uantum mechanics is a difficult subject to understand, but we can look forward to a very interesting world ahead. I think we need to make efforts to acquaint the public with the world of quantum mechanics. I am sure you feel a sense of adventure when you learn about the birth of the Universe and black holes. Those astronomical phenomena are also based on quantum mechanics. You can find mysterious phenomena that mirror the workings of the Universe even in electronic devices that are familiar to everyone. An electron recently discovered in molecular compound crystals by Naoya Tajima, Senior Research Scientist and a member of this laboratory, is an example of this, because the electron behaves in the same manner as a neutrino.” Kato and members of the laboratory are taking advantage of molecular compounds made mainly of organic materials to create new superconductors and materials for electronic devices.
Discovery of a new type of superconductor
“One typically mysterious phenomenon related to quantum mechanics and caused by electrons is superconductivity,” says Kato. Superconductivity is a phenomenon in which electrons are free to move without any electrical resistance when a substance is cooled below a certain temperature (its transition temperature). Since 1911, when superconductivity was first discovered in mercury, the phenomenon was found only in other metals and in alloys that were comparatively good electrical conductors.
However, in 1980, a superconductor (organic superconductor) was found in certain organic compounds that had been considered typical insulators. Then, in 1986, a superconductor was discovered from copper-oxide compounds that are resistant to electric current. The superconductor showed a transition temperature of 30 K (about -243 °C), which was above the highest transition temperature of 23 K then known. This ignited a worldwide fever of enthusiasm for high-temperature superconductors, followed by a development race for superconductors with higher transition temperatures. Currently a copper-oxide compound shows the highest known transition temperature, which is now 160 K (about -113°C). “The mechanisms of superconductivity for copper-oxide compounds and organic compounds are considered to have something in common. For over 20 years, worldwide research efforts have been directed toward understanding the mechanism, but no scientific theory acceptable to all has yet been presented.”
Superconductivity is caused by electrons when they form electron pairs (Cooper pairs). How can they form electron pairs in spite of the repulsive force caused by their negative charges? That is the biggest mystery in the mechanism of superconductivity.
In certain metals, some electrons separate from their atoms, yielding a crystal lattice in which positively charged ions are arranged in an orderly manner. Thus the crystal lattice itself is positively charged and acts to bind electrons when it vibrates. This theory, which is known as the BCS theory, was published in 1957, and the exponents of the theory were awarded the Nobel Prize in Physics in 1972. It is not considered, however, that the BCS theory can explain superconductivity in copper-oxide compounds or organic compounds. “Both copper-oxide and organic compounds are originally electrical insulators. What is important here is that they exhibit magnetic properties.” Here, the magnetic properties derive from the motion of the electron spins, similar to the rotation of the Earth around its axis. Electron spin exists in one of two possible states: upward or downward. An electron acts like a small bar magnet with its north pole pointing upward or downward.
An original copper-oxide compound or organic compound is stable when the directions of the electron spins are arranged alternately at low temperature. This is similar to the situation in which two bar magnets repel each other when arranged in parallel and in the same direction, whereas they stabilize when arranged alternately, in opposite directions. “Electrons in an electrical insulator begin to move very quickly and become a superconductor when some stimulating effect is given to the insulator to accelerate the motion of the electrons. Thus, the key to solving the mystery of why electrons in copper-oxide compounds or organic compounds form electron pairs is thought to be the state in which electron spins in these materials tend to arrange themselves alternately.”
‘Frustration’ causes electrons to form pairs
In 2007, a superconducting phenomenon originating from a different state from this was discovered by fellow members of our Condensed Molecular Materials Laboratory including Masafumi Tamura, former Vice Chief Scientist (Professor at Tokyo University of Science from this April), Yasuhiro Shimizu, former Special Postdoctoral Researcher (now Special Lecturer at the Institute for Advanced Research, Nagoya University), and Akiko Tajima, Contract Technical Scientist.
The new phenomenon was discovered in a compound made of molecules called Pd(dimt)2. The electron spins in the compound also tend to arrange themselves alternately. The crystal lattice of the compound, however, is a structure based on triangles. In this crystal lattice structure, electron spins cannot arrange themselves alternately, causing the direction of electron spins to be unstable. For example, consider three electrons, A, B, and C, in which spin A is directed upward whereas spin B is directed downward. Iin which direction can spin C be directed? If spin C is directed upward, the direction is the same as that of spin A, whereas if spin C is directed downward, the direction is the same as that of spin B. “This is a kind of state of frustration in which spin C cannot be directed in either direction.”
Just as human beings act unusually when frustrated, so also do electrons in materials. This ‘spin frustration’ system is now the center of attention in research into the physical properties of materials.
A surprising phenomenon was found in a compound known as Pd(dimt)2. “When the crystal lattice of the compound was cooled, the lattice deformed spontaneously; electrons came closer to each other to form electron pairs, and the electron spins of two electrons in a pair cancelled each other out, causing the resultant electron spins to seem to disappear.”
This mechanism is the same as that for two hydrogen atoms when they form a hydrogen molecule through covalent bonding. In other words, the two electrons succeeded in solving the ‘frustration’ problem by canceling out each other’s spin. In this state the material was an insulator, but it exhibited superconductivity when a slight force was applied to it, causing the electrons to come closer to each other and move more freely through the structure (Fig. 1b, right).
Aside from the route that leads to superconductivity from an alternately arranged spin state, we discovered a new route that leads to superconductivity from the state in which electron spins cancel out. “A new theory that explains the mechanism of superconductivity in copper-oxide or organic compounds should also be able to explain superconductivity through the new route. Theoretical scientists may be in trouble with this. In contrast, experimental scientists are very pleased to discover phenomena that are difficult for theoretical scientists to explain, because the discovery can lead to significant development in the theory.”
Advances in the theory of superconductivity contribute to the development of superconductors with higher transition temperatures. Today, high-temperature superconductors are being developed for applications that include highly efficient power lines and superconducting magnets. The current highest transition temperature, however, is 160 K, which means that superconductivity cannot be induced unless the temperature is reduced to 160 K or below. If a room-temperature superconductor is discovered, the discovery will revolutionize the world of electronics and will be significant in solving energy problems. “The discovery of high-temperature superconductors contributed to raising the transition temperature by a factor of almost ten, and what is needed is to increase the 160 K transition temperature to room temperature (about 300 K), which is only a factor of two. I think that room-temperature superconductors can be developed. We expect that the development may be triggered by our discovery of the canceling out of electron spins in the frustrated state.”
Fabricating 100-petabyte memory
In January 2008, the Condensed Molecular Materials Laboratory presented another major research result with the potential to revolutionize the world of electronics: Hiroshi Yamamoto, Senior Research Scientist, and other members successfully fabricated a synthesized structure of nanowires coated with insulating material.
Various high-performance electronic devices as well as small, lightweight electronic devices are supported by large-capacity memory devices, which are in turn based on the microfabrication of memory elements and wiring. The minimum width of a wire printed on semiconductor wafers is limited to several tens of nanometers, and nanowires are expected to serve as a future material for microfabrication because they are about 1 nm (10-9 m = one billionth of a meter) in diameter. At present, carbon nanotubes made of carbon atoms and semiconductor nanowires have been developed, but short circuits have been occurring between the nanowires because there has been no technique available to coat them with insulating material.
Yamamoto and other laboratory members combined conductive organic molecules and electrically insulating organic molecules in a successful synthesis of insulator-coated nanowires. These organic molecules come together spontaneously and function when they are combined by a weak force to form a complex structure. This process is called ‘self-organization.’
“For example, some molecules have a relationship like that between a key and a lock. They can spontaneously form a complex structure when mixed. We continued experiments with organic molecules to use self-organization in creating conductive molecular compounds. Insulator-coated nanowires are one of the research results,” says Kato. “It would not be truthful to say that we started off intending to form a complex structure. We tried different approaches and happened to find a prototype structure, which was then improved.”
Conventional semiconductor circuits have various elements and wires finely printed on a two-dimensional plate. When a circuit is fabricated in three dimensions with the insulator-coated nanowires developed by Yamamoto and other members of the laboratory, the circuit can be used as a memory device with an enormous storage capacity of 100 petabytes per cm3.
How can we imagine a storage capacity of 100 petabytes? The storage capacity of current USB memory devices, DVD media, or hard discs is measured in units of gigabytes (109 = 1 billion bytes). Tera (1012 = 1 trillion) is 1,000 times giga, and peta (1015 = 1,000 trillion) is still larger than tera. Thus 100 petabytes are equivalent to 100 million 1-gigabyte USB memories, or two million next-generation Blu-ray discs (50 gigabytes each). “Memory on this storage scale would remove the various operational constraints resulting from memory capacity. As a result, electronic devices with inconceivable functions could be produced.”
The biggest challenge in achieving enormous mass-storage is how to assemble the nanowires. Here, self-organization becomes the key. “I cannot say when it will be ready because it is the subject of future research. It might easily be developed by using self-organization if we have a technical breakthrough,” says Kato, giving his own views.
Learning from the functional mechanism of biological molecules
“An intelligent computer with the same functions as the human brain would be huge if it were made of inorganic materials such as silicon, which is the main material used in semiconductor devices, and the computer would consume enormous amounts of electrical energy. In contrast, the brain is compact and does not require much energy. At the present level, solar cells are not nearly as efficient as plant photosynthesis in energy use.”
Why do plants have such efficient systems? Biological molecules such as proteins, which are vital components of organisms, are also molecular compounds made mainly of organic materials. They feature flexibility and are amazingly abundant in variety. “Biological molecules are considered to be able to function very efficiently by drastically changing their characteristics in response to small conditional changes. This is similar, for example, to an organic molecule that changes drastically from an insulating state to a superconducting state in response to slight pressure.”
Proteins function properly when they are weakly combined with, or are separated from, other proteins or DNA. They change their molecular shapes in delivering or receiving electrons. “Biological molecules are similar to the conducting molecular compounds we are studying from the standpoint that biological molecules interact with each other to fulfill their function in the presence of a weak force. The function is based on chemical reactions and is driven by the motions of the electrons. This means that we need to understand electronic states before understanding the basic functioning of biological molecules.”
The Condensed Molecular Materials Laboratory formed the Research Group for Molecular Ensemble jointly with in-house research teams that are engaged in research into physical properties, structural and performance analyses of proteins and development of protein affecting low molecules. This group seeks to analyze the functional mechanism of biological molecules at the electron microscopic level.
“To be frank, the electronic states of conducting molecular compounds are very easy to understand. For example, it takes a large-scale computer to calculate precisely the electronic states of electrons that move freely in iron. In contrast, conducting molecular compounds are in a state in which electrons, which are originally at rest in their positions in the molecules, are forced to move through the molecules. Electronic states of this kind can be calculated by a computer or by computations on paper. Thus, we are beginning to understand that our method for analyzing electronic states in molecular compounds is effective also for biological molecules whose electrons are considered to have similar characteristics as those of molecular compounds.”
A new study from a new perspective will lead to an understanding of the functional mechanism of biological molecules at the electron microscopic level. This will trigger the development of high-performance, high-efficiency, and energy-saving electronic devices using molecular compounds as their key materials.
About the researcher
Reizo Kato was born in 1955 in Yamaguchi, Japan. He received his BSc degree in 1979, his MSc in 1981 and his DSc in 1984 from the University of Tokyo. He was appointed research associate of the Department of Chemistry at Toho University in 1984, and he was promoted to lecturer in 1988. He joined the Institute for Solid State Physics in the University of Tokyo as an associate professor in 1990. Since 1999 he has been a chief scientist, a director of the Condensed Molecular Materials Laboratory in RIKEN. He received the Chemical Society of Japan Award for Young Chemists in 1990 and the IBM Japan Science Prize in 1995 for his studies on molecular conductors. His research has been focused on the development of new molecular materials, especially molecular metals and superconductors.
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