Theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors

A density-functional theory of ferromagnetism in heterostructures of compound semiconductors doped with magnetic impurities is presented. The variable functions in the density-functional theory are the charge and spin densities of the itinerant carriers and the charge and localized spins of the impurities. The theory is applied to study the Curie temperature of planar heterostructures of III-V semiconductors doped with manganese atoms. The mean-field, virtual-crystal and effective-mass approximations are adopted to calculate the electronic structure, including the spin-orbit interaction, and the magnetic susceptibilities, leading to the Curie temperature. By means of these results, we attempt to understand the observed dependence of the Curie temperature of planar δ-doped ferromagnetic structures on variation of their properties. We predict a large increase of the Curie temperature by additional confinement of the holes in a δ-doped layer of Mn by a quantum well.

Optical spin transfer in ferromagnetic semiconductors

Circularly polarized laser pulses that excite electron-hole pairs across the band gap of (III,Mn)V ferromagnetic semiconductors can be used to manipulate and to study collective magnetization dynamics. The initial spin orientation of a photocarrier in a (III,V) semiconductors is determined by the polarization state of the laser. We show that the photocarrier spin can be irreversibly transferred to the collective magnetization, whose dynamics can consequently be flexibly controlled by suitably chosen laser pulses. As illustrations we demonstrate the feasibility of all optical ferromagnetic resonance and optical magnetization reorientation.

Singular Temperatures Connected to Charge Transport Mechanism Transitions in Perylene Bisimides from Steady-State Photocurrent Measurements

Perylene bisimides (PBIs) are n-type semiconducting and photogenerating materials widely used in a variety of optoelectronic devices. Particularly interesting are PBIs that are simultaneously water-soluble and liquid-crystalline (PBI-W+LC) and, thus, attractive for the development of high-performing easily processable applications in biology and “green” organic electronics. In this work, singular temperatures connected to charge transport mechanism transitions in a PBI-W+LC derivative are determined with high accuracy by means of temperature-dependent photocurrent studies. These singular temperatures include not only the ones observed at 60 and 110 °C, corresponding to phase transition temperatures from crystalline to liquid-crystalline (LC) and from LC to the isotropic phase, respectively, as confirmed by differential scanning calorimetry (DSC), but also a transition at 45 °C, not observed by DSC. By analyzing the photocurrent dependence simultaneously on temperature and on light intensity, this transition is interpreted as a change from monomolecular to bimolecular recombination. These results might be useful for other semiconducting photogenerating materials, not necessarily PBIs or even organic semiconductors, which also show transport behavior changes at singular temperatures not connected with structural or phase transitions.