Interlayer Exchange Coupling in (Ga,Mn)As-based Superlattices

The interlayer coupling between (Ga,Mn)As ferromagnetic layers in all-semiconductor superlattices is studied theoretically within a tight-binding model, which takes into account the crystal, band and magnetic structure of the constituent superlattice components. It is shown that the mechanism originally introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe superlattices, explains the experimental results observed in ferromagnetic semiconductor structures, i.e., both the antiferromagnetic coupling between ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer structures. The model allows also to predict (Ga,Mn)As-based structures, in which an antiferromagnetic interlayer coupling could be expected.Comment: 4 pages, 3 figure

Voltage controlled spin injection in a (Ga,Mn)As/(Al,Ga)As Zener diode

The spin polarization of the electron current in a p-(Ga,Mn)As-n-(Al,Ga)As-Zener tunnel diode, which is embedded in a light-emitting diode, has been studied theoretically. A series of self-consistent simulations determines the charge distribution, the band bending, and the current-voltage characteristics for the entire structure. An empirical tight-binding model, together with the Landauer- Buttiker theory of coherent transport has been developed to study the current spin polarization. This dual approach allows to explain the experimentally observed high magnitude and strong bias dependence of the current spin polarization.Comment: Submitted to Phys. Rev. B Rapid Communication

Structural and electronic properties of Pb1-xCdxTe and Pb1-xMnxTe ternary alloys

A systematic theoretical study of two PbTe-based ternary alloys, Pb1-xCdxTe and Pb1-xMnxTe, is reported. First, using ab initio methods we study the stability of the crystal structure of CdTe - PbTe solid solutions, to predict the composition for which rock-salt structure of PbTe changes into zinc-blende structure of CdTe. The dependence of the lattice parameter on Cd (Mn) content x in the mixed crystals is studied by the same methods. The obtained decrease of the lattice constant with x agrees with what is observed in both alloys. The band structures of PbTe-based ternary compounds are calculated within a tight-binding approach. To describe correctly the constituent materials new tight-binding parameterizations for PbTe and MnTe bulk crystals as well as a tight-binding description of rock-salt CdTe are proposed. For both studied ternary alloys, the calculated band gap in the L point increases with x, in qualitative agreement with photoluminescence measurements in the infrared. The results show also that in p-type Pb1-xCdxTe and Pb1-xMnxTe mixed crystals an enhancement of thermoelectrical power can be expected.Comment: 10 pages, 13 figures, submitted to Physical Review

Nanostructured, Alkaline Titanate‐Converted, and Heat‐Treated Ti6Al4V Microspheres via Wet‐Chemical Alkaline Modification and their ORR Electrocatalytic Response

This study describes the chemical conversion and heat treatment of Ti6Al4V microspheres (Ti6_MS), and the resulting effects on their electrocatalytic properties. The wet‐chemical conversion (5.0 m NaOH, 60 °C, 24 h; Sample label: Ti6_TC) converts the top surface of the Ti6_MS powder into an ≈820 nm thick sodium titanate surface. Heat‐treatment (Ti6_TC_HT) at 450 °C increases the stability of the surface, through partial titanate crystallization, while mitigating excess rutile formation. All samples are analyzed chemically (XPS, EDX, Raman, EELS), structurally (XRD and TEM), and morphologically (SEM, TEM), demonstrating the characteristic formation of sodium titanate dendritic structures, with minimal chemical, structural, and morphological differences due to the 450 °C heat‐treatment. The effect of the preparation methodology on oxygen reduction reaction (ORR) electrocatalytic performance is also tested. The introduction of the sodium titanate layer changes the mechanism of the ORR, from a mixed 4 electron/2 electron pathway to a predominantly 2‐electron pathway. By maintaining the microspherical nature of the material while also tuning the surface of the material toward different reaction mechanisms, a design strategy for new electrocatalyst materials is explored

Nanostructured, alkaline titanate‐converted, and heat‐treated Ti6Al4V microspheres via wet‐chemical alkaline modification and their ORR electrocatalytic response

This study describes the chemical conversion and heat treatment of Ti6Al4V microspheres (Ti6_MS), and the resulting effects on their electrocatalytic properties. The wet‐chemical conversion (5.0 m NaOH, 60 °C, 24 h; Sample label: Ti6_TC) converts the top surface of the Ti6_MS powder into an ≈820 nm thick sodium titanate surface. Heat‐treatment (Ti6_TC_HT) at 450 °C increases the stability of the surface, through partial titanate crystallization, while mitigating excess rutile formation. All samples are analyzed chemically (XPS, EDX, Raman, EELS), structurally (XRD and TEM), and morphologically (SEM, TEM), demonstrating the characteristic formation of sodium titanate dendritic structures, with minimal chemical, structural, and morphological differences due to the 450 °C heat‐treatment. The effect of the preparation methodology on oxygen reduction reaction (ORR) electrocatalytic performance is also tested. The introduction of the sodium titanate layer changes the mechanism of the ORR, from a mixed 4 electron/2 electron pathway to a predominantly 2‐electron pathway. By maintaining the microspherical nature of the material while also tuning the surface of the material toward different reaction mechanisms, a design strategy for new electrocatalyst materials is explored

Origin of ferromagnetic response in diluted magnetic semiconductors and oxides

This paper reviews the present understanding of the origin of ferromagnetic response of diluted magnetic semiconductors and diluted magnetic oxides as well as in some nominally magnetically undoped materials. It is argued that these systems can be grouped into four classes. To the first belong composite materials in which precipitations of a known ferromagnetic, ferrimagnetic or antiferromagnetic compound account for magnetic characteristics at high temperatures. The second class forms alloys showing chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent. To the third class belong (Ga,Mn)As, heavily doped p-(Zn,Mn)Te, and related semiconductors. In these solid solutions the theory built on p-d Zener's model of hole-mediated ferromagnetism and on either the Kohn-Luttinger kp theory or the multi-orbital tight-binding approach describes qualitatively, and often quantitatively many relevant properties. Finally, in a number of carrier-doped DMS and DMO a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation.Comment: review, 19 pages, 4 figure

Semiconductor Spintronics

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure

Vitamin A derivatives in the prevention and treatment of human cancer.

Vitamin A is essential for normal cellular growth and differentiation. A vast amount of laboratory data have clearly demonstrated the potent antiproliferative and differentiation-inducing effects of vitamin A and the synthetic analogues (retinoids). Recent in-vitro work has led to the exciting proposal that protein kinase-C may be centrally involved in many of retinoids' anticancer actions including the effects on ornithine decarboxylase induction, intracellular polyamine levels, and epidermal growth factor receptor number. Several intervention trials have clearly indicated that natural vitamin A at clinically tolerable doses has only limited activity against human neoplastic processes. Therefore, clinical work has focused on the synthetic derivatives with higher therapeutic indexes. In human cancer prevention, retinoids have been most effective for skin diseases, including actinic keratosis, keratoacanthoma, epidermodysplasia verruciformis, dysplastic nevus syndrome, and basal cell carcinoma. Several noncutaneous premaligancies, however, are currently receiving more attention in retinoid trials. Definite retinoid activity has been documented in oral leukoplakia, laryngeal papillomatosis, superficial bladder carcinoma, cervical dysplasia, bronchial metaplasia, and preleukemia. Significant therapeutic advances are also occurring with this class of drugs in some drug-resistant malignancies and several others that have become refractory, including advanced basal cell cancer, mycosis fungoides, melanoma, acute promyelocytic leukemia, and squamous cell carcinoma of the skin and of the head and neck. This report comprehensively presents the clinical data using retinoids as anticancer agents in human premalignant disorders and outlines the ongoing and planned studies with retinoids in combination and adjuvant therapy