SPring-8, the large synchrotron radiation facility

Release Date

16 Jul, 2009

Okayama University

Achievement:
Hiroyuki Okazaki, a part-time researcher (second-year Ph.D. student), and Prof. Takayoshi Yokoya, both of Okayama University Graduate School of Natural Science and Technology, jointly with Takayuki Muro and Tetsuya Nakamura, Senior Scientists at Japan Synchrotron Radiation Research Institute, Prof. Masahiro Toyoda of the Faculty of Engineering, Oita University, Yoshihiko Takano, a group leader at National Institute for Materials Science, and Prof. Tamio Oguchi of Hiroshima University, successfully clarified the cause of the high T_c of CaC₆, which has the highest T_c of all graphite intercalation compounds (GICs),³⁾ for the first time in the world.

Background:
Graphite is a laminar substance comprising carbon sheets (Fig. 1) and semimetal. GICs, in which atoms and ions are intercalated between carbon sheets, show vastly different characteristics from those of graphite, and some of them even exhibit superconductivity. Most GICs, however, have a T_c of 1 K or lower. CaC₆ is a new GIC superconductor discovered in 2005 with T_c = 11.5 K, one order higher than that of typical GIC superconductors. Unfortunately, the reason for this high T_c had long remained unclear. In this experiment, the research group succeeded in directly proving that the 3d electrons of Ca contributed to the superconductivity, by measuring CaC₆ crystals by a special experimental technique called resonant photoemission spectroscopy⁴⁾ using Soft X-ray Spectroscopy of Solid Beamline BL25SU (Fig. 2) of SPring-8.

Significance and ripple effect:
The result of this research reveals that CaC₆ is a very unusual GIC superconductor because no other GIC superconductors with 3d electrons contributing to superconductivity have yet been discovered (Fig. 3). This indicates that the 3d electrons are the cause of the high T_c and that CaC₆ is a new type of GIC superconductor. The discovery of the CaC₆ superconductor is expected to lead to the development of ultralight superconductive materials with higher T_c achieved through the intercalation of transition-metal elements with 3d electrons between carbon sheets.

This research was supported by Grants-in-Aid for Scientific Research and by the Core Research for Evolutional Science and Technology.

<Figure>

Fig. 1 Crystalline structures of graphite (left) and CaC₆ (right)
In graphite, carbon sheets consisting of carbon atoms in a honeycomb network (graphene) are laminated, whereas in CaC₆, Ca atoms are intercalated between carbon sheets.

Fig. 2 Experimental results of resonant photoemission spectroscopy of CaC₆ Measured data obtained by resonant photoemission spectroscopy.
The energy of incident light is also shown on the right. The intensity of the electron band⁶⁾ near the Fermi level (E_F)⁵⁾ (green ellipse in Fig. 3) increases at the light energy of 348.6 eV, which is equal to the energy difference between the 2p electron orbital and 3d electron band of Ca. This is direct evidence that the electrons with energy near E_F contributing to the superconductivity originate from the 3d electron orbital of Ca.

Fig. 3 Distributions of electron energy for conventional GIC superconductor (left) and CaC₆ (right)
In the alkali-metal-intercalated GIC superconductors discovered thus far, electrons occupy the electron band of s orbitals of the alkali metals as well as the electron band of graphite (light blue area in the figure). In the case of CaC₆, electrons occupy the Ca 3d electron band, probably because the band shifts to a lower energy level from the original high energy level through the interaction with the electron band of graphite. The result of this research indicates that the 3d electron band of the intercalated Ca atoms is occupied by electrons, and in this sense CaC₆ is very unusual. The number of 3d electrons per unit energy (density of states) is greater than that of other orbital electrons. T_c is related to the density of states at EF, and the 3d electrons with a high density of states can thus enable a high T_c.

<Glossary>

1) Superconductivity
The superconducting state is a state in which a certain type of metal exhibits zero electrical resistance at a certain temperature specific to it or lower. The temperature at which a substance transforms into a superconductor is called the superconducting critical temperature (T_c). For typical superconductors, T_c is nearly absolute zero (0 K = -273.15 ºC). Superconductivity is being used in, for example, transmission lines without energy loss, because electric current can flow with zero resistance. Superconductors can also be used in linear motor cars because of their capability to generate strong magnetic fields. The excellent aspects of superconductivity can be used at a lower cost if T_c can be increased.

2) Electron orbitals
The electrons of atoms have only discrete energy levels. In order of increasing energy level, the electron orbitals are 1s, 2s, 2p, 3s, 3p, 4s, 3d, ….

3) Graphite intercalation compound (GIC)
A laminar compound in which atoms and ions are intercalated between layers of laminar graphite.

4) Resonant photoemission spectroscopy
When high-energy light is irradiated onto a substance and the kinetic energy of photoelectrons emitted as a result of the photoelectric effect is measured, the electron energy inside the substance can be measured. Resonant photoemission spectroscopy is an experimental technique of measuring the kinetic energy of photoelectrons emitted upon the irradiation of light energy equivalent to the energy difference between the inner shell level6) specific to the elements in a compound and the electron band6) to be observed. A specific electron orbital can be highlighted for observation by this method.

5) Fermi level (E_F)
The maximum energy of electrons in a compound. Electrons with energy near E_F determine the electric properties of the compound.

6) Electron band
For electrons in a crystal, low energy levels are maintained in discrete states (inner shell level), whereas high energy levels form a continuous energy range because of interaction. This is called an electron band. Electrons occupy energy levels starting from the low energy level.