They are made from the orbital angular momentum of electrons orbiting their atomic nucleus – and the study of them has been dubbed ‘orbitronics’, analogous to spintronics for manipulating electron spins.
“This field holds great promise for memory devices, particularly because a large magnetisation could potentially be generated with relatively small charge currents, leading to energy-efficient devices. The million-dollar question now is identifying the right materials to generate flows of orbital angular momentum, a prerequisite for orbitronics.” according to the Paul Scherrer Institute, where such a material seems to have been discovered.
With Max Planck Institutes in Halle and Dresden, the institute has shown that ‘chiral topological semi-metals’, a class of materials discovered at PSI in 2019, it said, possess properties “that make them a highly practical choice for generating currents of orbital angular momentum”.
They have a helical atomic structure, giving them a natural handedness because such spirals can go left or right.
“This offers a significant advantage to other materials because you don’t need to apply external stimuli to get OAM [orbital angular momentum] textures – they’re an intrinsic property of the material,” said PSI scientist Michael Schüler (pictured). “This could make it easier to create stable and efficient currents of OAM without needing special conditions.”
Their particular chiral topological semi-metals are made from gallium and platinum, or gallium and palladium, where gallium atoms form a right-handed screw, while the platinum atoms form a left-handed screw.
Desirable among these textures, is a spherical collection of orbital angular momentums, with axis pointing to the centre, dubbed the orbital angular momentum monopole. These could exist in two states, with all the rotations clockwise when viewed from the outside, or all of them anti-clockwise. In effect, they have two possible polarities.
“At these monopoles, OAM radiates outwards from a centre point like the spikes of a hedgehog curled into a ball,” said the institute. “Why these monopoles are so tantalising is that OAM is uniform in all directions – isotropic – a very useful property as it means flows of OAMs could be generated in any direction.”
An analysis technique called ‘circular dichroism in angle-resolved photo-emission spectroscopy’, using variable energy x-ray photons from a synchrotron, yielded a lot of data, but no eureka moment because conventional wisdom was pointing in the wrong direction.
“At first, the data didn’t make sense,” said Schüler. “The signal seemed to be changing all over the place.”
What was expected was a spectroscopy signal that was directly proportional to the orbital angular momentum, but it transpired OAMs, as previously believed, but “rotated around the monopoles as the photon energy was changed”, said the institute. “In this way, they bridged the gap between theory and experiment and proved the presence of OAM monopoles.”
Having found monopoles, Schüler and colleagues went on to show that the polarity of a monopole can be flipped using a crystal with a mirror-image chirality. “This is a very useful property, as orbitronics devices could potentially be created with different directionality,” said Schüler.
