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

报道了κ-(BEDT-TTF)_2Cu[N(CN)_2]Br的非去氘化和部分去氘化的C-NMR研究，其中BEDT-TTF为双（乙二硫代）四噻吩。通过使用仅在中心碳位一侧富集^<13> c的BEDT-TTF分子，我们可以从Knight-shift和自旋-晶格弛豫速率估计态密度。虽然在^<13>的C-NMR中没有观察到比热测量所预测的降低质量效应，但在高磁场下，半氘化的κ-(BEDT-TTF)_2Cu[N(CN)_2]Br在18K左右发生了相分离，出现了金属到反铁磁的相变。这种跃迁不会在零场或低场下发生。这些结果表明存在一阶场致金属绝缘子跃迁。这些发现可以解释在强磁场作用下相界附近的复杂现象。与部分氘化的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^*处可以观察到相分离。前者的结果表明，电场诱导了AF跃迁。后一个结果表明，κ-(BEDT-TTF-d[4,4])_2Cu[N(CN)_2]Br中M-1相分离行为与场方向有关。

^<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.