Temperature dependence of the intrastack exchange interaction for a quasi-one-dimensional organic conductor

Abstract

The electron spin resonance ͑ESR͒ of (pyrene) 12 (SbF 6 ) 7 is measured as function of crystal orientation for temperatures above and below the Peierls transition temperature T P ϭ116 K. The principal values of the pyrene g tensor ͑2.00274, 2.00235, 2.00225͒ are determined. In spite of a defect concentration amounting to 2 -3ϫ10 Ϫ2 per formula unit, the ESR linewidth is below 60 mG in the metallic phase. The temperature dependence of the intrastack exchange interaction between localized defects and conduction electrons is derived. The exchange integral decreases by a factor of 8 between T P and room temperature.

Electron Spin Resonance: A Major Probe for Molecular Conductors

“…In the cases of perylene or fluoroanthene salts, Peierls phase transitions have been observed upon cooling, inducing a large modification of the spin relaxation. Moreover, because of the extremely narrow lines in the metallic phase, pulsed EPR was successfully applied to deduce the longitudinal and transverse relaxation times. − In particular, a decay of the correlation function as exp(−( t /τ) 3/2 ), characteristic of one-dimensional systems was observed . Accurate low field magnetometers were built with these materials taking advantage of the very small line width. , Unfortunately, this application has been limited by the instability of these materials at room temperature.…”

“…Moreover, because of the extremely narrow lines in the metallic phase, pulsed EPR was successfully applied to deduce the longitudinal and transverse relaxation times. [290][291][292][293][294][295][296][297][298][299][300] In particular, a decay of the correlation function as exp(-(t/τ) 3/2 ), characteristic of one-dimensional systems was observed. 301 Accurate low field magnetometers were built with these materials taking advantage of the very small line width.…”

Comparison of 2-D ESR imaging techniques in two frequency ranges

“…A fit of the well-known relation [28] M+(ri'2r) ( 2v 2y 91 (7) M+(r,, 0) = exp Ta 3 to the data M+(r,. 2r) yields as fit parameters the transverse relaxation time T 2 and, in the case of conduction electron imaging, the diffusion coefficient D. It is also possible to combine the imaging scheme with the inversion recovery sequence.…”

“…7b (for experimental parameters, see figure legend) reveals a difference of almost one order o f magnitude in the transverse relaxation times of these two crystals. Indeed, only the larger one is a (FA)zPF 6 crystal, while the smalter one is a pyrene radical cation salt, (PY)I2(SbF6) 7, with a much shorter T 2 [7].…”

“…While the fluoranthene cations arrange in closely spaced stacks along which a quasi-l-D conduction band is formed, the PF 6 anions are located in rows between these stacks [6]. At about Tp = 186 K, crystals of (FA)2PF 6 undergo a metal-semiconductor "Peierls transition" [3,5,7,8]. The ESR signal in the high-temperature metallic phase originates from the conduction electron spins [9].…”