Spin caloritronics in collinear ferromagnetic helical structures under irradiation

TL;DR

This work develops a tight-binding Floquet-Bloch NEGF framework to analyze spin-dependent thermoelectric transport in a ferromagnetic helical system under arbitrarily polarized light. The central finding is that light induces spin-split transmission and a light-generated spin gap near the Fermi level, dramatically enhancing the spin thermopower and the spin figure of merit $Z_sT$, while the charge counterpart remains comparatively modest. The spin FOM benefits from spin-channel crossings and high-frequency driving that suppresses electronic heat conduction; moreover, long-range hopping ($l_c$) strongly boosts spin caloritronic performance, with $Z_sT$ reaching very large values for suitable parameters. Phonon transport, influenced by lead materials (Si vs Ge), further shapes the total thermoelectric performance, highlighting design levers for spin-caloritronic energy conversion using irradiated ferromagnetic helices. Overall, the study provides a tunable platform where light polarization, geometric long-range hopping, and material choice combine to optimize spin-dependent thermoelectric response, offering routes toward efficient energy harvesting and cooling at the nanoscale.

Abstract

We study the charge and spin-dependent thermoelectric response of a ferromagnetic helical system irradiated by arbitrarily polarized light, using a tight-binding framework and the Floquet-Bloch formalism. Transport properties for individual spin channels are determined by employing the non-equilibrium Green's function technique, while phonon thermal conductance is evaluated using a mass-spring model with different lead materials. The findings reveal that that light irradiation induces spin-split transmission features, suppresses thermal conductance, and yields favorable spin thermopower and figure of merit (FOM). The spin FOM consistently outperforms its charge counterpart under various light conditions. Moreover, long-range hopping is shown to enhance the spin thermoelectric performance, suggesting a promising strategy for efficient energy conversion in related ferromagnetic systems.

Figures (17)

• Figure 1: (Color online). Schematic representation of an irradiated helical structure incorporating a collinear ferromagnetic spin configuration. The blue balls with the red arrow denoted the magnetic sites. At both ends of the helix, two semi-infinite metallic 1D non-magnetic leads (shown by the green arrows) are attached, referred to as the source ($S$) and drain ($D$). The blue waves represent an arbitrarily polarized light. $R$, $\Delta z$, and $\Delta \phi$ are respectively the radius, stacking distance, and twisting angle of the helix, respectively.
• Figure 2: (Color online). Spin-resolved transmission probability $\mathcal{T}_\sigma$$(\sigma = \uparrow, \downarrow)$ as a function of energy in the (a) absence and (b) presence of light. The light parameters are $A_x = 0.3$, $A_z = 0.2$, and $\theta = 0$. The number of sites in the helix is $N = 20$, with the SDS parameter set to $\mathpzc{h} = 0.25$. The physical parameters of the ferromagnetic helix are $R = 2.5\,$Å, $\Delta\phi = 5\pi/9$, $\Delta z = 1.5\,$Å, and $l_c = 0.9\,$Å. The region marked by the blue ellipse in panel (b) is magnified in the inset to highlight the behavior near $E = 0$. Black and red curves represent the up and down spin channels, respectively.
• Figure 3: (Color online). Behavior of various thermoelectric quantities as a function of Fermi energy at room temperature ($T = 300\,$K). The upper panels [(a)-(c)] correspond to the absence of light, while the lower panels [(d)-(f)] show the results in the presence of light. Panels (a) and (d) display the electrical conductance ($G_\alpha$), (b) and (e) show the thermopower ($S_\alpha$), and (c) and (f) present the electronic thermal conductance ($\kappa_\text{el}$). All system parameters are the same as those used in Fig. \ref{['trans']}. The subscript $\alpha$ denotes charge ($c$) and spin ($s$) components, represented by blue and green curves, respectively.
• Figure 4: (Color online). Phonon transmission probability $\mathcal{T}_{\text{ph}}$ as a function of phonon angular frequency $\omega$ for (a) silicon, and (b) germanium leads. (c) Phonon thermal conductance $k_{\text{ph}}$ as a function of temperature $T$ for the same set of leads. Black and red curves represent results for silicon and germanium leads, respectively. The central system is composed of carbon in all cases.
• Figure 5: (Color online). Variation of charge and spin figures of merit, $Z_cT$ and $Z_sT$, with Fermi energy at room temperature ($T = 300\,$K). The upper panel corresponds to the absence of light, while the lower panel shows the results in the presence of light. All system parameters are the same as those in Fig. \ref{['trans']}. The results correspond to systems with silicon and germanium leads, represented by black and red curves, respectively, highlighting the phononic contributions.