Restoration of Metallic Behavior in Organic Conductors by Small Electric Fields

Abstract

The electrical conductivity of two linear-chain organic compounds in their semiconducting states is found to increase drastically with the application of electric fields of ~ 10 mV/cm and with the increase of frequency to ~10 GHz. Also, the electron-spin-resonance line is found to switch abruptly from unresolvable to fully resolved (narrow and metallike) at fields of 0.1 V/cm. One speculative explanation of these observations is the presence of spin-density waves. PACS numbers: 71.30.+ h, 72.15.Nj, 76.30.PkMany organic charge-transfer salts exhibit anisotropic metallic conductivities and undergo transitions to semiconducting states below characteristic temperatures T MS . Recently Bechgaard and co-workers 1 have synthesized and studied some new salts, including (TMTSF) 2 PF 6 (tetramethyltetraselenafulvalenium hexafluorophosphate), whose dc conductivity increases beyond 10 5 (Q, cm)"* upon cooling to ~ 20 K and then decreases rapidly below T MS ~ 15 K. Subsequently Jerome etal. 2 have observed that hydrostatic pressure depresses T MS to 0 K near 9 kbar and that superconductivity appears at higher pressure (T c~0 .9 K at 12 kbar). Pedersen, Scott, and Bechgaard, 3 have examined the electron-spin resonance (ESR) of (TMTSF) 2 PF 6 and found that the resonance line narrows monotonically as the conductivity increases, then rapidly broadens and loses intensity near T MS .We have found that the conductivity and spin paramagnetism of (TMTSF) 2 PF 6 (and the isostructural AsF 6 compound) are drastically affected by small applied electric fields at atmospheric pressure and temperatures well below their nearly identical T MS values.Samples of (TMTSF) 2 PF 6 and (TMTSF) 2 AsF 6 were grown electrochemically from chlorobenzene with use of tetrabutylammonium salts as supporting electrolytes. TMTSF was prepared by a modified procedure not requiring carbon diselenide. 4 Typical samples were ~ lx 0.05x 0.02 cm 3 . Scanning electron microscopy revealed that a number of crystals in a batch were hollow with large voids extending deeply into the needles. These needles exhibited at least two types of morphology; flat (parallelepiped cross section) and polygonal (octagonal cross section). The (TMTSF) 2 PF 6 and the AsF 6 salts were strictly isomorphous with the monoclinic structure described by Jerome et al. 2

Physical phenomena in new organic conductors

“…Соответственно, ниже точки перехода быстро падает парамагнитный вклад в магнитную воспри-имчивость, измеряемый как по интенсивности сигнала ЭПР, так и в стати-ческих измерениях. В первых измерениях с поликристаллическими образцами 41 .…”

The structure of di(2,3,6,7-tetramethyl-1,4,5,8-tetraselenafulvalenium) hexafluorophosphate, (TMTSF)2PF6, the first superconducting organic solid

“…The absence of any precursor effects, such as diffuse X-ray scattering (Pouget, 1980), makes it unlikely that the metalto-insulator transition at 12-17 K in (TMTSF)2PF 6 is of the same Peierls nature as the transition observed in most other quasi one-dimensional conductors. Alternatively, it has been suggested that this transition originates in a distortion of spin-density waves (Walsh et al, 1980). Since information about the structural details is essential for the understanding of the phenomena © 198 i International Union of Crystallography observed in these compounds, we report the crystal structure of (TMTSF)2PF 6 at ambient pressure and temperature (295 K).…”

Vibrational spectra of two new organic semiconductors: tetrathiafulvalene (TTF) and tetramethyltetraselenafulvalene (TMTSF) salts of paranitrophenylmalononitrile (PNMA)

“…[2] and [3]), and defining dimer modes, known that ~i m o d e s of ions appear unchanged in frequency in the Raman spectrum, whereas these same modes can appear in [5] Qlg = QI + Q4, Q2, = Q2 + Q3 the IR spectrum but are considerably shifted in frequency due Q l u = Q l -Q 4 , Q 2 u z Q z -Q 3 Can. J. Chem.…”

“…This interaction produces a change in the equilibrium bond length only, i.e., there is no frequency change from this linear perturbation (9-13). The last equation [3] represents the vibrational energy of the four independent site modes of frequency oO.…”

“…The organic conducting salts of tetramethyltetraselenafulvalene (TMTSF) of the type (TMTSF),X, called Bechgaard salts, have attracted great attention due to the discovery of superconductivity in these systems below 10 K, the rich variety of electronic and magnetic ground states depending on the anion present, the temperature, and the pressure (1)(2)(3)(4)(5). In such compounds one finds in the presence of nonsymmetric inorganic anions a high temperature metallic state which extends down in temperature to 100 K below which an anion ordering transition is observed.…”