Anomalous Temperature Dependence of the Magnetic Susceptibility of Quasi-One-Dimensional Metals

Abstract

The temperature dependence of the g values measured for the series of alloys (TSeF) x -(TTF) 1 _ X (TCNQ) (where TSeF is tetraselenafulvalene, TTF is tetrathiafulvalene, and TCNQ is tetracyanoquinodimethane), 0^*^0.68, in the temperature range 60 K<T<300 K is presented. It is shown that the experimentally observed temperature dependence does not agree with the expected behavior based on significant effects of Peierls fluctuations on the magnetic susceptibility.One of the unique features of quasi-ID (one-dimensional) metals in comparison to 3D metals is the coexistence of an electrical conductivity that increases with decreasing temperature and a magnetic susceptibility that is strongly decreasing with decreasing temperature. The decrease in the susceptibility is typically 1 " 3 an order of magnitude bigger than the decrease that would be caused 4 by the singularity of the density of states of a one-dimensional band at the band edges. Therefore, the transport, if due to one-or twoelectron processes, indicates no gap in the single-particle excitation spectrum while the susceptibility data suggest that the spectrum of magnetic excitations has an appreciable gap which reduces significantly the measured susceptibility in the regime of metallic conductivity. For the case of TTF-TCNQ, for example, this reduction is by a factor of 2 in the temperature range 100 K< T < 300 K.This dichotomy has led to two conflicting points of view. In the one proposed by Lee, Rice, and Anderson, 5 the interactions between stacks are very weak. Peierls fluctuations develop slowly in amplitude and coherence length over a wide temperature range; and, only when the correlation lengths are very long, the weak interstack coupling locks the phases on the various stacks in a three-dimensional transition. The decreasing susceptibility is associated with the pseudogap that develops in this temperature range; and the failure to see evidence for a gap in the conductivity is attributed 2 to the increasing importance of the Frolich sliding conductivity. On the other side is the view 6 that the interstack interactions are not so weak, that they play an important part in the development of an appreciably amplitude and coherence length, and that, therefore, fluctuations occur in a much narrower temperature range. Just which of these conflicting views is nearer to the truth is crucial for our understand-ing of TTF-TCNQ.In this Letter we present new experimental results showing that the drop in susceptibility in the regime 60 K<T<300 K is not related to Peierls fluctuations. This is exactly opposite to the predictions of the first, quasi-one-dimensional, point of view. The experiments from which we derive this conclusion are the studies of the temperature dependence of the g value of the single EPR absorption line measured for the series of solid solutions (TSeF) x (TTPV x (TCNQ), 0^x ^ 0.68, in the temperature regime of metallic conductivity.Yafet showed 7 that the measured g shift is given by the g shift averaged over the Fermi surface under three assumpt...

Electron Spin Resonance: A Major Probe for Molecular Conductors

“…It is worth noting that resolved EPR lines have also be reported in HMTTF-TCNQ and HMTTF-TCNQF 4 (HMTTF stands for hexamethyl-TTF) − Note that this decomposition technique implies that the line width is frequency independent as seen for TTF-TCNQ by Tomkiewicz et al Some frequency dependence of the EPR line width has however been reported later . Figure shows the susceptibility decomposition for TTF-TCNQ with the contribution of each kind of stacks and the total susceptibility which is consistent with the static result .…”

Conduction electron spin resonance measurements on TTF-TNNQ and (TMTTF)2BF4 under hydrostatic pressure

“…For (TMTTF)2BF4 this is comprehensible since g is a molecular property, and contracting the lattice one does not expect to change molecular quantities. In the case of TTF-TCNQ the constancy of g means more, since the molecular g-factors of each chain are mixed together with the respective spin susceptibilities giving a common g value [20] :…”

Transverse and longitudinal resistivities of pure and irradiated TMTSF-DMTCNQ

“…Indeed, a few percent of chemical impurities have the same effect on the conductivity as defects created by irradiation [5,6]. On the other hand, alloying with isostructural molecules results in only a smooth variation of the conductivity, especially for low concentrations [20].…”