16 June 2017

Connection between superconductivity and insulator-metal transition

Alkali-doped fullerides (at left) and diagram (right) showing the phase transition between superconductivity (pink dome) and a three-dimensional Mott insulator (blue region), as shown by the yellow diamonds. Alkali-doped fullerides (at left) and diagram (right) showing the phase transition between superconductivity (pink dome) and a three-dimensional Mott insulator (blue region), as shown by the yellow diamonds.

The finding in fullerides opens a new way of exploring the role electron interactions play in high-temperature superconductivity

First, some background

Superconductivity — electric current that travels through a conductor without any resistance — is a hot area of research in physics. But conventional superconductors have limited applications because they operate only at extremely low temperatures.

That's why physicists are especially excited about high-temperature (also called exotic) superconductors (HTS), which operate at more practical temperatures. In HTS materials, electrons interact with each other in a stronger and different way than in conventional superconductors and researchers are exploring these mysteries. Physicists often explore this question by observing how a specific change — lowering the temperature, or tweaking the structure of a material by swapping out specific atoms (a technique called doping) — affects the state, behaviors and properties of the material.

Their work may one day lead to the discovery of materials that can superconduct at even higher temperatures suitable for power transmission and other energy applications.

Scientists have discovered different families of HTS; in most of them, electrons travel only within a two-dimensional plane. But in one known family of HTS materials called alkali-doped fullerides, the behavior is three-dimensional, giving scientists a new way to explore HTS. Alkali-doped fullerides are cubic crystals consisting of fullerenes — carbon molecules with a cage-like structure — that have been doped with one of the alkali metals, such as sodium or potassium.

What did scientists discover?

Scientists doped fullerenes by switching out the alkali metal that sits between the molecules. Then they exposed them to very high, pulsed magnetic fields to observe what changes would occur. They compared the temperature at which the alkali-doped fullerides transitioned into a superconductor (Tc) to the strength of the magnetic field needed to suppress superconductivity (Hc2). The ratio these two values revealed that the strength of the electron interaction in the material (i.e., the behavior that accounts for the superconductivity) increased near the point at which the material turned into metal, known as the Mott transition.

This finding suggests that the cooperative interplay between molecular electronic structure and strong electron interactions reinforces the robust superconductivity (high Tc and high Hc2) found in the alkali-doped fullerides.

Why is this important?

Understanding the relationship between the strength of the electronic interactions and superconductivity may one day enable scientists to design even more robust high-temperature superconductors. All other HTS exhibiting this relationship, including the world-record, copper-based superconductors, are two-dimensional, layered materials. The alkali-doped fullerides provide the first example of a transition from a three-dimensional Mott insulator to a superconductor, enabling scientists to explore how dimensionality and electron correlations affect superconductivity.

Who did the research?

Y. Kasahara1, Y. Takeuchi2, R.H. Zadik3, Y. Takabayashi4, R.H. Colman3, R.D. McDonald5, M.J. Rosseinsky6, K. Prassides4,7, Y. Iwasa2,8

1Kyoto University, Japan; 2QPEC & University of Tokyo, Japan; 3Durham University, UK; 4WPI-Tohoku University, Japan; 5National MagLab-LANL, USA; 6University of Liverpool, UK; 7JST-Tohoku University, Japan; 8RIKEN –CEMS, Japan

Why did they need the MagLab?

Very high pulsed magnetic fields — up to 90 tesla — are needed to suppress the superconductivity for the fulleride superconductors that were studied. In addition, the air sensitivity of the alkali-doped superconductors required novel radio-frequency instrumentation developed at the MagLab.


This research was conducted in the 65 Tesla Multi-Shot Magnet at the MagLab's Pulsed Field Facility.

Details for scientists


This research was funded by the following grants: G.S. Boebinger (NSF DMR-1157490)

For more information, contact Ross McDonald.


  • Research Area: Molecular Conductors, Superconductivity - Basic
  • Research Initiatives: Materials
  • Facility / Program: Pulsed Field
  • Year: 2017
Last modified on 17 July 2017