Text of a lecture by Dr. Junji Tominaga

In phase-change memory, which is Dr. Tominaga’s research area, it is safe to say that any point reached marks a new starting point. He is currently working on topological insulators for use in next-generation phase-change memory. What does this strange word “topology” mean? We will introduce the possibilities and application fields of topological insulators, to which the topology theory is applied.
*This article was released in “PR Magazine No.69 published in December, 2019”

The Nobel Prize in Physics 2016 was awarded to the following three people: Thouless, Haldane, and Kosterlitz. These three introduced the concept of topology, which is one of the geometric theories in mathematics, and discovered topological phase transitions in the basic characteristics of matter. While developing materials at the cutting edge of modern science in the 21st century, scientists all over the world are waging research battles in pursuit of great possibilities.
As part of this, research is underway on topological insulator materials, in which electricity flows on the surface despite the fact that no electricity flows inside.

Tominaga：In the fall of 2010, I had submitted a paper on superlattice energy-saving phase-change memory to a professional journal, and I was relieved that the paper had just been accepted. Because of the Great East Japan Earthquake of March 11, 2011, I could not conduct any experiments for three or four months, so I spent most of my time reading technical papers. One paper said that Sb2Te3, which I was working with, is a topological insulator. While I was reading technical papers, the expression “time-reversal symmetry” caught my attention. So I decided to apply a magnetic field. When I brought a magnet close to an ordinary ternary alloy, nothing happened. But when I brought a magnet close to the superlattice stacked film I had developed, its threshold voltage jumped from 0.8 to 2 V. When I removed the magnet, the threshold voltage returned to its original value. It was a weak magnet of around 0.1 Tesla, but I found that bringing this magnet close to the film changed its resistance value by two orders of magnitude. The change that occurs in MRAM is much smaller. Based on past experience, I had thought that phase change did not exhibit any magnetism. This experiment showed that destroying the time-reversal symmetry would cause some change.

In a topological insulator, the state of its electrons (wave function) is said to be “twisted,” unlike in ordinary insulators. An unimaginable phenomenon was confirmed in which this twist prevented electricity from flowing inside the material while allowing it to flow only on its surface.

Tominaga：The Ge₂Sb₂Te₅ ternary alloy is an ordinary insulator since it does not have any twist. In other words, there are two faces inside the alloy. Part of it is an ordinary insulator while another part is different. What about a superlattice? An ordinary topological insulator only has planar electrical conductivity. (GeTe)₂ is an ordinary insulator while Sb₂Te₃ is a topological insulator. When these two materials are repeatedly stacked, electricity will flow not only on the surfaces, but also on their interfaces. Since increasing the number of layers will proportionately increase the number of interfaces, we can extract more two-dimensional current and spin current. Furthermore, this can be accomplished at temperatures that are practical for  manufacturing instead of at super- low temperatures. It works fine at 470K. I cannot go into detail due to lack of space, but the technical paper in which I published my research results was cited in other papers around 300 times in 2017. There is now global competition to create materials like
this.
There are many kinds of memory, and the fastest types are CPU, SRAM, DRAM, etc. Below these, there are storage memory devices, such as optical discs and hard disks (HDs). In terms of processing speed, DRAM is faster than HD by three orders of magnitude. When handling big data, this difference will become a major problem. To solve this problem, storage class memory has emerged. It is phase-change memory.

The great potential of phase-change memory, to which topological insulator superlattices will be applied

• The next-generation phase-change memory will be a superlattice type and will be able to achieve significant energy savings.
•  Phase-change memory is ideal for AI chips.
•  It will become possible to carry out machine learning using big data and without using DRAM.
•  Superlattice films using van der Waals bonding can also be made using sputtering.
•  GeTe/SbTe superlattice film is a topological insulator.
•  If topological characteristics can be manifested successfully, they can be applied to spin memory in the future.
•  Advances in phase-change memory can be expected to be applied to fields beyond memory.

Research policy taking a unique point of view with the motto, “We can do it if we try”

Junji Tominaga, PhD, National Institute of Advanced Industrial Science and Technology

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