A Twist And A Spin

Harnessing two quantum properties transforms a neutron beam into a powerful probe of material structure.

By cleverly manipulating two properties of a neutron beam, scientists at the National Institute of Standards and Technology (NIST) and their collaborators have created a powerful probe of materials that have complex and twisted magnetic structures.

Penetrating deep inside heavyweight materials, yet still able to interact strongly with light elements, neutron beams image hydrogen-bearing liquids in engine parts, storage tanks and fuel cells. The beams can also map the shapes of polymers on the molecular scale, reveal the precise arrangement of atoms in a crystal and chart the distribution of water within growing plants.

Neutron beams became even stronger probes when scientists learned how to harness two quantum properties of the beams. One of these properties, formally known as orbital angular momentum, or OAM, refers to the twisting, or rotational motion of a neutron as it travels forward, similar to the whirlpool formed by water as it travels down a drain. The other quantum property, spin, is related to the neutron’s magnetic field, and can be likened to the spinning motion of a top.

In an unfiltered neutron beam, the spin of the neutron can point in a multitude of directions. By choosing the direction of spin and detecting changes in that direction when a neutron beam scatters off a material, researchers have examined the detailed magnetic structure of a host of materials.

Now NIST scientists and an international team of collaborators, including researchers at the Joint Quantum Institute and the University of Waterloo, have for the first time manipulated a neutron beam so that OAM and spin are correlated—each neutron that has a particular value of OAM has a particular pattern of spin associated with it.

The achievement promises to greatly expand the role that the beams play in studying the magnetic structure of materials that are twisted. For instance, the simultaneous selection of the two properties will allow neutron beams to more precisely examine helical molecules such as DNA and so-called topological materials, some of which act as conductors on their surface but as insulators beneath. The patterned neutron beam could also examine the shape of molecules that have a chiral structure—they do look the same as their mirror image. Just as your right hand cannot fit inside a left-handed glove, a right-handed molecule cannot be exchanged for its left-handed twin.

The researchers described their findings in a recent issue of the Proceedings of the National Academy of Sciences.

The team began their experiment by using two filters to ensure that the neutron beam was polarized—all the neutrons’ spins pointed in the same direction. To achieve that, the NIST team filled two chambers with a gas of helium-3 atoms, an isotope of helium that possesses spin. The researchers had manipulated these atoms so that nearly all of their spins aligned. Neutrons entering the chambers could only pass through if their spins matched that of the helium-3 atoms; all others were absorbed by the gas.

In the critical next step, the spin-polarized neutron beam traveled through a trio of triangular coils with varying magnetic fields. This arrangement shaped the polarized neutron beam into a helical wave. The number of windings of the helix over a given distance—how tightly the helix was wound—determined the numerical value of the OAM. In this way, the team endowed the neutron beam with a particular twist, or OAM.

The precise placement of the coils and their associated magnetic fields also imposed a geometric pattern on the direction of spin of the neutrons in the helical wave. In one such pattern, the spins traced out a circle around each winding, with the particular spin direction depending on how far each neutron resided from the center of the helix.