日時：2019年 1月25日（金） 16:00～
講師：Prof. Natalia Drichko
所属：The Johns Hopkins University, Baltimore, USA
題目：Signatures of quantum dipole liquid in an organic Mott insulator.
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 . 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 . Our results can be a key to understanding of organic triangular lattice spin liquid candidates.
 N. Hassan et al. Science 360, 1101 (2018).
 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）