Figure 1

(a) $U/W$-band filling phase diagram for the $\kappa$-BEDT-TTF system: blue, green and yellow lines denote band insulator, Mott insulator, and superconductor, respectively. The pale green area is thought to be metallic, although correlation-enhanced localization is very likely to occur. (b) Two-dimensional lattice of $\kappa$-Br viewed along the $b$ axis. The crystallographic $ac$ unit cell contains one red and one blue dimer in a layer (white square), each of which has one hole carrier. (c) Fermi surface of $\kappa$-Br. Since there are two dimers in the unit cell, the sectional area of the Fermi surface corresponds to 100% of the first Brillouin zone. Note that the degeneracy at the zone boundary is slightly broken resulting in anisotropic thermopower generation (Ref. 26). (d) $U/W$-$T$ phase diagram for the $\kappa$-BEDT-TTF system. PI, PM, AFI, and SC denote paramagnetic insulator, paramagnetic metal, antiferromagnetic insulator, and superconductor, respectively. Blue, black, and red arrows show temperature trajectories of $\kappa$-Br for the corresponding conditions (on SiO${}_2$/Si, bulk, and on polystyrene).Reuse & Permissions

Figure 2

(a) Schematic side view and (b) optical top view of the FET structure. (c) X-ray-diffraction pattern with incident beam normal to the substrate. The spots from a $\kappa$-Br monocrystal are observed among intense spots from Si. The bottom table shows the cell parameters from the literature (Ref. 21) and those calculated from the observations.Reuse & Permissions

Figure 3

(a) Temperature dependence of the resistance of $\kappa$-Br without a substrate or adhered to silicon or polystyrene. The data for a total of ten samples are shown. (b) Temperature difference between thermometers at two opposite sample ends.Reuse & Permissions

Figure 4

(a) Field-effect mobility estimated from ${\mu }_{\mathrm{FE}}=(1/C_i)$($d$${\sigma }_{\square }/$$d$$V_g)$. The curves of conductivity vs gate voltage were differentiated to obtain $d$${\sigma }_{\square }/$$d$$V_g$. Inset shows temperature dependence of ${\mu }_{\mathrm{FE}}$ at $V_g=120$ V. (b) Activation plots for the four-terminal sheet conductivity under various gate voltages. Extrapolations of the low-temperature part intersect at 1/$T=0$ with an identical prefactor ${\sigma }_0$.Reuse & Permissions

Figure 5

[(a),(b)] Transfer curves of $n$-polar (low $V_{\text{th}}$) and ambipolar (high $V_{\text{th}}$) samples, respectively. (c) Gate-voltage dependence of the activation energy derived from activation plots of the four-terminal conductivity. The field effect appears qualitatively similar, but the threshold voltage (horizontal position) and sensitivity to the gate voltage (slope) are sample dependent, probably due to surface conditions during fabrication.Reuse & Permissions

Figure 6

(a) Temperature dependence of the Hall coefficient and Hall mobility without gate voltage. The carrier density decreases with temperature since $n=1/e{R}_H$. (b) Gate-voltage dependence of $1/e{R}_H$ at 20 K. The blue line denotes the density of the injected carriers calculated by the capacitance model, $Q=C(V_g-V_{\text{th}})$. (c) Gate-voltage dependence of $R_H$ at various temperatures. Solid lines show simulations at each temperature using a classical model in which field-induced and thermally activated hole carriers coexist.Reuse & Permissions

Figure 7

(a) Schematic and optical images of the sample under the crossed Nichols condition. The heaters and thermometers are each connected with two Au wires, although the wires are omitted for clarity. (b) Temperature difference vs voltage plots at $T=20$ and 40 K, for $V_g=0$ and 120 V, along each crystallographic axis. The red curves show the thermopower for positive heater current (heater 1), while the blue curves show the case for heat reversal (heater 2). (c) Gate-voltage dependence of the thermopower along the $a$ axis. (d) Gate-voltage dependence of the thermopower along the $c$ axis. (e) Temperature dependence of the thermopower for $V_g=$ $-$80 and 120 V, along the $a$ axis.Reuse & Permissions

Figure 8

Schematics of the field effect in the Mott-insulating state of $\kappa$-Br. In the lower route, ESD enhances the screening effect and the effective Coulomb repulsive energy is reduced, resulting in a decrease in the hopping energy. The shape and amplitude of the DOS are simplified. The thickness of the shaded area in the middle of the band represents the viscosity of the carriers.Reuse & Permissions