J. E. Vazquez

Bio: J. E. Vazquez is an academic researcher from IBM. The author has contributed to research in topics: Oxygen & Superconductivity. The author has an hindex of 8, co-authored 14 publications receiving 4452 citations.

Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls

Abstract: CARBON exhibits a unique ability to form a wide range of structures. In an inert atmosphere it condenses to form hollow, spheroidal fullerenes. Carbon deposited on the hot tip of the cathode of the arc-discharge apparatus used for bulk fullerene synthesis will form nested graphitic tubes and polyhedral particles. Electron irradiation of these nanotubes and polyhedra transforms them into nearly spherical carbon 'onions'. We now report that covaporizing carbon and cobalt in an arc generator leads to the formation of carbon nanotubes which all have very small diameters (about 1.2 nm) and walls only a single atomic layer thick. The tubes form a web-like deposit woven through the fullerene-containing soot, giving it a rubbery texture. The uniformity and single-layer structure of these nanotubes should make it possible to test their properties against theoretical predictions.

Oxygen ordering, phase separation and the 60-K and 90-K plateaus in YBa2Cu3Ox

Abstract: A PLOT of the superconducting transition temperature (Tc) against oxygen content (x) for YBa2Cu3Ox exhibits two 'plateaus' when oxygen is removed from the material at low temperatures. Tc remains nearly constant at ∼60K for x= 6.6–6.7 and nearly constant at ∼90 K for x= 6.8–7.0. It is now common to assume that there are two distinct superconducting phases in YBa2Cu3Ox, the '60-K' and '90-K' phases, and that the two plateaus correspond to single-phase regions of the respective phases1,2. YBa2Cu3Ox samples prepared at low temperatures contain a variety of ordered oxygen superstructures3–5. Several theoretical studies have tried to predict the phase equilibria between these ordered structures6,7 and to explain how oxygen ordering leads to the Tc plateaus8,9, but no clear understanding of the interrelationships amongst these factors has yet emerged. This is partly because the techniques used previously to prepare the low-temperature samples do not control or monitor all of the key processing variables, particularly the oxygen partial pressure. Here we report on the ordered oxygen structures and superconducting properties of YBa2Cu3Ox samples prepared in precisely controlled oxygen environments using a solid-state ionic technique. We find no evidence for phase separ-ation between structures that have widely different oxygen content, but we do see electron diffraction evidence for phase separation between distinct phases that differ only slightly in oxygen content, and these regions of phase separation coincide with the Tc plateaus. These results show that the commonly held view that the two plateaus correspond to single-phase regions of respective 60-K and 90-K phases is incorrect: the changes in superconducting properties with oxygen content in YBa2Cu3Ox cannot be explained on this basis.

Evidence for superconductivity in La 2 CuO 4

Abstract: We report evidence for superconductivity in undoped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ obtained from resistivity, thermoelectric power, and susceptibility measurements. The onset temperature is near 40 K and we have determined its pressure and field dependence in resistivity. The superconducting behavior is of a trace and filamentary nature and is quite sensitive to stoichiometry. Its occurrence is extremely process dependent and an be controlled by oxygen pressure. We discuss several likely sources for the superconducting activity.

Superconductivity above 90 K in the compound YBa 2 Cu 3 O x : Structural, transport, and magnetic properties

Abstract: We report the structural, transport, and magnetic properties of the principal phase responsible for superconductivity in the recently discovered Y-Ba-Cu-O compounds with transition temperatures greater than 90 K.

Superconductivity above liquid nitrogen temperature: preparation and properties of a family of perovskite-based superconductors

Abstract: Over the last decade, the search for high-temperature superconducting materials remained virtually stagnant. This situation changed radically with the discovery of Bednorz and Mueller of superconductivity above 30 K in a layered perovskite oxide composed of La, Ba, and Cu. Improvements in the superconducting transition temperature (T/sub c/) to approx. 45 K by Sr substitution and identification of the phase responsible for superconductivity (La/sub 2-x/Ba(or Sr)/sub x/CuO/sub y/ where x is typically between 0.1-0.3) followed rapidly. The next major advance was immediate and dramatic. Wu, Chu, and co-workers reported a new material based on the starting composition Y/sub 1.2/Ba/sub 0.8/CuO/sub y/ with T/sub c/ well above 90 K. Nearly simultaneous reports by other groups confirmed these results. The Y-Ba-Cu material was a mixture of several unidentified phases and only a small fraction of the sample actually was superconducting. Recently, the structure of this new superconductor was identified as an oxygen-defect perovskite corresponding to the composition Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub y/. In this paper, they report on the synthesis of single-phase Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub y/ and show how the preparation conditions play a dramatic role in determining the superconducting properties. Also, based on their knowledge of the structure, they havemore » prepared a variety of new, single-phase high-temperature superconducting compounds in which Y and Ba are substituted by related elements, demonstrating that 90 + K superconductivity is a more general property of this structure.« less

Room-temperature transistor based on a single carbon nanotube

Abstract: The use of individual molecules as functional electronic devices was first proposed in the 1970s (ref 1) Since then, molecular electronics2,3 has attracted much interest, particularly because it could lead to conceptually new miniaturization strategies in the electronics and computer industry The realization of single-molecule devices has remained challenging, largely owing to difficulties in achieving electrical contact to individual molecules Recent advances in nanotechnology, however, have resulted in electrical measurements on single molecules4,5,6,7 Here we report the fabrication of a field-effect transistor—a three-terminal switching device—that consists of one semiconducting8,9,10 single-wall carbon nanotube11,12 connected to two metal electrodes By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state We have previously reported5 similar behaviour for a metallic single-wall carbon nanotube operated at extremely low temperatures The present device, in contrast, operates at room temperature, thereby meeting an important requirement for potential practical applications Electrical measurements on the nanotube transistor indicate that its operation characteristics can be qualitatively described by the semiclassical band-bending models currently used for traditional semiconductor devices The fabrication of the three-terminal switching device at the level of a single molecule represents an important step towards molecular electronics

Crystalline Ropes of Metallic Carbon Nanotubes

Abstract: The major part of this chapter has already appeared in [1], but because of the length restrictions (in Science), the discussion on why we think this form is given in only brief detail. This chapter goes into more depth to try to answer the questions of why the fullerenes form themselves. This is another example of the very special behavior of carbon. From a chemist’s standpoint, it is carbon’s ability to form multiple bonds that allows it to make these low dimensional forms rather than to produce tetrahedral forms. Carbon can readily accomplish this and it is in the mathematics and physics of the way this universe was put together, that carbon is given this property. One of the consequences of this property is that, if left to its own devices as carbon condenses from the vapor and if the temperature range is just right, above 1000°C, but lower than 1400°C, there is an efficient self-assembly process whose endpoint is C60.

Chemistry of Carbon Nanotubes

Abstract: Department of Materials Science, University of Patras, 26504 Rio Patras, Greece, Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, 116 35 Athens, Greece, Institut de Biologie Moleculaire et Cellulaire, UPR9021 CNRS, Immunologie et Chimie Therapeutiques, 67084 Strasbourg, France, and Dipartimento di Scienze Farmaceutiche, Universita di Trieste, Piazzale Europa 1, 34127 Trieste, Italy