Giving resonance to miracle superconductors

The possibility of resistance-free electrical transfer at higher temperatures would reduce the cost of MRI scanners. Yet commercial deployment of these conductors has failed to materialise as expected. We look at what went wrong and how new neutron research is shedding new light on the mysteries behind high temperature superconductivity

It is 100 years since the discovery of superconductivity, the phenomenon that allows certain materials to conduct electricity without resistance at low temperature and therefore transport electrical energy without loss.

Usually, the ‘critical’ temperature required to induce superconductivity is so low – near absolute zero (-273^oC) – that expensive liquid helium cooling is required to achieve it. This has restricted commercial application of superconductors to a few specialist industries where the benefits outweigh the significant costs.

MRI scanners use helium-cooled superconducting magnets made from niobium alloys to generate strong fields (1-2 Tesla) that line up the spins of hydrogen atoms in the body. Radio frequency fields are then used to systematically alter the alignment of this magnetisation. This produces an oscillating magnetic field detectable by the scanner, and this information is recorded to construct an image of the scanned area of the body. The technology has revolutionised the detection of diseased tissues such as brain tumours, however the maintenance of their superconductive magnets has driven the costs of MRI machines into millions of pounds, limiting their use in the financially-constrained healthcare environment.

In response, manufacturers have developed less expensive, low-magnetic-field-strength (0.2 Tesla and below) MRI scanners. Most of the cost saving comes from using an electromagnet instead of a superconducting one, however, the lower field strength results in poorer quality images and longer scan times.

In 1986, hopes for more easily usable and therefore more affordable superconductors were raised with the discovery – by IBM researchers Georg Bednorz and Alex Mueller – of superconductivity in copper oxide compounds at up to 138K (-135°C). These temperatures can be achieved using liquid nitrogen cooling. As nitrogen is a far more abundant resource than helium the whole process is much cheaper. With potential for even higher temperatures and therefore significant savings for industry, academic interest in superconductivity was renewed.

The discovery was awarded with the Nobel Prize in physics and led US president Ronald Regan to declare a new age of technology. As well as MRI machines, many in industry envisaged application in a range of new areas including levitating high-speed trains, super-efficient power generators and ultra-powerful supercomputers.

Whilst considerable progress has been made in the last quarter of a century, an understanding of the mechanism driving high temperature superconductivity remains elusive and continues to hold back its commercial deployment.

In all superconductors, electrons form pairs below a critical temperature. At these temperatures, atoms or obstacles in the crystal do not posses the energy required to break the pair and therefore can no longer deflect electrons. As a result, electron pairs are able to fly around the material unimpeded, giving rise to superconductivity. In conventional superconductors, the pairing is a subtle consequence of the interaction of the electrons with vibrations of the atomic lattice. However this theory does not hold for high temperature copper-oxide superconductors and fails to explain transition temperatures higher than a few Kelvin. It is thought that a magnetic attraction of the electrons gives rise to superconductivity in the copper oxides, but a deeper understanding is required.

The latest step forward in this understanding has taken place at the Institut Laue-Langevin (ILL) in Grenoble, the world’s flagship centre for neutron science. Using neutron spectroscopy, an international team of researchers from the Laboratoire Léon Brillouin and the University of Minnesota have proved experimentally the existence of unusual magnetic excitations that precede the transition to superconductivity at high temperatures.  The researchers used the IN20 polarised and IN8 unpolarised neutron spectrometers at ILL to conduct inelastic neutron scattering tests on three samples of co-aligned crystals of HgBa₂CuO_4+d.

Often knowing the atomic structure of a material will be sufficient to understand its nature. But to gain an insight into the underlying physics of a phase change, you need to understand the dynamics, i.e. the movements of atoms and magnetic moments. Inelastic neutron scattering measures the change in energy of the neutron as it scatters from a sample. It allows you to explore the underlying physics through greater understanding of atomic and magnetic dynamics and can be used to probe a wide variety of different physical phenomenon including quantum excitations.

Whilst more common light scattering techniques offer a cheap and reliable way of identifying compounds, they can only measure certain points in the atomic lattice (known as the Brillouin zone centre) and are only sensitive to certain vibrations. The inelastic neutron scattering offered at ILL provides more powerful ways of understanding the physics of a system and quantitatively describe the nature of the phase transitions to superconductivity.

The newly discovered magnetic excitations in copper oxide superconductors appear to be an intrinsic feature of a phenomenon known as the pseudo-gap phase. In this phase the materials demonstrate unusual properties that deviate considerably from the behaviour of normal metals. The explanation of the pseudo-gap phenomenon in high-transition temperature (high-Tc) copper oxide has been a major challenge in condensed matter physics for the past two decades.

“The pseudo-gap phase is an interesting phenomenon in its own right, but understanding the mechanism by which it arises is key to understanding the properties of high temperature superconductivity,” says Dr Paul Steffens, scientist at the ILL, who worked with the researchers from France and America.

Several theoretical models have been proposed to describe the pseudo-gap phase. One of them, put forward by Professor CM Varma in 1997 claims that the superconducting state emerges from a spontaneous formation of microscopic electrical current loops, thereby creating microscopic magnetic moments. The pseudo-gap phase results from the appearance of these loops of current. The team at the ILL have for the first time observed magnetic behaviour in a copper-oxide high temperature superconductor consistent with this spontaneous formation of tiny loops of current.

Their findings have the profound implication that the pseudo-gap transition represents a new phase of matter rather than a mere crossover phenomenon. The results from ILL, published in Nature, are furthering our understanding of the high critical temperatures for superconductivity observed in some copper-oxides and what creates this phenomenon in the first place.

 “This understanding could be helpful in the development of new superconductors with higher critical temperatures or improved physical properties, which would be of great benefit as it would increase the potential applications,” says Steffens.

The next step at ILL is to test further predictions made by Varma’s model. From there the team hope to find similar behaviour in other high temperature superconductors in the hope of developing an overarching theory.

First on their list is yttrium barium copper oxide (YBCO), one of the oldest known high temperature superconductors, discovered at the University of Houston by Paul Chau, the year after Bednorz and Mueller’s original discovery. Its critical temperature is still amongst the highest yet observed – around 100K. A potential valuable material for future industry, its commercial application is held back at the moment due to a range of physical properties which are common amongst many of the high temperature superconductors discovered so far.

YBCO has a low critical current density, allowing only a small current to be passed through while maintaining superconductivity. YBCO is also very brittle and therefore difficult to turn into flexible wires and films for electricity transport.

Whilst YBCO may not provide industry with the miracle material it seeks, a better understanding of how it can induce superconductivity at such high temperatures is maintaining both academic and commercial interest in this area of science.

Armed with more accurate theoretical models from the work at the ILL, future research stands a better chance of discovering that “ideal” superconductor – available at low production cost and with the right mechanical properties and critical temperatures. Looking even further into the future, there is no fundamental reason known why there couldn’t be superconductors able to operate in MRI machines at room temperature – removing the need for cooling completely and reducing the size and running costs of the scanner.

Though we have some way to go, continuing studies means we can realistically expect to see further progress in this area as researchers look to make the application of high temperature superconductors easier and cheaper. And the first step is a fuller understanding of this unique phenomenon.

In the 100 years since its discovery, the commercial applications of superconductivity have intrigued designers and technicians from various industries. MRI scanners represent one of the most successful applications so far but as we have seen, there are still challenges to overcome.  The world class research taking place at the ILL is helping solve this significant challenge and is giving science and industry the best possible chance of fully understanding superconductivity and unlocking its true potential.