Abstract
This paper summarizes an extensive study of the temperature dependence of magnetotransport in the fractional quantum Hall effect in GaAs-As heterostructure devices of varying mobility and density. For devices with electron mobility 400 000≲μ≲1 000 000 /V s, we find a single activation energy, , in the longitudinal transport coefficients, and , for Landau-level filling factors ν=(1/3, (2/3, (4/3, and (5/3, with a magnetic field dependence which is vanishingly small for B≲5.5 T and increases to .8 K at B=30 T. The observed is smaller by more than a factor of 3 than either the unbound quasiparticle-quasihole pair-creation energy gap or the magneto-roton energy gap, calculated for an ideal two-dimensional electron system. Observations for devices of mobility ≊300 000 /V s yield even smaller . Adequate fitting of all our results requires inclusion of finite electron layer thickness and disorder, with the effect of decreasing the energy gaps and providing a finite magnetic field threshold. At low temperatures and high magnetic fields, deviations from activated conduction are observed. These deviations are attributed to two-dimensional hopping conduction in a magnetic field. Samples of sufficiently low mobility, ≲150 000 /V s exhibit no evidence of activated conduction. Rather, the transport is qualitatively consistent with two-dimensional hopping alone. Studies at Landau-level filling factors ν=(2/5 and (3/5 also yield a single activation energy, , with a weak magnetic field dependence. Experimentally, we find Δ∼0.4, compared with an expected ratio of 0.28 from simple theoretical considerations.
- Received 29 April 1987
DOI:https://doi.org/10.1103/PhysRevB.36.7919
©1987 American Physical Society