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Study Reveals Limitations in Potential Quantum Computing Material

Quantum computers have the potential to carry out important operations that are considered to be impossible for current technology.

Deshpande and his team created devices using 3D TIs by stacking five few-atom-thin layers of various materials into sloppy sandwich-like structures. The core of the sandwich is the topological insulator, made from a few quintuple layers of bismuth antimony tellurium selenide (Bi2-xSbxTe3-ySey). (Image credit: Su Kong Chong)

Today’s computers are capable of processing data through transistors that carry one of two units of data, either a 0 or a 1. Quantum computing is predicated on the logic unit’s quantum mechanical behavior. Every quantum unit, or “qubit” for short, can exist in a quantum superposition instead of taking discrete values.

The qubits themselves are the biggest obstacles to quantum computing—creating logic units, which are sufficiently strong to carry instructions without being affected by the surrounding environment and ensuing errors, is an ongoing scientific challenge.

Physicists have hypothesized that a novel kind of material, known as three-dimensional (3D) topological insulator (TI), can serve as an excellent candidate from which qubits can be produced that would be adequately resilient from these kinds of errors and secured from losing their quantum data.

This material has an insulating interior as well as electricity-conducting metallic top and bottom surfaces. With regards to 3D topological insulators, the most significant property is that the conductive surfaces are assumed to be protected from the effect of the surroundings. There are several studies that have experimentally tested the way TIs behave in actual life.

As a matter of fact, a recent study performed at the University of Utah discovered that when the insulating layers are as thin as 16 quintuple atomic layers across, the bottom and top metallic surfaces start to affect one another and damage their metallic characteristics.

Such an experiment shows that the opposite surfaces start affecting one another at a relatively thicker insulating interior than earlier studies had demonstrated, potentially approaching a rare hypothetical phenomenon wherein the metallic surfaces also turn out to be insulating as the interior further thins out.

Topological insulators could be an important material in future quantum computing. Our findings have uncovered a new limitation in this system. People working with topological insulators need to know what their limits are. It turns out that as you approach that limit, when these surfaces start “talking” to each other, new physics shows up, which is also pretty cool by itself.

Vikram Deshpande, Study Corresponding Author and Assistant Professor, Department of Physics & Astronomy, University of Utah

The latest research has been reported in the journal Physical Review Letters on July 16th, 2019.

Sloppy Sandwiches Built from Topological Insulators

Assume a hardcover textbook as a 3D topological insulator, said Deshpande. Pages form the bulk of the book and serve as an insulator layer—they are incapable of conducting electricity. The hardcovers themselves indicate the metallic surfaces. Physicists discovered about 10 years ago that these surfaces can conduct electricity, and this led to an innovative topological field.

Along with his team, Deshpande used 3D TIs to fabricate devices and this was achieved by stacking five few-atom-thin layers of numerous materials into sloppy structures looking like a sandwich. The sandwich’s bulk core is the topological insulator, created from some quintuple layers of bismuth antimony tellurium selenide (Bi2-xSbxTe3-ySey).

This bulk core is packed by some layers of boron nitride, and is topped off with two graphite layers, both below and above. The graphite functions like metallic gates, fundamentally producing a pair of transistors that control conductivity.

In the previous year, Deshpande headed a study that revealed that this topological recipe constructed a device that acted like one would anticipate—bulk insulators that safeguard the metallic surfaces from the surrounding environment.

In the latest research, the 3D TI devices were manipulated by the team to observe the way the properties changes. The researchers initially constructed van der Waal heterostructures—the sloppy-like structures—and then subjected them to a magnetic field.

Deshpande’s group tested many of these heterostructures at his laboratory at the University of Utah, while the study’s first author Su Kong Chong, doctoral candidate at the U, traveled to the National High Magnetic Field Lab in Tallahassee to carry out the same kinds of experiments utilizing one of the highest magnetic fields available in the country.

A checkerboard pattern, in the presence of the magnetic field, evolved from the metallic surfaces, demonstrating the pathways through which electrical current will travel to the surface. In addition, the checkerboards—containing quantized conductivities versus voltages on the two gates—are clearly defined, with the grid intersecting at neat intersection points. This enables the investigators to monitor any distortion that may occur on the surface.

The researchers started with the insulator layer measuring 100 nm thick, which is roughly a thousandth of the diameter of a single strand of human hair, and this continuously became thinner down to 10 nm. The pattern began to distort until the insulator layer had a thickness of 16 nm, when the intersection points started to break up, producing a gap that denoted that the surfaces were not conductive anymore.

Essentially, we’ve made something that was metallic into something insulating in that parameter space. The point of this experiment is that we can controllably change the interaction between these surfaces. We start out with them being completely independent and metallic, and then start getting them closer and closer until they start ‘talking,’ and when they’re really close, they are essentially gapped out and become insulating.

Vikram Deshpande, Study Corresponding Author and Assistant Professor, Department of Physics & Astronomy, University of Utah

In 2010 and 2012, earlier experiments had also noticed the energy gap on the metallic surfaces when the insulating material becomes thin. However, a conclusion reached by those studies is that the energy gap appeared with relatively thinner insulating layers—that is, 5 nm in size. This analysis noted the breakdown of the metallic surface properties at relatively larger interior thickness of around 16 nm.

Different “surface science” techniques were used by other experiments where the materials were observed through a microscope with an extremely sharp metallic tip to look at each atom separately or examined with an intense energetic light.

These were extremely involved experiments which are pretty far removed from the device-creation that we are doing,” added Deshpande.

Deshpande and the team will look more thoroughly into the physics that produces that energy gap on the surfaces. Such gaps can be either negative or positive based on the thickness of the material.

Kyu Bum Han and Taylor Sparks from the U’s Department of Materials Science and Engineering are other authors who contributed to the study.


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