Clarification of Novel Phenomenon Expressed by Iron-Based High-Temperature Superconductor (Press Release)
- Release Date
- 15 Nov, 2010
- BL08W (High Energy Inelastic Scattering)
- First-Ever Observation of Disappearance of Superconductivity When Iron Atoms Become Magnetized
Japan Synchrotron Radiation Research Institute
Using the X-ray synchrotron radiation at SPring-8, researchers of Japan Synchrotron Radiation Research Institute (JASRI; Tetsuhisa Shirakawa, President), in collaboration with those of the University of Michigan (USA) and Zhejiang University (China), found, for the first time in the world, that the superconductivity of an iron-based high-temperature superconductor*1 disappears when iron atoms become ferromagnetic (magnet). This result was achieved through a high-accuracy magnetic Compton scattering*2 experiment (Fig. 1) using the stable high-brilliance and high-energy X-rays of SPring-8. The iron-based high-temperature superconductor EuFe2(As,P)2 examined in this study is a unique material showing both superconductivity*3 and ferromagnetism,*4 which have never been observed in conventional superconductors. The reason why it has both properties has not been clarified in detail. However, the constituent atoms of the superconductor may have different roles: the iron (Fe) atoms play the role of a superconductor and the europium (Eu) atoms play the role of a ferromagnet. This enables the coexistence of superconductivity and ferromagnetism. In this study, the researchers succeeded in detecting faint signals from the Fe atoms and found that the superconductivity temporarily disappears when the Fe atoms become ferromagnetic (magnet). This finding has promoted research to clarify the mechanism of iron-based high-temperature superconductors, which have recently attracted attention in materials science, and basic research to solve the longstanding issue of the coexistence of superconductivity and ferromagnetism. Also, this finding may lead to the development of new electronic devices that can switch between the expression and nonexpression of superconductivity by the on/off control of a magnetic field. Such new devices based on superconductivity can be operated at higher speeds than conventional semiconductor devices. This result was achieved through joint research by Masayoshi Ito, an associate senior scientist, and Yoshiharu Sakurai, an associate chief scientist, of JASRI; Guanchan Cao, a professor of Zhejiang University; and James Penner-Hahn, a professor, and Aniruddha Deb, a researcher, of the University of Michigan. Their achievement was published in the online version of the American scientific journal Physical Review Letters on 15 November 2010. （Publication） |
《Glossary》
*1 Iron-based high-temperature superconductor
This is one of high-temperature superconductors discovered in 2008. It was a great surprise to discover a high-temperature superconductor containing Fe atoms, which easily become ferromagnetic (magnet), as the main component because a strong magnetic field destroys superconductivity. After that, an iron-based high-temperature superconductor having both ferromagnetism and superconductivity was found, and research to clarify the coexistence mechanism has been progressing. While superconductivity is achieved only when the material is cooled, the name “high-temperature” here is used to indicate the high transition temperature (the temperature at which a material becomes superconductive) that cannot be explained by conventional superconductivity theories.
*2 Magnetic Compton scattering
Compton scattering is considered to occur as a result of an elastic collision between an electron and an X-ray photon, similar to the collision of billiard balls. By measuring the X-ray photon energy after Compton scattering, the momentum (or velocity) of an electron before Compton scattering can be determined. In magnetic Compton scattering, a method using Compton scattering, the spin magnetic moment*7 can be measured by aligning the directions of the electron spins (micromagnets) in a magnetic field induced by a superconducting electromagnet using circularly polarized*6 incident X-rays.
*3 Superconductivity
This is the phenomenon that, when a metalic material is cooled, the electrical resistance suddenly drops to zero at a low temperature in the vicinity of absolute zero.*5 From the currently established Bardeen, Cooper, and Schrieffer (BCS) superconductivity theory, the maximum superconducting transition temperature is approximately 30 K. Because the transition temperatures in cuprate and iron-based superconductors largely exceed this limit, they are called high-temperature superconductors.
*4 Ferromagnetism
An electron is an extremely small magnet. The phenomenon in which the N poles and S poles of all these small magnets are aligned in the same direction and the material, on the whole, takes on the characteristics of a large magnet is called ferromagnetism.
*5 Absolute zero and Kelvin
Absolute zero is the lower limit of temperature and nothing can be colder. The temperature scale where absolute zero is set as zero is called the absolute temperature scale and is expressed in Kelvin (K). Namely, absolute zero is 0 K, which is -273.15°C on the familiar Celsius scale. Also, 0°C is 273.15 K on the absolute temperature scale.
*6 Circularly polarized
Light and X-rays are transverse waves that travel while electric and magnetic fields are oscillating. If the direction of the electric field rotates 360° around the axis of propagation of light while the electric and magnetic fields travel a distance of one cycle of oscillation, the light is described as circularly polarized. When one looks in the direction of the source of the oncoming light and the electric field of the light rotates clockwise around the time axis, it is described as right circular polarization. If it rotates counterclockwise, it is described as left circular polarization.
*7 Spin magnetic moment
Electrons are charged negatively and are spinning at a constant rate. Therefore, electrons have the characteristics of a magnet with a certain magnetic strength because a constant current flows continuously around the spin axis. The magnetic strength of the electrons is called the spin magnetic moment.
《Figure》
Compton scattering occurs as a result of an elastic collision between an electron and an X-ray photon, similar to the collision of two billiard balls. By measuring the X-ray photon energy after Compton scattering, the momentum (or velocity) of the electron before Compton scattering can be measured. In magnetic Compton scattering, a method using Compton scattering, the spin magnetic moment can be measured by aligning the directions of electron spins (micromagnets) in a magnetic field induced by a superconducting electromagnet using circularly polarized incident X-rays.
(A) When two electrons are paired in a Cooper pair, the electron pair can move through a metal material without resistance. In a superconducting state, the electric resistance of the material becomes zero because such electron pairs are formed and move through without resistance.
(B) When a strong magnetic field is applied, the electron pairs (Cooper pairs) are broken and superconductivity does not occur. Since ferromagnetism generates a strong magnetic field inside a material, it is generally considered that superconductivity and ferromagnetism do not coexist in a material. The iron-based superconductor examined in this study is a unique material in which high-temperature superconductivity and ferromagnetism coexist at low temperature.
EuFe2(As0.73P0.27)2with coexisting superconductivity and ferromagnetism
The crystal structure of the iron-based high-temperature superconductor consists of two layers, namely, the iron-arsenic layer in which Fe atoms and arsenic (phosphorus) (As(P)) atoms are arrayed in a zigzag pattern and the europium (Eu) layer in which Eu atoms are arrayed flat. When superconductivity and ferromagnetism coexist, the iron-arsenic layer acts as the superconductor and the europium layer acts as the ferromagnet. When the Fe atoms in the iron-arsenic layer become ferromagnetic, superconductivity disappears.
superconductor EuFe2(As0.73P0.27)2 with coexisting superconductivity and ferromagnetism
When an iron-based high-temperature superconductor is cooled, the electric resistance sharply drops at 26 K (A), showing the transfer to superconductivity. When the superconductor is further cooled, the electric resistance temporarily increases at 18 K, the temperature at which the material becomes ferromagnetic (B), and superconductivity is about to disappear. The electric resistance becomes zero below 10 K where superconductivity and ferromagnetism coexist. The unit of electrical resistance is mΩ・cm.
in iron-based high-temperature superconductor EuFe2(As0.73P0.27)2
with coexisting superconductivity and ferromagnetism
The spin magnetic moments of the Fe and Eu atoms were determined by high-accuracy magnetic Compton scattering*2 However, the values at 0 K were obtained by theoretical calculations. The spin magnetic moments are proportional to the degree of ferromagnetism, namely, the magnetic strength of the material. Because the spin magnetic moment of the Eu atoms was significantly larger than that of the Fe atoms in the temperature range of this measurement, the Eu atoms were considered to play the role of the ferromagnet in the coexistent state of superconductivity and ferromagnetism. Whereas the electric resistance temporarily increases and superconductivity disappears at 18 K, as shown in Fig. 4, the spin magnetic moment of Fe atoms increases at the same temperature (inside the red dashed circle). These results indicate that superconductivity disappears because the superconducting Fe atoms temporarily tend to become ferromagnetic (magnet) at 18 K. The unit of spin magnetic moment is Bohr magneton.
For more information, please contact: Dr. Masayoshi Ito (JASRI) |
