Nature of electrons revealed with chip

Princeton University researchers have created the world's purest sample of gallium arsenide, a semiconductor used in devices that power cell phones and satellites. From a square no larger than the width of a pencil eraser, the study provided a new perspective into the quantum realm and deep insight into the very nature of electrons.

Fashioning the material down to one impurity for every 10 billion atoms, reaching a level of immaculate quality that outstrips even the world's purest silicon sample used in verifying the modern one-kilogram standard, the finished gallium arsenide chip allowed the team to observe the makeup of electrons like never before.

Rather than send the chip into space, which would offer the ideal environment for such experimentation, the researchers kept their studies terrestrial by taking the ultra-pure sample to the basement at Princeton's engineering quadrangle where they wired it up, froze it to colder-than-space temperatures, cloaked it in a powerful magnetic field, and applied a voltage to send electrons through the two-dimensional plane sandwiched between the material's crystalline layers.

As they shifted the magnetic field, surprising results began to happen.

Published in February in "Nature Materials," the results showed that many of the phenomena driving today's most advanced physics could be observed under far weaker magnetic fields than previously thought.

With lower magnetic fields, more labs could study the mysteries buried between the molecules that make up every piece of the physical universe.

According to the team, however, the most exciting part is that these less severe conditions presented new physics that have no established theoretical framework, possibly an entirely new branch of study never before conceived.

Besides the breakthrough in opening the gates for new physics, one of the surprises was when the electrons aligned into a lattice structure known as a Wigner crystal. Conventionally, scientists believed that the formation of Wigner crystals required extremely intense magnetic fields, roughly 14 Tesla (a unit of measurement for magnets). By way of comparison, the magnets in an MRI range between 0.5 to 3 Tesla.

The Princeton scientists' gallium arsenide study demonstrated that electrons can crystallize at less than one Tesla.

"We just needed the ultra-high quality to see them," said Kevin Villegas Rosales, one of the study's two first authors.

Furthermore, the team observed around 80% more "oscillations" in the system's electrical resistance and large "activation gap" of what is called the fractional quantum Hall effect (the quantum variant of the Hall effect we use to measure voltage).

This study came together as part of an ongoing collaboration between principal investigators Mansour Shayegan, professor of electrical and computer engineering, and Loren Pfeiffer, a senior research scholar at the same department.

"There has been a wonderful relationship between our labs," said Shayegan.

Until roughly a decade ago, he and Pfeiffer, who at the time worked at Bell Labs, had maintained a friendly competition in search of ever purer materials that allowed them to study ever more interesting physics problems. Then Pfeiffer joined Princeton.

As colleagues in the same department, they were then free to combine forces, quickly developing a natural divide-and-conquer approach to questions that, previously, they had been trying to answer on their own. As a result, in the 10-plus years since, Pfeiffer's group has built one of the world's finest material-deposition instruments while Shayegan's has refined leading methods to study the physics those ultra-pure materials reveal.

Although it's still too early to tell the overall ramifications such a discovery will have on the physics world, this is a remarkable step closer to unlocking the full potential of energy and the materials that will usher in the quantum age.