bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-dependent strain analysis of mirrors illuminated with intense femtosecond pulses in the soft x-ray spectral range
Authors: de Castro A. R. B. , Möller T.
Published: May 2005

We consider the three-dimensional time-dependent thermoelastic problem for a thin silicon blade illuminated by a single soft x-ray free-electron laser (FEL) pulse of fs duration. An exact analytical solution is given. It shows that the heat flow in the blade is very slow, which might pose problems for cooling if the FEL is operated to produce a train of pulses with short spacing. Surface mechanical oscillations are generated shortly after the FEL pulse is absorbed, and resonant enhancement by subsequent pulses in the pulse train is an issue. After the FEL pulse, but long before the heat front reaches the heat sink, the optical surface bulges outwards. The effects of a FEL pulse train with thousands of pulses need more detailed study, but experiments with single pulses at kHz rates are not expected to suffer from thermally induced surface figure distortion. The emphasis in this article is not on whether the blade will escape melting, but rather to what extent the profile of a high precision optical surface is preserved under FEL illumination, a question of major interest if focal spots with diameters in the micron range are needed.
Random partial orders, posts, and the causal set approach to discrete quantum gravity
Authors: Ash Avner , McDonald Patrick
Published: May 2005

We study a collection of Markov chains with values in the collection of partial orderings of the natural numbers. These systems arise naturally in the causal set approach to discrete quantum gravity and include the well-studied random partial orders of Alon, Bollobas, Brightwell, and Janson [Ann. Appl. Probab. 4, 108–123 (1994)]. We prove that under the dynamics associated to Markov chains in our collection, posts occur infinitely often, almost surely.
Measurement of the flow velocity in unmagnetized plasmas by counter propagating ion-acoustic waves
Authors: Ma J. X. , Li Yang-fang , Xiao De-long , Li Jing-ju , Li Yi-ren
Published: May 2005

The diffusion velocity of an inhomogeneous unmagnetized plasma is measured by means of the phase velocities of ion-acoustic waves propagating along and against the direction of the plasma flow. Combined with the measurement of the plasma density distributions by usual Langmuir probes, the method is applied to measure the ambipolar diffusion coefficient and effective ion collision frequency in inhomogeneous plasmas formed in an asymmetrically discharged double-plasma device. Experimental results show that the measured flow velocities, diffusion coefficients, and effective collision frequencies are in agreement with ion-neutral collision dominated diffusion theory.
A comparative study of contact resonance imaging using atomic force microscopy
Authors: Banerjee S. , Gayathri N. , Dash S. , Tyagi A. K. , Raj Baldev
Published: May 2005

We present here a comparative study of atomic force microscope (AFM) imaging in contact mode when either the cantilever carrying the probing tip or the sample is excited at ultrasonic frequencies. The cantilever or the sample can be excited by piezoelectric transducers attached to them. When the AFM tip is in contact with the sample surface the contact resonance curve depends on the local tip–sample contact stiffness. By measuring the contact resonance as a function of position one can image the local stiffness of the sample surface. We will report here imaging carried out on piezoelectric material and thin metal film using both the modes. The comparison shows that both give similar results.
Voltage-tunable four-color quantum-well infrared photodetectors
Authors: Li J. , Choi K. K. , Tsui D. C.
Published: May 2005

A voltage-tunable four-color quantum-well infrared photodetector (QWIP) has been demonstrated. The advantages of this detector include widely separated peaks, narrow linewidths, and negligible spectral cross-talk. The four colors demonstrated are widely distributed from mid-infrared to long-infrared, and are centered at 4.5, 5.3, 8.3, and 10.4μm. Each spectral peak is well defined and the corresponding linewidth is 0.58, 0.71, 0.83, and 1.01μm, respectively (corresponding to Δλ∕λ<0.14). The detector structure consists of two stacks of superlattice materials that are separated by a middle contact layer. Each material is designed to detect two specific wavelengths, tunable by the bias polarity. Four-color detection is achieved by applying different combinations of top and bottom bias relative to the common middle contact and reading out the optical signals sequentially from these two contacts. Using an appropriate bias sequence, the detector can be made sensitive to only one color at a time and the signals from the other colors are negligible. With this detection scheme, one can select different combinations of detection wavelengths and linewidths over a wide spectral range. This design thus greatly increases the flexibility of QWIPs in multicolor imaging.
Substitutional diatomic molecules NO, NC, CO, N2, and O2: Their vibrational frequencies and effects on p doping of ZnO
Authors: Limpijumnong Sukit , Li Xiaonan , Wei Su-Huai , Zhang S. B.
Published: May 2005

First-principles calculations show that AB defects substituting on an O site in ZnO where A, B=N, O, or C are an important class of defects whose physical properties cannot be described by the usual split interstitials but rather by substitutional diatomic molecules. The molecular natures of the (AB)O defects are reflected in their vibrational frequencies which are redshifted from those of the corresponding free molecules but only by about 10%. These calculated results agree with the frequency range recently observed by IR measurement on N-doped ZnO. Moreover, most (AB)O defects are donors in p-type samples. The (NC)O and (N2)O defects have sufficiently low energies to convert substituional NO acceptors into donors, thereby hindering the efforts of doping ZnO p type.
Synthesis and characterization of a single-crystal chemical-vapor-deposition diamond particle detector
Authors: Balducci A. , Marinelli Marco , Milani E. , Morgada M. E. , Pucella G. , Tucciarone A. , Verona-Rinati G. , Angelone M. , Pillon M.
Published: May 2005

The growth conditions and the detection properties of a homoepitaxial diamond film, deposited in Roma “Tor Vergata” University Laboratories by microwave chemical vapor deposition on a high-pressure high-temperature single-crystal substrate are reported. An energy resolution as low as 1.1% was achieved when irradiating the device with 5.5 MeV Am241 α-particles. The dependence of the charge collection efficiency and the energy resolution on the applied voltage are reported as well. A clear saturation plateau was observed in both curves. Preliminary results with 14.8 MeV neutron irradiation are reported, showing a well separated C12(n,α0)Be9 reaction peak.
Electro-optical response of a single-crystal diamond ultraviolet photoconductor in transverse configuration
Authors: De Sio A. , Achard J. , Tallaire A. , Sussmann R. S. , Collins A. T. , Silva F. , Pace E.
Published: May 2005

Diamond has been identified as a very promising material for X and ultraviolet sensing. In this Letter, a photoconductive device based on a freestanding homoepitaxial chemically vapor deposition (CVD) single-crystal diamond 500μm thick has been tested. Photoconductive measurements in coplanar and transverse configurations have been performed to characterize the device sensitivity in the 140–250 nm spectral range. Very high sensitivity values were achieved in both configurations. The sensitivity in the transverse configuration is at least 300 times higher than in the coplanar configuration.
Low-temperature growth of high-quality lead zirconate titanate thin films by 28 GHz microwave irradiation
Authors: Wang Z. J. , Kokawa H. , Takizawa H. , Ichiki M. , Maeda R.
Published: May 2005

Pb ( Zr x Ti 1 − x ) O 3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The elevated temperature generated by microwave irradiation to obtain the perovskite phase is only 480 °C, which is significantly lower than that of conventional thermal processing. X-ray diffraction analysis indicated that the PZT films crystallized well in the perovskite phase. A scanning electron microscopy image showed that the film has a spherulite grain structure and most of the grains are approximately 2 μm in size. The average values of the remanent polarization, coercive field, dielectric constant, and loss of the PZT films are 40μC∕cm2, 50 kV/cm, 1100, and 004, respectively. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures.
Electronic structure of Cs-doped tris(8-hydroxyquinoline) aluminum
Authors: Ding Huanjun , Gao Yongli
Published: May 2005

The evolution of the electronic structure of Cs-doped tris(8-hydroxyquinoline) aluminum (Alq) film has been investigated with photoemission spectroscopy. The results show that doping induces an energy level shift that can be divided into two stages. At the first stage, the Fermi level moves in the energy gap due to the charge transfer from Cs to Alq. Moreover, this energy level shift depends on the doping concentration in a semilogarithmic fashion. The second stage is characterized by a significant modification of the Alq electronic structure, manifested by the gap state and saturation of the energy level shift.
Integrated liquid crystal optical switch based on total internal reflection
Authors: Zhang Ailing , Chan Kam Tai , Demokan M. S. , Chan Victor W. C. , Chan Philip C. H. , Kwok Hoi S. , Chan Andy H. P.
Published: May 2005

An optical switch structure which combines a planar lightwave circuit and liquid crystal is proposed and experimentally demonstrated. The switching is realized by inducing total internal reflection through controlling the refractive index of the liquid crystal. The principle and the design of the integrated optical switch are described. Experimental results suggest that anchoring of the liquid crystal molecules on nonplanar structures is very crucial in affecting the switching performance.
Odd filling factor quantum Hall sequence in magnetic type-III quantum wells
Authors: Buhmann H. , Novik E. G. , Daumer V. , Liu J. , Gui Y. S. , Becker C. R. , Molenkamp L. W.
Published: May 2005

The magnetotransport properties of a series of Hg1−yMnyTe quantum wells (y=2%) reveal anomalous sequences of quantum Hall plateaus and corresponding minima in the longitudinal magnetoresistance which correspond solely to odd filling factors. This anomaly originates from the very large spin splitting in this material, which exceeds the cyclotron energy even at moderate magnetic fields (B>2T). An 8×8 band k∙p approach has been used to calculate the magnetic field dependence of the Landau levels. Finite temperature and Landau level broadening considerations confirm that subband crossings in the vicinity of the Fermi energy are responsible for the suppression of even filling factor quantum Hall states.
Static and dynamic measurements of the α-factor of five-quantum-dot-layer single-mode lasers emitting at 1.3μm on GaAs
Authors: Martinez A. , Lemaître A. , Merghem K. , Ferlazzo L. , Dupuis C. , Ramdane A. , Provost J.-G. , Dagens B. , Le Gouezigou O. , Gauthier-Lafaye O.
Published: May 2005

The “material” and “device” linewidth enhancement factor α of five-quantum-dot (QD)-layer single-mode lasers emitting at 1.3μm are investigated using two methods. The Hakki–Paoli method associated with pulsed analysis of Fabry–Perot modes below threshold demonstrates a record value of 0.7 at 1295 nm. High-frequency current modulation experiments showed a value of 2.0 just above threshold at 1.3μm with a steady increase with the current. Dynamic measurements on a three-QD layer device, with a reduced ground state optical gain, showed a similar increase of α with current but at a higher rate.
Dual lattice photonic-crystal beam splitters
Authors: Wu Lijun , Mazilu M. , Gallet J.-F. , Krauss T. F.
Published: May 2005

Light propagation in photonic crystals (PhC) is both sensitive to incident angle and wavelength. By combining two different PhC lattices, we utilize this effect to demonstrate a wavelength-dependent beam splitter with enhanced angular separation. The first lattice acts as a superprism that separates the incoming light according to wavelength, whereas the second lattice acts as an angular amplifier. We obtain 90° angular separation for two wavelengths separated by 70 nm (1300 nm regime) in a structure that is less than 10 μm long.
Improvement of the light output of InGaN-based light-emitting diodes using Cu-doped indium oxide/indium tin oxide p-type electrodes
Authors: Song June-O , Kwak J. S. , Park Y. , Seong Tae-Yeon
Published: May 2005

We report on the formation of highly transparent and low-resistance Cu-doped indium oxide(CIO)(3 nm)/indium tin oxide (ITO) (400 nm) ohmic contact to p-type GaN for high-brightness light-emitting diodes (LEDs) for solid-state lighting. The CIO/ITO contacts become ohmic with specific contact resistances of ∼10−4Ωcm2 and give transmittance higher than 98.7% at a wavelength of 405 nm when annealed at 630 °C for 1 min in air. Near UV LEDs fabricated with the annealed CIO/ITO p-type contact layers give a forward-bias voltage of 3.25 V at injection current of 20 mA. It is further shown that the output power of the LEDs with the CIO/ITO contacts is enhanced 78% at 20 mA as compared with that of LEDs with the conventional Ni∕Au contacts.
Mechanisms of dynamic range limitations in GaAs∕AlGaAs quantum-cascade lasers: Influence of injector doping
Authors: Jovanović V. D. , Indjin D. , Vukmirović N. , Ikonić Z. , Harrison P. , Linfield E. H. , Page H. , Marcadet X. , Sirtori C. , Worrall C. , Beere H. E. , Ritchie D. A.
Published: May 2005

The influence of doping density on the performance of GaAs∕AlGaAs quantum-cascade lasers is presented. A fully self-consistent Schrödinger–Poisson analysis, based on a scattering rate equation approach, was employed to simulate the above threshold electron transport in laser devices. V-shaped local field domain formation was observed, preventing resonant subband level alignment in the high pumping-current regime. The resulting saturation of the maximal current, together with an increase of the threshold current, limits the dynamic working range under higher doping. Experimental measurements are in good agreement with the theoretical predictions.
Controlled generation of neutral, negatively-charged and positively-charged excitons in the same single quantum dot
Authors: Ediger M. , Dalgarno P. A. , Smith J. M. , Gerardot B. D. , Warburton R. J. , Karrai K. , Petroff P. M.
Published: May 2005

We report the controlled generation of neutral, negatively-charged and positively-charged excitons in the same single InAs quantum dot. The control parameters are a vertical electric field applied to a capacitor-like structure, in which the quantum dots are embedded, and optical pump power. The strong Coulomb blockade in quantum dots can be exploited to control the charge of excitons containing one hole, the neutral exciton, X0, and singly negatively charged exciton, X1−. We show here how this concept can be extended to excitons containing two holes, the biexciton, 2X0, and significantly the singly positively charged exciton, X1+. We support all these assignments with a Coulomb blockade model. For all dots, the emission from the X1− is redshifted relative to the neutral exciton, but surprisingly we observe blueshifts as well as small redshifts for X1+.
Measurement of microwave electro-optic coefficient in Sr0.61Ba0.39Nb2O6 crystal fiber
Authors: Huang Chuanyong , Bhalla Amar S. , Guo Ruyan
Published: May 2005

A method is put forward to measure the linear electro-optic (EO) coefficient at high frequency using a light-coupled microwave cavity perturbation method of standing wave condition. The method is used to determine the EO coefficient of Sr0.61Ba0.39Nb2O6 (SBN) crystal fiber. The electro-optic coefficient r33 of SBN crystal fiber at 10 GHz is measured to be near 200(pm∕V), which is smaller than that measured at low frequency containing secondary contributions. This method is suitable for measuring the electro-optic property in a wide range of high frequencies depending on the availability of microwave cavity and suitable electronics.
Structure and luminescence of pyramid-shaped CdSe nanostructures grown by metalorganic chemical vapor deposition
Authors: Shan C. X. , Liu Z. , Ng C. M. , Hark S. K.
Published: May 2005

Sharply pointed pyramidal CdSe nanostructures were grown on Si substrates by metalorganic chemical vapor deposition using Au as a catalyst. Scanning electron microscopy analysis confirms their shape; x-ray diffraction, in combination with electron diffraction, reveals that they have the zinc blende structure and are single crystalline. Photoluminescence measurements on individual pyramids at room temperature show an intense near-band edge emission, confirming their good optical quality. Having the same zinc blende structure as ZnSe, these CdSe pyramids are potentially useful for fabricating nanometer-scaled II–VI heterostructures.
Observation of the biexponential ground-state decay time behavior in InAs self-assembled quantum dots grown on misoriented substrates
Authors: Shkolnik A. S. , Karachinsky L. Ya. , Gordeev N. Yu. , Zegrya G. G. , Evtikhiev V. P. , Pellegrini S. , Buller G. S.
Published: May 2005

Biexponential behavior of the time-resolved photoluminescence decay from the ground state has been studied over a temperature range of 77–300 K on samples with varying sized self-assembled InAs∕GaAs quantum dot ensembles controlled by substrate misorientation alone. The slower second decay component is considerably longer than the first one, and has been measured to be as long as 300 ns. This slow component is attributed to carrier recapturing and indirect radiative recombination processes.
Phonon characteristics and cathodolumininescence of boron nitride nanotubes
Authors: Zhi Chunyi , Bando Yoshio , Tang Chengchun , Golberg Dmitri , Xie Rongguo , Sekigushi Takashi
Published: May 2005

Large quantities of highly pure boron nitride nanotubes (BNNTs) are synthesized through a carbon-free method. Nanotube phonon features are investigated by Raman and Fourier-transformed infrared spectroscopies. Both methods indicate highly pure boron nitride phase. Intense ultraviolet light emission is observed when BNNTs are excited by an electron beam, which indicates that the present BNNTs have potential applications in ultraviolet optical devices.
Ultrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN
Authors: Mikulics M. , Marso M. , Javorka P. , Kordoš P. , Lüth H. , Kočan M. , Rizzi A. , Wu S. , Sobolewski R.
Published: May 2005

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to 200kV∕cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720fs in our LT GaN material.
Low temperature relaxations and effects on poling efficiencies of dendronized nonlinear optical side-chain polymers
Authors: Gray Tomoko , Overney René M. , Haller Marnie , Luo Jingdong , Jen Alex K.-Y.
Published: May 2005

Low temperature relaxations in a dendronized nonlinear optical (NLO) side-chain polymer were found to take place at more than 20 °C below the glass transition temperature. Relaxations of localized mobilities, removed from long range relaxations responsible for chromophore aggregation, are shown to offer new gateways for optimized acentric ordering of the chromophores. Supreme electro-optical (EO) activity was achieved by electrical poling close to the critical temperatures of localized mobilities identified as dendronized NLO side-chain relaxations. This study features, in particular, one new instrumental approach to relaxation studies of thin spin coated NLO polymer films; the shear-modulation force microscopy (SM-FM) method. Originating from scanning force microscopy (SFM), the SM-FM method grants access to the detection of low temperature relaxations in constrained thin NLO films not obtainable by conventional means.
Synthesis and field-electron-emission behavior of aligned GaAs nanowires
Authors: Zhi C. Y. , Bai X. D. , Wang E. G.
Published: May 2005

Large-area highly aligned GaAs nanowires were synthesized directly by etching GaAs wafer covered with Au film using H plasma. The characterizations by scanning electron microscopy and transmission electron microscopy reveal that the as-grown nanowires are uniform in distribution and the individual nanowires are well crystallized. The field-electron-emission behavior of the GaAs nanowire was studied and a low turn-on field of 2.0V∕μm was achieved, which suggests its potential application as cold electron sources. The mechanism for field-emission enhancement is also discussed.
Terahertz frequency range band-stop filters
Authors: Cunningham J. , Wood C. , Davies A. G. , Hunter I. , Linfield E. H. , Beere H. E.
Published: May 2005

We report the operation of band-stop filters in the terahertz (THz) frequency range, working at a center frequency of 600GHz. The filters were characterized by embedding them in a microstrip line attached to photoconductive switches which act as THz emitters and detectors. The filters have applications in sensing and detection. The chosen filter design allows cascading of several filters along the same microstrip line, which is of particular importance for a proposed molecular sensing array.
Superconducting vortex profile from fixed point measurements the “Lazy Fisherman” tunneling microscopy method
Authors: Kohen A. , Cren T. , Proslier Th. , Noat Y. , Sacks W. , Roditchev D. , Giubileo F. , Bobba F. , Cucolo A. M. , Zhigadlo N. , Kazakov S. M. , Karpinski J.
Published: May 2005

We introduce a mode of operation for studying the vortex phase in superconductors using scanning tunneling microscopy (STM). While in the conventional STM method, the tip is scanned over a sample in which a fixed vortex pattern is prepared, in our “Lazy Fisherman” method the STM tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunneling conductance spectra, dI∕dV(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface nonuniformity and allows one to study vortex dynamics. Using a statistical analysis of the spectra, we reconstruct the single vortex zero bias conductance profile. We apply the method to the c-axis face of a MgB2 single crystal sample and obtain a vortex profile with a coherence length, ξ of 57±2nm.
Influence of nitrogen growth pressure on the ferromagnetic properties of Cr-doped AlN thin films
Authors: Zhang Jun , Li X. Z. , Xu B. , Sellmyer D. J.
Published: May 2005

We report the magnetic properties of Cr-doped AlN thin films grown by reactive magnetron sputtering under various nitrogen pressures. Ferromagnetism is observed up to the highest temperature measured, 400 K, and shows strong dependence on the Cr concentration and, especially, the nitrogen growth pressure. By varying the nitrogen pressure during film growth, the magnetic properties of the films can be changed while keeping a constant Cr concentration. The ferromagnetism is enhanced in the films that were grown at low nitrogen pressures and thus nitrogen deficient, suggesting an important role of defects in the ferromagnetism of this material.
Urban Growth in Chengdu, Western China: Application of Remote Sensing to Assess Planning and Policy Outcomes
Authors: Schneider Annemarie , Seto Karen C , Webster Douglas R
Published: May 2005

The majority of studies on Chinese urbanization have been focused on coastal areas, with little attention given to urban centers in the west. Western provinces, however, will unquestionably undergo significant urban change in the future as a result of the ‘Go West’ policy initiated in the 1990s. In this paper the authors examine the relationship between drivers of urban growth and land-use outcomes in Chengdu, capital of the western province of Sichuan, China. In the first part of this research, remotely sensed data are used to map changes in land cover in the greater Chengdu area and to investigate the spatial distribution of development with use of landscape metrics along seven urban-to-rural transects identified as key corridors of growth. Results indicate that the urbanized area increased by more than 350% between 1978 and 2002 in three distinct spatial trends: (a) near the urban fringe in all directions prior to 1990, (b) along transportation corridors, ring roads, and near satellite cities after 1990, and, finally, (c) infilling in southern and western areas (connecting satellite cities to the urban core) in the late 1990s. In the second part of this paper the authors connect patterns of growth with economic, land, and housing market reforms, which are explored in the context of urban planning initiatives. The results reveal that, physically, Chengdu is following trends witnessed in coastal cities of China, although the importance of various land-use drivers differs from that in the east (for example, in the low level of foreign direct investment to date). The information provided by the land-use analysis ultimately helped tailor policies and plans for better land management and reduced fragmentation of new development in the municipality.
Necessary Space—Time Conditions for Human Interaction
Authors: Miller Harvey J
Published: May 2005

Key scientific and application questions concern the relationships between individual-level activities and their effects on broader human phenomena, such as transportation systems and cities. Continuing advances in geographic information science, location-aware technologies, and geosimulation methods offer great potential for observational and simulation studies of human activities at high levels of spatiotemporal resolution. The author contributes by developing rigorous statements of the necessary space–time conditions for human interaction by extending a measurement theory for time geography. The extended measurement theory identifies necessary conditions both for physical and for virtual interaction. The theory suggests elegant and tractable solutions that can be derived from data available from location-aware technologies or geosimulation methods. These conditions and their solutions could be used to infer the possibilities for human interaction from detailed space–time trajectories and prisms generated from observation or simulation studies.
Regulatory on/off minimization of metabolic flux changes after genetic perturbations
Authors: Shlomi Tomer , Berkman Omer , Ruppin Eytan
Published: May 2005

Predicting the metabolic state of an organism after a gene knockout is a challenging task, because the regulatory system governs a series of transient metabolic changes that converge to a steady-state condition. Regulatory on/off minimization (ROOM) is a constraint-based algorithm for predicting the metabolic steady state after gene knockouts. It aims to minimize the number of significant flux changes (hence on/off) with respect to the wild type. ROOM is shown to accurately predict steady-state metabolic fluxes that maintain flux linearity, in agreement with experimental flux measurements, and to correctly identify short alternative pathways used for rerouting metabolic flux in response to gene knockouts. ROOM's growth rate and flux predictions are compared with previously suggested algorithms, minimization of metabolic adjustment, and flux balance analysis (FBA). We find that minimization of metabolic adjustment provides accurate predictions for the initial transient growth rates observed during the early postperturbation state, whereas ROOM and FBA more successfully predict final higher steady-state growth rates. Although FBA explicitly maximizes the growth rate, ROOM does not, and only implicitly favors flux distributions having high growth rates. This indicates that, even though the cell has not evolved to cope with specific mutations, regulatory mechanisms aiming to minimize flux changes after genetic perturbations may indeed work to this effect. Further work is needed to identify metrics that characterize the complete trajectory from the initial to the final metabolic steady states after genetic perturbations.
Stable X chromosome inactivation involves the PRC1 Polycomb complex and requires histone MACROH2A1 and the CULLIN3/SPOP ubiquitin E3 ligase
Authors: Hernández-Muñoz Inmaculada , Lund Anders H. , van der Stoop Petra , Boutsma Erwin , Muijrers Inhua , Verhoeven Els , Nusinow Dmitri A. , Panning Barbara , Marahrens York , van Lohuizen Maarten
Published: May 2005

X inactivation involves the stable silencing of one of the two X chromosomes in XX female mammals. Initiation of this process occurs during early development and involves Xist (X-inactive-specific transcript) RNA coating and the recruitment of Polycomb repressive complex (PRC) 2 and PRC1 proteins. This recruitment results in an inactive state that is initially labile but is further locked in by epigenetic marks such as DNA methylation, histone hypoacetylation, and MACROH2A deposition. Here, we report that the E3 ubiquitin ligase consisting of SPOP and CULLIN3 is able to ubiquitinate the Polycomb group protein BMI1 and the variant histone MACROH2A. We find that in addition to MACROH2A, PRC1 is recruited to the inactivated X chromosome in somatic cells in a highly dynamic, cell cycle-regulated manner. Importantly, RNAi-mediated knock-down of CULLIN3 or SPOP results in loss of MACROH2A1 from the inactivated X chromosome (Xi), leading to reactivation of the Xi in the presence of inhibitors of DNA methylation and histone deacetylation. Likewise, Xi reactivation is also seen on MacroH2A1 RNAi under these conditions. Hence, we propose that the PRC1 complex is involved in the maintenance of X chromosome inactivation in somatic cells. We further demonstrate that MACROH2A1 deposition is regulated by the CULLIN3/SPOP ligase complex and is actively involved in stable X inactivation, likely through the formation of an additional layer of epigenetic silencing.
A direct intracranial record of emotions evoked by subliminal words
Authors: Naccache Lionel , Gaillard Raphaël , Adam Claude , Hasboun Dominique , Clémenceau Stéphane , Baulac Michel , Dehaene Stanislas , Cohen Laurent
Published: May 2005

A classical but still open issue in cognitive psychology concerns the depth of subliminal processing. Can the meaning of undetected words be accessed in the absence of consciousness? Subliminal priming experiments in normal subjects have revealed only small effects whose interpretation remains controversial. Here, we provide a direct demonstration of semantic access for unseen masked words. In three epileptic patients with intracranial electrodes, we recorded brain potentials from the amygdala, a neural structure that responds to fearful or threatening stimuli presented in various modalities, including written words. We show that the subliminal presentation of emotional words modulates the activity of the amygdala at a long latency (>800 ms). Our result indicates that subliminal words can trigger long-lasting cerebral processes, including semantic access to emotional valence.
Impact of an ectoparasite on the immunity and pathology of an invertebrate: Evidence for host immunosuppression and viral amplification
Authors: Yang Xiaolong , Cox-Foster Diana L.
Published: May 2005

Varroa mites (
Varroa destructor
) are ectoparasites of honey bees (
Apis mellifera
) and cause serious damage to bee colonies. The mechanism of how varroa mites kill honey bees remains unclear. We have addressed the effects of the mites on bee immunity and the replication of a picorna-like virus, the deformed wing virus (DWV). The expression of genes encoding three antimicrobial peptides (abaecin, defensin, and hymenoptaecin) and four immunity-related enzymes (phenol oxidase, glucose dehydrogenase, glucose oxidase, and lysozyme) were used as markers to measure the difference in the immune response. We have demonstrated an example of an ectoparasite immunosuppressing its invertebrate host with the evidence that parasitization significantly suppressed expression of these immunity-related genes. Given that ticks immunosuppress their vertebrate hosts, our finding indicates that immunosuppression of hosts may be a common phenomenon in the interaction and coevolution between ectoparasites and their vertebrate and invertebrate hosts. DWV viral titers were significantly negatively correlated with the expression levels of the immunity-related enzymes. All bees had detectable DWV. Mite-infested pupae developed into adults with either normal or deformed wings. All of the deformed-wing bees were greatly infected by DWV (≈10
6
times higher than varroa-infested but normal-winged bees). Injection with heat-killed bacteria dramatically promoted DWV titers (10
5
times in 10 h) in the mite-infested, normal-winged bees to levels similar to those found in mite-infested, deformed-wing bees. Varroa mites may cause the serious demise of honey bees by suppressing bee immunity and by boosting the amplification of DWV in bees exposed to microbes.
ACACNA1Fmutation identified in an X-linked retinal disorder shifts the voltage dependence of Cav1.4 channel activation
Authors: Hemara-Wahanui Ariana , Berjukow Stanislav , Hope Carolyn I. , Dearden Peter K. , Wu Shu-Biao , Wilson-Wheeler Jane , Sharp Dianne M. , Lundon-Treweek Patricia , Clover Gillian M. , Hoda Jean-Charles , Striessnig Jörg , Marksteiner Rainer , Hering Steffen , Maw Marion A.
Published: May 2005

Light stimuli produce graded hyperpolarizations of the photoreceptor plasma membrane and an associated decrease in a voltagegated calcium channel conductance that mediates release of glutamate neurotransmitter. The Cav1.4 channel is thought to be involved in this process. TheCACNA1Fgene encodes the poreforming subunit of the Cav1.4 channel and various mutations inCACNA1Fcause X-linked incomplete congenital stationary night blindness (CSNB2). The molecular mechanism of the pathology underlying the CSNB2 phenotype remains to be established. Recent clinical investigations of a New Zealand family found a severe visual disorder that has some clinical similarities to, but is clearly distinct from, CSNB2. Here, we report investigations into the molecular mechanism of the pathology of this condition. Molecular genetic analyses identified a previously undescribed nucleotide substitution inCACNA1Fthat is predicted to encode an isoleucine to threonine substitution at CACNA1F residue 745. The I745T CACNA1F allele produced a remarkable approximately –30-mV shift in the voltage dependence of Cav1.4 channel activation and significantly slower inactivation kinetics in an expression system. These findings imply that substitution of this wild-type residue in transmembrane segment IIS6 may have decreased the energy required to open the channel. Collectively, these findings suggest that a gain-of-function mechanism involving increased Cav1.4 channel activity is likely to cause the unusual phenotype.
Human-modified temperatures induce species changes: Joint attribution
Authors: Root Terry L. , MacMynowski Dena P , Mastrandrea Michael D. , Schneider Stephen H.
Published: May 2005

Average global surface-air temperature is increasing. Contention exists over relative contributions by natural and anthropogenic forcings. Ecological studies attribute plant and animal changes to observed warming. Until now, temperature–species connections have not been statistically attributed directly to anthropogenic climatic change. Using modeled climatic variables and observed species data, which are independent of thermometer records and paleoclimatic proxies, we demonstrate statistically significant “joint attribution,” a two-step linkage: human activities contribute significantly to temperature changes and human-changed temperatures are associated with discernible changes in plant and animal traits. Additionally, our analyses provide independent testing of grid-box-scale temperature projections from a general circulation model (HadCM3).
A prokaryotic superoxide dismutase paralog lacking two Cu ligands: From largely unstructured in solution to ordered in the crystal
Authors: Banci Lucia , Bertini Ivano , Calderone Vito , Cramaro Fiorenza , Del Conte Rebecca , Fantoni Adele , Mangani Stefano , Quattrone Alessandro , Viezzoli Maria Silvia
Published: May 2005

Little is known about prokaryotic homologs of Cu,Zn superoxide dismutase (SOD), an enzyme highly conserved among eukaryotic species. In 138ArchaeaandBacteriagenomes, 57 of these putative homologs were found, 11 of which lack at least one of the metal ligands. Both the solution and the crystal structures of the SOD-like protein fromBacillus subtilis, lacking two Cu ligands and found to be enzymatically inactive, were determined. In solution, the protein is monomeric. The available nuclear Overhauser effects, together with chemical-shift index values, allowed us to define and to recognize the typical Cu,Zn SOD Greek β-barrel but with largely unstructured loops (which, therefore, sample a wide range of conformations). On the contrary, in the crystal structure (obtained in the presence of slight excess of Zn), the protein is well structured and organized in covalent dimers held by a symmetric bridge consisting of a Zn ion bound to an Asp–His dyad in a tetrahedral geometry. Couples of dimers held by hydrophobic interactions and H bonds are further organized in long chains. The order/disorder transition is discussed in terms of metal binding and physical state.
Dimeric SecA is essential for protein translocation
Authors: Jilaveanu Lucia B. , Zito Christopher R. , Oliver Donald
Published: May 2005

SecA facilitates bacterial protein translocation by its association with presecretory or membrane proteins and the SecYEG translocon channel. Once assembled, SecA ATPase undergoes cycles of membrane insertion and retraction at SecYEG that drive protein translocation in a stepwise fashion. SecA exists in equilibrium between a monomer and dimer, and association with its translocation ligands shifts this equilibrium dramatically. Here, we examined the proposal that protein translocation can occur by means of a SecA monomer. We produced a mutant SecA protein lacking residues 2–11, which was found to exist mostly as a monomer, and it was unable to complement a conditional-lethal
secA
mutant, was inactive for
in vitro
protein translocation, and was poorly active for translocation ATPase activity. Furthermore, we developed a technique termed membrane trapping, where wild-type SecA subunits became trapped within the membrane by overproduction of membrane-stuck mutant SecA proteins, and, in one case, a membrane-associated SecA heterodimer was demonstrated. Finally, we examined both endogenous and reconstituted membrane-bound SecA and found a significant level of SecA dimer in both cases, as assessed by chemical crosslinking. Collectively, our results strongly suggest that membrane-bound SecA dimer is critical for the protein translocation cycle, although these results cannot exclude participation of SecA monomer at some stage in the translocation process. Our findings have important implications regarding SecA motor function and translocon assembly and activation.
Targeting quantum dots to surface proteins in living cells with biotin ligase
Authors: Howarth Mark , Takao Keizo , Hayashi Yasunori , Ting Alice Y.
Published: May 2005

Escherichia coli
biotin ligase site-specifically biotinylates a lysine side chain within a 15-amino acid acceptor peptide (AP) sequence. We show that mammalian cell surface proteins tagged with AP can be biotinylated by biotin ligase added to the medium, while endogenous proteins remain unmodified. The biotin group then serves as a handle for targeting streptavidin-conjugated quantum dots (QDs). This labeling method helps to address the two major deficiencies of antibody-based labeling, which is currently the most common method for targeting QDs to cells: the size of the QD conjugate after antibody attachment and the instability of many antibody–antigen interactions. To demonstrate the versatility of our method, we targeted QDs to cell surface cyan fluorescent protein and epidermal growth factor receptor in HeLa cells and to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in neurons. Labeling requires only 2 min, is extremely specific for the AP-tagged protein, and is highly sensitive. We performed time-lapse imaging of single QDs bound to AMPA receptors in neurons, and we compared the trafficking of different AMPA receptor subunits by using two-color pulse–chase labeling.
Dietary amino acids: effect of depletion and recovery on synthesis in vitro in rat skeletal muscle and liver
Authors: Omstedt P. T. , Von Der Decken Alexandra
Published: May 2005

1. Rats were given diets containing 200 g/kg of a complete or incomplete amino acid mixture or of high- or low-quality proteins. After 6 d the amino acid-incorporating activity of ribosomes from skeletal muscle and liver was studied.

2. The level of isotope incorporation relative to ribosomal RNA was similar for casein supplemented with methionine and for a complete amino acid mixture with the composition of whole-egg protein. Per wet weight of tissue there was a significant decrease after feeding with the complete amino acid mixture.

3. There was a significant decrease in activity after feeding with amino acid mixtures deficient in lysine, methionine or tryptophan. In skeletal muscle, but not in liver, the ribosomal activity was less than that obtained with wheat gluten. Activity per wet weight of both tissues was less than that obtained with wheat gluten.

4. Refeeding with methionine for 1 d resulted in complete restoration of ribosomal activity and activity per wet weight in skeletal muscle.

5. After lysine deficiency, protein synthesis per unit wet weight of both tissues and ribosomal activity in liver were not restored after 2 d of refeeding. Recovery of ribosomal activity in skeletal muscle was complete after 1 d.

6. Rats receiving the 200 g casein/kg diet supplemented with methionine at daily energy levels of 263, 176, 141 and 106 KJ (62.6, 42.1, 33.7 and 25.3 kcal) showed no changes in ribosomal activity, but there was a significant decrease in activity per wet weight when 106 KJ were given.
Digestion of concentrates in sheep
Authors: Orskov E. R. , Fraser C. , McDonald I. , Smart R. I.
Published: May 2005

1. In one experiment according to a latin square design five sheep with fistulas in the abomasum and terminal ileum were given diets based on barley. Five diets were compared, one without protein supplementation, two with different levels of fish meal and two with combinations of fish meal and urea.

2. The amount of non-ammonia crude protein passing through the abomasum and removed from the small intestine increased linearly with fish-meal supplementation but the only effect of supplementation with urea was to increase the crude protein removed before the abomasum. It was concluded that urea did not appear to have a sparing effect on the degradation of dietary protein.

3. Seventy-eight lambs were used in a second experiment. They were given thirteen diets made up from a basal barley diet and different combinations of fish meal and urea.

4. Digestibility of organic matter and efficiency of food conversion increased with each level of fish-meal supplementation. The effects of urea supplementation on organic matter digestibility and efficiency of food conversion depended upon the level of fish meal in the diet and at the highest level of fish-meal supplementation there were no effects.

5. The maximum digestibility of organic matter (about 820 g/kg) could be achieved both with urea alone and fish meal alone and was attained at a lower level of nitrogen in the diet with urea. In contrast, the maximum food conversion efficiency that could be achieved with a fish-meal supplement was much greater than could be obtained with a supplement of urea alone.
Ad lib. feeding during recovery from malnutrition
Authors: Ashworth Ann
Published: May 2005

1. Ten severely malnourished children were studied.

2. During recovery the mean energy intake was 916 kJ (219 kcal)/kg per d when the children were fed ad lib. on a high-energy milk preparation.

3. When a lower-energy milk preparation was given ad lib. the children voluntarily increased the volume consumed but the mean energy intake fell to 703 kJ (168 kcal)/kg per d.

4. After recovery the children no longer consumed an increased volume when the lower-energy milk preparation was offered.

5. The results provide further evidence of the importance of feeding with a high-energy preparation for the treatment of malnutrition, and demonstrate that additional benefits can be obtained by offering such mixtures ad lib.
The rejection of a diet which has been associated with a single administration of an histidine -free amino acid mixture
Authors: Simson P. C. , Booth D. A.
Published: May 2005

1. A protein-free diet, to which an odour had been added, was offered to rats immediately after giving a gastric load of an histidine-free, but otherwise balanced, amino acid mixture. The same diet with a different odour was offered to the rats on another day, after administration of a control load of saline or a balanced amino acid mixture. After access to stock diet for 6 h of 1 d, the rats were offered two samples of protein-free diet, each with one of the two odours.

2. The rate of consumption of odorized protein-free diet was depressed 2–4 h after administration of the histidine-free load. In the later preference test, the dietary sample with odour which had been offered after the deficient load was rejected in favour of the sample with the odour which had been offered after the control load.

3. Rejection of the deficiency-paired odour in the final preference test did not occur when the histidine-free load had been given 2 h before the rats were first offered odorized diet. Also, there was in these instances no depression of rate of food consumption 2–4 h after loading. This indicated that aversive reactions to the odour were established by association of the odour with some effect of the histidine-free load which had occurred within 2 h of its administration, and that the early depression of intake and the much later rejection during choice were both expressions of these acquired reactions.

4. This rapid conditioning of selective rejection did not depend on previous prolonged protein deprivation or on the use of immature rats but did depend upon an intermittent supply of amino acids during 3 d before conditioning. Subcutaneous injection of deficient amino acid mixture did not establish rejection, suggesting that conditioning depended on rapid delivery of the deficient mixture into the circulation.

5. It is concluded that the critical biochemical events which lead to the rejection of diets that are imbalanced or deficient in essential amino acids occur soon after ingestion of the diet, and may have been effective in producing a conditioned aversion before any suppression of food intake appears. It is suggested that the depression of food consumption, which is the normal response to an imbalanced diet, is in this instance the result of conditioned response to sensory qualities of the diet rather than to the direct effect of biochemical stimuli.