Superconductivity Advances

Landmark Experiment on "High" Temperature Superconductors

A Bristol-Toulouse-St Andrews collaboration including this School's Prof Andy Mackenzie has today published the results of a landmark experiment on high temperature superconductors.

Superconductivity is one of the most spectacular physical phenomena ever discovered. When certain metals are cooled, they suddenly change their properties, becoming perfect conductors of electricity and expelling magnetic fields. Superconductors already play a key role in some technologies such as the filtration of the china clays that are used in every piece of glossy paper in the world. Until 1986, superconductivity was thought to occur only at extremely low temperatures comparable to that of interstellar space. Then, an entirely new class of complex materials was discovered in which superconductivity appeared at a relatively balmy -150 ºC. These high temperature superconductors raise the hope of someday making superconductivity appear at room temperature. If this could be achieved, it would revolutionise a whole range of environmentally relevant technologies such as energy transmission and storage, so it remains a ‘holy grail’ of modern science.

One of the main barriers to progress in the field has been the fact that some experiments that are key to understanding high temperature superconductivity are notoriously difficult to perform. Traditionally, it has been accepted that in order to understand a superconductor one needs to understand its ‘parent’ metal, because the particles that pair up and participate in the superconductivity exist above the transition temperature in this metal. Understanding these metals has been a particularly difficult task in the field. In traditional superconductors the tactic was to destroy the superconductivity by raising the temperature or applying a magnetic field and then perform low temperature experiments on the parent metal. In the new materials this is particularly problematic because the superconductivity is so difficult to destroy – it protects itself from precisely the experiments that might be key to understanding it.

Sketch of the possible phase diagram of this type of superconductor.

The results reported here probe the region between the Superconductor and the Fermi liquid, using magnetic fields to tune the conditions.

One of the most profound ways to understand an unusual metal is to observe ‘quantum oscillations’, which arise from quantisation of the orbital motion of charge carriers that are subject to a magnetic field. If the charge carriers have a high probability of making a complete orbit ‘coherently’, that is without scattering from imperfections in their host crystal, the properties of the material oscillate as the magnetic field is changed. The charge carriers subject to this subtle effect do not need to be simple electrons, but can be delicate many-body excitations of metals in which the electrons interact strongly with each other. They are therefore the most useful probe of the exotic metals that are the parents of the high temperature superconductivity, but they are very difficult to observe. The experiments require exquisitely pure crystals and extremely large magnetic fields, and laboratories all over the world have been struggling for two decades to achieve the right combination of circumstances.

Over the past year a number of breakthrough experiments have been made around the world, but these still left important gaps in our knowledge. Now an international team of physicists from the University of Bristol, the High Magnetic Field Laboratory in Toulouse, France and the University of St Andrews have managed to fill those gaps, finding quantum oscillations in a series of high precision measurements on an exotic high temperature superconductor, Tl2Ba2CuO6.

Prof Andy Mackenzie The St Andrews member of the collaboration, Professor Andy Mackenzie, commented: ‘This work is something of a triumph of perseverance. My Bristol colleagues Nigel Hussey, Tony Carrington and I identified the material with the highest chance of yielding this result as young students and post-docs at Cambridge nearly twenty years ago, and I grew the crystals on which this year’s experiment succeeded as long ago as 1993. Looking for this signal is like searching for a very small needle in a very large haystack, and we have all tried to find it in many different experiments. It has only become possible because of some wonderful technical advances led by Cyril Proust at Toulouse. We feel a real sense of collaborative achievement. I should also say that the project has benefited enormously from the long-term, blue skies support provided by a Portfolio Partnership grant from the Engineering and Physical Sciences Research Council for collaborative work in the field between Bristol, Cambridge and St Andrews.’

The team’s breakthrough is described in the 16 October issue of the journal Nature.

First posted BDS 16.10.08