Study on the Mechanism of High-Temperature Cuprate Superconductivity Using Scanning Tunneling Microscope with Atomic Scale Spatial Resolution
Migaku Oda , Professor
Faculty of Science/Graduate School of Science (School of Science/Department of Physics)
High school : Hokkaido Prefectural Tomakomai Higashi High School
Academic background : Masters from Hokkaido University
- Research areas
- Solid state physics
- Research keywords
- high-temperature superconductivity, STM/STS
What are you aiming for?
“Superconductivity” is well-known phenomena demonstrated by the superconducting Maglev Train. It occurs when the electrical resistance of a metal reaches zero below a certain temperature (abbreviated to Tc). Superconductive materials (superconductors) also demonstrate the strange characteristic of being able to “float” above magnets. Fig. 1 shows a magnetic rail created by connecting square magnets, and a polystyrene train-shaped object into which a disk-shaped superconductor has been inserted. The train-shaped object (superconductor) is floating above the magnetic rail, and if it is hit from behind, it continues to operate without derailing, floating above the rail, even when entering a curve (Reference *1).
The strange phenomenon of superconductivity was discovered approximately 100 years ago by the Dutch physicist Onnes. Onnes discovered that the electric resistance of mercury suddenly reduced to zero at the ultra-low temperature of ‐269°C (4.2K). An explanation of the mechanism of superconductivity was not provided until around 50 years later in 1957, when three American physicists (Bardeen, Cooper and Schrieffer) described it. Their theory (the BCS Theory) states that superconductivity occurs when electrons (free electrons within metal), which have a negative electric charge and conventionally repulse one another, are drawn together with the assistance of ions (with a positive electric charge) and form pairs. When free electrons within metals (of which there are around 1022 per 1cc) form these pairs, they combine into a scrum and begin moving all at once, in contrast to when they move independently and become a liquid that is not subject to resistance (a superfluid). Furthermore, while Tc is related to the origin of electron attraction, the maximum Tc anticipated based on ion vibration according to BCS is at most roughly ‐240°C. In 1986, however, a Swiss researcher discovered that an oxide of copper demonstrated superconductivity at ‐237°C, and subsequently multiple high-temperature superconductors were discovered, one after another, from copper oxides, with Tc’s significantly exceeding that predicted by BCS. It is possible that the mechanism behind the electron attraction that occurs at such high Tc includes a new scientific concept. Once this mechanism is clarified, it may provide clues to discovering superconductors with even higher Tc’s. If we can find a substance that becomes a superconductor at ambient temperatures, it will become possible to transport power with no loss (superconductor power distribution) and run a superconductor linear motor car, without the use of highly expensive coolants (liquid helium). It would not be an exaggeration to call this a revolution in the world of electric power. We are engaged in studying the electronic properties of copper oxides in order to clarify the mechanisms of high-temperature superconductivity.
What sort of equipment are you using in your research?
Our main piece of equipment is the Scanning Tunneling Microscope (STM), which is a microscope that is able to look at atoms on the surface of a solid. When using STM, we place a sharply pointed metal probe at a perpendicular angle on the surface of the sample, and apply a voltage between the probe and the sample. As the probe approaches the surface of the sample, and the distance between the two reaches approximately 10-7cm, the (quantum-mechanical) tunnel effect of the electrons causes an electric current (tunnel current) to flow. The size of this current (~10-10A) is, put simply, dependent on the distance between the probe and the sample, as well as the electron density of the sample directly below the probe. If the distance between the probe and the sample is maintained at a fixed value, and the probe used to scan the surface of the sample, the tunnel current will vary according to changes in the electron density in the sample surface. These changes to the tunnel current, in response to electron density, are turned into an image using light and darkness, to create the STM image. Spatial differences in electron density are generally greater in locations where there are atoms and smaller in the spaces between atoms, and as such, using STM enables measurement of the position of atoms on the surface of the sample. Furthermore, STM can also be used in a measurement known as Scanning Tunneling Spectroscopy (STS), which allows us to look at electronic attributes such as the energy gap measured in superconductive states at a higher spatial resolution than atomic size.
Fig. 4 shows a sample STM image of a Bismuth-type copper oxide high-temperature superconductor. The lighter points correspond to atoms. The STS implemented during the process of measuring the STM image (Fig. 5), furthermore, clarifies some attributes of electron states that are interesting in terms of clarifying the occurrence mechanism of high-temperature superconductivity.
Fig. 4 STM image of high-temperature superconductivity
*1 Video of magnetic levitation train “UnderstandingHokkaidoUniversity through Video” (Faculty /GraduateSchool (Physics)):