^<13>C-NMR Study of the Organic Superconductor near the Superconductor-Insulator Phase Boundary

本文<13>报道了非氘代和部分氘代κ-（BEDT-TTF）_2Cu[N（CN）_2]Br的~ 1C-NMR研究，其中BEDT-TTF为双（亚乙基二硫代）四硫富瓦烯。通过使用<13>仅在中心碳位的一侧富集有^C的BEDT-TTF分子，我们可以从Knight位移和自旋-晶格弛豫速率估计态密度。在~ 1H-NMR谱中，虽然没有观察到比热测量中预测的质量效应<13>，但在强磁场下，半氘代κ-（BEDT-TTF）_2Cu[N（CN）_2]Br在18 K附近发生了金属-铁磁相变，并出现了相分离。这种转变在零场或低场下不会发生在 ** 自氘sal上。这些结果表明，一阶场诱导的金属-绝缘体转变。这些发现可以解释强磁场下相界附近的复杂现象。与部分氘代的κ-（BEDT-TTF-d[n，n]）_2Cu[N（CN）_2]Br不同，当外加电场平行于导电层时，在T^* 附近观察到相分离。前者的结果表明，场诱导AF转变。后一结果表明，κ-（BEDT-TTF-d[4，4]）_2Cu[N（CN）_2]Br的M-1相分离行为存在场方向依赖性。

^<13>C-NMR study on non-deulerated and partially deuterated κ-(BEDT-TTF)_2Cu[N(CN)_2]Br is reported, where BEDT-TTF is bis(ethylenedithio)tetra-thiafulvalene. By using the BEDT-TTF molecule having ^<13>C-enriched on only one side of the central carbon sites, we could estimate the density of states from the Knight-shift and the spin-lattice relaxation rate. Although the reduced mass effect predicted from the specific heat measurement was not observed in the ^<13>C-NMR, the melal-to-anliferromagnetic phase transition was observed with the phase separation around 18K on the half-deuterated κ-(BEDT-TTF)_2Cu[N(CN)_2]Br under a high magnetic field. Such a transition does not occur under a zero or low field on the **lf-deuterated sal. These results suggest a first order field-induced metal-insulator transition. These findings can explain the complex phenomena near the phase boundary under a high magnetic field. We also found that the phase separation is observed at around T^* with the applied field parallel to the conducting layer, unlike partially deuterated κ-(BEDT-TTF-d[n,n])_2Cu[N(CN)_2]Br. The former result suggests the field induces AF transition. the latter result suggests there is the field direction dependence of the M-1 phase separation behavior in κ-(BEDT-TTF-d[4,4])_2Cu[N(CN)_2]Br.