Hau H. Wang

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF)2Cu[N(CN)2]X (X=Cl, Br, I)

Abstract The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =11.6K, the Cl-salt becomes superconducting ( T c =12.8K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.

Effect of structural disorder on organic superconductors: a neutron diffraction body of "high-Tc" .beta.*-(bis(ethylenedithio)tetrathiafulvalene)2 triiodide at 4.5 K and 1.5 kbar.

In this paper the authors report the existence of a new ordered structural phase, denoted as ..beta..*-(ET)/sub 2/I/sub 3/, at T = 4.5 K and P = 1.5 kbar. To our knowledge, this is the first structural study of an organic superconductor with the sample at a temperature below the onset temperature for superconductivity. The structure of ..beta..*-(ET)/sub 2/I/sub 3/ derived from the data set obtained with the sample below the superconducting transition temperature, at 4.5 K and 1.5 kbar, is completely ordered but is not isostructural with the ..beta..-phase structures of the I/sub 2/Br/sup -/, AuI/sub 2//sup -/, and IBr/sub 2//sup -/ salts.

New cation-anion interaction motifs, electronic band structure, and electrical behavior in .beta.-(ET)2X salts [ET = bis(ethylenedithio)tetrathiafulvalene; X = dichloroiodide, chlorobromoiodide]

Etude structurale des composes du titre. Determination des temperatures de transition metal-isolant

Infrared optical properties of the 12K organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

Abstract Measurements of the polarized reflectively of κ-(BEDT-TTF) 2 -Cu[N(CN) 2 ]Br are presented, from 200 to 5500 cm −1 , at temperatures between 10 and 295 K. The optical conductivities, obtained after a Kramers-Kronig analysis, are very similar to those previously reported for the 10 K superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 , except for the clear evidence of narrower electronic bands in the Cu[N(CN) 2 ]Br - salt. The band structure is used to account for the position of the peak that appears in the mid-infrared interband conductivity. A comparison of the frequencies of the vibrational features in this 12 K superconductor, with those in the 10 K compound, shows an increase in the normally-inactive a g mode frequencies and a decrease in the normally-active b u mode frequencies. These can be related to the larger unit cell of the Cu[N(CN) 2 ]Br - salt which produces a softer lattice and narrower electronic bands, which in turn lead to a higher superconducting transition temperature.

Infrared study of the metal-insulator transition in the organic conductor K-(BEDT-TTF)2Cu[N(CN)2]Cl

Abstract Measurements of the polarized reflectivity of K -( BEDT - TTF ) 2 - Cu [ N ( CN ) 2 ] Cl are presented, from 200 to 5500 cm −1 , at temperatures between 10 and 295 K. The optical conductivities for E  a and E  c at room temperature are very similar, but develop differently in the mid-infrared as the sample is cooled. Below 50K a large temperature-dependent semiconductor energy-gap develops, with a value of approximately 900cm −1 at 10K. This has a large effect on the totally symmetric mode of the (BEDT-TTF) molecule near 1300 cm −1 , v 3 ( a g ). The possible cause and form of the energy gap are discussed.

Structural Characterization and Band Electronic Structure of α-(BEDT-TTF)2I3 below its 135 K Phase Transition

Abstract The nature of the 135 K metal-insulator (MI) transition in α-(BEDT-TTF)2l3, abbreviated α-(ET)2l3, was examined by determining the crystal structures above (298 K) and below (120 K) the phase transition and also by calculating the band electronic structures at both temperatures. This study demonstrates that both the crystal and band electronic structures of α-(ET)2l3 change only slightly upon passing through the MI transition. Within each sheetlike network of ET radical cations, the magnitudes of interactions between adjacent pairs of ET molecules (i-j) above and below the MI transition temperature (T MI) were evaluated by calculating their interaction energies, β ij = ⟨Ψ, H off|Ψ⟩, where Ψ1); and Ψ1 are the HOMOs of ET molecules i and j, respectively. The band electronic structures calculated by using single-zeta Slater type orbitals show that α-(ET)2l3 is a semiconductor with band gaps of 13 meV (298 K) and 35 meV (120 K). Thus, within the one-electron model, the apparently metallic properties...

Optical properties of the 10 K organic superconductor (BEDT-TTF)2[Cu(SCN)2]

Abstract Polarized reflectivity measurements at 295 K and 12 K are reported for (BEDT-TTF) 2 [Cu(SCN) 2 ], from 20 cm −1 to 8000 cm −1 . The optical conductivities obtained from Kramers-Kronig analyses show non-Drude behaviour, but plasma frequencies have been extracted from the region of the reflectivity minima. At 12 K, generlised Drude analysis gives a frequency-dependent effective mass of approximately 8 m e in the far-infrared similar to that found in the high- T c superconductor YBa 2 Cu 3 O 7 . A strong vibronically-activated totally-symmetric intramolecular mode is observed, the frequency of which shifts with the mid-infrared band in the two polarizations, and at the two temperatures. Other sharp phonon features are antiresonant in the a g band and resonant elsewhere.

Correlations of Anion Size and Symmetry with the Structure and Electronic Properties of β-(BEDTTTF)2X Conducting Salts with Trihalide Anions X = 13-, 12Br-, 1Br2-

Abstract In the isostructural series of β-(BEDT-TTF)2X organic metals, abbreviated β-(ET)2X, where X is a trihalide anion, I3-, I2Br- or IBr2-, and where ET is bis-(ethylenedithio) tetrathiafulvalene (C10S8H8), the salt containing the disordered asymmetric I2Br- anion has atypical structural and electronic properties compared to those containing the symmetrical and ordered I-3 and IBr2- anions.

Electron-phonon effects in the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

Abstract We report on room-temperature Raman measurements on κ-di[bis-ethylenedithiotetrathiafulvalene] copper dicyanamide bromide (κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br) and the isotopic analog 13 C(6)-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, in which the six central carbon atoms in the inner rings of the BEDT-TTF molecule are replaced with 13 C, and assign the features to the symmetric intramolecular modes of vibration, by comparison with a recently published study on neutral BEDT-TTF. We then used these data to help us make a more definitive assignment that has been previously possible of the features in the low-temperature IR conductivity spectra of (BEDT-TTF) 2 Cu[N(CN) 2 ]Br. It was also necessary to measure the low-temperature IR conductivity of additional isotopic analogs of BEDT-TTF, namely those containing 13 C(6), 13 C(2), 34 S(8) and d(8). We found, as expected, that the spectra are dominated by the electron-phonon activated totally symmetric (A g ) modes, but also that the nature of the strongest of these, ν 3 (A g ), is changed dramatically in (BEDT-TTF) 2 Cu[N(CN) 2 ]Br from what it is in neutral BEDT-TTF. We also found, unexpectedly, that one of the stronger IR features is most likely due to an activated non-totally-symmetric (B 3g ) mode, instead of a normally IR-active mode, which had been previously assigned, and further that several other (B 3g ) modes appear to be activated in the IR spectra.

Ambient pressure superconductivity at 4–5 K in β-(BEDT-TTF)2AuI2

Abstract Crystals of β-(BEDT-TTF) 2 AuI 2 , derived from the sulfur-based organic donor bis(ethylenedithio) tetrathiafulvalene [BEDT-TTF or “ET”] have been synthesized by electrocrystallization and studied by rf penetration depth measurements at ambient pressure and at temperatures down to 0.45 K. The crystals were found to be superconducting at ambient pressure with T c = 3.93–4.98 K, which represent the highest values of T c thus far observed at ambient pressure for an organic superconductor. Measurements of the upper critical field anisotropy are reported.