Tuneability and criticality in a three-dimensional stacked molecular system

Abstract

With the changing of physical properties through chemical substitution in mind, this work shows theapplicability of this approach to organic based spin-Peierls (SP) systems. To demonstrate this we have useda well known system, potassium TCNQ, that undergoes an SP transition atTSP=396 K. Simply substitutingprotons for fluorine, using TCNQF4, shows a decrease in the coupling strength between TCNQ anions where thespin-Peierls transition is dramatically reduced in temperature, with KTCNQF4showing aTSPat approximately160 K, which is due to changes of the electron spin density across the molecule. Muon spin relaxation is a moresuitable technique for determining the magnetic properties of these systems and measurements were conducted onboth KTCNQ and KTCNQF4in order to study the behavior of the SP transition. This has highlighted and shownthat both transitions exhibit the same behavior and so are indeed similar; however, we have also succeeded inbeing able to tune the transition. Estimates of the dynamic critical exponents from both samples are obtained fromthe muon data and are found to be∼0.33, corresponding to a three-dimensional (3D) antiferromagnetic systemimplying that spin fluctuations associated with the SP state are correlated not just along stacks of dimerizedTCNQ anions. This result extends the understanding of the SP phase transition in the KTCNQ system beyondthat determined from purely structural studies.