Pressure Effect on Fermi Surface in β″-(ET)(TCNQ)

In order to investigate the electronic state, we have performed the resistance measurements of β″-(ET)(TCNQ) under pressures. At ambient pressure, the resistance shows metallic behavior down to 0.5 K with three hump anomalies at T 1(=20 K), T 2(=60 K), and T 3(=250 K). As the sample is pressurized, the anomalies at T 1 and T 3 are strongly suppressed, and only the anomaly at T 2 is observed at 5.1 kbar. The angular-dependent magnetoresistance shows resonance structures under fields in both b*- c and b*- a planes, which arises from one-dimensional Fermi surfaces of the ET and TCNQ bands, respectively. The results are consistent with the band calculation, and also suggest that the Fermi surface is not reconstructed at T 2. The origin of the anomaly at T 2 is still an open question.