Can Magnetism Explain High Temperature Superconductivity?

Visualization of electron pair binding confirms predictions about how high temperature superconductivity works.

STM-measured anisotropy of energy gaps... — Image courtesy of J. C. Séamus Davis

The Science

The first experimental visualization of how strongly electrons are bound together into so-called Cooper pairs in an iron-based, high temperature superconductor was achieved for lithium iron arsenide, LiFeAs, using an electron-wave imaging technique derived from scanning tunneling microscopy (STM).

The Impact

These observations validated theoretical predictions that the pairs of electrons that flow with zero resistance through high temperature superconductors are the result of magnetic interactions between electrons, strengthening confidence that theory may one day be used to identify or design new, magnetically mediated, superconductors that can be used at temperatures far higher than today’s best “high-temperature” superconductors, which still must be chilled to below -130° C.

Summary

By measuring how strongly electrons are bound together, forming “Cooper pairs”, in an iron-based superconductor, scientists at the Center for the Emergent Superconductivity Energy Frontier Research Center, Brookhaven National Laboratory, Cornell University, and St. Andrews University, with materials developers at AIST in Japan, provide direct evidence that magnetism holds the key to this material’s ability to carry current with no resistance. Several groups of theorists had hypothesized that if the electrons in a superconductor have their magnetic moments pointing in opposite directions, they could overcome their mutual repulsion to join forces in so-called Cooper pairs — thus carrying current with no loss. According to the theory, the strength of the ‘glue’ holding electron pairs together would be different for specific electrons and depend on the direction that the electrons are traveling — with the pairing usually being stronger in a given direction than at 45° to that direction. The researchers figured out how to measure the predicted direction dependence (anisotropy) in the energy necessary to unbind a pair by using a specially developed application of scanning tunneling microscopy. Their novel method, known as “multi-band Bogoliubov quasiparticle scattering interference,” discovered the anisotropic pairing “signature” predicted for three electronic bands of a model superconductor, lithium iron arsenide.

Contact

J. C. Séamus Davis
Director of the Center for Emergent Superconductivity (CES) EFRC
jcdavis@ccmr.cornell.edu

Funding

DOE Office of Science, Office of Basic Energy Sciences, Energy Frontier Research Centers (EFRC) Program (Spectroscopic Imaging STM measurements of LiFeAs and subsequent analysis); UK Engineering and Physical Sciences Research Council (A.R.); Japan Society for the Promotion of Science (sample synthesis by H.E., K.K, C.H.L, A.I.); Fellowship support by National Science Foundation (Cornell Center for Materials Research, Y.X.), Academia Sinica Research Program on Nanoscience and Nanotechnology (T.-M.C.), and Royal Society-Wolfson Research Merit Award (A.P.M.).

Publications

M. P. Allan, A. W. Rost, A. P. Mackenzie, Yang Xie, J. C. Davis, K. Kihou, C. H. Lee, A. Iyo, H. Eisaki, and T.-M. Chuang, Science 336, 563 (2012). [DOI: 10.1126/science.1218726]