Spin-Selective Thermoelectric Transport in a Triangular Spin Ladder

TL;DR

This work develops a triangular spin-ladder model that maps spin to spatial channels, enabling controlled spin-resolved thermoelectric transport without external magnetic fields. Using nonequilibrium Green’s function techniques, it demonstrates that engineering hopping and onsite-energy asymmetries yields pronounced spin-channel separation, sharp energy features, and large spin Seebeck coefficients. The results show that the spin thermoelectric figure of merit $Z_S T$ can surpass the charge counterpart $Z_C T$ by up to a factor of two or more, with $Z_S T$ reaching values near 4 in optimized regimes. The findings highlight the interplay of geometry, interference, and coherence in enhancing spin-caloritronic performance, and point to lattice-design strategies for efficient low-dimensional spin-based thermoelectric devices, while noting WF-law deviations as a signature of enhanced thermoelectricity. $S^{0}$, $S^{0}$, $ au^{00}$, and $L_n^{0}$ are central quantities in the analysis, all evaluated within the linear-response NEGF framework.

Abstract

We theoretically investigate spin-resolved thermoelectric transport in a triangular ladder geometry hosting antiferromagnetic spin alignment, where lattice topology and magnetic ordering jointly enable highly efficient spin-selective energy conversion. The inherent geometric frustration of the ladder, together with intrinsic spin-filtering mechanisms, is shown to promote a pronounced separation between spin channels. Implementing spin-dependent onsite modulations, such as binary asymmetric potentials, induces pronounced spin splitting in the transmission spectrum, enabling controlled spin-selective transport and highlighting the role of lattice engineering in tailoring spin-dependent thermoelectric response. Additional control is achieved through modulation of the hopping amplitudes, which activates multiple transport pathways and allows fine tuning of spin-dependent conduction. A detailed evaluation of charge and spin thermoelectric coefficients reveals a strong enhancement of the thermoelectric performance, with the dimensionless figure of merit ZT reaching large values in optimized parameter regimes. Notably, the spin figure of merit systematically surpasses its charge counterpart, underscoring the decisive role of lattice geometry and antiferromagnetic order in amplifying spin thermoelectric efficiency. Our findings provide a versatile theoretical platform for designing low-dimensional spin-caloritronic devices with enhanced functionality.

Figures (16)

• Figure 1: (Color online). The schematic illustration presents a triangular spin ladder connected to source and drain electrodes. This unique configuration opens a new frontier in thermoelectric transport by capturing interplay between diagonal hopping and onsite energy.
• Figure 2: (Color online). The plot illustrates the transmission function as a function of energy under various hopping configurations. Panel (a) displays fully spin overlapped transmission spectra, indicating the absence of spin-dependent separation when the hopping amplitude is $t_u = 1$. In contrast, panel (b) shows partial overlap between the spin-resolved channels, with $t_u=0.6$, clearly revealing the onset of spin-channel differentiation. This behavior reflects the subtle interplay between the distinct hopping pathways and their collective influence on spin-dependent transport characteristics.
• Figure 3: (Color online). Seebeck coefficient as a function of Fermi energy is shown. Panel (a) displays the spin-resolved Seebeck response, showing distinct contributions from the up- and down-spin channels. Panel (b) presents the corresponding charge and spin Seebeck coefficients, thereby capturing the combined thermoelectric behavior arising from both transport channels.
• Figure 4: (Color online). Panel (a) presents the spin-resolved electronic conductance as a function of Fermi energy, clearly illustrating the distinct contributions from the up- and down-spin carriers. Panel (b) compares the corresponding total charge and spin conductances, offering insight into the system’s thermoelectric response arising from spin-dependent transport channels.
• Figure 5: (Color online). Thermal conductance as a function of Fermi energy, providing a comprehensive view of the system’s thermoelectric transport characteristics.