Plasmons (image) Rice University Caption The ability to sort carbon nanotubes by type through a process called "density gradient ultracentrifugation (DGU)" allowed Rice researchers to test purified batches of nanotubes to find the cause of terahertz peaks in spectroscopic experiments. They determined that free electrons formed plasmons that ripple at terahertz frequencies in metallic and doped nanotubes. Credit Kono Laboratory/Rice University Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.