第7回　物理工学科教室談話会(講師：Prof. Natalia Drichko)

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概要：
Mott insulators are commonly pictured with electrons localized on lattice sites. Their low-energy physics involves spins only. Recent theoretical work suggests that in molecular systems a new on-site charge degree of freedom can emerge. On a frustrated lattice with charge-spin coupling it would result in a new quantum spin liquid state. We experimentally demonstrate a presence of a charge degree of freedom in molecule-based Mott insulators k-(BEDT-TTF)2Hg(SCN)2X (X=Br,Cl).
In k-(BEDT-TTF)2Hg(SCN)2Br, when electrons localize on a triangular lattice of molecular dimers at temperatures below 100 K, they form electric dipoles which do not order at low temperatures and fluctuate, resulting in a so-called quantum dipole liquid state. A frequency of dipole fluctuations of 1.3 THz is detected experimentally in our Raman spectroscopy experiments through an observation of a related collective mode [1]. Magnetic properties of the system are currently investigated.
In k-(BEDT-TTF)2Hg(SCN)2Cl, electric dipoles order on dimer sites below 30 K. However, we demonstrate that below 15 K this charge order starts to melt, and the material shows paramagnetic behavior down to at least 2 K, suggestive of a spin liquid state.
We show that Raman spectroscopic response of Mott insulators with an on-site charge degree of freedom is different from the response of simple dimer Mott insulators. Raman spectra of the latter show two-magnon excitations at frequencies below 500 cm-1 expected for a S=1/2 antiferromagnet on a triangular lattice with J≈250 K [2]. Our results can be a key to understanding of organic triangular lattice spin liquid candidates.

[1] N. Hassan et al. Science 360, 1101 (2018).
[2] N. Hassan et al. Crystals 8, 233 (2018).
紹介教員：鹿野田 一司 教授（物理工学専攻）、賀川 史敬 准教授（物理工学専攻）