Volume 1, Issue 2, June 2011
 REGULAR ARTICLES


Structural and magnetic properties of bulk nanocrystalline Erbium metal
View Description Hide DescriptionBulk nanocrystalline Erbium metals were prepared via Spark Plasma Sintering (SPS) and subsequent annealing process. The nanocrystalline Er metals have the same hexagonal close packed structure as that of coarsegrained sample. Decrease in grain size results in remarkable changes in the three magnetic orderingtemperatures of the nanocrystalline Er metal. At 5 K, the magnetization drops by 10.9%, while the coercivity increases by 4 times for nanocrystalline Er compared with those of coarsegrained sample. These results indicate the remarkable influence of the nanostructure on the magnetism of Er due to finite size effect.

Change in carbon nanofiber resistance from ambient to vacuum
View Description Hide DescriptionThe electrical properties of carbon nanofibers (CNFs) can be affected by adsorbed gas species. In this study, we compare the resistance values of CNF devices in a horizontal configuration in air and under vacuum. CNFs in air are observed to possess lower current capacities compared to those in vacuum. Further, Joule heating due to current stressing can result in desorption of gas molecules responsible for carrier trapping, leading to lower resistances and higher breakdown currents in vacuum, where most adsorbed gaseous species are evacuated before any significant readsorption can occur. A model is proposed to describe these observations, and is used to estimate the number of adsorbed molecules on a CNF device.

Fabrication of Liintercalated bilayer graphene
View Description Hide DescriptionWe have succeeded in fabricating Liintercalated bilayer graphene on silicon carbide. The lowenergy electron diffraction from Lideposited bilayer graphene shows a sharp × 30° pattern in contrast to Lideposited monolayer graphene. This indicates that Li atoms are intercalated between two adjacent graphene layers and take the same wellordered superstructure as in bulk C6Li. The angleresolved photoemission spectroscopy has revealed that Li atoms are fully ionized and the π bands of graphene are systematically folded by the superstructure of intercalated Li atoms, producing a snowflakelike Fermi surface centered at the Γ point. The present result suggests a high potential of Liintercalated bilayer graphene for application to a nanoscale Liion battery.

Determination of the decay exponent in mechanically stirred isotropic turbulence
View Description Hide DescriptionDirect numerical simulation is used to investigate the decay exponent of isotropic homogeneous turbulence over a range of Reynolds numbers sufficient to display both high and low Re number decay behavior. The initial turbulence is generated by the stirring action of the flow past many small randomly placed cubes. Stirring occurs at 1/30^{th} of the simulation domain size so that the lowwavenumber and large scale behavior of the turbulent spectrum is generated by the fluid and is not imposed. It is shown that the decay exponent in the resulting turbulence matches the theoretical predictions for a k^{2} lowwavenumber spectrum at both high and low Reynolds numbers. The transition from high Reynolds number behavior to low Reynolds number behavior occurs relatively abruptly at a turbulentReynolds number of around 250 ().

Nitrogen is a deep acceptor in ZnO
View Description Hide DescriptionZinc oxide is a promising material for blue and UV solidstate lighting devices, among other applications. Nitrogen has been regarded as a potential ptype dopant for ZnO. However, recent calculations [Lyons, Janotti, and Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)] indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce ptype ZnO. A broad photoluminescence(PL) emission band near 1.7 eV, with an excitation onset of ∼2.2 eV, was observed, in agreement with the deepacceptor model of the nitrogen defect. The deepacceptor behavior can be explained by the low energy of the ZnOvalence band relative to the vacuum level.

Optimized photolithographic fabrication process for carbon nanotube devices
View Description Hide DescriptionWe have developed a photolithographic process for the fabrication of large arrays of single walled carbon nanotubetransistors with high quality electronic properties that rival those of transistorsfabricated by electron beam lithography. A buffer layer is used to prevent direct contact between the nanotube and the novolacbased photoresist, and a cleaning bake at 300C effectively removes residues that bind to the nanotube sidewall during processing. In situ electrical measurement of a nanotubetransistor during a temperature ramp reveals sharp decreases in the ONstate resistance that we associate with the vaporization of components of the photoresist. Data from nearly 2000 measured nanotubetransistors show an average ONstate resistance of 250 ± 100 kΩ. This new process represents significant progress towards the goal of highyield production of large arrays of nanotubetransistors for applications including chemical sensors and transducers, as well as integrated circuit components.

Effect of DC bias on electrical conductivity of nanocrystalline αCuSCN
View Description Hide DescriptionThe grain boundary space charge depletion layer in nanocrystalline alpha phase CuSCN is investigated by studying electrical properties using impedance spectroscopic analysis in frequency domain. The measurements were performed at room temperature in wide frequency range 1 Hz to 1 MHz under various DC bias applied voltages ranges from 0 V to 2.1 V. The effect of bias on grain and grain boundary contribution electrical conductivity has been investigated by equivalent circuit model using nonlinear least squares (NLLS) fitting of the impedance data. Three order of magnitude variation of grain boundary conductivity was observed for varying 0 V to 2.1 V. Variations in the σ_{ac} clearly elucidate the DC bias is playing crucial role on grain boundary double Schottky barriers of nanocrystalline αCuSCN.

Precise ab initio calculations of the 3d transitionmetal clusters: Sc_{2}
View Description Hide DescriptionThe ground state of Sc_{2} was studied by the valence multireference configuration interaction method with single and double excitations plus Davidson correction (MRCISD(+Q)) at the complete basis set limit. The calculations were made under C _{2v } symmetry restrictions, which allowed us to obtain at the dissociation limit the Sc atoms in different states (in all previous studies of Sc_{2} the D _{2h } symmetry group was employed). From the Mulliken population analysis and energy calculations follows that in the ground state Sc_{2} dissociates in one Sc in the ground state and the other in the second excited quartet state, ^{4}F_{u}. The corrected parameters of the ground potential curve are the following: R _{ e } = 5.2 bohr, D _{ e } = 50.37 kcal/mol, and ω _{ e } = 234.5 cm^{1}. The dissociation energy in respect to the dissociation on two Sc in the ground states was estimated as D _{ e } = 9.98 kcal/mol.

A new route to graphene layers by selective laser ablation
View Description Hide DescriptionSelectively creating regions of spatially varying thickness may enable the utilization of the electronic properties of Nlayer (N=1 or more) graphene and other similar layered materials (e.g., topological insulators or layered superconductors) for novel devices and functionalities on a single chip. The ablation threshold energy density increases dramatically for decreasing layer numbers of graphene originating from the dimensional crossover of the specific heat. For the 2D regime of graphite (up to N≈7) the dominant flexural mode specific heat (due to its N^{1} dependence) gives rise to a strong layer numberdependence on the pulsed laser ablation threshold energy density, while for 3D regime (N>>7) the ablation threshold saturates due to dominant acoustic mode specific heat. As a result, several energy density windows exist between the minimum energy densities that are required for ablating single, bi, or more layers of graphene, allowing layer number selectivity.

An inverse problem of temperature estimation for the combination of the linear and nonlinear resistances
View Description Hide DescriptionThe subject of the theoretical analysis presented in this paper is an analytical approach to the temperature estimation, as an inverse problem, for different thermistors – linear resistances structures: series and parallel ones, by the STFT  Special Trans Functions Theory (S.M. Perovich). The mathematical formulae genesis of both cases is given. Some numerical and graphical simulations in MATHEMATICA program have been realized. The estimated temperature intervals for strongly determined values of the equivalent resistances of the nonlinear structures are given, as well.

Effect of solage on the surface and optical properties of solgel derived mesoporous zirconia thin films
View Description Hide DescriptionMesoporous ZrO_{2}thin films have been deposited by a modified solgel dip coating technique using HCl as catalyst. Effects of solage on the surface and on the optical properties are studied. Transmission electron micrographs of the films reveal the pore dimensions in mesoporous regime. A strong correlation in surface topography with solage has been observed where increase in solage induces a systematic enhancement in the value of root mean square roughness of the films. Optical study shows that deposited films have high transmittance and an enhancement of 5.6 times in porosity in films prepared with solage of 10 days with respect to that of 1 day. Band gap estimation by Tauc's plots of films is observed to 5.74 eV, which shows invariance with the solage.

A simple method to prepare selfassembled organicorganic heterobilayers on metal substrates
View Description Hide DescriptionWe demonstrate a selfassembly based simple method to prepare organicorganic heterobilayers on a metal substrate. By either sequential or codeposition of parasexiphenyl (p6P) and pentacene molecules onto the Cu(110) surface in ultrahigh vacuum, p6P/pentacene/Cu(110) heterobilayer is synthesized at room temperature. The layer sequence of the heterostructure is independent of the growth scenario indicating the p6P/pentacene/Cu(110) is a selfassembledstructure with lowest energy. Besides, the bilayer shows a very high orientational ordering and is thermally stable up to 430K.

Local spin valve effect in lateral (Ga,Mn)As/GaAs spin Esaki diode devices
View Description Hide DescriptionWe report here on a local spin valve effect observed unambiguously in lateral allsemiconductor allelectrical spin injection devices, employing p ^{+} −(Ga,Mn)As/n ^{+} −GaAs Esaki diode structures as spin aligning contacts. We discuss the observed local spinvalve signal as a result of the interplay between spintransportrelated contribution and the tunneling anisotropic magnetoresistance of the magnetic contacts. The magnitude of the spinrelated magnetoresistance change is equal to 30 Ω which is twice the magnitude of the measured nonlocal signal.

The positivecolumn plasma in lowpressure noble gas d.c. discharge as an integral plasmafield system
View Description Hide DescriptionA onedimension model for positivecolumn plasma is analyzed. In the framework of this model, a complete, selfconsistent set of equations for the plasma column is proposed and justified. Basic prerequisites for the model and the equations used in it are discussed at length to clarify the mathematics and physics that underlie the proposed generalized description of plasma states. A study of the equations has unveiled the existence of two structurally stable types of steady states and three integrals of motion in the plasma system. The first type of states corresponds to spatially homogeneous plasma, and the second type, to the selfforming plasma structure with striations. Analysis of spatiotemporal plasma structures (spatially homogeneous and stratified stationary plasma states) and their attendant phenomena is given in detail. It is shown that the equations offer a more penetrating insight into the physical states and properties of positivecolumn plasma in dcdriven gas discharges, and into the various phenomena proceeding in the discharge system. Such a behavior is intimately related to the influence which the electric field has on the rate of ionization reactions. The theoretical results are compared to experimental data and can be used for to place the great body of experimental data in their proper framework. The modern fluid bifurcation model proposed to describe the properties of nonisothermic positivecolumn plasma in dcdriven lowpressure noblegas discharges proved to be rather realistic, capable of adequately reproducing the basic properties of real fieldplasma systems.

Equilibrium states of homogeneous sheared compressible turbulence
View Description Hide DescriptionEquilibrium states of homogeneous compressible turbulence subjected to rapid shear is studied using rapid distortion theory (RDT). The purpose of this study is to determine the numerical solutions of unsteady linearized equations governing double correlations spectra evolution. In this work, RDT code developed by authors solves these equations for compressible homogeneous shear flows. Numerical integration of these equations is carried out using a secondorder simple and accurate scheme. The two Mach numbers relevant to homogeneous shear flow are the turbulentMach numberM _{ t }, given by the root mean square turbulentvelocityfluctuations divided by the speed of sound, and the gradient Mach numberM _{ g } which is the mean shear rate times the transverse integral scale of the turbulence divided by the speed of sound. Validation of this code is performed by comparing RDT results with direct numerical simulation (DNS) of [A. Simone, G.N. Coleman, and C. Cambon, Fluid Mech. 330, 307 (1997)] and [S. Sarkar, J. Fluid Mech. 282, 163 (1995)] for various values of initial gradient Mach numberM _{ g } _{0}. It was found that RDT is valid for small values of the nondimensional times St (St < 3.5). It is important to note that RDT is also valid for large values of St (St > 10) in particular for large values of M _{ g } _{0}. This essential feature justifies the resort to RDT in order to determine equilibrium states in the compressible regime.

Indications of energetic consequences of decoherence at short times for scattering from open quantum systems
View Description Hide DescriptionDecoherence of quantum entangled particles is observed in most systems, and is usually caused by systemenvironment interactions. Disentangling two subsystems A and B of a quantum systemAB is tantamount to erasure of quantum phase relations between A and B. It is widely believed that this erasure is an innocuous process, which e.g. does not affect the energies of A and B. Surprisingly, recent theoretical investigations by different groups showed that disentangling two systems, i.e. their decoherence, can cause an increase of their energies. Applying this result to the context of neutronCompton scattering from H_{2} molecules, we provide for the first time experimental evidence which supports this prediction. The results reveal that the neutronproton collision leading to the cleavage of the HH bond in the subfemtosecond timescale is accompanied by larger energy transfer (by about 3%) than conventional theory predicts. It is proposed to interpreted the results by considering the neutronproton collisional system as an entangled open quantum system being subject to decoherence owing to the interactions with the “environment” (i.e., two electrons plus second proton of H_{2}).

Squeezing out hydrated protons: lowfrictionalenergy triboelectric insulator charging on a microscopic scale
View Description Hide DescriptionThough triboelectric charging of insulators is common, neither its mechanism nor the nature of the charge is well known. Most research has focused on the integral amount of charge transferred between two materials upon contact, establishing, e.g., a triboelectric series. Here, the charge distribution of tracks on insulating polymer films rubbed by polymercovered pointed swabs is investigated in high resolution by Kelvin probe force microscopy. Pronounced bipolar charging was observed for all nine rubbing combinations of three different polymers, with absolute surface potentials of up to several volts distributed in streaks along the rubbing direction and varying in polarity on μmlength scales perpendicular to the rubbing direction. Charge densities increased considerably for rubbing in higher relative humidity, for higher rubbing loads, and for more hydrophilic polymers. The ends of rubbed tracks had positively charged rims. Surface potential decay with time was strongly accelerated in increased humidity, particularly for polymers with high water permeability. Based on these observations, a mechanism is proposed of triboelectrification by extrusions of prevalently hydrated protons, stemming from adsorbed and dissociated water, along pressure gradients on the surface by the mechanical action of the swab. The validity of this mechanism is supported by explanations given recently in the literature for positive streaming currents of water at polymer surfaces and by reports of negative charging of insulators tapped by accelerated water droplets and of potential built up between the front and the back of a rubbing piece, observations already made in the 19^{th} century. For more brittle polymers, strongly negatively charged microscopic abrasive particles were frequently observed on the rubbed tracks. The negative charge of those particles is presumably due in part to triboemission of electrons by polymer chain scission, forming radicals and negatively charged ions.

Multidimensional wave field signal theory: Mathematical foundations
View Description Hide DescriptionMany important physical phenomena are described by wave or diffusionwave type equations. Since these equations are linear, it would be useful to be able to use tools from the theory of linear signals and systems in solving related forward or inverse problems. In particular, the transform domain signal description from linear system theory has shown concrete promise for the solution of problems that are governed by a multidimensional wave field. The aim is to develop a unified framework for the description of wavefields via multidimensional signals. However, certain preliminary mathematical results are crucial for the development of this framework. This first paper on this topic thus introduces the mathematical foundations and proves some important mathematical results. The foundation of the framework starts with the inhomogeneous Helmholtz or pseudoHelmholtz equation, which is the mathematical basis of a large class of wavefields. Application of the appropriate multidimensional Fourier transform leads to a transfer function description. To return to the physical spatial domain, certain mathematical results are necessary and these are presented and proved here as six fundamental theorems. These theorems are crucial for the evaluation of a certain class of improper integrals which arise in the evaluation of inverse multidimensional Fourier and Hankel transforms, upon which the framework is based. Subsequently, applications of these theorems are demonstrated, in particular for the derivation of Green's functions in different coordinate systems.

Stereoscopic full aperture imaging in nuclear medicine
View Description Hide DescriptionImages of planar scintigraphy and single photon emission computerized tomography (SPECT) used in nuclear medicine are often low quality. They usually appear to be blurred and noisy. This problem is due to the low spatial resolution and poor sensitivity of the acquisition technique with the gamma camera (GC). Other techniques, such as coded aperture imaging (CAI) reach higher spatial resolutions than GC. However, CAI is not frequently used for imaging in nuclear medicine, due to the decoding complexity of some images and the difficulty in controlling the noise magnitude. Summing up, the images obtained through GC are low quality and it is still difficult to implement CAI technique. A novel technique, full aperture Imaging (FAI), also uses gamma rayencoding to obtain images, but the coding system and the method of images reconstruction are simpler than those used in CAI. In addition, FAI also reaches higher spatial resolution than GC. In this work, the principles of FAI technique and the method of images reconstruction are explained in detail. The FAI technique is tested by means of Monte Carlo simulations with filiform and spherical sources. Spatial resolution tests of GC versus FAI were performed using two different sourcedetector distances. First, simulations were made without interposing any material between the sources and the detector. Then, other more realistic simulations were made. In these, the sources were placed in the centre of a rectangular prismatic region, filled with water. A rigorous comparison was made between GC and FAI images of the linear filiform sources, by means of two methods: mean fluence profile graphs and correlation tests. Finally, threedimensional capacity of FAI was tested with two spherical sources. The results show that FAI technique has greater sensitivity (>100 times) and greater spatial resolution (>2.6 times) than that of GC with LEHR collimator, in both cases, with and without attenuating material and long and shortdistance configurations. The FAI decoding algorithm reconstructs simultaneously four different projections which are located in separate image fields on the detector plane, while GC produces only one projection per acquisition. Simulations have allowed comparison of both techniques under ideal identical conditions. Our results show it is possible to apply an extremely simple encoded imaging technique, and get threedimensional radioactivity information for simplistic geometry sources. The results are promising enough to evaluate the possibility of future research with more complex sources typical of nuclear medicine imaging.

Towards weakening of the Coulomb blockade in artificially prepared clusters of superconducting grains
View Description Hide DescriptionThe relative role of dipoledipole interactions between induced polarization moments in the total charge balance of a model system of two clusters (each cluster contains Nsuperconducting grains) is analyzed. It is found that, due to its orientational nature, the dipole energy between clusters may overcome the direct Coulomb coupling between grains (within a single cluster). To verify the model predictions experimentally, specially prepared clusters with markedly distinctive graingrain (ε_{ c }) and clustercluster (ε_{ d }) dielectric properties are needed. More specifically, clusters of Albased superconducting grains embedded into SrTiO _{3}dielectric matrix (with ε_{ c } ≃ 10^{4}) and separated by a dielectric layer of Al _{2} O _{3} (with ε_{ d } ≃ 10) would result in a noticeable decrease of the total charging energy of the system.
