Sebastian Elsasser, Dan Wu, Martin Dressel, John A. Schlueter
The Mott-insulator \kappa-(BEDT-TTF)_2Cu_2(CN)_3 is the prime candidate of a quantum spin liquid with puzzling magnetic properties. Our THz and infrared investigations reveal that also the charge dynamics does not follow the expectations for a Mott insulator. The frequency-dependent conductivity exhibits a power-law behavior $\sigma_1(\omega)\propto \omega^n$ that grows stronger as the temperature decreases and extends all the way through the far-infrared. With $n\approx 0.8$ to 1.5 we obtain a significantly smaller exponent than predicted by Ng and Lee [Phys. Rev. Lett. {\bf 99}, 156402 (2007)]. We suggest fluctuations becomes important in the spin-liquid state and couple to the electrodynamic properties differently compared to the antiferromagnetic Mott insulator \kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl. We discuss the various possibilities of how charge fluctuations are influenced by the presence or absence of magnetic order.
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http://arxiv.org/abs/1208.1664
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