Hop Length Research Articles

Dielectric Relaxation and Conductivity of CdGa2S4 Single Crystal Grown by the CTR Method

CdGa2S4 single crystals (a = 5.555 ± 0.002 A, c = 10.190 ± 0.005 A) have been grown by the chemical transport reaction (CTR) method, and their dielectric properties have been analyzed. An experimental study of the frequency dependence of the dielectric coefficients and conductivity of the CdGa2S4 single crystal has made it possible to reveal the nature of dielectric losses, establish the hopping charge-transfer mechanism, and estimate the parameters of localized states (density and energy spread of states near the Fermi level; mean duration and length of hops; and concentration of deep traps, which implement ac conductivity).

Path Formation Time in the Noise-Limited Fractionated Spacecraft Network with FDMA

Connectivity and path formation time are very important for the design and optimization in fractionated spacecraft network. Taking frequency division multiple access (FDMA) with subcarrier binary phase-shift keying (BPSK) modulation as an example, this paper focuses on the issues of constraint to orbital elements and path formation time for the noise-limited fractionated spacecraft network percolating. First, based on the proposed evolution of the dynamic topology graph in the fractionated spacecraft network, we prove the constraint condition of orbital elements for noise-limited fractionated spacecraft network percolating, and the definition of path formation time is provided and the mobility model is established. Next, we study the relationship between first docking time and spatial initial distribution and the relationship between first separating time and spatial initial distribution. These relationships provide an important basis for the orbit design in the fractionated spacecraft network. Finally, the numerical results show that the network topology for fractionated spacecraft is time-varying and dynamic. The path formation time and hop length scale linearly with path length within each orbital hyperperiod and change periodically. Besides, the time constant gradually tends to a stable value with path formation time increasing, that is, path length. These results powerfully support percolation theory further under the environment of the noise-limited fractionated spacecraft network.

Optimal Hop Lengths to Ensure Minimum Energy Consumption in Wireless Sensor Networks

In wireless sensor networks, nodes have a low computing capacity, a small antenna and a very limited energy source; thereby batteries are considered as a critical resource and should be used efficiently. On the other hand, the antennas are the biggest consumers of energy, therefore, and their use must be very efficient to minimize energy consumption. In a dense WSN, each node may route messages to destination nodes either through short-hops or long-hops, by using a short or a long radio range. Thus, the hop length optimization can save energy. In this article, the authors propose a theorem to optimize the hop lengths and a routing algorithm to improve the WSN power consumption. The theorem establishes a simple condition to ensure the optimal hop lengths which guarantees the minimum energy consumption. And the proposed algorithm based on that condition is used to find the optimal routing path. The simulation results are obtained by applying the condition and the algorithm on WSNs and reveals a high performance regarding WSNs energy consumption and network lifetime.

Geography and Routing in the Internet

The Internet is a network of networks consisting of tens of thousands of Autonomous Systems (ASes). These ASes connect to each other in different forms to enable the “global” Internet communication. In this study, we investigate the geographical characteristics of the visible Internet as well as examine the relation between geography and intra-AS and inter-AS routing policies. We show that the ingress-to-egress subpaths have lower circuitousness compared to the end-to-end paths. Our findings not only demonstrate the efficient backbone infrastructures and routing schemes deployed by ASes but also show the consequences of economical incentives on the adoption of inter-AS paths. We present and examine the existence of a strong correlation between the geographical distance and round trip delay time as well as the lack of a correlation between the geographical distance and hop length in the Internet. We investigate the relation between the geographical distance and intra-AS routing policies by employing cross-AS (X-AS) Internet topology maps. Our results show that more than two thirds of the intra-AS subpaths are congruent with the shortest geographical distance whether or not geographical distance is employed as a custom parameter in routing decisions. Our results provide new insights into the relations between geography and Internet routing, which allow the network researchers and practitioners to improve their networking infrastructures, reevaluate their routing policies, deploy geography-aware network overlays, and develop more realistic network simulation processes.

Investigation on structural, electrical and magnetic properties of titanium substituted cobalt ferrite nanocrystallites

The doping of parent compound with selective dopants usually enhances the properties of the parent compound. In this paper, titanium doped cobalt ferrite nanoparticles (Co1−xTixFe2O4 with x = 0–0.075) via the sol-gel route is reported. The effect of titanium ions on the properties of cobalt ferrite is detailed. Structural evolution is confirmed using XRD technique and the titanium ion induced changes are given in terms of lattice constant, grain size, X-ray density, tetrahedral and octahedral hopping length and tetrahedral and octahedral bond length. The X-ray diffraction pattern displayed the formation of cubic inverse spinal structure belonging to the Fd3m space group. FTIR analysis confirmed the presence of characteristic peaks of ferrites around 400 cm−1 and 600 cm−1. Raman spectroscopy revealed reduction in peak intensities of Raman active modes. SEM characterization had shown the formation of well resolved spherical particles. Elemental composition was evaluated by EDAX spectrum. TEM images also confirmed the spherical morphology of the nanocrystallites and crystallite size calculated is in agreement with that calculated from the XRD analysis. The SAED pattern shows uniform fringes of width 0.428 nm belonging (222) plane. The diffraction pattern of HRTEM confirmed cubic spinel structure of the samples. Magnetic properties are evaluated from room temperature hysteresis loops. Increase in the saturation magnetization and coercivity was observed with increasing titanium content. Gradual increase in impedance with the increase in titanium concentration was estimated by impedance spectroscopy.

Effect of Sb doping on the transport and electrochemical properties of partially amorphous SnO2 thin films

Sb-doped SnO2 thin films synthesized by sol-gel method were characterized physically and electrochemically. The crystallization was promoted by Sb incorporation in the lattice and the optical gap is blue-shifted due to the quantum confinement. The thermal variation of the electrical conductivity indicates a semiconducting behavior with relaxation process shifted to higher frequencies, typical of hopping mechanism of charge carriers. The hopping length R averages ~ 1.4 nm and the maximum barrier height (Wm) for the electronic jumps is 878 meV. The interfacial capacitance is characteristic of n-type behavior with an electrons density of 2.7 × 1019 cm−3 and a space charge region of 150 nm. The valence and conduction bands were found to be −0.89 and +2.95 VSCE respectively; they are pH sensitive and sweeping with respect to redox system. The impedance data revealed three distinct electrochemical behaviors with high diffusion process in doped samples marked by the increase of Warburg impedance (0.91–202.3 kΩ·cm2). The optimal band bending with respect to levels (H2O/H2, O2/H2O) • ‐(O2/O2• −, OH•/H2O), was achieved with localized states mediating the charge exchange.

Enhanced dielectric and piezoelectric properties of BNT-KNNG piezoelectric ceramics

The structural, morphological and electric properties of (1-x) Bi0.5Na0.5TiO3 – x K0.5Na0.5NbO3 + 1 wt. % Gd2O3 (BNT-KNNG; x = 0–0.02) ceramics were investigated in detail. XRD studies revealed the rhombohedral phase, which is also confirmed by HRTEM. The diffuse phase transition of BNT-KNNG samples exhibited a relaxor behaviour supported by modified Curie Weiss law and Vogel–Fulcher law. The AC-Conductivity of BNT-KNNG ceramics in terms of hopping length, hopping energy and density of states across the Fermi level was analyzed by Mott's variable range hopping mechanism. The addition of KNNG effectively reduced grain size, transition temperature, leakage current and improved dielectric (ε′ = 1074 and tanδ = 0.059 @ 1 kHz) and piezoelectric properties; electromechanical coupling factors (k33, k31, kt ∼ 48% and kp = 53%), and piezoelectric constants (d33 = 108 pC/N and g33 = 1.14 × 10−2 Vm/N) are observed for x = 0.01 sample, which is a potential candidate for high power electromechanical applications.

Stability enhancement of yttrium substituted nickel zinc ferrite/PZT magnetoelectric gyrators under high power conditions

The influence of yttrium (Y) ion substitution on the magneto-mechanical properties of nickel-zinc ferrite (Ni0.7Zn0.3YxFe2-xO4) or NZFO-Y and the power efficiency (η) of magnetoelectric (ME) gyrators consisting of NZFO-Y/Pb(Zr, Ti)O3 layers were studied. X-ray diffraction (XRD) of polycrystalline samples of NZFO-Y indicated that yttrium incorporated into the crystal lattice resulted in an enlargement of the lattice parameter. Also, the density and ionic hopping length were increased, and the porosity was decreased. Together, these resulted in significant changes in the magneto-mechanical coupling (k33,m) and mechanical quality (Qm) factors. It was found that k33,m decreased with the increasing Y content for 0 ≤ x ≤ 0.08, whereas Qm was increased. For x > 0.05, the values of k33,m and Qm were found to be nearly stable with increasing input power density (PD-Ms) up to 47 W/in3. Under this high power driving condition for 20 min, we found that the power conversion stability of ME gyrators was increased with the increasing Y content in the ferrite and that a maximum power conversion efficiency occurred near 77%. These findings offer the potential for ferrite-based ME gyrator applications in power electronic conversion devices.

Phase transition and conduction mechanism study by overlapping large-polaron tunnelling model in Li2MoO4 ceramic compound

The Lithium-ionic conductor Li2MoO4 compound was prepared by the conventional solid-state reaction method. A Rietveld analysis of powder X-ray diffraction at room temperature has exhibited the title compound crystallizes in the trigonal system (space group) R3¯ with a phenacite-like structure. The calorimetric and Raman spectroscopy-studies revealed a single-phase transition at 635 K. The electrical conductivity was measured in the frequency range from 200 Hz to 1 MHz and temperatures between 578 and 723 K using impedance spectroscopy. Besides, the analysis of Nyquist plots revealed the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries. The electrical conductivity for the grain (σg) follows an Arrhenius behavior with two different activation energies Ea(I) = 1.038 eV for T < 635 K and Ea(II) = 0.481 eV for T > 635 K. The frequency dependence of the conductivity was interpreted in terms of Jonscher's law. Temperature dependence of the power law exponent s suggests that the overlapping large-polaron tunneling (OLPT) model is the dominant transport process in this material. The optimum hopping length of the polaron (1.8 <Rω < 3.4 Å), is large compared with the interatomic spacing (LiO = 1.964 Å; MoO = 1.8 Å).

Probing variable range hopping lengths by magneto conductance in carbonized polymer nanofibers

Using magneto transport, we probe hopping length scales in the variable range hopping conduction of carbonized polyacetylene and polyaniline nanofibers. In contrast to pristine polyacetylene nanofibers that show vanishing magneto conductance at large electric fields, carbonized polymer nanofibers display a negative magneto conductance that decreases in magnitude but remains finite with respect to the electric field. We show that this behavior of magneto conductance is an indicator of the electric field and temperature dependence of hopping length in the gradual transition from the thermally activated to the activation-less electric field driven variable range hopping transport. This reveals magneto transport as a useful tool to probe hopping lengths in the non-linear hopping regime.

Effect of A-site cation ordering on the thermoelectric properties of the complex cobalt oxides Gd1-xSrxCoO3-δ (x = 0.8 and 0.9)

The effect of substitution of Sr2+ ions for Gd3+ ions on the phase composition, electrical resistivity, thermoelectricity, and thermal conductivity of rare-earth cobalt oxides Gd1-xSrxCoO3-δ (x = 0.8 and 0.9) has been investigated. It has been determined that at the investigated strontium concentrations, the single-phase disordered nonstoichiometric cubic perovskites and superstructures with ordered Sr2+/Gd3+ ions and anion vacancies can be formed. The influence of ordering/disordering of Gd and Sr cations over crystal-lattice A- sites on the thermoelectric figure of merit and sample stability at high temperatures has been studied. The thermoelectric figure of merit of the disordered samples was found to exceed by far the analogous parameter of the ordered samples, which allows us to consider the disordering as a way of improving the thermoelectric parameters. Two contributions to the conductivities are discussed: high-temperature thermoactivation and low-temperatures variable range hopping. The parameters of the Mott electronic structure, including DOS N(εF), hopping energy ε (the energy of hopping conductivity activation), and hopping length Rh, have been estimated

Magnetoreresistance of carbon nanotube-polypyrrole composite yarns

Three types of samples, carbon nanotube yarn and carbon nanotube-polypyrrole composite yarns had been investigated by measurement of the electrical conductivity as a function of temperature and magnetic field. The conductivity was well explained by 3D Mott variable range hopping (VRH) law at T < 100 K. Both positive and negative magnetoresistance (MR) were observed by increasing magnetic field. The MR data were analyzed based a theoretical model. A quadratic positive and negative MR was observed for three samples. It was found that the localization length decreases with applied magnetic field while the density of states increases. The increasing of the density of states induces increasing the number of available energy states for hopping. Thus the electron hopping probability increases in between sites with the shorter distance that results to small the average hopping length.

Dielectric relaxation and study of electrical conduction mechanism in BaZr0.1Ti0.9O3ceramics by correlated barrier hopping model

AbstractThis work aims to study the electrical conduction mechanism in the dielectric material BaZr0.1Ti0.9O3(BZT) ceramics by applying AC signal in the frequency range of 102Hz to 106Hz. The phase purity and microstructure of the sample have been studied by X-ray diffraction refinement and field-emission scanning electron microscope (FE-SEM) analysis. The appearance of resonance peaks in the loss tangent at high temperature is due to inherent dielectric relaxation processes of this oxide. The temperature dependent Cole-Cole plot has been studied in details to determine both the grain and grain boundary contribution to the conductivity. Electrical modulus analysis reveals that the hopping of charge carriers is the most probable conduction mechanism in BZT ceramics. The obtained data of AC conductivity obey the universal double power law and have been discussed in terms of microstructural network characteristics. The behavior of frequency exponent n of AC conductivity as a function of temperature verify the applicability of the correlated barrier hopping (CBH) model. The AC conductivity data are used to estimate the minimum hopping length, density of states at Fermi level, thermal conductivity and apparent activation energy. The value of activation energy confirms that the oxygen vacancies play a vital role in the conduction mechanism.

Dielectric Properties and Conductivity of Ag-Doped TlGaS2 Single Crystals

The effect of silver ions (2 mol %) on the dielectric properties and electrical conductivity of TlGaS2 single crystals grown by the Bridgman–Stockbarger method is investigated. The experimental results of studying the frequency dispersion of the dielectric coefficients of TlGaS2 single crystals (2 mol % Ag) makes it possible to establish the nature of dielectric losses and the charge-transfer mechanism, to evaluate the density of states near the Fermi level, the spread of states, the average hopping time and length, and the concentration of deep traps responsible for ac conductivity. The Ag doping of the TlGaS2 single crystals results in an increase in the density of states near the Fermi level and in a decrease in the average hopping time and length.

Reliability and energy-efficiency analysis of safety message broadcast in VANETs

Safety applications in vehicular ad hoc networks (VANETs) rely heavily on broadcast schemes for the exchange of status messages. Reliable and on-time delivery of safety messages is critical in VANETs that requires extremely efficient broadcast models. In this paper, we model the reliability of flooding used as the underlying data dissemination protocol for time-critical safety messages. We consider a multi-hop VANET and model end-to-end reliability provided by the network layer. The analytical results unleash important insights about the flooding mechanism. For instance, the reliability drops exponentially beyond a certain threshold value of packet loss probability. Secondly, the impact of hop length on reliability can be catastrophic if not handled appropriately. Keeping in view of the fact that energy-efficient protocols is a key requirement in the upcoming Internet of Vehicles (IoV), we also show that restrictive flooding is more energy-efficient than plain flooding under the same reliability constraint. We validate our theoretical findings through simulations and propose modifications to plain flooding to make it more reliable.

Диэлектрические свойства и электропроводность легированного серебром монокристалла TlGaS-=SUB=-2-=/SUB=-

AbstractThe effect of silver ions (2 mol %) on the dielectric properties and electrical conductivity of TlGaS_2 single crystals grown by the Bridgman–Stockbarger method is investigated. The experimental results of studying the frequency dispersion of the dielectric coefficients of TlGaS_2 single crystals (2 mol % Ag) makes it possible to establish the nature of dielectric losses and the charge-transfer mechanism, to evaluate the density of states near the Fermi level, the spread of states, the average hopping time and length, and the concentration of deep traps responsible for ac conductivity. The Ag doping of the TlGaS_2 single crystals results in an increase in the density of states near the Fermi level and in a decrease in the average hopping time and length.

On experimental censorship of particle hops in bed-load transport

Observation of incomplete particle hops in imaging-based experiments with bed-load transport does not just occur with hops that are longer than an area of observation. Even short hops can be in fact incompletely measured if they cross the boundaries of the focus area. Experimental censorship biases the statistical distribution and moments of bed-load particle hops. This paper presents: a theoretical quantification of an expected effect of experimental censorship; based on this quantification, a method to correct the statistical distribution of hop lengths; an application of the method to experimental data. Applying the correction leads to significantly different values of the mean hop length, compensating the biasing effect of censorship. By contrast, nothing can be done for experimental truncation that is associated with hops longer than an applied area of observation. Experimental truncation must be avoided a priori, by appropriate design of the measurement method.

Efros-Shklovskii variable range hopping and nonlinear transport in 1T/1T′−MoS2

We have studied temperature- and electric-field dependent carrier transport in single flakes of MoS$_{2}$ treated with n-butyllithium. The temperature dependence of the four-terminal resistance follows the Efros-Shklovskii variable range hopping conduction mechanism. From measurements in the Ohmic and non-Ohmic regime, we estimate the localization length and the average hopping length of the carriers, as well as the effective dielectric constant. Furthermore, comparison between two- and four-probe measurements yield a contact resistance that increases significantly with decreasing temperature.

Structural and dielectric properties of the fluorite-type LaxCe1−xO2−δ ceramics

High density (95–97%) Lax () ceramics were tested for structural and dielectric properties. Dense microstructure and controlled phase-purity were obtained for all the -doped samples. A surprising three-fold increase was noticed in the frequency dependent dielectric permittivity behavior for the ceramic, associated with the space-charge polarization at grain boundaries or inhomogeneous microstructure of the ceramic. High resistivity (∼108 Ωm) at room temperature was noticed for the ceramics (at 100 Hz). Hopping of electrons within the oxygen vacancies were noted as the main transport mechanism; strong localization of charge carriers were noticed at high frequencies. The temperature dependent ac-resistivity analysis (T) provides strong evidence for Mott’s variable range hopping process of charge carriers, between the localized states with an average hopping length of RH of 3 nm and hopping energy of WH of 0.67 eV. Moreover, the effect of grains and grain boundaries on the correlation between ac-conductivity and angular-frequency ω has been discussed in detail.

Properties of amorphous carbon thin films grown by ion beam sputtering

The electrical performance of amorphous carbon thin films obtained by the ion beam sputtering of a carbon target in argon has been investigated. It has been shown by the Raman spectroscopy method that these films have a graphite-like structure. It has also been found by conductivity and thermopower studies that the hopping mechanism of conductivity with a variable length of hops over localized states near the Fermi level changes to the mechanism of hopping over the nearest neighbors as the temperature rises from 77 to 190 K. Near room temperature, electrotransport is provided by variable-length hops over localized states at the tail of the valence band.