Fig. 2: Measurement of the temperature dependence of resistance in α-(BETS)₂I₃.

a Standard and inverted configuration for the four-terminal in-plane resistance measurement. Excitation currents were applied from the two terminals on the top surface of the crystal, and the voltage drop between the two terminals on the top surface (standard configuration) and a back surface (inverted configuration) was measured. The red and blue arrows represent the current flows via the surface and the bulk, respectively. b Setup for the out-of-plane resistance measurement. c Temperature dependence of the standard in-plane resistance (blue curve) for Sample #1 and the out-of-plane resistance (red curve) for Sample #2. Both data are normalized by the resistance value at 300 K. The out-of-plane resistance exhibits a drop between 35 and 10 K (orange region), indicating the appearance of the metallic surface conduction. d Temperature dependence of the in-plane resistance of α-(BETS)₂I₃ measured by the standard configuration for Sample #1 (blue curve), the inverted configuration for Sample #3 (green curve), and the non-local configuration for Sample #4 (orange curve), respectively. The resistance measured by the inverted and non-local configurations shows a flattened resistance at low temperatures because of the large contribution of the surface metallic state.