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2017.05.09 "Peculiar Paramagnetic Phase Adjacent to Superconducting Phase in Lambda-(BEDT-STF)2GaCl4" 南舘 孝亮博士（北海道大学 理学研究院 低次元電子物性研究室 博士研究員）
The organic quasi-two-dimensional conductor lambda-(BETS)2GaCl4 shows the superconducting (SC) phase below Tc=5.5 K. To clarify the adjacent phase of the SC phase in lambda-(BETS)2GaCl4 and discuss the origin of this phase, we have measured the electrical resistivity under pressure and the magnetic properties at ambient pressure in lambda-(STF)2GaCl4 (STF = bis(ethylenedithio)diselenadithiafulvalene), which was expected to be placed at the negative pressure region of the lambda-(STF)2GaCl4.
At first, we found the SC phase above 1.2GPa. We also found that the ground state of the STF salt at ambient pressure is the insulating phase without any magnetic ordering. Accordingly, the phase neighboring the SC phase is the non-AF-ordered paramagnetic insulating (PMI) phase.
Remarkably, in this PMI phase, the temperature dependence of the spin susceptibility obeys to the two-dimensional AF Heisenberg model with the regular triangle lattice. This is the typical behavior of the spin-liquid system. In the seminar, to explain the peculiar spin-liquid-like behavior in lambda-(STF)2GaCl4, we discuss the relation between the charge disproportionation and the geometry of the spin-interaction.
2017.04.13 "Symmetry Breaking Characteristic of Tight Dimerization in Antiferromagnetic Molecular Solid" 山本 貴博士（愛媛大学 理工学研究科 固体物理化学研究室 准教授）
Antiferromagnetic state of β'-Me4P[Pd(dmit)2]2 has been known as the obtuse and isosceles triangular lattice consisting of tight dimers with S = 1/2. Vibrational spectroscopy focused on electrical densities of C=C double bonds revealed symmetry breakings in lattice and molecule, which are discussed from the viewpoint of molecular orbitals specific to the tight dimerization.