bluuebunny/crossref_metadata_2025 huggingface dataset:
Monitoring the thermal and nonthermal excitation of fibers
Authors: Gretarsson Andri M. , Saulson Peter R.
Published: April 2005

The vibrations of well-isolated tungsten and fused silica fibers under tension were examined with a highly optimized shadow sensor. We adapted event search and analysis techniques used with resonant gravitational wave detectors to search for burstlike excitation events in the fibers. The method is optimized to distinguish rapid excitations against a background of thermal driving. The level and statistics of thermal noise were consistent with the fluctuation-dissipation theorem. No excess noise fiber events were seen. The energy innovation threshold for events was 1.8 kT in the case of a fused silica fiber loaded at 518 MPa (54% of breaking strength) during a 24 h run and 27 kT in the case of a tungsten fiber loaded at 326 MPa (94% of breaking strength) during a 6 h run.
Three-dimensional hydrodynamic experiments on the National Ignition Facility
Authors: Blue B. E. , Robey H. F. , Glendinning S. G. , Bono M. J. , Burkhart S. C. , Celeste J. R. , Coker R. F. , Costa R. L. , Dixit S. N. , Foster J. M. , Hansen J. F. , Haynam C. A. , Hermann M. R. , Holder J. P. , Hsing W. W. , Kalantar D. H. , Lanier N. E. , Latray D. A. , Louis H. , MacGowan B. J. , Maggelssen G. R. , Marshall C. D. , Moses E. I. , Nikitin A. J. , O'Brien D. W. , Perry T. S. , Poole M. W. , Rekow V. V. , Rosen P. A. , Schneider M. B. , Stry P. E. , Van Wonterghem B. M. , Wallace R. , Weber S. V. , Wilde B. H. , Woods D. T. , Young B. K.
Published: April 2005

The production of supersonic jets of material via the interaction of a strong shock wave with a spatially localized density perturbation is a common feature of inertial confinement fusion and astrophysics. The behavior of two-dimensional (2D) supersonic jets has previously been investigated in detail [J. M. Foster, B. H. Wilde, P. A. Rosen, T. S. Perry, M. Fell, M. J. Edwards, B. F. Lasinski, R. E. Turner, and M. L. Gittings, Phys. Plasmas 9, 2251 (2002)]. In three dimensions (3D), however, there are new aspects to the behavior of supersonic jets in compressible media. In this paper, the commissioning activities on the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. Sorem, Laser Focus World 30, 75 (1994)] to enable hydrodynamic experiments will be presented as well as the results from the first series of hydrodynamic experiments. In these experiments, two of the first four beams of NIF are used to drive a 40Mbar shock wave into millimeter scale aluminum targets backed by 100mg∕cc carbon aerogel foam. The remaining beams are delayed in time and are used to provide a point-projection x-ray backlighter source for diagnosing the three-dimensional structure of the jet evolution resulting from a variety of 2D and 3D features. Comparisons between data and simulations using several codes will be presented.
Effects of partial substitution of transition metals for cobalt on the high-temperature thermoelectric properties of Ca3Co4O9+δ
Authors: Yao Q. , Wang D. L. , Chen L. D. , Shi X. , Zhou M.
Published: April 2005

Polycrystalline samples of Ca3Co4−xMxO9+δ (M=Ni, Fe, Mn, Cu; x=0–0.6) have been prepared using sol-gel procedure followed by spark plasma sintering. Their thermoelectric properties have been carefully examined from room temperature to 1000 K. Substitution of Ni and Fe for Co caused the significant increase of electrical resistivity, but the thermopower changed only slightly. The magnitude of the increases in electrical resistivity and thermopower for the Mn-substituted samples was the most pronounced as compared with Ni- and Fe-substituted samples in the same substitution content, which could be explained by the strong localization of charge caused by Mn substitution. Temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for all samples. The activation energy increased with increasing substitution content. On the other hand, the Cu substitution resulted in decreases of both electrical resistivity and thermopower, and the activation energy remained unchanged for Cu-substituted sample. These results suggest that Ni, Fe, and Mn substitute the Co in the CoO2 layers, which is the conducting path in the Ca3Co4O9+δ structure, while Cu substitutes the Co in the Ca2CoO3 layers.
Spectroscopy with multichannel correlation radiometers
Authors: Harris A. I.
Published: April 2005

Correlation radiometers make true differential measurements in power with high accuracy and small systematic errors. This receiver architecture has been used in radio astronomy for measurements of continuum radiation for over 50 years; in this article, spectroscopy over broad bandwidths using correlation techniques is examined. After general discussions of correlation and the choice of hybrid phase experimental results from tests with a simple laboratory multichannel correlation radiometer are shown. Analysis of the effect of the input hybrid’s phase shows that a 90° hybrid is likely to be the best general choice for radio astronomy, depending on its amplitude match and phase flatness with frequency. The laboratory results verify that the combination of the correlation architecture and an analog lag correlator is an excellent method for spectroscopy over very wide bandwidths.
Photoluminescence enhancement in nanocomposite thin films of CdS–ZnO
Authors: Ayyub Pushan , Vasa Parinda , Taneja Praveen , Banerjee Rajarshi , Singh B. P.
Published: April 2005

We show that the photoluminescence emitted from a dense, two-component quantum dot ensemble on a thin film is significantly higher and decays much faster than that from quantum dots of either of the two pure systems (CdS and ZnO). The semiconductor nanocomposite, in which the characteristic grain size of each species was 2–3nm, was deposited directly on Si wafers by high-pressure magnetron sputtering, and exhibits a single, relatively sharp optical absorption edge.
Dynamics of plasma vortices: The role of the electron skin depth
Authors: Mentink J. H. , Bergmans J. , Kamp L. P. J. , Schep T. J.
Published: April 2005

The drift-Alfvén equations for a quasineutral plasma that is permeated by a strong background magnetic field describe the plasma dynamics in the transverse plane. Three scalar fields—which are linear combinations of parallel current, fluid vorticity, and density—are incompressibly advected by three streaming potentials. These three “generalized vorticities” are point-wise conserved. Their dynamics is studied numerically using the method of Contour Dynamics, which for this purpose is generalized from one to three different types of vorticity. In the regime where the electron inertial skin depth de is smaller than the ion sound gyroradius, the interaction dynamics of patches of generalized vorticity shows a preference for the formation of structures with length scale O(de). In this regime quasiperiodic splitting-merging cycles of single vorticity patches occur. These vortex interactions are considered to be the paradigm for the interactions of separated vortical structures in a strongly magnetized turbulent plasma.
Refractive index, sound velocity and thickness of thin transparent films from multiple angles picosecond ultrasonics
Authors: Côte R. , Devos A.
Published: April 2005

We present a method for refractive indices and longitudinal sound velocity measurements from picosecond ultrasonic experiments made at different probe incidence angles. For transparent or semitransparent materials such as dielectrics or semiconductors, picosecond ultrasonic experiments can lead to oscillations in the reflectivity curves whose frequency depends on the refractive indices, the sound velocity and the experiments angle. From these data we establish a simple method for the calculation of the refractive indices and verify it on a GaAs sample. We show on fluorinated silica glass and aluminum nitride practical applications of this method on thin films. From two experiments we measure the refraction index and the sound velocity of these materials, with no assumption on the materials properties or on the sample layers’ thicknesses. Here the materials are buried under a thin aluminum film. It illustrates the fact that the method can be applied to multilayers. From the same experiments we then derive the thickness of the layers. It shows that this method can render picosecond ultrasonic experiments independent from other characterization means.
Laser plasma acceleration of electrons: Towards the production of monoenergetic beams
Authors: Krushelnick K. , Najmudin Z. , Mangles S. P. D. , Thomas A. G. R. , Wei M. S. , Walton B. , Gopal A. , Clark E. L. , Dangor A. E. , Fritzler S. , Murphy C. D. , Norreys P. A. , Mori W. B. , Gallacher J. , Jaroszynski D. , Viskup R.
Published: April 2005

The interaction of high intensity laser pulses with underdense plasma is investigated experimentally using a range of laser parameters and energetic electron production mechanisms are compared. It is clear that the physics of these interactions changes significantly depending not only on the interaction intensity but also on the laser pulse length. For high intensity laser interactions in the picosecond pulse duration regime the production of energetic electrons is highly correlated with the production of plasma waves. However as intensities are increased the peak electron acceleration increases beyond that which can be produced from single stage plasma wave acceleration and direct laser acceleration mechanisms must be invoked. If, alternatively, the pulse length is reduced such that it approaches the plasma period of a relativistic electron plasma wave, high power interactions can be shown to enable the generation of quasimonoenergetic beams of relativistic electrons.
Effect of internal residual stress on the dielectric properties and microstructure of sputter-deposited polycrystalline (Ba,Sr)TiO3 thin films
Authors: Morito Kentaro , Suzuki Toshimasa
Published: April 2005

The role of internal residual stress in the dielectric properties of sputter-deposited polycrystalline (Ba,Sr)TiO3 thin films was studied experimentally using test samples in the form of parallel-plate capacitors on platinized silicon substrate. Different deposition-pressure conditions provided a systematic change in the film stress and we found that a reduction in the in-plane tensile stress of the films (primarily originating from the thermal expansion mismatch) was effective for increasing the dielectric constant in the out-of-plane direction. On the other hand, a large in-plane tensile stress (exceeding 1GPa) cannot be elastically accommodated in a 250-nm-thick (Ba,Sr)TiO3 thin film and the film relaxes through the optically visible microcrack formation. The relationship between the internal residual stress and the dielectric constant of the crack-free (Ba,Sr)TiO3 thin films could be well explained by the phenomenological thermodynamic theory of Landau–Ginsburg–Devonshire, without a transition to the in-plane polar axis phase.
Vortical structures in a laminar V-notched indeterminate-origin jet
Authors: New T. H. , Lim K. M. K. , Tsai H. M.
Published: April 2005

A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a V-notched indeterminate-origin jet with two peaks and two troughs. The laminar jet (Re=2000) was studied under forcing and nonforcing conditions to investigate the resultant dynamics of coherent large- and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex pairs which spread outwards into the ambient fluid and the troughs generating similar vortex pairs but entraining ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex pairs. Laser cross sections further showed that the subsequent formation of azimuthal ring vortices causes these streamwise vortex pairs to be entrained. This entrainment causes the streamwise vortex pairs to “roll-up” together with the ring vortices, leading to intense flow interactions between them. Interestingly, in a comparison with the experimental study reported by Longmire et al. [“Control of jet structure by crown-shaped nozzles,” AIAA J. 30, 505 (1992)] using higher Reynolds number air jet (Re=19000), it was found that forced jet flows with four peaks and four troughs yielded practically the same flow observations as the present nozzles with two peaks and two troughs. An updated flow model based on instantaneous and time-averaged evidence is presented to explain how the interaction of the vortex structures will give rise to the present new observations.
Magnetic and electrical properties of La0.7Sr0.3MnO3–Zn0.8Co0.2Al0.01O junctions on silicon substrates
Authors: Yan L. , Goh W. C. , Ong C. K.
Published: April 2005

p - n and n-p junctions based on the p-type La0.7Sr0.3MnO3 (LSMO) and the n-type Zn0.8Co0.2Al0.01O (ZCAO) have been fabricated on silicon substrates via pulsed laser deposition. Both LSMO and ZCAO thin layers deposited on Si had good remanent magnetic characteristics at room temperature. The metal-semiconductor transition temperature (Tp) of the ZCAO∕LSMO n-p junction is lower than that of the pure LSMO layer, which is due to the influence of the top ZCAO layer. The low-field (±3kOe) magnetoresistance (LFMR) value of the ZCAO∕LSMO n-p junction is about −15% at 80 K, which is lower than that of the pure LSMO layer on Si (−21%). As deposited on top of ZCAO∕Si, the LSMO layer has no LFMR property. The LSMO∕ZCAO p-n junction has good current flow versus voltage (I–V) property with a threshold voltage of about 1.5 V at 80–300 K, which is very similar to normal Si-based p-n diode. Its I–V characteristics cannot be modulated by low external magnetic fields. However, the ZCAO∕LSMO n-p junction demonstrated a nonlinear I–V behavior, and its current flow can be modulated by low external magnetic fields. The resistance variation of the ZCAO∕LSMO n-p junction was −15% under forward bias voltages as the external magnetic field varied from 0 to 3 kOe.
Penetration of a negatively buoyant jet in a miscible liquid
Authors: Philippe P. , Raufaste C. , Kurowski P. , Petitjeans P.
Published: April 2005

We report experimental results on the evolution of a laminar liquid jet injected with negatively buoyant condition in a miscible surrounding liquid. Since molecular diffusion is negligible, the only significant miscible effect is the absence of any surface tension. After an initial intrusion phase, the jet reaches a steady-state characterized by a constant penetration depth. A simple theoretical model is derived which successfully predicts the transient phase as well as the subsequent steady state in terms of stationary penetration depth and jet’s profile. All the experimental points collapse on a master curve involving two dimensionless numbers: the densimetric Froude number Fr and S, a number comparing viscous friction to buoyancy. Finally, this curve obtained for laminar flows is compared to classical results on turbulent fountains.
Measuring the top–bottom effect of a tilted thick specimen in an ultrahigh-voltage electron microscope
Authors: Zhang Hai-Bo , Yang Chao , Takaoka Akio
Published: April 2005

From the 40 nm gold particles on the surface of a 5μm thick amorphous specimen tilted by a 360° tilt holder, we have observed the top–bottom effect (TBE) in a 3 MV ultrahigh-voltage electron microscope. Because of the TBE, object points located on the top surface of the specimen have poorer image quality than those on the bottom. Measurements of the diameter and contrast of gold-particle images show that the TBE becomes strong when increasing the effective thickness of the specimen by tilting the specimen. The TBE can produce additional image blurring of about 10, 27, and 74 nm in the gold-particle diameter for the effective thickness of 5.0, 10.0, and 14.6μm, respectively. The need for further investigations is thus suggested to understand and determine the TBE limitation on the resolution or fidelity of electron tomography for thick specimens.
Alternative form for the pseudo-Voigt peak shape
Authors: Végh János
Published: April 2005

A number of different approximations are in use in different fields of spectroscopy for calculating the Voigt line shape. The most popular among them is the pseudo-Voigt line shape. Its main disadvantage is that it is quite complicated to conclude from its parameters the natural width of the line and the spectrometer contribution. Furthermore there exists no simple possibility to replace the line shape with a more accurate approximation to the Voigt shape. An alternative form for this peak shape is presented, which is parametrized with the natural width and spectrometer contribution and transparently maps these parameters to the parameters “Gauss/Lorentz mixing ratio” and “total width.” This representation keeps the calculation simple and quick. Furthermore it allows a smooth transition to the more accurate Voigt approximations.
Analog
+
digital
phase and frequency detector for phase locking of diode lasers
Authors: Cacciapuoti L. , de Angelis M. , Fattori M. , Lamporesi G. , Petelski T. , Prevedelli M. , Stuhler J. , Tino G. M.
Published: April 2005

We describe a type of phase and frequency detector employing both an analog phase detector and a digital phase and frequency detector. The analog and digital detectors are mutually exclusive so that only one of them is active at any given time, resulting in a phase detector with both the broad capture range of digital circuits and the high speed and low noise of analog mixers. The detector has been used for phase locking the diode lasers generating the sequence of Raman pulses in an atom interferometer. The rms phase error of the phase lock is about 100 mrad in a 5 Hz–10 MHz bandwidth. The limit set on the interferometer phase resolution by the residual phase noise is 1.1 mrad. Since the digital circuitry is implemented with a programmable logic device the detector can be easily adapted to other experiments requiring frequency/phase stabilization of lasers sources.
Neutral density map of Hall thruster plume expansion in a vacuum chamber
Authors: Walker Mitchell L. R. , Gallimore Alec D.
Published: April 2005

A neutral background pressure map of the large vacuum test facility (LVTF) is presented. The LVTF is mapped at cold anode flow rates of 5.25, 10.46, and 14.09mg∕s. In addition, neutral background pressure maps are created at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46mg∕s for discharge voltages of 300 and 500V, corresponding to P5 Hall thruster operating conditions ranging from 1.5to5.0kW. The chamber pressure is mapped at nominal xenon pumping speeds of 140 000 and 240000l∕s. The pressure map is performed with a rake consisting of five calibrated Bayard–Alpert hot-cathode ionization gauges. The plume expansion appears to be independent of anode flow rate and facility background pressure. Analysis of axial pressure profiles on the LVTF’s centerline shows that the plume pressure decreases from a maximum at the thruster exit plane down to the facility background pressure at approximately 2m downstream of the exit plane. Comparison of axial pressure profiles on the LVTF’s centerline shows that the neutral density is nearly the same for cold flow and hot flow. The study shows that a cold flow neutral density background map accurately characterizes the neutral density in an operating Hall thruster plume.
Space-variant polarization manipulation of a thermal emission by a SiO2 subwavelength grating supporting surface phonon-polaritons
Authors: Dahan Nir , Niv Avi , Biener Gabriel , Kleiner Vladimir , Hasman Erez
Published: April 2005

Space-variant polarization manipulation of enhanced nondirectional thermal emission in a narrow spectral peak is presented. The emission is attributed to surface phonon-polariton excitation from space-variant subwavelength SiO2 gratings. Polarization manipulation was obtained by discretely controlling the local orientation of the grating. We experimentally demonstrated thermal emission in an axially symmetric polarization distribution. Theoretical calculations based on rigorous coupled-wave analysis are presented along with experimental results.
Optical properties of highly ordered AlN nanowire arrays grown on sapphire substrate
Authors: Zhao Qing , Zhang Hongzhou , Xu Xiangyu , Wang Zhe , Xu Jun , Yu Dapeng , Li Guohua , Su Fuhai
Published: April 2005

Highly ordered AlN nanowire arrays were synthesized via a simple physical vapor deposition method on sapphire substrate. The nanowires have an extremely sharp tip <10nm, with the average length around 3 μm. Raman spectroscopy analysis on the AlN nanowire arrays revealed that the lifetime of the phonons is shorter than that in bulk AlN. The transmission spectra of the AlN nanowires showed a blueshift ∼0.27eV at the absorption edge with that of the bulk AlN, which is closely related to the small size of the nanowires.
In situ investigation of growth and thermal stability of ultrathin Si layers on the HfO2∕Si (100) high-κ dielectric system
Authors: Lebedinskii Yu. Yu. , Zenkevich A. , Gusev E. P. , Gribelyuk M.
Published: April 2005

We report on growth behavior and thermal stability of ultrathin (0.5–12nm) Si layers grown on HfO2 (3nm)∕Si (100) by pulsed laser deposition (PLD) as investigated by in situ x-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy. Layer-by-layer growth of ultrathin Si overlayer was observed during room-temperature deposition, while an island-like growth behavior dominated at T⩾900K. Vacuum annealing of the Si(3.5nm)∕HfO2 (3nm)∕Si (100) structure resulted in Si crystallization and coarsening. The process was accompanied by hafnium oxide reduction at the surface causing HfO2 layer decomposition and hafnium silicide formation. These interface reactions depended on the thickness of the Si cap layer—the thicker (12nm) Si layer on HfO2 (40nm)∕Si (100) appeared to be stable at least up to T=1200K. An ultrathin (0.3nm) Al2O3 layer PLD deposited between HfO2 and the Si overlayer also effectively inhibited the reactions up to T=1200K.
Band structure and alignment of the AlN∕SiC heterostructure
Authors: Choi Jongwoo , Puthenkovilakam Ragesh , Chang Jane P.
Published: April 2005

Valence-band structures and band alignments at the AlN∕SiC interface are studied by x-ray photoelectron spectroscopy (XPS) combined with first-principle calculations. Theoretical valence-band structures of SiC and AlN, and band offsets at their interface, were obtained by plane-wave pseudopotential method within the framework of density functional theory. The conduction- and valence-band offsets determined by XPS are 1.3 and 1.7eV, respectively, in excellent agreement with the theoretical values. These relatively large band offsets indicate that AlN is suitable as a gate dielectric or a lattice-matched interfacial layer on SiC.
Role of surface-to-volume ratio of metal nanoparticles in optical properties of Cu:Al2O3 nanocomposite films
Authors: Requejo-Isidro J. , del Coso R. , Solis J. , Gonzalo J. , Afonso C. N.
Published: April 2005

We report on the role of the surface-to-volume ratio of Cu nanoparticles (NPs) both in the linear and nonlinear optical properties of Cu:Al2O3 nanocomposite films. The results show that when the shape of the NPs deviates sufficiently from that of a sphere, the increase of the fraction of metal atoms present at the surface (NS) with respect to the total amount of atoms (NT) in the NP leads to a substantial reduction of the enhancement of the local field. As a consequence, for NS∕NT values above a certain threshold (≈0.4–0.5), the surface–plasma resonance is smeared out and the nonlinear optical response of the nanocomposite film becomes very weak and independent of the dimensions of the NPs or their volume fraction in the matrix.
Effects of nonequally biaxial misfit strains on the phase diagram and dielectric properties of epitaxial ferroelectric thin films
Authors: Wang Jie , Zhang Tong-Yi
Published: April 2005

The effects of nonequally biaxial in-plane misfit strains on the equilibrium polarization states and the dielectric properties of single-domain epitaxial ferroelectric thin films are investigated by a nonlinear thermodynamic theory. The “misfit strain-misfit strain” and “temperature-misfit strain” phase diagrams for single-domain BaTiO3 (BT) and PbTiO3 (PT) thin films grown on tetragonal substrates are developed by minimizing the Helmholtz free energy. The nonequally biaxial misfit strains cause the presence of two in-plane tetragonal ferreoelectric phases, a1 (P1≠0,P2=P3=0) and a2 (P2≠0,P1=P3=0), in both BT and PT thin films, which do not exist if the misfit strains are equally biaxial.
Demonstration of a 320×256 two-color focal plane array using InAs/InGaAs quantum dots in well detectors
Authors: Krishna Sanjay , Forman Darren , Annamalai Senthil , Dowd Philip , Varangis Petros , Tumolillo Tom , Gray Allen , Zilko John , Sun Kathy , Liu Mingguo , Campbell Joe , Carothers Daniel
Published: April 2005

We report the demonstration of a two-color infrared focal plane array based on a voltage-tunable quantum dots-in-well (DWELL) design. The active region consists of multiple layers of InAs quantum dots in an In0.15Ga0.85As quantum well. Spectral response measurements yielded a peak at 5.5μm for lower biases and at 8–10μm for higher biases. Using calibrated blackbody measurements, the midwavelength and long wavelength specific detectivity (D*) were estimated to be 7.1×1010cmHz1∕2∕W(Vb=1.0V) and 2.6×1010cmHz1∕2∕W(Vb=2.6V) at 78 K, respectively. This material was processed into a 320×256 array and integrated with an Indigo 9705 readout chip and thermal imaging was achieved at 80 K.
Blue-purplish InGaN quantum wells with shallow depth of exciton localization
Authors: Akasaka Tetsuya , Gotoh Hideki , Nakano Hedetoshi , Makimoto Toshiki
Published: April 2005

Temperature-dependent time-resolved PL measurements were performed for blue-purplish InGaN multiple quantum wells grown on various kinds of underlying layers (ULs). By using an InGaN UL, excitons recombined radiatively at low temperatures, being confined in the shallow potential minima (7.1meV), while they radiatively recombined two-dimensionally with high luminescent efficiency at around room temperature, being delocalized thermally from the potential minima. Therefore, the exciton localization is not necessary in order to obtain high luminescent efficiency, but it is important to annihilate the nonradiative recombination centers by incorporation of indium atoms into ULs.
Low-resistance magnetic tunnel junctions prepared by partial remote plasma oxidation of 0.9nm Al barriers
Authors: Ferreira Ricardo , Freitas Paulo P. , MacKenzie Maureen , Chapman John N.
Published: April 2005

Current perpendicular to the plane read-head elements suitable for high-density magnetic storage require low resistance while maintaining a reasonable magnetoresistive (MR) signal (R×A<1Ωμm2 and MR>20% for areal densities >200Gb∕in2). This letter shows that competitive low R×A junctions can be produced using underoxidized barriers starting from 0.9nm thick Al layers. For as-deposited junctions, tunneling magnetoresistance (TMR) ∼20% for R×A∼2–15Ωμm2 is obtained, while in the R×A∼60–150Ωμm2 range, TMR values between 40% to 45% are achieved. A limited number of junctions exhibits considerably lower R×A values with respect to the average, while keeping a similar MR (down to 0.44Ωμm2 with TMR of 20% and down to 2.2Ωμm2 with TMR of 52%). Experimental data suggest that current confinement to small regions (barrier defects/hot spots) may explain these results.
Lines That Induce Phenomenal Transparency
Authors: Grieco Alba , Roncato Sergio
Published: April 2005

Three neighbouring opaque surfaces may appear split into two layers, one transparent and one opaque beneath, if an outline contour is drawn that encompasses two of them. The phenomenon was originally observed by Kanizsa [1955 Rivista di Psicologia69 3–19; 1979 Organization in Vision: Essays on Gestalt Psychology (New York: Praeger)], for the case where an outline contour is drawn to encompass one of the two parts of a bicoloured figure and a portion of a background of lightest (or darkest) luminance. Preliminary observations revealed that the outline contour yields different effects: in addition to the stratification into layers described by Kanizsa, a second split, opposite in depth order, may occur when the outline contour is close in luminance to one of the three surfaces. An initial experiment was designed to investigate what conditions give rise to the two phenomenal transparencies: this led to the conclusion that an outline contour superimposed on an opaque surface causes this surface to emerge as a transparent layer when the luminances of the contour and the surface differ, in absolute value, by no more than 13.2 cd m−2. We have named this phenomenon ‘transparency of the intercepted surface’, to distinguish it from the phenomenal transparency arising when the contour and surface are very different in luminance. When such a difference exists, the contour acts as a factor of surface definition and grouping: the portion of the homogeneous surface it bounds emerges as a fourth surface and groups with a nearby surface if there is one close in luminance. The transparency phenomena (‘transparency of the contoured surface’) perceived in this context conform to the constraints of Metelli's model, as demonstrated by a second experiment, designed to gather ‘opacity’ ratings of stimuli. The observer judgments conformed to the values predicted by Metelli's formula for perceived degree of transparency, a. The role of the outline contour in conveying figural and intensity information is discussed.
Structured Perceptual Input Imposes an Egocentric Frame of Reference—Pointing, Imagery, and Spatial Self-Consciousness
Authors: Marcel Anthony , Dobel Christian
Published: April 2005

Perceptual input imposes and maintains an egocentric frame of reference, which enables orientation. When blindfolded, people tended to mistake the assumed intrinsic axes of symmetry of their immediate environment (a room) for their own egocentric relation to features of the room. When asked to point to the door and window, known to be at mid-points of facing (or adjacent) walls, they pointed with their arms at 180° (or 90°) angles, irrespective of where they thought they were in the room. People did the same when requested to imagine the situation. They justified their responses (inappropriately) by logical necessity or a structural description of the room rather than (appropriately) by relative location of themselves and the reference points. In eight experiments, we explored the effect on this in perception and imagery of: perceptual input (without perceptibility of the target reference points); imaging oneself versus another person; aids to explicit spatial self-consciousness; order of questions about self-location; and the relation of targets to the axes of symmetry of the room. The results indicate that, if one is deprived of structured perceptual input, as well as losing one's bearings, (a) one is likely to lose one's egocentric frame of reference itself, and (b) instead of pointing to reference points, one demonstrates their structural relation by adopting the intrinsic axes of the environment as one's own. This is prevented by providing noninformative perceptual input or by inducing subjects to imagine themselves from the outside, which makes explicit the fact of their being located relative to the world. The role of perceptual contact with a structured world is discussed in relation to sensory deprivation and imagery, appeal is made to Gibson's theory of joint egoreception and exteroception, and the data are related to recent theories of spatial memory and navigation.
Optic-Flow-Based Perception of Two-Dimensional Trajectories and the Effects of a Single Landmark
Authors: Bertin René J V , Israël Isabelle
Published: April 2005

Human observers can detect their heading direction on a short time scale on the basis of optic flow. We investigated the visual perception and reconstruction of visually travelled two-dimensional (2-D) trajectories from optic flow, with and without a landmark. As in our previous study, seated, stationary subjects wore a head-mounted display in which optic-flow stimuli were shown that simulated various manoeuvres: linear or curvilinear 2-D trajectories over a horizontal plane, with observer orientation either fixed in space, fixed relative to the path, or changing relative to both. Afterwards, they reproduced the perceived manoeuvre with a model vehicle, whose position and orientation were recorded. Previous results had suggested that our stimuli can induce illusory percepts when translation and yaw are unyoked. We tested that hypothesis and investigated how perception of the travelled trajectory depends on the amount of yaw and the average path-relative orientation. Using a structured visual environment instead of only dots, or making available additional extra-retinal information, can improve perception of ambiguous optic-flow stimuli. We investigated the amount of necessary structuring, specifically the effect of additional visual and/or extra-retinal information provided by a single landmark in conditions where illusory percepts occur. While yaw was perceived correctly, the travelled path was less accurately perceived, but still adequately when the simulated orientation was fixed in space or relative to the trajectory. When the amount of yaw was not equal to the rotation of the path, or in the opposite direction, subjects still perceived orientation as fixed relative to the trajectory. This caused trajectory misperception because yaw was wrongly attributed to a rotation of the path: path perception is governed by the amount of yaw in the manoeuvre. Trajectory misperception also occurs when orientation is fixed relative to a curvilinear path, but not tangential to it. A single landmark could improve perception. Our results confirm and extend previous findings that, for unambiguous perception of ego-motion from optic flow, additional information is required in many cases, which can take the form of fairly minimal, visual information.
Novel Visual Illusions Related to Vasarely's ‘Nested Squares’ Show That Corner Salience Varies with Corner Angle
Authors: Troncoso Xoana G , Macknik Stephen L , Martinez-Conde Susana
Published: April 2005

Vasarely's ‘nested-squares’ illusion shows that 90° corners can be more salient perceptually than straight edges. On the basis of this illusion we have developed a novel visual illusion, the ‘Alternating Brightness Star’, which shows that sharp corners are more salient than shallow corners (an effect we call ‘corner angle salience variation’) and that the same corner can be perceived as either bright or dark depending on the polarity of the angle (ie whether concave or convex: ‘corner angle brightness reversal’). Here we quantify the perception of corner angle salience variation and corner angle brightness reversal effects in twelve naive human subjects, in a two-alternative forced-choice brightness discrimination task. The results show that sharp corners generate stronger percepts than shallow corners, and that corner gradients appear bright or dark depending on whether the corner is concave or convex. Basic computational models of center – surround receptive fields predict the results to some degree, but not fully.
Winter disruption of the circadian clock in chestnut
Authors: Ramos Alberto , Pérez-Solís Estefanía , Ibáñez Cristian , Casado Rosa , Collada Carmen , Gómez Luis , Aragoncillo Cipriano , Allona Isabel
Published: April 2005

Circadian clock performance during winter dormancy has been investigated in chestnut by using as marker genesCsTOC1andCsLHY, which are homologous to essential components of the central circadian oscillator inArabidopsis. During vegetative growth, mRNA levels of these two genes in chestnut seedlings and adult plants cycled daily, as expected. However, during winter dormancy,CsTOC1andCsLHYmRNA levels were high and did not oscillate, indicating that the circadian clock was altered. A similar disruption was induced by chilling chestnut seedlings (to 4°C). Normal cycling resumed when endodormant or cold-treated plants were returned to 22°C. The behavior ofCsTOC1andCsLHYduring a cold response reveals a relevant aspect of clock regulation not yet encountered inArabidopsis.
Static and dynamic quantum effects in molecular liquids: A linearized path integral description of water
Authors: Poulsen Jens Aage , Nyman Gunnar , Rossky Peter J.
Published: April 2005

Structure, transport properties, and IR spectra including quantum effects are calculated for a flexible simple point charge model of liquid water. A recently introduced combination of a variational local harmonic description of the liquid potential surface and the classical Wigner approximation for the dynamics is used. The potential energy and interatomic radial distribution functions are in good agreement with accurate results from the literature and are significantly closer to experiment than predictions found from classical theory. The oxygen and hydrogen velocity correlation functions are also calculated, and the corresponding molecular diffusion coefficient is in good accord with existing theoretical estimates including quantum effects. Of most interest, anab initioquantum correction factor is obtained to correct the far IR spectrum of water. When corrected, a spectrum based on a classical simulation yields results that agree well with experiment. Combined with internal tests of consistency, these observations indicate that this quite flexible approach will be effective for a variety of molecular problems involving the dynamics of light nuclei.
Not for the Profit, but for the Satisfaction? – Evidence on Worker Well‐Being in Non‐Profit Firms
Authors: Benz Matthias
Published: April 2005

Non‐profit firms are often seen as workplaces where people not only work for money, but also find substantial satisfaction in the kind of work they do. Studies looking at compensating wage differentials, however, have only found limited support for this notion. In this paper, a novel approach is undertaken to compare the utilities of non‐profit and for‐profit employees, by using measures of job satisfaction. The results show that in both the United States and Great Britain over the 1990's, non‐profit workers were generally more satisfied with their jobs than for‐profit workers. The robustness of the results is explored in detail, and implications for the governance of non‐profit firms are shortly discussed.
The Role of the State in Economic Development
Authors: Tabellini Guido
Published: April 2005

This paper discusses the recent literature on the role of the state in economic development. It concludes that government incentives to enact sound policies are key to economic success. It also discusses the evidence on what happens after episodes of economic and political liberalizations, asking whether political liberalizations strengthen government incentives to enact sound economic policies. The answer is mixed. Most episodes of economic liberalizations are indeed preceded by political liberalizations. But the countries that have done better are those that have managed to open up the economy first, and only later have liberalized their political system.
PACT: an efficient and powerful algorithm for generating area cladograms
Authors: Wojcicki Maggie , Brooks Daniel R.
Published: April 2005

Aim  To introduce and describe the functioning of a new algorithm, phylogenetic analysis for comparing trees (PACT), for generating area cladograms that provide accurate representation of information contained in taxon–area cladograms.

Methods  PACT operates in the following steps. Convert all phylogenies to taxon–area cladograms. Convert all taxon–area cladograms to Venn diagrams. Choose any taxon–area cladogram from the set of taxon–area cladograms to be analysed and determine its elements. This will be the template area cladogram. Select a second taxon–area cladogram. Determine its elements. Document which elements in the second tree occur in the template tree (denoted by ‘Y’) and which do not (denoted by ‘N’). Each ‘Y’ indicates a match with previous pattern and these are combined. Each ‘N’ is a new element and is attached to the template area cladogram at the node where it is linked with a Y. This requires two rules: (1) ‘Y + Y = Y’ (combine common elements) as long as they are connected at the same node; and (2) ‘Y + N = YN’ (add novel elements to the template area cladogram at the node where they first appear). Once the novel elements in the second taxon–area cladogram have been added to the template area cladogram, see if any of them can be further combined. This requires three additional rules: (1) ‘Y(Y− = Y(Y−’ (do not combine Y's if they are attached at different nodes on the template area cladogram); (2) ‘Y + YN = YN’ (Y is part of group YN); and (3) ‘YN + YN = YNN’ (Y is the same for each, but each N is different). Repeat for all available taxon–area cladograms.

Results  Three exemplars demonstrate that PACT provides the most accurate area cladograms for vicariance‐driven biotic diversification, dispersal‐driven biotic diversification and taxon pulse‐driven biotic diversification. PACT can also be used as an a priori method of biogeographical analysis.

Main conclusions  PACT embodies all the strong points and none of the weaknesses of previously proposed methods of historical biogeography. It is most useful as an a posteriori method, but it is also superior to all previous a priori methods because it does not specify costs, or weights or probabilities, or likelihoods of particular biogeographical processes a priori and is thus sensitive to clade‐specific historical contingencies.
Rapoport effect and biomic specialization in African mammals: revisiting the climatic variability hypothesis
Authors: Fernández Manuel Hernández , Vrba Elisabeth S.
Published: April 2005

Aim One of the mechanisms proposed to explain the tendency for geographical range size to increase from the equator to the poles, known as the Rapoport effect, is the climatic variability hypothesis. It states that, towards higher latitudes, greater seasonal climatic variability is the most important pressure that selectively promotes greater general climatic tolerance of species, and therefore also more extensive species ranges. In order to test this hypothesis, we explore the influence of climate, area and biome diversity on the latitudinal gradient of climatic specialization.

Location The study used the large mammal assemblage from Africa.

Methods The degree of climatic specialization of African large mammals (Primates, Carnivora, Proboscidea, Perissodactyla, Hyracoidea, Tubulidentata, Artiodactyla and Pholidota) is investigated using the biomic specialization index (BSI) for each mammal species, based on the number of biomes it inhabits. We studied the influence of 11 climatic and biogeographical predictors in the latitudinal pattern of biomic specialization. Stepwise multiple regressions were used to identify the strongest predictors of biomic specialization in Africa and, separately, in both continental hemispheres. We also studied differences among taxonomical groups (primates, carnivores and artiodactyls). We used correlograms generated using Moran's I coefficients to control for spatial autocorrelation in all these analyses.

Results Average BSI values for successive 1°‐latitude bands generally decline towards the equator and temperature variability emerged as the most predictive factor in the regression model for the whole continent, thus supporting the climatic variability hypothesis. Nevertheless, there are differences between hemispheres and among taxa. While temperature variability is the most important predictor of latitudinal variability in biomic specialization in most of the regression models for the northern hemisphere, continental area for each latitudinal band is the best predictor in all the regression models in the southern hemisphere.

Main conclusions It appears that similar patterns in latitudinal variation in average BSI may be caused by different factors in the two hemispheres. We suggest that the strong north–south geographical asymmetry of Africa, which influences its biogeographical structure, and the presence of land connections with Eurasia in the northern hemisphere are responsible for the observed patterns. Our data illustrate the influence of continental biogeographical structure and history on macroecological patterns.
The distribution of ground spiders (Araneae, Gnaphosidae) along the altitudinal gradient of Crete, Greece: species richness, activity and altitudinal range
Authors: Chatzaki M. , Lymberakis P. , Markakis G. , Mylonas M.
Published: April 2005

Aim  To study the altitudinal variation of ground spiders (Araneae, Gnaphosidae) of Crete, Greece, as far as species composition, species richness, activity and range of distribution are concerned.

Location  Altitudinal zones (0–2400 m) along the three main mountain massifs of the island of Crete.

Methods  Thirty‐three sampling sites were located from 0 to 2400 m a.s.l. on Crete, and sampled using pitfall traps. Material from the high‐activity period of Gnaphosidae (mid‐spring to mid‐autumn) was analysed. Sampling sites were divided into five altitudinal zones of 500 m each. Statistical analysis involved univariate statistics (anova) and multivariate statistics, such as multidimensional scaling (MDS) and cluster analysis (UPGMA) using binomial data of species presence or absence.

Results  Species richness declines with altitude and follows a hump‐shaped pattern. The activity pattern of the family, as a whole, is not correlated with altitude and is highly species‐specific. In the highest zone, both species richness and activity decline dramatically. The altitudinal range of species distribution increases with altitude. On the Cretan summits live highly tolerant lowland species and isolated residents of the high mountains of Crete. Two different patterns of community structure are recorded.

Main conclusions  Communities of Gnaphosidae on Crete present two distinct structures following the altitudinal gradient, these being separated by a transitional zone between 1600 and 2000 m. This study supports previous results which show a hump‐shaped decline in species richness of Gnaphosidae along altitudinal gradients, leading to a peak at 400–700 m, where an optimum of environmental factors exists. This makes this zone the meeting point of the often opportunistic lowland species with the older and most permanent residents of the island. Rapoport's rule on the positive correlation of the altitudinal range of species distributions with altitude is also supported. The high activity recorded for the species that persist on the high mountains of Crete is indicative of a tolerant arachnofauna, and is considered to result from relaxation of competitive interactions with other species. This is related to a reduction in species numbers, shortening of the activity period on high mountains and the unique presence of high mountain species that thrive only there. As shown in our study, strategies to cope with altitude are species‐specific. Therefore, there cannot exist one single model to describe how animals react to the change in altitude, even under the same environmental conditions.