We study the Fermi-edge singularity appearing in the current-voltage characteristics for resonant tunneling through a localized level at finite temperature. An explicit expression for the current at low temperature and near the threshold for the tunneling process is presented which allows to coalesce data taken at different temperatures to a single curve. Based on this scaling function for the current we analyze experimental data from a GaAs-AlAs-GaAs tunneling device with embedded InAs quantum dots obtained at low temperatures in high magnetic fields.

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 =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 =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 K agrees well with early predictions for a Fermi liquid of composite fermions. DOI: 10.1103/PhysRevB.76.153311 PACS number͑s͒: 73.43.Ϫf, 72.10.Di, 73.20.Mf In the fractional quantum Hall ͑FQH͒ effect the Coulomb interaction induces the formation of new quasiparticle states characterized by a fractional charge and a finite excitation gap.1-4 A nice description of the FQH effect is given by the composite fermion ͑CF͒ theory 5 where the FQH effect is regarded as an integer quantum Hall effect of new quasiparticles. In particular, at a filling factor =1/2 these composite fermions can be described as a quasi-free Fermi sea in an effective zero-magnetic field. 6One of the experimental challenges lies in accessing the ground state properties of a two-dimensional electron system ͑2DES͒ by means of thermodynamic quantities such as magnetization, [7][8][9] thermal conductivity, 10,11 thermopower, 12,13 or specific heat. 14,15 It is, however, far from being straightforward to measure the specific heat C of a 2DES directly. In general C is strongly dominated by the contribution of the surrounding substrate, and, in order to obtain a reasonable signal from the 2DES, multilayer structures have to be used. 14,15 An alternative method lies in the use of time-resolved phonon-spectroscopy experiments. 16,17,26 Here, a defined amount of energy is dissipated inside the 2DES in a short period of time ͑typically a few nanoseconds͒. Measuring the temperature of the 2DES long before it reaches equilibrium with the substrate then allows us to determine its specific heat directly. 18In this Brief Report we will show how we can use phonon-absorption experiments to deduce the temperature dependence of C of a 2DES in the FQH regime. Results at fractional filling factors suggest the excitation across a quasiparticle gap in quantitative agreement with theoretical predictions.19 At filling factor =1/2 we essentially find a linear temperature dependence predicted for a Fermi liquid of composite fermions.6 Slight deviations from linearity will be tracked down to spin splitting in the CF system.The sample used in this experiment is a 2DES embedded in a modulation-doped AlGaAs/ GaAs heterostructure. Using its persistent photoconductivity the electron concentration n e inside the 2DES can be tuned be means of an infrared photodiode. The sample is mounted on the cold finger of a dilution refrigerator and inserted into a 13 T superconducting magnet. In order to heat up the 2DES on short time sc...

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