Sound Velocity Measurements in (Tmttf) 2 X Salts, X = ReO 4 , AsF 6 , SbF 6
- PDF / 339,586 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 96 Downloads / 210 Views
SOUND VELOCITY MEASUREMENTS IN (TMTTF) 2 X SALTS, X = ReO 4 , AsF 6, SbF 6 S.E. Brown* and H.H.S. Javadi, Los Alamos National Laboratory, Los Alamos, N.M. 87545 R. Laversanne, Centre de Recherche Paul Pascal, CNRS, Domaine Universitaire de bordeaux 1, 33405 Talence Cedex, France
ABSTRACT Young's modulus E measurements using the vibrating reed technique are reported for salts of the (TMTTF) 2 X series, where X = ReO 4 , SbF 6 , AsF 6 . All three exhibit softening of the modulus at the structureless transition, with the largest effect in the material with the non-centrosymmetric anion ReO 4. The behavior is typical of quadratic coupling to an order parameter plus effects from fluctuations. There is a large stiffening of E below the anion ordering temperature of (TMTTF) 2 ReO 4 . INTRODUCTION The charge transfer salts (TMTCF) 2 X (C = Se, S) consist of flat molecules of TMTCF which are stacked in a zig-zag pattern to form a conducting path along the stacking direction. A complete charge transfer occurs from the conducting stacks to the counterions X, which are positioned in structural cavities and separate the sheets of TMTCF stacks from each other. A combination of interactions in these highly tunable systems leads to a wide variety of low temperature phases, including the formation of charge/spin density waves (CDW/SDW), superconductivity (SC), spin-Peierls (SP) and anion ordering (AO). Some of the phase transitions are linked to the type of counterion, which has two varieties: non-centrosymmetric [dipolar, (SCN', NO 3 ,...), or tetrahedral(ReO 4 -, C10 4 -,...)], or centrosymmetric [octahedral (PF6-, AsF 6 -, SbF6-.), or single ions (Br-,...)]. For example, the salts with non-centrosymmetric anions are observed to order at various temperatures. However, despite the concentrated effort of many researchers, some questions concerning the transitions remain unanswered. The importance of electron-electron as compared to electron-phonon interactions in driving some of these instabilities is still controversial. An outstanding question regarding the TMTTF salts is the role of charge localization in the competition between different instabilities. Perhaps related to the more localized nature of (TMTTF) 2 X (compared to the selenium analogs) is the observation of a "structureless transition" (SLT) in several species (e.g. AsF 6 , SbF 6 , and ReO 4 ). The accepted signature of the SLT is a sharp change in slope of the conductivity and thermopower but no observed change in the Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
246
magnetic susceptibility [1]. Below the SLT the conductivity is activated, reminiscent of many systems with Peierls instabilities [2], although no evidence for a structural transition has been found in x-ray scattering [3] or infrared spectroscopy [4] experiments. However, more recent x-ray studies suggest that more thorough investigations are in order [5]. The observed distinction between charge and spin degrees of freedom (as suggested by the substantial effect of the structu
Data Loading...
