Future info storage could use tiny valleys

In the push to store ever more information in digital storage materials, physicists have made a breakthrough in a new technique called valleytronics.

Mallika Randeria from Princeton University in the US and her colleagues explored how electrons interacted with energy “valleys” on the surface of a crystal of pure bismuth. Their results are published in the journal Nature Physics.

Electronics is based on electric charge and current, spintronics interacts with electron spin, but valleytronics seeks to store information by moving electrons between separate areas in which they tend to rest within a crystal.{%recommended 7023%}

Randeria’s experiments used the tiny tip of a scanning tunneling electron microscope to detect where electrons sat on the surface of the crystal. Calculations had predicted  that the hexagonal structure of the bismuth would lead to six elliptical valleys arranged in a circle like the petals of a daisy. However, previous experiments lacked the precision to pick out the individual valleys, a mere 20 nanometres wide by 200 nanometres long.

The two-storey-high Princeton apparatus, fully isolated from vibration, cooled to only thousandths of a degree above absolute zero, and encased in a chamber pumped to ultra-high vacuum, was able to for the first time pick out the shape of the valleys.

Crucially, the electrons did not spread evenly amongst the six dips, but tended to cluster in one. This is an important characteristic for the success of valleytronics, which would store information by moving electrons between distinct valleys. It would have been much more difficult had they been evenly spread between all of them.

The experiments also confirmed that the ellipses were not completely symmetric, but pointier at one end. The electrons tended to accumulate at the rounder end, meaning the charge was not uniformly distributed over the surface, a property known as “ferro-electrical behaviour”. 

Randeria says that the ferroelectric behaviour detected was too weak for practical uses, but, more broadly, valleytronics is a promising approach to encoding more information into materials, when electronics reaches its limits.

“At some point you end up with problems with heating by having all these charges moving around,” she says.