Pressure and temperature dependence of interlayer spin diffusion and electrical conductivity in the layered organic conductorsκ-(BEDT-TTF)2Cu[N(CN)2]X(X = Cl, Br)

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Abstract

A high frequency (111.2--420 GHz) electron spin resonance study of the interlayer spin diffusion is presented in the conducting phases of the layered organic compounds, $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]$X$ ($\ensuremath{\kappa}$-ET${}_{2}$-$X$), $X=\text{Cl}$ or Br. The interlayer spin cross relaxation time ${T}_{x}$ and the intrinsic spin relaxation time ${T}_{2}$ of single layers are measured as a function of temperature and pressure. Spin diffusion is two dimensional in the high temperature bad-metal phase (i.e., electrons are confined to a single molecular layer for longer than ${T}_{2}$). The interlayer electron hopping frequency ${\ensuremath{\nu}}_{\ensuremath{\perp}}=1/(2{T}_{x})$ decreases along the bad-metal to Mott insulator crossover and increases along the bad-metal to normal metal (or superconductor) crossover. The density of states (DOS) is determined from a comparison of ${T}_{x}$ and the interlayer resistivity. In the bad-metal phase it is four to five times larger than the DOS calculated from the electronic structure neglecting electron correlations. In $\ensuremath{\kappa}$-ET${}_{2}$-$X$ the DOS increases with pressure along the bad-metal to normal metal crossover. Results are compared with predictions of the dynamical mean field theory.

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