Spin-Peierls transition

Radicals have been known for many years to form organic paramagnetic materials with numerous magnetic properties (ferro- or ferri-magnetism, spin Peierls transition, spin frustration, spin ladder systems) (see [51-60] for verdazyl radicals, [61-68] for thiazyl radicals, [69] for nitronyl nitroxide and [70-78] for Tempo radicals) (Fig. 6). When they are in their cationic form, they are valuable candidates for an association with the M(dmit)2 systems they will then provide the magnetic properties thanks to their free electron(s), whereas the M(dmit)2 moieties will provide the electrical properties. 

According to the factor-group analysis, there are 18 Raman and 9 IR active new vibrational modes in the low-temperature structure of CuGe03, below the temperature of the spin-Peierls transition Tc=14 K. While two of the Raman-active folded modes were clearly observed in the very first optical experiments, no traces of the IR folded modes could be found for a long time. We have observed IR folded modes for the first time, measuring... 

Fig. 10. Illustration of the molecular displacements occurring in the a,-Cp plane in [TTF] [Cu(tfd)2] below the spin-Peierls transition temperature of 12 °K. Only the [TTF]+ units are shown for clarity however, the translation of the center of mass of the [Cu(tfd)2] units is indicated (Ref. 51)...

An additional feature of the temperature dependent X-ray scattering is the persistence above Tc of intensity at the superlattice positions51). This is consistent with a soft phonon mode at a wave vector commensurate with the changes that occur on dimerization. It has been suggested that this low frequency lattice mode may be a requirement for the observation of a spin-Peierls transition. 

The spin-Peierls transition (18, 19) is a peculiar magnetoelastic transition that may take place in a limited series of quasi ID insulating systems (20, 286). It is interesting to note that, for once, this behavior had been theoretically predicted for organic free radicals (18, 19) long before its experimental observation. It happened that the first experimental observation of a spin-Peierls system was reported for dithiolene systems, namely, (TTF)[M(tfd)2], where M is Cu or Au (25, 26). However, since bis(l,2-dithiolene) complex-based compounds have a tendency to form ID stacks (see above), this priority is not truely amazing. 

A large number of papers have appeared on this research reporting additional theoretical and experimental studies on the (TTF)[M(tdf)2] (M = Cu, Au) compounds, and detailed reviews are available (20, 286). Note, however, that these dithiolene complex-based compounds were the first members of the quite limited series of spin-Peierls systems. Among these, another related, selenium analogue dithiolene complex-based compound, (TTF) Cu[Se2C2(CF3)2]2, also undergoes a spin-Peierls transition at 6 K (290). 

Coupled ID electronic and magnetic properties have been reported in (Per)[M(mnt)2] complexes (131). Some of these systems undergo simultaneous Peierls and spin-Peierls transitions, but the existence of a real interplay is not yet established. This work was reviewed in Sections II and III. 

There are four different potential instabilities in quasi-one-dimensional conductors at low temperature the charge density wave, spin density wave, spin-Peierls transition, and superconducting transitions ST (superconductivity triplet) and SS (superconductivity singlet). Since superconductivity is treated in another chapter, we ignore it in this contribution. 

Note In the spin-Peierls transitions on the Ni, Pt, the driving forces seem to be electron-phonon interactions and not spin-phonon, since the field dependence of the transition temperatures are of Peierls type and not spin-Peierls. 

Spin-Peierls Transition in MEM(TCNQ)2 and Related Compounds... 

In any case the spin-Peierls transition is driven by one-dimensional pre-transitional structural fluctuations [46]. Such fluctuations start to develop at some temperature TF above TsP. The effect of these critical fluctuations is to induce a local pairing of the spins which leads to an observable deviation of the magnetic susceptibility x below TF from the general Bon-... 

The spin-Peierls transition has been the object of several recent papers. Examples of the first case, with p = 1, are provided by the insulating alkali metal simple salts [47] (see below), and examples of the second case, with p = 3 or I, are given by the salt MEM(TCNQ)2 [17-19,46], by the salts of the (TMTTF)2X series [46,48], or by the salts of the (BCPTTF)2X series (BCPTTF = benzocyclopentyltetrathiafulvalene) [46,49]. 

In ideally one-dimensional systems, only intrachain electron-phonon and spin-phonon couplings are, within mean-field approximation, at the origin of electronic-Peierls and/or spin-Peierls transitions, respectively. In real systems, such as the TCNQ salts under concern here, it is clear, however, that one should take properly into account the coupling of the electrons to external potentials also and, in the first case, to the periodic electrostatic cation potential. 

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