Thermodynamics of the spin-density wave transition in (TMTSF)2PF6

Abstract

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...

Spin-density-wave state in(TMTSF)2PF6: AValfells1, Kuhns2, Kleinhammes3 et al. 1997Phys. Rev. B

“…This suggests that fluctuations are important over temperature scales on the order of at least 1 K around T SDW . We contrast this with calorimetric data 27 where the signature of the incipient transition starts at 0.3 K above T SDW .…”

Colossal Seebeck Coefficient of Hopping Electrons in(TMTSF)2 PF6

“…works (See the supplement). It is concomitant with a sharp jump in resistivity and a lambda-anomaly in specific heat [25,26], all three confined to a very narrow window near the critical temperature.…”

Glass transition in the spin-density wave phase of (TMTSF)2PF6

“…With this technique, a PT law is recovered (with a small variation of p from 18.5 to 19 m3/molK between 2.5 and 8 K, in excellent agreement with our data above our anomaly at 3.5 K). In addition it also sho~s the SD%' transition as a small bump at 12.1 K. Other specific heat measurements by Coroneus, Alavi, and Brown [15] give between 8 and 12 K a lattice PT contribution which is almost 3 times smaller.…”