Measurement of the specific heat of a fractional quantum Hall system

Using a time-resolved phonon absorption technique, we have measured the relative specific heat of a two-dimensional electron system in the fractional quantum Hall effect regime. For filling factors ν=5∕3, $4∕3$, $2∕3$, $3∕5$, $4∕7$, $2∕5$, and $1∕3$ the specific heat displays a strong exponential temperature dependence in agreement with excitations across a quasiparticle gap. At filling factor ν=1∕2 we were able to measure the relative specific heat of a composite fermion system. The observed nearly linear temperature dependence on temperature down to T=0.140.3em0exK agrees well with early predictions for a Fermi liquid of composite fermions.