4 research outputs found
Anomalous structure in the single particle spectrum of the fractional quantum Hall effect
The two-dimensional electron system (2DES) is a unique laboratory for the
physics of interacting particles. Application of a large magnetic field
produces massively degenerate quantum levels known as Landau levels. Within a
Landau level the kinetic energy of the electrons is suppressed, and
electron-electron interactions set the only energy scale. Coulomb interactions
break the degeneracy of the Landau levels and can cause the electrons to order
into complex ground states. In the high energy single particle spectrum of this
system, we observe salient and unexpected structure that extends across a wide
range of Landau level filling fractions. The structure appears only when the
2DES is cooled to very low temperature, indicating that it arises from delicate
ground state correlations. We characterize this structure by its evolution with
changing electron density and applied magnetic field. We present two possible
models for understanding these observations. Some of the energies of the
features agree qualitatively with what might be expected for composite
Fermions, which have proven effective for interpreting other experiments in
this regime. At the same time, a simple model with electrons localized on
ordered lattice sites also generates structure similar to those observed in the
experiment. Neither of these models alone is sufficient to explain the
observations across the entire range of densities measured. The discovery of
this unexpected prominent structure in the single particle spectrum of an
otherwise thoroughly studied system suggests that there exist core features of
the 2DES that have yet to be understood.Comment: 15 pages, 10 figure
Spin gap in the two-dimensional electron system of GaAs/AlxGa1-xAs single heterojunctions in weak magnetic fields
We study the interaction-enhanced spin gaps in the two-dimensional electron gas confined in GaAs/AlGaAs single heterojunctions subjected to weak magnetic fields. The values are obtained from the chemical potential jumps measured by magnetocapacitance. The gap increase with parallel magnetic field indicates that the lowest-lying charged excitations are accompanied with a single spin flip at the odd-integer filling factor nu=1 and nu=3, in disagreement with the concept of skyrmions