Reversed Peierls transition

Karutz FO, von Schutz JU, Wachtel H, Wolf HC (1998) Optically reversed Peierls transition in crystals of Cu(dicyanoquinonediimine)2. Phys Rev Lett 81 140-143... 

Problem 9.1. The optically-induced reversed Peierls transition ... 

In the Cu(DCNQI)2 radical-anion salt in the neighbourhood of the Peierls phase-transition temperature, it was found that the insulating state can be switched optically on a time scale of less than 20 ps into the conducting state (F. O. Karutz, H. C. Wolf et al, Phys. Rev. Lett. 81,140 (1998)). This optical switching process was termed a reversed Peierls transition . From the current transients, it was found that the switched volume must be at least 100 times larger than the directly photo-... 

The Peierls transition can be reversed in some DCNQI crystals through optical excitation. The conductivity of the crystal is in this case increased by up to eight orders of magnitude by a short light pulse. To detect this effect, the (transient) electric conductivity must be measured in the excited state. More on this subject will likewise be presented in Chap. 12. 

A nice example is provided by the radical-ion salts of DCNQI. We have already pointed out in Sect. 9.5 that these include a great variety of salts which differ in terms of the metal ions or in terms of the substituents on DCNQI (CH3,1, Br and others), but which have essentially the same crystal structures (Fig. 1.7). The minor differences in the intermolecular spacings and the mutual orientations of the molecules within the crystal can however lead to great differences in their physical properties [13]. In Figs. 9.15 and 12.5, it becomes clear that in contrast to normal Cu dimethyl DCNQI, i.e. hs, the deuteration of the six CH3 protons to <4 leads to a Peierls transition in the range of ca. 70 K from a metallic to an insulating phase, reversibly and in a very narrow temperature interval. This behaviour could be made use of for a molecular switch. If, as in hg, the phase transition is suppressed, then this is because the Cu ions act as bridges between the DCNQI stacks and therefore convert the one-dimensional system into a three-dimensional one (c Sect. 9.5). 

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