Coexistence of Dirac and massive carriers in $\alpha$-(BEDT-TTF)${}_2$I${}_3$ under hydrostatic pressure

Transport measurements were performed on the organic layered compound $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$ under hydrostatic pressure. The carrier types, densities, and mobilities are determined from the magnetoconductance of $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$. While evidence of high-mobility massless Dirac carriers has already been given, we report here their coexistence with low-mobility massive holes. This coexistence seems robust as it has been found up to the highest studied pressure. Our results are in agreement with recent DFT calculations of the band structure of this system under hydrostatic pressure. A comparison with graphene Dirac carriers has also been done.

Figure 1

Typical magnetoconductivity curve of aI3 (open circles) that can be understood as a two-carrier system (blue line: A-carrier conductance, green line: B-carrier conductance, black line: conductance of a system with both A and B carriers). The mobility of each carrier type is determined as the crossover from a constant conductivity at low fields to the $H^{-2}$ regime at high fields. The left axis shows the square (2D) conductivity of each BEDT-TTF plane while the right axis shows the “bulk” (3D) longitudinal conductivity (see text). Inset: Photograph of one sample.

Figure 2

Magnetoconductivity curves of aI3 at $P=2.3$ GPa for different temperatures, from bottom to top: $1.5$, $2.2$, $3.0$, $3.9$, 6, 8, 9, 12, 15, 20, and 27 K. The left axis shows the square (2D) conductivity of each BEDT-TTF plane while the right axis shows the “bulk” (3D) longitudinal conductivity.

Figure 3

Temperature dependencies of the densities for A and B carrier types (circles: $1.6$ GPa, triangles: $2.3$ GPa, and squares: $3.0$ GPa; blue thin symbols for A carriers and red thick symbols for B carriers). The left axis shows the density for each BEDT-TTF plane ($n_{2D}$) while the right axis shows the bulk density ($n_{3D}$). The lines represent power-law fits which yield exponents $0.9$ (A carriers) and $2.2$ (B carriers). Inset: Band structure calculations at $1.7$ GPa where both Dirac cones (A) and parabolic bands (B) cross the Fermi level (adapted from Ref. 11).

Figure 4

Temperature dependencies of the mobilities for A and B carrier types (circles: $1.6$ GPa, triangles: $2.3$ GPa, and squares: $3.0$ GPa; blue thin symbols for A carriers and red thick symbols for B carriers). The continuous lines represent power-law fits which yield exponents $-1.9$ (A carriers) and $-1.0$ (B carriers). The low-temperature dispersion of the data points for A carriers is due to a decrease of the saturating mobility (dotted line) by increasing pressure.

Figure 5

Thermopower for a pressure of 1.5 GPa as function of temperature for sample 2. The sign change observed as sweeping temperature confirms the two-carrier picture.