H. Hau Wang

Organic Superconductors—New Benchmarks

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures (Tc ≈ 13 kelvin) reported for these systems. These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions. Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high Tc values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths. The largest number of organic superconductors, including those with the highest Tc values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene]. The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well.

Rapid Raman spectroscopic determination of the stoichiometry of microscopic quantities of BEDT–TTF-based organic conductors and superconductors

An approximately linear dependence exists between the degree of charge transfer and the Raman-active CC stretching frequencies in insulating, conducting, and superconducting salts of the electron-donor molecule BEDT–TTF, which may be used to rapidly deduce the stoichiometry of newly synthesized materials.

The assignment of the normal modes of the BEDT-TTF electron-donor molecule using the infrared and Raman spectra of several isotopic analogs

Abstract Room-temperature infrared and Raman measurements on neutral bis(ethylenedithio) tetrathiafulvalene, abbreviated to BEDT-TTF, and four of its isotopic analogs are reported. The analogs are 13C(6) in which the central six carbon atoms are substituted with 13C; 13C(2) in which only the two central atoms are substituted and 34S(8) and d(8) in which all of the sulphur or all of the hydrogen atoms are substituted; respectively. The frequencies and frequency shifts of the Raman features and the infrared powder absorption features are measured. The polarized infrared reflectivity of a single crystal, and the intensities of the Raman features, help in assigning symmetries to certain resonances. By comparing these shifts to those calculated using a realistic valence force model the assigment of most of the in-plane fundamental intermolecular vibrational modes, and a few of the out-of-plane ethylene modes is achieved. The force constants are refined to produce excellent overall agreement between the measured and calculated frequencies and isotope shifts.

Single-crystal X-ray and neutron diffraction investigations of the temperature dependence of the structure of the Tc = 10 K organic superconductor κ-(ET)2Cu(NCS)2

Abstract The crystal structure of κ-(ET) 2 Cu(NCS) 2 [ET or BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, C 10 H 8 S 8 ] has been examined by single-crystal neutron and X-ray diffraction at temperatures between 298 and 15 K. Comparison of the low temperature ordered structures determined by use of X-ray and neutron diffraction with the previously reported crystallographically disordered room temperature X-ray structure indicates the avoidance of close H···H contacts as the reason for the conformational disorder of the terminal ethylene groups of the ET molecules at high temperatures. The space group is monoclinic noncentrosymmetric P 2 1 , Z = 2. Unit cell parameters at 118 K are a = 16.359(4), b = 8.418(2), c = 12.855(3) A, β = 111.21(2)°, and V = 1650.3(7) A 3 ; at 15 K, a = 16.373(5), b = 8.375(3), c = 12.775(6) A, β = 111.45(4)°, and V = 1630(1) A 3 . The interlayer spacing a · sin β remains constant upon cooling from 298 to 15 K even though the a axis increases slightly in length.

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.

Isotope effect in BEDT-TTF based organic superconductors

Abstract The results of our comprehensive isotope effect studies, in which seven different isotopically labeled (involving 13 C, 34 S and 2 H labeling) BEDT-TTF derivatives and isotopically labeled anion [Cu( 15 N 13 CS) 2 ] − were utilized, are summarized. For the first time, convincing evidence for a genuine BCS-like mass isotope effect in an organic superconductor is revealed in our studies.

Synthesis, purification, and characterization of the 1 : 1 addition product of C60 and anthracene

A cleanly separated Diels–Alder product of C60, a 1 : 1 adduct of anthracene, has been synthesized and characterized by thermogravimetric analysis, and NMR, UV–VIS, IR and mass spectroscopies.

The correlation of lattice phonon frequencies with the superconducting transition temperatures of some (BEDT-TTF) salts

The far-infrared powder absorption spectra, between 10 cm-’ and 350 cm-‘, of seven BEDT-TTF salts that exhibit superconductivity at or near ambient pressure, have been measured as a function of temperature, between 5 K and 300 K. The temperature dependences of the peak frequencies, along with the measured deuterium isotopic frequency shifts, separated the lattice from the internal modes. The frequencies of the observed lattice modes were found to increase with T, in the three K-phase salts in agreement with the reduced volume of the unit cell, and to have a mixed response in the three B-phase compounds. Furthermore, the genera1 range and magnitude of the frequencies in the higher-T, K-phase compounds were found to be larger than in the lower T, pphase compounds. Consequently, no evidence was found for a correlation between higher values of T, and a softer lattice in these organic superconductors.

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.

Frequencies of fundamental vibrations of BEDT-TTF, using infrared and Raman spectra of 13C, 34S and 2H substituted molecules

Abstract The frequencies of the in-plane fundamental vibrations of BEDT-TTF have been assigned on the basis of observed frequencies and isotope shifts in Raman and infrared spectra, and by comparison with frequencis and shifts from a refined normal-coordinate calculation.