Localization lengths of ultrathin disordered gold and silver nanowires

The localization lengths of ultrathin disordered Au and Ag nanowires are estimated by calculating the wire conductances as functions of wire lengths. We study Ag and Au monoatomic linear chains, and thicker Ag wires with very small cross sections. For the monoatomic chains we consider two types of disorder: bounded random fluctuations of the interatomic distances, and the presence of random substitutional impurities. The effect of impurity atoms on the nanowire conductance is much stronger. Our results show that electrical transport in ultrathin disordered wires may occur in the strong localization regime, and with relatively small amounts of disorder the localization lengths may be rather small. The localization length dependence on wire thickness is investigated for Ag nanowires with different impurity concentrations.Comment: 6 pages, postscript figures included, submitted to PR

Free-electron Model for Mesoscopic Force Fluctuations in Nanowires

When two metal electrodes are separated, a nanometer sized wire (nanowire) is formed just before the contact breaks. The electrical conduction measured during this retraction process shows signs of quantized conductance in units of G_0=2e^2/h. Recent experiments show that the force acting on the wire during separation fluctuates, which has been interpreted as being due to atomic rearrangements. In this report we use a simple free electron model, for two simple geometries, and show that the electronic contribution to the force fluctuations is comparable to the experimentally found values, about 2 nN.Comment: 4 pages, 3 figures, reference correcte

Low-field magnetic resonance imaging and multislice computed tomography for the detection of cervical syringomyelia in dogs

Background: Syringomyelia (SM) is defined as the presence of fluid- containing cavities within the parenchyma of the spinal cord. Sagittal magnetic resonance (MR) images have been described as the preferred technique for visualizing SMin dogs and humans. Objective: To investigate whether computed tomography (CT) can be used to diagnose SM. Animals: Thirty- two client- owned dogs referred for investigation of the cervical spine on magnetic resonance imaging (MRI) and CT. Methods: Two reviewers retrospectively analyzed sagittal and transverse T1- weighted spin echo (T1WSE) MR images and CT images from each dog for the presence of SM and, if SM was present, the width (mm, syrinx width [ SW]) was measured. The results were analyzed statistically. Results: For the presence of SM there was a moderate interobserver agreement for MR (81%, j = 0.54) and almost perfect agreement for CT (94%, j = 0.87). There was a moderate intramodality agreement for both observers (observer 1 81%, j = 0.59; observer 2 81%, j = 0.57). For measurement of SW the repeatability was the best on the midsagittal T1WSE images (95% repeatability coefficient < 0.52 mm) and the reproducibility was the best on midsagittal images in both modalities (95% limits of agreement 0.55- 0.45; P = 0.002). Conclusion and Clinical Importance: Both techniques can be used to detect SM. Midsagittal MR and CT images are best used for measuring SW. Computed tomography can be used as a diagnostic tool for SM when MRI is not available, but CT cannot replace MRI as the standard screening technique for the detection of SM in Cavalier King Charles Spaniel for breeding purposes

From favorable atomic configurations to supershell structures: a new interpretation of conductance histograms

Title: From favorable atomic configurations to supershell structures: a new interpretation of conductance histograms Authors: A. Hasmy (IVIC), E. Medina (IVIC), P.A. Serena (CSIC,IVIC) Comments: 7 pages, 3 figures, cond-mat.anwar.10825 Subj-class: Soft Condensed MatterComment: 7 pages, 3 figuresSubject: fput HMS.tex HMS-FIG1.ps HMS-FIG2.ps HMS-FIG3.p

Structure and stability of finite gold nanowires

Finite gold nanowires containing less than 1000 atoms are studied using the molecular dynamics simulation method and embedded atom potential. Nanowires with the face-centered cubic structure and the (111) oriented cross-section are prepared at T=0 K. After annealing and quenching the structure and vibrational properties of nanowires are studied at room temperature. Several of these nanowires form multi-walled structures of lasting stability. They consist of concentrical cylindrical sheets and resemble multi-walled carbon nanotubes. Vibrations are investigated by diagonalization of the dynamical matrix. It was found that several percents of vibrational modes are unstable because of uncompleted restructuring of initial fcc nanowires.Comment: 4 figures in gif forma

Quantum Conductance in Semimetallic Bismuth Nanocontacts

Electronic transport properties of bismuth nanocontacts are analyzed by means of a low temperature scanning tunneling microscope. The subquantum steps observed in the conductance versus elongation curves give evidence of atomic rearrangements in the contact. The underlying quantum nature of the conductance reveals itself through peaks in the conductance histograms. The shape of the conductance curves at 77 K is well described by a simple gliding mechanism for the contact evolution during elongation. The strikingly different behaviour at 4 K suggests a charge carrier transition from light to heavy ones as the contact cross section becomes sufficiently small.Comment: 5 pages including 4 figures. Accepted for publication in Phys. Rev. Let

Connective neck evolution and conductance steps in hot point contacts

Dynamic evolution of the connective neck in Al and Pb mechanically controllable break junctions was studied during continuous approach of electrodes at bias voltages V_b up to a few hundred mV. A high level of power dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in the constriction lead to overheating of the contact area, electromigration and current-enhanced diffusion of atoms out of the "hot spot". At a low electrode approach rate (10 - 50 pm/s) the transverse dimension of the neck and the conductance of the junction depend on V_b and remain nearly constant over the approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists of a few atoms only and the quantum nature of conductance manifests itself in abrupt steps and reversible jumps between two or more levels. These features are related to an ever changing number of individual conductance channels due to the continuous rearrangement in atomic configuration of the neck, the recurring motion of atoms between metastable states, the formation and breaking of isolated one-atom contacts and the switching between energetically preferable neck geometries.Comment: 21 pages 10 figure

Stiff monatomic gold wires with a spinning zigzag geometry

Using first principles density functional calculations, gold monatomic wires are found to exhibit a zigzag shape which remains under tension, becoming linear just before breaking. At room temperature they are found to spin, what explains the extremely long apparent interatomic distances shown by electron microscopy.The zigzag structure is stable if the tension is relieved, the wire holding its chainlike shape even as a free-standing cluster. This unexpected metallic-wire stiffness stems from the transverse quantization in the wire, as shown in a simple free electron model.Comment: 4 pages, latex, 5 figures, submitted to PR

Energetics, forces, and quantized conductance in jellium modeled metallic nanowires

Energetics and quantized conductance in jellium modeled nanowires are investigated using the local density functional based shell correction method, extending our previous study of uniform in shape wires [C. Yannouleas and U. Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable shaped constricted region. The energetics of the wire (sodium) as a function of the length of the volume conserving, adiabatically shaped constriction leads to formation of self selecting magic wire configurations. The variations in the energy result in oscillations in the force required to elongate the wire and are directly correlated with the stepwise variations of the conductance of the nanowire in units of 2e^2/h. The oscillatory patterns in the energetics and forces, and the correlated stepwise variation in the conductance are shown, numerically and through a semiclassical analysis, to be dominated by the quantized spectrum of the transverse states at the narrowmost part of the constriction in the wire.Comment: Latex/Revtex, 11 pages with 5 Postscript figure

Soil biochemistry and microbial activity in vineyards under conventional and organic management at Northeast Brazil.

The São Francisco Submedium Valley is located at the Brazilian semiarid region and is an important center for irrigated fruit growing. This region is responsible for 97% of the national exportation of table grapes, including seedless grapes. Based on the fact that orgThe São Francisco Submedium Valley is located at the Brazilian semiarid region and is an important center for irrigated fruit growing. This region is responsible for 97% of the national exportation of table grapes, including seedless grapes. Based on the fact that organic fertilization can improve soil quality, we compared the effects of conventional and organic soil management on microbial activity and mycorrhization of seedless grape crops. We measured glomerospores number, most probable number (MPN) of propagules, richness of arbuscular mycorrhizal fungi (AMF) species, AMF root colonization, EE-BRSP production, carbon microbial biomass (C-MB), microbial respiration, fluorescein diacetate hydrolytic activity (FDA) and metabolic coefficient (qCO2). The organic management led to an increase in all variables with the exception of EE-BRSP and qCO2. Mycorrhizal colonization increased from 4.7% in conventional crops to 15.9% in organic crops. Spore number ranged from 4.1 to 12.4 per 50 g-1 soil in both management systems. The most probable number of AMF propagules increased from 79 cm-3 soil in the conventional system to 110 cm-3 soil in the organic system. Microbial carbon, CO2 emission, and FDA activity were increased by 100 to 200% in the organic crop. Thirteen species of AMF were identified, the majority in the organic cultivation system. Acaulospora excavata, Entrophospora infrequens, Glomus sp.3 and Scutellospora sp. were found only in the organically managed crop. S. gregaria was found only in the conventional crop. Organically managed vineyards increased mycorrhization and general soil microbial activity