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We study the ground state properties of a quantum antiferromagnet on thekagome lattice in the presence of a magnetic field, paying particular attentionto the stability of the plateau at magnetization 1/3 of saturation and thenature of its ground state. We discuss fluctuations around classical groundstates and argue that quantum and classical calculations at the harmonic leveldo not lead to the same result in contrast to the zero-field case. For spinS=1/2 we find a magnetic gap below which an exponential number of non-magneticexcitations are present. Moreover, such non-magnetic excitations also have a(much smaller) gap above the three-fold degenerate ground state. We provideevidence that the ground state has long-range order of valence-bond crystaltype with nine spins in the unit cell.Comment: 5 pages including 4 figures, uses REVTeX4; final version with some  small extensions; to appear in Phys. Rev.

Quantum kagomé antiferromagnet in a magnetic field: Low-lying nonmagnetic excitations versus valence-bond crystal order