Bayreuth 1998 – wissenschaftliches Programm

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CP: Chemische Physik

CP 12: Spektroskopie von Molekülen und Kristallen

CP 12.17: Poster

Dienstag, 10. März 1998, 18:00–20:00, P1

ESR studies on the quasi one-dimensional organic spin systems (TMTTF)₂X (X=PF₆ and ClO₄) — •M. Dumm¹, M. Dressel¹, A. Loidl¹, and L. K. Montgomery² — ¹Experimentalphysik V, Universität Augsburg, Universitätsstr. 1, D-86135 Augsburg — ²Department of Chemisty, Indiana University, Bloomington, IN 47405, U.S.A.

We investigated the magnetic properties of single crystals of the low-dimensional organic spin chain compounds (TMTTF)₂PF₆ and (TMTTF)₂ ClO₄ by X-band ESR spectroscopy at temperatures 4 K ≤ T ≤ 300 K. The results of the tetramethyltetrathiafulvalene (TMTTF) compounds are compared with those of the one-dimensional metals (TMTSF)₂X. The ESR linewidth of the sulfur salts is two orders of magnitude smaller than it is in the selenium analogs. This is in good agreement with the fact, that the conductivity of the 1-dim semiconductors with localized spins on the TMTTF dimers is significantly lower. At high temperatures (T>20 K) the ESR intensity of (TMTTF)₂X can be well described by an alternating spin 1/2 antiferromagnetic Heisenberg chain. In (TMTTF)₂ClO₄ the alternation parameter α changes at the first-order structural phase transition (T_AO = 72 K). Below 20 K we find an additional transition of unknown origin. At T_SP=19 K (TMTTF)₂PF₆ undergoes a spin-Peierls transition. Below T_SP the ESR signal broadens and the integrated ESR intensity vanishes with decreasing temperature. This behavior can be well described by mean field theory. We determinated a singlet-triplet gap Δ(0) ≈ 30 K.