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Published online: 13 May 2009 | doi:10.1038/nchina.2009.96

Superconductivity: Isotopes in action

Felix Cheung

Introduction

The recent discovery of superconductivity in pnictides with a critical transition temperature (T_C) higher than 39 K (the maximum value predicted by the conventional theory of superconductivity) has generated plenty of excitement. Experiments performed by Xianhui Chen at University of Science and Technology of China in Hefei and co-workers¹ now show that lattice vibrations (phonons) and spin waves (magnons) are both involved in the superconducting mechanism.

In conventional superconductors, phonons bind electrons into so-called Cooper pairs, which carry the superconducting current. However, theoretical calculations indicated that the electron–phonon interaction is not strong enough to make pnictides superconducting. Scientists have suggested that magnons in the crystal lattice may also be at play.

The researchers studied the effect of oxygen- and iron-isotope substitution on two pnictide systems, SmFeAsO_1-x F_x and Ba_1-xK_xFe₂As₂. They found that although the replacement of ¹⁶O with the heavier isotope ¹⁸O had very little effect on the pnictides, the substitution of ⁵⁴Fe with the heavier isotope ⁵⁶Fe resulted in an increase in T_C and spin-density wave transition temperature (T_SDW).

Because lattice vibrations are dependent on the mass of the atoms that make up the lattice, the change in T_C after isotope substitution indicates that electron–phonon interaction plays a role in the superconducting mechanism. In addition, the surprising finding of T_SDW indicates that magnons are also important.

The researchers emphasize that resolving the interplay between phonons and magnons will be the key to understanding high-temperature superconductivity.

The authors of this work are from:
Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, China; Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China; National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, China.

Reference

1. Liu, R. H. et al. A large iron isotope effect in SmFeAsO_1-x F_x and Ba_1-xK_xFe₂As₂. Nature doi:10.1038/nature07981 (2009). | Article |