205322 (2005)] is employed. The model combines the disorder-free
Landauer-B\"uttiker formalism with the tight-binding description of the host
band structure. The obtained results show how much the spherical 4x4 kp model
[Nguyen, Shchelushkin, and Brataas, cond-mat/0601436] overestimates DWR in the
adiabatic limit, and reveal the dependence of DWR on the magnetization profile
and crystallographic orientation of the wall.Comment: 4 pages, 4 figures, submitted to Phys. Rev. B - Rapid Com
Theory of quantum dot spin-lasers
We formulate a model of a semiconductor Quantum Dot laser with injection of
spin-polarized electrons. As compared to higher-dimensionality structures, the
Quantum-Dot-based active region is known to improve laser properties, including
the spin-related ones. The wetting layer, from which carriers are captured into
the active region, acts as an intermediate level that strongly influences the
lasing operation. The finite capture rate leads to an increase of lasing
thresholds, and to saturation of emitted light at higher injection. In spite of
these issues, the advantageous threshold reduction, resulting from spin
injection, can be preserved. The "spin-filtering" effect, i.e., circularly
polarized emission at even modest spin-polarization of injection, remains
present as well. Our rate-equations description allows to obtain analytical
results and provides transparent guidance for improvement of spin-lasers.Comment: 7 pages, 3 figure
Robust Magnetic Polarons in Type-II (Zn,Mn)Te Quantum Dots
We present evidence of magnetic ordering in type-II (Zn, Mn) Te quantum dots.
This ordering is attributed to the formation of bound magnetic polarons caused
by the exchange interaction between the strongly localized holes and Mn within
the dots. In our photoluminescence studies, the magnetic polarons are detected
at temperatures up to ~ 200 K, with a binding energy of ~ 40 meV. In addition,
these dots display an unusually small Zeeman shift with applied field (2 meV at
10 T). This behavior is explained by a small and weakly temperature-dependent
magnetic susceptibility due to anti-ferromagnetic coupling of the Mn spins
Characterization of CdSe nanocrystals coated with amphiphiles. A capillary electrophoresis study
We have synthesized CdSe nanocrystals (NCs) possessing a trioctylphosphine surface passivation layer and modified with amphiphilic molecules to form a surface bilayer. The NCs covered with single amphiphiles are not stable in aqueous solution, but a mixed amphiphilic system is shown to provide stability in solution over several months. The solutions of the modified NCs were characterized by UV-Vis absorbance, photoluminescence, and transmission electron microscopy. An electrophoretic study revealed two operational modes. The first relies on the enrichment of NCs using a micellar plug as a tool. The accumulation of NCs at the plug-electrolyte buffer interface results in a sharp peak. By controlling the electrophoretic conditions, nanocrystals were forced to exit a micellar plug into an electrolyte buffer. We conclude that a system consisting of modified nanocrystals and a micellar plug can act as a mixed pseudomicellar system, where modified nanocrystals play the role of pseudomicelles
Capillary electrophoretic separation of nanoparticles
In the present work, CdSe nanocrystals (NCs) synthesized with a trioctylphosphine surface passivation layer were modified using amphiphilic molecules to form a surface bilayer capable of providing stable NCs aqueous solutions. Such modified nanocrystals were used as a test solute in order to analyze new electrophoretic phenomena, by applying a micellar plug as a separation tool for discriminating nanocrystals between micellar and micelle-free zones during electrophoresis. The distribution of NCs between both zones depended on the affinity of nanocrystals towards the micellar zone, and this relies on the kind of surface ligands attached to the NCs, as well as electrophoretic conditions applied. In this case, the NCs that migrated within a micellar zone can be focused using a preconcentration mechanism. By modifying electrophoretic conditions, NCs were forced to migrate outside the micellar zone in the form of a typical CZE peak. In this situation, a two-order difference in separation efficiencies, in terms of theoretical plates, was observed between focused NCs (N ~ 107) and a typical CZE peak for NCs (N ~ 105). By applying the amino-functionalized NCs the preconcentration of NCs, using a micellar plug, was examined, with the conclusion that preconcentration efficiency, in terms of the enhancement factor for peak height (SEFheight) can be, at least 20. The distribution effect was applied to separate CdSe/ZnS NCs encapsulated in silica, as well as surface-modified with DNA, which allows the estimation of the yield of conjugation of biologically active molecules to a particle surface