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Posted: June 11, 2010

Observation of shell effects in superconducting nanoparticles of tin

(Nanowerk News) The loss of electrical resistance of a metal particle is also a matter of its size. A group of researchers has now proven that the temperature below which a material becomes a superconductor can increase dramatically, when the material is present as spherical nanoparticles ("Observation of shell effects in superconducting nanoparticles of Sn").
This was proven by observing tin nanoparticles with a scanning tunneling microscope. Thus, quantum effects in the tiny particles can intensify superconductivity up to 60 %, but only if a "magical" size is reached that can be predicted accurately. These results provide new starting points for superconductivity at room temperature.
Materials that carry current without loss would save a lot of energy. Superconductor are capable of doing so, however, the best superconductors first give up their resistance at a temperature below 170 degrees Celsius. The team of researchers has now succeeded in increasing the critical temperature below which a material becomes a superconductor by creating nanoparticles of a specific size.
The superconductivity increases due to a quantization of the energy states in a nanoparticle. The quantum states change the properties of nanoscopic systems unexpectedly and abruptly. The so-called "shell effects" that intensify superconductivity are one of the most surprising consequences.
Physicists have predicted those shell effects for a long time. According to that, metallic nanoparticles form electronic shells that are set with electrons. At a certain amount, the electrons join together to Cooper pairs more easily. Those Cooper pairs move without resistance through the material. The gathering to that ,magical' amount of electrons depends on the size and form of the particles.
At first, the researchers have grown exact hemispheres from tin and lead in a vacuum. Their heights were adjusted to 1-50 nanometres. Afterwards, the researchers observed the electronic properties of the nanoparticles with a specific scanning tunneling microscope at a temperature of about -273 degree Celsius and identified the superconducting energy gap that provides information about the critical temperatures.
The experiments showed that the superconducting energy gap of the tiny nanoparticles reacted particularly sensitively to the size of a particle. However, it neither rose nor decreased continuously but it showed very strong fluctuations.
This effect didn't occur with lead. This is due to the fact that the coherence length of tin is much bigger then the one of lead, which makes it far more sensitive with regard to quantum effects.
Since the quantum mechanical shells occur in all kinds of material, they can be used to enforce the superconductivity in many materials. Therefore, the quantum-engineering opens up new perspectives for superconductivity by targeted nanostructuring.
Source: Max Planck Society for the Advancement of Science
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