Skyrmion Physics Beyond the Lowest Landau Level Approximation

The effects of Landau level mixing and finite thickness of the two-dimensional electron gas on the relative stability of skyrmion and single spin-flip excitations at Landau level filling factor $\nu=1$ have been investigated. Landau level mixing is studied by fixed-phase diffusion Monte Carlo and finite thickness is included by modifying the effective Coulomb interaction. Both Landau level mixing and finite thickness lower skyrmion excitation energies and favor skyrmions with fewer spin flips. However, the two effects do not work `coherently'. When finite thickness is included the effect of Landau level mixing is strongly suppressed.Comment: 4 pages, 4 figure

Optimization of ground and excited state wavefunctions and van der Waals clusters

A quantum Monte Carlo method is introduced to optimize excited state trial wavefunctions. The method is applied in a correlation function Monte Carlo calculation to compute ground and excited state energies of bosonic van der Waals clusters of upto seven particles. The calculations are performed using trial wavefunctions with general three-body correlations

Quantum Hall Fluids on the Haldane Sphere: A Diffusion Monte Carlo Study

A generalized diffusion Monte Carlo method for solving the many-body Schr\"odinger equation on curved manifolds is introduced and used to perform a `fixed-phase' simulation of the fractional quantum Hall effect on the Haldane sphere. This new method is used to study the effect of Landau level mixing on the $\nu=1/3$ energy gap and the relative stability of spin-polarized and spin-reversed quasielectron excitations.Comment: 13 pages, Revtex + psfig, figures include

Edge reconstruction in the fractional quantum Hall regime

The interplay of electron-electron interaction and confining potential can lead to the reconstruction of fractional quantum Hall edges. We have performed exact diagonalization studies on microscopic models of fractional quantum Hall liquids, in finite size systems with disk geometry, and found numerical evidence of edge reconstruction under rather general conditions. In the present work we have taken into account effects like layer thickness and Landau level mixing, which are found to be of quantitative importance in edge physics. Due to edge reconstruction, additional nonchiral edge modes arise for both incompressible and compressible states. These additional modes couple to electromagnetic fields and thus can be detected in microwave conductivity measurements. They are also expected to affect the exponent of electron Green's function, which has been measured in tunneling experiments. We have studied in this work the electric dipole spectral function that is directly related to the microwave conductivity measurement. Our results are consistent with the enhanced microwave conductivity observed in experiments performed on samples with an array of antidots at low temperatures, and its suppression at higher temperatures. We also discuss the effects of the edge reconstruction on the single electron spectral function at the edge.Comment: 19 pages, 12 figure

Fermi-sea-like correlations in a partially filled Landau level

The pair distribution function and the static structure factor are computed for composite fermions. Clear and robust evidence for a $2k_F$ structure is seen in a range of filling factors in the vicinity of the half-filled Landau level. Surprisingly, it is found that filled Landau levels of composite fermions, i.e. incompressible FQHE states, bear a stronger resemblance to a Fermi sea than do filled Landau levels of electrons.Comment: 23 pages, revte

Hamiltonian Description of Composite Fermions: Calculation of Gaps

We analytically calculate gaps for the 1/3, 2/5, and 3/7 polarized and partially polarized Fractional Quantum Hall states based on the Hamiltonian Chern-Simons theory we have developed. For a class of potentials that are soft at high momenta (due to the finite thickness of the sample) we find good agreement with numerical and experimental results.Comment: 4 pages, 2 eps figures. One reference added, some typos (one in equation 7) corrected, and minor notational modification

Two-Roton Bound State in the Fractional Quantum Hall Effect

The true nature of the lowest-energy, long-wavelength neutral excitation of the fractional quantum Hall effect has been a long outstanding problem. In this Letter, we establish that it is a two-roton bound state.Comment: 4 pages, 4 figures; Physical Review Letters, in pres

Magnons and skyrmions in fractional Hall ferromagnets

Recent experiments have established a qualitative difference between the magnetization temperature-dependences $M(T)$ of quantum Hall ferromagnets at integer and fractional filling factors. We explain this difference in terms of the relative energies of collective magnon and particle-hole excitations in the two cases. Analytic calculations for hard-core model systems are used to demonstrate that, in the fractional case, interactions suppress the magnetization at finite temperatures and that particle-hole excitations rather than long-wavelength magnons control $M(T)$ at low $T$.Comment: 4 pages, no figure

Spontaneous Magnetization of Composite Fermions

It is argued that the composite fermion liquid is a promising candidate for an observation of the elusive, interaction driven magnetization first proposed by Bloch seven decades ago. In analogy to what is theoretically believed to be the case for the idealized electron gas in zero magnetic field, this spontaneously broken symmetry phase is predicted to occur prior to a transition into the Wigner crystal.Comment: 5 pages, 4 figure

Study of Low Energy Spin Rotons in the Fractional Quantum Hall Effect

Motivated by the discovery of extremely low energy collective modes in the fractional quantum Hall effect (Kang, Pinczuk {\em et al.}), with energies below the Zeeman energy, we study theoretically the spin reversed excitations for fractional quantum Hall states at $\nu=2/5$ and 3/7 and find qualitatively different behavior than for $\nu=1/3$. We find that a low-energy, charge-neutral "spin roton," associated with spin reversed excitations that involve a change in the composite-fermion Landau level index, has energy in reasonable agreement with experiment.Comment: Postscript figures included. Accepted in Phys. Rev. B (Rapid Communication