The work, currently theoretical, contends that hourglass fermions exist in topological insulators – which are insulators in their interiors and on their top and bottom surfaces, but perfect conductors on two of their sides, where fermions create hourglass-shaped channels that enable electrons to flow.
In their study, the researchers theorise that the laws of physics prohibit current from flowing in the crystal’s bulk and top and bottom surfaces, but permit electron flow in completely different ways on the side surfaces through the hourglass-shaped channels. This type of channel, known as a dispersion, was previously unknown.
“Our hourglass fermion is curiously movable, but unremovable,” said associate professor Andrei Bernevig. “It is impossible to remove the hourglass channel from the surface of the crystal.”
The property is believed to arise from the intertwining of spatial symmetries – characteristics of the crystal structure – with the band theory of crystals.
A branch of mathematics mathematics called cohomology has determined that 230 crystal types exist in three spatial dimensions. Of these, 157 have the potential to host what the team calls ‘interesting electronic behaviour’, such as the hourglass fermion.
“The exploration of the behaviour of these interesting fermions, their mathematical description and the materials where they can be observed, is poised to create an onslaught of activity in quantum, solid state and material physics,” said Professor Robert Cava. “We are just at the beginning.”
