Spin-Selective Orbital Reconfiguration and Colossal Nonlinear Anisotropy in Defect-Engineered Atomically Thin Quantum Dots

Abstract

Spin-texture and wavefunction modification is still a central problem in the attempt to control spin orientation and distribution, effective spin transport, minimized energy dissipation, and improve functionality in spin-based information processing. Here, we studied a specific combination of atomically thin, nonmagnetic, group-13-based post-transition-metal--chalcogenide (PTMC) quantum dots (M$_{10}$X$_{12}$; M = Ga, In, Tl; X = S, Se, Te) with a single transition-metal (Zr, Mo, Mn) introduced at the central site using density functional theory. It is observed that the spatial distributions of frontier orbitals are not only spin-selective but also site-selective in real-space due to the induced TM--X$_3$ hybridization, which could allow independent tuning of both spin and spatial characteristics simultaneously. The normalized density overlap can differ by up to $10^{-3}$, whereas the normalized signed-amplitude overlap of the corresponding wavefunctions can reach values less than or equal $10^{-4}$. The local and effective magnetic moments of M$_9$TMX$_{12}$ structure can range from 1 $\rightarrow$ 7 $\mu_B$ and 1.62 $\rightarrow$ 20.93 $\mu_B$, respectively, while the spatial extent of identical-spin orbitals can differ by up to $\sim197$ a.u. Moreover, energy-gap of the pristine combination ranges between 0.89--5.02 eV, and it widens upon TM-substitution for $E_{g}^{\uparrow}$ (2.6--5.41) while narrowing for $E_{g}^{\downarrow}$ (1.62--5.36), which demonstrates that one spin channel can consistently remain energetically more accessible. A clear directional imbalance in the nonlinear optoelectronic response is observed, with the induced polarization switching between in-phase and out-of-phase (phase inversion) based on the field direction. The hyperpolarizability components reach maximum values of up to $\sim 10^{6}$ ($\beta_{\parallel}$) and $\sim 10^{9}$ ($\gamma_{\parallel},\gamma_{\perp}$), while the relative distribution between parallel and perpendicular directions remains consistent. Such multifunctional responses resulting from a single-site TM perturbation facilitate fundamental insight into how localized electronic changes simultaneously modulate spin selectivity, orbital anisotropy, and nonlinear polarization as interdependent quantum variables in low-dimensional nanoflakes, which holds promise for the design of multivariable quantum information processing architectures and next-generation nano-spintronic materials.