Suppressed absolute negative conductance and generation of high-frequency radiation in semiconductor superlattices

We show that space-charge instabilities (electric field domains) in semiconductor superlattices are the attribute of absolute negative conductance induced by small constant and large alternating electric fields. We propose the efficient method for suppression of this destructive phenomenon in order to obtain a generation at microwave and THz frequencies in devices operating at room temperature. We theoretically proved that an unbiased superlattice with a moderate doping subjected to a microwave pump field provides a strong gain at third, fifth, seventh, etc. harmonics of the pump frequency in the conditions of suppressed domains.Comment: 8 pages. Development of cond-mat/0503216 . Version 2: Final version, erratum is include

A molten salt test loop for component and instrumentation testing

Molten salt is an effective coolant for a wide range of applications, including nuclear reactors, concentrated solar power, and other high temperature industrial heat transfer processes. The technical readiness level of components and instrumentation for high-temperature molten salt applications needs improvement for molten salt to be more widely adopted. A molten salt test loop was designed, built, and commissioned as a test bed to address these issues. The molten salt test loop at Abilene Christian University was built out of 316 stainless steel with a forced flow centrifugal-type pump, and was instrumented for remote operation. A low-temperature molten nitrate salt was used in this system, which was designed to operate at temperatures up to 300 ◦C and flow rates up to 90 liters per minute. This paper describes the loop design, computational fluid dynamics modeling, construction, and commissioning details. An outline of the data acquisition and control systems is presented. Salt samples were taken before and after introduction into the loop, and melting points were measured both before and after salt circulation. Performance of the system is discussed as well as improvements required for higher temperature loops envisioned for the future

Electronic structure, linear, nonlinear optical susceptibilities and birefringence of CuInX2 (X = S, Se, Te) chalcopyrite-structure compounds

The electronic structure, linear and nonlinear optical properties have been calculated for CuInX2 (X=S, Se, Te) chalcopyrite-structure single crystals using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. We present results for band structure, density of states, and imaginary part of the frequency-dependent linear and nonlinear optical susceptibilities. We find that these crystals are semiconductors with direct band gaps. We have calculated the birefringence of these crystals. The birefringence is negative for CuInS2 and CuInSe2 while it is positive for CuInTe2 in agreement with the experimental data. Calculations are reported for the frequency-dependent complex second-order non-linear optical susceptibilities . The intra-band and inter-band contributions to the second harmonic generation increase when we replace S by Se and decrease when we replace Se by Te. We find that smaller energy band gap compounds have larger values of in agreement with the experimental data and previous theoretical calculations.Comment: 17 pages, 6 figure

NON-STOICHIOMETRY AND SOLID SOLUTION IN ADAMANTINE TERNARY COMPOUNDS

La considération des phases possibles de structure adamantine, qui satisfont à la règle des 4 électrons par site, conduit à des diagrammes de solutions solides qui indiquent comment différents atomes d'impuretés peuvent être incorporés dans les composés I-III-VI2 et II-IV-V2 sans effet de dopage.A consideration of the possible adamantine phases which satisfy the four electrons per site rule, leads to solid solution diagram which indicate how various impurity atoms may be incorporated into I-III-VI2 and II-IV-V2 compounds without a doping effect

GROWTH OF II IV V2 COMPOUNDS AND ALLOYS BY ACCELERATED CRUCIBLE ROTATION TECHNIQUES

On décrit la croissance de monocristaux de composés de structure chalcopyrite à partir de solutions métalliques. Le tube du four est animé d'un mouvement de rotation alternatif qui assure l'agitation du bain ; la température est abaissée avec des interruptions accompagnées de légers réchauffements afin de limiter les nucléations. Une tentative de croissance de ZnCdGeP4 est décrite.The growth of single crystals of chalcopyrite compounds from solution in liquid metals is described. The furnace tube is rotated to and fro to provide stirring and the temperature is lowered with interruptions and slight reheating to limit nucleations. An attempt to grow ZnCdGe2P4 is described

OPTICAL ABSORPTION IN ZnSiP2

On a étudié le coefficient d'absorption de ZnSiP2 au voisinage de sa bande interdite pour les deux polarisations. On a trouvé trois singularités pour E ⊥ c qui sont attribuées à des transitions pseudo-directes. L'absorption pour E // c est attribuable principalement à des transitions indirectes. Une courbe théorique a été ajustée aux résultats expérimentaux à l'aide des intensités relatives des transitions pseudo-directes calculées à partir du modèle quasi cubique. Ceci conduit pour l'énergie de liaison de l'exciton à une valeur approximative de 22 meV, et pour l'élément de matrice des transitions pseudo-directes à une valeur à peu près mille fois plus petite que celle des transitions directes dans les composés III-V. Les différences d'énergie entre les bandes de valence donnent des valeurs pour l'énergie du couplage spin-orbite de 0,056 eV et pour l'énergie du champ cristallin de - 0,130 eV. La bande interdite de ZnSiP2 à la température ambiante est de 2,082 eV après avoir tenu compte de l'énergie de liaison de l'exciton.The polarised absorption coefficient of ZnSiP2 near the fundamental absorption edge has been studied. Three singularities were found for E ⊥ c which are attributed to pseudodirect transitions. The absorption for E // c is mainly due to indirect transitions. A theoretical curve was fitted to the experimental results with the aid of the relative theoretical intensities for pseudodirect transitions calculated from the quasicubic model. This yields approximate values for the exciton binding energy of 22 meV, and for the matrix element for pseudodirect transitions about a thousand times smaller than that for direct transitions in III-V compounds. The splittings of the valence bands give values for the spin-orbit splitting of 0.056 eV and of the crystal field splitting of - 0.130 eV. The lowest band gap in ZnSiP2 is pseudodirect at 2.082 eV at room temperature after making allowance for the exciton binding energy