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Atomic Imaging of Magnetic Properties

Science may be a step closer to solving one of the outstanding mysteries of solid state physics after an international team of researchers led by Dr Peter Wahl of this School achieved atomic-scale imaging of the magnetic structure in the parent compound of an unconventional superconductor. Superconductors are extraordinary materials which under certain conditions, usually very low temperatures, can conduct electricity without any energy losses. Although superconductivity was first observed over 100 years ago, science still can’t explain exactly why some materials behave this way.

superconductor


Research has been taking place around the world to try to find out exactly how “high temperature” or "unconventional" superconductors, discovered around 30 years ago, work. The “high temperature” in this moniker is actually very cold (around minus 196 °C) compared with room temperature, but is very much higher than the earlier superconductors that operated just a few degrees above absolute zero. The search for an explanation for this “high temperature” superconductivity and a means of making superconductors work at room temperature is considered the Holy Grail of condensed matter physics. Lossless power transmission would revolutionise mankind’s use of energy.

Now a team of researchers, led by Dr Peter Wahl of this School, has started to shed new light on the behaviour of high-temperature superconductors. From the beginning, it was clear that superconductivity in these materials is driven by a mechanism different from the one in the earlier “conventional” superconductors, in which lattice vibrations have been identified as the enabler of superconductivity. Fluctuations of magnetic order appear to play an important role for the high temperature superconductivity.

Dr Wahl and his colleagues from the Max Planck Institute for Solid State Research and University of Ausburg in Germany used ultra- low temperature scanning tunneling microscopes (STM) which operate at temperatures down to only a few hundreds of a degree above absolute zero. In these microscopes, the surface of the material is probed with a tip which hovers just a few atomic radii above the material. To enable imaging of the magnetic properties of the material the researchers needed to have the tip acting like a magnet. To do this the researchers had to apply a trick: rather than covering the tip with magnetic material, a technologically demanding process, they picked up some magnetic material from the surface – almost like a vacuum cleaner - but with the magnetic material sticking to the end of the tip.

“With this novel approach, we hope to shed new light into the mysteries of unconventional superconductivity, by being able to directly visualize both magnetic order and detect superconductivity in the same measurement,” said Dr Wahl. The researchers were able to image the magnetic order in iron tellurium, the parent compound of an iron-based superconductor, at the atomic scale.

This is the first spin-polarised STM measurement on a strongly correlated electron material of a type similar to high temperature superconductors. The structure of this particular material was already known through neutron scattering experiments, but the team’s STM measurements provide a real-space image of the surface and its magnetic order.

It is expected that these techniques will be important in ongoing studies to determine whether magnetism and superconductivity coexist or compete in high temperature (unconventional) superconductors. It may be that identifying the relation between magnetism and superconductivity will allow us to solve the mystery of the effects behind superconductivity in the high temperature (strongly correlated electron) superconductors.

Dr Wahl’s research will be boosted by the construction of a new cutting edge ultra-low vibration (ULV) facility in the School of Physics and Astronomy at St Andrews. The ULV lab represents an investment of more than £2 million for the University of St Andrews and is expected to be completed early next year.

First results of the team’s experiments are published in Science Express.

First posted BDS 04.08.14