James D. Dudek

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.

The first organic cation-radical salt superconductor (Tc= 4 K) with an organometallic anion: superconductivity, synthesis and structure of κL-(BEDT-TTF)2Cu(CF3)4·TCE

Superconductivity at ambient pressure with onset Tc= 4.00 ± 0.05 K is reported in a unique organic cation-radical salt κL-(ET)2Cu(CF3)4·TCE, [ET = bis(ethylenedithio)tetrathiafulvalene, TCE = 1,1,2-trichloroethane] with the Cu(CF3)4– species being the first Cu3+ and F containing organometallic anion in an organic superconductor.

A new 9 K superconducting organic salt composed of the bis (ethylenedithio) tetrathiafulvalene (ET) electron-donor molecule and the tetrakis (trifluoromethyl) cuprate (III) anion, [Cu(CF3)4]-

Abstract We report the discovery of a second, higher- T c superconducting organic charge-transfer salt derived from the electron-donor molecule BEDT-TTF (or ET), the novel organometallic anion [Cu(CF 3 ) 4 ] − , and the neutral solvent molecule 1, 1, 2-trichloroethane (TCE). We have very recently reported that this charge-transfer system yields a new superconducting ϰ phase salt, ϰ L - (ET) 2 Cu(CF 3 ) 4 ·TCE, with inductive onset T crmc =4.0 K at ambient pressure. This ϰ phase salt (ϰ denotes a particular packing arrangement of the ET organic donor molecules) is electrocrystallized in the habit of hexagonal plates. Crystals possessing a needle-like habit electrocrystallize simultaneously with these plates, and we find these needles to be a distinctly different superconducting phase with diamagnetic onset T c =9.2±0.1 K at ambient pressure. On the basis of our experiments, we denote this new superconducting phase as ϰ H −( ET ) 2 Cu ( CF 3 ) 4 ·( TCE ) x , X

The synthesis and characterization of radical cation salts of bis(ethylenedithio)tetraselenafulvalene

Abstract Four 2:1 radical cation salts (SbF 6 , CF 3 SO 3 , GaCl 4 , Cu[N(CN) 2 ]Br) of bis(ethylenedithio)-tetraselenafulvalene (BETS) have been prepared and scrutinized by X-ray crystallography, DC resistivity measurements (300-10K), rf penetration depth experiments (0.5K), pressure studies (0.5–5kbar, 4K), and tight-binding band calculations. The salts are κ-phase, and three are metallic to low temperatures. κ-(BETS) 2 Cu[N(CN) 2 ]Br is isostructural with κ-(ET) 2 Cu[N(CN) 2 ]Br.

Synthesis and characterization of two new organic superconductors, κL- and κH-[bis(ethylenedisulfanyl)tetrathiafulvalene]2Au(CF3)4·(1,1,2-trichloroethane)via microelectrocrystallization

A novel microelectrocrystallization (MEC) procedure (ten-fold decrease in halogenated solvent use, micromolar quantities of electron donor molecule and supporting electrolyte, and Hg free electrodes), necessitated by the very small available quantities of the novel organometallic Au(CF3)4– anion, has been developed to prepare two distinct phases of κ-(ET)2Au(CF3)4·(TCE)[ET = bis(ethylenedisulfanyl)tetrathiafulvalene, TCE = 1,1,2-trichloroethane] which exhibit ambient pressure superconductivity with onset temperatures of 2.1 and 10.5 K.

Synthesis and Characterization of Radical Cation Salts Derived from Tetraselenafulvalene and Bis(Ethy1enedithio)tetraselenafulvalene

Abstract New synthetic routes to bis(ethylenedithio)tetraselenafulvalene (BETS) were explored, starting from tetraselenafulvalene (TSF). Several radical cation salts of TSF and BETS were prepared by electrocrystallization. A new kappa phase salt of BETS, K-(BETS)2GaCl4, displayed an interesting resistance-temperature profile from 300-35K.

New Organic Superconductors in the System (ET)2M(CF3)4(Solvent) (M = Cu, Ag, Au): Dramatic Effects of Organometallic Anion and Solvent Replacement

Abstract Over twenty molecular-based superconductors have been synthesized recently via electrocrystallization of the organic bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) electron donor molecule with the novel organometallic M(CF3)4 − (M = Cu, Ag, and Au) anions in a variety of 1,1,2-trihaloethane solvents. The highly anisotropic physical properties of these solvated (ET)2 M(CF3)4(1,1,2-trihaloethane) salts can be tuned via modifications of each of their three discrete molecular components. These unique salts crystallize in two distinct structural modifications of the κ-type packing motif (sheets of dimerized electron-donor molecules arranged in a parquet fashion). Crystals of the lower T c phase (κL) have a plate-like morphology and superconducting transition temperatures in the 2–6 K range, while the transitions of the higher T c phase (κH), which typically have a needle-like morphology, are in the 9–11 K range. The synthesis, structure, and physical properties of this new family of materials ...

In search of an isotope effect on Tc in κ-(ET)2X superconductors with 34S isotopic substitution in the eight sulfur atoms of bis (ethylenedithio) tetrathiafulvalene (ET)

Abstract We describe the first studies of the superconducting isotope effect with 34 S substitution for all eight sulfur atoms of the ET molecule in κ-(ET) 2 Cu[N(CN) 2 ]Br( T c =11.6 K ) and κ-(ET) 2 Cu(NCS) 2 ( T c =9.6 K ). The frequencies of two symmetric normal mode vibrations associated with the C-S stretching motions (∼500cm −1 ) of ET decrease by about 2.7% with this substitution. Furthermore, the mass of ET is increased by about 16 amu. A small shift, ΔT c =-0.08±0.07 K , indicates that the C-S stretching motions of ET do not provide a dominant exchange mechanism for Copper pairing. The results rule out the possibility of a conventional BCS isotope effect with ET as the relevant mass entity, but they are consistent with a small BCS mass effect with α =0.17±0.15 for κ -( ET ) 2 Cu [ N ( CN ) 2 ] Br .

Fullerene derivatives and fullerene superconductors

Abstract A series of 1:1 C 60 cycloaddition adducts, C 60 A (A = anthracene, butadiene, cyclopentadiene and methylcyclopentadiene), has been synthesized. The products are cleanly separated and characterized by use of TGA, 1 H-NMR, IR, and mass spectrometry. Among these adducts, C 60 , (methylcyclopentadiene) showed the highest thermal stability and was doped with three equivalents of rubidium. The resulting Rb 3 C 60 (MeCp) is a semiconductor but can be thermally converted to the superconducting Rb 3 C 60 through a retro-Diels-Alder reaction. A one-step doping process to prepare Rb 3 C 60 crystals has been developed. The optimal doping condition occurs at 300°C. High superconducting shielding fractions between 60 and 90% and sharp transition widths (Δ T 10 - 90 between 4 and 0.7 K) were measured for these samples.