Shape dependence of Edelstein and magnetoelectric effects in the V-shaped model

Abstract: We theoretically investigate the shape dependence and microscopic mechanism of the magnetoelectric effect, including both nonmagnetic (Edelstein-type) and magnetic origins, in a V-shaped one-dimensional chain model. Numerical calculations based on the Kubo formula reveal that the magnitude of the nonmagnetic-driven magnetoelectric response reaches a maximum at an apex angle of $θ\approx 0.6π$. To clarify the microscopic origin of this behavior, we construct a low-energy effective Hamiltonian by projecting onto the $s$-orbital subspace and demonstrate that the polarity induced by the V-shaped geometry manifests as an effective spin--orbit interaction. An analytical derivation of the Green's function shows that the geometric effect appears as a $T$-matrix contribution, reflecting the local breaking of translational symmetry at the V-shaped edge. Furthermore, by employing a multipole-basis representation, we identify the selection rules that govern the magnetoelectric tensor and reveal that the coupling between the effective spin--orbit interaction and the orbital angular momentum generated across the apex plays an essential role. The resulting angular dependence, $\sinθ\sin{θ/2}$, peaks at $θ= 2\tan^{-1}\sqrt{2} \approx 0.608π$, in good agreement with the numerical results. We also analyze a ferromagnetic V-shaped model including the Zeeman interaction and show that the magnetic-driven magnetoelectric response originates from the spin magnetization induced by the coupling between the electric-field--driven charge-potential gradient and the Zeeman term. These findings demonstrate that the V-shaped geometry gives rise to distinct magnetoelectric mechanisms depending on the presence or absence of time-reversal symmetry and provide a microscopic framework for understanding shape-induced multipole phenomena in mesoscopic and bulk systems.