Sound Velocity Study of Bechgaard Salts (TMTSF) 2 PF 6 and (TMTSF) 2 ClO 4
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SOUND VELOCITY STUDY OF BECHGAARD SALTS (TMTSF) 2PF 6 AND (TMTSF) 2C10
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3 X. D. Shi(,2), L.Chiang( ), R.Upasani(3), P.M.Chaikin(2-3)
(0) Dept. of Physics, Univ. of Pennsylvania, Phila. , PA 19104 (2)
Dept. of Physics, Princeton Univ. , Princeton, NJ 08544
(3) Exxon Research and Eng. Co. , Route 22 East,Annandale, NJ 08801
Abstract We have studied the elastic properties of the Bechgaard salts (TMTSF) 2PF6 and (TMTSF) 2C1O 4 through their Spin Density Wave (SDW) and anion ordering transitions. Using a vibrating reed technique to measure the sound velocity and attenuation, we observed a large softening of sound velocity immediately above the SDW transition temperature, 12K, of (TMTSF) 2PF 6 followed by a large hardening below the transition. The softening results from a strong dependence of T, on stress in the most conducting direction, while the hardening results from the loss of screening from the conduction electrons and yields a value for the long wavelength electron-phonon coupling constant. The anion ordering transition at 24K in (TMTSF)2 ClO 4 is marked by a hardening of the lattice that is considerably larger than expected from the lattice constant changes. Our most interesting finding, however, is the observation of a series of steps in the sound velocity as the Field Induced Spin Density Wave (FISDW) transitions are crossed by increasing the magnetic field at low temperatures. The fact that the sound velocity steps are small at the beginning of the cascade of transitions and increase with field, even as extrapolated to low temperature, is an unexpected result from present theory and may indicate a more direct coupling of the lattice stiffness to the SDW order parameter.
The Bechgaard salts, (TMTSF) 2X where X are anions such as PF 6 , C10 4, etc, have been of great interest for the past decade because of the richness of physical phenomena they have presented [1-5]. They provide an unique opportunity for the study of interacting electron systems in reduced dimensions. Although they represented the first organic superconductors, Spin Density Wave (SDW) transitions and magnetic Field Induced Spin Density Wave (FISDW) transitions have proven to be the most interesting and unusual properties for these systems. Numerous experiments and calculations have shown that they are extremely anisotropic and have only open orbits at their Fermi surfaces. In (TMTSF) 2PF 6 at ambient pressure, a Peierls like instability leads to a SDW ground state with a T, of 12K [7]. (TMTSF) 2C10 4 is metallic and superconducting at low temperature, and application of a magnetic field (much larger than that required to destroy the superconductivity) perpendicular to the most conducting plane has dramatic effects on its electronic properties . The field reduces the effective dimensionality of the electron gas to one and leads to a density wave instability. The competition between the magnetic length and the SDW wavelength introduces a cascade of FISDW transitions [2,3,4]. Elastic measurements are very sensitive probes of second order
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