Neighbor Hopping Research Articles

Phase diagram for the Harper model of the honeycomb lattice

The Harper equation arising out of a tight-binding model of electrons on a honeycomb lattice subject to a uniform magnetic field perpendicular to the plane is studied. Contrasting and complementary approaches involving von Neumann entropy, fidelity, fidelity susceptibility, and multifractal analysis are employed to characterize the phase diagram. Remarkably even in the absence of the quasi-periodic on-site potential term, the Hamiltonian allows for a metal-insulator transition. The phase diagram consists of three phases: two metallic phases and an insulating phase. A variant model where next nearest neighbor hopping is included, exhibits a mobility edge and does not allow for a simple single phase diagram characterizing all the eigenstates.

A Study on Optical and Dielectric Properties of Ni-ZnO nanocomposite

Nickel-zinc oxide (Ni-ZnO) nanoparticles were synthesized via low-cost sol-gel auto combustion route using ethylene glycol as a reducing agent. The structural and surface morphological analysis were confirmed by XRD, FESEM and TEM micrographs which exhibits pure phase formation, unit cell crystal structure and inhomogeneous grain growth in nm range (43.20 nm). The EDXS elemental mappings and SAED patterns exposed the presence of the fundamental element with stoichiometric ratios and double phase formation of Ni-ZnO nanocomposite. The dielectric and conduction mechanism were characterized by impedance, modulus, dielectric and conductance spectra with frequency and temperature evolution. The suppressed Nyquist plot of impedance and modulus with temperature demonstrates the curtailment of grain boundary resistance and dominance of grain contribution. The dielectric constant and loss both lessened with high-frequency evolution and utmost value of dielectric constant 300 was observed at 95 °C. The ion hopping conduction mechanism (Nearest neighbor hopping and Variable range hopping) was proposed to explain its conduction processes and activation energy (1.02 eV) of dc conduction was calculated from Arrhenius plot. Finally, the direct and indirect optical band gap of Ni-ZnO was estimated from UV-DRS absorption spectra using Kubelka-Munk equation.

Growth of [formula omitted] thin films by selenization of RF sputtered binary precursors

In this work we present a method to grow Sb2Se3 thin films with a potential use as absorber layers in solar cell structures. The films were grown on several substrates: soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursor's films were deposited by RF magnetron sputtering and then selenized under a H2Se gas flow. Different selenization temperatures were tested and analysed. Compositional and morphological analyses were performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Phase identification and structural characterization were done by X-ray Diffraction and Raman scattering spectroscopy showing that Sb2Se3 is the dominant phase with an orthorhombic crystalline structure. Traces of rhombohedral and amorphous Se secondary phases were also observed supported by their Se-rich compositions. Visible-NIR reflectance measurements allowed to extract a direct bandgap with a value close to 1.06 eV. Photoluminescence spectroscopy shows an emission with a broad band at 0.85 eV for samples selenized at lower temperatures and an intense peak at 0.75 eV for the sample selenized at higher temperatures. Electrical characterization shows low free hole concentrations and mobilities. At low temperatures, the nearest neighbour hopping is the dominant mechanism for the electronic transport for the analysed samples. Both electrical and optical properties are influenced by the type of defects present on samples. A discussion is made on the properties that need to be improved in order that these films can be integrated into thin film solar cells.

A multi-hop pointer forwarding scheme for efficient location update in low-rate wireless mesh networks

Recently, a pointer forwarding scheme (PFS) was proposed to reduce location update overhead in wireless mesh networks. Using PFS, a location update is replaced with a simple forwarding pointer setup between two neighboring mesh routers (MRs). However, in PFS, if the two MRs are not one hop neighbors, PFS fails to set up a forwarding pointer, thus increasing location update overhead. To improve PFS, we present a multi-hop pointer forwarding scheme (MPFS). MPFS allows forwarding pointers to be constructed over multi-hop at once even if MRs are not one hop neighbor by using logical tree distance constructed during network formation. The tree distance is used to relay forwarding pointer packets over multi-hop links without additional control overhead during forwarding pointer setup and to estimate hop distance between two MRs. By doing so, MPFS improves the probability of success in forwarding pointer setup while ensuring k≤km, resulting in lowering the location update overhead. Also, we analyze pointer forwarding success probability and average chain length and discuss why MPFS is suitable for resource-constrained LRWMNs. Using ns-2, we show that MPFS significantly reduces the number of location update events, location update delay and signaling overhead, and packet losses during location updates. With real-world implementation, we also confirm feasibility of MPFS.

Intersection and Complement Set (IACS) Method to Reduce Redundant Node in Mobile WSN Localization.

The majority of the Wireless Sensor Network (WSN) localization methods utilize a large number of nodes to achieve high localization accuracy. However, there are many unnecessary data redundancies that contributes to high computation, communication, and energy cost between these nodes. Therefore, we propose the Intersection and Complement Set (IACS) method to reduce these redundant data by selecting the most significant neighbor nodes for the localization process. Through duplication cleaning and average filtering steps, the proposed IACS selects the normal nodes with unique intersection and complement sets in the first and second hop neighbors to localize the unknown node. If the intersection or complement sets of the normal nodes are duplicated, IACS only selects the node with the shortest distance to the blind node and nodes that have total elements larger than the average of the intersection or complement sets. The proposed IACS is tested in various simulation settings and compared with MSL* and LCC. The performance of all methods is investigated using the default settings and a different number of degree of irregularity, normal node density, maximum velocity of sensor node and number of samples. From the simulation, IACS successfully reduced 25% of computation cost, 25% of communication cost and 6% of energy consumption compared to MSL*, while 15% of computation cost, 13% of communication cost and 3% of energy consumption compared to LCC.

A surface potential based compact model for two-dimensional field effect transistors with disorders induced transition behaviors

A surface potential based compact model for two-dimensional field effect transistors (2D-FETs) is proposed to incorporate the structural disorders induced transition behaviors among variable range hopping (VRH), nearest neighbor hopping (NNH), and band-like transport in most 2D materials. These functions coupled with effective transport energy and multiple trapping and releasing theory enable our developed model to predict the temperature and carrier density dependent current characteristics. Its validity is confirmed by the experimental results such as the metal insulator transition (MIT) in transition metal dichalcogenides and VRH-NNH transition in black phosphorus nanoribbon. Based on this model, the band-tail effects on the crossover gate voltage of MIT behavior are quantitatively investigated. It is found that the transition behavior is closely related to the distribution of the band-tail states. Furthermore, this model is implemented in Verilog-A for circuit-level prediction and evaluation of 2D-FETs to provide deeper insight into the relationship between material properties, device physics, and circuit performances.

Dirac states from px,y orbitals in the buckled honeycomb structures: A tight-binding model and first-principles combined study**Project supported by the National Key Research and Development Projects of China (Grant No. 2016YFA0202300), the National Natural Science Foundation of China (Grant No. 61390501), the Science Fund from the Chinese Academy of Sciences (Grant No. XDPB0601), and the US Army Research Office.

Dirac states composed of px,y orbitals have been reported in many two-dimensional (2D) systems with honeycomb lattices recently. Their potential importance has aroused strong interest in a comprehensive understanding of such states. Here, we construct a four-band tight-binding model for the px,y-orbital Dirac states considering both the nearest neighbor hopping interactions and the lattice-buckling effect. We find that px,y-orbital Dirac states are accompanied with two additional narrow bands that are flat in the limit of vanishing π bonding, which is in agreement with previous studies. Most importantly, we analytically obtain the linear dispersion relationship between energy and momentum vector near the Dirac cone. We find that the Fermi velocity is determined not only by the hopping through π bonding but also by the hopping through σ bonding of px,y orbitals, which is in contrast to the case of pz-orbital Dirac states. Consequently, px,y-orbital Dirac states offer more flexible engineering, with the Fermi velocity being more sensitive to the changes of lattice constants and buckling angles, if strain is exerted. We further validate our tight-binding scheme by direct first-principles calculations of model-materials including hydrogenated monolayer Bi and Sb honeycomb lattices. Our work provides a more in-depth understanding of px,y-orbital Dirac states in honeycomb lattices, which is useful for the applications of this family of materials in nanoelectronics.

An Energy Efficient Receiver-based Flooding Scheme Using 1-Hop Neighbours Geographical Information for MANETs

Flooding is a fundamental, critical, and indispensable operation to support various applications and protocols in wireless ad hoc networks. The traditional flooding scheme generates excessive redundant packet retransmissions, causing contention and packet collisions, and ultimately wasting limited bandwidth and energy. Some recent flooding schemes that avoid those problems have been studied. They can achieve local optimality and have lower computational complexity. However, drawbacks limit the efficiency of these schemes. In this paper, we propose an efficient flooding protocol that minimizes flooding traffic, leveraging location information of 1-hop neighbor nodes. Our scheme is receiver-based; it does not piggyback any neighbor information. We prove theoretically that the proposed scheme achieves 100 percent deliverability. Simulation shows our scheme to be highly efficient. It consumes less energy, reduces the number of forwarding nodes almost to that of the benchmark, but maintains a high delivery ratio.

An Energy Efficient Receiver-based Flooding Scheme Using 1-Hop Neighbours Geographical Information for MANETs

Flooding is a fundamental, critical, and indispensable operation to support various applications and protocols in wireless ad hoc networks. The traditional flooding scheme generates excessive redundant packet retransmissions, causing contention and packet collisions, and ultimately wasting limited bandwidth and energy. Some recent flooding schemes that avoid those problems have been studied. They can achieve local optimality and have lower computational complexity. However, drawbacks limit the efficiency of these schemes. In this paper, we propose an efficient flooding protocol that minimizes flooding traffic, leveraging location information of 1-hop neighbor nodes. Our scheme is receiver-based; it does not piggyback any neighbor information. We prove theoretically that the proposed scheme achieves 100 percent deliverability. Simulation shows our scheme to be highly efficient. It consumes less energy, reduces the number of forwarding nodes almost to that of the benchmark, but maintains a high delivery ratio.

FJADA: Friendship Based JellyFish Attack Detection Algorithm for Mobile Ad Hoc Networks

TCP and UDP are considered the most popular and well known transport layer protocols to facilitate the end to end communication between two nodes in the network. TCP is used as the transport layer protocol in packet delivery and error sensitive applications, where packet loss cannot be compromised. However, low-rate TCP targeted Denial of Service (DoS) attacks exploit the retransmission timeout and congestion control features of TCP protocol. These low-rate TCP targeted Denial of Service (DoS) attacks are also called JellyFish (JF) attacks. These attacks perform the malicious activities either by delaying, or periodically dropping or mis-ordering the data packets on the route from source to destination node in the network, and cause severe degradation in end-to-end throughput in the network. JellyFish attack is further classified as JF-Delay Variance Attack, JF-Periodic Drop Attack and JF-Reorder Attack based on the type of the malicious activities being performed. JellyFish attack conforms to all existing routing and packet forwarding protocol specifications, and therefore it becomes very difficult to detect its presence in the network. In this paper, a Friendship Based JellyFish Attack Detection Algorithm (FJADA) is presented for Mobile Ad Hoc Networks, where the basic concept of friendship mechanism is added to the existing Direct Trust-based Detection (DTD) algorithm to save the valuable resources of a node in monitoring the activities of its one hop neighbours, through promiscuous mode. FJADA also minimizes the possibility of overestimating the malicious behaviour of innocent nodes due to radio transmission errors, network congestion or packet collisions. The results obtained throughout the simulation experiments clearly show the feasibility and effectiveness of the proposed detection algorithm.

Secure Non-Consensus Based Spectrum Sensing in Non-Centralized Cognitive Radio Networks

In a non-centralized cognitive radio network, secondary users (SUs) cooperate in order to determine the presence of the primary user (PU) signal. This can be accomplished by each node sharing its spectrum sensing result with its neighboring SUs. In this paper, a non-consensus based distributed spectrum sensing (NCSS) scheme is presented, in which a node uses the raw energy level of the PU signal from its $h$ -hop neighbors for determining the status of the PU. A message passing scheme is used rather than the conventional consensus scheme. A comparison with an existing consensus algorithm is also shown. Further, two types of spectrum sensing data falsification attacks are considered in which the attackers inject very high or very low energy values. To catch such abnormal behavior of a node, an approach called secure NCSS algorithm is proposed, which isolates a node that generates extreme energy values so that it is removed from future computation. For this, an outlier detection technique is adopted. The approach is based on the idea that a malicious node’s energy level deviates significantly from those of genuine nodes, whereas all genuine nodes share nearby estimated energy levels.

Optimal doping for d-wave superconducting ground states within the generalized Hubbard model

A single-band generalized Hubbard model that describes two-dimensional superconductivity with d-wave symmetry on a square lattice within the BCS formalism is considered. For a set of Hamiltonian parameters and varying the ratio between nearest-neighbor and next-nearest neighbor hoppings (t'/t), an optimal doping (nop) can be found for each t'/t value, where the critical temperature is maximum (Tc-max). After calculating the superconducting gap at T=0K and the corresponding ground state (Eg ) for all the carrier concentrations, a ground state energy minimum (Eg-min) is found close to half filling. Since Tc-max is the highest critical temperature for a given ratio t'/t, the minimum of all the Tc-max values defines a supreme of this set of temperatures, named as Tc-max-sup. The corresponding optimal doping for Tc-max-sup will be called nop-sup, and the the results show that Eg-min is located at nop-sup. The Fermi surface (FS) is analyzed for carrier concentrations close to nop-sup and it is suggested that the location for over (OD) and under (UD) doping regimes (nOD>nop-sup>nUD) could define a pseudogap zone for high critical temperature superconductors.

A localized algorithm for clustering in cognitive radio networks

With the steep increase in wireless devices, the requirement of the spectrum has increased for communication. However, despite huge demand, the available spectrum is underutilized. The cognitive radio network (CRN) is more challenging environment as compared to conventional mobile ad hoc network because of node mobility and dynamic channel availability. Therefore, we propose a localized clustering scheme, which aims to provide better stability, scalability, efficient spectrum management, and reduce communication overhead. Each node computes its weight and shares with its one hop neighbours, and a node with maximal weight becomes the cluster head. Subsequently, the neighbour nodes sharing the channel(s) with a cluster head join it to form a cluster. To provide fault tolerance, vice-cluster head is also selected along with cluster head. We compare the performance of our protocol with competing protocols in the CRN. Finally, we present analytic as well as simulation study of our protocol and provide a method to handle the cluster dynamics.

Fuzzy-controlled energy-efficient single hop clustering scheme with (FESC) in ad hoc networks

A mobile ad hoc network (MANET) is formed when group of mobile wireless nodes collaborate between them to communicate through wireless links in the absence of the fixed infrastructure and any centralized control. These characteristics make it able to adapt and operate in difficult conditions. It is vital to keep the topology stable as long as possible. An effective clustering scheme renders longer cluster lifetime and incurs low maintenance overhead. However, the mobility of nodes adversely affects the hierarchical structure. Therefore, electing nodes with small relative velocity with respect to it’s 1-hop downlink neighbors or cluster members, for the role of cluster head is a preferred option that makes the clusters more stable. In the proposed scheme FESC, we present a relatively stable clustering scheme by considering more powerful however less mobile (w.r.t. the cluster members) nodes as cluster heads. The cluster heads can themselves estimate the competency value of the cluster. If it is small enough, then it advises the nodes with other cluster options to join some better clusters so that they don’t arbitrarily join and leave available clusters. The simulation analysis confirms that our scheme delivers more stable clusters with low maintenance effort.

QL-EEBDG: QLearning based energy balanced routing in underwater sensor networks

In this paper, we propose a Q-Learning based efficient and balanced energy consumption data gathering routing protocol (QLEEBDG) for underwater sensor networks (USNs). We set an optimal next hop forwarder for each node to transmit its the sensed data. This helps to reduce distance between sender and receiver. The energy consumption is minimum. Furthermore, a node is considered an eligible forwarder node only if its next hop neighbour exists. We incorporate this mechanism to avoid void hole problem. Our technique minimizes energy consumption in the network, hence, lifespan increases. The performance of our proposed technique is validated through extensive simulations.

A dynamical mean-field study of orbital-selective Mott phase enhanced by next-nearest neighbor hopping

The dynamical mean-field theory is employed to study the orbital-selective Mott transition (OSMT) of the two-orbital Hubbard model with nearest neighbor hopping and next-nearest neighbor (NNN) hopping. The NNN hopping breaks the particle-hole symmetry at half filling and gives rise to an asymmetric density of states (DOS). Our calculations show that the broken symmetry of DOS benefits the OSMT, where the region of the orbital-selective Mott phase significantly extends with the increasing NNN hopping integral. We also find that Hund's rule coupling promotes OSMT by blocking the orbital fluctuations, but the influence of NNN hopping is more remarkable.

Low intrinsic carrier density LSMO/Alq3/AlOx/Co organic spintronic devices

The understanding of spin injection and transport in organic spintronic devices is still incomplete, with some experiments showing magnetoresistance and others not detecting it. We have investigated the transport properties of a large number of tris-(8-hydroxyquinoline)aluminum-based organic spintronic devices with an electrical resistance greater than 5 MΩ that did not show magnetoresistance. Their transport properties could be described satisfactorily by known models for organic semiconductors. At high voltages (>2 V), the results followed the model of space charge limited current with a Poole-Frenkel mobility. At low voltages (∼0.1 V), that are those at which the spin valve behavior is usually observed, the charge transport was modelled by nearest neighbor hopping in intra-gap impurity levels, with a charge carrier density of n0 = (1.44 ± 0.21) × 1015 cm−3 at room temperature. Such a low carrier density can explain why no magnetoresistance was observed.

Zero modes of the Kitaev chain with phase-gradients and longer range couplings

We present an analytical solution for the full spectrum of Kitaev's one-dimensional p-wave superconductor with arbitrary hopping, pairing amplitude and chemical potential in the case of an open chain. We also discuss the structure of the zero-modes in the presence of both phase gradients and next nearest neighbor hopping and pairing terms. As observed by Sticlet et al, one feature of such models is that in a part of the phase diagram, zero-modes are present at one end of the system, while there are none on the other side. We explain the presence of this feature analytically, and show that it requires some fine-tuning of the parameters in the model. Thus as expected, these ‘one-sided’ zero-modes are neither protected by topology, nor by symmetry.

Low cost localization using Nyström extended locally linear embedding

In this article, we employ Nyström method with locally linear embedding (LLE) to approximate the node location and develop an iterative localization technique for accurate node localization. Initially, a group of three adjacent anchor nodes forms a reference structure and nodes in one hop neighborhood of this reference structure are localized using LLE. After this, remaining sensor nodes are localized using Nyström extended LLE. Experimental results show that the proposed Nyström extended LLE (NLLE) is able to localize the nodes accurately for different value of anchors, connectivity and varying size of the neighborhood. It is also observed that the NLLE is able to localize sensor nodes with an increased accuracy at low cost as compared to existing manifold learning techniques.

Kesterite Cu2ZnSnS4 thin films by drop-on-demand inkjet printing from molecular ink

Inkjet printing of kesterite Cu2ZnSnS4 (CZTS) thin films on glass from molecular ink is described. CZTS ink consists of copper acetate, zinc acetate, tin chloride and thiourea dissolved in a mixture of ethylene glycol and isopropyl alcohol. The printed precursor films are vacuum dried and thermolysed at 200 °C in air to obtain CZTS films. X-ray diffraction and Raman spectroscopy of films confirm the formation of kesterite CZTS without any secondary phases. The band gap of the films is 1.48 eV as deduced from transmission spectrum using Tauc plot. The films are p-type with hole density and mobility of 2.65 × 1019 cm−3 and 0.3 cm2V−1s−1, respectively. Measurement of electrical conductivity of films in the temperature range from 77 to 300 K show that dominant mechanisms of conduction are Mott-Variable Range Hopping, Nearest Neighbour Hopping and Thermally Activated Band Conduction in the temperature ranges of 77–155 K, 180–240 K and 250–300 K, respectively.