The conductance of multiwalled carbon nanotubes (MWNTs) was found to be quantized. The experimental method involved measuring the conductance of nanotubes by replacing the tip of a scanning probe microscope with a nanotube fiber, which could be lowered into a liquid metal to establish a gentle electrical contact with a nanotube at the tip of the fiber. The conductance of arc-produced MWNTs is one unit of the conductance quantum G0 5 2e 2 /h 5 (12.9 kilohms) ‐1 . The nanotubes conduct current ballistically and do not dissipate heat. The nanotubes, which are typically 15 nanometers wide and 4 micrometers long, are several orders of magnitude greater in size and stability than other typical room-temperature quantum conductors. Extremely high stable current densities, J . 10 7 amperes per square centimeter, have been attained.

http://www.nanoscience.gatech.edu/zlwang/paper/1998/98_sci_1.pdf

Carbon nanotube quantum resistors

http://myweb.rz.uni-augsburg.de/~eckern/WEH-287/ABSTRACTS/ruitenbeek.pdf

Quantum properties of atomic-sized conductors

As the scale of microelectronic engineering continues to shrink, interest has focused on the nature of electron transport through essentially one-dimensional nanometre-scale channels such as quantum wires1 and carbon nanotubes2,3. Quantum point contacts (QPCs) are structures (generally metallic) in which a ‘neck’ of atoms just a few atomic diameters wide (that is, comparable to the conduction electrons' Fermi wavelength) bridges two electrical contacts. They can be prepared by contacting a metal surface witha scanning tunnelling microscope (STM)4,5,6,7 and by other methods8,9,10,11,12, and typically display a conductance quantized in steps of 2e2/h(∼13 kΩ−1)13,14, where e is the electron charge and h is Planck's constant. Here we report conductance measurements on metal QPCs prepared with an STM that we can simultaneously image using an ultrahigh-vacuum electron microscope, which allows direct observation of the relation between electron transport and structure. We observe strands of gold atoms that are about one nanometre long and one single chain of gold atoms suspended between the electrodes. We can thus verify that the conductance of a single strand of atoms is 2e2/h and that the conductance of a double strand is twice as large, showing that equipartition holds for electron transport in these quantum systems.

Quantized conductance through individual rows of suspended gold atoms

A large variety of nanometre-scale devices have been investigated in recent years that could overcome the physical and economic limitations of current semiconductor devices. To be of technological interest, the energy consumption and fabrication cost of these 'nanodevices' need to be low. Here we report a new type of nanodevice, a quantized conductance atomic switch (QCAS), which satisfies these requirements. The QCAS works by controlling the formation and annihilation of an atomic bridge at the crossing point between two electrodes. The wires are spaced approximately 1 nm apart, and one of the two is a solid electrolyte wire from which the atomic bridges are formed. We demonstrate that such a QCAS can switch between 'on' and 'off' states at room temperature and in air at a frequency of 1 MHz and at a small operating voltage (600 mV). Basic logic circuits are also easily fabricated by crossing solid electrolyte wires with metal electrodes.

Quantized conductance atomic switch

A few years after the invention of the scanning tunneling microscope (STM), the atomic force microscope (AFM) was developed Instead of measuring tunneling current, a new physical quantity could be investigated with atomic-scale resolution: the force between a small tip and a chosen sample surface This paper reviews progress and recent results obtained with AFM and other closely related techniques in the field of nanotribology, and attempts to point out many of the unresolved questions that remain The goal of this paper is to demonstrate that AFM is capable of producing atomic-scale knowledge As such, the authors will focus upon some of the contributions of the AFM to nanotribology They will almost exclusively discuss results that shed light on the actual atomic and molecular processes taking place, as opposed to the more applied investigations of microscale properties which are also carried out with AFM They will accompany this discussion by mentioning related theoretical efforts and simulations, although their main emphasis will be upon experimental results and the techniques used to obtain them, as well as suggested future directions In many ways, AFM techniques for quantitative, fundamental nanotribology are only in a nascent stage; certain key issues such as force calibration,more » tip characterization, and the effects of the experimental environment, are not fully resolved or standardized The authors thus begin with a critical discussion of the relevant technical aspects with using AFM for nanotribology 289 refs« less

/pdf/scratching-the-surface-fundamental-investigations-of-4kjq5s243s.pdf

Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy.

We present direct measurements at room temperature of the conductance of a point contact between a scanning tunneling microscope tip and Ni, Cu, and Pt surfaces. As the contact is stretched the conductance jumps in units of 2${\mathit{e}}^{2}$/h. Atomistic simulations of the stretch of the contact combined with calculations of the conductance using the Landauer formula show that the observed behavior is due to the quantization of the transverse electron motion in a contact which contains between one and ten atoms.

/pdf/quantized-conductance-in-an-atom-sized-point-contact-2m35kfa5qj.pdf

Quantized conductance in an atom-sized point contact.

We present experimental and theoretical results for the conductance and mechanical properties of atom-sized wires between two metals. The experimental part is based on measurements with a scannng tunneling microscope (STM) where a point contact is created by indenting the tip into a gold surface. When the tip is retracted, a 10--20 \AA{} long nanowire is formed. Our measurements of the conductance of nanowires show clear signs of a quantization in units of 2${\mathit{e}}^{2}$/h. The scatter around the integer values increases considerably with the number of quanta, and typically it is not possible to observe more than up to four quanta in these experiments. A detailed discussion is given of the statistical methods used in the analysis of the experimental data. The theoretical part of the paper addresses some questions posed by the experiment: Why can conductance quantization be observed, what is the origin of the scatter in the experimental data, and what is the origin of the scaling of the scattering with the number of conductance quanta? The theoretical discussion is based on a free-electron-like model where scattering from the boundary of the nanowire is included. The configurations of the nanowires are deduced from molecular dynamics simulations, which also give information about the mechanical properties of the system. We show that such a model can account semiquantitatively for several of the observed effects. One of the main conclusions of the theoretical analysis is that, due to the plastic deformation of the nanowires formed by the STM, the typical length scale of the variations in the shape of the boundary is not an atomic radius but rather five times that value. This is the reason why scattering is sufficiently small to make conductance quantization observable by STM.

/pdf/quantized-conductance-in-atom-sized-wires-between-two-metals-1lgzcwrhv5.pdf

Quantized conductance in atom-sized wires between two metals.

We present measurements of the electrical conductance through breaking point contacts in commercial and home-built electromechanical relays under ambient conditions. Inspired by an experiment published by Costa-Kr\"amer et al. [Surf. Sci. 342, L1144 (1995)], we constructed an automated setup, which makes it possible to record the conductance in a breaking contact 25 000 times during 24 h. Quantization of the conductance in units of ${G}_{0}{=2e}^{2}/h$ is shown to be intimately correlated with the choice of contact material and experimental conditions. This and previous studies clearly demonstrate that Au is a benchmark system, which exhibits quantized conductance independent of the conditions. Peaks in the Au histogram are shown to be asymmetric, which is interpreted as another indicator of conductance quantization. For the more reactive metals Ag and Cu, quantization is also observed, but not as clearly as in Au. In contrast, we find no evidence of quantization in a number of transition metals. It is tentatively suggested that the presence and/or absence of quantization is correlated with impurities in the nanowires. Other deviations from perfect quantization are discussed. It is revealed how differential nonlinearity of analog-to-digital converters introduces significant noise in the corresponding conductance histograms unless a proper compensation is made.

Quantized conductance in relays

We present measurements of current–voltage (I–V) curves on gold quantum point contacts (QPCs) with a conductance up to 4 G0 (G0=2e2/h is the conductance quantum) and voltages up to 2 V. The QPCs are formed between the gold tip of a scanning tunneling microscope and a Au(110) surface under clean ultra-high-vacuum conditions at room temperature. The I–V curves are found to be almost linear in contrast to previous reports. Tight-binding calculations of I–V curves for one- and two-atom contacts are in excellent agreement with our measurements. On the other hand, clearly nonlinear I–V curves are only observed when the sample has been cleaned in air.

/pdf/current-voltage-curves-of-gold-quantum-point-contacts-heibt12vbq.pdf

I. Stensgaard

Papers

Quantized conductance in an atom-sized point contact.

Quantized conductance in atom-sized wires between two metals.

Quantized conductance in relays

Current-voltage curves of gold quantum point contacts revisited

Metallic Nanowires: Formation and Quantized Conductance