Magnetism in Closed-shell Quantum Dots: Emergence of Magnetic Bipolarons

Similar to atoms and nuclei, semiconductor quantum dots exhibit formation of shells. Predictions of magnetic behavior of the dots are often based on the shell occupancies. Thus, closed-shell quantum dots are assumed to be inherently nonmagnetic. Here, we propose a possibility of magnetism in such dots doped with magnetic impurities. On the example of the system of two interacting fermions, the simplest embodiment of the closed-shell structure, we demonstrate the emergence of a novel broken-symmetry ground state that is neither spin-singlet nor spin-triplet. We propose experimental tests of our predictions and the magnetic-dot structures to perform them.Comment: 4 pages, 4 figures; http://link.aps.org/doi/10.1103/PhysRevLett.106.177201; minor change

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

A Coherent Nonlinear Optical Signal Induced by Electron Correlations

The correlated behavior of electrons determines the structure and optical properties of molecules, semiconductor and other systems. Valuable information on these correlations is provided by measuring the response to femtosecond laser pulses, which probe the very short time period during which the excited particles remain correlated. The interpretation of four-wave-mixing techniques, commonly used to study the energy levels and dynamics of many-electron systems, is complicated by many competing effects and overlapping resonances. Here we propose a coherent optical technique, specifically designed to provide a background-free probe for electronic correlations in many-electron systems. The proposed signal pulse is generated only when the electrons are correlated, which gives rise to an extraordinary sensitivity. The peak pattern in two-dimensional plots, obtained by displaying the signal vs. two frequencies conjugated to two pulse delays, provides a direct visualization and specific signatures of the many-electron wavefunctions.Comment: 2 figure