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The temperature (T) dependence of electrical resistivity in graphene has been experimentally investigated between 10 and 400 K for samples prepared on various substrates: HfO2, SiO2, and h-BN. The resistivity of graphene shows a linear T-dependence at low T and becomes superlinear above a substrate-dependent transition temperature. The results are explained by remote interfacial phonon scattering by surface optical phonons at the substrates. The use of an appropriate substrate can lead to a significant improvement in the charge transport of graphene. [ABSTRACT FROM AUTHOR]

The substitution of nitrogen atoms in the lattice plane of the graphene can adjust the electronic properties of the graphene to translate it from the half-metal to the n-type semiconductor. Here, we report a practicable growth of nitrogen-doped graphene films with the nitrogen atoms doped content of 4.4–7.5% by the sole solid precursor based chemical vapor deposition method. After the post-annealing process at high temperature, the morphology and crystallization quality of the nitrogen-doped graphene film were considerably improved. The as-synthesized nitrogen-doped graphene films exhibit typical n-type behavior with the electron carrier density of approximately 6.55×1012 cm-2 and the Hall mobility of around 522 cm2V-1 s-1. [ABSTRACT FROM AUTHOR]

Experiments are conducted in a linear stratified fluid with a momentum source modeled via a nozzle jet moving horizontally. The generation mechanism of the quasi-two-dimensional dipolar vortex streets is investigated and their evolution characteristics are analyzed. Observation shows that the formation of a dipolar vortex street requires a nonzero motion of the nozzle in addition to conditions of the Reynolds and Froude number (Re, Fr). The (Re, Fr) condition that the dipolar vortex streets can be generated is determined via experimental measurements. The explanation for the absence of such a vortex street can be the low energy of the jet and the strong body-effect disturbance of the solid nozzle. The dependence of the vortex street dimensionless formation time τ and the Strouhal number St on the Froude number Fr or the Reynolds number Re is analyzed. This analysis shows that τ and St appear to be independent of Re and approximately have power-law relations with Fr via data fitting. The exponents of Fr in the two power-law functions are -0.27 for τ and -0.21 for St, while the constant coefficients are 65 and 0.21. [ABSTRACT FROM AUTHOR]

We present an all-optical method for measuring the average size, density, and mass fraction of large (>50 nm) clusters produced in cryogenically cooled, continuous gas jets. The technique combines three optical diagnostics--forward/backward Mie scattering detection, 90° scattering imaging, and neutral gas interferometry. Our measurement shows that cryogenic cooling and high backing gas pressure greatly enhance both cluster size and clustering ratio in continuous gas flow. Such a cluster source can be used as a potential target for intense, high-repetition-rate (>kHz) laser pulses. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM), field-emission scanning electron microscopy, and energy dispersive X-Ray spectroscopy are used to study morphological and compositional variations of metal-organic interfaces in organic bistable devices. The results show that bias voltage causes rougher interfaces with new protrusions, and the switching phenomena origins from the evolution of these protrusions under external electric field. In order to exclude other possible factors, three types of bistable devices are designed and examined. In addition, metal-coated AFM probes are utilized to simulate the switching process, which yields similar results and corroborates our conclusion. [ABSTRACT FROM AUTHOR]

The effect of electric field and temperature on bistable characteristics of indium tin oxide/tris-(8-hydroxyquinoline) aluminum (Alq3)/aluminum (Al) device has been investigated. The switching time, during which the device turns from a high resistive (OFF) state to a low resistive (ON) state, increases with decreasing field and temperature. The observed phenomena are accounted for by a phenomenological model that metallic nano-tips may evolve under the influence of the electric field and temperature. [ABSTRACT FROM AUTHOR]

The electrical performance of suspended few-layer MoTe2 field-effect-transistors with ionic liquid gating has been investigated. The suspended structure not only enhances the mobility of MoTe2 by removing the influence of the substrate but also allows ions to accumulate on both the top and the bottom surface of MoTe2. The consequent increase in the gate capacitance resulted in an improved subthreshold swing (∼73 mV/dec) and on–off ratio (106) at room temperature for suspended MoTe2 compared to substrate-supported devices. Suspended transistors with ionic liquid gating enable a larger charge density compared to ionic liquid gated supported devices and may provide a useful platform to study screening physics in 2D materials. [ABSTRACT FROM AUTHOR]

Single-layer organic memories made of organic material with good conductance have been characterized. Asymmetrical bistable behaviors under biases of opposite polarities are observed for devices with asymmetric electrodes. It is experimentally confirmed that a close correlation exists between the conductivity of the organic layer and the asymmetric bistability of the device under opposite biases. Inserting a block layer or thickening the organic layer will result in negative differential resistance under positive biases, leading to reversible symmetrical bistability. The phenomena are ascribed to the presence of filamentary microconducting channels in the organic layer. [ABSTRACT FROM AUTHOR]

In this letter, we show how high-resolution scanning tunneling microscopy (STM) imaging can be used to reveal that certain edges of micromechanically exfoliated single layer graphene crystals on silicon oxide follow either zigzag or armchair orientation. Using the cleavage technique, graphene flakes are obtained that very often show terminating edges seemingly following the crystallographic directions of the underlying honeycomb lattice. Performing atomic resolution STM-imaging on such flakes, we were able to directly prove this assumption. Raman imaging carried out on the same flakes further validated our findings. [ABSTRACT FROM AUTHOR]

Resonant Raman spectroscopy studies of (Mn, Co)-codoped ZnO films were carried out using the 325 nm laser as the excitation source. The mixed mode character of the longitudinal optical (LO) phonon reveals that the resonant Raman spectra of the films can be used to test the c-axis orientation degree of their crystallite grains. The ratio of integrated Raman intensities between the 2LO to LO changes as a function of annealing temperatures, which reaches maximum at about 800 °C and demonstrates the variation of electron-LO phonon coupling in the films. These can provide helpful information for the fabrication of ZnO based functional films and for the development of their future applications. [ABSTRACT FROM AUTHOR]

Experiments are performed for the characteristics of internal waves generated by four horizontally towed models, including a sphere and three slender models with different aspect ratios in a density stratified fluid with a strong halocline. The Reynolds number ranges within 3500 < Re = UD/v < 126 000, and the Froude number ranges within 0.4 < Fr =U/NmaxD < 18. Based on the time histories of the density perturbation and analysis of the correlation velocity of internal waves generated by the towed models, it is shown that the body-generated waves are stationary multiple-mode Lee waves, while the wake-generated waves are non-stationary wake waves mainly produced by the large-scale coherent structure in turbulent wakes acting as a moving excitation source. The transition between body-generated and wake-generated internal waves occurs at a critical Froude number Frc which is linearly dependent on the model aspect ratio. Before the transition, when the wave height H changes with the variation of Fr, a peak value of H can be observed and its corresponding Froude number Frp increases linearly with growing aspect ratio. After the transition, the Froude number Friw associated with the internal wave correlation velocity fluctuates within a certain range. The wave height linearly increases with increasing Fr regardless of the aspect ratios, and the wave pattern is dominated by a non-stationary wake-generated wave. According to the experimental configuration, a theoretical model is proposed to simulate the body-generated and wake-generated internal waves. The numerical results obtained by this theoretical model show general agreement with the experiments. [ABSTRACT FROM AUTHOR]

Broadband terahertz generation via two-color femtosecond laser filamentation is studied with laser input energies up to 60 mJ. In the small f-number focusing regime, the output THz energy strongly saturates, which is attributed to ionization-induced laser defocusing in filamentation. This saturation effect can be minimized by elongating the plasma filament with weak focusing. A conversion efficiency of >10-4 is achieved in elongated filamentation. [ABSTRACT FROM AUTHOR]

We demonstrate high-field (>8 MV/cm) terahertz generation at a high-repetition-rate (1 kHz) via two-color laser filamentation. Here, we use a cryogenically cooled femtosecond laser amplifier capable of producing 30 fs, 15 mJ pulses at 1 kHz as a driver, along with a combination of a thin dual-wavelength half-waveplate and a Brewster-angled silicon window to enhance terahertz generation and transmission. We also introduce a cost-effective, uncooled microbolometer camera for real-time terahertz beam profiling with two different modes. [ABSTRACT FROM AUTHOR]

Distributed-loss gyrotron traveling-wave amplifiers (gyro-TWTs) with high-gain, broadband, and millimeter-wave capabilities have been demonstrated. Most structures with distributed wall losses are stabilized in gyro-TWTs that operate at low beam currents. Attenuating severs are added to the interaction circuit of a distributed-loss gyro-TWT to prevent high beam currents that result in mode competition. Simulation results show that gyrotron backward-wave oscillations (gyro-BWOs) are not effectively suppressed by the lossy section; in contrast, the severed sections can effectively enhance the start-oscillation threshold of gyro-BWOs in the proposed gyro-TWT. Meanwhile, localized reflective oscillations seem not to occur in the gyro-TWT unless it operates at a high magnetic field or with a high interaction length. The stable gyro-TWT, operating in the low-loss TE01 mode, is predicted to yield a peak output power of 405 kW at 33 GHz with an efficiency of 20%, a saturated gain of 77 dB and a 3 dB bandwidth of 2.5 GHz for a 100 kV, 20 A electron beam with an axial velocity spread of Δvz/vz=5%. [ABSTRACT FROM AUTHOR]

Metastable filamentary superconductivity with Tc = 10.2 K was observed in SrPt2Ge2. This is the highest Tc reported outside the iron-based 122-layer compounds. The stable phase for this compound is a non-superconducting monoclinic LaPt2Ge2-type structure with lattice parameters a = 0.4401(4) nm, b = 0.4435(4) nm, c = 0.9764(10) nm, β = 90.5°-90°. Weak diamagnetic shielding signal indicates that superconductivity is not from the major monoclinic phase, but from the minor BaFe2As2-type body-center-tetragonal (122-bct) phase with a=0.44 nm, c=0.98 nm,β=90°. Investigation on the pseudoternary system Sr(Pd1-xPtx)2Ge2 suggests that the stable bct to monoclinic structure phase transition occurs near x = 0.15. For x<0.15 with bct phase, bulk Tc increases from 3.2 K for SrPd2Ge2 to 3.6 K for SrPd1.8Pt0.2Ge2. For x>0.15 with metastable filamentary bct phase, Tc increases from 7.5 K for x = 0.5 to 9.2 K for x = 0.7 and reaches a Tc maximum of 10.2 K for SrPt2Ge2. High Tc is due to a strong quasi-2D Pt-Ge-Pt 5dxz,yz-4p-5dxz,yz hybridization in the Pt-Ge layer with squeezed-along-c-axis PtGe4 tetrahedral crystal field and Pt-5d8 spin-orbital interaction in the metastable filamentary bct phase and is another example of multi-band (dxz/dyz/dxy) non-conventional s-wave superconductor. [ABSTRACT FROM AUTHOR]

Efficiency of tris (8-hydroxyquinolato) aluminum (Alq3) based organic light emitting diodes has been obviously improved by inserting a parylene-N (PPXN) buffer layer with proper thickness and location in the Alq3 layer. For an optimized PPXN inserted structure, current efficiency of 6.89 cd/A and luminous power efficiency of 1.85 lm/W were achieved; both are 42% higher than the 4.84 cd/A and 1.30 lm/W of the buffer free structure, respectively. The light emitting spectra of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped Alq3 (Alq3:DCM) strips showed the electron current enhancement resulted in the observed efficiency improvement of PPXN inserted devices. The device current evolutions were explained by nonuniform potential distribution in the organic layer and the tunneling barrier reduction. [ABSTRACT FROM AUTHOR]

Due to their high aspect ratio, well-defined cylindrical structure, and good electrical conductivity, carbon nanotubes (CNTs) are ideal probes for "true" local imaging of electric domain structures at the nanoscale. By performing force-distance measurements and tip-shape profiling with a uniformly charged oxide square, we clearly demonstrate the local nature of the CNT tip for electrostatic force microscopy. We show that CNTs can be used to probe long-range electrostatic forces with a lateral resolution better than 5 nm. [ABSTRACT FROM AUTHOR]

Synchrotron radiation photoemission study showed that the energy level alignment at the interface between N, N'-bis(naphthalene-1-y1)-N, N'-bis(phenyl) benzidine (NPB), a typical hole transport material, and Al could be adjusted by precovering a thin C60 layer on Al. The interface dipoles so formed could shift both the highest occupied molecular orbital level of NPB and the secondary electron cutoff measured at the early stage of the NPB deposition. The barrier height for hole injection from Al to NPB could thus be lowered by as much as 0.98 eV, and the optimal thickness of the inserted C60 layer was found to be 8-12 Å. [ABSTRACT FROM AUTHOR]

Presents a study which examined the field dependence of magnetic transitions in magnetic superconductors. Discussion on the anisotropy of the magnetic transitions; Requirement for field-induced weak ferromagnetic transitions; Details of the experimental techniques used.

Presents a study which examined the systematic variation of anomalous praseodymium (Pr) antiferromagnetic ordering temperature for two-CuO[sub2]-layer cuprate systems. Details of the experimental techniques used; Discussion on the systematic of anomalous Pr ordering.

High-resolution synchrotron radiation photoemission spectroscopy was used to investigate the chemical properties of Al–LiF interfaces. An electronic state appeared at the Al/LiF interface with a binding energy 4.8 eV higher than that of the metallic Al 2p core level, but the state was hardly found to be present at the LiF/Al interface. This indicates that intensive chemical reaction could occur at the Al/LiF interface, while the reaction occurring at the LiF/Al interface would be weak. This result explains well the unsymmetrical electron injection from different sides of the symmetrical device of indium-tin-oxide\Al\LiF\tris(8-hydroxyquinoline) aluminum\LiF\Al showing unsymmetrical current-voltage characteristics. [ABSTRACT FROM AUTHOR]

A parylene-N (PPXN) buffer layer inserted between anode and organic layers in typical organic light-emitting diodes (OLEDs) based on N, N′-bis(naphthalene-1-yl)-N, N′-bis(phenyl) benzidine (NPB) and tris (8-hydroxyquinolato) aluminum (Alq3) results in significant enhancement of the current efficiency. The enhancement can be 1.7 times higher compared to that of the buffer-free structure. Analyzing indicates that the buffer plays a role of blocking both the hole and electron current. Whether hole injection can be enhanced depends on electron accumulation at the buffer-organic interface. Inserting the PPXN buffer modifies carrier balance in the device, leading to the observed efficiency enhancement. [ABSTRACT FROM AUTHOR]

Lasing at 460, 485, and 510 nm is demonstrated in optically pumped, vacuum-deposited amorphous thin films of a carbazole derivative doped with Coumarin 47, perylene, and Coumarin 30, respectively. Efficient, nonradiative Fo¨rster energy transfer between host and dopant organic molecules results in low lasing thresholds (5 μJ/cm[sup 2]), high differential quantum efficiencies (15%), high peak output powers (20 W), and long operational lifetimes (>10[sup 5] pulses at 100 times the threshold power). © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]