Supercurrents in Josephson junctions with chiral molecular potentials

Abstract

The influence of chiral molecular potentials on phase-coherent transport in superconducting Josephson junctions is investigated. Within a Bogoliubov-de Gennes tight-binding framework, an SNS junction functionalized by adsorbed chiral molecules is modeled, where electrostatic gradients generated by the molecules induce spin-orbit coupling in the normal region. The equilibrium charge current-phase relation is found to remain largely insensitive to molecular chirality in symmetric, zero-field configurations. In contrast, the spin supercurrent exhibits a pronounced chirality-dependent response, with opposite enantiomers producing distinct and anisotropic spin-polarized Josephson currents. The resulting handedness contrast can be enhanced through control parameters such as molecular orientation and the strength of the induced spin-orbit coupling. The temperature dependence of these currents further shows that the chirality-dependent signatures persist across a range of temperatures well below the superconducting critical temperature. These results establish Josephson interferometry as a phase-sensitive and accessible platform for detecting molecular chirality and highlight spin-polarized superconducting transport as a promising route toward integrating chiral molecular functionality into superconducting spintronic devices.

Figures (6)

• Figure 1: (a) Schematic of the Josephson junction considered throughout this work. Two superconducting leads with order-parameter phases $\mp\phi/2$ are connected through a normal weak link (dark region) with adsorbed chiral molecular structures on top. The equilibrium supercurrent $I_\phi$ flows along the transport direction in response to the gauge-invariant phase bias $\phi$. An out-of-plane magnetic field $B\hat{\mathbf{z}}$ threads flux through the junction area and produces interference of transverse supercurrent paths. (b) Current--phase relation at zero field, $I_c(\phi)\equiv I(\phi,B{=}0)$. While the molecular texture visibly renormalizes the overall Josephson response relative to the no-molecule reference, the two enantiomers yield overlapping charge supercurrents in this parameter set, consistent with chirality entering predominantly through spin-dependent structure of the Andreev states, robust chirality readout is therefore obtained from symmetry-breaking configurations such as magnetic-field interference and spin-sensitive transport observables studied in the following sections. (c) Fraunhofer response: critical current $I_c(B)=\max_{\phi}|I(\phi,B)|$ normalized to its zero-field value, comparing the reference junction without molecules (blue) to junctions functionalized by left- and right-handed enantiomers (orange and green).
• Figure 2: Dependence of charge and spin currents on the effective spin--orbit coupling strength $\alpha_{\mathrm{SO}}$. (a) The normalized charge critical current varies only weakly with handedness, indicating that the primary molecular effect in the charge sector is a renormalization of the junction transparency. (b--d) In contrast, finite spin currents emerge in the presence of the molecular potential and grow with increasing $\alpha_{\mathrm{SO}}$, with opposite enantiomers producing spin currents of opposite sign. Parameters: $W=8$, $L=26$, $\mu=0.5$, $\Delta_0=0.2$, $h_z=0$, $\theta=\pi/4$, and $B=0$.
• Figure 3: Angular dependence of the charge and spin currents on the molecular tilt angle $\theta$. Rotating the molecular electrostatic texture modifies the effective spin--orbit field experienced by quasiparticles in the junction. While the charge current remains only weakly sensitive to chirality, the spin-current components show a pronounced orientation dependence and opposite responses for the two enantiomers. Parameters: $W=8$, $L=26$, $\mu/t=0.5$, $\Delta_0/t=0.2$, $\alpha_{\mathrm{SO}}/t=2$, $h_z=0$, and $B=0$.
• Figure 4: Temperature dependence of charge and spin currents in the chiral-molecule Josephson junction. (a) The normalized charge current decreases with increasing temperature and vanishes as the superconducting gap closes near $T_c$. (b--d) The spin-current components are progressively modified with temperature but retain a residual offset above $T_c$. This offset reflects an equilibrium spin-current background generated by the spin--orbit-coupled molecular potential rather than a phase-coherent Josephson contribution. Parameters: $W=8$, $L=26$, $\mu/t=0.5$, $\Delta_0/t=0.2$, $\alpha_{\mathrm{SO}}/t=2$, $\theta=\pi/4$, and $B=0$.
• Figure A1: Decomposition of the spin current into normal-state and superconducting contributions. The total spin current $I_s^a$ (solid lines) is separated into a normal-state background $I_s^a(\Delta=0)$ (dotted lines) and a superconducting component $I_{s,\mathrm{SC}}^a = I_s^a - I_s^a(\Delta=0)$ (dash--dotted lines) for the left- and right-handed molecular enantiomers.