20 K crystal structure, electrical transport, electronic band structure, scanning tunnelling microscopy and pressure–RF impedance studies on the organic conducting salt κ-(BEDT–TSF)₂Cu[N(CN)₂]Br

Abstract

The electrochemical synthesis of κ-(BEDT–TSF)2Cu[N(CN)2]Br, which is virtually isostructural (Pnma) with the 12.5 K ambient pressure superconductor κ-(BEDT–TTF)2Cu[N(CN)2]Br, has been systematically investigated. The electrical resistivity of κ-(BEDT–TSF)2Cu[N(CN)2]Br was measured in the a[ρ300= 2(1)× 10–2Ω cm] and b[ρ300= 1.7(0.7)× 101Ω cm] crystallographic directions. The resistivity decreased monotonically from 300 to 4 K following a T2 relationship, but a superconducting state was never achieved. Tight-binding band calculations showed that the Fermi surface of κ-(BEDT–TSF)2Cu[N(CN)2]Br is indistinguishable from the BEDT–TTF analogue. STM images were obtained of both the radical cation and the polymeric anion layers. The images were in agreement with partial electron density plots. A 20 K X-ray structure of κ-(BEDT–TSF)2Cu[N(CN)2]Br was obtained and is discussed in detail. One of the BEDT–TSF ethylenedithio groups was still disordered at 20 K; this may have been responsible for the failure to observe superconductivity.