ESR Study of Neutral-Ionic Transition of Mixed-Stacked Charge-Transfer Salts

混合堆积的电荷转移盐由交替排列的给体和受体分子组成，具有中性和离子相。在最近合成的(BEDT-TTF)(ClMeTCNQ)和(Bedo-TTF)(C_12TCNQ)中，由于BEDT-TTF和Bedo-TTF分子具有很强的二维相互作用，自旋-Peierls转变被抑制。在分析(BEDT-TTF)(C1MeTCNQ)的中性-离子转变时，我们假设转变温度与压力无关。这导致实际观察到的自旋磁化率在1 Gpa以上达到饱和。为了证实这一假设，我们测量了磁化率在2.5 GPa时的压力依赖关系，并能够观察到预期的饱和度。在(Bedo-TTF)(Cl2TCNQ)中，观察到了几种压力下的自旋磁化率与温度的关系。根据这些结果，我们得到了压力-温度相图，以及Q和W波段的ESR谱。结果表明，(Bedo-TTF)(Cl2TCNQ)的自旋体系在8 GPa时为三维自旋气体，在10 GPa时为三维自旋液体。有机电荷转移化合物β‘-(BEDT-TTf)_2ICl_2是Mott绝缘体，在常压下T_N=22K发生反铁磁相变，在14.2K下发生超导相变。在低于3 GPa时用电子自旋共振测量了顺磁自旋磁化率随温度的变化，以研究绝缘到金属态的电子态。我们正在准备一种新的系统，用于在压力下使用立方砧室进行ESR，这使我们能够测量高达10 Gpa的ESR。该系统将用于研究β-(BEDT-TTf)_2ICl_2从反铁磁性到金属态的电子态。

Mixed-stacked charge-transfer salts, composed of alternatively aligned donor and acceptor molecules, have neutral and ionic phases. In recently synthesized (BEDT-TTF)(ClMeTCNQ) and (BEDO-TTF)(C_12TCNQ), spin-Peierls transition is suppressed due to BEDT-TTF and BEDO-TTF molecules with strong 2-dimensional interaction. In the analysis of the neutral-ionic transition of (BEDT-TTF)(C1MeTCNQ), we had assumed that the transition temperature is independent of pressure. This leads to the saturation of the spin susceptibility above 1 GPa, which was actually observed. To confirm this assumption, we measured the pressure dependence of the susceptibility up to 2.5 GPa and was able to observed the expected saturation. In (BEDO-TTF)(Cl_2TCNQ), the temperature dependences of the spin susceptibility under several pressures were observed. We obtained the pressure-temperature phase diagram from those results, together with Q-and W-band ESR spectra. It was suggested that the spin system of (BEDO-TTF)(Cl_2TCNQ) is 3-dimensional spin gas under 8 GPa and 3-dimensional spin liquid above 10 GPa. An organic charge-transfer compound, β'-(BEDT-TTF)_2ICl_2, is a Mott insulator, undergoing an antiferromagnetic transition at T_N=22 K under ambient pressure and superconducting transition under the pressure higher than 8 GPa at 14.2 K. The temperature dependences of paramagnetic spin susceptibility have been measured by ESR below 3 GPa to study the electronic states in the region from the insulating to the metallic state. We are preparing a new system for ESR under pressure with a cubic anvil cell, which enables us to measure ESR up to 10 GPa. This system will be used to study the electronic states in the region from the antiferromagnetic to the metallic states of β-(BEDT-TTF)_2ICl_2.