An expandable kinetic Monte Carlo platform for modelling electron transport through chiral molecules

TL;DR

The paper introduces an expandable kinetic Monte Carlo platform for modelling electron transport through chiral molecules under bias, explicitly incorporating spin channels and spin–chirality couplings relevant to eMChA and CISS. It demonstrates that standard resistive behavior emerges from stochastic hopping and that a phenomenological current-induced spin–orbit coupling term, $E_{\mathrm{eMChA}} = \Gamma \cdot s \cdot \tanh\left( \frac{qV}{k_B T} \right)$, yields voltage-dependent spin asymmetries consistent with magnetochiral phenomena, while vanishing in the linear-response limit. The model’s modular design enables integration with CupFlow and DAISY to explore dielectric magnetochiral anisotropy and related effects, providing a controlled framework to test competing microscopic theories. By linking microscopic hopping dynamics to macroscopic observables through a tunable energy bias and spin terms, the platform offers a practical tool for benchmarking against experiments and guiding future theory development on CISS and eMChA.

Abstract

Molecular chirality interacting in a non-negligible manner with the spin angular momentum of subatomic particles, mainly electrons or photons, is the cause of a variety of spin-dependent filtering effects in quantum transport. Among them, spin-selective transport at room temperature is clearly one of the most promising properties in the quest for functional spintronic devices. In this context, two main effects have been experimentally investigated in the past 25 years and have attracted significant interest within the community: the so-called Electronic Magnetochiral Anisotropy (eMChA) and Chirality Induced Spin Selectivity (CISS). Despite extensive research, there is still a lack of consensus in the modeling of their microscopic mechanisms. As a consequence, it remains unclear whether the two are truly distinct or if they originate from a common physical cause. With the long-term goal of modelling the main different theories and to test them against the available experimental evidence, we programmed the core of an efficient kinetic Monte Carlo code. The current code models electron transport under an external voltage, distinguishes between $α$ and $β$ spin currents, and parametrizes molecules by their intrinsic electron mobility and the effective coupling between electron movement, spin and chirality. The code allows obtaining spin filtering values arising from the effective coupling between these three. We obtain an effect that vanishes at low voltages, with the asymmetry between positive and negative voltages typically found in electrical magnetochiral anisotropy experiments.

Figures (1)

• Figure 1: Current-voltage curve obtained from the kinetic Monte Carlo simulation for $\alpha$ (red curve) and $\beta$ (purple curve) spin channels, together with the corresponding unpolarized current. The results are shown for a system comprising $N=5500$ electrons distributed over identical molecular strands, a dimensionless diffusion coefficient $D=0.1$ controlling the overall hopping rate, and a phenomenological spin-chirality coupling strength $Q_{\mathrm{CISS}}=0.4$ that introduces spin-dependent asymmetry in the hopping probabilities. The currents are reported in arbitrary units and exhibit a nonlinear voltage dependence with spin-resolved asymmetry characteristic of electric magnetochiral anisotropy.