Spin-Textured Neutron Beams with Orbital Angular Momentum

TL;DR

This paper develops a theoretical framework for generating and characterizing neutron beams with orbital angular momentum using magnetic devices, enabling new quantum magnetic material probes.

Contribution

It introduces a path-integral approach to relate neutron OAM states to spin textures, extending SEMSANS techniques for complex spin-OAM entangled neutron beams.

Findings

The technique produces complex spin-OAM entangled patterns.

A path-integral approach accurately describes OAM generation.

Comparison with Larmor precession clarifies underlying physics.

Abstract

We present a rigorous theoretical framework underpinning the technique of spin-echo modulated small-angle neutron scattering (SEMSANS), and show how the technique can be extended in order to generate spin-textured neutron beams with orbital angular momentum (OAM) via birefringent neutron spin-polarization devices known as magnetic Wollaston prisms. Neutron OAM beams are mathematically characterized by a ``cork-screw'' phase singularity $e^{i \ell \phi}$ about the propagation axis where $\ell$ is the OAM quantum number. To understand the precise relationship between the emergent OAM state and the variety of spin textures realized by various setups, we have developed a path-integral approach that in the interferometric limit makes a judicious use of magnetic Snell's law. We show that our proposed technique produces a complex two-dimensional pattern of spin-OAM entangled states which may be useful as a probe of quantum magnetic materials. We compare our path-integral approach to the well-known single-path Larmor precession model and present a pedagogical derivation of magnetic Snell's law of refraction for both massive and massless particles based on Maupertuis's action principle.