Number Of Conducting Channels Research Articles

Signature of weak-antilocalization in sputtered topological insulator Bi2Se3 thin films with varying thickness

We report a low-temperature magneto transport study of Bi2Se3 thin films of different thicknesses (40, 80 and 160 nm), deposited on sapphire (0001) substrates, using radio frequency magnetron sputtering technique. The high-resolution x-ray diffraction measurements revealed the growth of rhombohedral c-axis {0003n} oriented Bi2Se3 films on sapphire (0001). Vibrational modes of Bi2Se3 thin films were obtained in the low wavenumber region using Raman spectroscopy. The surface roughness of sputtered Bi2Se3 thin films on sapphire (0001) substrates were obtained to be ~ 2.26–6.45 nm. The chemical and electronic state of the deposited Bi2Se3 was confirmed by X-ray photoelectron spectroscopy and it showed the formation of Bi2Se3 compound. Resistivity versus temperature measurements show the metallic nature of Bi2Se3 films and a slight up-turn transition in resistivity at lower temperatures < 25 K. The positive magneto-resistance value of Bi2Se3 films measured at low temperatures (2–100 K) confirmed the gapless topological surface states in Bi2Se3 thin films. The quantum correction to the magnetoconductivity of thin films in low magnetic field is done by employing Hikami–Larkin–Nagaoka theory and the calculated value of coefficient ‘α’ (defining number of conduction channels) was found to be 0.65, 0.83 and 1.56 for film thickness of 40, 80 and 160 nm, respectively. These observations indicate that the top and bottom surface states are coupled with the bulk states and the conduction mechanism in Bi2Se3 thin films varied with the film thicknesses.

Quantum conductance in edge functionalized stanene nanoribbons: A first-principle study

The electronic and transport properties in an antiferromagnetic junction of edge functionalized stanene nanoribbons (SnNRs) were studied using first-principle density functional theory. The behaviour of quantum conductance at zero bias voltage with varying ribbon widths in bare and edge hydrogenated SnNRs is demonstrated. The conductance gap near the Fermi level decreases with increasing the width of edge hydrogenated and pristine zigzag and armchair SnNRs. Moreover, the number of conductance channels also increases with increasing the ribbon width. The observed density of states (DOS) are completely corroborating the conductance profile in all the SnNRs. The conductance profile in all zigzag SnNRs exhibits a semiconducting behaviour in antiferromagnetic configuration. In contrast to other armchair SnNRs, 3n+2 armchair SnNRs exhibit a conducting behaviour, which shows the metallic property of corresponding SnNRs. These predictions indicating that stanene nanoribbons of appropriate width could be useful for spintronic applications.

Spin—Orbit Coupling in Single-Walled Gold Nanotubes

The effect of spin—orbit coupling on the electron levels of (5, 3), (8, 7), (11, 3), (18, 11), (10, 5), (8, 8), and (13, 0) gold nanotubes of various diameters and chirality was studied in the framework of the relativistic version of the linear augmented cylindrical wave method. Spin-dependent band structures and electron state densities were calculated. Spin—orbit coupling is manifested as splitting of nonrelativistic dispersion curves. For the dispersion curve that crosses the Fermi level in the (5, 3) tube of the minimum radius, this splitting reaches 0.5 eV and decreases on going to low-lying valence band states. An increase in the radius and a decrease in the cylindrical surface curvature of these nanotubes suppress the spin—orbit splitting. All tubes possess a metal type band structure in which the number of conduction channels increases with increasing radius of the nanotube.

Electro-thermal RF modeling and performance analysis of graphene nanoribbon interconnects

This paper presents an electro-thermal radio frequency (RF) model and performance analysis for multilayer graphene nanoribbon (MLGNR) interconnects. The number of conduction channels is calculated as a function of temperature and Fermi energy. A comprehensive model is developed to calculate the temperature dependent effective mean free path (MFP) considering different scattering mechanisms. The RF model of doped and undoped metallic top-contact (TC), as well as side-contact (SC) MLGNR interconnects is demonstrated using ABCD parameter based multi-conductor transmission line formalism. The RF performance of arsenic pentafluoride ( $$\text {AsF}_5$$ ), lithium (Li) and ferric chloride ( $$\text {FeCl}_3$$ ) intercalation doped TC-MLGNR interconnects is investigated and compared with pristine (undoped) TC and SC-MLGNR interconnects for different temperatures. For the first time, our investigation shows that the electro-thermal RF performance of TC-MLGNR can be improved by intercalation doping. It is found that $$\text {AsF}_5$$ , Li and $$\text {FeCl}_3$$ intercalated top-contact MLGNR can operate up to a few GHz for semi-global interconnects ( $$100\,\upmu $$ m) and several MHz for global interconnects ( $$500\,\upmu $$ m). Our analysis also proves that the Li intercalated TC-MLGNR shows the best RF performance as compared to conventional copper, pristine, and other type of intercalation doped TC-MLGNR interconnects over the chip operating temperature range from 233 to 378 K. The performance of Li intercalated TC-MLGNR has been found to be improved further by increasing the specularity during fabrication.

Magnetotransport in antidot arrays of three-dimensional topological insulators

Three-dimensional topological insulators are a new kind of quantum matter featured with gapless Dirac-like energy-dispersive surface states in the insulating bulk band gaps. However, in experiment, it is difficult to study quantum interference effect of surface states due to considerable contribution from bulk carriers in thick bulk material. To suppress such a bulk state contribution, nanostructures, such as ultra-thin films, nanowires and nanoribbons, have been employed in the study of quantum interference effects of the surface states. Here, we report on a magnetotransport measurement study of nanoscaled antidot array devices made from three-dimensional topological insulator Bi2Se3 thin films. The antidot arrays with hundreds of nanometers in diameter and edge-to-edge distance are fabricated in the thin films by utilizing the focused-ion beam technique, and the magnetotransport properties of the fabricated devices are measured at low temperatures. The results of the magnetotransport measurements for three representative devices, denoted as Dev-1 (with no antidot array fabricated), Dev-2 (with an antidot array of a relatively large period), and Dev-3 (with an antidot array of a relatively small period), are reported in this work. Weak anti-localization indicated by a sharp peak of conductivity at zero magnetic field is observed in all the three devices. Through theoretical fitting to the measurement data, the transport parameters in the three devices, such as spin-orbit coupling length Lso, phase coherence length L, and the number of conduction channels , are extracted. The extracted Lso value is tens of nanometers, which is consistent with the presence of the strong spin-orbit interaction in the Bi2Se3 thin film. The extracted L value is hundreds of nanometers and increases exponentially with temperature decreasing. It is found that the magnetotransports in Dev-1 and Dev-2 are well characterized by the coherent transport through a single conduction channel. For Dev-3, the magnetotransport at low temperatures is described by the coherent transport through two independent conduction channels, while at elevated temperatures the magnetotransport is dominantly described by the transport through one single conduction channel. Unlike the case where the transport occurs dominantly through a single conduction channel, the transport through two independent conduction channels in Dev-3 implies that at least one surface channel is present in the device.

Weak antilocalization effect due to topological surface states in Bi2Se2.1Te0.9

We have investigated the weak antilocalization (WAL) effect in the p-type Bi2Se2.1Te0.9 topological system. The magnetoconductance shows a cusp-like feature at low magnetic fields, indicating the presence of the WAL effect. The WAL curves measured at different tilt angles merge together when they are plotted as a function of the normal field components, showing that surface states dominate the magnetoconductance in the Bi2Se2.1Te0.9 crystal. We have calculated magnetoconductance per conduction channel and applied the Hikami-Larkin-Nagaoka formula to determine the physical parameters that characterize the WAL effect. The number of conduction channels and the phase coherence length do not change with temperature up to T = 5 K. In addition, the sample shows a large positive magnetoresistance that reaches 1900% under a magnetic field of 35 T at T = 0.33 K with no sign of saturation. The magnetoresistance value decreases with both increasing temperature and tilt angle of the sample surface with respect to the magnetic field. The large magnetoresistance of topological insulators can be utilized in future technology such as sensors and memory devices.

Modeling and Performance Analysis of Graphene Nanoribbon Interconnects

Attempts have been made to propose a compact resistive model for graphene nanoribbon interconnects. Top-contact (TC-GNR) and side-contact (SC-GNR) GNR interconnects have been modeled. The number of conduction channels, mean free path, and resistance are calculated for different Fermi energies and interconnect widths. Present study showed that the resistance of SC-GNR is ~10× less and delay is ~2–16× less than that of TC-GNR.

Structural and electronic properties of chiral single-wall copper nanotubes

The structural, energetic and electronic properties of chiral (n, m) (3⩽n⩽6, n/2⩽m⩽n) single-wall copper nanotubes (CuNTs) have been investigated by using projector-augmented wave method based on density-functional theory. The (4, 3) CuNT is energetically stable and should be observed experimentally in both free-standing and tip-suspended conditions, whereas the (5, 5) and (6, 4) CuNTs should be observed in free-standing and tip-suspended conditions, respectively. The number of conductance channels in the CuNTs does not always correspond to the number of atomic strands comprising the nanotube. Charge density contours show that there is an enhanced interatomic interaction in CuNTs compared with Cu bulk. Current transporting states display different periods and chirality, the combined effects of which lead to weaker chiral currents on CuNTs.

The electrical resistance of vacuum

This paper deals with the physics of electrical conduction in vacuum between two parallel conducting planes (planar vacuum diode). After reviewing known features of conduction in the high-voltage range, we turn to the low-voltage range. An ohmic current–voltage characteristic is calculated in the case of identical cathodic and anodic electrodes, whence an electrical resistance of the vacuum gap can be defined. The inverse resistance involves the elemental conductance 2e2/h and the number of conductance channels between the two electrodes. The channels are thermally populated from the electrodes and the population is analytically calculable from the Poisson equation of electrostatics and the Boltzmann law of thermal equilibrium. The observed resistance of a real vacuum diode (Mullard's EB 91) is accounted for without adjusting parameters. The paper also examines the link-up between Joule's law, involving dissipation, and Ohm's law, with vacuum being contrasted with a material conducting medium; the origin of dissipation in vacuum is understood. Quantum and statistical physics are kept at the undergraduate level. Finally, the results obtained for the vacuum diode shed light upon the quantized conductance of nanoscale semiconductor wires, a topic usually handled only in graduate courses.

High sensitivity temperature sensor based on a long, suspended single-walled carbon nanotube array

Considering the electron–phonon scattering mechanisms as well as the number of conduction channels, the long single-walled carbon nanotube (SWCNT) has larger temperature coefficient of resistivity than that of the short SWCNT or that of the multi-walled carbon nanotube (CNT). A tens of micrometres length, suspended SWCNT array-based temperature sensor was proposed, and its fabrication technique was presented. The theoretical analysis and the experimental results showed that the long SWCNT array device had high sensitivity and low power consumption than ordinary platinum thermal resistor. The long SWCNT array sensor also featured a simple and application-oriented process and was easier to use than a single CNT-based device. Thus, it has the potential to be applied in high sensitivity temperatures or flow measurements in an ultra small scale.

Electron Transport Modeling for Junctions of Zigzag and Armchair Graphene Nanoribbons (GNRs)

Conductance of a seamless string of zigzag/armchair/zigzag graphene nanoribbons (GNRs) is modeled using nonequilibrium Green's function methodology as a function of width, Fermi level and the metallic or semiconducting nature of the armchair GNR. Such a structure with a semiconductor armchair GNR combined with a top gate can potentially form a transistor whereas a structure with a metallic armchair GNR can be used for interconnecting nearby transistors. It is shown that the bends in such structures can be quite transparent for electrons (80% to 90% of ideal conductance) so long as there is the same number of conduction channels in the zigzag and armchair GNRs.

Comparison of electronic and geometric structures of nanotubes with subnanometer diameters: A density functional theory study

All-electron calculations based on density functional theory have been carried out to study the electronic structures of single-walled nanotubes with subnanometer diameters. Present studies suggest the need for performing all-electron calculations, specifically for the nanotubes with very small diameters. Complete geometry optimization is found to be very crucial for predicting the electronic properties. We report here the electronic properties of two of the smallest single-walled carbon nanotubes (SWCNTs)---an armchair (2,2) and a zigzag (4,0) SWCNT---with comparable diameters of about $3\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. It is observed that, both geometrically and electronically, they are quite different. The discussion on the properties of zigzag $(n,0)$ SWCNTs, with diameters in the subnanometer regime, as a function of $n$, involves the elucidation of their geometric and band structures. In the band structures, systematic variations of a nearly-free-electron-like state and a nearly-dispersion-free state are shown as a function of $n$ and their implications have been discussed. The Fermi surfaces calculated for the metallic SWCNTs show signature of a collection of quasi-Fermi-points. This shows the quasi-one-dimensional nature of these nanotubes (NTs). From the present calculations, it is expected that the number of conduction channels is restricted between 2 and 3 for the NTs studied here.

Physical Modeling of Temperature Coefficient of Resistance for Single- and Multi-Wall Carbon Nanotube Interconnects

Equivalent circuit models are presented for the resistance of single- and multi-wall carbon nanotubes (MWCNs) that capture various electron-phonon scattering mechanisms as well as changes in the number of conduction channels as a function of temperature. For single- and few-wall nanotubes, the temperature coefficient of resistance (TCR) is always positive and increases with length. It reaches 1/(T-200 K) for lengths much larger than the electron mean free path, where T is the temperature in kelvin. For MWCNs with large diameters (>20 nm), TCR varies from -1/T to +0.66/(T-200 K) as the length varies from zero to very large values

Molecular Origins of Conduction Channels Observed in Shot-Noise Measurements

Measurements of shot noise from single molecules have indicated the presence of various conduction channels. We present three descriptions of these channels in molecular terms showing that the number of conduction channels is limited by bottlenecks in the molecule and that the channels can be linked to transmission through different junction states. We introduce molecular-conductance orbitals, which allow the transmission to be separated into contributions from individual orbitals and contributions from interference between pairs of orbitals.

Ab initiostudy of helical silver single-wall nanotubes and nanowires

We report results for the electronic structures of extended silver single-wall nanotubes (AgSWNTs) within a first-principles, all-electron self-consistent local density functional approach adapted for helical symmetry. We carried out calculations on twenty-one different AgSWNTs ranging in radii from approximately $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ to $3.6\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. AgSWNTs with radii greater than $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ were also calculated with a silver atomic chain inserted along the nanotube axis; we refer to these composite structures as silver nanowires (AgNWs). Energetic trends for the AgSWNTs are not as predictable as expected. For example, the total energy does not necessarily decrease monotonically as nanotube radius increases, as is the case for single-wall carbon nanotubes. The conductivity of these AgSWNTs and AgNWs is also addressed. Similar to the case for helical gold nanowires, the number of conduction channels in the AgSWNTs does not always correspond to the number of atom rows comprising the nanotube. However, for all AgNWs considered, the additional silver atomic chain placed along the tube's axis results in one additional conduction channel.

First-Principles Study of Helical Silver Single-Wall Nanotubes and Nanowires

ABSTRACTWe investigate the electronic structures of extended helical silver single-wall nanotubes (AgSWNTs). Because these helical nanotubes are essentially comprised of n-atom strands winding about the nanotube's axis, we systematically examine, strand by strand, the electronic properties and the number of conduction channels associated with these structures. Herein, we study a special case of high-symmetry nanotubes. Nanotubes with sufficiently large radii were also calculated with a silver atomic chain inserted along the nanotube's axis. The analysis is carried out within a first-principles, all-electron self-consistent local density functional approach (LDF) adapted for helical symmetry. Modeling helical silver (or gold) single-wall nanotubes entails rolling up a sheet of atoms and mapping the atoms onto the surface of a cylinder, comparable to rolling up a graphite sheet for a carbon nanotube. It is well known that controlling the size and shape of silver and gold nanostructures results in the ability to tailor the optical and catalytic properties of these materials. In this preliminary study, we consider changes in the electronic structures of these materials as each nanotube is built strand by strand.

Enhancement of persistent currents due to confinement in metallic samples.

Confinement and surface roughness (SR) effects on the magnitude of the persistent current are analyzed for ballistic bidimensional metallic samples. Depending on the particular geometry, localized border states can show up at half-filling. These border states contribute coherently to the persistent current and its magnitude is enhanced with respect to their value in the absence of confinement. A linear scaling of the typical current I(typ) with the number of conduction channels M is obtained. This result is robust with respect to changes in the relevant lengths of the samples and to the SR. Possible links of our results to experiments are also discussed.

The signature of chemical valence in the electrical conduction through a single-atom contact

Fabrication of structures at the atomic scale is now possible using state-of-the-art techniques for manipulating individual atoms1, and it may become possible to design electrical circuits atom by atom. A prerequisite for successful design is a knowledge of the relationship between the macroscopic electrical characteristics of such circuits and the quantum properties of the individual atoms used as building blocks. As a first step, we show here that the chemical valence determines the conduction properties of the simplest imaginable circuit—a one-atom contact between two metallic banks. The extended quantum states that carry the current from one bank to the other necessarily proceed through the valence orbitals of the constriction atom. It thus seems reasonable to conjecture that the number of current-carrying modes (or ‘channels’) of a one-atom contact is determined by the number of available valence orbitals, and so should strongly differ for metallic elements in different series of the periodic table. We have tested this conjecture using scanning tunnelling microscopy and mechanically controllable break-junction techniques2,3 to obtain atomic-size constrictions for four different metallic elements (Pb, Al, Nb and Au), covering a broad range of valences and orbital structures. Our results demonstrate unambiguously a direct link between valence orbitals and the number of conduction channels in one-atom contacts.