bluuebunny/crossref_metadata_2025_split huggingface dataset:
Tracking quintessence, WIMP relic density, PAMELA and Fermi LAT
Authors: Pallis C. 
Published: February 2010

AbstractThe generation of an early kination dominated (KD) era within a tracking quintessential model is investigated, the relic density of the Weakly Interacting Massive Particles (WIMPs) is calculated and we show that it can be enhanced with respect to (w.r.t) its value in the Standard Cosmology (SC). By adjusting the parameters of the quintessential scenario, the cold dark matter abundance in the universe can become compatible with large values for the annihilation cross section times the velocity of the WIMPs. Using these values and assuming that the WIMPs annihilate predominantly to µ+µ‐, we calculate the induced fluxes of e±cosmic rays and fit the current PAMELA and Fermi‐LAT data. We achieve rather good fits in conjunction with a marginal fulfillment of the restriction arisen from the Cosmic Microwave Background (CMB).
Sgluons and gluinos of the hybrid N = 1 / N = 2 SUSY model
Authors: Kalinowski J. 
Published: February 2010

AbstractProspects for observation of scalar gluons (sgluons) and Dirac gluinos, as predicted by the hybrid N = 1/N = 2 supersymmetric model, at the CERN Large Hadron Collider are discussed.
Effects of shallow traps on the reverse current of diamond Schottky diode: An electrical transient study
Authors: Garino Yiuri , Teraji Tokuyuki , Koizumi Satoshi , Koide Yasuo , Ito Toshimichi 
Published: February 2010

AbstractWe investigated the performance of the reverse characteristics of diamond Schottky diodes at high voltage. We observed that when the diode was in reverse bias voltage conditions, the negative current decayed with a stretched exponential behaviour, which can be ascribed to charging effects. During the discharging phase, where no bias was applied, we observed a positive current that shows again a decaying behaviour, but with a power law trend. These behaviours can be rationalized in terms of trapping/detrapping processes occurring at shallow level traps.
Thermal analysis and characterization of the effect of substrate thinning on the peformances of GaN‐based light emitting diodes
Authors: Lee H. K. , Yu J. S. , Lee Y. T. 
Published: February 2010

AbstractWe report the effect of substrate thickness on the optical and thermal characteristics of InGaN/GaN light emitting diodes (LEDs), operating at λ ∼ 450 nm, with different mesa sizes. For various mesa sizes with different substrate thicknesses, the junction temperature (Tj) is measured as a function of injection current by the forward voltage method and the characteristic temperature is also investigated by measuring the temperature‐dependent electroluminescence spectrum. Based on the experimentally determined heat source density, the junction temperature, heat flux, and thermal resistance (Rth) are calculated theoretically by using a three‐dimensional anisotropic heat dissipation model. The use of a thin substrate thickness of 150 µm effectively improves the heat extraction capability due to the shorter heat transfer path length. For 150 µm (443 µm) thick substrate, the Tj value of LEDs is experimentally estimated as 307.7 K (322.6 K), 311.6 K (329.5 K), and 323 K (355.9 K) under an injection current of 80 mA for 450 × 450, 400 × 400, and 300 × 300 µm2, respectively. At a high injection current of 400 mA, the Tj of 450 × 450 µm2 LED with 150 µm thick substrate is reduced by ∼25.7% compared to that of LED with 443 µm substrate thickness, indicating a value of Tj = 384.5 K. The Rth value is decreased for larger mesa size and thinner substrate thickness, resulting in Rth < 30 K/W for 450 × 450 µm2 LED with 150 µm thick substrate. The theoretically calculated Rth values indicate a good agreement with the experimentally measured data.
Effect of buffer thickness on DC and microwave performance of AlGaN/GaN heterojunction field‐effect transistors
Authors: Chevtchenko Serguei A. , Brunner Frank , Würfl Joachim , Tränkle Günther 
Published: February 2010

AbstractThe authors compared DC and microwave performance of AlGaN/GaN heterojunction field‐effect transistors (HFETs) fabricated on epitaxial structures with different thickness of GaN buffer layer. The structures were grown by low‐pressure metal‐organic vapour phase epitaxy on semi‐insulating SiC substrates. An increase of the average off‐state breakdown voltage from 39 to 142 V with reduction of buffer thickness from 1.5 to 0.5 µm was measured. The increase of breakdown voltage was explained by reduction of buffer leakage current due to higher concentration of dislocations threading to the surface of thin buffer. Our results revealed no degradation of AlGaN/GaN interface and device performance including dispersion for the case of reduced buffer thickness. A comparison of the electron sheet resistance, maximum drain‐to‐source current, transconductance, threshold voltage, gate and drain leakage currents is provided. RF maximum power performance was improved by ∼1 W/mm with higher power added efficiency for transistors fabricated on epitaxial structure with 0.5‐µm‐thick GaN buffer. Current slump was measured by pulsed I–V characteristics. A nearly unchanged dispersion demonstrated the possibility of GaN buffer thickness reduction approach for suppression of punch‐through effect without compromise in current collapse commonly observed in GaN‐based HFETs.
Fabrication of ordered carbon nanotube structures by unconventional lithography
Authors: Greco Pierpaolo , Facchini Massimo , Cavallini Massimiliano , Ruani Giampiero , Dionigi Chiara , Biscarini Fabio 
Published: February 2010

AbstractWe use stamp‐assisted deposition to align single‐walled carbon nanotubes (CNTs) starting from a water suspension of CNTs in sodium dodecyl sulphate (SDS) solution. The driving force of the process is the interplay between capillary flow and spatial confinement, which leads CNTs to self‐organize into bundles aligned in rows hundreds of micrometres long. Lithographically controlled wetting, where the solution is confined in menisci pinned to the stamp protrusions, yields patterns with feature size as small as single nanotubes. Using micromolding in capillaries instead we fabricate patterns of CNTs lines, with nanometric size, preferentially aligned along the microchannel axis and exhibiting a conductivity of about 100 S/cm.
Electrodeposited PdNi as possible ferromagnetic contacts for carbon nanotubes
Authors: Usgaocar A. R. , de Groot C. H. 
Published: February 2010

AbstractA process for electrodepositing PdNi alloys and subsequent characterisation studies are reported. PdNi alloys are deposited on 0.019–0.021 and 1–2 Ω cm n‐type silicon from a bath of Pd‐ethylenediamine dichloride and Ni sulphate. The deposited films form excellent Schottky barriers on 1–2 Ω cm Si with leakage currents of the order of µA/cm2 and forward current higher than the reverse current by about six orders of magnitude at 1 V. Ni atomic fractions in the deposited films are studied for different bath concentrations and the deposition potential was found to play an important role in deciding the composition of the deposited film. For high Ni concentration solutions, it is possible to deposit films with a wide range of Ni content by varying the deposition potential, while this method cannot be used with low Ni concentration solutions. Films with Ni concentrations above 30% were observed to be ferromagnetic at room temperature and ferromagnetic properties strengthened with Ni content. A structure for a carbon nanotube (CNT) transistor is described and electrical characterisation of the device using PdNi alloy contacts is presented. The Schottky barrier characteristics and ferromagnetic character make PdNi alloys a good material for ferromagnetic contacts to CNTs on a Si substrate.
Magnetically driven superconducting pairing interaction in the two‐dimensional Hubbard model within a spin‐rotationally invariant approach
Authors: Kopeć T. K. , Apinyan V. A. 
Published: February 2010

AbstractWe implement the rotationally invariant formulation of the two‐dimensional Hubbard model, with nearest‐neighbors hopping t, which allows for the analytic study of the system in the low‐energy limit. Both U(1) and SU(2) gauge transformation are used to factorize the charge and spin contribution to the original electron operator in terms of the corresponding gauge fields. The Hubbard Coulomb energy U‐term is then expressed in terms of quantum phase variables conjugate to the local charge and variable spin quantization axis, providing a useful representation of strongly correlated systems. It is shown that these gauge fields play a similar role as phonons in the BCS theory: they act as the “glue” for fermion pairing.
Development of the critical exponent at the antiferromagnetic phase transition of YbRh2Si2 under chemical pressure
Authors: Krellner Cornelius , Klingner Christoph , Geibel Christoph , Steglich Frank 
Published: February 2010

AbstractWe present specific‐heat measurements in the vicinity of the antiferromagnetic phase transition on single crystals of the alloy Yb(Rh1−xCox)2Si2 for x ≤ 0.38. This study was motivated by the violation of critical universality in the undoped YbRh2Si2 [Krellner et al., Phys. Rev. Lett. 102, 196402, 2009] where we have found a large critical exponent α = 0.38. For Co‐doped samples we observe a drastic change of the critical fluctuations resulting in a negative α, explainable within the universality classes of phase transitions. The development of α under chemical pressure gives strong indication that the violation of critical universality in YbRh2Si2 is due to the nearby quantum critical point.
Coupled spin–phonon excitations in helical multiferroics
Authors: Jia Chenglong , Berakdar Jamal 
Published: February 2010

AbstractBoth the Dzyaloshiskii–Moriya interaction and the exchange striction are shown to affect dynamically the magnetoelectric excitations in the helical multiferroics. The exchange striction results in a biquadratic interaction between the spins and the transverse phonons, giving rise to quantum fluctuations of the ferroelectric polarization P. This leads to low‐lying phonon modes that are perpendicular to P and to the helical spins at small wave vector but are parallel to P at a wave vector close to the magnetic modulation vector. For spin‐1/2 helimagnet, the local polarization can be completely reversed by the spin fluctuation, and so does the direction of the on‐site spin chirality, which allows for a finite differential scattering intensity of polarized neutrons from a cycloidal magnet.
Photoinduced nonequilibrium spin, charge polarizations and spin‐dependent current in quantum rings
Authors: Zhu Z. G. , Berakdar J. 
Published: February 2010

AbstractWe investigate the spin‐dependent dynamical response of a semiconductor quantum ring with a spin orbit interaction upon the application of a single and two linearly polarized, picosecond, asymmetric electromagnetic pulses in the presence of a static magnetic flux. We find that the pulse‐generated electric dipole moment is spin dependent. It is also shown that the spin orbit interaction induces an extra SU(2) effective flux in addition to the static external magnetic flux which is reflected in an additional periodicity of the spin‐dependent dipole moment. Furthermore, the pulses may induce a net dynamical charge currents and dynamical spin currents when the clockwise and anti‐clockwise symmetry of the carrier is broken upon the pulse application.
Phase diagram of the sodium‐rich NaxCoO2 cobaltates
Authors: Zorkovská A. , Baran A. , Kajňaková M. , Feher A. , Šebek J. , Šantavá E. , Lin C. T. , Peng J. B. 
Published: February 2010

AbstractSpecific heat of layered NaxCoO2 single crystals (x = 0.76–0.87) down to 100 mK and the dc‐susceptibility down to 2 K in magnetic field applied parallel to the layers has been measured. The specific heat data of single crystals of exceptional high quality with 0.71 ≤ x ≤ 0.747 reported in L. Balicas et al. [Phys. Rev. Lett., 100, 126405 (2008)] with the kind permission of the authors have been also analyzed. Non‐Fermi liquid (NFL) features observed in Na concentration dependent temperature region have been analyzed and discussed in terms of disorder induced NFL behavior with quantum Griffiths phases in the background. The phase diagram is proposed, the results indicate possible quantum critical point below x = 0.7 and closeness to metal–insulator transition at doping above x = 0.87.
Quantum criticality of Ce1−xLaxRu2Si2: The magnetically ordered phase
Authors: Raymond Stéphane , Knafo William , Flouquet Jacques , Bourdarot Frédéric , Lejay Pascal 
Published: February 2010

AbstractWe report specific heat and neutron scattering experiments performed on the system Ce1−xLaxRu2Si2 on the magnetic side of its quantum critical phase diagram. The Kondo temperature does not vanish at the quantum phase transition and elastic scattering indicates a gradual localization of the magnetism when x increases in the ordered phase.
Low temperature properties of the electron spin resonance in YbRh2Si2
Authors: Sichelschmidt J. , Kambe T. , Fazlishanov I. , Zakharov D. , Krug von Nidda H.‐A. , Wykhoff J. , Skvortsova A. , Belov S. , Kutuzov A. , Kochelaev B.I. , Pashchenko V. , Lang M. , Krellner C. , Geibel C. , Steglich F. 
Published: February 2010

AbstractWe present the field and temperature behavior of the narrow electron spin resonance (ESR) response in YbRh2Si2 well below the single ion Kondo temperature. The ESR g‐factor reflects a Kondo‐like field and temperature evolution of the Yb3+ magnetism. Measurements toward low temperatures (> 0.5 K) have shown distinct crossover anomalies of the ESR parameters upon approaching the regime of a well‐defined heavy Fermi liquid. Comparison with the field dependence of specific heat and electrical resistivity reveal that the ESR parameters can be related to quasiparticle mass and cross section and, hence, contain inherent heavy electron properties.
Single crystal growth of CeNi2Ge2 using the floating zone technique
Authors: Bergmann C. , Jeevan H. S. , Schubert M. , Geibel C. , Gegenwart P. 
Published: February 2010

AbstractIn order to investigate the low‐temperature electronic properties of the heavy‐fermion metal CeNi2Ge2, single crystals were grown using the floating zone technique. Previous investigations on polycrystals of CeNi2Ge2 have revealed that the low‐temperature properties, i.e., non‐Fermi liquid (NFL) behavior and incipient superconductivity, are strongly sample dependent, possibly due to a slight variation of the Ni and Ge content. We prepared polycrystalline feed rods of slightly off‐stoichiometric composition by induction or arc‐melting methods and grew single crystals using a four‐mirror optical furnace under high‐purity argon atmosphere. As a result, large single crystals of few centimeter size have been obtained. Annealed pieces were investigated by measurements of the temperature dependence of the electrical resistivity ρ(T). A clear correlation between the starting composition and the obtained resistivity ratio (RR2K) was found with the highest ratio for a nominal starting composition of Ce0.98Ni2.025Ge1.975 (${\rm RR}_{2{\rm K}} = 29$). The temperature dependence of the electrical resistivity is similar as reported previously and displays NFL behavior below 1 K.
Competition and/or coexistence of antiferromagnetism and superconductivity in CeRhIn5 and CeCoIn5
Authors: Knebel Georg , Aoki Dai , Brison Jean‐Pascal , Howald Ludovic , Lapertot Gerard , Panarin Justin , Raymond Stephane , Flouquet Jacques 
Published: February 2010

AbstractThe Ce compounds CeCoIn5 and CeRhIn5 are ideal model systems to study the competition of antiferromagnetism (AF) and superconductivity (SC). Here we discuss the pressure–temperature and magnetic field phase diagrams of both compounds. In CeRhIn5 the interesting observation is that in zero magnetic field a coexistence AF + SC phase exist inside the AF phase below the critical pressure $p_{\rm c}^{\rm *} \approx 2$ GPa. Above $p_{\rm c}^{\rm *} $ AF is suppressed in zero field but can be re‐induced by applying a magnetic field. The collapse of AF under pressure coincides with the abrupt change of the Fermi surface. In CeCoIn5 a new phase appears at low temperatures and high magnetic field (LTHF) which vanishes at the upper critical field Hc2. In both compounds the paramagnetic pair breaking effect dominates at low temperature. We discuss the evolution of the upper critical field under high pressure of both compounds and propose a simple picture of the glue of reentrant magnetism to the upper critical field in order to explain the interplay of antiferromagnetic order and SC.
Effect of chemical substitution and pressure on YbRh2Si2
Authors: Nicklas M. , Macovei M. E. , Ferstl J. , Krellner C. , Geibel C. , Steglich F. 
Published: February 2010

AbstractWe carried out electrical resistivity experiments on (Yb,La)Rh2Si2 and on Yb(Rh,Ir)2Si2 under pressure and in magnetic fields. YbRh2Si2 exhibits a weak antiferromagnetic transition at atmospheric pressure with a Néel temperature of only $T_N \approx 70$ mK. By applying a small magnetic field TN can be continuously suppressed to $T = 0$ at $B_c = 60$ mT ($B \bot c$) driving the system to a quantum critical point (QCP). On applying external pressure the magnetic phase is stabilized and TN (p) is increasing as usually observed in Yb‐based heavy‐fermion metals. Substituting Yb by La or Rh by Ir allows to create a negative chemical pressure, La (Ir) being smaller than Yb (Rh), and eventually to drive YbRh2Si2 to a pressure controlled QCP. In this paper we compare the effect of external hydrostatic pressure and chemical substitution on the ground‐state properties of YbRh2Si2.
Power law specific heat divergence in Sr3Ru2O7
Authors: Rost A. W. , Berridge A. M. , Perry R. S. , Mercure J.‐F. , Grigera S. A. , Mackenzie A. P. 
Published: February 2010

AbstractWe present measurement and analysis of field‐dependent specific heat measurements on Sr3Ru2O7, showing that, at low temperatures, an incipient divergence is cut off by the formation of a new phase previously identified to show the transport properties of an electronic nematic. We discuss how to interpret a specific heat divergence in a system with van Hove singularities, and caution against a simple‐minded comparison of experimentally determined power laws with the predictions of quantum critical theories.
Different nature of instabilities in BaFe2As2 and BaNi2As2 as revealed by optical spectroscopy
Authors: Chen Z. G. , Hu W. Z. , Wang N. L. 
Published: February 2010

AbstractThe interplay between different instabilities, such as structural distortions, magnetic orderings, and superconductivity, is of great interest in condensed matter physics. The Fe‐ and Ni‐based layered pnictides offer a new opportunity to study the intriguing interplay between those instabilities. Both the FeAs‐ and NiAs‐based 122 compounds show rather sharp phase transitions, and similar anomalies can be found from resistivity and specific heat measurements. Here we show by optical spectroscopy measurements that the charge dynamics across the transitions in BaFe2As2 and BaNi2As2 are significantly different. The study reveals that the driving mechanisms for the instabilities are different.
Hidden low‐temperature instability in PrOs4Sb12
Authors: Grube K. , Zaum S. , Bauer E. D. , Maple M. B. , Löhneysen H. v. 
Published: February 2010

AbstractThe low‐temperature behavior of many correlated electron systems is characterized by the appearance of macroscopically ordered states that show a dome‐shaped evolution of their transition temperatures as a function of non‐thermal control parameters. In the heavy‐fermion compound PrOs4Sb12, a field‐induced phase emerges in a magnetic field range between $B \approx 4.5$ and 13.5 T. The phase is characterized by antiferroquadrupolar (AFQ) ordering of the Pr3+ ions. We measured the specific heat of PrOs4Sb12 single crystals in magnetic fields up to $B = 14$ T parallel to the crystallographic [100] direction to determine the entropy S as a function of T and B. Above the maximum AFQ transition temperature, the entropy $S(T = {\rm const},B)$ reveals a pronounced maximum. We show that this maximum can be related to a low‐temperature instability which is produced by the degeneracy of the lowest crystalline electric field (CEF) energy levels.
Statistical resource needs to be increased in the European regulatory agencies
Authors: Keene Oliver N. , Hughes Sara H. , Howitt Nigel ,   on behalf of the PSI and EFSPI Boards of Directors
Published: February 2010

AbstractThe following viewpoint from PSI and EFSPI regarding the current level of statistical resource in the European regulatory agencies was first presented as a position paper to a meeting of the EU Heads of Agencies in July 2009, and was endorsed by EFPIA. Copyright © 2010 John Wiley & Sons, Ltd.
Microcontact Printing onto Oxide‐Free Silicon via Highly Reactive Acid Fluoride‐Functionalized Monolayers
Authors: Scheres Luc , ter Maat Jurjen , Giesbers Marcel , Zuilhof Han 
Published: February 2010

AbstractThis work describes a new route for patterning organic monolayers on oxide‐free silicon by microcontact printing (µCP) on a preformed, reactive, acid‐fluoride‐terminated monolayer. This indirect printing approach is fast and easily preserves the oxide‐free and well‐defined monolayer–silicon interface, which is the most important property for potential applications in biosensing and molecular electronics. Water‐contact‐angle measurements, ellipsometry, attenuated total reflection infrared spectroscopy, and X‐ray photoelectron spectroscopy (XPS) demonstrate the formation of the initial acid‐fluoride‐terminated monolayers without upside‐down attachment. Subsequent printing for twenty seconds with an N‐hexadecylamine‐inked poly(dimethylsiloxane) stamp results in well‐defined 5‐µm N‐hexadecylamide dots, as evidenced by atomic force microscopy and scanning electron microscopy. Printing with a flat stamp allows investigation of the efficiency of amide formation by µCP and water‐contact‐angle measurements, ellipsometry, and XPS reveal the quantitative conversion of the acid fluoride groups to the corresponding amide within twenty seconds. The absence of silicon oxide, even after immersion in water for 16 h, demonstrates that the oxide‐free monolayer–silicon interface is easily preserved by this patterning route. Finally, it is shown by fluorescence microscopy that complex biomolecules, like functionalized oligo‐DNA, can also be immobilized on the oxide‐free silicon surface via µCP.