H.H. Wang

A new ambient-pressure organic superconductor: (BEDT-TTF)2(NH4)Hg(SCN)4

Abstract A new ambient-pressure organic superconducting charge-transfer salt, (BEDT-TTF) 2 (NH 4 )Hg(SCN) 4 , is reported. The unit cell parameters of this salt indicate that it is isostructural to (BEDT-TTF) 2 KHg(SCN) 4 , which possesses an α-type packing motif. The broad ESR line width of 56 to 84 G at 300 K as well as its temperature dependence further indicates that this salt has an α-type packing. Conductivity measurements show that (BEDT-TTF) 2 (NH 4 )Hg(SCN) 4 is metallic to low temperatures and inductive studies by RF penetration depth measurements demonstrate that it is superconducting with an onset at 1.15 K.

New κ-phase materials, κ-(ET)2Cu[N(CN)2]X.X=Cl, Br and I. The synthesis, structure and superconductivity above 11 K in the Cl (Tc = 12.8 K, 0.3 kbar) and Br(Tc = 11.6 K) salts

Abstract The syntheses, structures, selected physical properties, and band electronic structures of three copper (I) dicyanamide halide salts of bis(ethylenedithio)tetrathiafulvalene [κ-(ET) 2 Cu[N(CN) 2 ]X, where X = Cl, Br, and I] are discussed. X-ray crystallographic studies demonstrate that the three derivatives are isostructural. The bromide salt is an ambient pressure superconductor with an inductive onset at 11.6 K and a resistive onset at 12.5 K. κ-(ET) 2 Cu[N(CN) 2 ]Cl exhibits the highest reported superconducting transition temperature (T c = 12.8 K. 0.3 kbar) for an organic superconductor, once a semiconductor-semiconductor transition (42 K) is suppressed. The application of GE varnish or Apiezon N grease to crystals of κ-(ET) 2 Cu[N(CN) 2 ]Cl provides sufficient stress to induce superconductivity at “ambient pressure”. Crystals of the iodide remain metallic to ∼150 K, where they become weakly semiconductive. No sign of superconductivity was detected at pressures (hydrostatic and shearing) up to 5.2 kbar and at temperatures as low as 1.1 K. The band electronic structures of the three salts are essentially identical. The differences in superconducting properties are explained in terms of differences in lattice softness, which are strongly influenced by short C-H…donor and C-H…anion contacts.

LOW-TEMPERATURE PENETRATION DEPTH OF KAPPA -(ET)2CUN(CN)2BR AND KAPPA -(ET)2CU(NCS)2

We present high precision measurements of the penetration depth \lambda of single crystals of \kappa-(ET)_2Cu[N(CN)2]Br and \kappa-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, \lambda\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, \lambda(0)=100\pm 20 \mu m.

Pressure dependence of the superconducting transition temperature of potassium fullerene, KxC60

Abstract The pressure dependence of the superconducting transition temperature in K x C 60 is determined using solid He pressure techniques to be d T c /d P =-0.63±0.08 K/kbar. Crude scaling would predict Rb x C 60 and CsC 60 to have T c values of ∼35 and ∼60 K, respectively.

Charge-Transfer Salts Derived from the New Electron-Donor Molecule BEDO-TTF: ESR, Superconductivity and Electrical Properties, and Crystal and Band Electronic Structure

Abstract Charge transfer salts of bis(ethylenedioxy)tetrathiafulvalene, BEDO-TTF, namely (BEDO-TTF)2AuBr2, and charge transfer salts (BEDO-TTF)mXn where X− is I3, AuI2 −, Au(CN)2−, ClO4 −, BrO4 −, BF4 −, PF6 −, AsF6 −, NO3 −, C(CN)3 −, and HgBr3− have been synthesized. The AuBr2− salt is the first BEDO-TTF salt to be structurally characterized. Crystallographic investigations of several of the other (BEDO-TTF)mXn salts have shown that (with the exception of the I3− salt) these new synmetals are structurally very similar to (BEDO- TTF)2AuBr2. Therefore, a 2:1 stoichometry is expected for BEDO-TTF salts. The observation of similar ESR linewidths for all the BEDO-TTF salts reinforces this conclusion. While (BEDO-TTF)2PF6 is semiconducting and the AuBr2−, Au(CN)2−, and ClO4− salts are metallic only near room temperature and semiconducting at low temperatures, (BEDO-TTF)2AuI2 shows metallic conductivity to low temperatures. RF penetration depth measurements on crystals derived from BEDO-TTF/KSCN/CuSCN show the...

Effect of pressure on the superconducting transition temperature of κ-(BEDT-TTF)2Cu(NCS)2

Abstract The effect of hydrostatic pressure on the superconducting transition temperature at ambient pressure of κ-(BEDT-TTF) 2 Cu(NCS) 2 is found to be −3 K/kbar initially. This is the largest pressure derivative ever observed for a superconducting transition.

The role of anions on the crystal structures and electrical properties of the organic metals and superconductors, (BEDT-TTF)2X (X = trihalide anions)

Abstract A systematic study of the crystal structures of (BEDT-TTF) 2 X salts [BEDT-TTF is bis-(ethylenedithio) tetrathiafulvalene, C 10 S 8 H 8 , and X is a linear trihalide anion] reveal that the different packing motifs for the BEDT-TTF molecules are directly related to the electrical properties of these materials. The packing of the organic molecules is influenced by the cation-anion interactions of the -CH 2 …X − type and is observed to change systematically with anion size.

(MDT-TTF)2 AuI2: An ambient pressure organic superconductor (Tc = 4.5 K) based on an unsymmetrical electron donor

Abstract R f penetration depth measurements show (MDT-TTF) 2 AuI 2 to be an ambient pressure superconductor with a T c ∼4.5 K. The effectof hydrostatic pressure on T c was found to be −0.92 K/kbar, which suggests that anions longer than AuI 2 - may puff up the lattice and may yield cation-radical salts of MDT-TTF with even higher superconducting transition temperatures. (MDT-TTF) 2 AuI 2 has a room temperature ESR linewidth of ∼ 100 G, which gradually increases as the temperature is lowered. The structure and properties of (MDT-TTF) 2 AuI 2 bear a close resemblance to those of the 10 K organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . The band electronic structure of (MDT-TTF) 2 AuI 2 is two-dimensional, with a closed Fermi surface. The MDT-TTF donor molecule prossesses only four CH bonds to form short contacts with anions, as compared to eight for BEDT-TTF. Thus, the translational and/or librational modes of vibration are expected to be substantially different in (MDT-TTF) 2 AuI 2 and κ-(BEDT-TTF) 2 Cu(NCS) 2 .

Determination of a mass isotope effect on Tc in an electron-donor-based organic superconductor, κ-(ET)2Cu(NCS)2, where ET represents bis(ethylenedithio)tetrathiafulvalene

Abstract We describe the first determination of a genuine mass isotope effect on T c arising from the isotopic substitution of atoms in the ET [bis(ethylenedithio)tetrathiafulvalene] molecule of an electron-donor-based organic superconductor, κ-(ET) 2 Cu(NCS) 2 ( T c = 9.6 K, inductive onset). The isotopic substitution in the ET molecule involves concurrent replacement of the four carbon atoms in the terminal 1,2-ethanediyl groups with 13 C and all eight sulfuir atoms with 34 S. This substitution, 13 C(4) 34 S8), increases the mass of the ET molecule by 20 amu, which is a 5% increase in the normal mass. With the use of AC susceptibility measurements on a large sampling of single crystals, including both undeuterated and fully deuterated salts, we obtain ΔT c = −0.12 ± 0.05 K for this isotope effect. Assuming a BCS-like mass effect with ET as the relevant mass entity ( M ), this effect gives α = 0.26 ± 0.11 for T c αM − α . Additionally, our experiments with fully deuterated κ-(ET) 2 Cu(NCS) 2 ( 2 H replacing eight hydrogen atoms in ET) confirm the occurrence of a large inverse isotope effect for the deuteration, and yield the first definitive value for this effect, ΔT c = +0.28 ± 0.06 K. The isotope effect for 13 C(4) 34 S(8) substitution, however, is unaffected by deuteration. Contrary to an earlier report by others, we find a zero isotope effect within ±0.06 K for 13 C(4) substitution alone.

Superconductivity up to 11.1 K in three solvated salts composed of [Ag (CF3)4]- and the organic electron-donor molecule bis (ethylenedithio) tetrathiafulvalene (ET)

Abstract Electrocrystallization of the electron-donor molecule BEDT-TTF [bis (ethylenedithio) tetrathiafulvalene] with the square planar organometallic anion [Ag(CF 3 ) 4 ] − in TCE (1,1,2-trichloroethane) results in the formation of at least four crystalline phases. A plate-like phase, κ L -(ET) 2 Ag(CF 3 ) 4 · TCE, is isostructural to κ L -(ET) 2 Cu(CF 3 ) 4 · TCE T c = 4.0 K with a slightly expanded unit cell, but a lower critical temperature of 2.6 K. Two superconducting needle phases of (ET) 2 Ag(CF 3 ) 4 · TCE have been observed, with critical temperatures of 9.4 and 11.1 K. The critical temperature of this latter phase is within 0.5 K of the current record for ambient pressure organic superconductors, 11.6 K in κ-(ET) 2 Cu[N(CN) 2 ]Br. A fourth phase, (ET) 2 Ag(CF 3 ) 4 , crystallizes simultaneously with the plates and needles, contains no solvent molecules, and does not superconduct down to 0.7 K.