The electron distribution over Landau levels is calculated in the regime of the quantum Hall effect at strong Coulomb interaction. For this purpose, a modified scheme of exact diagonalization with a significantly reduced basis of many-particle configurations has been developed, which makes it possible to adequately take into account the mixing of states in several Landau levels. The behavior of the electron distribution function over Landau levels is studied as a function of the filling factor and the Wigner–Seitz radius rs. It is shown that the Landau quantization at a fixed electron density significantly suppresses the smearing of the electron distribution function as compared to the case of zero magnetic field. For example, for rs 1 and low filling factors, the quasiparticle contribution to the Migdal jump 1 − Z in the distribution function is an approximately linear function of ν and a quadratic function of rs. Simultaneously, as ν decreases, the tails of the distribution function become longer. The mechanism of rearrangement of the distribution function in the regime of the quantum Hall effect is described in terms of the generation of magnetoplasmon fluctuations involved in the structure of the ground state of the system.
The electron distribution over Landau levels is calculated in the regime of the quantum Hall effect at strong Coulomb interaction. For this purpose, a modified scheme of exact diagonalization with a significantly reduced basis of many-particle configurations has been developed, which makes it possible to adequately take into account the mixing of states in several Landau levels. The behavior of the electron distribution function over Landau levels is studied as a function of the filling factor and the Wigner–Seitz radius rs. It is shown that the Landau quantization at a fixed electron density significantly suppresses the smearing of the electron distribution function as compared to the case of zero magnetic field. For example, for rs 1 and low filling factors, the quasiparticle contribution to the Migdal jump 1 − Z in the distribution function is an approximately linear function of ν and a quadratic function of rs. Simultaneously, as ν decreases, the tails of the distribution function become longer. The mechanism of rearrangement of the distribution function in the regime of the quantum Hall effect is described in terms of the generation of magnetoplasmon fluctuations involved in the structure of the ground state of the system.
The electron distribution over Landau levels is calculated in the regime of the quantum Hall effect at strong Coulomb interaction. For this purpose, a modified scheme of exact diagonalization with a significantly reduced basis of many-particle configurations has been developed, which makes it possible to adequately take into account the mixing of states in several Landau levels. The behavior of the electron distribution function over Landau levels is studied as a function of the filling factor and the Wigner–Seitz radius rs. It is shown that the Landau quantization at a fixed electron density significantly suppresses the smearing of the electron distribution function as compared to the case of zero magnetic field. For example, for rs < 1 and low filling factors, the quasiparticle contribution to the Migdal jump 1 − Z in the distribution function is an approximately linear function of ν and a quadratic function of rs. Simultaneously, as ν decreases, the tails of the distribution function become longer. The mechanism of rearrangement of the distribution function in the regime of the quantum Hall effect is described in terms of the generation of magnetoplasmon fluctuations involved in the structure of the ground state of the system.
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.
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.
The fractional quantum Hall effect was experimentally discovered in 1982. It was observed that the Hall conductivity σyx of a two-dimensional electron system is quantized, σyx=e2/3h, in the vicinity of the Landau level filling factor ν=1/3. In 1983, Laughlin proposed a trial many-body wave function, which he claimed described a “new state of matter”—a homogeneous incompressible liquid with fractionally charged quasiparticles. Here, I develop an exact diagonalization theory that allows one to calculate the energy and other physical properties of the ground and excited states of a system of N two-dimensional Coulomb interacting electrons in a strong magnetic field. I analyze the energies, electron densities, and other physical properties of the systems with N≤7 electrons continuously as a function of magnetic field in the range 1/4≲ν<1. The results show that both the ground and excited states of the system resemble a sliding Wigner crystal whose parameters are influenced by the magnetic field. Energy gaps in the many-particle spectra appear and disappear as the magnetic field changes. I also calculate the physical properties of the ν=1/3 Laughlin state for N≤8 and compare the results with the exact ones. This comparison, as well as an analysis of some other statements published in the literature, show that the Laughlin state and its fractionally charged excitations do not describe the physical reality, neither at small N nor in the thermodynamic limit. The results obtained shed new light on the nature of the ground and excited states in the fractional quantum Hall effect.
We study the nature of the many-body electron states (and quantum phase transitions between them) in a double-quantum-well structure under a strong external magnetic field as a function of the materials parameters that define the two-layer system, namely, the thickness of individual layers, the separation between them, the individual well potentials confining the electrons, and the potential barrier between them. Motivated by two recent experiments, we consider two different situations, one with (almost) no quantum tunneling between the wells, and the other with substantial interwell quantum tunneling. We use the spherical system finite-size exact diagonalization technique for our calculations. By calculating the overlap of our exact small-system numerical wave functions with various (postulated) analytic wave functions, we comment on the nature of the incompressibility for various Landau-level filling factors (\ensuremath{\nu}). We investigate, in particular details, the possibility of \ensuremath{\nu}=1/2 (i.e., 1/4 average occupancy in each well), and, 1 (i.e., 1/2 occupancy in each well) incompressible states where \ensuremath{\nu} is the total filling factor for the system. We also provide results for the \ensuremath{\nu}=2/3 situation. Our conclusion, based on our use of realistic system parameters in our calculations, is that in both the recent experimental realizations of the \ensuremath{\nu}=1/2 fractional quantum Hall effect, the relevant ground state is the so-called ``331'' state which is stabilized by the competition between intrawell and interwell electron-electron correlations.We calculate for various double-quantum-well structural parameters and magnetic fields the \ensuremath{\nu}=1/2 excitation spectra and the energy gaps, and compare it with the recent experimental data, obtaining excellent quantitative agreement. We provide a detailed quantum phase diagram for the \ensuremath{\nu}=1 state as a function of the interwell Coulomb interaction and the symmetric-antisymmetric single-particle gap, and find that the \ensuremath{\nu}=1 quantum Hall effect observed in the two recent experiments, in contrast to the \ensuremath{\nu}=1/2 effect, belongs in some sense to two different universality classes. We also predict the existence of a reentrant \ensuremath{\nu}=1 quantum Hall effect in double-quantum-well systems.
Exact diagonalization of a two-dimensional electron gas in a strong magnetic field in the disk geometry shows that there exists a filling-factor range in the second Landau level where the states significantly differ from those in the lowest Landau level. We show that the difference arises because the interaction between composite fermions is not negligible in higher Landau levels, as indicated by a substantial mixing between composite-fermion Landau-like levels, or $\ensuremath{\Lambda}$ levels. We find that the exact ground state is well reproduced by composite-fermion theory with $\ensuremath{\Lambda}$-level mixing incorporated at the lowest level of approximation. Using the same variational approach in the spherical geometry we estimate the excitation gap at filling 1∕3 in the second Landau level (which corresponds to 7∕3 of experiment).