L. N. Hall

Inductive And Resistive Studies Of The Ambient Pressure Organic Superconductor β-(BEDT-TTF)2I3

Abstract Inductive studies based on rf penetration depth measurements and resistive studies based on 4-lead electrical resistivity measurements are reported for the sulfur-based ambient pressure superconductor β-(BEDT-TTF)2I3. The inductive studies show that the bulk superconducting transition temperature (Tc) is sample dependent, with values of Tc ranging from 1.2 K to 1.4 K. The critical magnetic fields Hc2 show considerable anisotropy in directions perpendicular and parallel to the crystallographic ab plane. The electrical resistivity as a function of temperature shows linear and quadratic dependences on T and unusual structure near 8 K and 4 K.

Superconducting and Electrical Properties of (Bedt-Ttf)2I3 at Ambient Pressure

Abstract Studies of the of field penetration depths and electrical conductivity are described for the sulfur-based ambient pressure organic superconductor (BEDT-TTF)2I3.

Superconductivity above 2 K At Ambient Pressure in Iododibromide (IBr2 −) Charge-Transfer Salts of Bis (ETHYLENEDITHIO) TETRATHIAFULVALENE, BEDT-TTF

Abstract Crystals of the charge-transfer salt (BEDT-TTF)2IBr2, derived from the sulfur-based organic donor bis(ethylenedithio) tetrathiafulvalene [BEDT-TTF], have been synthesized by electrocrystallization and studied by rf penetration depth measurements at ambient pressure and temperatures down to 0.45 K. One of two phases was found to be superconducting with Tc = 2.3-2.8 K. This phase, denoted as β-(BEDT-TTF)2 IBr2, is isostructural with the superconducting triiodide salt, β-(BEDT-TTF)2I3. Evidence for the existence of a metastable superconducting phase of the BEDT-TTF/IBr2 system with Tc near 4.2 K is given.

High-field superconductivity in U6Fe and U6Co☆

Upper critical field data for U6Fe and U6Co are found to be markedly inconsistent with the weak coupling WHHM model. Nevertheless, excellent quantitative fits of Hc2 vs. T are obtained by using a modification of the WHHM model in which the orbital field is multiplied by a temperature-dependent enhancement factor.

Low Temperature Magnetoresistance of UPt3, U3Fe and U6Co

The transverse magnetoresistance Δρ(T,H)≡ρ(T,H)-ρ(T,0) has been measured at fixed temperatures in the range 0.5K \leqslant T \leqslant 40K as a function of magnetic field H \leqslant 15T for polycrystalline samples of U6Fe and U6Co, and a single crystal of UPt3 (\overrightarrowH||\overrightarrowa and \overrightarrowH||\overrightarrowc, \overrightarrowJ||\overrightarrowb). The sign of Δρ(T,H) changes from positive to negative as T increases through ~25K for U6Fe and U6Co, and \i~18K for UPt3. At low temperatures the data obey a novel universal form Δρ=AH2/B+H for all three materials, where A and B are T- and H-independent constants. Such behavior is postulated as typical of materials entering a heavy Fermi liquid state at low temperatures.

Magnetoresistivity in single crystal ErRh4B4

The magnetoresistance of single-crystal ErRh_4B_4 has been measured from 0.05 to 10 K with magnetic fields from 0 to 5 T applied along the magnetically easy a direction. The spin disorder resistivity in the normal state is interpreted with a crystal field model which also fits the magnetic anisotropy and specific heat. The data show that substantial spin disorder remains in ErRh_4B_4 even at T = 0 K in the ferromagnetic state.

Organic superconductors: structure-property relations and new materials design

Most known organic materials are electrical insulators having extremely low electrical conductivities of δ < 10-10 Ω-1 cm-1. A small number of organic materials are semiconductors having, for classification purposes, conductivities of δ ≈ 10-10-1 Ω-1 cm-1. A very small, but growing, number of organic substances are metallic in nature, i.e. having conductivities that rise with decreasing tem perature (δ ≈ 1−1010 Ω-1 cm-1). The latter systems comprise a class of intensely studied materials known as ‘organic metals’ of which fewer than ten can display the complete absence of electrical resistance at low temperatures, i.e. superconductivity (δ ≈ infinity). The known organic superconductors are novel, being derived from radical-cation donors and monovalent anions, X. The donors are derived from two kinds of molecules, neither of which contain any metallic elements. These are TMTSF (tetramethyltetraselenafulvalene) and BED T-TTF (bis-ethylenedithiotetrathiafulvalene, or ‘ET’ in abbreviated form). Most of the (TMTSF)2X and (ET)2X conducting materials require applied pressure to induce superconductivity that is thus far observed at very low temperatures (Te ≈ 1–2 K). However, two materials, (TMTSF)2ClO4 and (ET)2I3 are ambient pressure organic superconductors (Te = 1.2 and 1.4 K, respectively). Within each class the crystal structures have many similarities, the most important being a complex ‘infinite sheet networkߣ of short Se-Se interactions in (TMTSF)2X and a ‘corrugated sheet network’ of short S-S interactions in ET 2X. In this paper we discuss structure-property relations of the (TMTSF)2X salts, and of the (ET)2X salts as far as is known. In addition, we attempt to provide insight and guidelines for the synthesis of new highly conducting anionic derivatives of TMTSF and ET. It appears that while highly conducting (TMTSF)2X materials can be designed before synthesis, the onset of superconductivity depends heavily on the presence of anion order in the crystal, which is a parameter not easily controlled. For the (ET)2X systems the structural disorder apparent at 298 and 125 K may persist to very low tem perature, making it difficult to correlate structural order with superconductivity as is the case for (TMTSF)2X systems.