bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of source-drain electric field on the Meyer–Neldel energy in organic field effect transistors
Authors: Ullah Mujeeb , Pivrikas A. , Fishchuk I. I. , Kadashchuk A. , Stadler P. , Simbrunner C. , Sariciftci N. S. , Sitter H.
Published: June 2011

We studied the influence of the lateral source-drain electric field on the Meyer–Neldel phenomenon observed for the charge mobility measured in C60-based organic field effect transistors (OFETs). It was found that the characteristic Meyer-Neldel temperature notably shifts with applied source drain electric field. This finding is in excellent agreement with an analytic model recently extended to account also for the field dependence of the charge carrier mobility in materials with a Gaussian density-of-states distribution. As the theoretical model to predict charge carrier mobility is not limited to zero-electric field, it provides a more accurate evaluation of energetic disorder parameters from experimental data measured at arbitrary electric fields.
Surface potential measurement of organic thin film on metal electrodes by dynamic force microscopy using a piezoelectric cantilever
Authors: Satoh Nobuo , Katori Shigetaka , Kobayashi Kei , Watanabe Shunji , Fujii Toru , Matsushige Kazumi , Yamada Hirofumi
Published: June 2011

We describe applications of a cantilever with a lead zirconate titanate (PZT) piezoelectric film as self-sensing to dynamic force microscopy (DFM) combined with Kelvin probe force microscopy (KFM). We adopted a frequency modulation (FM) detection method not only to stabilize the imaging conditions in our DFM but also to enhance the sensitivity for the detection of electrostatic forces in KFM measurement. We deposited Alq3 [tris (8-hydroxyquinolinato) aluminum] thin films and aluminum (Al) electrode patterns on an indium tin oxide (ITO)/glass substrate by vacuum evaporation using shadow masks. The surface structures and local surface potential of Alq3 films on metals were investigated using our DFM/KFM instrument to study the local electrical properties at the molecule–metal interface. The photosensitive organic material sample can be in a completely dark environment because no optics are required for cantilever deflection sensing in our experimental setup.
Predictions of electromagnetic field-induced forces utilizing adiabatic invariance
Authors: Ryan J. D. , Wanis S.
Published: June 2011

The Boltzmann–Ehrenfest (BE) principle of adiabatic invariance, a form of energy conservation, is utilized for the purposes of predicting field-induced forces acting on a neutral particle placed in an electromagnetic cavity resonator. In a simulation environment, the force was determined using the BE principle and compared to both experimental direct force measurements and to analytical approximation. The results were found to be in good agreement. The BE principle was also applied to measure the force profile in lossless systems. Since this technique of predicting and measuring field-induced forces involves only a knowledge of the adiabatic invariant, resonance frequency shift, and particle position within the cavity, the BE principle offers a noncontact method of measuring noncontact forces. It also has the unique capability of measuring forces that arise as a result of particle-particle and particle-wall force interactions that would, otherwise, be poorly categorized by a single scattering regime for an analytical description and might be difficult to measure directly using physical contact. Furthermore, this capability allows for accurate predictions of the force on more interesting configurations of particles of arbitrary shape or composition placed into arbitrary cavity modes.
Powered oscillator using ignitron switches
Authors: Nonn P. D. , Blair A. P. , McCollam K. J. , Sarff J. S. , Stone D. R.
Published: June 2011

A 10-MVA-scale resonant oscillator, powered by a pulse-forming network and switched with a pair of commutating mercury ignitrons, was developed for the MST reversed-field pinch plasma-confinement experiment. A novel feature of this circuit is its commutation mechanism, wherein each turning on of one ignitron causes a reverse voltage transient that turns off the other. Two of these oscillators are used in oscillating-field current-drive tests, in which they are capable of nearly 1MW net input power to the plasma, with resonant frequencies of a few 100 Hz for pulse durations of a few tens of ms, being precharged for immediate full amplitude. We describe the circuit and its operation, and discuss features that allow reliable, high-current commutation of the ignitrons and exploit their low switching impedance.
Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO0.85F0.15: Evidence of strong anisotropy in the order parameter
Authors: Dmitriev V. M. , Khlybov E. P. , Kondrashov D. S. , Terekhov A. V. , Rybaltchenko L. F. , Khristenko E. V. , Ishchenko L. A. , Kostyleva I. E. , Zaleski A. J.
Published: June 2011

Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 with an unexpectedly low superconducting transition temperature Tc ≈ 11.3 K compared to related FeAs compounds based on Sm and Gd surrounding Eu in lanthanide series. The nearly fivefold lower Tc, compared to the expected value, is attributed to the divalent properties of Eu ions; the weakly magnetic Eu3+ ions may be present and the highly magnetic Eu2+ ions are strong suppressors of superconductivity. Most of the measured spectra have features corresponding to two energy gaps whose values varied, from contact to contact, over 2Δs/kTc = 2.2 −4.7 and 2Δ1/kTc = 5.1−11.7 for small and large gaps, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6−6.4. The relatively large crystallite sizes (at least ∼25 μm) and the large number of measured contacts (several tens) suggest, with a high degree of probability, that these spectra account quite fully for the gap distribution in essentially all the crystallographic directions. These data and the absence of zero gaps in the measured spectra indicate anisotropic s− or s±-symmetry of the order parameter in EuAsFeO0.85F0.15 that has been found in other, similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides.
Estimating the coherence length in the electron-doped superconductor Nd2−xCexCuO4+δ
Authors: Charikova T. B. , Harus G. I. , Shelushinina N. G. , Sochinskaya O. E. , Ivanov A. A.
Published: June 2011

The low-temperature upper critical field is measured for single crystal Nd2−xCexCuO4+δ with different values of x and nonstoichiometric disorder (δ). The coherence length for pair correlation ξ and the product kFξ, where kF is the Fermi wave vector, are estimated. It is shown that the parameter kFξ≅100 for these single crystals, so that, phenomenologically, the NdCeCuO-system is in the domain of Cooper-pair-based (BCS) superconductivity.
Distorted diamond Ising-Hubbard chain
Authors: Lisnii B. M.
Published: June 2011

The ground state and thermodynamics of distorted Ising-Hubbard chains are studied with on-site Coulomb repulsion taken into account. A decoration-iteration transformation method is used to obtain exact results for the free energy, entropy, specific heat, magnetization of the Ising and Hubbard subsystems, and magnetic susceptibility. The effect of Coulomb repulsion on the ground state, field and temperature dependences of the magnetization, magnetic susceptibility, and specific heat is studied for the case of a geometrically frustrated system. Strong repulsion leads to formation of an additional high-temperature peak in the specific heat. Independently of any repulsion, the temperature dependence of the specific heat can have two low-temperature peaks.
Origin of an enhanced colossal magnetoresistance effect in epitaxial Nd0.52Sr0.48MnO3 thin films
Authors: Prokhorov V. G. , Kaminsky G. G. , Kim J. M. , Eom T. W. , Park J. S. , Lee Y. P. , Svetchnikov V. L. , Levtchenko G. G. , Nikolaenko Yu. M. , Khokhlov V. A.
Published: June 2011

Nd0.52Sr0.48MnO3 films of various thicknesses have been prepared by dc magnetron sputtering on single crystal LaAlO3 (001) substrates. Reducing the film thickness leads to a significant suppression of ferromagnetic (FM) ordering and the Curie point falls below the antiferromagnetic (AFM) transition temperature. When this occurs, a huge rise of the magnetoresistance ratio from 400 to 60 000% is observed in an applied magnetic field of 5 T. We surmise that this new kind of the enhanced colossal magnetoresistance effect originates in the FM/AFM competition and the collapse of the charge-ordered state at high magnetic fields, rather than in the regular double-exchange mechanism.
Impact of discharge voltage on wall-losses in a Hall thruster
Authors: Mazouffre S. , Dannenmayer K. , Blank C.
Published: June 2011

Calibrated infrared thermal imaging is used to investigate the temperature of the BN-SiO2 discharge chamber walls of the high-power PPSX000-ML Hall thruster over a broad voltage range. The energy flux deposited by charged particles onto the channel walls is assessed by means of a semi-empirical time-dependent thermal model. Equilibrium temperature as well as power losses onto the channel walls are determined for low and high voltage operation states. For a given input power, the two quantities strongly depend upon the discharge voltage. Above ~ 500 V, losses augment in an exponential way whereas they vary linearly at low voltages. As suggested by many theoretical works, secondary electron emission yield and associated near-wall sheath potential lowering could explain experimental outcomes. The contribution of ion bombardment, however, cannot be fully ruled out, as shown here.
Theoretical insight of adsorption cooling
Authors: Chakraborty Anutosh , Leong Kai Choong , Thu Kyaw , Saha Bidyut Baran , Ng Kim Choon
Published: June 2011

This letter proposes and presents a thermodynamic formulation to calculate the energetic performances of an adsorption cooler as a function of pore widths and volumes of solid adsorbents. The simulated results in terms of the coefficient of performance are validated with experimental data. It is found from the present analysis that the performance of an adsorption cooling device is influenced mainly by the physical characteristics of solid adsorbents, and the characteristics energy between the adsorbent-adsorbate systems. The present study confirms that there exists a special type of silica gel having optimal physical characteristics that allows us to obtain the best performance.
Electrical characterization of n-type Al0.30Ga0.70N Schottky diodes
Authors: Arehart A. R. , Allerman A. A. , Ringel S. A.
Published: June 2011

The carrier trapping properties and current transport behavior of Ni/n-Al0.30Ga0.70N Schottky diodes were quantitatively characterized by a combination of deep level optical spectroscopy (DLOS), thermally based deep level transient spectroscopy (DLTS), current-voltage-temperature (I-V-T), and internal photoemission (IPE) measurements. High quality Schottky diode behavior was observed with an IPE-determined barrier height of 1.66 eV and the observed temperature-independent reverse leakage current behavior was found to be consistent with field emission in reverse bias and thermionic-field emission in forward bias as the dominant transport mechanisms. The trap spectroscopy measurements revealed the presence of several bandgap states located at EC–0.9 eV (seen by both DLOS and DLTS), EC–1.5, EC–3.11, and EC–3.93 eV—all via DLOS. The EC–3.10 level, which is present in very high concentration, is found to correlate with the energy position expected for the cation vacancy in AlGaN, based on the vacuum referred binding energy model for the AlxGa1−xN alloy. The relatively shallow trap at EC–3.93/EV+0.15 eV, which is possibly Mg-related, is also present in significant concentration. The total observed trap concentration in this sample is in excess of the net doping extracted from capacitance-voltage, which will likely impact device behavior and is consistent with the observed I-V-T behavior.
Solar energy enhancement using down-converting particles: A rigorous approach
Authors: Abrams Ze’ev R. , Niv Avi , Zhang Xiang
Published: June 2011

The efficiency of a single band-gap solar cell is specified by the Shockley-Queisser limit, which defines the maximal output power as a function of the solar cell’s band-gap. One way to overcome this limit is by using a down-conversion process whereupon a high energy photon is split into two lower energy photons, thereby increasing the current of the cell. Here, we provide a full analysis of the possible efficiency increase when placing a down-converting material on top of a pre-existing solar cell. We show that a total 7% efficiency improvement is possible for a perfectly efficient down-converting material. Our analysis covers both lossless and lossy theoretical limits, as well as a thermodynamic evaluation. Finally, we describe the advantages of nanoparticles as a possible choice for a down-converting material.
Magnetic order-order phase transitions in itinerant magnets: Fe2−xMnxAs
Authors: Valkov V. I. , Golovchan A. V. , Dyakonov V. P. , Szymczak H.
Published: June 2011

Magnetic-field induced magnetic order-order phase transitions in Fe0.69Mn1.31As are studied using first principles calculations of the electronic structure of Fe2−xMnxAs system alloys and a two-band model of itinerant magnets. It is shown that, within this model, the low-temperature ground and magnetic-field induced states can be described in terms of a coexistence of uniform ferromagnetic and periodic antiferromagnetic components of the total magnetic moment of the d-band. The effect of hydrostatic pressure on order-order transitions arises from a structural renormalization of the electron density of states when the material is compressed. The model predicts that collinear ferromagnetism in the Fe2−xMnxAs system is a metastable state, transitions to which can only occur in ultrahigh magnetic fields.
Mixed 1D-2D quantum electron transport in percolating gold film
Authors: Beliayev E. Yu. , Belevtsev B. I. , Kolesnichenko Yu. A.
Published: June 2011

A gold film (mean thickness ≈ 3.5 nm) was deposited in high vacuum on a single-crystal sapphire substrate at a temperature of 70 K. The transport properties of the film at low temperature reveal simultaneous 1D and 2D quantum interference effects of weak localization and electron-electron interaction. This behavior is found to be determined by inhomogeneous electron transport at the threshold of a thickness-controlled metal-insulator transition.
Non-invasive determination of the parameters of strongly coupled 2D Yukawa liquids
Authors: Ott T. , Stanley M. , Bonitz M.
Published: June 2011

We derive a method for identifying the Yukawa model parameters of two-dimensional (2D) strongly coupled dusty plasmas. The method is based on non-invasive observation of the dust component of the plasma and comparison with a large body of simulation data in the form of simple analytical formulas. This approach requires only input from a time-series of configuration snapshots and particle velocities with no recourse to additional information about the system. In addition, we derive an effective coupling parameter for 2D Yukawa systems based on the height of the first maximum of the pair distribution function. Two variants—one valid in the high-coupling range, the other for arbitrary couplings of the liquid—are derived. Comparison to previous approaches to Yukawa coupling parameters shows that the present concept is more general and more accurate. Our results should be directly applicable as a simple, yet reliable diagnostic method for a variety of experiments, including dusty plasmas, colloidal suspensions, and ions in traps, and can be employed to facilitate comparisons between experiments, theory, and simulations.
Effect of PbZr0.52Ti0.48O3 thin layer on structure, electronic and magnetic properties of La0.65Sr0.35 MnO3 and La0.65Ca0.30MnO3 thin-films
Authors: Hezareh T. , Razavi F. S. , Kremer R. K. , Habermeier H.-U. , Lebedev O. I. , Kirilenko D. , Tendeloo G. Van
Published: June 2011

Epitaxial thin film heterostructures of high dielectric PbZr1-xTixO3 (PZT) and La1-xAxMnO3 (A-divalent alkaline earth metals such as Sr (LSMO) and Ca (LCMO)) were grown on SrTiO3 substrates and their structure, temperature dependence of electrical resistivity, and magnetization were investigated as a function of the thickness of the LSMO(LCMO) layer. The microstructures of the samples were analyzed by TEM. By applying an electric field across the PZT layer, we applied a ferrodistortive pressure on the manganite layer and studied the correlations between lattice distortion and electric transport and magnetic properties of the CMR materials.
Low-temperature specific heat of single crystal bismuth oxyhalides
Authors: Bunda V. V. , Bunda S. A. , Kajňaková M. , Feher A. , Syrkin E. S. , Feodosyev S. B.
Published: June 2011

The low-temperature specific heat of layered tetragonal bismuth oxyhalide crystals, prepared by the method of chemical gas transport reactions, is measured. The specific heat is found to have Debye behavior at temperatures below 4 K. The Debye temperatures θD of BiOCl (205 K), BiOBr (168 K), and BiOI (146 K) single crystals are determined. The entropy and Debye temperatures of these compounds are found to depend linearly on the corresponding correlations parameters. The low values of the Debye temperatures with the rather high values of the upper boundaries of the phonon spectrum of these compounds may be caused by a high density of phonon states at low frequencies and a substantial deviation of their phonon spectrum from a Debye spectrum at frequencies above 10 cm−1.
Theoretical analysis of telescopic oscillations in multi-walled carbon nanotubes
Authors: Zavalniuk Vladimir , Marchenko Sergey
Published: June 2011

A simplified theory of telescopic oscillations in multi-walled carbon nanotubes is developed. Explicit expressions are derived for the telescopic force constants (longitudinal rigidity) and the frequencies of telescopic oscillations. The contribution of small-amplitude telescopic oscillations to the low-temperature specific heat of nanotubes is estimated.
The effect of O2 impurities on the low-temperature radial thermal expansion of bundles of closed single-walled carbon nanotubes
Authors: Dolbin A. V. , Esel’son V. B. , Gavrilko V. G. , Manzhelii V. G. , Popov S. N. , Vinnikov N. A. , Sundqvist B.
Published: June 2011

The effect of oxygen impurities on the radial thermal expansion αr of bundles of closed, single-walled carbon nanotubes has been investigated at temperatures of 2.2-48 K by a dilatometric method. Saturation of bundles of nanotubes with oxygen causes an increase in the positive αr-values over the entire range of temperatures studied here. Also, several peaks appeared in the αr(T) curve above 20 K. Low-temperature desorption of oxygen from powders consisting of bundles of single-walled nanotubes with open and closed ends has been investigated.
Accurate determination of second order nonlinear optical coefficients from powder crystal monolayers
Authors: Aramburu I. , Ortega J. , Folcia C. L. , Etxebarria J. , Illarramendi M. A. , Breczewski T.
Published: June 2011

An accurate procedure for the determination of second harmonic generation (SHG) efficiency from powder crystal monolayers is proposed. The method is based on the analysis of the dependence of the SHG output on the powder particle size. Unlike with conventional powder methods, the amount of material required is very small, and the scattering of the primary beam by the powder particles is negligible, allowing for an accurate analysis of the collected data. The experimental results demonstrate that the proposed method can provide reliable values for the nonlinear optical coefficients, particularly in the case of phase-matchable uniaxial materials, for which simple analytical expressions can be derived.
Thermodynamics of second phase conductive filaments
Authors: Karpov V. G. , Nardone M. , Simon M.
Published: June 2011

We present a theory of second phase conductive filaments in phase transformable systems; applications include threshold switches, phase change memory, resistive memory, and shunting in thin film structures. We show that the average filament parameters can be described thermodynamically. In agreement with the published data, the predicted filament current-voltage characteristics exhibit negative differential resistance that vanishes at high currents where the current density becomes a bulk material property. Our description is extendible to filament transients and allows for efficient numerical simulation.
Anomalies in the temperature dependence of the contribution to the speed of sound from hybridized electronic states of transition element impurities
Authors: Okulov V. I. , Gudkov V. G. , Zhevstovskikh I. V. , Lonchakov A. T. , Paranchich L. D. , Paranchich S. Yu.
Published: June 2011

The temperature dependence of the speed of sound in crystalline mercury selenide with low concentrations of iron impurities is studied. Experiments are conducted in the ranges of concentration and temperature where hybridized electronic states in iron impurities have been observed previously. It is found that at temperatures below 10 K the speed of slow transverse ultrasonic waves has an anomalous nonmonotonic segment of its temperature variation that is related to the influence of the impurities and reflects the existence of hybridized states. The observed anomalies in the sound speed are described in terms of a theory for the electron contribution to the elastic moduli that includes hybridization of impurity states and electron-electron interactions. Fits of the theoretical dependences to the experimental data yield quantitative information on the parameters of the hybridized states and of the Fermi-liquid interaction.
Infrared spectroscopy of solid normal hydrogen doped with CH3F and O2 at 4.2 K: CH3F:O2 complex and CH3F migration
Authors: Abouaf-Marguin L. , Vasserot A.-M.
Published: June 2011

Double doping of solid normal hydrogen with CH3F and O2 at about 4.2 K gives evidence of (ortho-H2)n:CH3F clusters and of O2:CH3F complex formation. FTIR analysis of the time evolution of the spectra in the region of the v3 C-F stretching mode indicates that these clusters behave very differently from (ortho-H2)n:H2O clusters. The main point is the observed migration of CH3F molecules in solid para-H2 at 4.2 K which differs from that of H2O under identical experimental conditions. This is confirmed by an increase over time of the integrated intensity of the CH3F:O2 complex with a rate constant K = 2.7(2) · 10−4 s−1.
Low-temperature anomalies in the dynamic elastic moduli of cubic AIIBVI crystals with 3d-transition metal impurities
Authors: Lonchakov A. T.
Published: June 2011

A negative paramagnetic contribution to the dynamic elastic moduli is identified in AIIBVI:3d wide band-gap compounds for the first time. It appears as a paramagnetic elastic, or, briefly, paraelastic, susceptibility. These compounds are found to have a linear temperature dependence for the inverse paraelastic susceptibility. This is explained by a contribution from the diagonal matrix elements of the orbit-lattice interaction operators in the energy of the spin-orbital states of the 3d-ion as a function of applied stress (by analogy with the Curie contribution to the magnetic susceptibility). The inverse paraelastic susceptibility of AIIBVI crystals containing non-Kramers 3d-ions is found to deviate from linearity with decreasing temperature and reaches saturation. This effect is explained by a contribution from nondiagonal matrix elements (analogous to the well known van Vleck contribution to the magnetic susceptibility of paramagnets).
Multiferroism in orientational engineered (La, Mn) co-substituted BiFeO3 thin films
Authors: Wang D. Y. , Chan N. Y. , Zheng R. K. , Kong C. , Lin D. M. , Dai J. Y. , Chan H. L. W. , Li S.
Published: June 2011

Orientational engineered (La, Mn) co-substituted BiFeO3 (LMBFO) thin films were epitaxially grown on CaRuO3 electroded (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) single crystal substrates by pulsed laser deposition. The experimental results demonstrate that the crystallographic orientation is the critical dominant of the structural and multiferroic properties of LMBFO thin films. Giant remanent polarization of 65, 92, and 106 μC/cm2 for [001], [110], [111]-oriented films, respectively, were demonstrated at room temperature. Saturated magnetization is also significantly dependent on film orientation with the highest value of 12.8 emu/cm3 along [001] direction. The variation in leakage current density and ferroelectric coercivity were ascribed to the substantially difference of ferroelectric domain structures in variously oriented LMBFO thin films.
Quantum dynamics of rovibrational transitions in H2-H2 collisions: Internal energy and rotational angular momentum conservation effects
Authors: Santos S. Fonseca dos , Balakrishnan N. , Lepp S. , Quéméner G. , Forrey R. C. , Hinde R. J. , Stancil P. C.
Published: June 2011

We present a full dimensional quantum mechanical treatment of collisions between two H2 molecules over a wide range of energies. Elastic and state-to-state inelastic cross sections for ortho-H2 + para-H2 and ortho-H2 + ortho-H2 collisions have been computed for different initial rovibrational levels of the molecules. For rovibrationally excited molecules, it has been found that state-to-state transitions are highly specific. Inelastic collisions that conserve the total rotational angular momentum of the diatoms and that involve small changes in the internal energy are found to be highly efficient. The effectiveness of these quasiresonant processes increases with decreasing collision energy and they become highly state-selective at ultracold temperatures. They are found to be more dominant for rotational energy exchange than for vibrational transitions. For non-reactive collisions between ortho- and para-H2 molecules for which rotational energy exchange is forbidden, the quasiresonant mechanism involves a purely vibrational energy transfer albeit with less efficiency. When inelastic collisions are dominated by a quasiresonant transition calculations using a reduced basis set involving only the quasiresonant channels yield nearly identical results as the full basis set calculation leading to dramatic savings in computational cost.
Quantized characteristics in carbon nanotube-based single-hole memory with a floating nanodot gate
Authors: Ohori Takahiro , Ohno Yasuhide , Maehashi Kenzo , Inoue Koichi , Hayashi Yutaka , Matsumoto Kazuhiko
Published: June 2011

We have fabricated floating-gate carbon nanotube field-effect transistors (CNTFETs), in which Au floating dots are expected to act as charge storage nodes. The fabricated floating-gate CNTFETs clearly exhibited the memory effect, in addition to single-hole memory operation. A staircase relation between the gate threshold voltage and the charging voltage was obtained. Quantized shifts in the gate threshold voltage as a function of charging time and retention time were also observed. The quantized characteristics are attributed to the effect of single-hole charging in the Au floating dot.
FePt L1/A1 graded media with a rough interphase boundary
Authors: Lee Jehyun , Alexandrakis Vasilis , Fuger Markus , Dymerska Barbara , Suess Dieter , Niarchos Dimitris , Fidler Josef
Published: June 2011

A graded media consisting of FePt L10(hard) and A1(soft) phases separated by a rough wedge-shaped interphase boundary, the “phase graded media” is suggested. The rough interface helps domain wall propagation from the soft to the hard phase, owing to the easily reversed wedge tips of the hard phase. The reversed domain expands in the hard phase with a small additional field. As a result, the switching field of the phase graded media was reduced to 13 kOe which is 16% of FePt L10 single phase (79 kOe), the reduction is comparable with the stacked graded media.
Energy harvesting by nonlinear capacitance variation for a relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer
Authors: Zhu H. , Pruvost S. , Cottinet P. J. , Guyomar D.
Published: June 2011

The present letter describes the investigation of the electrostatic energy harvesting through nonlinear capacitance variation caused by changes in temperature for a poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer. Owing to the electric tunability of the terpolymer, the harvested energy can-using an Ericsson cycle-be simulated from permittivity under a dc electric field. When going from 25 to 0 °C, it was found, from simulation, that the harvested energy increased up to 240 mJ/cm3 when raising the electric field at 80 kV/mm. Experimental measurement was also carried out, thus confirming the feasibility of electrostatic energy harvesting through low temperature Ericsson cycle.
Multilayer graphene under vertical electric field
Authors: Kumar S. Bala , Guo Jing
Published: June 2011

We study the effect of vertical electric field (E-field) on the electronic properties of multilayer graphene. We show that the effective mass, electron velocity, and density-of-state of a bilayer graphene are modified under the E-field. We also study the transformation of the band structure of multilayer graphenes. E-field induces finite (zero) band gap in the even (odd)-layer ABA-stacking graphene. On the other hand, finite band gap is induced in all ABC-stacking graphene. We also identify the optimum E-field to obtain the maximum band gap in the multilayer graphenes. Finally, we compare our results with the experimental results of a field-effect-transistor.
Interface engineering of high-Mg-content MgZnO/BeO/Si for p-n heterojunction solar-blind ultraviolet photodetectors
Authors: Liang H. L. , Mei Z. X. , Zhang Q. H. , Gu L. , Liang S. , Hou Y. N. , Ye D. Q. , Gu C. Z. , Yu R. C. , Du X. L.
Published: June 2011

High-quality wurtzite MgZnO film was deposited on Si(111) substrate via a delicate interface engineering using BeO, by which solar-blind ultraviolet photodetectors were fabricated on the n-MgZnO(0001)/p-Si(111) heterojunction. A thin Be layer was deposited on clean Si surface with subsequent in situ oxidation processes, which provides an excellent template for high-Mg-content MgZnO growth. The interface controlling significantly improves the device performance, as the photodetector demonstrates a sharp cutoff wavelength at 280 nm, consistent with the optical band gap of the epilayer. Our experimental results promise potential applications of this technique in integration of solar-blind ultraviolet optoelectronic device with Si microelectronic technologies.
Room-temperature operation of a unipolar nanodiode at terahertz frequencies
Authors: Balocco Claudio , Kasjoo Shahrir R. , Lu Xiaofeng F. , Zhang Linqing Q. , Alimi Yasaman , Winnerl Stephan , Song Aimin M.
Published: June 2011

We report on the room-temperature electrical rectification at 1.5 THz of a unipolar nanodiode based on symmetry breaking in a nanochannel. The exploitation of its nonlinear diodelike characteristic and intrinsically low parasitic capacitance enables rectification at ultrahigh speed. The zero-voltage threshold and unique planar layout make the nanodiode suitable for building large arrays. This is the highest speed reported in nanorectifiers to date.
In situ nanomechanical testing in focused ion beam and scanning electron microscopes
Authors: Gianola D. S. , Sedlmayr A. , Mönig R. , Volkert C. A. , Major R. C. , Cyrankowski E. , Asif S. A. S. , Warren O. L. , Kraft O.
Published: June 2011

The recent interest in size-dependent deformation of micro- and nanoscale materials has paralleled both technological miniaturization and advancements in imaging and small-scale mechanical testing methods. Here we describe a quantitative in situ nanomechanical testing approach adapted to a dual-beam focused ion beam and scanning electron microscope. A transducer based on a three-plate capacitor system is used for high-fidelity force and displacement measurements. Specimen manipulation, transfer, and alignment are performed using a manipulator, independently controlled positioners, and the focused ion beam. Gripping of specimens is achieved using electron-beam assisted Pt-organic deposition. Local strain measurements are obtained using digital image correlation of electron images taken during testing. Examples showing results for tensile testing of single-crystalline metallic nanowires and compression of nanoporous Au pillars will be presented in the context of size effects on mechanical behavior and highlight some of the challenges of conducting nanomechanical testing in vacuum environments.
Bragg polariton luminescence from a GaN membrane embedded in all dielectric microcavity
Authors: Trichas E. , Pelekanos N. T. , Iliopoulos E. , Monroy E. , Tsagaraki K. , Kostopoulos A. , Savvidis P. G.
Published: June 2011

We report on the development of a band gap-selective photochemical etching technique capable of producing 200 nm thick optical quality freestanding GaN membranes. The use of low electrolyte concentration combined with intense excitation by a laser source are shown to yield highly anisotropic etch profile with optical quality etched surfaces. Using this technique, high quality GaN microcavity is fabricated by embedding the GaN membrane inside an all-dielectric mirror cavity. In these structures, Bragg polariton photoluminescence is observed at room temperature.
Surfactant-mediated Si quantum dot formation on Ge(001)
Authors: Pachinger D. , Groiss H. , Teuchtmann M. , Hesser G. , Schäffler F.
Published: June 2011

Stranski–Krastanow growth, modified by the presence of submonolayer coverages of carbon or antimony, is investigated for tensile strained silicon epilayers on Ge(001) substrates. We find a reduction in the wetting layer thickness from >10 monolayers without surfactant, to 7.5 and 4.5 monolayers, respectively, with Sb and C predeposition. Very small islands with an aspect ratio of 0.05 and a narrow size distribution were found for Sb-mediated growth. For both adatom species the wetting layer is free of dislocations, whereas the Si islands are almost completely strain relaxed, mainly via sessile 90° misfit dislocations. We show that early dislocation nucleation is an inherent property of a (001) oriented cubic heterosystem under tensile strain.
From positive to negative magnetoresistance in graphene with increasing disorder
Authors: Zhou Yang-Bo , Han Bing-Hong , Liao Zhi-Min , Wu Han-Chun , Yu Da-Peng
Published: June 2011

Artificial disorder was introduced gradually into monolayer graphene by controlling Ga+ ion irradiation and the corresponding electronic transport properties regulated by gate voltage, source-drain voltage, temperature, and magnetic field were studied experimentally. An unsaturated positive magnetoresistance (MR) up to 100% at ∼5 T was observed in as-fabricated graphene, while there is significant negative MR in disordered graphene. This phenomenon was attributed to the monocrystalline breaking and crystallite-boundary scattering in disordered graphene.
Highly catalytic counter electrodes for organic redox couple of thiolate/disulfide in dye-sensitized solar cells
Authors: Wang Liang , Wu Mingxing , Gao Yurong , Ma Tingli
Published: June 2011

We synthesized a low-cost, highly catalytic tungsten carbide that was embedded in ordered mesoporous carbon (WC-OMC). This was used as a counter electrode (CE) in dye-sensitized solar cells (DSCs). For comparison, we also evaluated the catalytic activity of bare WC, and several normal carbon materials for the organic redox couple of thiolate/disulfide (T−/T2). The DSCs showed highly photovoltaic conversion efficiencies, ranging from 4.75% to 5.34%. The efficiency of the DSC composed of WC-OMC was 45% higher than that of Pt. Based on kinetic studies, the increased efficiency was caused by the increased exchange current density and decreased charge transfer resistance.
Effective passivation of Si surfaces by plasma deposited SiOx/a-SiNx:H stacks
Authors: Dingemans G. , Mandoc M. M. , Bordihn S. , van de Sanden M. C. M. , Kessels W. M. M.
Published: June 2011

Very low surface recombination velocities <6 and <11 cm/s were obtained for SiOx/a-SiNx:H stacks synthesized by plasma-enhanced chemical vapor deposition on low resistivity n- and p-type c-Si, respectively. The stacks induced a constant effective lifetime under low illumination, comparable to Al2O3 on p-type Si. Compared to single layer a-SiNx:H, a lower positive fixed charge density was revealed by second-harmonic generation measurements, while field-effect passivation was absent for a reference stack comprising thermally grown SiO2. The results indicate that hydrogenation of interface states played a key role in the passivation and remained effective up to annealing temperatures >800 °C.
High Q-factor sapphire whispering gallery mode microwave resonator at single photon energies and millikelvin temperatures
Authors: Creedon Daniel L. , Reshitnyk Yarema , Farr Warrick , Martinis John M. , Duty Timothy L. , Tobar Michael E.
Published: June 2011

The microwave properties of a crystalline sapphire dielectric whispering gallery mode resonator have been measured at very low excitation strength (E/ℏω≈1) and low temperatures (T≈30 mK). The measurements were sensitive enough to observe saturation due to a highly detuned electron spin resonance, which limited the loss tangent of the material to about 2×10−8 measured at 13.868 and 13.259 GHz. Small power dependent frequency shifts were also measured which correspond to an added magnetic susceptibility of order 10−9. This work shows that quantum limited microwave resonators with Q-factors >108 are possible with the implementation of a sapphire whispering gallery mode system.
Demonstration of low leakage current and high polarization in ultrathin AlN/GaN high electron mobility transistors grown on silicon substrate
Authors: Medjdoub F. , Zegaoui M. , Rolland N. , Rolland P. A.
Published: June 2011

High quality AlN/GaN heterostructures grown on silicon substrate are demonstrated. It is found that high carrier concentration can be achieved whereas circular diodes showed a low leakage current up to 200 V reverse bias. 200 nm gate length AlN/GaN transistors exhibited a drain current density of 1.3 A/mm with a pinchoff leakage current below 20 μA/mm and a record GaN-on-silicon extrinsic transconductance of 470 mS/mm. These results demonstrate the possibility to achieve a unique combination of large polarization with a barrier thickness as low as 3 nm while preserving a remarkably low device leakage current without using any gate insulator.
Singlet state relaxation via intermolecular dipolar coupling
Authors: Pileio G.
Published: June 2011

The intermolecular contribution to the relaxation of singlet states has been derived on the basis of a translational-rotational diffusion model that describes molecules as impenetrable spheres which translate and rotate in an isotropic low-viscosity medium. The equations for the relaxation rate constants obtained are discussed and the dependence on physical parameters is exploited. Theoretical predictions are compared with experiments when the intermolecular relaxation is due to both protons and deuterons present in the sample. An agreement between experiments and theory of ±4% was obtained when the physical parameters are estimated from first-principles calculation.
Analysis of gene expression at the single-cell level using microdroplet-based microfluidic technology
Authors: Mary Pascaline , Dauphinot Luce , Bois Nadège , Potier Marie-Claude , Studer Vincent , Tabeling Patrick
Published: June 2011

In the present work, we have measured the messenger RNA expression of specific genes both from total RNA and cells encapsulated in droplets. The microfluidic chip introduced includes the following functionalities: RNA/cell encapsulation, lysis, reverse transcription and real-time polymerase chain reaction. We have shown that simplex and duplex gene expression measurements can be carried out over a population of 100 purified RNA samples encapsulated simultaneously in 2 nl droplets in less than 2 h. An analysis of 100 samples containing one to three cells has shown excellent consistency with standard techniques regarding average values. The cell-to-cell distributions of the E-cadherin expression suggest fluctuations on the order of 80% in the number of transcripts, which is highly consistent with the general findings from the literature. A mathematical model has also been introduced to strengthen the interpretation of our results. The present work paves the way for the systematic acquisition of such information in biological and biomedical studies.
Electrochemically grown metallic nanocomb structures on nanoporous alumina templates
Authors: Kaya Savas , Atar Erdem
Published: June 2011

Electrochemical growth of metallic nanocomb structures on anodized alumina templates is described. Nanocombs originate from the orderly growth and merger of very thin (d=15∓5 nm) metallic nanowires which do not completely fill much larger pores (d∼100 nm) in the alumina template (t≤3 μm). Instead, the nanowires prefer growing along the inner corners of the hexagonal pores, coalescing into a highly ordered structure as they emerge, resulting a metallic form reminiscent of the topology of the original template. We disclose here the typical processing conditions and the microstructure of this previously unknown material as observed with a scanning electron microscope (SEM) and energy dispersive x-ray (EDX) spectroscopy. It is shown that Au nanocombs have an anomalous EDX spectra and can emit electrons at a field of ∼1 kV/cm.
Using the peak effect to pick the good organic couplers
Authors: Katzir Eran , Yochelis Shira , Paltiel Yossi
Published: June 2011

Nanostructures are likely to become primary components of future electronic devices. Self assembled molecular electronics is a route to achieve this goal. One central issue in molecular electronics deals with charge transport across molecular elements. In this letter we present a hybrid system sensitive to the coupling and consequently to charge or energy transfer through organic molecules. Our system uses gold nanoparticles coupled through organic molecules layers to type II superconductor. We correlate the organic capping ligands of the gold nanoparticles with the vortices pinning efficiency. This sensitive phenomenon distinguishes between different organic molecules coupling efficiency.
High sensitivity carbon monoxide sensors made by zinc oxide modified gated GaN/AlGaN high electron mobility transistors under room temperature
Authors: Hung S. C. , Chen C. W. , Shieh C. Y. , Chi G. C. , Fan R. , Pearton S. J.
Published: June 2011

AlGaN/GaN high electron mobility transistors (HEMTs) with zinc oxide (ZnO) nanowires modified gate exhibit significant changes in channel conductance upon expose to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition (CVD) with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change in surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm in the open cavity with continuous gas flow using a 50×50 μm2 gate sensing area.
Current-limiting behavior in multijunction solar cells
Authors: Braun Avi , Szabó Nadine , Schwarzburg Klaus , Hannappel Thomas , Katz Eugene A. , Gordon Jeffrey M.
Published: June 2011

Experimental measurements on tandem GaInAsP/InGaAs concentrator solar cells are presented that demonstrate how the short-circuit current can shift from that of the higher current subcell to that of the lower current subcell as irradiance increases. Theoretical modeling illustrates how this can occur when the current-limiting subcell has a noticeably nonzero slope in its current-voltage curve near short-circuit, and should be general to all series-connected multijunction cells of this nature.
Two-dimensional carrier density distribution inside a high power tapered laser diode
Authors: Pagano R. , Ziegler M. , Tomm J. W. , Esquivias I. , Tijero J. M. G. , O’Callaghan J. R. , Michel N. , Krakowski M. , Corbett B.
Published: June 2011

The spontaneous emission of a GaAs-based tapered laser diode emitting at λ=1060 nm was measured through a window in the transparent substrate in order to study the carrier density distribution inside the device. It is shown that the tapered geometry is responsible for nonuniform amplification of the spontaneous/stimulated emission which in turn influences the spatial distribution of the carriers starting from below threshold. The carrier density does not clamp at the lasing threshold and above it the device shows lateral spatial hole-burning caused by high stimulated emission along the cavity center.
Terahertz spectroscopy of Ni–Ti alloy thin films
Authors: Jameson A. D. , Kevek J. W. , Tomaino J. L. , Hemphill-Johnston M. , Paul M. J. , Koretsky M. , Minot E. D. , Lee Yun-Shik
Published: June 2011

We investigate the carrier dynamics in nickel–titanium (Ni–Ti) alloy thin films using terahertz transmission spectroscopy. Analyzing the power transmission data and the transmitted waveforms, we obtained the alloy resistivity as a function of Ti concentration. Sharp changes in the resistivity were observed at the Ti fractions of 22%, 44%, and 62%, indicating that structural disorder is greatly enhanced when the alloy undergoes a phase transition.
MgZnO-based metal-semiconductor-metal solar-blind photodetectors on ZnO substrates
Authors: Zheng Qinghong , Huang Feng , Ding Kai , Huang Jin , Chen Dagui , Zhan Zhibing , Lin Zhang
Published: June 2011

Using lattice matched ZnO substrates, wurtzite single crystalline Mg0.49Zn0.51O films were obtained by reactive magnetron cosputtering method, and the heterostructures of MgZnO/ZnO were fabricated into metal-semiconductor-metal solar-blind photodetectors (SBPDs). Calculated and experimental results demonstrate that the response of the ZnO substrate can be suppressed by adopting a thick MgZnO epilayer. The SBPD with a 2 μm thick MgZnO epilayer shows a peak responsivity of 304 mA/W at 260 nm under 10 V bias, which is comparable to the highest value ever reported in MgZnO-based SBPDs. A rejection ratio (R260 nm/R365 nm) over 5×102 is also observed, indicating fully suppression of the signal from ZnO substrate.