Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals without Inversion Center

Atomistic sp3d5s* tight-binding theory of PbSe and PbS nanocrystals is developed. It is demonstrated, that the valley splittings of confined electrons and holes strongly and peculiarly depend on the geometry of a nanocrystal. When the nanocrystal lacks a microscopic center of inversion and has T_d symmetry, the splitting is strongly suppressed as compared to the more symmetric nanocrystals with O_h symmetry, having an inversion center.Comment: 5 pages, 4 figures, 1 tabl

The Zel'dovich effect and evolution of atomic Rydberg spectra along the Periodic Table

In 1959 Ya. B. Zel'dovich predicted that the bound-state spectrum of the non-relativistic Coulomb problem distorted at small distances by a short-range potential undergoes a peculiar reconstruction whenever this potential alone supports a low-energy scattering resonance. However documented experimental evidence of this effect has been lacking. Previous theoretical studies of this phenomenon were confined to the regime where the range of the short-ranged potential is much smaller than Bohr's radius of the Coulomb field. We go beyond this limitation by restricting ourselves to highly-excited s states. This allows us to demonstrate that along the Periodic Table of elements the Zel'dovich effect manifests itself as systematic periodic variation of the Rydberg spectra with a period proportional to the cubic root of the atomic number. This dependence, which is supported by analysis of experimental and numerical data, has its origin in the binding properties of the ionic core of the atom.Comment: 17 pages, 12 figure

Thermal emission spectroscopy of the middle atmosphere

The general objective of this research is to obtain, via remote sensing, simultaneous measurements of the vertical distributions of stratospheric temperature, ozone, and trace constituents that participate in the catalytic destruction of ozone (NO(sub y): NO, NO2, NO3, HNO3, ClONO2, N2O5, HNO4; Cl(sub x): HOCl), and the source gases for the catalytic cycles (H2O, CH4, N2O, CF2Cl2, CFCl3, CCl4, CH3Cl, CHF2Cl, etc.). Data are collected during a complete diurnal cycle in order to test our present understanding of ozone chemistry and its associate catalytic cycles. The instrumentation employed is an emission-mode, balloon-borne, liquid-nitrogen-cooled Michelson interferometer-spectrometer (SIRIS), covering the mid-infrared range with a spectral resolution of 0.020 cm(exp -1). Cryogenic cooling combined with the use of extrinsic silicon photoconductor detectors allows the detection of weak emission features of stratospheric gaseous species. Vertical distributions of these species are inferred from scans of the thermal emission of the limb in a sequence of elevation angles. The fourth SIRIS balloon flight was carried out from Palestine, Texas on September 15-16, 1986 with 9 hours of nighttime data (40 km). High quality data with spectral resolution 0.022 cm(exp -1), were obtained for numerous limb sequences. Fifteen stratospheric species have been identified to date from this flight: five species from the NO(sub y) family (HNO3, NO2, NO, ClONO2, N2O5), plus CO2, O3, H2O, N2O, CH4, CCl3F, CCl2F2, CHF2Cl, CF4, and CCl4. The nighttime values of N2O5, ClONO2, and total odd nitrogen have been measured for the first time, and compared to model results. Analysis of the diurnal variation of N2O5 within the 1984 and 1986 data sets, and of the 1984 ClONO2 measurements, were presented in the literature. The demonstrated ability of SIRIS to measure all the major NO(sub y) species, and therefore to determine the partitioning of the nitrogen family over a continuous diurnal cycle, is a powerful tool in the verification and improvement of photochemical modeling

Surface scattering velocities in III-nitride quantum well laser structures via the emission of hybrid phonons

We have theoretically and numerically studied nitride-based quantum well (QW) laser structures. More specifically, we have used a QW made with III-nitride where the width of the barrier region is large relative to the electron mean free path, and we have calculated the electron surface capture velocities by considering an electron flux which is captured into the well region. The process is assisted by the emission of the longitudinal optical phonons as predicted by the hybrid (HB) model. The results of surface capture velocities via the emission of HB phonons are compared to the emission of the dielectric continuum phonons (Zakhleniuk et al 1999 Phys. Status Solidi a 176 79). Our investigation shows that the two different phonon models predict almost the same results for the non-retarded limit. Furthermore, the surface capture velocities strongly depend on the size of the structure and the heterostructure materials. Lastly, a comparison to the recent experimental values shows that our model could accurately describe the experimentally measured parameters of the quantum capture processes

Interaction of intense vuv radiation with large xenon clusters

The interaction of atomic clusters with short, intense pulses of laser light to form extremely hot, dense plasmas has attracted extensive experimental and theoretical interest. The high density of atoms within the cluster greatly enhances the atom--laser interaction, while the finite size of the cluster prevents energy from escaping the interaction region. Recent technological advances have allowed experiments to probe the laser--cluster interaction at very high photon energies, with interactions much stronger than suggested by theories for lower photon energies. We present a model of the laser--cluster interaction which uses non-perturbative R-matrix techniques to calculate inverse bremsstrahlung and photoionization cross sections for Herman-Skillman atomic potentials. We describe the evolution of the cluster under the influence of the processes of inverse bremsstrahlung heating, photoionization, collisional ionization and recombination, and expansion of the cluster. We compare charge state distribution, charge state ejection energies, and total energy absorbed with the Hamburg experiment of Wabnitz {\em et al.} [Nature {\bf 420}, 482 (2002)] and ejected electron spectra with Laarmann {\em et al.} [Phys. Rev. Lett. {\bf 95}, 063402 (2005)]

Dampak Teknologi Sistem USAha Pertanian Padi terhadap Peningkatan Produksi dan Pendapatan USAhatani di Jawa Timur

Assessment was carried out in the areas of the rice-based farming system assessment, namely in Lamongan,Nganjuk, Jombang, Blitar and Malang regencies lasting from 1997 until September, 2000. Technology packageapplied in those areas were: (1) use of improved varieties and seed, (2) planting method, (3) rational fertilization, and(4) weed control. Data collection was done using a survey method since July to September, 2002. This assessmentaimed (1) to get information on the stage of technology adoption and diffusion of the rice-based farming systemtechnology, and (2) to find information on impacts of the activities of the rice-based farming system assessmentrelated with dissemination of new improved varieties, application of double-row planting (jajar legowo) method,numbers of adopting farmers, planted areas, yields, and farmers' income. The results showed that adoption anddiffusion of recommended technology package on rice by the farmers in the assessment areas were high, especiallyon the method of planting and use of new improved varieties. Rice-based farming system technology conducted infive regencies gave positive impacts to (1) new improved varieties diffusion, (2) use of jajar legowo planting method,(3) number of adopting farmers, and planted areas, and (4) yields and farmers' income. Training and extensionthrough farmers' groups are necessary to sustain the program.Key words: technology impacts, farming system, oryza sativa, yield, incomeKajian dampak teknologi SUP padi ini dilakukan di wilayah yang sama dengan kegiatan SUP padi, yaitu diKabupaten Lamongan, Nganjuk, Jombang, Blitar dan Malang. yang dilakukan pada tahun 1997 sampai 2000. Rakitanteknologi yang diterapkan pada saat kegiatan tersebut meliputi : (1) penggunaan varietas dan bibit unggul, (2) caratanam, (3) pemupukan rasional dan (4) pengendalian gulma. Kajian dampak ini menggunakan metode survai dandilakukan pada bulan Juli – September 2002. Data yang dikumpulkan meliputi data sekunder dan primer. Tujuan daripengkajian adalah (1) diperolehnya informasi tingkat adopsi dan difusi teknologi SUP padi dan (2) diperolehnyainformasi dampak kegiatan SUP padi terhadap penyebaran varietas unggul baru, cara tanam jajar legowo, jumlahpetani adopter dan luas areal tanam, serta produktivitas dan pendapatan USAhatani. Hasil kajian menunjukkan bahwaadopsi dan difusi paket teknologi yang dianjurkan pada SUP padi oleh petani di kabupaten tersebut cukup tinggi,terutama dalam hal cara tanam dan penggunaan varietas unggul baru. Pengkajian SUP padi yang telah dilakukan dilima kabupaten telah berdampak positif terhadap ; (1) penyebaran varietas unggul baru, (2) cara tanam jajar legowo,(3) jumlah petani adopter dan luas areal tanam, serta (4) produktivitas dan pendapatan USAhatani. Agar supayakegiatan adopsi teknologi SUP tersebut berlanjut, maka kegiatan bimbingan dan pembinaan melalui kelompok taniperlu selalu dilakukan

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