A student pointed me to a recent article on physicsweb. This article discusses a new (scientific) ranking system developed by a German student (Michael Banks) in Max Planck Institute of Solid State Physics to characterize the "hotness" of the scientific subject. If, after reading the popular physicsweb article linked above, you are interested in more details you may wish to read the attached original article posted by Banks. "Carbon nanotubes" emerges at the top of the list.