Geometric Phases and Persistent Spin Currents from nonminimal couplings

Abstract

We investigate a class of nonminimal derivative couplings between fermions and electromagnetic fields that generate Rashba-like spin--orbit interactions in one-dimensional quantum rings. Starting from a generalized Dirac Lagrangian containing two independent axial structures built from the field strength $F_{\mu\nu}$ and its dual $\tilde{F}_{\mu\nu}$, we perform a systematic nonrelativistic expansion and show that both couplings induce effective Hamiltonians of the form $\boldsymbol{\mathcal{F}}\cdot(\boldsymbol{p}\times\boldsymbol{\sigma})$. This reveals that magnetic as well as electric background fields may give rise to Rashba-type interactions, in contrast with standard condensed-matter scenarios. Before passing to the nonrelativistic limit, we analyze the relativistic content of the model in detail: the canonical structure of the deformed Dirac operator, the admissible background classes, the effective bilinear current, and the branch splitting of the relativistic dispersion relation, which constitutes the primary relativistic signature of the theory. We derive exact analytical energy levels and normalized eigenspinors for the resulting ring Hamiltonian, compute Aharonov--Anandan geometric phases, and analyze persistent spin currents together with the associated differential spin response $\mathcal{G}_s = \partial\mathcal{J}_\varphi^z/\partial\xi$. Exploiting the analytical control offered by the model, we derive the first systematic order-of-magnitude bounds on the two Lorentz-invariant couplings $\mathfrak{g}_1$ and $\mathfrak{g}_2$ from both spectroscopic and mesoscopic scenarios, identifying the experimental channels most sensitive to the new physics encoded in these operators. We discuss physical implications, signatures, and possible experimental analogs, and outline several promising directions involving disorder, noise, and nonequilibrium spin dynamics.