Current Induced Spin-Polarization in Chiral Molecules

Abstract

The inverse spin-galvanic effect or current-induced spin-polarization is mainly associated with interfaces between different layers in semiconducting heterostructures, surfaces of metals, and bulk semiconducting materials. Here, we theoretically predict that the inverse spin-galvanic effect should also be present in chiral molecules, as a result of the chiral induced spin selectivity effect. As proof-of-principle, we calculate the nonequilibrium properties of a model system that previously has been successfully used to explain a multitude of aspects related to the chiral induced spin selectivity effect. Here we show that current driven spin-polarization in a chiral molecule gives rise to a magnetic moment that is sensitive to external magnet field. The chiral molecule then behaves like a soft ferromagnet. This, in turn, suggests that magnetic permeability measurement in otherwise nonmagnetic systems may be used noninvasively to detect the presence of spin-polarized currents.

Figure 1

Current induced spin-polarization as a function of the voltage bias for different magnetic fields ranging between ±0.1 T applied to a chiral molecule comprising 2 × 8 sites. The inset shows the corresponding current–voltage characteristics. Here, we have used t₀ = 1 eV, ε₀–μ = 0, λ₀ = t₀ × 10^–3, t₁ = λ₀/10, 10^–2 ≤ ω_n/t₀ ≤ 1, n = 1, 2, ..., 30, Γ^χ = t₀/20, and T = 300 K.

Figure 2

(a) Total nonequilibrium density of electron states DOS(ω) = ∑_mDOS_m(ω) corresponding to the 2 × 8 sites helix in Figure 1 at B = 0 and a source-drain voltage of 1 V. (b) Corresponding total nonequilibrium spin-polarization ρ^z(ω) = ∑_mρ_m^z(ω) for external magnetic fields ranging between ±0.1 T. In panel (b), the plots are offset for clarity.

Figure 3

Site resolved current induced spin-polarization for clockwise (cw – red) and counterclockwise (ccw – blue) helices with 2 × 8 sites at the bias voltage V = 1 V. (a) Total moment |⟨S_m⟩|, (b–d) ⟨S_m^z⟩, ⟨S_m^x⟩, and ⟨S_m^y⟩. Inset in panel (c) shows the corresponding charge densities N_m. Other parameters are given in Figure 1.

Figure 4

Nonequilibrium properties as a function of molecular length and a voltage bias of 1 V. (a) Induced magnetic moment S_Mol (red) and corresponding longitudinal spin susceptibility dS_Mol/dB_z (purple) and (b) spin-susceptibility as a function of polar angle (θ) orientation of the magnetic field relative to the length direction of the molecule, of length 4 × 8. (c) Site resolved charge distributions for M = 4, 6, 9, and 14, and (d) corresponding site-resolved spin-polarization. The insets in panels (a), (b), and (c) show the transverse spin susceptibility dS_Mol/dB_x, a schematic of the setup, and the charge current, respectively. The plots in panels (c) and (d) are offset for clarity. Other parameters are as shown in Figure 1.