DC Hall Effect of the Low-TcState of β-(BEDT-TTF)2I3

The Hall effect was studied in β-(BEDT-TTF) 2 I 3 in order to survey the problem of the low- T c and the high- T c states and the general interest to the Fermi surface of this material. i) Our measurement revealed that this material is a metal with almost constant hole numbers down to 20 K. By the estimate of R H =1/ nec , the hole number seems to be less than one per unit cell. ii) In the temperature dependence of R H in detail, we found a pronounced stepwise decrease by 8% in Hall voltage when temperature is lowered through 175 K but not through 110 K. iii) Further, below 20 K, Hall voltage was found to decrease. Possible reasons for such phenomena are discussed.