Spin caloritronics in spin semiconducting armchair graphene nanoribbons

We designed a spin-caloritronics device based on armchair graphene nanoribbons (AGNRs). We theoretically show that a trapezoidal-shaped graphene flake consisting of four zigzag edges bridged between two AGNRs becomes a spin semiconductor with a tunable spin-dependent transmission gap. The results indicate that the appearance of spin semiconducting properties with spin-dependent localized transmission peaks around the Fermi level could produce the spin-caloritronics effects of AGNRs. Interestingly, the values of the spin Seebeck coefficient $S_S$ are comparable to values obtained for zigzag graphene nanoribbons, and also $S_S$ sensitively increases as the transmission gap increases. Furthermore, by engineering the position and orientation of a triangular antidot only in the scattering region, both $S_S$ and the spin figure of merit could be separately enhanced at room temperature.

Figure 1

(a) Schematic structure of a spin-caloritronics device based on AGNR, which is divided into left (18-AGNR) and right (7-AGNR) contacts and the scattering region (a trapezoidal nanoflake). A (black) and B (yellow) atoms illustrate sublattice sites. (b) Other configurations considered in this paper, which are different in terms of the size of their left contact and scattering region, with the same right contact.

Figure 2

Transmission coefficients for spin-up (red filled areas) and spin-down (blue filled areas) channels as a function of $E-E_F$. The configuration type is specified in each panel. The panels, arranged from (a) to (d), correspond to the smallest through the largest configurations. Colored lines show the spin-dependent conductance as a function of the chemical potential for spin-up (solid symbol) and spin-down (open symbol) channels.

Figure 3

(a) Thermally induced spin-polarized currents ($I_{\uparrow }$, $I_{\downarrow }$) and (b) spin and charge currents ($I_S$ and $I_C$) as a function of $\mathrm{\Delta }T$ at $T_L=300\mathrm{K}$ for different configurations.

Figure 4

(a) The electron thermal conductance as a function of the chemical potential at $T=300$ K and (b) the phonon thermal conductance as a function of temperature for the structures shown in Fig. 1.

Figure 5

Spin (colored solid line) and charge (black dashed line) Seebeck coefficients and spin-up (solid symbol) and spin-down (open symbol) Seebeck coefficients as a function of the chemical potential at room temperature. The color of $S_S$ changes according to Seebeck polarization.

Figure 6

(a) Spin and (b) charge FOMs as a function of the chemical potential at $T=300\mathrm{K}$.

Figure 7

Schematic view of configurations with antidots with different positions and orientations in the channel. The antidots are created in the 18A-5Z-7A configuration.

Figure 8

The phonon thermal conductance as a function of temperature for antidot-included configurations.

Figure 9

Transmission coefficients for spin-up (red filled areas) and spin-down (blue filled areas) channels as a function of $E-E_F$. Spin-up (solid symbols) and spin-down (open symbols) electrical conductance values as a function of the chemical potential for the antidot-included configuration at $T=300\mathrm{K}$.

Figure 10

(a) The spin Seebeck coefficient and (b) spin FOM as a function of the chemical potential at $T=300\mathrm{K}$ for the configurations presented in Fig. 7.

Figure 11

The spin FOM for the 15A-3Z-7A, 21A-6Z-7A, and 24A-8Z-7A configurations with antidots of the AD1 type. The results regarding configurations without antidots are also provided for comparison (the results are calculated at $T=300\mathrm{K}$).