bluuebunny/crossref_metadata_2025 huggingface dataset:
Amplitude and frequency noise sensitivities of optical frequency discriminators based on Fabry–Pérot interferometers and the frequency modulation technique
Authors: Bava Elio , Galzerano Gianluca , Svelto Cesare
Published: December 2006

We report on the sensitivities to frequency and amplitude noise of optical frequency discriminators based on Fabry–Pérot interferometers and the frequency modulation (Pound–Drever–Hall) technique. The response to frequency and amplitude noise is obtained by analyzing the demodulated transients to phase or amplitude steps impressed on the incident frequency modulated light. The responses in the Fourier frequency domain are then obtained by performing the Laplace transform of the demodulated transients. As expected, when the laser frequency is coincident with the Fabry–Pérot resonance, this discriminator is completely immune to amplitude noise. To simulate real operations, the amplitude noise immunity is evaluated in the presence of small detunings between the laser frequency and the Fabry–Pérot resonance. Diagrams of the responses to both types of noise are reported for symmetric and asymmetric resonators. The Pound–Drever–Hall discriminator turns out to be insensitive to the asymmetry of the Fabry–Pérot resonator. The performed evaluations lead to a simple estimation of the minimum detectable frequency noise in terms of a given laser intensity noise.
Magnetostriction studies in an antiferromagnetic polycrystalline Mn42Fe58 alloy
Authors: Peng W. Y. , Zhang J. H.
Published: December 2006

Magnetostriction “jump” effect often occurs in ferromagnetic materials. However, in this letter, the jump effect is observed in antiferromagnetic face-centered-cubic austenitic polycrystalline Mn42Fe58 alloy at room temperature. The magnetostriction under an applied field of 1.0T is 169ppm parallel to the field. The jump magnetostriction of −581ppm, which is antiparallel to the field, is obtained when the sample is subjected to the compressive stress of 1.52MPa. The relation between the magnetostriction and the field strength is discussed.
Determination of complex absorbing potentials from the electron self-energy
Authors: Henderson Thomas M. , Fagas Giorgos , Hyde Eoin , Greer James C.
Published: December 2006

The electronic conductance of a molecule making contact to electrodes is determined by the coupling of discrete molecular states to the continuum electrode density of states. Interactions between bound states and continua can be modeled exactly by using the (energy-dependent) self-energy or approximately by using a complex potential. We discuss the relation between the two approaches and give a prescription for using the self-energy to construct an energy-independent, nonlocal, complex potential. We apply our scheme to studying single-electron transmission in an atomic chain, obtaining excellent agreement with the exact result. Our approach allows us to treat electron-reservoir couplings independent of single-electron energies, allowing for the definition of a one-body operator suitable for inclusion into correlated electron transport calculations.
Mechanism of leakage current reduction of tantalum oxide capacitors by postmetallization annealing
Authors: Lau W. S. , Khaw K. K. , Han Taejoon , Sandler Nathan P.
Published: December 2006

In this letter, the authors will point out that defect states related to impurities or structural defects in tantalum oxide capacitors can be passivated by hydrogen during postmetallization anneal (PMA) while oxygen vacancies are enhanced by PMA such that some will observe a decrease while other may observe an increase in the leakage current after PMA. The PMA process can be tuned such that the hydrogen passivation of defect states dominates over the enhancement of oxygen vacancies, resulting in significant leakage current reduction.
Grating-bicoupled plasmon-resonant terahertz emitter fabricated with GaAs-based heterostructure material systems
Authors: Otsuji Taiichi , Meziani Yahya Moubarak , Hanabe Mitsuhiro , Ishibashi Takuma , Uno Tomohiro , Sano Eiichi
Published: December 2006

A grating-bicoupled plasmon-resonant terahertz emitter was fabricated using InGaP∕InGaAs∕GaAs heterostructure material systems. The device structure is based on a high-electron mobility transistor and incorporates doubly interdigitated grating gates that periodically localize the two-dimensional (2D) plasmon in 100nm regions with a submicron interval. Photoexcited electrons, injected to the 2D plasmon cavities, extensively promoted the plasmon instability, resulting in observation of emission of terahertz electromagnetic radiation at room temperature.
Modulational transparency and femtosecond pulse train in Bragg reflectors with time-varying dielectric constant
Authors: Wu Jiu Hui , Cheng T. H. , Liu A. Q. , Wang Y. X.
Published: December 2006

A theoretical study is conducted to investigate the nonlinear properties of a one-dimensional (1D) periodic stratified structure with time-varying dielectric constants. When the dielectric constant is modulated temporally, the distributed Bragg reflector exhibits different throughputs depending on the modulation amplitude. Below or at the critical value, a periodic and stable femtosecond pulse train is generated; above this critical value, the modulational transparency is observed, as the reflectivity decreases rapidly, so that all incident radiation is completely transferred through the layered media. When the layer number is up to 15, the pulse train generated at the critical value is periodic with perfect reflectivity and zero rejection. These excellent optical properties may enhance the potential applications of the 1D periodic stratified structure in optical switches and laser optics.
Temperature effect on impact ionization characteristics in metamorphic high electron mobility transistors
Authors: Lai Po-Hsien , Fu Ssu-I , Hung Ching-Wen , Tsai Yan-Ying , Chen Tzu-Pin , Chen Chun-Wei , Liu Wen-Chau
Published: December 2006

Comprehensive studies of temperature-dependent gate-metal-related impact ionization in metamorphic high electron mobility transistors (MHEMTs) are demonstrated. It is known that, from experimental results, the electric field and temperature dependences of impact ionization mechanisms in MHEMT’s operation are dominated by the ionization threshold energy and hot electron population. The peak impact ionization-induced gate current could be substantially decreased by the presence of specific gate contact with a higher Schottky barrier height. Therefore, the suppressions of bell-shaped behavior and related impact ionization effect are observed by using the appropriate Schottky gate contacts.
Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface
Authors: Lin H. T. , Chen Y. F. , Huang P. W. , Wang S. H. , Huang J. H. , Lai C. H. , Lee W. N. , Chin T. S.
Published: December 2006

An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As∕IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed.
Doping of germanium nanowires grown in presence of PH3
Authors: Tutuc E. , Chu J. O. , Ott J. A. , Guha S.
Published: December 2006

The authors study the Au-catalyzed chemical vapor growth of germanium (Ge) nanowires in the presence of phosphine (PH3), used as a dopant precursor. The device characteristics of the ensuing nanowire field effect transistors (FETs) indicate n-type, highly doped nanowires. Using a combination of different nanowire growth sequences and their FET characteristics, the authors determine that phosphorus incorporates predominately via the conformal growth, which accompanies the acicular, nanowire growth. As such, the Ge nanowires grown in the presence of PH3 contain a phosphorus doped shell and an undoped core. The authors determine the doping level in the shell to be ≃(1−4)×1019cm−3.
Polyimide amplified piezoelectric scanning mirror for spectral domain optical coherence tomography
Authors: Zara Jason M. , Patterson Paul E.
Published: December 2006

The authors present polyimide amplified piezoelectric bimorph scanning mirrors for application in optical coherence tomography (oct). These devices use piezoelectric bimorph actuators to drive microfabricated polyimide structures at resonance. These devices have tilting tables (either 1.125 or 2.25mm wide) that tilt on 3μm thick torsion hinges to amplify the motion of the bimorph actuators and produce large scan ranges. These devices have been integrated into the scanning arm of a spectral domain OCT imaging system. Preliminary in vivo images have been obtained with scans of 40°–50° at real-time imaging rates of 25–41frames∕s.
High performance ZnO nanowire field effect transistor using self-aligned nanogap gate electrodes
Authors: Cha S. N. , Jang J. E. , Choi Y. , Amaratunga G. A. J. , Ho G. W. , Welland M. E. , Hasko D. G. , Kang D.-J. , Kim J. M.
Published: December 2006

A field effect transistor (FET) using a zinc oxide nanowire with significantly enhanced performance is demonstrated. The device consists of single nanowire and self-aligned gate electrodes with well defined nanosize gaps separating them from the suspended nanowire. The fabricated FET exhibits excellent performance with a transconductance of 3.06μS, a field effect mobility of 928cm2∕Vs, and an on/off current ratio of 106. The electrical characteristics are the best obtained to date for a ZnO transistor. The FET has a n-type channel and operates in enhancement mode. The results are close to those reported previously for p-type carbon nanotube (CNT) FETs. This raises the possibility of using ZnO as the n-type FET with a CNT as the p-type FET in nanoscale complementary logic circuits.
Subpicosecond time-resolved Raman studies of field-induced transient transport in an InxGa1−xAs-based p-i-n semiconductor nanostructure
Authors: Tsen K. T. , Kiang Juliann G. , Ferry D. K. , Morkoç H.
Published: December 2006

Electron transient transport in an InxGa1−xAs-based (x=0.53) p-i-n nanostructure under the application of an electric field has been studied by time-resolved Raman spectroscopy on a subpicosecond time scale and at T=300K. The experimental results reveal the time evolution of the electron distribution function and electron drift velocity with subpicosecond time resolution. These experimental results are compared with those of both InP-based and GaAs-based p-i-n nanostructures and provide a consistent understanding and better insight of electron transient transport phenomena in semiconductors.
Electron transport in naphthylamine-based organic compounds
Authors: Tse S. C. , Kwok K. C. , So S. K.
Published: December 2006

Two naphthylamine-based hole transporters, namely, N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′diamine (NPB) and 4,4′,4″-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine (2TNATA), were found to possess electron transporting (ET) abilities. From time-of-flight measurements, values of electron mobilities for NPB and 2TNATA are (6–9)×10−4 and (1–3)×10−4cm2∕Vs, respectively, under an applied electric field range of 0.04–0.8MV∕cm at 290K. An organic light-emitting diode that employed NPB as the ET material was demonstrated. The electron conducting mechanism of NPB and 2TNATA in relation to the Marcus theory [Rev. Mod. Phys. 65, 599 (1993)] from quantum chemistry will be discussed.
Label-free photoelectrochemical strategy for hairpin DNA hybridization detection on titanium dioxide electrode
Authors: Lu Wu , Wang Geng , Jin Yan , Yao Xin , Hu Jianqiang , Li Jinghong
Published: December 2006

A new photoelectrochemical strategy for hairpin DNA hybridization was devised, in which TiO2 served as the anchor and signal transducer, and no label or redox couples were required. Once the hybridization between hairpin DNA probe and target DNA occurred, the photocurrent would decrease, utilizing which the sequence of the target DNA could be identified. The sequence specificity experiment showed that one or more mismatches of DNA bases could be discriminated. This photoelectrochemical method would be a potential tool in DNA hybridization detection due to its great advantages: label-free, high sensitivity, specific recognition, low cost, and easy fabrication.
Impact of disorder on high quality factor III-V nitride microcavities
Authors: Christmann G. , Simeonov D. , Butté R. , Feltin E. , Carlin J.-F. , Grandjean N.
Published: December 2006

The authors report on the micron scale characterization of a monolithic GaN microcavity (MC) with lattice matched AlInN∕GaN distributed Bragg reflectors by means of a microtransmission setup. This technique allows extracting very high quality factors (Q up to 2800), in accordance with theoretical predictions, contrary to what was previously reported for nitride based MCs. Furthermore, two-dimensional mappings of the MC transmission spectrum allow probing the disorder in this MC. The direct relationship between an increased disorder and a reduction in the Q factor is clearly observed.
Current-voltage behavior in hole-only single-carrier devices with self-assembling dipole molecules on indium tin oxide anodes
Authors: Ganzorig Chimed , Sakomura Masaru , Ueda Kazuyoshi , Fujihira Masamichi
Published: December 2006

The authors report the use of chemically modified indium tin oxide (ITO) with different binding groups (–COCl and –PO2Cl2) of p-chlorobenzene derivatives forming effective monolayers to control the work function of ITO and hence to enhance the hole injection. The enhanced hole injection is studied by measuring current density–voltage (J-V) characteristics. The behavior of J-V characteristics caused by varying the ITO work function in hole-only single-carrier devices with a hole transport layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine is examined. Upon grafting with p-chlorophenylphosphoryl dichloride, the J-V characteristics show a space-charge-limited conduction behavior. Such modified ITO anodes lead to improvements in the device properties.
Structural and magnetic properties of epitaxial thin films of the ordered double perovskite La2CoMnO6
Authors: Guo H. Z. , Gupta A. , Calvarese T. G. , Subramanian M. A.
Published: December 2006

Epitaxial thin films of the ordered ferromagnetic double perovskite La2CoMnO6 have been grown on (001)-oriented SrTiO3 substrates by pulsed laser deposition. X-ray photoemission spectroscopy analysis suggests that the Mn and Co ions are primarily in the 4+ and 2+ valence states. Magnetic measurements indicate the presence of two phases: a majority and a minority ferromagnetic phase with transition temperatures of 230 and 80K, respectively. A spin-glass-type magnetic behavior is also indicated for the films based on time-dependent measurements. The spin-glass behavior likely arises from a competition between ferromagnetic and antiferromagnetic interactions because of local disorder, even in films grown under optimal conditions.
Mask-shift filtering for forming microstructures with irregular profile
Authors: Dong Xiaochun , Du Chunlei , Wang Changtao , Deng Qiling , Zhang Yudong , Luo Xiangang
Published: December 2006

A method of mask-shift filtering is presented for forming microstructures with irregular profile. When a three-dimensional (3D) microstructure is quantized to a number of microslices in one direction with equal interval, their two-dimensional average projections are calculated, respectively. A shift filtering mask is constructed by arranging the submasks converted from the corresponding subprojections. Filtering is carried out during exposure by moving the mask in the quantized direction to form a 3D microprofile on resist. The experimental results for forming irregular continuous relief profile in the larger areas are demonstrated and analyzed in this letter.
Compression behavior of nanosized nickel and molybdenum
Authors: R Selva Vennila. , Kulkarni Shrinivas R. , Saxena Surendra K. , Liermann Hans-Peter , Sinogeikin Stanislav V.
Published: December 2006

X-ray diffraction studies of nanomolybdenum and nanonickel were conducted using a synchrotron source under nonhydrostatic compression up to pressures of 37 and 56GPa, respectively. No phase transitions were found up to the maximum pressure studied. Bulk modulus and its pressure derivative were calculated from the pressure-volume data using the third-order Birch-Murnaghan equation of state. A decrease in compressibility in both systems with decrease in particle size has been observed. Compressibility behavior in nanometals studied is similar to the effect explained by Hall [Proc. Phys. Soc. London, Sect. B 64, 747 (1951)] and Petch [J. Iron Steel Inst., London 174, 25 (1953)].
Temperature response of C13 atoms in amorphized 6H–SiC
Authors: Jiang W. , Zhang Y. , Shutthanandan V. , Thevuthasan S. , Weber W. J.
Published: December 2006

Implantation of C2+13 ions was employed to produce a concentration profile in 6H–SiC at 140K. In situ study of C13-implanted species was performed using the resonant reaction of C13(p,γ)N14 at Ep=1.748MeV. Significant C13 diffusion in the amorphized SiC does not occur up to 1130K. The presence of Au implants (1.9at.%) does not affect the C13 behavior. High-energy H+ irradiation also does not promote the C13 diffusion. The results suggest that C atoms are readily trapped locally in the SiC structure during disordering, which is important in understanding the amorphization and recrystallization processes in SiC.
Nanowire metamaterials with extreme optical anisotropy
Authors: Elser Justin , Wangberg Robyn , Podolskiy Viktor A. , Narimanov Evgenii E.
Published: December 2006

The authors study perspectives of nanowire metamaterials for negative-refraction waveguides, high-performance polarizers, and polarization-sensitive biosensors. They demonstrate that the behavior of these composites is strongly influenced by the concentration, distribution, and geometry of the nanowires, derive an analytical description of electromagnetism in anisotropic nanowire-based metamaterials, and explore the limitations of their approach via three-dimensional numerical simulations. Finally, they illustrate the developed approach on the examples of nanowire-based high-energy-density waveguides and nonmagnetic negative-index imaging systems with far-field resolution of one-sixth of vacuum wavelength.
Three-dimensional self-organization of crystalline gold nanoparticles in amorphous alumina
Authors: Jin Chunming , Zhou Honghui , Wei Wei , Narayan Roger
Published: December 2006

Multilayered heterostructures containing gold nanoparticles embedded in amorphous alumina matrices were deposited on silicon (001) substrates using pulsed laser deposition. The three-dimensional ordering of gold nanoparticles within these multilayered heterostructures was investigated using cross-sectional transmission electron microscopy and image Fourier transformation. Self-organization of gold nanoparticles along the vertical direction was observed in films grown at 20 and at 320°C. Self-organization occurred by means of two different growth modes; both vertically correlated growth (top-on-top) and anticorrelated growth (top-on-middle) mechanisms were observed. The results of these studies suggest that the driving force for vertical ordering in this material is related to the long-range elastic interactions among the nanoparticles within the growing films.
Molecular beam epitaxy of phase pure cubic InN
Authors: Schörmann J. , As D. J. , Lischka K. , Schley P. , Goldhahn R. , Li S. F. , Löffler W. , Hetterich M. , Kalt H.
Published: December 2006

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01Å. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.
Comment on “Legendre-transform functionals for spin-density-functional theory” [J. Chem. Phys. 124, 224108 (2006)]
Authors: Holas A. , Balawender R.
Published: December 2006

In this Comment the authors propose an alternative formulation of the starting point to the analysis performed in the commented paper [P. W. Ayers and W. Yang, J. Chem. Phys.124, 224108 (2006)]. An important role of the z component of the vector of the total spin angular momentum operator for the considered many-electron system is indicated. The authors believe their approach to be mathematically cleaner and conceptually easier than the original treatment.
Enhanced plasticity in Mg-based bulk metallic glass composite reinforced with ductile Nb particles
Authors: Pan D. G. , Zhang H. F. , Wang A. M. , Hu Z. Q.
Published: December 2006

The authors report the synthesis of Mg-based metallic glass composite reinforced with Nb particles which are simply added during melting process. The ductile Nb particles effectively impede shear band propagation and upon yielding, deformed Nb particles distribute the load uniformly to the surrounding glassy matrix to promote the initiation and branching of abundant secondary shear bands. In contrast to the previous Mg-based metallic glass composites which fracture with very little plasticity, the composite shows great resistance to crack growth. The high strength of 900MPa and large plasticity of 12.1±2% have made it comparable to excellent Zr- or Ti-based metallic glass composite.
Direct observation of the epitaxial growth of molecular layers on molecular single crystals
Authors: Sassella A. , Borghesi A. , Campione M. , Tavazzi S. , Goletti C. , Bussetti G. , Chiaradia P.
Published: December 2006

In this letter the authors use the evolution of reflectance anisotropy spectra with film thickness during the growth of organic molecular films of a prototype molecular system (α-quarterthiophene grown onto a single crystal of the same material) to demonstrate homoepitaxy. The real time monitoring of the optical anisotropy of a thin film during deposition by organic molecular beam epitaxy is assessed as a powerful tool to achieve an effective in situ control of the growth starting from the very early deposition stages.
Selective epitaxial growth of 4H-SiC at reduced temperatures using halo-carbon precursor
Authors: Krishnan Bharat , Das Hrishikesh , Lin Huang-De , Koshka Yaroslav
Published: December 2006

Selective homoepitaxial growth of 4H-SiC polytype at temperatures down to 1300°C was investigated using novel halo-carbon epitaxial growth technique. More than three times higher growth rate than in blanket epitaxy without morphology degradation was due to the local loading effect, which demonstrated that the growth rate of SiC homoepitaxy at low temperatures is limited by the gas-phase reaction mechanisms rather than surface mechanisms. It was established that the step-flow growth without undesirable nucleation may be maintained at growth rates in excess of 6μm∕h. Strong dependence of the growth rate on the ratio of the seed area to the masked area indicated that the growth rate is predominantly mass transport limited. Facet formation and conditions for suppressing defect generation at the mesa walls were investigated.
Femtosecond laser fabrication of apertures on two-dimensional microlens arrays
Authors: Langer G. , Brodoceanu D. , Bäuerle D.
Published: December 2006

Two-dimensional lattices of microspheres formed by self-assembly from colloidal solutions can be employed as microlens arrays. With applications in confocal microscopes, some types of parallel laser-surface processing or devices based on anomalous optical transmission, it is useful to improve the imaging properties of microspheres by generating an aperture on each of these lenses. This has been achieved by means of laser-induced forward transfer using femtosecond Ti:sapphire laser radiation. Some of the peculiarities observed in the experiments can be understood on the basis of field intensity calculations.
Raman scattering and electroluminescence of ZnS:Cu,Cl phosphor powder
Authors: Nien Yung-Tang , Chen In-Gann
Published: December 2006

Raman spectra of ZnS:Cu,Cl powders show a structural transition from hexagonal to cubic and a crystal growth with increasing Cu addition (>400ppm), which is supposed to result from Cu incorporation or CuxS precipitation. The redshift of longitudinal optical mode in ZnS:Cu,Cl with increased amounts of Cu suggests that the excess Cu ions were located interstitially in the lattice and created a tensile strain. Under an electric field (3.75×106V∕m), a broad emission spectrum superposing of green, self-activated blue, and blue bands can be observed in the powders (Cu⩾400ppm), which agrees with the luminescence centers of CuZn+, vacancies, and Cui+, respectively.
Strong magnetic scattering from TiOx adhesion layers
Authors: Trionfi A. , Lee S. , Natelson D.
Published: December 2006

Electronic phase coherence in normal metals is incredibly sensitive to magnetic scattering. As a result, the weak localization magnetoresistance and time-dependent universal conductance fluctuations are powerful probes of magnetic impurities. The authors report measurements of these effects in Au and Ag nanowires with a 1.5nm thick Ti adhesion layer underneath the deposited metal. The results indicate an anomalously large magnetic impurity concentration due to the Ti layer. The results also suggest that this magnetic scattering and its evolution are related to the oxidation state of the Ti.
Defect reduction in (112¯) a-plane GaN by two-stage epitaxial lateral overgrowth
Authors: Ni X. , Özgür Ü. , Fu Y. , Biyikli N. , Xie J. , Baski A. A. , Morkoç H. , Liliental-Weber Z.
Published: December 2006

The authors report a two-stage epitaxial lateral overgrowth (ELO) method to get uniformly coalesced (112¯0) a-plane GaN using metal organic chemical vapor deposition by employing a relatively lower growth temperature in the first stage followed by conditions leading to enhanced lateral growth in the second. Using a two-stage ELO method the average Ga-polar to N-polar wing growth rate ratio has been reduced from 4–6 to 1.5–2, which consequently reduced the height difference between the two approaching wings at the coalescence front that resulted from the wing tilt (0.44° for Ga and 0.37° for N wings, measured by x-ray diffraction), thereby making their coalescence much easier. Transmission electron microscopy showed that the threading dislocation density in the wing areas was 1.0×108cm−2, more than two orders of magnitude lower than that in the window areas (4.2×1010cm−2). However, high density of basal stacking faults of 1.2×104cm−1 was still present in the wing areas as compared to c-plane GaN where they are rarely observed away from the substrate. Atomic force microscopy and photoluminescence measurements on the coalesced ELO a-plane GaN sample also indicated improved material quality.
Vacancy induced structural and magnetic transition in MnCo1−xGe
Authors: Wang Jian-Tao , Wang Ding-Sheng , Chen Changfeng , Nashima O. , Kanomata T. , Mizuseki H. , Kawazoe Y.
Published: December 2006

The authors report ab initio total energy calculations on the first-order structural transition of the ferromagnetic MnCo1−xGe(0.00⩽x⩽0.25) intermetallic compound. They show that increasing Co vacancies induce a transition from an orthorhombic structure at 0⩽x⩽0.08 to a hexagonal structure at x>0.08. A concomitant high-to-low moment magnetic transition and a large magnetovolume effect occur due to the change of the symmetry and the resulting coupling distance between the magnetic atoms. These results provide an excellent account for the experimental results and reveal the crucial role of the Co vacancies in determining the relative structural stability and the magnetic properties of MnCo1−xGe.
The Iranian petroleum crisis and United States national security
Authors: Stern Roger
Published: December 2006

The U.S. case against Iran is based on Iran's deceptions regarding nuclear weapons development. This case is buttressed by assertions that a state so petroleum-rich cannot need nuclear power to preserve exports, as Iran claims. The U.S. infers, therefore, that Iran's entire nuclear technology program must pertain to weapons development. However, some industry analysts project an Irani oil export decline [e.g., Clark JR (2005)
Oil Gas J
103(
18
):34–39]. If such a decline is occurring, Iran's claim to need nuclear power could be genuine. Because Iran's government relies on monopoly proceeds from oil exports for most revenue, it could become politically vulnerable if exports decline. Here, we survey the political economy of Irani petroleum for evidence of this decline. We define Iran's export decline rate (
edr
) as its summed rates of depletion and domestic demand growth, which we find equals 10–12%. We estimate marginal cost per barrel for additions to Irani production capacity, from which we derive the “standstill” investment required to offset
edr
. We then compare the standstill investment to actual investment, which has been inadequate to offset
edr
. Even if a relatively optimistic schedule of future capacity addition is met, the ratio of 2011 to 2006 exports will be only 0.40–0.52. A more probable scenario is that, absent some change in Irani policy, this ratio will be 0.33–0.46 with exports declining to zero by 2014–2015. Energy subsidies, hostility to foreign investment, and inefficiencies of its state-planned economy underlie Iran's problem, which has no relation to “peak oil.”
Risk assessment for invasive species produces net bioeconomic benefits
Authors: Keller Reuben P. , Lodge David M. , Finnoff David C.
Published: December 2006

International commerce in live organisms presents a policy challenge for trade globalization; sales of live organisms create wealth, but some nonindigenous species cause harm. To reduce damage, some countries have implemented species screening to limit the introduction of damaging species. Adoption of new risk assessment (RA) technologies has been slowed, however, by concerns that RA accuracy remains insufficient to produce positive net economic benefits. This concern arises because only a small proportion of all introduced species escape, spread, and cause harm (i.e., become invasive), so a RA will exclude many noninvasive species (which provide a net economic benefit) for every invasive species correctly identified. Here, we develop a simple cost:benefit bioeconomic framework to quantify the net benefits from applying species prescreening. Because invasive species are rarely eradicated, and their damages must therefore be borne for long periods, we have projected the value of RA over a suitable range of policy time horizons (10–500 years). We apply the model to the Australian plant quarantine program and show that this RA program produces positive net economic benefits over the range of reasonable assumptions. Because we use low estimates of the financial damage caused by invasive species and high estimates of the value of species in the ornamental trade, our results underestimate the net benefit of the Australian plant quarantine program. In addition, because plants have relatively low rates of invasion, applying screening protocols to animals would likely demonstrate even greater benefits.
Structure and quantum chemical characterization of chloroperoxidase compound 0, a common reaction intermediate of diverse heme enzymes
Authors: Kühnel Karin , Derat Etienne , Terner James , Shaik Sason , Schlichting Ilme
Published: December 2006

We have determined the crystal structure of the chloroperoxidase (CPO) hydroperoxo reaction intermediate (CPO compound 0) at 1.75-Å resolution. The intermediate was generated through controlled photoreduction of the CPO oxygen complex during x-ray data collection, which was monitored by recording of the crystal absorption spectra. Initially, the peroxo-anion species was formed and then protonated to yield compound 0. Quantum chemical calculations indicate that the peroxo-anion species is not stable and collapses instantaneously to compound 0. Compound 0 is present in the ferric low-spin doublet ground state and is characterized by a long O

O bond length of 1.5 Å and a Fe

O bond distance of 1.8 Å, which is also observed in the crystal structure.
Regulation of innate antiviral defenses through a shared repressor domain in RIG-I and LGP2
Authors: Saito Takeshi , Hirai Reiko , Loo Yueh-Ming , Owen David , Johnson Cynthia L. , Sinha Sangita C. , Akira Shizuo , Fujita Takashi , Gale Michael
Published: December 2006

RIG-I is an RNA helicase containing caspase activation and recruitment domains (CARDs). RNA binding and signaling by RIG-I are implicated in pathogen recognition and triggering of IFN-α/β immune defenses that impact cell permissiveness for hepatitis C virus (HCV). Here we evaluated the processes that control RIG-I signaling. RNA binding studies and analysis of cells lacking RIG-I, or the related MDA5 protein, demonstrated that RIG-I, but not MDA5, efficiently binds to secondary structured HCV RNA to confer induction of IFN-β expression. We also found that LGP2, a helicase related to RIG-I and MDA5 but lacking CARDs and functioning as a negative regulator of host defense, binds HCV RNA. In resting cells, RIG-I is maintained as a monomer in an autoinhibited state, but during virus infection and RNA binding it undergoes a conformation shift that promotes self-association and CARD interactions with the IPS-1 adaptor protein to signal IFN regulatory factor 3- and NF-κB-responsive genes. This reaction is governed by an internal repressor domain (RD) that controls RIG-I multimerization and IPS-1 interaction. Deletion of the RIG-I RD resulted in constitutive signaling to the IFN-β promoter, whereas RD expression alone prevented signaling and increased cellular permissiveness to HCV. We identified an analogous RD within LGP2 that interacts in trans with RIG-I to ablate self-association and signaling. Thus, RIG-I is a cytoplasmic sensor of HCV and is governed by RD interactions that are shared with LGP2 as an on/off switch controlling innate defenses. Modulation of RIG-I/LGP2 interaction dynamics may have therapeutic implications for immune regulation.
Genetic regulation of patterned tubular branching in
Drosophila
Authors: Hatton-Ellis E. , Ainsworth C. , Sushama Y. , Wan S. , VijayRaghavan K. , Skaer H.
Published: December 2006

A common theme in organogenesis is the branching of epithelial tubes, for example in the lung, liver, or kidney. The later morphogenesis of these branched epithelia dictates the final form and function of the mature tissue. Epithelial branching requires the specification of branch cells, the eversion process itself, and, frequently, patterned morphogenesis to produce branches of specific shape and orientation. Using the branching of renal tubule primordia from the hindgut in
Drosophila
, we show that these aspects are coordinately regulated. Cell specification depends on Wnt signaling along the tubular gut and results in the spatially restricted coexpression of two transcription factors, Krüppel and Cut, in the hindgut, whose activity drives cells toward renal tubule fate. Significantly, these transcription factors also confer the competence to respond to a second signal; TGF-β induces branching to form the four renal tubule buds. Differential activation of the TGF-β pathway also patterns the tubules, resulting in the asymmetry in size and positioning that is characteristic of the two tubule pairs. High levels of TGF-β promote the expression of
Dorsocross1–3
and anterior tubule growth, whereas low levels allow the expression of the transcriptional repressor, Brinker, and thus promote posterior tubule identity. We show that patterning of the tubule primordium into two distinct pairs is critical for the eversion of tubule branches, as well as for their asymmetric morphogenesis.
Activity-dependent neurotransmitter-receptor matching at the neuromuscular junction
Authors: Borodinsky Laura N. , Spitzer Nicholas C.
Published: December 2006

Signaling in the nervous system requires matching of neurotransmitter receptors with cognate neurotransmitters at synapses. The vertebrate neuromuscular junction is the best studied cholinergic synapse, but the mechanisms by which acetylcholine is matched with acetylcholine receptors are not fully understood. Because alterations in neuronal calcium spike activity alter transmitter specification in embryonic spinal neurons, we hypothesized that receptor expression in postsynaptic cells follows changes in transmitter expression to achieve this specific match. We find that embryonic vertebrate striated muscle cells normally express receptors for glutamate, GABA, and glycine as well as for acetylcholine. As maturation progresses, acetylcholine receptor expression prevails. Receptor selection is altered when early neuronal calcium-dependent activity is perturbed, and remaining receptor populations parallel changes in transmitter phenotype. In these cases, glutamatergic, GABAergic, and glycinergic synaptic currents are recorded from muscle cells, demonstrating that activity regulates matching of transmitters and their receptors in the assembly of functional synapses.
A unique configuration of genome-wide DNA methylation patterns in the testis
Authors: Oakes C. C. , La Salle S. , Smiraglia D. J. , Robaire B. , Trasler J. M.
Published: December 2006

In the mammalian lifecycle, the two periods of genome-wide epigenetic reprogramming are in the early embryo, when somatic patterns are set, and during germ cell development. Although some differences between the reprogrammed states of somatic and germ cells have been reported, overall patterns of genomic methylation are considered to be similar. Using restriction landmark genomic scanning to examine ≈2,600 loci distributed randomly throughout the genome, we find that the methylation status of testicular DNA is highly distinct, displaying eightfold the number of hypomethylated loci relative to somatic tissues. Identification and analysis of >300 loci show that these regions are generally located within nonrepetitive sequences that are away from CpG islands and 5′ regions of genes. We show that a contributing factor for these differences is that the methylation state of non-CpG-island DNA is correlated with the regional level of GC content within chromosomes and that this relationship is inverted between the testis and somatic tissues. We also show that in
Dnmt3L
-deficient mice, which exhibit infertility associated with abnormal chromosomal structures in germ cells, this unique testicular DNA methylation pattern is not established. These special properties of testicular DNA point to a broad, distinct epigenetic state that may be involved in maintaining a unique chromosomal structure in male germ cells.
Parasitic inhibition of cell death facilitates symbiosis
Authors: Pannebakker Bart A. , Loppin Benjamin , Elemans Coen P. H. , Humblot Lionel , Vavre Fabrice
Published: December 2006

Symbiotic microorganisms have had a large impact on eukaryotic evolution, with effects ranging from parasitic to mutualistic. Mitochondria and chloroplasts are prime examples of symbiotic microorganisms that have become obligate for their hosts, allowing for a dramatic extension of suitable habitats for life. Out of the extraordinary diversity of bacterial endosymbionts in insects, most are facultative for their hosts, such as the ubiquitousWolbachia, which manipulates host reproduction. Some endosymbionts, however, have become obligatory for host reproduction and/or survival. In the parasitoid waspAsobara tabidathe presence ofWolbachiais necessary for host oogenesis, but the mechanism involved is yet unknown. We show thatWolbachiainfluences programmed cell death processes (a host regulatory feature typically targeted by pathogens) inA. tabida, making its presence essential for the wasps' oocytes to mature. This suggests that parasite strategies, such as bacterial regulation of host apoptosis, can drive the evolution of host dependence, allowing for a swift transition from parasitism to mutualism.
Multiple-motor based transport and its regulation by Tau
Authors: Vershinin Michael , Carter Brian C. , Razafsky David S. , King Stephen J. , Gross Steven P.
Published: December 2006

Motor-based intracellular transport and its regulation are crucial to the functioning of a cell. Disruption of transport is linked to Alzheimer's and other neurodegenerative diseases. However, many fundamental aspects of transport are poorly understood. An important issue is how cells achieve and regulate efficient long-distance transport. Mounting evidence suggests that many
in vivo
cargoes are transported along microtubules by more than one motor, but we do not know how multiple motors work together or can be regulated. Here we first show that multiple kinesin motors, working in conjunction, can achieve very long distance transport and apply significantly larger forces without the need of additional factors. We then demonstrate
in vitro
that the important microtubule-associated protein, tau, regulates the number of engaged kinesin motors per cargo via its local concentration on microtubules. This function of tau provides a previously unappreciated mechanism to regulate transport. By reducing motor reattachment rates, tau affects cargo travel distance, motive force, and cargo dispersal. We also show that different isoforms of tau, at concentrations similar to those in cells, have dramatically different potency. These results provide a well defined mechanism for how altered tau isoform levels could impair transport and thereby lead to neurodegeneration without the need of any other pathway.
Independent control of grafting density and conformation of single-stranded DNA brushes
Authors: Opdahl Aric , Petrovykh Dmitri Y. , Kimura-Suda Hiromi , Tarlov Michael J. , Whitman Lloyd J.
Published: December 2006

We describe self-assembly of ssDNA brushes that exploits the intrinsic affinity of adenine nucleotides (dA) for gold surfaces. The grafting density and conformation of these brushes is deterministically controlled by the length of the anchoring dA sequences, even in the presence of thymine nucleotides (dT). We produce and characterize brushes of model block-oligonucleotides, d(T

m

-A

n

), with systematically varied lengths
m
and
n
of the thymine and adenine blocks [denoted d(T

m

) and d(A

n

), respectively]. The hairpin conformation, dominant for self-complementary d(T

m

-A

n

) oligos in solution, is disrupted by the high preferential affinity of dA for gold surfaces. As a result, the d(T

m

-A

n

) oligos adsorb as a brush of d(T) strands immobilized via the d(A) blocks. Quantitative analysis by FTIR spectroscopy and x-ray photoelectron spectroscopy (XPS) reveals a unique feature of DNA immobilization via d(A) blocks: The surface density of dA nucleotides is close to saturation and is nearly independent of d(A) block length. Accordingly, the lateral spacing (grafting density) of the d(T) blocks is determined by the length of the d(A) blocks. The d(T) blocks extend away from the surface in a brush-like conformation at a lateral spacing 2–3 times larger (a grafting density 5–10 times lower) than in analogous films immobilized via standard thiol linkers. This combination of brush-like conformation and low saturation grafting density is expected to increase the efficiency of DNA hybridization at surfaces. Therefore, immobilization via d(A) blocks offers a method of producing DNA brushes with controlled properties for applications in biotechnology and nanotechnology.
A singular enzymatic megacomplex from
Bacillus subtilis
Authors: Straight Paul D. , Fischbach Michael A. , Walsh Christopher T. , Rudner David Z. , Kolter Roberto
Published: December 2006

Nonribosomal peptide synthetases (NRPS), polyketide synthases (PKS), and hybrid NRPS/PKS are of particular interest, because they produce numerous therapeutic agents, have great potential for engineering novel compounds, and are the largest enzymes known. The predicted masses of known enzymatic assembly lines can reach almost 5 megadaltons, dwarfing even the ribosome (≈2.6 megadaltons). Despite their uniqueness and importance, little is known about the organization of these enzymes within the native producer cells. Here we report that an 80-kb gene cluster, which occupies ≈2% of the
Bacillus subtilis
genome, encodes the subunits of ≈2.5 megadalton active hybrid NRPS/PKS. Many copies of the NRPS/PKS assemble into a single organelle-like membrane-associated complex of tens to hundreds of megadaltons. Such an enzymatic megacomplex is unprecedented in bacterial subcellular organization and has important implications for engineering novel NRPS/PKSs.
Conserved markers of fetal pancreatic epithelium permit prospective isolation of islet progenitor cells by FACS
Authors: Sugiyama Takuya , Rodriguez Ryan T. , McLean Graeme W. , Kim Seung K.
Published: December 2006

Prospective isolation and characterization of progenitor cells is a paradigmatic strategy for studies of organ development. However, extraction of viable cells, fractionation of lineages, and
in vitro
analysis of progenitors from the fetal pancreas in experimental organisms like mice has proved challenging and has not yet been reported for human fetal pancreas. Here, we report isolation of pancreatic islet progenitor cells from fetal mice by FACS. Monoclonal antibodies that recognize cell-surface proteins on candidate stem cells in brain, skin, and other organs enabled separation of major pancreatic cell lineages and isolation of native pancreatic cells expressing neurogenin 3, an established marker of islet progenitors. New
in vitro
cell culture methods permitted isolated mouse islet progenitors to develop into hormone-expressing endocrine cells. Insulin-producing cells derived
in vitro
required or expressed factors that regulate fetal β cell differentiation; thus, the genetic programs normally controlling
in vivo
mouse islet development are similarly required in our system. Moreover, antibodies that recognize conserved orthologous cell-surface epitopes in human fetal pancreas allowed FACS-based enrichment of candidate islet progenitor cells expressing neurogenin 3. Our studies reveal previously undescribed strategies for prospective purification and analysis of pancreatic endocrine progenitor cells that should accelerate studies of islet development and replacement.
Penetrating arterioles are a bottleneck in the perfusion of neocortex
Authors: Nishimura Nozomi , Schaffer Chris B. , Friedman Beth , Lyden Patrick D. , Kleinfeld David
Published: December 2006

Penetrating arterioles bridge the mesh of communicating arterioles on the surface of cortex with the subsurface microvascular bed that feeds the underlying neural tissue. We tested the conjecture that penetrating arterioles, which are positioned to regulate the delivery of blood, are loci of severe ischemia in the event of occlusion. Focal photothrombosis was used to occlude single penetrating arterioles in rat parietal cortex, and the resultant changes in flow of red blood cells were measured with two-photon laser-scanning microscopy in individual subsurface microvessels that surround the occlusion. We observed that the average flow of red blood cells nearly stalls adjacent to the occlusion and remains within 30% of its baseline value in vessels as far as 10 branch points downstream from the occlusion. Preservation of average flow emerges 350 μm away; this length scale is consistent with the spatial distribution of penetrating arterioles. We conclude that penetrating arterioles are a bottleneck in the supply of blood to neocortex, at least to superficial layers.