Power-law dependence of the optical conductivity observed in the quantum spin-liquid compound \kappa-(BEDT-TTF)2Cu2(CN)3

TL;DR

This study reveals a power-law frequency dependence of optical conductivity in the quantum spin-liquid compound ppa-(BEDT-TTF)2Cu2(CN)3, indicating unconventional charge dynamics and fluctuations distinct from typical Mott insulators.

Contribution

It demonstrates that charge fluctuations in the quantum spin-liquid ppa-(BEDT-TTF)2Cu2(CN)3 follow a power-law behavior, challenging existing theoretical predictions and highlighting the role of fluctuations in spin-liquid states.

Findings

Optical conductivity exhibits a power-law dependence with an exponent between 0.8 and 1.5.

The observed exponent is smaller than theoretical predictions by Ng and Lee.

Charge fluctuations are influenced by magnetic order or its absence, affecting electrodynamic properties.

Abstract

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.