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Electrodynamics and first-principles phonon calculations for the spin-liquid system κ-(BEDT-TTF)2Cu2(CN)3

The dimer Mott insulator κ-(BEDT-TTF)2Cu2(CN)3 showing an anomalous dielectric [1] and optical response [2] has been in the focus of intense activity in the last years. No definitive understanding has been achieved since experimental evidence [3] excludes theoretically predicted electric dipoles on molecular dimers [4]. Variable-range hopping in dc transport and dielectric response featuring relaxor-like ferroelectricity demonstrated an unforeseen inherent heterogeneity suggested to originate in the anion network [5]. Here we present broad-band electrodynamic investigations combined with first-principles density-functional calcula¬tions of the ground state and phonon vibrations [6]. The obtained results give insight into the origin of disorder within the anion network and the resulting domain structure. Moreover, a central role of the cation-anion coupling and the lattice phonons based on their joint movements in the anomalous terahertz response is revealed. We feel that the quantum spin liquid state in κ-(BEDT-TTF)2Cu2(CN)3 can only be understood be taking the particular formation and coupling of the BEDT-TTF molecules to the Cu2(CN)3 anions into account. [1] M. Abdel-Jawad et al., Phys.Rev. B 82, 125119 (2010). [2] K. Itoh et al., Phys. Rev. Lett. 110, 106401 (2013). [3] K. Sedlmeier et al., Phys. Rev. B 86, 245103 (2012). [4] C. Hotta, Phys. Rev. B 82, 241104 (2010) ; M. Naka and S. Ishihara, J. Phys. Soc. Jpn. 79, 063707 (2010) and 82, 023701 (2013) ; H. Li et al., J. Phys.: Cond. Matt. 22, 272201 (2010) ; S. Dalay et al., Phys. Rev. B 83, 245106 (2011). [5] M. Pinteric et al., Phys.Rev. B 90, 195139 (2014). [6] M. Dressel, to be submitted (2015).

electrodynamic response; DFT calculations; phonon vibrations; spin liquid

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