  Since the discovery of high transition-temperature (Tc) superconductivity in
copper oxides two decades ago, continuous efforts have been devoted to
searching for similar phenomenon in other compounds. With the exception of MgB2
(Tc =39 K), however, Tc is generally far lower than desired. Recently,
breakthrough has been made in a new class of oxypnictide compounds. Following
the initial discovery of superconductivity in LaO1-x FxFeAs (Tc =26 K), Tc
onset has been raised to 55 K in ReO1-xFxFeAs (Re: Ce, Pr, Nd, Sm). Meanwhile,
unravelling the nature of the energy associated with the formation of
current-carrying pairs (Cooper pairs), referred to as the superconducting
energy gap, is the first and vital step towards understanding why the
superconductivity occurs at such high temperature and is also important for
finding superconductors with still higher Tc. Here we show that, on the basis
of the nuclear magnetic resonance (NMR) measurements in PrO0.89F0.11FeAs (Tc
=45 K), the Cooper pair is in the spin-singlet state (two spins are
anti-paralleled), with two energy gaps opening below Tc. The results strongly
suggest the existence of nodes (zeros) in the gap. None of superconductors
known to date has such unique gap features, although copper-oxides and MgB2
share part of them.
