bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of novel non-nucleoside reverse transcriptase (RT) inhibitor resistance mutations at residues 132 and 135 in the 51 kDa subunit of HIV-1 RT
Authors: Nissley Dwight V. , Radzio Jessica , Ambrose Zandrea , Sheen Chih-Wei , Hamamouch Noureddine , Moore Katie L. , Tachedjian Gilda , Sluis-Cremer Nicolas
Published: April 2007

Several rare and novel NNRTI [non-nucleoside reverse transcriptase (RT) inhibitor] resistance mutations were recently detected at codons 132 and 135 in RTs from clinical isolates using the yeast-based chimaeric TyHRT (Ty1/HIV-1 RT) phenotypic assay. Ile132 and Ile135 form part of the β7–β8 loop of HIV-1 RT (residues 132–140). To elucidate the contribution of these residues in RT structure–function and drug resistance, we constructed twelve recombinant enzymes harbouring mutations at codons 132 and 135–140. Several of the mutant enzymes exhibited reduced DNA polymerase activities. Using the yeast two-hybrid assay for HIV-1 RT dimerization we show that in some instances this decrease in enzyme activity could be attributed to the mutations, in the context of the 51 kDa subunit of HIV-1 RT, disrupting the subunit–subunit interactions of the enzyme. Drug resistance analyses using purified RT, the TyHRT assay and antiviral assays demonstrated that the I132M mutation conferred high-level resistance (>10-fold) to nevirapine and delavirdine and low-level resistance (∼2–3-fold) to efavirenz. The I135A and I135M mutations also conferred low level NNRTI resistance (∼2-fold). Subunit selective mutagenesis studies again demonstrated that resistance was conferred via the p51 subunit of HIV-1 RT. Taken together, our results highlight a specific role of residues 132 and 135 in NNRTI resistance and a general role for residues in the β7–β8 loop in the stability of HIV-1 RT.
Allosteric activation of the extracellular Ca2+-sensing receptor byL-amino acids enhances ERK1/2 phosphorylation
Authors: Lee Heather J. , Mun Hee-Chang , Lewis Narelle C. , Crouch Michael F. , Culverston Emma L. , Mason Rebecca S. , Conigrave Arthur D.
Published: April 2007

The calcium-sensing receptor (CaR) mediates feedback control of Ca2+o (extracellular Ca2+) concentration. Although the mechanisms are not fully understood, the CaR couples to several important intracellular signalling enzymes, including PI-PLC (phosphoinositide-specific phospholipase C), leading to Ca2+i (intracellular Ca2+) mobilization, and ERK1/2 (extracellular-signal-regulated kinase 1/2). In addition to Ca2+o, the CaR is activated allosterically by several subclasses of L-amino acids, including the aromatics L-phenylalanine and L-tryptophan. These amino acids enhance the Ca2+o-sensitivity of Ca2+i mobilization in CaR-expressing HEK-293 (human embryonic kidney) cells and normal human parathyroid cells. Furthermore, on a background of a physiological fasting serum L-amino acid mixture, they induce a small, but physiologically significant, enhancement of Ca2+o-dependent suppression of PTH (parathyroid hormone) secretion. The impact of amino acids on CaR-stimulated ERK1/2, however, has not been determined. In the present study, we examined the effects of L-amino acids on Ca2+o-stimulated ERK1/2 phosphorylation as determined by Western blotting and a newly developed quantitative assay (SureFire). L-Amino acids induced a small, but significant, enhancement of Ca2+o-stimulated ERK1/2. In CaR-expressing HEK-293 cells, 10 mM L-phenylalanine lowered the EC50 for Ca2+o from approx. 2.3 to 2.0 mM in the Western blot assay and from 3.4 to 2.9 mM in the SureFire assay. The effect was stereoselective (L>D), and another aromatic amino acid, L-tryptophan, was also effective. The effects of amino acids were investigated further in HEK-293 cells that expressed the CaR mutant S169T. L-Phenylalanine normalized the EC50 for Ca2+o-stimulated Ca2+i mobilization from approx. 12 mM to 5.0 mM and ERK1/2 phosphorylation from approx. 4.6 mM to 2.6 mM. Taken together, the data indicate that L-phenylalanine and other amino acids enhance the Ca2+o-sensitivity of CaR-stimulated ERK1/2 phosphorylation; however, the effect is comparatively small and operates in the form of a fine-tuning mechanism.
Purification and identification of the STAT5 protease in myeloid cells
Authors: Schuster Björn , Hendry Lisa , Byers Helen , Lynham Steven F. , Ward Malcolm A. , John Susan
Published: April 2007

STAT (signal transducer and activator of transcription) proteins are critical regulators of cytokine-induced cell proliferation, differentiation and survival. STAT functional activity can be variably regulated by post-translational modifications, including phosphorylation, acetylation, methylation and sumoylation. Additionally, limited proteolytic digestion of full-length STAT proteins (STATα) generates C-terminally truncated forms (STATγ) in different cell lineages, which have significantly reduced transcriptional activity due to the lack of the transactivation domain. Previously, it has been shown that STAT5γ, generated by an unidentified nuclear serine protease, plays an important role in myeloid cell differentiation and is aberrantly expressed in acute myeloid leukaemia. To better understand this regulatory mechanism for STAT5 function, we have purified the STAT5 protease from the immature myeloid cell line 32D and identified it by MS analysis as the granule-derived serine protease, CatG (cathepsin G). We show that purified CatG can specifically cleave full-length STAT5 to generate STAT5γ, and this activity can be inhibited by AEBSF [4-(2-aminoethyl)benzenesulfonyl fluoride] in an in vitro protease assay. Importantly, preparation of nuclear and cytoplasmic extracts from immature myeloid cell lines, 32D and FDC-P1, in the presence of a specific inhibitor for CatG results in the identification of STAT5α only. These studies indicate that nuclear STAT5γ does not naturally exist in immature myeloid cells and is artificially generated from STAT5α during the preparation of extracts due to the abundance of CatG in these cells. Therefore in contrast with earlier studies, our data suggest that STAT5α, rather than STAT5γ is the active form in immature myeloid cells.
Regulation of brain anandamide by acute administration of ethanol
Authors: Ferrer Belen , Bermúdez-Silva Francisco Javier , Bilbao Ainhoa , Alvarez-Jaimes Lily , Sanchez-Vera Irene , Giuffrida Andrea , Serrano Antonia , Baixeras Elena , Khaturia Satishe , Navarro Miguel , Parsons Loren H. , Piomelli Daniele , Rodríguez de Fonseca Fernando
Published: April 2007

The endogenous cannabinoid acylethanolamide AEA (arachidonoylethanolamide; also known as anandamide) participates in the neuroadaptations associated with chronic ethanol exposure. However, no studies have described the acute actions of ethanol on AEA production and degradation. In the present study, we investigated the time course of the effects of the intraperitoneal administration of ethanol (4 g/kg of body mass) on the endogenous levels of AEA in central and peripheral tissues. Acute ethanol administration decreased AEA in the cerebellum, the hippocampus and the nucleus accumbens of the ventral striatum, as well as in plasma and adipose tissue. Parallel decreases of a second acylethanolamide, PEA (palmitoylethanolamide), were observed in the brain. Effects were observed 45–90 min after ethanol administration. In vivo studies revealed that AEA decreases were associated with a remarkable inhibition of the release of both anandamide and glutamate in the nucleus accumbens. There were no changes in the expression and enzymatic activity of the main enzyme that degrades AEA, the fatty acid amidohydrolase. Acute ethanol administration did not change either the activity of N-acyltransferase, the enzyme that catalyses the synthesis of the AEA precursor, or the expression of NAPE-PLD (N-acylphosphatidylethanolamine-hydrolysing phospholipase D), the enzyme that releases AEA from membrane phospholipid precursors. These results suggest that receptor-mediated release of acylethanolamide is inhibited by the acute administration of ethanol, and that this effect is not derived from increased fatty acid ethanolamide degradation.
Mapping binding sites for the PDE4D5 cAMP-specific phosphodiesterase to the N- and C-domains of β-arrestin using spot-immobilized peptide arrays
Authors: Baillie George S. , Adams David R. , Bhari Narinder , Houslay Thomas M. , Vadrevu Suryakiran , Meng Dong , Li Xiang , Dunlop Allan , Milligan Graeme , Bolger Graeme B. , Klussmann Enno , Houslay Miles D.
Published: April 2007

β2-ARs (β2-adrenoceptors) become desensitized rapidly upon recruitment of cytosolic β-arrestin. PDE4D5 (family 4 cAMP-specific phosphodiesterase, subfamily D, isoform 5) can be recruited in complex with β-arrestin, whereupon it regulates PKA (cAMP-dependent protein kinase) phosphorylation of the β2-AR. In the present study, we have used novel technology, employing a library of overlapping peptides (25-mers) immobilized on cellulose membranes that scan the entire sequence of β-arrestin 2, to define the interaction sites on β-arrestin 2 for binding of PDE4D5 and the cognate long isoform, PDE4D3. We have identified a binding site in the β-arrestin 2 N-domain for the common PDE4D catalytic unit and two regions in the β-arrestin 2 C-domain that confer specificity for PDE4D5 binding. Alanine-scanning peptide array analysis of the N-domain binding region identified severely reduced interaction with PDE4D5 upon R26A substitution, and reduced interaction upon either K18A or T20A substitution. Similar analysis of the β-arrestin 2 C-domain identified Arg286 and Asp291, together with the Leu215–His220 region, as being important for binding PDE4D5, but not PDE4D3. Transfection with wild-type β-arrestin 2 profoundly decreased isoprenaline-stimulated PKA phosphorylation of the β2-AR in MEFs (mouse embryo fibroblasts) lacking both β-arrestin 1 and β-arrestin 2. This effect was negated using either the R26A or the R286A mutant form of β-arrestin 2 or a mutant with substitution of an alanine cassette for Leu215–His220, which showed little or no PDE4D5 binding, but was still recruited to the β2-AR upon isoprenaline challenge. These data show that the interaction of PDE4D5 with both the N- and C-domains of β-arrestin 2 are essential for β2-AR regulation.
Selenophosphate synthetase 2 is essential for selenoprotein biosynthesis
Authors: Xu Xue-Ming , Carlson Bradley A. , Irons Robert , Mix Heiko , Zhong Nianxin , Gladyshev Vadim N. , Hatfield Dolph L.
Published: April 2007

Selenophosphate synthetase (SelD) generates the selenium donor for selenocysteine biosynthesis in eubacteria. One homologue of SelD in eukaryotes is SPS1 (selenophosphate synthetase 1) and a second one, SPS2, was identified as a selenoprotein in mammals. Earlier in vitro studies showed SPS2, but not SPS1, synthesized selenophosphate from selenide, whereas SPS1 may utilize a different substrate. The roles of these enzymes in selenoprotein synthesis in vivo remain unknown. To address their function in vivo, we knocked down SPS2 in NIH3T3 cells using small interfering RNA and found that selenoprotein biosynthesis was severely impaired, whereas knockdown of SPS1 had no effect. Transfection of SPS2 into SPS2 knockdown cells restored selenoprotein biosynthesis, but SPS1 did not, indicating that SPS1 cannot complement SPS2 function. These in vivo studies indicate that SPS2 is essential for generating the selenium donor for selenocysteine biosynthesis in mammals, whereas SPS1 probably has a more specialized, non-essential role in selenoprotein metabolism.
Nucleic acids potentiate Factor VII-activating protease (FSAP)-mediated cleavage of platelet-derived growth factor-BB and inhibition of vascular smooth muscle cell proliferation
Authors: Shibamiya Aya , Muhl Lars , Tannert-Otto Susanne , Preissner Klaus T. , Kanse Sandip M.
Published: April 2007

FSAP (Factor VII-activating protease) can cleave and inactivate PDGF-BB (platelet-derived growth factor-BB) and thereby inhibits VSMC (vascular smooth-muscle cell) proliferation. The auto-activation of FSAP is facilitated by negatively charged polyanions such as heparin, dextransulfate or extracellular ribonucleic acids. Since auto-activation is essential for the anti-proliferative function of FSAP, the influence of nucleic acids as cofactors for the FSAP-mediated inhibition of PDGF-BB was investigated. Natural or artificial RNA was an effective cofactor for FSAP mediated PDGF-BB degradation, whereas the effect of DNA was weak. RNA-induced cleavage of PDGF-BB was inhibited by serine protease inhibitors. The pattern of PDGF-BB cleavage was identical with either heparin or RNA as a cofactor. One of the cleavage sites in PDGF-BB was at the positions 160–162 (R160KK162), which is an important region for receptor binding and activation. In VSMCs, PDGF-BB-stimulated DNA synthesis was inhibited by FSAP in the presence of RNA. RNA was more effective than DNA and the cofactor activity of RNA was neutralized after pretreatment with RNase. FSAP binding to RNA protected the nucleic acid from degradation by RNase. These data are relevant to situations where extracellular nucleic acids released from necrotic or apoptotic cells could activate local FSAP, leading to inhibition of PDGF-BB.
Deuterium gas-puff Z-pinch implosions on the Z accelerator
Authors: Coverdale C. A. , Deeney C. , Velikovich A. L. , Davis J. , Clark R. W. , Chong Y. K. , Chittenden J. , Chantrenne S. , Ruiz C. L. , Cooper G. W. , Nelson A. J. , Franklin J. , LePell P. D. , Apruzese J. P. , Levine J. , Banister J.
Published: April 2007

Experiments on the Z accelerator with deuterium gas-puff implosions have produced up to 3.7×1013 (±20%) neutrons at 2.34MeV (±0.10MeV). Although the mechanism for generating these neutrons was not definitively identified, this neutron output is 100 times more than previously observed from neutron-producing experiments at Z. Dopant gases in the deuterium (argon and chlorine) were used to study implosion characteristics and stagnated plasma conditions through x-ray yield measurements and spectroscopy. Magnetohydrodynamic (MHD) calculations have suggested that the dopants improved the neutron output through better plasma compression, which has been studied in experiments increasing the dopant fraction. Scaling these experiments, and additional MHD calculations, suggest that ∼5×1014 deuterium-deuterium (DD) neutrons could be generated at the 26-MA refurbished Z facility.
Bulk FePt∕Fe3Pt nanocomposite magnets prepared by spark plasma sintering
Authors: Rong Chuan-Bing , Nandwana Vikas , Poudyal Narayan , Liu J. Ping , Saito Tetsuji , Wu Yaqiao , Kramer Matthew J.
Published: April 2007

Fe Pt ∕ Fe 3 Pt bulk nanocomposite magnets have been prepared by the spark plasma sintering of chemically synthesized FePt and Fe3O4 nanoparticles under 100MPa at 400–600°C. It was found that the phase transition of the FePt compound from the face centered cubic to the L10 started at 500°C and was almost completed at 600°C during the sintering. The activated atomic diffusion during the phase transition led to a significant increase in density. The density about 70% of the theoretical value has been obtained. Grain size of the sintered samples remains nanoscale even after a postannealing at 700°C. Pressure rather than temperature was more effective in enhancing intergrain exchange coupling compared to temperature.
Thermal stability, sensitivity, and noise characteristics of MgO-based magnetic tunnel junctions (invited)
Authors: Mazumdar Dipanjan , Liu Xiaoyong , Schrag B. D. , Shen Weifeng , Carter Matthew , Xiao Gang
Published: April 2007

Thermal stability, sensitivity, and noise of micron-scale magnetic tunnel junctions based on MgO tunnel barriers have been studied for both the memory and sensing configurations. Junctions show solid high-temperature performance with substantial magnetoresistance observed even at 500°C. At temperatures above 375°C, the junctions begin to experience irreversible degradation due to interlayer diffusion. The thermal stability of these devices depends strongly on the exchange bias of the device and hence on the properties of the antiferromagnetic layer. Sensitivities as high as 3.3%∕Oe have been obtained at room temperature for junctions configured as low-field sensors. Sensitivity values are constant up to temperatures of 300°C, above which performance decays due to a loss of exchange bias and overall magnetoresistance. Noise spectra are 1∕f at frequencies up to 51kHz, and sensors have a resultant field noise better than 1nT∕Hz0.5 at 100kHz. A comparison is made with devices fabricated with alumina tunnel barriers.
Systematic tuning of magnetization reversal in Permalloy nanowires using sloped ends
Authors: Petracic O. , Read D. E. , Cowburn R. P.
Published: April 2007

The magnetization reversal of Permalloy (Ni81Fe19) nanowires has been investigated by magneto-optic Kerr effect (MOKE) magnetometry, where one end of the wire exhibits a slope in the thickness. Straight nanowires with a thickness of 7.5nm, widths of 150nm, and length of 100μm were prepared by electron-beam lithography. The sloped ends were achieved by using a penumbra shadow mask during NiFe deposition. The topography of the wires has been studied by atomic force microscopy. One finds that the slope profile can be tuned by the position under the mask, mask-to-sample distance, and angle of deposition. Corresponding MOKE hysteresis loops show a systematic reduction of the coercive field with increasing length of the sloped part. For example, wires where the slope has a length of 45μm exhibit a coercive field of 11Oe, whereas nanowires without sloped ends show 107Oe.
Experimental study of the formation and scaling of a round synthetic jet
Authors: Shuster Jennifer M. , Smith Douglas R.
Published: April 2007

The flow field from a piston-cylinder synthetic jet actuator was investigated in detail over a range of dimensionless stroke values, L∘∕D∘, and Reynolds numbers, ReU∘. In each of the test flows examined, only one of these dimensionless groups was varied. The flow fields were examined using particle image velocimetry measurements in a plane bisecting the jet. A slug flow model was used to determine scaling parameters for the jet flow field. In the near-field of the orifice, the flow was dominated by the vortex ring formed during the expulsion phase of the actuator cycle, and the flow field scaled exclusively on the actuator stroke, L∘. For distances from the orifice greater than L∘, the flow field resembled a conventional, round turbulent jet. The resemblance was not complete as the synthetic jet had a faster spreading rate with a correspondingly more rapid decline in the mean centerline velocity. The dimensionless jet momentum was comparable at the higher stroke values for the same Reynolds numbers, and the jet momentum increased with Reynolds number. It is apparent from this work that the slug flow model provides suitable scaling parameters for the near- and far-field regions of a synthetic jet flow. Moreover, the study conclusively demonstrates that the dimensionless stroke, L∘∕D∘, uniquely identifies a synthetic jet flow.
Wave induced barrier transparency and melting of quasi-crystalline structures in two dimensional plasma turbulence
Authors: Das Amita
Published: April 2007

The conservation of energy and enstrophy in two dimensional inviscid hydrodynamics leads to dual cascade behavior. The energy cascades towards long scales and the enstrophy is transferred to shorter scales. The interplay of these dynamical processes leads to self organization and formation of coherent patterns in the two dimensional hydrodynamic turbulence. It was shown by Kukharkin et al. [Phys. Rev. Lett. 25, 2486 (1995)] that this process of self organization occurs in an even more interesting fashion in the Hasegawa Mima (HM) equation [Phys. Fluids 21, 21 (1978)] This equation is a generalization of the two dimensional Navier Stokes hydrodynamics model in which there is a characteristic natural scale in the system (e.g., Larmor radius in the drift wave context). Kukharkin et al. observed that this scale acts as a barrier in the energy cascade, such that the cascade rate at the longer wavelength side of the barrier is smaller. This work has also shown that the accumulation of energy around the intrinsic scale leads to the formation of quasi-crystalline patterns. In the present paper it has been demonstrated that the presence of wave excitations leads to an increased cascade towards longer scales past the natural length scale barrier. It has also been demonstrated that wave excitations lead to the melting of quasi-crystalline structures. Another intriguing but interesting observation is that even though the faster cascade is induced by waves arising through an anisotropic inhomogeneity in one of the plasma parameters, the spectrum of the fluctuations continues to remain predominantly isotropic. A physical understanding of the observations is provided by illustrating a close connection between the Kelvin–Helmholtz destabilization of shear flows and the phenomenon of inverse cascade in 2D fluid flows.
Quantum dust-acoustic double layers
Authors: Moslem W. M. , Shukla P. K. , Ali S. , Schlickeiser R.
Published: April 2007

The quantum dust-acoustic double layers (QDADLs) are studied in an unmagnetized, collisionless quantum dusty plasma whose constituents are the electrons, ions, and negatively/positively charged dust particles. By employing the quantum hydrodynamical equations and the reductive perturbation technique, a quantum extended Korteweg–de Vries equation is derived. A steady-state double-layer solution of the latter is presented by taking into account the quantum-mechanical effects. It is numerically found that both compressive and rarefactive QDADLs can exist only for positive charged dust particles under the condition ni0∕ne0<1, where ni0(ne0) is the unperturbed number density of the ions (electrons). It is further noted that the formation of the compressive and the rarefactive double layers depends on the quantum plasma parameters. The relevance of the present investigation to the dust charge impurities in laser-solid interactions is discussed. In general, this study should be useful for the diagnostics of charged dust impurities in ultrasmall microelectronic and nanoelectronic components, as well as in astrophysical objects where charged dust particles are inherently present.
A regime diagram for premixed flame kernel-vortex interactions
Authors: Echekki Tarek , Kolera-Gokula Hemanth
Published: April 2007

Direct numerical simulations of flame kernel-vortex interactions are implemented in an axisymmetric configuration using a two-step global mechanism to study the different combustion regimes of the interactions. Four combustion regimes have been identified. They include: (1) the “laminar kernel” regime, (2) the “wrinkled kernel” regime, (3) the “breakthrough” regime, and (4) the “global extinction” regime. Transitions from different regimes are achieved through variations of the vortex strength, and operation in each regime is governed by two key parameters, the ratio of the vortex translational velocity to the laminar flame speed and the ratio of the kernel size to the vortex size at the onset of the interactions. Qualitative and quantitative comparisons between the flame responses in the different regimes are presented. A regime diagram is constructed based on the key parameters that control transition between the different regimes. The diagram bears some similarities with other diagrams based on planar flame-vortex interactions. However, it also offers additional features that constitute refinements to the existing diagrams of which the role of interaction of a vortex with an already curved flame is important.
Gyrokinetic theory and simulation of mirror instability
Authors: Qu Hongpeng , Lin Zhihong , Chen Liu
Published: April 2007

The finite Larmor radius (FLR) effects play an important role in determining the threshold and the growth rate of the mirror instability. In this study, a general dispersion relation of the mirror mode with FLR effects is derived by using gyrokinetic theory. It shows that both the FLR effects and the coupling to the slow sound wave are stabilizing. A gyrokinetic particle simulation code has been developed for simulation of compressible magnetic turbulence driven by the mirror instability. Results of the linear simulation of mirror mode agree well with the analytic dispersion relation.
Enhancement of sensitivity in gas chemiresistors based on carbon nanotube surface functionalized with noble metal (Au, Pt) nanoclusters
Authors: Penza M. , Cassano G. , Rossi R. , Alvisi M. , Rizzo A. , Signore M. A. , Dikonimos Th. , Serra E. , Giorgi R.
Published: April 2007

Multiwalled carbon nanotube (MWCNT) films have been fabricated by using plasma-enhanced chemical vapor deposition system onto Cr–Au patterned alumina substrates, provided with 3nm thick Fe growth catalyst, for NO2 and NH3 gas sensing applications, at sensor temperature in the range of 100–250°C. Nanoclusters of noble metal surface catalysts (Au and Pt) have been sputtered on the surface of MWCNTs to enhance the gas sensitivity with respect to unfunctionalized carbon nanotube films. It was found that the gas sensitivity of Pt- and Au-functionalized MWCNT gas sensors significantly improved by a factor up to an order of magnitude through a spillover effect for NH3 and NO2 gas detections, respectively. The metal-functionalized MWCNT sensors exhibit very high gas sensitivity, fast response, reversibility, good repeatability, and sub-ppm range detection limit with the sensing properties of the MWCNT films tailored by surface catalyst used to functionalize the MWCNT sensors.
On the behavior of ultraintense laser produced hot electrons in self-excited fields
Authors: Yabuuchi T. , Adumi K. , Habara H. , Kodama R. , Kondo K. , Tanimoto T. , Tanaka K. A. , Sentoku Y. , Matsuoka T. , Chen Z. L. , Tampo M. , Lei A. L. , Mima K.
Published: April 2007

A large number of hot electrons exceeding the Alfvén current can be produced when an ultraintense laser pulse irradiates a solid target. Self-excited extreme electrostatic and magnetic fields at the target rear could influence the electron trajectory. In order to investigate the influence, we measure the hot electrons when a plasma was created on the target rear surface in advance and observe an increase of the electron number by a factor of 2. This increase may be due to changes in the electrostatic potential formation process with the rear plasma. Using a one-dimensional particle-in-cell simulation, it is shown that the retardation in the electrostatic potential formation lengthens the gate time when electrons can escape from the target. The electron number escaping within the lengthened time window appears to be much smaller than the net produced number and is consistent with our estimation using the Alfvén limit.
Heart-shaped bubbles rising in anisotropic liquids
Authors: Zhou Chunfeng , Yue Pengtao , Feng James J. , Liu Chun , Shen Jie
Published: April 2007

This Letter reports on numerical simulations motivated by experimental observations of an unusual inverted-heart shape for bubbles rising in an anisotropic micellar solution. We explain the bubble shape by assuming that the micelles are aligned into a nematic phase, whose anchoring energy on the bubble competes with the interfacial tension and the bulk elasticity of the nematic to modify the interfacial curvature. Numerical results show that bubbles with sufficiently strong planar anchoring rising in a vertically aligned nematic indeed assume the observed shape. The parameter values required are compared with the experimental materials and conditions.
Is plane-channel flow a friendly case for the testing of large-eddy simulation subgrid-scale models?
Authors: Meyers Johan , Sagaut Pierre
Published: April 2007

We present the grid-convergence behavior of channel-flow direct numerical simulations (DNS) at coarse resolutions typically encountered in large-eddy simulation subgrid-model testing. An energy-conservative discretization method is used to systematically vary the streamwise (Nx) and spanwise (Nz) resolution. We observe that the skin friction does not converge monotonously, and at coarse resolutions, a line of Nx–Nz combinations is found where the error on the skinfriction is zero. Along this line, mean profiles are evaluated and found to fit surprisingly well fully resolved DNS results. The location of this line is shown to depend on the Reynolds number and the wall-normal resolution.
Self-similar two-particle separation model
Authors: Lüthi Beat , Berg Jacob , Ott Søren , Mann Jakob
Published: April 2007

We present a new stochastic model for relative two-particle separation in turbulence. Inspired by material line stretching, we suggest that a similar process also occurs beyond the viscous range, with time scaling according to the longitudinal second-order structure function S2(r), e.g.; in the inertial range as ε−1∕3r2∕3. Particle separation is modeled as a Gaussian process without invoking information of Eulerian acceleration statistics or of precise shapes of Eulerian velocity distribution functions. The time scale is a function of S2(r) and thus of the Lagrangian evolving separation. The model predictions agree with numerical and experimental results for various initial particle separations. We present model results for fixed time and fixed scale statistics. We find that for the Richardson-Obukhov law, i.e., ⟨r(t)2⟩=gεt3, to hold and to also be observed in experiments, high Reynolds numbers are necessary, i.e., Reλ>O(1000), and the integral scale needs to be large compared to initial separation, i.e., L∕r0>30 and d∕L>3 need to be fulfilled, where d is the size of the field of view. Removing the constraint of finite inertial range, the model is used to explore separation dynamics in the asymptotic regime. As Reλ→∞, the distance neighbor function takes on a constant shape, almost as predicted by the Richardson diffusion equation. For the Richardson constant we obtain that g→0.95 as Reλ→∞. This asymptotic limit is reached at Reλ>1000. For the Richardson constant g, the model predicts a ratio of gb∕gf≈1.9 between backwards and forwards dispersion.
Spin-charge separation, tunneling, and spin transport in one-dimensional metals
Authors: Halperin Bertrand I.
Published: April 2007

Electron-electron interactions have strong effects on the low-energy excitations of a one-dimensional metal. Luttinger liquid theory, which is supposed to describe this situation, predicts, among other things, that an injected electron will split into separate charge and spin excitations, which propagate at different velocities. We shall review some experiments and theoretical analyses where spin-charge separation can have manifest consequences, including discussion of the spin-incoherent regime, which can occur in low-density electron systems, when the temperature is low compared to the energy scale for charge excitations, but larger than the spin-exchange energy for neighboring electrons.
Collective state of the Bose gas of interacting dipolar excitons
Authors: Timofeev V. B. , Gorbunov A. V.
Published: April 2007

Experiments connected with dipolar exciton Bose condensation in ring lateral traps are presented. The long-range spatial coherence of condensate along the sample surface is observed and discussed for the first time.
On the ultrastrong vacuum Rabi coupling of an intersubband transition in a semiconductor microcavity
Authors: Ciuti Cristiano , Carusotto Iacopo
Published: April 2007

In this invited paper of the 28th International Conference of the Physics of Semiconductors (ICPS-28), we discuss the peculiar quantum electrodynamical properties of a semiconductor microcavity system, in which a cavity photon mode is strongly coupled to an intersubband transition of a doped quantum well system. In this kind of semiconductor system, it is possible to achieve an unprecedented ultrastrong coupling regime, in which the vacuum Rabi frequency is comparable to the electronic transition frequency. We discuss the anomalous quantum properties of the quantum ground state (a squeezed vacuum) and of the intersubband cavity polariton excitations. We address the role of dissipation and point out some future perspectives concerning the investigation of quantum vacuum radiation effects induced by an ultrafast time modulation of the quantum vacuum.
Entangled photons from the biexciton cascade of quantum dots
Authors: Young R. J. , Stevenson R. M. , Shields A. J. , Atkinson P. , Cooper K. , Ritchie D. A.
Published: April 2007

Single InAs quantum dots embedded in a planar cavity, formed by mismatched sets of GaAs/AlAs distributed Bragg reflectors, can be a useful source of triggered polarization-entangled photon pairs. We demonstrate this with a fidelity exceeding 70% for the expected entangled state. Quantum dot based devices may therefore be of great use in quantum communications and quantum information processing.
Correlated and entangled pairs of single photons from semiconductor quantum dots
Authors: Akopian N. , Lindner N. H. , Poem E. , Berlatzky Y. , Avron J. , Gershoni D. , Gerardot B. D. , Petroff P. M.
Published: April 2007

Entangled photon pairs are emitted from a biexciton decay cascade of single quantum dots when spectral filtering is applied. We show this by experimentally measuring the density matrix of the polarization state of the photon pair emitted from a continuously pumped quantum dot. The matrix clearly satisfies the Peres criterion for entanglement. By applying in addition a temporal window, the quantum dot becomes an entangled light source.
Spin properties of charged Mn-doped quantum dot
Authors: Besombes L. , Léger Y. , Maingault L. , Mariette H.
Published: April 2007

The optical properties of individual quantum dots doped with a single Mn atom and charged with a single carrier are analyzed. The emission of the neutral, negatively and positively charged excitons coupled with a single magnetic atom (Mn) are observed in the same individual quantum dot. The spectrum of the charged excitons in interaction with the Mn atom shows a rich pattern attributed to a strong anisotropy of the hole-Mn exchange interaction slightly perturbed by a small valence-band mixing. The anisotropy in the exchange interaction between a single magnetic atom and a single hole is revealed by comparing the emission of a charged Mn-doped quantum dot in longitudinal and transverse magnetic field.
Quantum versus classical hyperfine-induced dynamics in a quantum dot
Authors: Coish W. A. , Loss Daniel , Yuzbashyan E. A. , Altshuler B. L.
Published: April 2007

In this article we analyze spin dynamics for electrons confined to semiconductor quantum dots due to the contact hyperfine interaction. We compare mean-field (classical) evolution of an electron spin in the presence of a nuclear field with the exact quantum evolution for the special case of uniform hyperfine coupling constants. We find that (in this special case) the zero-magnetic-field dynamics due to the mean-field approximation and quantum evolution are similar. However, in a finite magnetic field, the quantum and classical solutions agree only up to a certain time scale t<τc, after which they differ markedly.
Optically manipulating spins in semiconductor quantum dots
Authors: Yao Wang , Liu Ren-Bao , Sham L. J.
Published: April 2007

Physics considered here is the active control of a quantum system and of its decoherence by its environment. The relevance is in the quantum nature of nanoscience and how coherent optics in semiconductor quantum dots can contribute to quantum control. This article reviews: (1) The more recent theory of control of a set of dot spins through cavity quantum electrodynamics and (2) the quantum basis for control of decoherence of the electron spin interacting with the nuclei in the quantum dot.
Quantum ballistic phenomena in nanostructures of paraelectric PbTe
Authors: Grabecki G.
Published: April 2007

This article reviews recent developments in the physics of lead telluride nanostructures. PbTe is a IV–VI narrow gap paraelectric semiconductor, characterized by the huge static dielectric constant ε>1000 at helium temperatures. Methods of gating and nanostructure fabrication of modulation doped PbTe∕Pb1−xEuxTe quantum wells are described. Specific properties of PbTe, make it possible to put into evidences a number of interesting effects in the quantum ballistic regime, not encountered in standard semiconductor nanostructures. It is found that conductance of narrow constrictions shows precise zero-field quantization in 2e2∕h units, despite a significant amount of charged defects in the vicinity of the conducting channel. This unusual result is a consequence of a strong suppression of Coulomb potential fluctuations in PbTe, an effect confirmed by numerical simulations. Furthermore, orbital degeneracy of electron waveguide modes can be controlled by the width of PbTe∕Pb1−xEuxTe quantum wells, so that unusual sequences of plateau conductance are observed. Finally, because of a rather large Zeeman splitting in PbTe, significant spin splitting of the conductance plateaus is observed already in the magnetic fields of about 0.2 T. It is shown that PbTe quantum wires act as efficient local spin-filters, in which the spin polarized current is carried by a large number of wire modes.
Quantum dots with coherent interfaces between rocksalt-PbTe and zincblende-CdTe
Authors: Heiss W. , Groiss H. , Kaufmann E. , Hesser G. , Böberl M. , Springholz G. , Schäfﬂer F. , Leitsmann R. , Bechstedt F. , Koike K. , Harada H. , Yano M.
Published: April 2007

The formation of PbTe quantum dots (QDs) in a crystalline CdTe host matrix is demonstrated by the annealing of a coherent, heteroepitaxial PbTe layer clad between CdTe layers. The resulting QDs have a centrosymmetric shape and they exhibit intense room-temperature mid-infrared photoluminescence due to an electron-hole pair recombination in the narrow-gap PbTe. The intense luminescence approves the high quality of the QD interfaces, between the sixfold coordinated rocksalt structure of PbTe and the fourfold coordinated zincblende structure of CdTe. To gain further insight into the structural interface properties, we compare quantitatively multislice simulations of HRTEM images with first-principles total-energy calculations in the repeated-slab approximation. The most drastic effect occurs at the electrostatically neutral (110) interface, where we find a lateral spatial offset between the two crystal halves due to rebonding across the interface. For the two polar (001) interfaces, significantly different lattice plane spacings are observed, depending on whether the polar CdTe (001) face is cation or anion-terminated. The agreement between the first-principles calculations and the HRTEM data is excellent.
Transition-metal silicides as materials for magnet-semiconductor heterostructures
Authors: Kratzer Peter , Hashemifar S. Javad , Wu Hua , Hortamani Mahbube , Scheffler Matthias
Published: April 2007

The injection of a spin-polarized current into a semiconductor, one of the key requirements for spintronics, poses the challenge for computational materials science to possibly screen suitable materials. In a number of theoretical investigations, we have put forward magnetic intermetallic compounds grown epitaxially on Si as promising candidates. We employed density functional theory calculations with the GGA-PBE exchange-correlation functional and the full-potential augmented plane wave plus local orbital (FP-APW+lo) method, as implemented in the WIEN2k package. In the spirit of computational materials science, we investigated the stability and magnetic properties of thin films of the Heusler alloy Co2MnSi, as well as of binary late transition metal monosilicides, in contact with the Si surface. For the Heusler alloy Co2MnSi, we could show that the (001) surface retains the half-metallic character of the bulk if a fully Mn-terminated surface is prepared. At interfaces with Si, a finite density of states at the Fermi energy was found for both spin channels, but the half-metallic behavior recovers only a few layers away from the interface. For the monosilicides of the late 3d-transition metals (Mn, Fe, Co, Ni), we predict a CsCl-like structure that has not yet been observed as bulk compound but may be stabilized epitaxially on Si(001). For very thin films of CoSi and MnSi grown in this structure, our calculations find a ferromagnetic ground state. Recently, we identified the atomic structure of MnSi films on Si(111) which is close to the natural crystal structure of bulk MnSi (B20), and also shows large magnetic moments of the Mn atoms at the surface and interface. All MnSi films have a high degree of spin polarization (between 30% and 50%, depending on film thickness) at the Fermi level, and are thus promising materials for fabricating electrical contacts for spin injection into Si.
Multi-wavelength operation and vertical emission in THz quantum-cascade lasers
Authors: Scalari Giacomo , Sirigu Lorenzo , Terazzi Romain , Walther Christoph , Amanti Maria I. , Giovannini Marcella , Hoyler Nicolas , Faist Jérôme , Sadowski Marcin L. , Beere Harvey , Ritchie David , Dunbar L. Andrea , Houdré Romuald
Published: April 2007

Multi-wavelength laser action in the terahertz (THz) region from a quantum cascade structure is demonstrated. Laser emission is obtained at 1.39 and 2.3 THz by using a structure based on a large single quantum well. A strong perpendicular magnetic field is employed to increase the gain and achieve laser action. In the second part of the work, a vertically emitting THz quantum cascade laser device that exploits an in-plane optical resonator based on a two-dimensional photonic crystal is demonstrated. Stable single mode vertical emission is reported. Simulations based on the block-iterative frequency-domain method on a plane wave basis account for the observed results.
Electro-optically tunable second-harmonic-generation gratings in ion-exfoliated thin films of periodically poled lithium niobate
Authors: Djukic Djordje , Cerda-Pons Guiem , Roth Ryan M. , Osgood Richard M. , Bakhru Sasha , Bakhru Hassaram
Published: April 2007

The authors demonstrate that thin, single-crystal films fabricated by ion exfoliation of a bulk periodically poled LiNbO3 (PPLN) crystal reduced the tuning voltages for electro-optically tunable harmonic generation in PPLN devices. The tuning voltage of ±150V resulted in 0.5nm total tuning of the second harmonic signal in the authors’ 10-μm-thick device, compared to an order of magnitude higher tuning voltage value needed in a comparable bulk device.
Atomic arrangement variations of [0001]-tilt grain boundaries in ZnO thin films grown on p-Si substrates due to thermal treatment
Authors: Shin J. W. , Lee J. Y. , No Y. S. , Jung J. H. , Kim T. W. , Choi W. K.
Published: April 2007

The plane-view high-resolution transmission electron microscopy (HRTEM) images in ZnO thin films grown on p-Si substrates showed that (101¯0) asymmetric grain boundaries with a periodic array of strain contrast features existed in a sparse columnar structure for as-grown ZnO thin films and that (112¯0) asymmetric grain boundaries and (8513¯0) symmetric grain boundaries existed in a dense columnar structure for annealed ZnO thin films. The atomic arrangement variations of [0001]-tilt grain boundaries in ZnO thin films grown on Si substrates due to thermal treatment are described on the basis of the HRTEM results.
Surface plasmon resonance of gold nano-sea-urchin
Authors: Su Yen Hsun , Lai Wei Hao , Chen Wei-Yu , Hon Min Hsiung , Chang Shih-Hui
Published: April 2007

The authors synthesized high-quality gold nano-sea-urchin in aqueous solution with an environment-friendly method. They found that the gold nano-sea-urchin can induce the interaction of surface plasmon resonance (SPR) mode with substrate. The SPR peak splits and blueshifts from 630to440nm and the result has potential application for enhanced-Raman scattering, optical communications, and solar cells.
Damage threshold of inorganic solids under free-electron-laser irradiation at 32.5nm wavelength
Authors: Hau-Riege S. P. , London R. A. , Bionta R. M. , McKernan M. A. , Baker S. L. , Krzywinski J. , Sobierajski R. , Nietubyc R. , Pelka J. B. , Jurek M. , Juha L. , Chalupský J. , Cihelka J. , Hájková V. , Velyhan A. , Krása J. , Kuba J. , Tiedtke K. , Toleikis S. , Tschentscher Th. , Wabnitz H. , Bergh M. , Caleman C. , Sokolowski-Tinten K. , Stojanovic N. , Zastrau U.
Published: April 2007

Samples of B4C, amorphous C, chemical-vapor-deposition-diamond C, Si, and SiC were exposed to single 25fs long pulses of 32.5nm free-electron-laser radiation at fluences of up to 2.2J∕cm2. The samples were chosen as candidate materials for x-ray free-electron-laser optics. It was found that the threshold for surface damage is on the order of the fluence required for thermal melting. For larger fluences, the crater depths correspond to temperatures on the order of the critical temperature, suggesting that the craters are formed by two-phase vaporization.
Influence of molecular structure and microstructure on device performance of polycrystalline pentacene thin-film transistors
Authors: Cheng Horng-Long , Mai Yu-Shen , Chou Wei-Yang , Chang Li-Ren
Published: April 2007

The authors have fabricated the pentacene thin films on polymethylmethacrylate (PMMA) and on silicon dioxide dielectric surfaces featuring similar surface energy and surface roughness. On both surfaces the pentacene films displayed high crystal quality from x-ray diffraction scans, although the film on PMMA had significantly smaller grain size. The pentacene transistors with PMMA exhibited excellent electrical characteristics, including high mobility of above 1.1cm2∕Vs, on/off ratio above 106, and sharp subthreshold slope below 1V∕decade. The analysis of molecular microstructure of the pentacene films provided a reasonable explanation for the high performance using resonance micro-Raman spectroscopy.
Improvement in charge retention in Au-nanocrystal-based memory structures by employing (Ba0.5Sr0.5)TiO3 as control oxide
Authors: Wang Chen-Chan , Liang Chun-Sheng , Tseng Jiun-Yi , Wu Tai-Bor
Published: April 2007

The use of (Ba0.5Sr0.5)TiO3 (BST) as the control oxide in a Au-nanocrystal-based memory structure was investigated. The Au nanocrystals, surrounded with a core shell of Au-doped BST, were self-assembled in a BST∕SiO2 stack. The leakage of the gate dielectric is reduced by a partial dissolution of Au into the BST matrix, which enlarges the energy band gap of BST. Moreover, direct tunneling through the tunnel oxide is suppressed because the internal field induced by the charges in the Au nanocrystals can be screened by the ultrahigh-k shell of BST. Therefore, the retention property of the Au-nanocrystal-based memory structure is improved.
Generation of 10μJ ultrashort terahertz pulses by optical rectification
Authors: Yeh K.-L. , Hoffmann M. C. , Hebling J. , Nelson Keith A.
Published: April 2007

Generation of near single-cycle pulses centered at 0.5THz frequency with up to 10μJ energy, 100μW average power, and 5.0MW peak power was demonstrated by tilting the intensity front of a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal. Terahertz pulse intensity as high as 10MW∕cm2 was achieved. The photon conversion efficiency was 45% and the calculated peak electric field strength at the focus of an off-axis parabolic mirror was 250kV∕cm.
Solubility-driven thin film structures of regioregular poly(3-hexyl thiophene) using volatile solvents
Authors: Yang Hoichang , LeFevre Scott W. , Ryu Chang Y. , Bao Zhenan
Published: April 2007

Using atomic force microscopy and grazing-incidence x-ray diffraction, the authors found that the use of volatile CH2Cl2 not only offers an advantage in minimizing regioregular poly(3-hexyl thiophene) (RR P3HT) film deposition time, but also directs the desirable parallel orientation of π-π stacking planes of RR P3HT with respect to solid substrates, for both spin and drop castings. The substrate temperature effects have been investigated to support claims that the substrate-induced crystallization of RR P3HT preseeds the parallel oriented crystals prior to spin casting, when the warm CH2Cl2 solution is loaded on a “cold” substrate held at room temperature.