New material may yield quantum computers

Scientists at the US Department of Energy’s Pacific Northwest National Laboratory have created a semiconductor material that may propel research closer to realising the potential of quantum computing.

Scientists at the US Department of Energy’s Pacific Northwest National Laboratory have created a semiconductor material that has superior magnetic properties at room temperature and that may propel research one step closer to realising the potential of quantum computing.

Using a special synthesis technique, PNNL scientists created a thin-film semiconductor material made of titanium, oxygen and cobalt.

In collaboration with scientists at the IBM Almaden Research Centre, they showed that the materials required for quantum computing and the emerging area of spintronics can be obtained.

‘Although other scientists have created similar materials, their films had considerably poorer magnetic properties,’ said Scott Chambers, a chemist and PNNL senior chief scientist.

‘Our synthesis technique, while difficult to perform, is more controllable at the atomic level, and therefore yields better results. The next step is to refine the growth process.’

The PNNL work builds upon experiments conducted by scientists in Japan who created the same material using laser ablation; an effective but less controlled thin-film synthesis method.

The strength of laser ablation is that it allows researchers to cover a wide range of growth conditions and film compositions rather quickly when used in combinatorial mode.

In this way, several materials can be screened relatively rapidly to determine promising candidates. This kind of search by the Japanese group revealed that the material the PNNL team has synthesised in a more controlled fashion has significant potential for the applications at hand.

The current generation of computers uses an electron’s charge to store and process information, but this approach limits the ultimate speed and storage density that can be achieved.

Magnetic storage, such as that found in a computer hard drive, relies on the magnetic properties created by an electron’s spin.

However, if an electron’s spin can be harnessed within a semiconductor, the potential exists to create entirely new ways of computing and signal processing that will greatly increase speed and data storage densities. The exploitation of an electron’s spin to carry information, rather than its charge, often is referred to as spintronics.

Spintronics would provide the basic properties required for advanced technologies, such as on-chip integration of magnetic storage and electronic processing functions and quantum computing, which depends on coherent spin states to transmit and store information. A material is permanently magnetic if the majority of its electrons spin in the same direction.

In order to be practical, spintronics will need to use semiconductors that maintain their magnetic properties at room temperature.

This is a challenge because most magnetic semiconductors lose their magnetic properties at temperatures well below room temperature, and would require expensive and impractical refrigeration in order to work in an actual computer.

Chambers and his team of scientists achieved these properties in anatase titanium dioxide that is infused with a small amount of cobalt, a magnetic impurity.

Chambers and his team created this magnetic semiconductor material using molecular beam epitaxy. In this growth method, individual beams of atoms – in this case, titanium, oxygen and cobalt – are generated in a highly controlled vacuum environment and directed onto a crystalline surface of strontium titanate where the atoms condense and form a crystalline film with dimensions on the nanoscale.

After the material was created, a team of scientists at IBM, led by research staff scientist Robin Farrow, validated the results by characterising the material’s magnetic properties.

In the material synthesised at PNNL and characterised at IBM, each cobalt atom’s magnetic moment, which is a measure of the material’s magnetic strength, is about five times larger than in the Japanese scientists’ material.

While early results are promising, PNNL scientists will continue their research to determine the material’s ideal growth temperature, growth rate, composition and choice of substrate, and then optimise the structural properties required to achieve the desired magnetic properties.