Intercalate Superconductivity and van der Waals Equation.

Superconductivity in two single-element intercalated compounds has been investigated with the van der Waals equation. For Cu x TiSe2 and YBa2Cu3O6+x , the van der Waals term characterizing the attractive energy per particle (i.e., electrons), aN/V, is calculated from concentration-dependent transition temperature plots derived from experiment. It is shown that two times the attractive energy per intercalant valence electron (2aN val/V unit) is equal to the energy gap predicted by BCS theory (Δ) for these superconductors. This realization allows another way to estimate the energy gap of superconducting intercalated insulators and semiconductors, this time, directly from physical real-space properties of the superconductor and the applied external pressure. The physical properties of importance are shown to be the intercalant concentration, transition temperature, and the number of intercalant valence electrons per unit cell volume.

Estimating the Single-Element Concentration of Intercalated Insulators for the Emergence of Superconductivity.

To predict whether a compound will superconduct and to predict its transition temperature T c prior to measurement have always been desires of the materials science community. Matthias was first to report the necessary conditions for the occurrence of superconductivity in elements, compounds, and alloys in terms of density (valence electrons per atom). This current report is motivated by somewhat similar empirical observations concerning the importance of valence electrons per unit cell; more specifically, dopant valence electrons per unit cell within intercalated insulators. In this article, though not exhaustive, a representative list of 40 superconductors will be used to show that the onset of superconductivity (insulator-superconductor boundary) within intercalated insulators can easily be modeled, almost exactly, by the ideal gas law equation. Given this observation, in contrast to Matthias, interactions are semiclassically accounted for to ultimately determine the single-element onset concentration needed to bring about superconductivity within many intercalated insulators known to date. The 13 compounds which were previously intercalated and will be discussed include inorganics, TiSe2, C60, YBa2Cu3O6, IrTe2, Bi2Se3, MoS2, ZrNCl, HfNCl, BP (black phosphorus), HoTe3, and Y2Te5, and organics, C22H14 and C14H10. In essence, the overall objective of this report is to offer a slightly different viewpoint on superconductivity, led by empirical observations, which seemingly leads to predictable experimental outcomes. If newly discovered materials further validate this approach to intercalated superconductors, with minor refinements, a route to purposefully designing superconductors may be accessible through onset conditions outlined in this article.

From Ta2S5 Wires to Ta2O5 and Ta2O5-x S x.

Synthesis routes to forming novel materials are oftentimes complicated and indirect. For example, Ta2S5 has only been found as an unwanted byproduct of certain chemical reactions, and its properties were unknown. However, here we demonstrate the growth of Ta2S5 wires with steel-like tensile strength, which are also precursors for the first controlled synthesis of long, mesoscopic Ta2O5 wires and superconducting Ta2O5-x S x wires. Single-crystal wires of tantalum pentasulfide, Ta2S5, were first grown using vapor transport from polycrystalline XTa2S5, sulfur, and TeCl4 in fused-quartz tubes, where X = Ba or Sr. Crystals form as long wires with lengths on the order of a few centimeters and varying cross sections as small as 25 µm2. They were found to have steel-like tensile strength, and their crystal structure was determined using X-ray diffraction to be monoclinic with space group P2/m and with lattice parameters a = 9.91(7) Å, b = 3.82(5) Å, and c = 20.92(2) Å. Electrical resistivity measurements reveal Ta2S5 to be a narrow band gap semiconductor with E g = 110 meV, while a Debye temperature ΘD = 97.0(5) K is observed in specific heat. Tantalum pentasulfide wires were then converted to insulating tantalum pentoxide (Ta2O5) wires after calcinating them for 30 min in air at 900 °C. Finally, tantalum pentoxide wires were converted to tantalum oxysulfide (Ta2O5-x S x ) wires after annealing them in CS2 vapor for 30 min at 900 °C. The oxysulfide crystal structure was determined using X-ray diffraction to be that of ß-Ta2O5. Electrical and magnetic measurements reveal Ta2O5-x S x to be metallic and superconducting with T c = 3 K.

Photomagnetic response in highly conductive iron(II) spin-crossover complexes with TCNQ radicals.

Co-crystallization of a cationic Fe(II) complex with a partially charged TCNQ(.δ-) (7,7',8,8'-tetracyanoquinodimethane) radical anion has afforded molecular materials that behave as narrow band-gap semiconductors, [Fe(tpma)(xbim)](X)(TCNQ)(1.5)⋅DMF (X=ClO4(-) or BF4(-); tpma=tris(2-pyridylmethyl)amine, xbim=1,1'-(α,α'-o-xylyl)-2,2'-bisimidazole). Remarkably, these complexes also exhibit temperature-and light-driven spin crossover at the Fe(II) center, and are thus the first structurally defined magnetically bistable semiconductors assembled with the TCNQ(.δ-) radical anion. Transport measurements reveal the conductivity of 0.2 S cm(-1) at 300 K, with the low activation energy of 0.11 eV.

Variable range hopping in single-wall carbon nanotube thin films: a processing-structure-property relationship study.

By varying the ultrasonication and ultracentrifugation conditions, single-walled carbon nanotube (SWCNT) dispersions with a broad range of SWCNT length and diameter (L = 342-3330 nm; d = 0.5-12 nm) were prepared and characterized by a preparative ultracentrifuge method (PUM) and dynamic light scattering (DLS) technique. The well-characterized dispersions were then fabricated into SWCNT thin films by spray coating. Combined optical, spectroscopic, and temperature-dependent electrical measurements were performed to study the effect of SWCNT structures on the charge transport behavior of SWCNT thin films. Regardless of SWCNT size in the dispersion and the thin film thickness, the three-dimensional variable range hopping (3D VRH) conduction model was found to be appropriate in explaining the temperature-dependent sheet resistance results for all SWCNT thin films prepared in this study. More importantly, with the SWCNT structural information determined by the PUM method, we were able to identify a strong correlation between the length of SWCNTs and the 3D VRH parameter T0, the Mott characteristic temperature. When the SWCNT length is less than ∼700 nm, the T0 of SWCNT thin films shows a drastic increase, but when the length is greater than ~700 nm, T0 is only weakly dependent on the SWCNT length. Under the framework of traditional VRH, we further conclude that the electron localization length of SWCNT thin films shows a similar dependence on the SWCNT length.