“Massless” Electrons Could Make Way for Faster Electronic Devices

August 27, 2020 by Luke James

The discovery of “massless” electrons in a phase-change material could bring us faster electronic devices, researchers say.

Phase-change materials have attracted a great deal of attention from researchers due to the contrast in optical and electrical properties between their two phases. Their atomic structures shift from amorphous to crystalline and back again when heated and cooled, and this makes them useful for electronic devices where information needs to be written and rewritten constantly. 

Now, researchers at Hiroshima University say that they have found Dirac electrons—electrons that behave as if they have no mass—in a phase-change material commonly used in CDs and DVDs. It is thought that the discovery of “massless” electrons could lead to faster electronic devices.


A “3D Analog” of Graphene

The material in question is GeSb2Te4, a topologically nontrivial phase-change compound commonly used as a memory material in phase-change optical disks. It’s unique because its crystalline phase has Dirac electrons that behave like a “3D analog of graphene.”

In the published research paper, the researchers explain that amorphous phase demonstrates a semiconducting behavior with a large electrical resistivity. In contrast, the crystalline phase behaves like a metallic with lower electrical resistivity. 


The crystal structure for the intermixed crystalline phase of GeSb2Te4

On the left is the crystal structure for GeSb2Te4's intermixed crystalline phase. The center image depicts an "angle-resolved photoemission spectrum of crystalline GeSb2Te4." The right-side image shows a schematic band structure of crystalline GeSb2Te4. Image used courtesy of Akio Kimura, Hiroshima University

Although graphene is already widely used as a high-speed conductor, it has a few shortcomings in electronic devices. However, the researchers say that GeSb2Te4 is a more efficient material that pairs both speed and flexibility. And because each phase has individual reversible properties, it can be used in electronic devices where information is repeated multiple times over. 


Chalcogenide Phase-Change Materials

Researchers were already well aware of the contrasting optical and electrical properties in chalcogenide phase-change materials. This has led to their extensive use in memory devices. 

However, by performing spin-, time-, and angle-resolved photoemission spectroscopy, the research team found in experimental results that the material’s crystalline phase became topologically nontrivial near the Dirac semimetal phase. This results in linearly dispersive bulk Dirac-like bands that cross the Fermi level and are responsible for the conductivity in the stable crystalline phase of GeSb2Te4.

In addition, the researchers found that the crystalline structure’s surface shares characteristics with a topological insulator, where the internal structure is static while the surface acts as a conductor. 

As a 3D version of graphene, GeSb2Te4 integrates speed with the flexibility to make way for the next generation of electrical switching devices.