Nature 392, 51–54 (1998)

In this Letter in the 5 March issue, some of the superscript lettering on Figs 2–4 was printed incorrectly. In addition, the contrast was low in some copies of the journal. Reprints with the figures correctly produced are available, and they are reprinted here.

Figure 2
figure 1

In-phase charge (10−18CV−1

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Figure 3
figure 2

Figure 3

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Figure 4
figure 3

Figure 4

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