This article was originally published online, on 25 February 2008, with errors in the caption to Fig. 11: The value in item (c) should have been “16 400” rather than “164 000,” and the value in item (d) should have been “18 400” rather than “184 000.” AIP apologizes for these errors. All online versions of the article were corrected on 26 February 2008; the article was correct as it appeared in the printed version of the journal.
© 2008 American Institute of Physics.
2008
American Institute of Physics
