Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

When is a metal not a metal?

23.05.2003


Niobium clusters display non-metallic properties at ultra-cold temperatures



The May 23 issue of the journal Science answers that question with an account of the surprising behavior exhibited by nanometer-scale clusters of the metal niobium. When the clusters are cooled to below 20 degrees Kelvin, electrical charges in them suddenly shift, creating structures known as dipoles.

"This is very strange, because no metal is supposed to be able to do this," said Walter de Heer, a professor in the School of Physics at the Georgia Institute of Technology and co-author of a paper to be published on the topic in Science. "These clusters become spontaneously polarized, with electrons moving to one side of the cluster for no apparent reason. One side of each cluster becomes negatively-charged, and the other side becomes positively-charged. The clusters lock into that behavior and stay that way."


This ferroelectric phenomenon has so far been observed in clusters of niobium, vanadium and tantalum – three transition metals that in bulk form become superconducting at about the same temperature that the researchers observe formation of dipoles in the tiny clusters. De Heer believes this discovery will open up a new field of research – and provide clues to the mystery of superconductivity.

In bulk metals – and even in niobium clusters at room temperature -- electrical charge is normally distributed equally throughout the sample unless an electric field is applied to them. But in the clusters of up to 200 niobium atoms created by de Heer and collaborators Ramiro Moro, Xiaoshan Xu and Shuangye Yin, that changes when the particles are cooled to less than 20 degrees Kelvin.

The Georgia Tech researchers discovered this "spontaneous symmetry breaking" while searching for signs of superconductivity in the nanometer-scale clusters. It was completely unexpected – and de Heer admits he has no explanation for it.

"When this happens, these particles that are made out of metal atoms no longer behave as if they were metallic," he said. "Something changes the particles from a metal into something else."

For the smallest clusters, the strength of the dipole effect varies dramatically according to size. Clusters composed of 14 atoms display strong effects, while those made up of 15 atoms show little effect. Above 30 atoms, clusters with even numbers of atoms display stronger dipole effects than clusters with odd numbers of atoms.

"Structure matters greatly to this process," de Heer said. "A small change can affect the position of the phase transition rather profoundly, and the exact arrangement of atoms really does matter to these systems."

He attributes the size sensitivity to the quantum size regime, which is related to restrictions on how electrons can move in very small clusters.

De Heer sees strong "circumstantial evidence," but no solid proof, that the phenomenon is connected to superconductivity in these metals.

"Our assumption is that superconductivity in the bulk materials has something to do with the spontaneous production of dipole in the small particles," he said. "At this point, it is circumstantial evidence – the same materials and the same temperature regime, and the odd phase transitions occurring in both. By studying several different metals, we found that those that are superconducting in bulk have this effect, and those that are not superconducting do not have it. That strengthens our belief that this is connected to superconductivity in some way that we don’t yet understand."

To produce and study the tiny clusters, the researchers use a custom-built apparatus that includes a laser, large vacuum chamber, liquid helium and a specially designed detector able to count and characterize several million particles per hour.

First, a laser beam is aimed at a niobium rod held within the vacuum chamber. Pulses from the laser vaporize the niobium, creating a cloud of metallic vapor. A stream of very cold helium gas is then injected into the chamber, causing the niobium gas to condense into particles of varying sizes. Under pressure from the ultra-cold helium, the particles exit through a small hole in the chamber’s wall, creating a one millimeter-wide jet of particles that passes between two metal plates before hitting the detector.

At intervals one minute apart, the metal plates are energized with 15,000 volts, creating a strong electrical field. The field interacts with the polarized niobium nanoclusters, causing them to be deflected away from the detector. Unpolarized clusters remain in the beam and are counted by the detector

By comparing detector readings while the plates are energized against the readings when no field is applied, the researchers learn which clusters carry the dipole. The continuous production of particles allows de Heer’s research team to gather data on millions of particles during each experiment. By varying the temperature and voltage, they study the impact of these changes on the effect.

So far, they have studied in detail clusters of up to 200 atoms, though de Heer believes the effect should continue in larger clusters, perhaps up to 500 atoms or as many as 1,000.

"This is just the beginning of what will ultimately be a very exciting story," he said. "We certainly have a lot of work to do.



Technical contact: Walter de Heer (404-894-7879); E-mail: (deheer@electra.physics.gatech.edu)

John Toon | EurekAlert!
Further information:
http://gtresearchnews.gatech.edu/

More articles from Physics and Astronomy:

nachricht When fluid flows almost as fast as light -- with quantum rotation
22.06.2018 | The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences

nachricht Thermal Radiation from Tiny Particles
22.06.2018 | Universität Greifswald

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Temperature-controlled fiber-optic light source with liquid core

In a recent publication in the renowned journal Optica, scientists of Leibniz-Institute of Photonic Technology (Leibniz IPHT) in Jena showed that they can accurately control the optical properties of liquid-core fiber lasers and therefore their spectral band width by temperature and pressure tuning.

Already last year, the researchers provided experimental proof of a new dynamic of hybrid solitons– temporally and spectrally stationary light waves resulting...

Im Focus: Overdosing on Calcium

Nano crystals impact stem cell fate during bone formation

Scientists from the University of Freiburg and the University of Basel identified a master regulator for bone regeneration. Prasad Shastri, Professor of...

Im Focus: AchemAsia 2019 will take place in Shanghai

Moving into its fourth decade, AchemAsia is setting out for new horizons: The International Expo and Innovation Forum for Sustainable Chemical Production will take place from 21-23 May 2019 in Shanghai, China. With an updated event profile, the eleventh edition focusses on topics that are especially relevant for the Chinese process industry, putting a strong emphasis on sustainability and innovation.

Founded in 1989 as a spin-off of ACHEMA to cater to the needs of China’s then developing industry, AchemAsia has since grown into a platform where the latest...

Im Focus: First real-time test of Li-Fi utilization for the industrial Internet of Things

The BMBF-funded OWICELLS project was successfully completed with a final presentation at the BMW plant in Munich. The presentation demonstrated a Li-Fi communication with a mobile robot, while the robot carried out usual production processes (welding, moving and testing parts) in a 5x5m² production cell. The robust, optical wireless transmission is based on spatial diversity; in other words, data is sent and received simultaneously by several LEDs and several photodiodes. The system can transmit data at more than 100 Mbit/s and five milliseconds latency.

Modern production technologies in the automobile industry must become more flexible in order to fulfil individual customer requirements.

Im Focus: Sharp images with flexible fibers

An international team of scientists has discovered a new way to transfer image information through multimodal fibers with almost no distortion - even if the fiber is bent. The results of the study, to which scientist from the Leibniz-Institute of Photonic Technology Jena (Leibniz IPHT) contributed, were published on 6thJune in the highly-cited journal Physical Review Letters.

Endoscopes allow doctors to see into a patient’s body like through a keyhole. Typically, the images are transmitted via a bundle of several hundreds of optical...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Munich conference on asteroid detection, tracking and defense

13.06.2018 | Event News

2nd International Baltic Earth Conference in Denmark: “The Baltic Sea region in Transition”

08.06.2018 | Event News

ISEKI_Food 2018: Conference with Holistic View of Food Production

05.06.2018 | Event News

 
Latest News

Graphene assembled film shows higher thermal conductivity than graphite film

22.06.2018 | Materials Sciences

Fast rising bedrock below West Antarctica reveals an extremely fluid Earth mantle

22.06.2018 | Earth Sciences

Zebrafish's near 360 degree UV-vision knocks stripes off Google Street View

22.06.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>