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
analysis was used to show that the effects are not due to sulfur or carbon diffusing to the surface. Also, the process is reversible; room-temperature data is repeated when the sample cools to room temperature.Presumably, the surface-state binding energy decreases with increasing temperature permitting electrons to spill out of this state and thus reducing its emission amplitude. The fact that an observable shoulder remains at 400 °C (above the Curie temperature) indicates that the temperature-dependent shift in binding energy for this surface state is less than its binding energy, but clearly not zero.In summary, this paper reports a previously unobserved feature in the Ni(100) photoemission spectra. Within experimental limits this feature satisfies all conditions for a surface state, and it is identified as resulting from a magnetic surface state near I\ This new result places important constraints on the interpretation of ESP experiments in terms of bulk band structure.The compound ditetramethyltetraselenafulvalene-hexafluorophosphate [(TMTSF) 2 PF 6 ] is a highly conducting linear-chain organic crystal. At atmospheric pressure, Bechgaard etal. 1 have observed a conductivity o^ 10 5 (12 cm)" 1 near a temperature of 20 K. Below about 15 K the material enters a semiconducting state which resembles the Peierls state observed in a number of similar compounds. 2 At a hydrostatic pressure P = 1.2 GPa, Jerome etal. 3 have found that the semiconducting transition is absent and that there 444 (1976). . n The resonance lamp radiation was not polarized. At Hoo = 21.2 eV, incident angles corresponding to k\\ = 0.0 A" 1 , 0.35 A" i , and 0.69 A" > were 33°, 23°, and 3° which produce incident "p" components of approximately 27%, 20%, and 2% of the total field. 12 Resolution in the retardation mode is given by 0.007^ (small aperture) or 0.02^ (large aperture), where E p = pass energy of the analyzer. Based on source size and pass-energy settings, selected spectra were obtained using an estimated resolution of approximately 40 meV. These spectra did not differ from spectra taken with use of an estimated resolution of approximately 60 meV. At values of resolution worse than 100 meV the Fermi edge smears out and the resolved peak at E? becomes a shoulder (at what appears to be a higher binding energy; see Ref. 9).is a new transition at 0.9 K where a increases by over 10 5 . With the application of magnetic fields of order 200 Oe, the transition temperature T c drops, and the magnitude of the rise in a decreases to ~ 5. Subsequent ac susceptibility measurements 4 at a frequency v-68 Hz show an anomaly indicative of a transition into a diamagnetic state. Jerome etal? have suggested that (TMTSF) 2 PF 6 at P = 1.2 GPa is a BCS superconductor. 5 Although there have been several theoretical models for high conductivity in one-dimensional A partial Meissner effect, fully diamagnetic shielding signals, and large anisotropics in the upper and lower critical fields in ditetramethyltetraselenafulvalene-hexafluorophosphate [ (TMTSF) 2 PF 6...
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