A team of researchers from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley have constructed a material whose theorization and laboratory studies since the late 2000s, classify it among the IT substances of the future.
A New State of Matter
Two areas of materials science were involved in the production of the material: two-dimensional materials, such as graphene, and topological materials. Two-dimensional materials behave differently than the three-dimensional types. As for the topological materials, discovered in 2005, they are becoming increasingly popular for their potential contributions to quantum computing.
The new typology is paving the way for a new epistemology of materials physics. The materials behave as insulators in their interior and the edges act as conductors, hence the name topological insulation. Their edges have a highly metallic nature making them useful in electronic applications based on the electron’s spin. They are called topological insulators because their electronic properties are related to their topological characteristics.
Each type of material has an invariant configuration of atoms based on a well-defined symmetry. This physical representation—classifying materials as solids, liquids, etc., according to their topology or, in this case, the amount of symmetry they present—has been shattered by the discovery of topological insulators where using atomic symmetry as a means of description is no longer relevant. They are de facto considered to be a new state of matter. Since the crystal structure of conductive metals and non-conductive metals are different, topological insulators are said to have a variant topology.
Emergent properties of topological materials and insulators
1T’-WTe2: A New Era in Spintronics
In 2007, researchers succeeded in changing the topological configuration of a material by inverting surface electronic bands in relation to their volume. Also, the electron’s inherent property, called spin, makes it possible to impact the electrical resistance and the magnetism of the metallic edge.
Spin, known as the electron’s fourth property, is in fact the electron rotating on its axis. At the edges of the material the electron’s spin orientation functions much like a compass needle pointing either north or south. This quantum phenomenon can be used to produce a magnetic effect. Although new technologies in material growth and radiology have allowed these states to be observed and handled, the production of a magnetic topological insulator has long been a major challenge. The Berkeley Lab team finally succeeded.
In spintronics, as in traditional electronics, the electron’s electric charge and its spin are both used. The emergence of spintronics in the 1980s introduced the principle of magnetic data storage. The data storage potential was confirmed not only within the electron but also in its spin or, in other words, in the electron’s polarized states. However, the development of this type of material, in which the electron spin can be controlled, was not possible since it required a nanometric structure like that found in two-dimensional materials.
Future prospects for spintronics
The material, created by the team from 1T’-WTe2, being both two-dimensional as well as a magnetized topological insulator, can thus carry data faster using less energy and with less heat buildup.
It was manufactured and studied at Berkeley Lab’s Advanced Light Source (LAS), using a facility called the synchrotron. Basically, the synchrotron is an instrument that allows for the manipulation of charged particles at high energy.
Shujie Tang, author and co-lead in the study, was involved in growing 3-atom-thick crystalline samples of the material in a highly purified vacuum-sealed compartment at the ALS, using the molecular beam epitaxy process. The high-purity samples obtained were subsequently studied using ARPES, or angle-resolved photoemission spectroscopy, which makes it possible to probe materials’ electron properties.
Tang claims his team is the first ever to have produced two-dimensional topological insulation and to confirm its structural characteristics with ARPES.
Indeed, since the edge of the material, which is the conductive part, is atomically thin—measuring only a few nanometres or thousands of times thinner than an X-ray beam—it was difficult to identify all the electronic properties of the material. For example, UC Berkeley collaborators performed additional atomic scale measurements using the scanning tunneling microscopy (STM) technique to accurately measure the edge state of the material.
The research began in 2015 and involved more than two dozen researchers, in addition to collaborative computer research within Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC). The team is now aiming to develop larger samples of the material and to discover how to selectively tune and emphasize certain properties. Besides its conductive properties, the material is light-sensitive and can be used in the photovoltaic and optoelectronic fields.
The study, entitled “Quantum spin Hall state in monolayer 1T’-WTe2“, was published in the Nature Physics journal on June 26, 2017. Along with the above-mentioned university scientists, other researchers in this study came from the State Key Laboratory of Functional Materials for Informatics at the Shanghai Institute of Microsystem and Information Technology, the Department of Physics at Pusan National University and the Max Planck POSTECH Center for Complex Phase Materials (Korea).
