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Charge order in dimerized κ-(BEDT-TTF)2Hg(SCN)2X: examining the magnetic anisotropy

Electronic multiferroicity is at the forefront of investigations in solid state physics due to the promise of practical applications as well as its fundamental character. Among many candidate materials there has been a recent increase of interest in the κ-(BEDT-TTF)2CuX family of quasi-2D dimerized organic conductors thanks to the pronounced dielectric response found in the magnetically-ordered phase [1]. Interestingly, in these compounds optical and NMR spectroscopies do not find any charge disproportionation, which is a basic prerequisite for electric dipoles [2, 3]. On the other hand, a mercury-based system κ-(BEDT-TTF)2Hg(SCN)2Cl shows clear evidence of charge ordering at TCO = 30 K [4]. Below that temperature, the ESR parameters change sharply and an antiferromagnetic-resonance-like response is found [5]. To reveal the nature of this novel low-temperature phase, we present a detailed investigation of magnetic anisotropy in κ-(BEDT-TTF)2Hg(SCN)2Cl. The obtained data complements infrared and preliminary dielectric results. We contrast the results with sister compound κ-(BEDT-TTF)2Hg(SCN)2Br where charge order is absent, and discuss our findings in light of recent theoretical descriptions. [1] P. Lunkenheimer et al., Nat. Mater. 11, 755–758 (2012). ; S. Tomić et al., J. Phys.: Condens. Matter 25, 436004 (2013) ; M. Pinterić et al., Phys. Rev. B 90, 195139 (2014). [2] K. Sedlmeier et al., Phys. Rev. B 86, 245103 (2012) [3] Y. Shimizu et al., Phys. Rev. B 73, 140407 (2006). [4] N. Drichko et al., Phys. Rev. B 89, 075133 (2014). [5] S. Yasin et al., Physica B 407, 1689 (2012).

charge order; magnetic anisotropy; organic conductors

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