Anharmonic multi-phonon nonradiative transition: An ab initio calculation approach

Nonradiative carrier recombinations at deep centers in semiconductors are of great importance for both fundamental physics and device engineering. In this article, we provide a revised analysis of Huang’s original nonradiative multi-phonon (NMP) theory with ab initio calculations. First, we confirmed at the first-principles level that Huang’s concise formula gives the same results as the matrix-based formula, and that Huang’s high-temperature formula provides an analytical expression for the coupling constant in Marcus theory. Secondly, we correct for anharmonic effects by taking into account local phonon-mode variations for different charge states of a defect. The corrected capture rates for defects in GaN and SiC agree well with experiments

Coupled optical interface modes in a Fibonacci dielectric superlattice

The coupled optical interface modes in a Fibonacci dielectric superlattice are studied. In the dielectric continuum approximation, the dispersion relation is found to have two bands of dual triadic Cantor structures, each being nonuniform scaling. For most of the eigenfrequencies, the amplitude profiles of electrostatic potential in this quasiperiodic structure are critical. Moreover, an invariant is analytically derived and is used to describe the general features of the frequency spectra and potential profiles.published_or_final_versio

Persistent currents in mesoscopic Fibonacci rings

In the framework of a tight-binding model, we study energy spectra and persistent currents in mesoscopic Fibonacci rings threaded by a magnetic flux. It is found that the flux-dependent electron eigenenergies E(Φ) in mesoscopic Fibonacci rings still form "bands" with respect to the flux Φ, but there is a scaling relation between the total "bandwidth" and the Fibonacci number. When the strength of the one-dimensional quasiperiodic potential increases, the persistent current decreases rapidly. Interestingly, for a generalized mixing model of mesoscopic Fibonacci rings, free-electron-like persistent current may appear if the number of electrons of the system takes a specific value.published_or_final_versio

Quantum waveguide theory of serial stub structures

The electronic behaviors in quantum wires with serial stubs are studied. A general theory of quantum waveguide based on transfer matrix method is developed and is used to treat periodic stub structures, serial stub structures with a defect stub, and Fibonacci stub structures. A number of interesting physical properties in connection with electronic transmission, energy spectra, and charge density distributions in these structures, are found theoretically. In particular, we find that whether there are periodicity and symmetry in the transmission and energy spectra depends on the commensurability of the length parameters. If one length ratio is incommensurate, then the transmission and energy spectra do not exhibit periodicity and symmetry even for periodic stub structures. In particular, the quasiperiodic behaviors are shown in Fibonacci stub structures proposed by us whenever the length parameters are commensurate. The experimental relevance is also addressed briefly. © 1999 American Institute of Physics.published_or_final_versio

Ab Initio Investigation of Charge Trapping Across the Crystalline- Si -Amorphous- Si O2 Interface

Accurate microscopic description of the charge-trapping process from semiconductor to defects in the dielectric-oxide layer is of paramount importance for understanding many microelectronic devices such as complementary metal-oxide-semiconductor (CMOS) transistors, as well as electrochemical reactions. Unfortunately, most current microscopic descriptions of such processes are based on empirical models with parameters fitted to experimental device performance results or simplified approximations like the Wentzel-Kramers-Brillouin (WKB) method. Some critical questions are still unanswered, including: What controls the charge-hopping rate, the coupling strength between the defect level to semiconductor level, or the energy difference? How does the hopping rate decay with defect-semiconductor distance? What is the fluctuation of the defect level, especially in amorphous dielectrics? Many of these questions can be answered by ab initio calculations. However, to date, there are few ab initio studies for this problem mainly due to technical challenges from atomic-structure construction to large-system calculations. Here, using the latest advances in calculation methods and codes, we study the carrier-trapping problem using density-functional theory (DFT) based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional. The valence bond random-switching method is used to construct the crystalline-Si-amorphous-SiO2 (c-Si/a-SiO2) interfacial atomic structure, and the HSE yields a band offset that agrees well with experiments. The hopping rate is calculated with the Marcus theory, and the hopping-rate dependences on the gate potential and defect distances are revealed, as well as the range of fluctuation results from amorphous structural variation. We also analyze the result with the simple WKB model and find a major difference in the description of the coupling constant decay with the defect-semiconductor distance. Our results provide the ab initio simulation insights for this important carrier-trapping process for device operation

Curcumin Enhances Bortezomib Treatment of Myeloma by Inhibiting Heat Shock Protein 90 Expression

Purpose: To investigate whether curcumin augments bortezomib-induced apoptosis in myeloma cells (MM1.R line), and to explore the molecular mechanism with regard to heat shock protein 90 (HSP90) expression.Methods: MTT cell viability assay was used to assess growth inhibition of MM1.R cells at different concentrations of curcumin alone and also combined with 0.01 mM bortezomib. Annexin V and propidium iodide (PI) labeling were used to detect apoptosis. Caspase 3, caspase 9, NF-κB, and HSP 90 protein expression were measured by Western blotting.Results: Curcumin alone inhibited MM1.R cell growth and increased apoptosis in a concentration dependent manner. When curcumin was combined with low concentration (0.01 mM) bortezomib, both effects（viability inhibition and apoptosis induction increased (p < 0.05), whereas bortezomib alone had no effect (p > 0.05). Western blotting revealed that for curcumin and combined treatments, expression of the apoptotic markers, caspase 3 and caspase 9, increased while expression of NF-κB and HSP 90 decreased (p < 0.05). Again, low concentration bortezomib alone had no effect, whereas the combined treatment showed the largest effect, thus suggesting that the actions of curcumin and bortezomib are synergistic.Conclusion: Curcumin increased MM1.R cell sensitivity to bortezomib, which may be due to suppression of NF-κB and HSP90 activity.Keywords: Curcumin, Bortezomib, Myeloma cells, Cell growth, Apoptosis, Heat shock protein 9

Association of a genetic polymorphism in the gene encoding fibrinogen beta chain with hypertension in Hong Kong Chinese

published_or_final_versionThe 15th Annual Research Conference of the Department of Medicine, The University of Hong Kong, Hong Kong, 16 January 2010. In Hong Kong Medical Journal, 2010, v. 16, suppl. 1, p. 51, abstract no. 8

Microstructures and resistivity of cuprate/manganite bilayer deposited on SrTiO3 substrate

Thin Yba[SUB2]Cu[SUB3]O[SUB7-δ/La[SUB0.67]Ca[SUB0.33]MnO[SUB3] (YBCO/LCMO) films were grown on SrTiO[SUB3](STO)substrates by magnetron sputtering technique. The microstructures of the bilayers were characterized and a standard four-probe technique was applied to measure the resistivity of the samples. The interdiffusions at the YBCO/LCMO and LCMO/STO interfaces formed two transient layers with the thickness of about 3 and 2 nm, respectively. All the bilayers were well textured along the c axis. At low temperature, the superconductivity can only be observed when the thickness of YBCO is more than 25 nm. When the thickness of YBCO is less than 8 nm, the bilayers show only ferromagnetism. The superconductivity and ferromagnetism perhaps coexist in the bilayer with the YBCO thickness of 12.5 nm. These interesting properties are related to the interaction between spin polarized electrons in the manganites and the cooper pairs in the cuprates. © 2003 American Institute of Physics.published_or_final_versio