Berkeley Lab Develops 3D Racetrack for Electrons in Nanoscale Crystal Slices

A scanning electron microscope image shows triangular (red) and rectangular (blue) samples of a semimetal crystal known as cadmium arsenide. The rectangular sample is about 0.8 microns (thousandths of a millimeter) thick, 3.2 microns tall and 5 microns long. The triangular sample has a base measuring about 2.7 microns. The design of the triangular samples, fabricated at Berkeley Lab’s Molecular Foundry, proved useful in mapping out the strange electron orbits exhibited by this material when exposed to a magnetic field. The red scale bar at lower right is 50 microns. (Credit: Nature, 10.1038/nature18276)

A unique 3D racetrack for electrons in extremely thin slices of a nanomaterial has been developed by researchers. This nanomaterial was created at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

For the very first time, the global team of researchers from Berkeley Lab, UC Berkeley, and Germany observed unique behavior where electrons rotate around one surface, and then through the volume of the material to its opposite surface and back.

The likelihood of developing the “topological matter” that is capable of carrying electrical current on its surface without any loss at room temperature has gained immense interest in the research community. The main aim is to approach the lossless conduction of a different class of materials called superconductors, but without the requirement for the extreme, freezing temperatures that superconductors need.

Microchips lose so much energy through heat dissipation that it’s a limiting factor. The smaller they become, the more they heat up.

James Analytis, Staff Scientist, Berkeley Lab

An inorganic semimetal called cadmium arsenide (Cd3As2), which is the material under study, displays quantum properties, which are not explained by the standard laws of physics. These properties provide a new system that will help to decrease waste energy in microchips. In 2014, researchers identified that cadmium arsenide shares a few electronic properties with graphene, which is a single-atom-thick material vital for next-generation computer components, but in a 3D form.

“What’s exciting about these phenomena is that, in theory, they are not affected by temperature, and the fact they exist in three dimensions possibly makes fabrication of new devices easier,” Analytis said.

A quantum property called chirality was displayed by the samples of cadmium arsenide. This property merges an electron’s basic property of spin to its momentum, especially providing it right- or left-handed traits. The experiment offered an initial step toward the aim of using chirality to transmit energy and charge through a material without any loss.

As part of the experiment, researchers manufactured and analyzed how electric current passes in slices of a cadmium arsenic crystal just 150 nanometers in thickness - about 600 times smaller compared to the width of a strand of human hair - based on a high magnetic field.

The samples of the crystal were crafted at Berkeley Lab’s Molecular Foundry, which aims to construct and analyze nanoscale materials. The 3D structure of these materials was studied using X-rays at Berkeley Lab’s Advanced Light Source.

Several mysteries still exits in respect to the unique features of the material under study. The researchers are now identifying other fabrication methods that will help to construct a similar material with built-in magnetic features, preventing the need for an external magnetic field.

“This isn’t the right material for an application, but it tells us we’re on the right track,” Analytis said.

The success of their adjustments determines the possibility of using a material like this to develop interconnects between multiple computer chips, for example, for next-generation computers that rely on an electron’s spin to process data called spintronics, and also for manufacturing thermoelectric devices capable of transforming waste heat to electric current.

Analytis stated that at first it was not possible to obtain a clear picture whether the researchers were even capable enough to develop pure sample at the smallest scale needed to perform the experiment.

We wanted to measure the surface states of electrons in the material. But this 3-D material also conducts electricity in the bulk—it’s central region—as well as at the surface. As a result, when you measure the electric current, the signal is swamped by what is going on in the bulk so you never see the surface contribution.

James Analytis, Staff Scientist, Berkeley Lab

Hence, the researchers shrunk the sample from millionths of a meter to the nanoscale to provide them increased surface area and guarantee that the surface signal would be the leading one in an experiment.

“We decided to do this by shaping samples into smaller structures using a focused beam of charged particles,” he said. “But this ion beam is known to be a rough way to treat the material—it is typically intrinsically damaging to surfaces, and we thought it was never going to work.”

But Philip J.W. Moll, currently at the Max Planck Institute for Chemical Physics of Solids in Germany, discovered a method to minimize this damage and offer surfaces that are finely polished in the small slices using tools at the Molecular Foundry. “Cutting something and at the same time not damaging it are natural opposites. Our team had to push the ion beam fabrication to its limits of low energy and tight beam focus to make this possible.”

When an electric current was applied to the samples, the team discovered that the electrons race around in circles - just like how they orbit the nucleus of an atom, but their path travels through the bulk and the surface of the material.

The electrons are pushed around the surface by the applied magnetic field. Once the same momentum and energy of the bulk electrons is reached, they are dragged by the chirality of the bulk and then get pushed through to the other surface, repeating this unique twisting path until the electrons are scattered by defects in the material.

This experiment symbolizes a successful marriage of theoretical approaches with the ideal techniques and materials, Analytis said.

“This had been theorized by Andrew Potter on our team and his co-workers, and our experiment marks the first time it was observed,” Analytis said. “It is very unusual—there is no analogous phenomena in any other system. The two surfaces of the material ‘talk’ to each other over large distances due to their chiral nature.”

We had predicted this behavior as a way to measure the unusual properties expected in these materials, and it was very exciting to see these ideas come to life in real experimental systems. Philip and collaborators made some great innovations to produce extremely thin and high-quality samples, which really made these observations possible for the first time.

Andrew Potter, Assistant Physics Professor, University of Texas

The team also studied that disorder in the patterning of the crystal surface of the material does not influence the behavior of electrons there, even the disorder in the central material develops an impact on whether the electrons pass through the material from one surface to the other surface.

A dual handedness is exhibited by the movement of the electrons, with a few electrons passing around the material in a single direction and others twisting in an opposite direction.

Researchers are currently advancing this work by focusing on designing new materials for futuristic studies, Analytis said. “We are using techniques normally restricted to the semiconductor industry to make prototype devices from quantum materials.”

Berkeley Lab’s Molecular Foundry and Advanced Light Source are both DOE Office of Science User Facilities.

The Department of Energy’s Office of Science, the Gordon and Betty Moore Foundation, and the Swiss Federal Institute of Technology in Zurich (ETH Zurich) supported this study.

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