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 $\nu =5∕3$, $4∕3$, $2∕3$, $3∕5$, $4∕7$, $2∕5$, and $1∕3$ the specific heat displays a strong exponential temperature dependence in agreement with excitations across a quasiparticle gap. At filling factor $\nu =1∕2$ we were able to measure the relative specific heat of a composite fermion system. The observed nearly linear temperature dependence on temperature down to $T=0.14\mathrm{K}$ agrees well with early predictions for a Fermi liquid of composite fermions.

• Received 7 June 2007

DOI:https://doi.org/10.1103/PhysRevB.76.153311