Effects of random vacancies on the spin-dependent thermoelectric properties of silicene nanoribbon

TL;DR

This work addresses how random vacancies in silicene nanoribbon central regions influence spin-dependent thermoelectric performance under magnetic proximity. It models the system with a single-band tight-binding Hamiltonian including hopping $t$, intrinsic SOC $\lambda_{SO}$, and Zeeman splitting $M$, solving via Green's functions in the linear-response regime, and incorporates $\kappa_{ph}(T)$ from QuantumATK with disorder averaging. The key finding is that a moderate vacancy concentration around $C=3\%$ combined with a ferromagnetic exchange field of about $M=0.6$ eV enhances both charge and spin Seebeck coefficients while reducing total thermal conductance, leading to higher $ZT$ at room temperature; antiferromagnetic configurations show negligible spin thermoelectric response. This demonstrates a robust route to engineer high-efficiency thermoelectric and spin-caloritronic devices in silicene nanoribbons, with effects persisting across sizes and geometries.

Abstract

The spin-dependent thermoelectric properties of silicene nanoribbon heterostructures are investigated, in which the central conductor contains a random distribution of vacancies and is connected to two pristine leads of the same material, placed in proximity to ferromagnetic insulators. The magnetic moments of the leads are analyzed in both parallel and antiparallel configurations. A tight-binding Hamiltonian and the Green's function formalism are employed to calculate the spin-resolved thermoelectric properties of the system as functions of geometrical confinement and vacancy concentration. The results demonstrate an enhancement in charge and spin-dependent thermopower, resulting in an improved thermoelectric efficiency at room temperature, which overcomes the limitations imposed by the classical Wiedemann-Franz law. These findings indicate that defective silicene nanoribbons are promising platforms for the development of efficient thermoelectric and spin-caloritronic devices.

Figures (6)

• Figure 1: Schematic view of the proposed device. $LC$ and $W$ denote the length and width of the central ribbon, respectively. In this configuration, three atoms are removed, corresponding to a vacancy concentration of 3%. Red and blue colors indicate the possible magnetic polarizations of the leads.
• Figure 2: Spin-dependent transmission $T_\sigma(\varepsilon)$ for ferromagnetic and antiferromagnetic configurations in A-SNRs with $N=12$, $LC=20$ ($L \approx 26$ nm) and $C=3\%$. Left panels: $M=0.2$ eV; right panels: $M=0.6$ eV. In both cases, systems with no vacancies (0%) and with 14 vacancies (3%) are considered.
• Figure 3: Phonon thermal conductance $\kappa_\text{ph}$ as a function of temperature for $N=12$, $L=26$ nm, computed with QuantumATK 2022.12 ATK.
• Figure 4: [(a) and (b)] Electronic conductance $\mathcal{G}_{c,s}$ in units of $\mathcal{G}_{0}=e^2/h$, [(c) and (d)] total thermal conductance $\kappa_{\text{tot}}$, [(e) and (f)] Seebeck coefficient $S_{c,s}$, and [(g) and (h)] figure of merit $ZT_{c,s}$ for $N=12$, $L\approx 26$ nm, 0$\%$ and 3$\%$ vacancies, and $M=0.6$ for the ferromagnetic configuration.
• Figure 5: Comparison of the charge and spin Seebeck coefficients for an N=12 A-SNR. Left panels correspond to the system with diluted disorder of 3% and ferromagnetic contacts (for different exchange field intensities), while right panels show the defect-free case with ferromagnetic leads. The dashed red curves correspond to charge Seebeck to the pristine case, with $M=0$ and $C=0$ at $T = 300$ K.