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Physical review. B, Condensed matter

Size dependence in small-system calculations for fractional quantum Hall states

Allan H. MacDonald, G. C. Aers

Published Aug 15, 1986• 19 citations
Publisher

Abstract

We discuss the size dependence of small-system periodic-boundary-condition calculations of ground-state energies and excitation energies for two-dimensional electron systems in a strong magnetic field. The discussion is based on an examination of the Hartree-Fock approximation for a finite-size system. We find that finite-size-system calculations overestimate the magnitude of the ground-state energy and underestimate excitation energies, especially in the case of higher-orbital Landau levels. Numerical estimates of the finite-size corrections are given for 4-, 9-, and 16-electron systems.

DOI
10.1103/physrevb.34.2906

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Physical Review Letters•Jan 21, 1985•448 citations
We discuss the size dependence of small-system periodic-boundary-condition calculations of ground-state energies and excitation energies for two-dimensional electron systems in a strong magnetic field. The discussion is based on an examination of the Hartree-Fock approximation for a finite-size system. We find that finite-size-system calculations overestimate the magnitude of the ground-state energy and underestimate excitation energies, especially in the case of higher-orbital Landau levels. Numerical estimates of the finite-size corrections are given for 4-, 9-, and 16-electron systems.
Two dimensional electrons in a strong magnetic field show the fractional quantum Hall effect at low temperatures. This effect is explained successfully by a discovery of a new liquid type ground state. However the infinitely strong magnetic field has been assumed in existing theories. Here the ground state around one third filling of the lowest Landau level is investigated at a finite magnetic field by numerical diagonalization of the Hamiltonian. When the cyclotron energy is not too small compared to a typical Coulomb energy, no qualitative change of the ground state is found: A natural generalization of the liquid state at the infinite magnetic field describes the ground state. Excitation energies of quasiparticles decrease as the magnetic field decreases.
Finite-size two-dimensional systems are investigated by numerical diagonalization of the Hamiltonian. Both fermion systems and boson systems are investigated. The energy of the liquidlike ground state of the fermion system shows a downward cusp at 13\frac{1}{3}31​ filling of the lowest Landau level. This behavior is consistent with the observed fractional quantum Hall effect. The short-range behavior of the pair distribution function of the ground state is analyzed and good agreement with Laughlin's trial wave function is obtained.
The formalism introduced in a previous paper is used for discussing the Coulomb interaction of many electrons moving in two space-dimensions in the presence of a strong magnetic field. The matrix element of the Coulomb interaction is evaluated in the new basis, whose states are invariant under discrete translations (up to a gauge transformation). This paper is devoted to the case of low filling factor, thus we limit ourselves to the lowest Landau level and to spins all oriented along the magnetic field. For the case of filling factor νf = 1/u we give an Ansatz on the state of many electrons which provides a good approximated solution of the Hartree–Fock equation. For general filling factor νf = u′/u a trial state is given which converges very rapidly to a solution of the self-consistent equation. We generalize the Hartree–Fock equation by considering some correlation: all quantum states are allowed for the u′ electrons with the same translation quantum numbers. Numerical results are given for the mean energy and the energy bands, for some values of the filling factor (νf = 1/2, 1/3, 2/3, 1/4, 3/4, 1/5, 2/5, 3/5, 4/5). Our results agree numerically with the Charge Density Wave approach. The boundary conditions are shown to be very important: only large systems (degeneracy of Landau level over 200) are not affected by the boundaries. Therefore results obtained on small scale systems are somewhat unreliable. The relevance of the results for the Fractional Quantum Hall Effect is briefly discussed.
The Hamiltonian describing interacting two-dimensional electrons in a high magnetic field is diagonalized numerically for a small number of particles to obtain the low-lying excitation spectra. The results include estimates of energy gaps for values of $\ensuremath{\nu}$ (the lowest-Landau-level filling factor) equal to certain multiples of 15\frac{1}{5}51​, 17\frac{1}{7}71​, 19\frac{1}{9}91​, and 111\frac{1}{11}111​. These $\ensuremath{\nu}'\mathrm{s}$ are characterized by the existence of periodic rigid parent states which generate maximum phase space. The even-denominator cases are markedly different.
We compute energy gaps for spin-polarized fractional quantum Hall states in the lowest Landau level (LL) at filling fractions $\ensuremath{\nu}=\frac{1}{3},$ 25,\frac{2}{5},52​, 37,\frac{3}{7},73​, and 49\frac{4}{9}94​ using exact diagonalization of systems with up to 16 particles and extrapolation to the infinite system-size limit. The gaps calculated for a pure Coulomb interaction and ignoring finite width effects, disorder and LL mixing agree well with the predictions of composite fermion theory provided the logarithmic corrections to the effective mass are included. This is in contrast with previous estimates, which, as we show, overestimated the gaps at $\ensuremath{\nu}=2/5$ and 3/7 by around 15%. We also study the reduction of the gaps as a result of the nonzero width of the two-dimensional (2D) layer. We show that these effects are accurately accounted for using either Gaussian or ``$z\ifmmode\times\else\texttimes\fi{}$ Gaussian'' (zG)(z\mathrm{G})(zG) trial wave functions, which we show are significantly better variational wave functions than the Fang-Howard wave function. The Gaussian and zGz\mathrm{G}zG wave functions give Haldane pseudopotential parameters which agree with those of self-consistent local density approximation calculations to better than $\ifmmode\pm\else\textpm\fi{}0.2%.$ For quantum well parameters typical of heterostructure samples, we find gap reductions of around 20%. The experimental gaps, after accounting heuristically for disorder, are still around 40% smaller than the computed gaps. However, for the case of tetracene layers in metal-insulator-semiconductor (MIS) devices we find that the measured activation gaps are close to those we compute. We discuss possible reasons why the difference between computed and measured activation gaps is larger in GaAs heterostructures than MIS devices. Finally, we present calculations using systems with up to 18 electrons of the gap at $\ensuremath{\nu}=\frac{5}{2}$ including width corrections.
The incompressible states of interacting two-dimensional electrons in a high magnetic field has been studied by finite-size calculations in the spherical geometry. The excitation spectrum at 13\frac{1}{3}31​ Landau-level filling is described, and the Laughlin-Jastrow character of the Coulomb-interaction ground state is unambiguously confirmed. As the interaction is varied, a transition to a gapless compressible state is observed.