M. E. Kelly

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.

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 ...

New directions in the synthesis and exploration of novel organic superconductors: inverse isotope effect in the organic superconductor κL-(ET)2Ag(CF3)4(1-bromo-1,2-dichloroethane) [ET = bis(ethylenedithio)tetrathiafulvalene]

Abstract We report the first study of the effect of isotopic substitution on the superconducting transition temperature in the κ-(ET)2M(CF3)4(1,1,2-trihaloethane) [ET = bis(ethylenedithio)tetrathiafulvalene; M = Cu, Ag, and Au] family of organic superconductors, where M(CF3)4− is a large, discrete (non-polymeric) anion. This work also represents the first single crystal determinations of Tc in this series of superconductors. Substitution of the eight hydrogen atoms of the ET electron-donor molecule by deuterium causes the Tc of κL-(ET)2Ag(CF3)4(1-bromo-1,2-dichloroethane) to increase from 2.90 ± 0.04 K to 3.11 ± 0.04 K (here κL signifies a phase with a lower Tc, between 2 and 6 K, and κH is a different phase with higher Tc, between 9 and 12 K). Thus, this is the first example which demonstrates that the inverse isotope effect previously observed in κ-(ET)2Cu(SCN)2 (this salt contains a “self-assembled” polymeric anion and was the first organic superconductor with Tc>10 K) is also present in a κ-phase ET-based superconductor containing a discrete anion. This finding is significant for the future development of organic superconductors with novel physical properties, because entirely new directions for the synthesis of unique superconducting systems are now possible with the used of large discrete anions which can be synthesized in a rational and repeatable fashion (as opposed to randomly synthesized self-assembled polymeric anions).

Synthesis, Structure, Raman, and ESR Characterization of a New Organic Charge Transfer Salt, (BEDT-TTF)2[N(SO2CF3)2]

Abstract The new charge transfer salt (BEDT-TTF)2[N(SO2CF3)2] was synthesized and structually characterized. Its unit cell parameters at 295 K are: a = 6.6394(13) A, b = 8.658(2) A, c = 17.349(4) A, α = 94.70(2)°, β = 97.21(2)°, γ = 68.95(2)°, V = 922.7(4) A3, space group P 1, and Z = 1. The anion, N(SO2CF3)2 −, was found to be disordered, which may relate to the absence of the V4 and V5 Ag modes of the BEDT-TTF molecule in the Raman spectrum. ESR measurements reveal its semiconductive behavior with a small energy gap of 18.7 meV.

Electron-phonon coupling in κ-(bedt-ttf)2cu(ncs)2 organic superconductor studied by raman scattering spectroscopy

Abstract Low frequency Raman data for κ-(ET) 2 Cu(NCS) 2 (T c = 10.4 K) are reported. Measurements have been performed in a wide range of temperatures (1.5–100 K) and frequency hardening related to the superconducting transition has been observed for all the low-lying coupled phonons. The measured relative frequency shifts are lower than those previously reported for κ-(ET) 2 Cu[N(CN) 2 ]Br (T c =11.6 K), but the behavior of the two systems is similar and indicates a significant strength of the intermolecular electron-phonon coupling. The effects of isotopic substitutions ( 13 C 4 34 S 8 and 2 H 8 ) in ET molecules on the low frequency Raman active phonons of κ-(ET) 2 Cu(NCS) 2 are also reported. They give the first direct experimental confirmation to the suggested lattice softening induced by deuteration.

Rational design of organic superconductors through the use of the large, discrete molecular anions M(CF3)4-(M = Cu, Ag, Au) and SO3CF2CH2SF5-

A new approach to the synthesis of organic superconductors has recently been pioneered which involves the use of large, discrete, molecular anions as the charge-compensating entities in these charge transfer salts. The organic electron-donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) has been electrocrystallized with the novel organometallic M(CF3)4 (M = Cu, Ag, and Au) anions in a variety of 1,1,2-trihaloethane solvents. Over twenty organic superconductors have been synthesized which can be described by the general formula (ET) 2 M(CF 3 ) 4 (1,1,2-trihaloethane). These solvated salts are shown to have highly anisotropic physical properties which can be tuned via modifications of each of their three molecular components: ET electron donor molecule, M(CF3)4 anion, and neutral 1,1,2-trihaloethane solvent molecule. Superconductivity has also been observed in an ET salt containing the discrete SF 5 CH 2 CF 2 SO 3 - anion with an onset temperature near 5.2 K.

Dynamic disorders and its relaxation in organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

Abstract In this paper, we report direct evidence of dynamic structural disorders introduced by quenching a crystal of charge transfer salt κ-(BEDT-TTF)2Cu[N(CN)2]Br superconductor from high temperatures to 77 K. The sample quenched from different high temperatures relaxes to an equilibrium state with a single relaxation time constant. The magnitude of the relaxation amplitude is dependent on the quenching temperature up to 140 K, above which all relaxation curves overlap with each other. The results suggest strongly that the 140 K is associated with the onset temperature of the order-disorder transition of ethylene groups and the transition is responsible for the anomalous lattice expansion observed at this temperature.

Polymorphism in κ-(BEDT-TTF)2M(CF3)4(solvent) superconductors

Abstract A new crystallographic modification (κ L ′) was found in the BEDT-TTF: M (CF 3 ) 4 − :solvent ( M = Cu, Ag, Au) system. The structure of monoclinic κ L ′-(BEDT-TTF) 2 Cu(CF 3 ) 4 (1,2-dibromo-1-chloroethane = DBCE) is reported. It differs from the orthorhombic κ L − phase by having all BEDT-TTF molecules tilted in the same direction with respect to the conducting plane normal, whereas in κ L the tilt direction alternates between layers.

Isotope Effect Study of κ-(BEDT-TTF)2Cu(NCS)2: Labeling in the Anion

Abstract Isotopic labeling in the anion of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, with 13C and 15N isotopes in the thiocyanate groups, has been carried out. This labeling does not lead to an isotope effect on the superconducting transition temperature (T c).

The effect of spin fluctuations on two phonons lines in the Raman scattering from (ET)2Cu (NCS)2

Abstract The temperature-dependence of the intensity and frequency of two intense features in the Raman spectrum of χ-(BEDT-TTF)2[Cu(NCS)2] have been measured. The frequencies of these modes were observed to soften below 80K, in the temperature range where antiferromagnetic fluctuations have been observed, providing evidence of interactions between the phonons and the magnetism.