bluuebunny/crossref_metadata_2025 huggingface dataset:
Late-Glacial and Holocene Glacier Fluctuations and Environmental Change on South Georgia, Southern Ocean
Authors: Clapperton Chalmers M. , Sugden David E. , Birnie Jacqueline , Wilson Mandy J.
Published: December 2004

South Georgia provides a terrestrial record of postglacial environmental change from a largely oceanic zone of the Earth. The record is representative of the southern westerlies and provides a link between Antarctica and the temperate zones of southern South America. Evidence from glacial geomorphology, slope stratigraphy, and analyses of environmental indicators in peat and lake cores is used to interpret this record. Wastage of the full-glacial ice cap was interrupted by a late-glacial stade of the outlet and valley glaciers before ca. 10,000 yr B.P. Plant growth had begun at low altitude (<50 m) on the sheltered (lee side) northeast coast within the late-glacial moraine limits by 9700 yr B.P. Environmental conditions on slopes above 80 m probably were too rigorous for a stable vegetation cover until ca. 6400 yr B.P. This was followed by a period from 5600 to 4800 yr B.P. when conditions were warmer than at present by up to 0.6°C. Periods of climatic cooling occurred at ca. 4800-3800 yr B.P., ca. 3400-1800 yr B.P., and within the last 1400 yr. The most extensive Holocene advance of South Georgia glaciers culminated just before 2200 yr B.P. These Holocene temperature changes of between 0.5 and 1.0°C are comparable in scale and timing to those identified from recent analyses of Vostok ice cores from the Antarctic ice sheet.
Accuracy of North American Human Skeleton Ages
Authors: Stafford Thomas W. , Hare P.E. , Currie Lloyd , Jull A. J. T. , Donahue Douglas
Published: December 2004

Accelerator mass spectrometry (AMS) radiocarbon dates fail to provide conclusive evidence that all New World human fossils are younger than approximately 11,000 yr. Because fossil bones vary widely in preservation, their radiocarbon dates are not equally accurate. Molecular-level radiocarbon dating, which used individual amino acids to assess fossil diagenesis, revealed that dates on known-age, noncollagenous bone were underestimated by at least 2000 to 9000 yr. The significance is that >11,000-yr-old fossil bones with poor preservation would yield Holocene and not Pleistocene radiocarbon ages, regardless of what chemical pretreatment or 14C counting method was used. Irreplaceable evidence for Pleistocene-age fossils in the New World could be lost if the diagenesis of fossil bones is not evaluated before the bones are radiocarbon dated. In contrast, radiocarbon ages for collagenous fossils can be determined more accurately if 14C is measured in several individual amino acids that are isolated from collagenous bone protein. Molecular-level radiocarbon dating will greatly improve not only the accuracy of chronologies for human migrations and animal extinctions, but of all late Quaternary chronologies that are based upon the 14C dating of fossil proteins.
Correlation of the Holocene Jarvis Creek, Tangle Lakes, Cantwell, and Hayes Tephras in South-Central and Central Alaska
Authors: Begét James E. , Reger Richard D. , Pinney DeAnne , Gillispie Tom , Campbell Kathy
Published: December 2004

The geochemistry, petrography, and distribution of the Jarvis Creek Ash (Péwé, 1965, 1975a) indicate that this tephra from the lower Delta River area of central Alaska is correlative with vol volcanic ash from sites in south-central Alaska near Tangle Lakes (upper Delta River area) and the Cantwell ash from Hayes volcano found in the upper Nenana River area (Riehle et al., 1990). Volcanic glass compositions of distal Jarvis Creek and Tangle Lakes tephra samples are compositionally restricted, while several discrete glass populations are present in some samples are compositionally collected nearer Hayes volcano. These correlations extend the known distribution of Hayes volcano tephras across the Alaska Range and into central Alaska, a distance of more than 650 km. New geochronologic data for the Jarvis Creek Ash suggest it was deposited ca. 3660 ± 125 yr B.P., consistent with previous age estimates of tephra eruptions at the Hayes volcano. The name “Jarvis Creek Ash” has well-established priority with respect to “Cantwell ash” or other local names for this tephra layer from the Hayes volcano.
Periodic Variations in Late Pleistocene Speleothem Abundance in Britain
Authors: Kashiwaya Kenji , Atkinson Timothy C. , Smart Peter L.
Published: December 2004

Temporal change in the growth of speleothems in the United Kingdom during the past 150,000 years is shown to be related to the insolation variation by the Milankovitch theory. The speleothem data compiled by Gordon et al. (1989. Quarternary Research 31, 14–26) have two Milanokovitch frequencies: the ca. 40,000-yr period related to change in the earth's obliquity and the ca. 20,000-yr period related to the precession of the equinoxes. The abundance of speleothem growth was, in general, large during the last interglaciation and small during the last glaciation. In both periods, however, speleothem abundance was greater during periods of strong insolation and less during weak insolation.
Land-Sea Correlations and Evolution of the Cambridge Fiord Marine Basin during the Last Deglaciation of Northern Baffin Island
Authors: Stravers Jay A. , Syvitski James P.M.
Published: December 2004

Cambridge Fiord functions as a major outlet trough whenever Laurentide ice approaches the eastern coast of Baffin Island. During the last glacial maximum (late Foxe glaciation), an outlet glacier occupied most of the fiord, possibly terminating as far seaward as Buchan Gulf. Glacier retreat began prior to 8800 yr B.P. and was punctuated by the formation of two onshore moraines with upper age limits of 7900 and 7200 yr B.P. Correlative offshore moraines (recognized on seismic reflection profiles) formed prominent submarine sills which partitioned the fiord into three basins. These formed effective sediment traps that restricted the seaward migration of debris flows and turbidites, thus facilitating the accumulation of thick (> 100 m) stratified glacialmarine sediments in the inner fiord. Correlations of onshore and offshore moraines allow time lines to be placed within an offshore seismic stratigraphic section that is inaccessible to piston coring. Resultant estimates of sediment accumulation rates give a mean value of 2.34 × 106 m3/yer for deposits trapped in the inner fiord basins. These thick sequences of sediments do not represent extended Foxe/Wisconsin time periods, but, rather, were rapidly deposited in the inner fiord during the last deglaciation.
Source Areas and Transport Mechanisms for Freshwater and Brackish-Water Diatoms Deposited in Pelagic Sediments of the Equatorial Atlantic
Authors: Pokras Edward M.
Published: December 2004

Distributions of freshwater and brackish-water diatoms from dust samples and modern sediments of the equatorial Atlantic demonstrate different transport mechanisms and source areas. Both Melosira spp. and Stephanodiscus spp. are transported via winds from the southern Sahara and Sahel in Northern Hemisphere winter. The core-top distribution of Cyclotella striata delineates the extent of the low-salinity plume formed by runoff from the Zaire River into the extreme eastern Atlantic. The transport mechanism and source areas for Melosira spp. during arid phases have not changed appreciably in the last 130,000 yr. There is no evidence for long-distance transport of freshwater diatoms by the southeast trade winds. This study confirms the validity of paleoclimatic research which inferred eolian transport of Melosira spp. in the winter dust plume from source regions north of the equator, although fluviatile input of Melosira valves into nearshore sediments cannot be ruled out.
What can be learned from positive data? Insights from an ‘ideal learner’. Commentary on ‘A Multiple process solution to the logical problem of language acquisition’ by Brian MacWhinney
Authors: CHATER NICK
Published: December 2004

MacWhinney's stimulating discussion suggests that there are many lines of argument that may address concerns raised by theorists who are concerned that there is a logical problem of language acquisition. This commentary argues: (1) that if the ‘logical problem’ applied to language, it would apply, with curious consequences to any learning by experience; (2) that the logical problem does not apply – given sufficient positive data from any reasonable language, language can be learned, in a probabilistic sense, by an ‘ideal learner’ using a simplicity principle; and (3) that a simplicity, or minimum description length, principle may provide a useful methodology for assessing claims concerning learnability of particular linguistic structures.
What if Chomsky were right?
Authors: HAUSSER ROLAND
Published: December 2004

The outcome of scientific research depends on how a phenomenon is viewed and how the questions are phrased. This applies also to the nativist view of language acquisition. As a complement to MacWhinney's discussion of nativism from the viewpoint of cognitive psychology, I would like to devote this commentary to the question of the title from the viewpoint of computational linguistics.
Something old, something new
Authors: MARCUS GARY F.
Published: December 2004

MacWhinney's list of proposed mechanisms for language acquisition seems so all-encompassing that it might appear churlish to dwell on a mechanism that's left out, but on his ultimate list of critical mechanisms there is an indeed an important (and intended) omission: innate, linguistically-specialized constraints. Such constraints are indeed mentioned, but if I understand the point of the article correctly, it is to argue that seven cognitive mechanisms, ranging from competition to cue construction, indirect negative evidence and monitoring, collectively obviate the need for any innate machinery that it specialized for language. Why not include room for innate, domain-specific constraints, too?
Comment on ‘A multiple process solution …’ (B. Macwhinney)
Authors: NIYOGI PARTHA
Published: December 2004

The central question posed by the so called ‘logical problem of language acquisition’ is how it comes to be that children are able to GENERALIZE from a finite set of linguistic data to acquire (learn, develop, grow) a computational system (grammar) that applies to novel examples not encountered before. The difficulty of this generalization problem was first posed cogently by Gold and while Macwhinney discusses the Gold framework and the linguistic literature on this matter, it is worth noting that the Gold framework is not the only one. There are at least two important new sources of insight from computational learning theory in the decades following Gold that need to be kept in mind. First, there is the development of empirical process theory that forms the basis of any analysis of statistical learning (see summary in Vapnik, 1998). Applying this approach to language (see Niyogi, 1998 for a treatment), one concludes that the family of learnable grammars must have a finite Vapnik Chervonenkis (VC) dimension. The VC dimension is a combinatorial measure of the complexity of a class of functions. Grammars may be viewed as functions mapping sentences to their grammaticality value. In this more sophisticated sense of the VC dimension, the class of grammars must be constrained. Second, there is the development of the theory of computational complexity suggesting that while a learning algorithm might exist, it may not be efficient, i.e. run in polynomial time. These two developments come together in the influential Probably Approximately Correct (PAC) model (Valiant, 1984).
Inversion without grammar
Authors: O'GRADY WILLIAM
Published: December 2004

MacWhinney's paper focuses on the role of input and learning strategies in explaining how language is acquired, essentially denying the poverty of stimulus assumption on which the so-called ‘logical problem of language acquisition’ is based.
Modeling the transition of the inner plasma sheet from weak to enhanced convection
Authors: Wang Chih‐Ping , Lyons Larry R. , Chen Margaret W. , Toffoletto Frank R.
Published: December 2004

We seek to determine whether the adiabatic plasma transport and energization resulting from electric and magnetic drift can quantitatively account for the plasma sheet under weak and enhanced convection observed by Geotail presented in the companion paper [Wang et al., 2004]. We use a modified Magnetospheric Specification Model to simulate the dynamics and distributions of protons originating from the deep tail and low‐latitude boundary layer (LLBL) under an assigned, slowly increasing convection electric field. The magnetic field is Tsyganenko 96 model, modified so that force balance is maintained along the midnight meridian. Our simulation results reproduce well the observed radial profiles and magnitudes of pressure and magnetic field. The changes of these parameters with convection strength are also well reproduced, indicating that the electric and magnetic drift control the large‐scale structure of the plasma sheet. The plasma flows near midnight are diverted toward dusk by diamagnetic drift. We obtain a steady state plasma sheet under strong and steady convection, showing that magnetic drift and field line stretching bring the plasma sheet away from possible convection disruption. The protons from the LLBL strongly affect the plasma sheet density and temperature during quiet times but not during enhanced convection. For the same cross–polar cap potential, stronger shielding of the convection electric field results in smaller energization. The penetration electric field is important in moving the plasma sheet to smaller geocentric radial distance. Our results suggest that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet because of strong diamagnetic drift.
Novel method to preserve freshly cleaved surfaces
Authors: Fink M. , Smith V. R.
Published: December 2004

A novel method is presented by which one can ensure that a freshly cleaved surface is protected from contamination. The surface, cleaved in a protective atmosphere like argon, is dipped into molten camphor while still in the inert environment. The surface can then be transported out of the hood and exposed to regular air. The camphor coating has the advantage of ease of removal and can protect the surface for a time scale as long as hours. The camphor can be removed including the last monolayer without exposing the surface to any stressful treatment as shown by computer controlled thermal desorption measurements.
High sensitivity bulk electro-optic modulator field sensor for high voltage environments
Authors: Huang Mao-Sheng , Lu Mao-Hong , Shy Jow-Tsong
Published: December 2004

An optical electric field sensor is an effective instrument for surveying the electric or magnetic field around a high voltage electrical system. A Mach–Zender interferometer type modulator is generally used in this kind of sensor. The sensor has good sensitivity to electric or magnetic fields, but an unexpected high field could easily destroy the modulator owing to its short electrode separation. A bulk modulator usually has long separation between the two electrodes, which can prevent modulator breakdown, but its sensitivity is usually worse than the Mach–Zender interferometer type sensor. To solve this problem, a Fabry–Perot cavity is used to improve the sensitivity of the bulk modulator type sensor. This work also discusses the optimization of the sensor sensitivity. When the sensor works on the point where a cavity resonance has maximal slope, the proposed sensor has approximately the same sensitivity for sensing a field as the Mach–Zender interferometer type sensor.
Ranque–Hilsch vortex tube thermocycler for fast DNA amplification and real-time optical detection
Authors: Ebmeier Ryan J. , Whitney Scott E. , Sarkar Amitabha , Nelson Michael , Padhye Nisha V. , Gogos George , Viljoen Hendrik J.
Published: December 2004

An innovative polymerase chain reaction (PCR) thermocycler capable of performing real-time optical detection is described below. This device utilizes the Ranque–Hilsch vortex tube in a system to efficiently and rapidly cycle three 20 μL samples between the denaturation, annealing, and elongation temperatures. The reaction progress is displayed real-time by measuring the size of a fluorescent signal emitted by SYBR green/double-stranded DNA complexes. This device can produce significant reaction yields with very small amounts of initial DNA, for example, it can amplify 0.25 fg (∼5 copies) of a 96 bp bacteriophage λ-DNA fragment 2.7×1011-fold by performing 45 cycles in less than 12 min. The optical threshold (150% of the baseline intensity) was passed 8 min into the reaction at cycle 34. Besides direct applications, the speed and sensitivity of this device enables it to be used as a scientific instrument for basic studies such as PCR assembly and polymerase kinetics.
emiT: An apparatus to test time reversal invariance in polarized neutron decay
Authors: Mumm H. P. , Garcia A. , Grout L. , Howe M. , Parazzoli L. P. , Robertson R. G. H. , Sundqvist K. M. , Wilkerson J. F. , Freedman S. J. , Fujikawa B. K. , Lising L. J. , Dewey M. S. , Nico J. S. , Thompson A. K. , Chupp T. E. , Cooper R. L. , Coulter K. P. , Hwang S. R. , Welsh R. C. , Broussard L. J. , Trull C. A. , Wietfeldt F. E. , Jones G. L.
Published: December 2004

We describe an apparatus used to measure the triple-correlation term (Dσ̂n⋅pe×pν) in the beta decay of polarized neutrons. The D coefficient is sensitive to possible violations of time reversal invariance. The detector has an octagonal symmetry that optimizes electron–proton coincidence rates and reduces systematic effects. A beam of longitudinally polarized cold neutrons passes through the detector chamber, where a small fraction undergo beta decay. The final-state protons are accelerated and focused onto arrays of cooled semiconductor diodes, while the coincident electrons are detected using panels of plastic scintillator. Details regarding the design and performance of the proton detectors, beta detectors, and the electronics used in the data collection system are presented. The neutron beam characteristics, the spin-transport magnetic fields, and polarization measurements are also described.
Micromachined fountain pen for atomic force microscope-based nanopatterning
Authors: Deladi S. , Tas N. R. , Berenschot J. W. , Krijnen G. J. M. , de Boer M. J. , de Boer J. H. , Peter M. , Elwenspoek M. C.
Published: December 2004

We present a tool that can be used in standard atomic force microscope and that enables chemical, chemical/mechanical, or physical surface modification using continuous liquid supply. The device consists of a reservoir micromachined into the probe support that is connected to fluidic channels embedded in a V-shaped cantilever. Via the fluidic channels, the liquid reaches the tip. The fluid transport to the sample surface is demonstrated and fountain pen lithography applications are presented.
Efficient and stable blue light-emitting diodes based on an anthracene derivative doped poly(N-vinylcarbazole)
Authors: Niu Yu-Hua , Chen Baoquan , Kim Tae-Dong , Liu Michelle S. , Jen Alex K.-Y.
Published: December 2004

Single-layer blue light-emitting diodes (LEDs) are fabricated by spin coating a blend of 9,10-bis(3′,5′-diaryl)phenyl anthracene in poly(N-vinylcarbazole) (PVK) or in the mixture of PVK and an electron-transporting molecule, 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole. The Commission Internationale de I’Eclairage coordinates of the resulting LEDs are very close to that of the blue standard from the National Television Standards Committee. These devices also show excellent color stability when operated at a voltage span from 6to22V. High external quantum efficiency (>1.5%) and brightness (>3000cd∕m2) can be obtained in these devices.
Highly a-axis-oriented Nb-doped Pb(TixZr1−x)O3 thin films grown by sol–gel technique for uncooled infrared dectors
Authors: Han Hui , Song Xiuyu , Zhong Jian , Kotru Sushma , Padmini P. , Pandey R. K.
Published: December 2004

Pb ( Nb 0.02 Zr 0.2 Ti 0.8 ) O 3 thin films with thickness of 900nm were spincoated on platinized silicon substrate using the sol–gel method. X-ray diffraction and field emission scanning electron microscopy show that the films are highly a-axis-oriented with columnar structure. A large remnant polarization (Pr+) ∼92.2μC∕cm−2 was observed for as-grown films. The coercive field (Ec+) for these films was 159.3kV∕cm. Good pyroelectric properties are obtained as a result of large remnant and spontaneous polarization and their strong temperature dependence. The pyroelectric coefficient and figure of merit at room temperature (22°C) are 10.8×10−4Cm−2K−1 and 2.34×10−5Pa−1∕2, respectively.
Zinc oxide single-crystal microtubes
Authors: Cheng Jiping , Guo Ruyan , Wang Qing-Ming
Published: December 2004

ZnO single-crystal microtubes were fabricated using an encapsulated microwave growth method. The ZnO crystals are grown in hexagonal hollow tube form with a well faceted end and side surfaces, which have cross-sectional dimensions of 100to250μm, lengths of 3–5mm, and wall thickness of 1–2μm. Under optical excitation, a strong near-band-edge emission was obtained at a peak wavelength of 377.8nm with a full width at half maximum of 11nm. The ZnO microtubes exhibited a highly selective UV light response with a cut-off wavelength at ∼370nm, and excellent electron field emission properties with an emission current density of 11mA∕cm2 at an applied field of ∼20V∕μm.
Cross-sectional structure of carrot defects in 4H–SiC epilayers
Authors: Zhang X. , Ha S. , Benamara M. , Skowronski M. , O’Loughlin M. J. , Sumakeris J. J.
Published: December 2004

Surface morphology of carrot defects in 4H–SiC epilayers is described based on optical microscopy and molten potassium hydroxide etching. Its crystallographic structure is investigated using cross-sectional transmission x-ray topography. A threading screw dislocation in substrate serves as the nucleation source of a carrot. At the beginning of the epitaxial growth, the source dislocation is deflected toward the step-flow direction where a basal plane dislocation and a vertical planar defect nucleate together. The vertical planar defect fills the space between the basal plane dislocation and the deflected threading dislocation. This structure forms characteristic morphology on the epilayer surface, such as groove, shallow trench, etch line of the groove, and hexagonal and oval etch pits at each end of the groove.
As-doped p-type ZnO produced by an evaporation∕sputtering process
Authors: Look D. C. , Renlund G. M. , Burgener R. H. , Sizelove J. R.
Published: December 2004

Strongly p-type ZnO is produced by the following sequence of steps: (1) evaporation of Zn3As2 on a fused-quartz substrate at 350°C; and (2) sputtering of ZnO with substrate held at 450°C. The electrical characteristics include: resistivity of 0.4Ωcm, a mobility of 4cm2∕Vs, and a hole concentration of about 4×1018cm−3. This resistivity is among the best (lowest) ever reported for p-type ZnO. Secondary-ion mass spectroscopic analysis gives an average As concentration about 5×1019cm−3, and a simple one-band fit of the temperature-dependent mobility curve yields an acceptor concentration of about 9×1019cm−3. This is strong evidence that the p-type dopant involves As, although it is not clear whether the acceptor is simply AsO or the recently suggested AsZn−2VZn.
Field emission from AlN nanoneedle arrays
Authors: Zhao Q. , Xu J. , Xu X. Y. , Wang Z. , Yu D. P.
Published: December 2004

AlN nanoneedles with an average tip dimension of ∼15nm were synthesized via a simple vapor deposition method. The AlN nanoneedles exhibit excellent field-emission properties with a low turn-on field of 3.1V∕μm and a high current density of 4.7mA∕cm2 at the field of 9.9V∕μm. The field enhancement factor for a single nanoneedle is estimated to be as high as 30 000 due to its small tip radius. These features make the AlN nanoneedles a competitive candidate for field-emission-based displays.
Fragmentation of nanowires driven by Rayleigh instability
Authors: Toimil Molares M. E. , Balogh A. G. , Cornelius T. W. , Neumann R. , Trautmann C.
Published: December 2004

Rayleigh instability of copper nanowires has been experimentally demonstrated. After annealing 30–50-nm-diam wires at temperatures between 400 and 600°C, different stages of the fragmentation process are observed by scanning electron microscopy. At 400°C, the wires start to fragment, forming shorter sections at 500°C, and finally decaying into a chain of nanospheres at 600°C. Average diameter and spacing of the spheres are in agreement with theoretical predictions. The Rayleigh instability applied to nanowires provides a structuring technique producing long chains of nanospheres, which should find interesting applications, for instance, by guiding light below the diffraction limit via coherent coupling of surface-plasmon polaritons.
Blue phosphorescent dye as sensitizer and emitter for white organic light-emitting diodes
Authors: Lei Gangtie , Wang Liduo , Qiu Yong
Published: December 2004

We demonstrate high-efficiency white organic light-emitting diodes with blue electrophosphorescent and red phosphor-sensitized electrofluorescent emission. The phosphorescent dye, bis[(4,6-difluorophenyl)-pyridinato-N,C2] (picolinato) Ir(III) (FIrpic) and the fluorescent dye, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl) (DCJTB) are co-doped into host material, N,N′-dicarbazolyl-1, 4-dimethene-benzene (DCB). The device with 8% (weight ratio) FIrpic and 0.4% (weight ratio) DCJTB exhibits white emission and the maximum current efficiency and luminance are 9.2cd∕A and 18 200cd∕m2, respectively. For comparison, a device without FIrpic is fabricated which has poor performance. These results indicate that the blue phosphorescent dye acts not only as emitter but also as phosphorescent sensitizer for fluorescent dye.
True Young modulus of Pb(Zr,Ti)O3 films measured by nanoindentation
Authors: Delobelle P. , Guillon O. , Fribourg-Blanc E. , Soyer C. , Cattan E. , Rèmiens D.
Published: December 2004

In order to assess transverse elastic modulus of lead zirconate titanate (PZT) thin films, we used a nanoindenter with constant monitoring of the force. It is shown that the true elastic modulus, i.e., the low pressure elastic modulus, can be measured by this method. This permitted a determination that sputter deposited PZT (54∕46) films display a Young modulus close to a ferroelectric hard type PZT bulk ceramic. Moreover, this low pressure modulus is independent of grain size and thickness of the film, but this is no more true for high pressure modulus. This provides a step toward a better understanding of the properties of these PZT films and the possibility to better use them in microsystems.
Improved pinning effect in PtMn/NiFe system by Cr addition into PtMn
Authors: Dai B. , Cai J. W. , Lai W. Y. , Ge X. , Zhang Z.
Published: December 2004

The exchange bias and thermal properties of Pt1−xMnx–Cr∕NiFe films were investigated. Adding Cr into Pt1−xMnx film by inserting thin Cr layers was found to decrease the exchange bias considerably for x>0.5 but lead to a great enhancement of the pinning field along with unvarying coercivity and better thermal stability for x⩽0.5. An optimum pinning field of ∼180 Oe was obtained for Pt0.55Mn0.45–Cr(4%) pinning 150 Å NiFe—almost twice that of pure Pt0.55Mn0.45, and even larger than that of Pt0.5Mn0.5, by 20%. Although possessing a smaller grain size, Pt0.55Mn0.45–Cr(4%) has a higher blocking temperature than either pure Pt0.55Mn0.45 or Pt0.5Mn0.5. Structure characterization revealed that adding Cr promoted the ordering process of Pt1−xMnx (x⩽0.5) greatly, and a perfect ordering phase of Pt(MnCr) was formed. The present results suggest that PtMn–Cr has favorable properties as a pinning layer.
Designable buried waveguides in sapphire by proton implantation
Authors: Laversenne L. , Hoffmann P. , Pollnau M. , Moretti P. , Mugnier J.
Published: December 2004

Buried and stacked planar as well as buried single and parallel channel waveguides are fabricated in sapphire by proton implantation. Good control of the implantation parameters provides excellent confinement of the guided light in each structure. Low propagation losses are obtained in fundamental-mode, buried channel waveguides without postimplantation annealing. Choice of the implantation parameters allows one to design mode shapes with different ellipticity and∕or mode asymmetry in each orthogonal direction, thus demonstrating the versatility of the fabrication method. Horizontal and vertical parallelization is demonstrated for the design of one- or two-dimensional waveguide arrays in hard crystalline materials.
Cleaved-edge overgrowth of aligned quantum dots on strained layers of InGaAs
Authors: Wasserman D. , Lyon S. A.
Published: December 2004

Strain aligned InAs quantum dots were grown on the cleaved edges of first growth samples containing strained InxGa(1−x)As layers of varying thickness and indium fraction. The formation of the cleaved-edge quantum dots was observed by means of atomic force microscopy. 100% linear alignment of InAs quantum dots over the InGaAs strain layers of the first growth sample is demonstrated. Linear density of the aligned dots was found to depend on the properties of the underlying InGaAs strain layers. Vertical alignment of an additional InAs quantum dot layer over the buried, linearly aligned, initial dot layer was observed for thin GaAs spacer layers.
Collection of transverse and longitudinal fields by means of apertureless nanoprobes with different metal coating characteristics
Authors: Descrovi E. , Vaccaro L. , Nakagawa W. , Aeschimann L. , Staufer U. , Herzig H. P.
Published: December 2004

The coupling and transmission of transverse and longitudinal fields into apertureless microfabricated near-field optical probes is investigated. Two kinds of probes with different metal coating roughness are considered. Transverse and longitudinal field distributions are obtained by focusing azimuthally and radially polarized beams produced by means of a liquid crystal plate. The focal plane is scanned using microfabricated probes in a collection mode configuration. It is found that the roughness of the metal coating plays an important role in the coupling strength of transverse fields into the probes: the relative coupling efficiency for transverse fields diminishes with a rough metal coating, while that of longitudinal fields does not.
Electron scattering dependence of dendritic magnetic instability in superconducting MgB2 films
Authors: Ye Z. X. , Li Qiang , Hu Y. F. , Pogrebnyakov A. V. , Cui Y. , Xi X. X. , Redwing J. M. , Li Qi
Published: December 2004

Magnetic instability in both ultrapure and carbon-doped MgB2 films is investigated by magneto-optical imaging, transport, and bulk magnetization measurements. In the carbon-doped MgB2 thin films, familiar dendritic flux-jump patterns were observed at low temperature as reported in previous experiments. In the ultrapure MgB2 thin film, however, a remarkably stable flux penetration was observed, clearly showing the classic behavior of the critical state model. Such different behavior indicates that the electron scattering ultimately controls the magnetic stability of the MgB2 films.
The quantum efficiency of dispenser photocathodes: Comparison of theory to experiment
Authors: Jensen Kevin L. , Feldman Donald W. , O’Shea Patrick G.
Published: December 2004

The quantum efficiency (QE) characteristics of commercially available dispenser cathodes were measured, giving QEs of (for Scandate) 6.5×10−5, 2.0×10−4, and 8.0×10−4, and (for M-type) 3.0×10−4, 1.4×10−3, and 2.6×10−3, for wavelengths of 532, 355, and 266nm, respectively, corresponding to harmonics of an Nd:YAG laser. A time-dependent photoemission model was developed to analyze the data, as well as dispenser and metal photocathode data in the literature, and quantitatively good agreement is found, demonstrating the utility of the code as a predictive estimator of performance.
Efficient optical parametric oscillation based on periodically poled 1.0 mol % MgO-doped stoichiometric LiTaO3
Authors: Yu Nan Ei , Kurimura Sunao , Nomura Yoshiyuki , Nakamura Masaru , Kitamura Kenji , Takada Yasutoshi , Sakuma Jun , Sumiyoshi Tetsumi
Published: December 2004

Effective optical parametric oscillation (OPO) at room temperature was demonstrated by a periodically poled 1.0 mol% MgO-doped stoichiometric lithium tantalate (Mg:SLT) crystal. The slope conversion efficiency of total output power was ∼65% and the maximum output power was 655 mW at 1.3 W pumping without photorefraction. The difference of the OPO performance was investigated between 0.5 and 1.0 mol % Mg:SLT crystals, and a 1.0 mol % MgO-doped device exhibited better OPO performance and high resistance of photorefraction. The effective nonlinear coefficient of deff was found to be 10pm∕V by single-pass first-order quasiphase matched second-harmonic generation at 1064 nm.
The influence of time, temperature, and grain size on indentation creep in high-purity nanocrystalline and ultrafine grain copper
Authors: Zhang Kai , Weertman J. R. , Eastman J. A.
Published: December 2004

Microhardness measurements have been carried out on high purity Cu samples with average grain sizes ranging from ∼10 to ∼200nm, over temperatures from liquid nitrogen to ambient, and dwell-times of the indenter in the sample from 5 s to 39 h. The Vickers hardness diminishes approximately linearly with the logarithm of the dwell-time. At short dwell-times the hardness increases significantly with decreasing grain size and with decreasing temperature, but the influence of these variables substantially diminishes at longer times. Investigation by transmission electron microscopy shows that rapid grain growth under the indenter most likely is responsible for the strong and long-lasting indentation creep.
Optical and electrical step-recovery study of minority-carrier transport in an InGaN∕GaN quantum-well light-emitting diode grown on sapphire
Authors: Kaplar R. J. , Kurtz S. R. , Koleske D. D.
Published: December 2004

Forward-to-reverse bias step-recovery experiments were performed on an InGaN∕GaN single-quantum-well light-emitting diode grown on sapphire. With the quantum well sampling the minority-carrier hole density at a single position, the optical emission displayed a two-stage decay. Using a solution to the diffusion equation to self-consistently describe both the optical and electrical recovery data, we estimated values for the hole lifetime (758±44ns), diffusion length (588±45nm), and mobility (0.18±0.02cm2∕Vs) in GaN grown on sapphire. This low value of the minority-carrier mobility may reflect trap-modulated transport, and the lifetime is suggestive of slow capture and emission processes occurring through deep levels.
Mechanics and geometry of nanoasperity contacts in organic fluids
Authors: Mann Adrian B.
Published: December 2004

Nanoindentation measurements show that immersion of GaAs (100) in a long-chain alcohol causes an apparent increase in both the hardness and reduced elastic modulus of the GaAs. In contrast to the results for alcohol, immersion in distilled water has no discernable effect on the mechanical behavior of the surface. The results are explained by the ability of the long-chain alcohols to sustain elastic strains when they are in the highly confined geometry around a nanoasperity contact. This “elastic fluid” behavior modifies both the contact area and elastic properties of the nanoasperity contact.
Nonvolatile organic field-effect transistor memory element with a polymeric gate electret
Authors: Singh Th. B. , Marjanović N. , Matt G. J. , Sariciftci N. S. , Schwödiauer R. , Bauer S.
Published: December 2004

Organic field-effect transistors with a polymeric electret as gate insulator and fullerenes as a molecular semiconductor were fabricated. We observed an amplification of the drain–source current Ids on the order of 104 upon applying a gate voltage Vg. Reversing the gate voltage Vg features large metastable hysteresis in the transfer characteristics Ids(Vg) with a long retention time. The observation of a switchable channel current Ids is proposed to originate from charge storage in the organic electret. As such, this device is a demonstration of an organic nonvolatile memory element switchable with the gate voltage.
Dopant activation and ultralow resistance ohmic contacts to Si-ion-implanted GaN using pressurized rapid thermal annealing
Authors: Yu Haijiang , McCarthy L. , Xing H. , Waltereit P. , Shen L. , Keller S. , Denbaars S. P. , Speck J. S. , Mishra U. K.
Published: December 2004

Activation annealing of Si implants in metalorganic-chemical-vapor-deposition-grown GaN has been studied for use in ohmic contacts. Si was implanted in semi-insulating GaN at 100keV with doses from 5×1014to1.5×1016cm−2. Rapid thermal annealing at ∼1500°C with 100bar N2 overpressure was used for dopant activation, resulting in a minimum sheet resistance of 13.9Ω∕square for a dose of 7×1015cm−2. Secondary-ion-mass-spectroscopy measurements showed a post-activation broadening of the dopant concentration peak by 20nm (at half the maximum), while x-ray triple-axis ω–2θ scans indicated nearly complete implant damage recovery. Transfer-length-method measurements of the resistance of Ti∕Al∕Ni∕Au contacts to activated GaN:Si (5×1015cm−2 at 100keV) indicated contact resistances of 0.07 and 0.02Ωmm for as-deposited and subsequently annealed contacts, respectively.
Electric-field-induced ferroelastic single domaining of SrTiO318
Authors: Dec Jan , Kleemann Wolfgang , Itoh Mitsuru
Published: December 2004

A method for engineering the nearly perfect single domain state in the ferroelastic phase of strontium titanate single crystal is presented. It combines shaping and the action of a strong dc electric field which is not conjugate with the order parameter. The single domain formation is attributed to domain wall motion due to anisotropic electrostriction, which switches residual odd-oriented domains into the axially oriented state. This feature seems to hint at ferrobielectric or ferroelastobielectric behavior.
Determination of the coefficient of friction between metal and diamond under high hydrostatic pressure
Authors: Crowhurst J. C. , Darnell I. M. , Goncharov A. F. , Lassila D. H. , Zaug J. M.
Published: December 2004

We have developed an experimental method to obtain the coefficient of friction between diamond and metal under high confining pressure in the diamond anvil cell. A metal ring is compressed between the diamond anvils and its inner and outer diameters are measured as a function of the compression. Measured dimensions are then compared to the results of finite element simulations, in which the coefficient of friction is a free parameter. As an example, we apply the method to polycrystalline molybdenum up to a hydrostatic pressure of 6GPa.
High-sheet-charge–carrier-density AlInN∕GaN field-effect transistors on Si(111)
Authors: Dadgar A. , Schulze F. , Bläsing J. , Diez A. , Krost A. , Neuburger M. , Kohn E. , Daumiller I. , Kunze M.
Published: December 2004

Al In N ∕ Ga N heterostructures have been proposed to possess advantageous properties for field-effect transistors (FETs) over AlGaN∕GaN [Kuzmík, IEEE Electron Device Lett. 22, 501 (2001); Yamaguchi et al., Phys. Status Solidi A 188, 895 (2001)]. A major advantage of such structures is that AlInN can be grown lattice-matched to GaN while still inducing high charge carrier densities at the heterointerface of around 2.7×1013cm−3 by the differences in spontaneous polarization. Additionally, it offers a higher band offset to GaN than AlGaN. We grew AlInN FET structures on Si(111) substrates by metalorganic chemical vapor phase epitaxy with In concentrations ranging from 9.5% to 24%. Nearly lattice-matched structures show sheet carrier densities of 3.2×1013cm−2 and mobilities of ∼406cm2∕Vs. Such Al0.84In0.16N FETs have maximum dc currents of 1.33A∕mm for devices with 1μm gate length.
Improved device efficiency and color purity: Spectral redshift and line narrowing for poly [2-methoxy,5-(2-ethylhexyloxy)-1,4-phenylenevinylene] via blending with phenyl-substituted poly [p-phenylene vinylene] derivatives
Authors: Zhang Yong , Peng Junbiao , Mo Yueqi , Cao Yong
Published: December 2004

Highly efficient light-emitting diodes with redshifted narrow emission were realized based on blends of poly [2-methoxy,5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and phenyl-substituted poly [p-phenylene vinylene] derivative (P-PPV). EL emissions of blend device were redshifted about 18nm from the peak emission of 582nm for neat MEH-PPV device to around 600nm for devices from the blend film. The full width at half maximum of MEH-PPV emission from blend film was reduced from 91 to 36nm. The external quantum and luminous efficiencies of devices from blend films (MEH-PPV:P-PPV=20:80) reached 5.27% and 4.71cd∕A, respectively. The effective energy transfer and solid-state dilution effect of MEH-PPV in P-PPV host were thought of as the origin of the performance enhancement.
Energy-band diagram of metal/Lu2O3/silicon structures
Authors: Seguini G. , Bonera E. , Spiga S. , Scarel G. , Fanciulli M.
Published: December 2004

Internal photoemission spectroscopy has been used to determine the band alignment in Lu2O3 based metal-oxide-semiconductor structures. The Si/Lu2O3 interface conduction- and valence-band offsets were determined to be 2.1±0.1 and 2.6±0.1eV, respectively. The energy barrier for electrons at the Al/Lu2O3 interface is 2.4±0.1eV. The value of the Lu2O3 transport band gap, obtained by photoconductivity measurements, was found to be 5.8±0.1eV. Optical absorption spectroscopy gave a value of 4.89±0.02eV for the Lu2O3 optical band gap.
Nanocantilevers made of bent silicon carbide nanowire-in-silicon oxide nanocones
Authors: Lin Ming , Loh Kian Ping , Boothroyd Chris , Du Anyan
Published: December 2004

We report the plasma-assisted synthesis of nanocones on a nickel-coated silicon substrate using tetramethyl silane as the gas precursor. These nanocones consist of coaxially aligned, crystalline β-SiC nanowires surrounded by conical-shaped, amorphous silicon oxide precipitates. The propensity of the SiC wire to undergo changes in the growth axis directs the bending of the nanocones, giving rise to structures resembling nanocantilevers. The growth mechanism that controls the nanostructure formation is discussed.
Optimized atom injection in a Hall effect thruster
Authors: Garrigues L. , Hagelaar G. J. M. , Boniface C. , Boeuf J. P.
Published: December 2004

An improvement of the neutral gas injection in order to increase the Hall thruster lifetime capabilities is demonstrated using a two-dimensional model. An additional atom injection through the channel ceramics in a region located between the ionization and the acceleration zones leads to an efficient ionization of the neutral flux, with a flattening of the plasma density and potential profiles in the radial direction. Thanks to this modified injection of the atom flow, a focusing electric field is produced, reducing the ion flux impacting on ceramic walls.
Crystal structure and magnetic properties of SmCo7−xHfx compounds
Authors: Luo J. , Liang J. K. , Guo Y. Q. , Yang L. T. , Liu F. S. , Zhang Y. , Liu Q. L. , Rao G. H.
Published: December 2004

The crystal structure and magnetic properties of SmCo7−xHfx (x=0.05,0.1,0.2,0.3) compounds were studied by means of x-ray powder diffraction and magnetic measurements. The as-cast compounds SmCo7−xHfx with x=0.1 and 0.2 crystallize in the TbCu7-type structure with the space group P6∕mmm. According to the structure refinement, the doping element Hf prefers to occupy the 2e site. The compounds have high Curie temperatures (1080K for x=0.1, 1075K for x=0.2), large saturation magnetization (104emu∕g for x=0.1, 102emu∕g for x=0.2 at 5K), and large magnetic anisotropy fields (244kOe for x=0.1, 282kOe for x=0.2 at 5K) with strong uniaxial magnetocrystalline anisotropy.
Molecular molds
Authors: Mayne A. J. , Soukiassian L. , Commaux N. , Comtet G. , Dujardin G.
Published: December 2004

A problem in nanometer-scale applications that combine silicon technology with organic molecules, is the selective adsorption of organic molecules on the silicon surface. Reducing the active device size down to a single molecule requires a very precise geometry of the molecule–surface connection. Here, we report the room temperature adsorption of biphenyl or Trima molecules in artificially created individual molecular molds on a passivated silicon surface. These molecular molds are areas of clean silicon atoms with well defined geometries, fabricated by sequential removal of single hydrogen atoms from the hydrogenated silicon surface using the scanning tunneling microscope (STM) tip. The mold geometry can be controlled to adsorb the molecules at a selected place and in a desired orientation. The potential selection of molecules according to physical size or chemical properties is a step towards combined molecular and semiconductor chip technology.
Free-standing, optically pumped, GaN∕InGaN microdisk lasers fabricated by photoelectrochemical etching
Authors: Haberer E. D. , Sharma R. , Meier C. , Stonas A. R. , Nakamura S. , DenBaars S. P. , Hu E. L.
Published: December 2004

GaN-based, mushroom-shaped microdisk lasers were fabricated using band-gap selective photoelectrochemical etching. The optically pumped microdisks had well-defined, distinct modes at excitation powers ranging from about 8to16W∕cm2. Modal linewidths of 0.09nm were reported, which was near the resolution of the measurement equipment. Quality factors for the microdisks were >4600. The observed lasing threshold was 12.1W∕cm2. At higher excitation powers, heating effects and degradation were observed in the optical response of the microdisks.
Temperature dependence of thermal conductivity of AlxGa1−xN thin films measured by the differential 3ω technique
Authors: Liu Weili , Balandin Alexander A.
Published: December 2004

Reliable values of thermal conductivity of thin films made of GaN and its alloys are important for further development of nitride technology due to the problem of self-heating in GaN-based power transistors and optical devices. Using the differential 3ω technique we measured the thermal conductivity of AlxGa1−xN thin films (x=0 and 0.4) grown by the hydride vapor phase epitaxy. Thermal conductivity of the examined Al0.4Ga0.6N alloy, which is about 25W∕mK at 300K, displays a rather unusual temperature dependence. A noticeable growth of the thermal conductivity with temperature up to 350K is more characteristic for amorphous or completely disordered materials. The measured high-temperature thermal conductivity data are in good agreement with predictions based on the virtual crystal model. Obtained results are important for modeling the self-heating effects in GaN transistors and can be used for the device structure optimization.
Subwavelength multichannel imaging using a solid immersion lens: Spectroscopy of excitons in single quantum dots
Authors: Hewaparakrama K. P. , Wilson A. , Mackowski S. , Jackson H. E. , Smith L. M. , Karczewski G. , Kossut J.
Published: December 2004

We demonstrate subwavelength imaging of excitons confined to single CdTe quantum dots. By combining slit-confocal microscopy with a hemispherical solid immersion lens, we simultaneously map the emission of thousands of single quantum dots with a spatial resolution of 400nm. By analyzing the linear polarization of the quantum dot emissions at B=0T, we find that the distribution of the exchange splitting is centered at zero with a standard deviation of ±340μeV. Similar experiments performed at B=3T give an average value of the exciton effective g factor of 3.1±0.4. This experimental approach provides an effective means to gain statistical information about the quantum-dot exciton fine structure in the ensemble.