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TT: Fachverband Tiefe Temperaturen

TT 6: Postersession Correlated Electrons: (General) Theory, Low-Dimensional Systems, Kondo Physics, Heavy Fermions, Quantum-Critical Phenomena

TT 6.30: Poster

Montag, 23. März 2009, 13:00–16:45, P1A

Multi-frequency ESR studies on (TMTTF)₂X: evidence of the anisotropic Zeeman interaction in the charge ordered state — •S. Yasin^1,2, B. Salameh², M. Dumm², and M. Dressel² — ¹Institut Hochfeld-Magnetlabor Dresden, Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany — ²1. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany

We studied the charge ordered (CO) state of the quasi 1-D S = 1/2 quantum spin chains (TMTTF)₂X (X=SbF₆ and AsF₆) by comprehensive W-Band (95 GHz), Q-band (34 GHz) and X-Band (9.5 GHz) ESR experiments between 4 and 300 K in order to explore the nature of the exchange interaction in the CO state. At high temperatures, both compounds show a linear decrease of the linewidth with decreasing temperature; this behavior does not depend on the applied microwave frequency as well as the anisotropy of both, linewidth and g-value. Below T_CO, the breaking of the inversion symmetry of the (TMTTF)₂-dimers results in additional contributions Δ H_CO to the ESR linewidth. While the linewidth is frequency independent along the three principle magnetic axes a, b′, and c*, it is substantially enhanced for the Q- and W-band measurements along the diagonal of a−b′ plane. The enhanced linewidth along 45^o in the a−b′ plane below T_CO shows a quadratic frequency dependence which is characteristic for anisotropic Zeeman interaction. From this finding we can conclude that the charge order leads to two inequivalent magnetic sites. We will compare this result to one obtained on anion-ordered TMTTF salts where a different charge-order pattern was proposed.