We report on calorimetric measurements of the organic spin-density wave (SDW) conductor (TMTSF)2PF6. The experiments were carried out from 8 to 20 K, with emphasis on the neighborhood of the 12.1 K SDW transition. The transition anomaly was less than 1% of the total specific heat at that temperature, and deviates substantially from the shape expected for mean-field theory for temperatures approximately 0.15 K on either side of the transition. A comparison to the thermodynamics of the 2D Heisenberg antiferromagnet is made.PACS numbers: 75.40.Cx, 65.50.+m, 71.45.Gm, 75.30.Fv The organic conductor (TMTSF)2PF6 is representative of a series of materials, known as the Bechgaard salts, that display a fascinating variety of electronic and structural behavior, including spin-density waves (SDW), superconductivity, magnetic-field-induced spin-density waves (FISDW), anion order-disorder transitions, and more [l]. Other members of the family are created by replacing the PF6 anion with another, such as NO3, SbF 6 , AsF6, or ReC>4. In many of these, a metal-insulator transition is known to occur at about 10 K at ambient pressure, all with very similar physical properties. For instance, the conductivities are highly anisotropic, which is a characteristic of materials unstable to density wave formation. As in antiferromagnets, the magnetic susceptibility is anisotropic below the transition temperature [2]; this, coupled with the lack of any evidence for structural changes [3], indicates that the transition is of the spindensity wave type. The wave vector associated with the ground state has been established by NMR measurements to be incommensurate with the lattice, but close to (Ki,0)[4l.Our investigation of the specific heat of (TMTSF)2PF6 in the transition region was motivated by questions regarding the formation of the spin-density wave phase, and for purposes of comparison to other phase transitions involving electron structure. These include magnetic transitions, charge-density wave (CDW) systems, superconductivity, and the field-induced spin-density wave states.Superconductors, CDW systems, and SDW systems share the feature that the overall free energy of the system is lowered by the opening of a gap in the conduction electron system. The density wave states are characterized by their low dimensionality, and hence fluctuations often contribute to the thermodynamic properties over a wide temperature range. In the case of CDW systems in particular, a large enhancement over that expected from the electronic degrees of freedom is observed [5]. The behavior is believed to be the result of phonon contributions, as suggested by MacMillan [6], and hence, should not be present for SDW materials. For the CDW systems, as in superconductors, deviations from the weak-coupling (mean-field) theory lead to the same behavior as for the 3D XY model. However, this may not be true for the SDW systems, which seem to be more like Heisenberg systems.Magnetic transitions arising from the ordering of local moments are often characterized by ver...
Kwok, Griiner, and Brown Reply: Mozurkewich 1 is correct when he points out that our specific-heat 2 data are not consistent with fluctuations in the Gaussian approximation. There are several reasons why we did not elaborate on this in our Letter. Most importantly, we did not believe that a specific analysis of our data would result in an unambiguous interpretation, as we will discuss below. In addition, the main point of the paper was to identify a scaling between the magnetic susceptibility X and the specific heat c in the fluctuation regime of a Peierls transition. The calculations by Chandra, 3 which predicted the relation dx/dT -c, are indeed limited to the region where the Gaussian approximation is appropriate. Although detailed calculations have not been performed beyond this approximation, it was argued that the relation between the specific heat and magnetic susceptibility has a more general validity. This conjecture is supported by our results.From the experimental side, we preferred to emphasize the scaling between the susceptibility and specific heat, rather than the issue of critical behavior. The classic experimental studies demonstrating critical behavior and the evaluation of exponents relied on one or more thermodynamic quantities obeying a power law over decades of reduced temperature. 4 Aronovitz, Goldbart, and Mozurkewich 5 recently pointed out that a correction to scaling
We have observed a softening of the Young's modulus accompanied by an increase in the internal friction, when the dc bias exceeds the threshold for nonlinear conduction in the spin-density-wave state of (TMTSF)&PF6. No field dependence of the elasticity is detected above the spin-density-wave transition near 12 K. The results are reminiscent of effects associated with the depinning of charge-density waves, except that here, the extrinsic coupling to the lattice may have profound effects on the dynamics of the system.
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