Composite fermions traversing a potential barrier

Using a composite fermion picture, we study the lateral transport between two two-dimensional electron gases, at filling factor 1/2, separated by a potential barrier. In the mean field approximation, composite fermions far from the barrier do not feel a magnetic field while in the barrier region the effective magnetic field is different from zero. This produces a cutoff in the conductance when represented as a function of the thickness and height of the barrier. There is a range of barrier heights for which an incompressible liquid, at $\nu =1/3$, exists in the barrier region.Comment: 3 pages, latex, 4 figures available upon request from [email protected]. To appear in Physical Review B (RC) June 15t

Measurements of the Composite Fermion masses from the spin polarization of 2-D electrons in the region 1<ν<21<\nu<2

Measurements of the reflectivity of a 2-D electron gas are used to deduce the polarization of the Composite Fermion hole system formed for Landau level occupancies in the regime 1<\nu<2. The measurements are consistent with the formation of a mixed spin CF system and allow the density of states or `polarization' effective mass of the CF holes to be determined. The mass values at \nu=3/2 are found to be ~1.9m_{e} for electron densities of 4.4 x 10^{11} cm^{-2}, which is significantly larger than those found from measurements of the energy gaps at finite values of effective magnetic field.Comment: 4 pages, 3 fig

Partially spin polarized quantum Hall effect in the filling factor range 1/3 < nu < 2/5

The residual interaction between composite fermions (CFs) can express itself through higher order fractional Hall effect. With the help of diagonalization in a truncated composite fermion basis of low-energy many-body states, we predict that quantum Hall effect with partial spin polarization is possible at several fractions between $\nu=1/3$ and $\nu=2/5$. The estimated excitation gaps are approximately two orders of magnitude smaller than the gap at $\nu=1/3$, confirming that the inter-CF interaction is extremely weak in higher CF levels.Comment: 4 pages, 3 figure

Hund's Rule for Composite Fermions

We consider the ``fractional quantum Hall atom" in the vanishing Zeeman energy limit, and investigate the validity of Hund's maximum-spin rule for interacting electrons in various Landau levels. While it is not valid for {\em electrons} in the lowest Landau level, there are regions of filling factors where it predicts the ground state spin correctly {\em provided it is applied to composite fermions}. The composite fermion theory also reveals a ``self-similar" structure in the filling factor range $4/3>\nu>2/3$.Comment: 10 pages, revte

Fractional Quantum Hall States in Low-Zeeman-Energy Limit

We investigate the spectrum of interacting electrons at arbitrary filling factors in the limit of vanishing Zeeman splitting. The composite fermion theory successfully explains the low-energy spectrum {\em provided the composite fermions are treated as hard-core}.Comment: 12 pages, revte

New insulating phases of two-dimensional electrons in high Landau levels: observation of sharp thresholds to conduction

The intriguing re-entrant integer quantized Hall states recently discovered in high Landau levels of high-mobility 2D electron systems are found to exhibit extremely non-linear transport. At small currents these states reflect insulating behavior of the electrons in the uppermost Landau level. At larger currents, however, a discontinuous and hysteretic transition to a conducting state is observed. These phenomena, found only in very narrow magnetic field ranges, are suggestive of the depinning of a charge density wave state, but other explanations can also be constructed.Comment: 5 pages, 5 figure

Cosmic coincidence problem and variable constants of physics

The standard model of cosmology is investigated using time dependent cosmological constant $\Lambda$ and Newton's gravitational constant $G$. The total energy content is described by the modified Chaplygin gas equation of state. It is found that the time dependent constants coupled with the modified Chaplygin gas interpolate between the earlier matter to the later dark energy dominated phase of the universe. We also achieve a convergence of parameter $\omega\to-1$, with minute fluctuations, showing an evolving $\omega$. Thus our model fairly alleviates the cosmic coincidence problem which demands $\omega=-1$ at present time.Comment: 27 pages, 15 figure

Pulsed Magnetic Field Measurements of the Composite Fermion Effective Mass

Magnetotransport measurements of Composite Fermions (CF) are reported in 50 T pulsed magnetic fields. The CF effective mass is found to increase approximately linearly with the effective field $B^*$, in agreement with our earlier work at lower fields. For a $B^*$ of 14 T it reaches $1.6m_e$, over 20 times the band edge electron mass. Data from all fractions are unified by the single parameter $B^*$ for all the samples studied over a wide range of electron densities. The energy gap is found to increase like $\sqrt{B^*}$ at high fields.Comment: Has final table, will LaTeX without error

Structures for Interacting Composite Fermions: Stripes, Bubbles, and Fractional Quantum Hall Effect

Much of the present day qualitative phenomenology of the fractional quantum Hall effect can be understood by neglecting the interactions between composite fermions altogether. For example the fractional quantum Hall effect at $\nu=n/(2pn\pm 1)$ corresponds to filled composite-fermion Landau levels,and the compressible state at $\nu=1/2p$ to the Fermi sea of composite fermions. Away from these filling factors, the residual interactions between composite fermions will determine the nature of the ground state. In this article, a model is constructed for the residual interaction between composite fermions, and various possible states are considered in a variational approach. Our study suggests formation of composite-fermion stripes, bubble crystals, as well as fractional quantum Hall states for appropriate situations.Comment: 16 pages, 7 figure