Metals superconducting transition

Rhenium hexafluoride is a cosdy (ca 3000/kg) material and is often used as a small percentage composite with tungsten or molybdenum. The addition of rhenium to tungsten metal improves the ductility and high temperature properties of metal films or parts (11). Tungsten—rhenium alloys produced by CVD processes exhibit higher superconducting transition temperatures than those alloys produced by arc-melt processes (12). 

It will be intriguing to theoretically examine the possibility of superconductivity in CNT prior to the actual experimental assessment. A preliminary estimation of superconducting transition temperature (T ) for metallic CNT has been performed considering the electron-phonon coupling within the framework of the BCS theory [31]. It is important to note that there can generally exist the competition between Peierls- and superconductivity (BCS-type) transitions in lowdimensional materials. However, as has been described in Sec. 2.3, the Peierls transition can probably be suppressed in the metallic tube (a, a) due to small Fermi integrals as a whole [20]. 

Electronic structures of SWCNT have been reviewed. It has been shown that armchair-structural tubes (a, a) could probably remain metallic after energetical stabilisation in connection with the metal-insulator transition but that zigzag (3a, 0) and helical-structural tubes (a, b) would change into semiconductive even if the condition 2a + b = 3N s satisfied. There would not be so much difference in the electronic structures between MWCNT and SWCNT and these can be regarded electronically similar at least in the zeroth order approximation. Doping to CNT with either Lewis acid or base would newly cause intriguing electronic properties including superconductivity. 

The idea that new phenomena could be present in 3He at very low temperatures arose from thermal measurements. The first observation was the anomaly in the specific heat at the normal superfluid transition which reminded the behaviour of specific heat at the superconductive transition in metals (Fig. 2.11) [34-36]. 

In 1968, an international agreement was reached about the definition of an official (practical) scale of temperature for T> 14 K. This temperature scale IPTS-68, corrected in 1975 [11], was defined by reference fixed points given by transitions of pure substances. To extend the low-temperature range of IPTS-68, the EPT 76 [12-13] gave nine reference temperatures defined by phase transition of pure substances in particular the superconductive transition (between 0.5 and 9K) of five pure metals was introduced. Moreover,... 

TES suffer from some limitations such as the small useful temperature range and the non-linearity of the transition curve. The latter drawback is especially evident in roughly patterned TES, as in the case shown in Fig. 15.5 [25], Feedback techniques, similar to those used in electronic amplifiers, minimize these drawback, reducing also the TES time response [26], The superconducting transition temperature (sometimes quite different from those of the bulk metal) of a TES made with one metal layer (single layer) depends on the metal used and on the film thickness. 

The normal spinel Li[Ti2]04 is a metallic oxide with a superconducting transition temperature of 13.7 K. The nominal formula is Li+[Ti3+Ti4+]04, in which the Li+ ions occupy the tetrahedral sites while the octahedral sites contain titanium with an average charge Ti3 5+, although as the material is metallic at room temperature the electrons are delocalized in a partly filled 3d band. 

Metallic appearance in massive form, black to metallic color in powdered state or in electrodeposited form hexagonal crystal system density 20.53 g/cm3 hardness (Brinell) 250 melts at 3,180°C vaporizes at 5,627°C (estimated) vapor pressure 4.6x10- torr at 2,500°C electrical resistivity 19.14 microhm -cm modulus of elasticity 67x10 psi at 20°C specific magnetic susceptibility 0.369x10 thermal neutron absorption cross section 86 barns/atom superconductivity transition temperature 1.7°K insoluble in water and hydrochloric acid soluble in dilute nitric acid and hydrogen peroxide slightly soluble in sulfuric acid. 

Those readers not familiar with superconductivity in organic materials may find the Tc values rather low. However, they are comparable to values for inorgaiuc metallic elements. Here is a list of some selected examples FcCNb) = 9.25 K, rc(Pb) = 7.20 K, rc(a-Hg) = 4.15 K, rc(Sn) = 3.72 K, Tc(Al) = 1.17 K, Tc(ri) = 0.40 K, etc. It is interesting to note that copper does not exhibit a superconducting transition. The highest known Tc values of any material correspond to the copper-oxide series with Tc 138 K as the absolute record for the thallium-doped mercury-cuprate compound. 

GPa) [239], a-(ET)2l3 was able to be converted to mosaic poly crystal with Tc 8 K by tempering at 70-100 °C for more than 3 days. The Tc 8 K phase thus obtained exhibited a similar NMR pattern to that of the /in-salt however, it was isolated at ambient pressure so was designated as (Xj-salt [240], RDP films composed of ET in polycarbonate (2 wt%) were treated with CH2CI2/I2 vapors and then annealed at 137 or 155 °C to convert the a-(ET)2l3 to the superconducting a -(ET)2l3. The film is metallic and exhibits a broad superconducting transition below 7 K [241],... 

Ito H, Ishiguro T, Kuhota M, Saito G (1996) Metal-nonmetal transition and superconductivity localization in the two-dimensional conductor /c-(BEDT-TTE)2Cu[N(CN)2]Cl under pressure. J Phys Soc Jpn 65 2987-2993... 

The superconducting transition temperatures for selected chemical elements and certain metallic alloys are presented in Table 1. These data and those presented throughout this Chapter have been taken, for the most part, from the excellent compilation by B. W. Roberts (3). 

High-pressure experiments on this oxide superconductor suggested (83) that a re-entrant superconducting transition occurred between the grains of the superconducting particles. The authors further concluded that the mechanism of superconductivity in this oxide system remained unknown. Other resistance measurements were also carried out (84) using pressures of -125 kbar indicating a possible onset of metallic behavior in this material at room temperature. 

Uchida, S., Kitazawa, K. and Tanaka, S., Superconductivity and Metal-Semiconductor Transition in BaPbj.xBix03. Phase Transitions 8 95 (1987). 

Khan, Y., Nahm, K., Rosenberg, M. and Willner, H., Superconductivity and Semiconductor-Metal Phase Transition in the System BaPbj.xBixOs. Phys. Stat. Sol.(a) 39 79 (1977). 

---