Scientists at the US Department of Energy’s Brookhaven National Laboratory are studying a material that may lead to significant advances in the miniaturisation of electronics.
The material – a perovskite-related oxide containing calcium, copper, titanium, and oxygen in the formula CaCu3Ti4O12 – is said to be unusual in that it has an extremely high dielectric constant, a property that determines its ability to become electrically polarised.
A high dielectric constant means that more charge can be stored whilst electronic circuits can be made smaller.
Unlike other dielectric materials, this one retains its high dielectric constant over a wide range of temperatures, from 100 to 600 Kelvins (K).
Yet the material’s dielectric constant drops precipitously – 1,000-fold – below 100K, with no evidence of structural or phase changes in the atoms.
‘Such a large change in the way charge is distributed within the material implies that the atomic structure should change as well,’ said Christopher Homes, the lead physicist on the Brookhaven study. ‘It’s difficult to imagine how one property can undergo such a large change while the other remains unaffected.’
Previously, scientists have looked for hints of changes using x-rays, neutron beams, and other methods without success. But Homes’ technique, measuring optical conductivity, or the material’s ability to reflect and absorb varying frequencies of infrared light, revealed a number of unusual changes in the way the atomic structure vibrates.
The scientists detected the vibrations by illuminating samples of the substance with varying wavelengths of infrared light at Brookhaven’s National Synchrotron Light Source, and measuring which wavelengths were reflected and which were absorbed.
The absorbed wavelengths are those that match the atoms’ natural resonance. As the temperature of the substance was cooled below the 100K mark, the absorbed frequencies – and therefore the vibrations – changed.
‘Since the vibrations in a solid depend a great deal on how the charges are distributed, the changes in vibrations suggest that the charges can be rearranged without causing a structural distortion,’ said Homes. ‘The fact that we see these changes offers the first real glimpse of why this material has such a large dielectric constant, and the mechanism by which it decreases so dramatically below 100K.’
The scientists speculate that at temperatures above 100K, pairings of dipoles can flip around quickly, independent of one another.
This property and the high density of dipoles within the solid are both said to contribute to the large dielectric constant. If the material is placed in an electric field, all the individual dipoles flip into alignment to separate the charges.
But as the material cools, the dipoles ‘freeze out’ in random positions, losing their ability to flip quickly into alignment. This ‘electronic phase transition’ happens in the absence of a structural change. ‘Additional research will help us understand this effect and the range of ways this material might be used in microelectronics and other fields,’ concluded Homes.