The new index is based on the "Hirsch index", which was devised last year by Jorge Hirsch of the University of California at San Diego as a way of quantifying the performance of individual scientists. Hirsch's h-index is derived from the number of times that papers by a particular scientist are cited. A scientist with a h-index of 10, say, will have published 10 papers that have received at least 10 citations each. The best researchers should therefore have the highest h-indexes.

Banks has now taken this method a step further by applying the h-index to particular topics or compounds mentioned in the abstract of a paper, rather than to people. A topic or compound with a h-b index of 10 means that there are at least 10 papers on that topic, each of which has been cited at least 10 times. Since some topics and compounds have been around longer than others, Banks divides the h-b by the number of years that papers on that topic or compound have been published. This normalises the result to yield a number, m, which indicates how important a particular topic is today -- that is, how many researchers are actively working on it.

Like the original h-index, the h-b index is calculated by searching the ISI Web of Knowledge database, which takes only takes a few seconds. The method involves searching the topic field on the database and then sorting the results in terms of citations. "This is the only method available where you can compare different compounds used in solid state physics or even topics in physics as a whole," explains Banks.

Banks lists two tables for various compounds and topics according to how big their m and h-b are. Carbon-60 tops the table of chemical compounds, with an m of 5.2, followed by gallium nitride (2.12) in second.

In the list of hot topics, carbon nanotubes are top with an m of 12.85. This is followed by nanowires, quantum dots, fullerenes, giant magnetoresistance, M-theory and quantum computation, which have m numbers of 8.75, 7.84, 7.78, 6.82, 6.58, and 5.21 respectively.

Banks says that an m number greater than three means that a topic is hot. Moreover, a large m number combined with a large h-b (greater than 100) represents a topic that was popular in the past and still is today. Examples of such topics include porous silicon and spin glasses. Finally, a small m but large h-b reflects an older topic that was popular for many years but is now less so, such as perovskites and amorphous silicon.

The new index might help potential PhD students to choose their future area of research, suggests Banks. It could also provide a useful yardstick to compare different fields when awarding funds and grants. However, he warns that his index should not become the only way to assess the importance of a particular subject.