It’s all in the spin

Japanese researchers show subtle fluctuations in electron spins are the origin of magnetism and superconductivity in a common oxide

Magnetism and superconductivity are material properties that generally exclude each other. The reason is that in a magnet, the electron spins—tiny magnets responsible for the material’s magnetism—align in a common direction. On the other hand, superconductivity requires the pairing of electrons with opposing spins.

In that respect, sodium cobalt oxide, NaxCoO2, is unusual. It has a lattice structure with crystal planes formed by cobalt (Co) and oxygen (O) atoms. Along those planes it is ferromagnetic. However, when brought into contact with water, water molecules integrate into the material’s crystal structure forming a slightly modified hydrated compound, NaxCoO2 • yH2O. And, this compound shows superconductivity along the CoO2 planes at low temperatures.

However, a team of researchers from RIKEN’s Discovery Research Institute in Wako, and colleagues from the universities of Chofu and Nagoya, now propose that magnetism and superconductivity in the non-hydrated and hydrated forms of NaxCoO2 actually share a common origin (1).

Typically, the origin of properties such as magnetism or superconductivity lies in the way electrons occupy the internal electronic states of a material. Like water filling an empty bucket, electrons in a material occupy all available electronic states beginning with the ones having the lowest electronic energy. Those electrons ending up at the ‘top’ of the bucket, also called the ‘Fermi surface’ (Fig. 1 - click on link below), are responsible for the electronic behavior of a material.

This Fermi surface can take quite complicated shapes. It can even consist of completely separate regions, particularly in crystals that are not symmetric in all directions. This is the case for NaxCoO2.

The researchers studied the theoretical electron interaction between the disconnected parts of the Fermi surface and found that electrons can jump between these ‘islands’. This is made possible by so-called ‘spin fluctuations’—tiny changes in the way electron spins are aligned with each other. Through these synchronized tilts in their spins, electrons can gather just the right amount of energy to bridge the gaps at the Fermi surface. This electron interaction is crucial to mediate the material’s electronic properties and, according to Ryotaro Arita from the RIKEN team, “provides the necessary incentive for the electrons to create either ferromagnetism or superconductivity”.

Such a common origin for both phenomena in NaxCoO2 is quite rare, although Arita is convinced that if proven experimentally, this might lead to the discovery of other superconducting materials with disconnected Fermi surfaces.
Reference

1. Kuroki, K., Ohkubo, S., Nojima, T., Arita, R., Onari, S. & Tanaka, Y. Unified origin for the 3D magnetism and superconductivity in NaxCoO2. Physical Review Letters 98, 136401 (2007).

For more information, please contact
Saeko Okada
Email: [email protected]

Published: 11 Jul 2007

Institution:

Contact details:

2-1, Hirosawa, Wako, 351-0198

+81-48-462-1225
Country: 
News topics: 
Content type: 
Collaborator: 
Websites: 
Reference: 

Physical Review Letters