  Specific heat and the magnetocaloric effect are used to probe the
field-induced up-up-down phase of Cs2CuBr4, a quasi-two-dimensional spin-1/2
triangular antiferromagnet with near-maximal frustration. The shape of the
magnetic phase diagram shows that the phase is stabilized by quantum
fluctuations, not by thermal fluctuations as in the corresponding phase of
classical spins. The magnon gaps determined from the specific heat are
considerably larger than those expected for a Heisenberg antiferromagnet,
probably due to the presence of a small Dzyaloshinskii-Moriya interaction.
