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

In thermal devices the capacity to transform heat into electricity is characterized

by the thermoelectric efficiency described by the figure of merit  ZT = S²σT/κ [1], where

S is the Seebeck coefficient, σ is the electronic conductance and κ is the total thermal conductance

(electron and phonon contributions) at temperature T [2] . In order enhance the thermoelectric

efficiency, it is necessary reduce both contributions to the thermal conductance

without affecting electronic conduction [3]. This can not be obtained in bulk metallic materials due to

the classical Wiedemann-Franz law. Thus, several possible routes to improve the thermoelectric

materials have been proposed, such as: i) a controlled reduction of the lattice thermal conductance κ_ph

by increasing phonon scattering (nanopatterning with antidots, defects, or edge modification),

ii) enhancing the electron-hole asymmetry at the Fermi energy (by edge roughness or by proximitized

ferromagnetic insulator substrates) and iii) the effects of disorder produced by imputiries,

point defects or vacancies, among others [4].

In this work we investigate the spin-dependent thermoelectrical properties of Silicene nanoribbons,

composed by a central conductor with a random distribution of vacancies, connected to two pristine

leads of the same material, placed over ferromagnetic insulators.

We considered two configurations for the leads, with their magnetic moments being parallel or antiparallel.

Besides, there is a temperature difference between the leads, giving rise to thermoelectric effects.

By using a tight binding Hamiltonian and within the Green´s function formalism, we calculate charge

and spin-dependent Seebeck coefficient, thermal conductance (phonon and spin-resolved electronic

contribution) and charge and spin figure of merit, as a function of the geometrical confinement

and the vacancy concentration. Our results exhibit an enhancement of charge and spin dependent

thermopower and the consequent improvement of the thermoelectrical efficiency at room temperature,

suggesting that defected silicene nanorribons can be efficient for thermoelectric devices.

[1] H J Goldsmid, Springer, Berlin, 2010

[2] C Villagonzalo, et al, Eur Phys J. B 12,1999

[3] C Gayner and K Kar, Prog Mater Sci 83, 330, 2016

[4] C Nunez et al, J Phys: Cond Mat, 32, 275301, 2020