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Physical review. B, Condensed matter

Chiral phase states of the Hubbard Hamiltonian

Fedor Vasilievich Kusmartsev, MANFRED L. RISTIG

Published Sep 1, 1991• 6 citations
Publisher

Abstract

We present a variational approach different from that based on Gutzwiller's ansatz by investigating chiral flux-phase states of the Hubbard Hamiltonian in analogy to the treatment of the fractional quantum Hall effect. The proposed class of generalized Laughlin trial functions is specialized to permit detailed consideration of a set of states that includes a ferromagnetic ground state generated by a spontaneous gauge field in conjunction with a fictitious magnetic field having a flux commensurate with the filling. We evaluate the trial energy expectation values and demonstrate that the treatment is, at least, appropriate for the Hubbard model with sufficiently large on-site Coulomb repulsion and low electron densities. The members of the special set of trial states may be suitably classified by the flux quanta of the associated field and may be characterized either by integer or fractional quantum numbers. The excitations designated by fractional quantum numbers, which are not commensurate with the filling, are identified as flux-phase states breaking the symmetries of the lattice.

DOI
10.1103/physrevb.44.5351

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Generalized flux states and the quantum Hall effect (abstract)

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Journal of Applied Physics•Apr 15, 1991•0 citations
We present a variational approach different from that based on Gutzwiller's ansatz by investigating chiral flux-phase states of the Hubbard Hamiltonian in analogy to the treatment of the fractional quantum Hall effect. The proposed class of generalized Laughlin trial functions is specialized to permit detailed consideration of a set of states that includes a ferromagnetic ground state generated by a spontaneous gauge field in conjunction with a fictitious magnetic field having a flux commensurate with the filling. We evaluate the trial energy expectation values and demonstrate that the treatment is, at least, appropriate for the Hubbard model with sufficiently large on-site Coulomb repulsion and low electron densities. The members of the special set of trial states may be suitably classified by the flux quanta of the associated field and may be characterized either by integer or fractional quantum numbers. The excitations designated by fractional quantum numbers, which are not commensurate with the filling, are identified as flux-phase states breaking the symmetries of the lattice.
We propose a variational approach for the study of chiral flux-phase states. We present a class of variational wave functions of the generalized Laughlin-type. The generalization consists of the introduction of Gutzwiller projections and of a fictitious magnetic field. We classify antiferromagnetic chiral flux-phase states and evaluate analytically the corresponding expectation values of the t-J Hamiltonian at any value of filling. There are two types of chiral flux-phase states: commensurate and fractional. We have found that the minimal energy has a commensurate-flux-phase state for which the value of the flux of the orbital magnetic field equals the filling.
We propose a variational approach for the study of chiral flux-phase states. We present a class of variational wave functions of the generalized Laughlin-type. The generalization consists of the introduction of Gutzwiller projections and of a fictitious magnetic field. We classify antiferromagnetic chiral flux-phase states and evaluate analytically the corresponding expectation values of the t-J Hamiltonian at any value of filling. There are two types of chiral flux-phase states: commensurate and fractional. We have found that the minimal energy has a commensurate-flux-phase state for which the value of the flux of the orbital magnetic field equals the filling.
In this paper, the Bose-Hubbard model (BHM) with the nearest-neighbor (NN) repulsions is studied from the viewpoint of possible bosonic analogs of the fractional quantum Hall (FQH) state in the vicinity of the Mott insulator (MI). First, by means of the Gutzwiller approximation, we obtain the phase diagram of the BHM in a magnetic field. Then, we introduce an effective Hamiltonian describing excess particles on a MI and calculate the vortex density, momentum distribution, and the energy gap. These calculations indicate that the vortex solid forms for small NN repulsions, but a homogeneous featureless ``Bose metal'' takes the place of it as the NN repulsion increases. We consider particular filling factors at which the bosonic FQH state is expected to form. Chern-Simons (CS) gauge theory to the excess particle is introduced, and a modified Gutzwiller wave function, which describes bosons with attached flux quanta, is introduced. The energy of the excess particles in the bosonic FQH state is calculated using that wave function, and it is compared with the energy of the vortex solid and Bose metal. We found that the energy of the bosonic FQH state is lower than that of the Bose metal and comparable with the vortex solid. Finally, we clarify the condition that the composite fermion appears by using CS theory on the lattice that we previously proposed for studying the electron FQH effect.
New trial wave functions corresponding to half filling quantum Hall states are proposed. These wave functions are constructed by first pairing up the quasielectrons of the 1/3 Laughlin quantum Hall state, with the same relative angular momentum for each pair, and then making the paired quasielectrons condense into a 1/4 Laughlin state. The quasiparticle excitations of the proposed wave functions carry ±1/4\pm1/4±1/4 of electron charge, and obey Abelian fractional statistics. In the spherical geometry, the total flux quanta NϕN_ϕNϕ​ is shown to be related to the number of electrons NNN by Nϕ=2N−(5−q)N_ϕ = 2N-(5-q)Nϕ​=2N−(5−q) with qqq being the relative angular momentum between the quasielectrons in each pair which takes values of non-negative even integers. The overlaps are calculated between the proposed trial wave functions, including the ground state, quasiexciton states, and quasihole states, and the exact states of the finite size systems at Nϕ=2N−3N_ϕ = 2N-3Nϕ​=2N−3. The near unity overlaps are obtained in the lowest Landau level, while the moderate overlaps are obtained in the second Landau level. The relevance of the wave functions to the yet to be discovered fractional quantum Hall effect in the lowest Landau level, as well as the 5/2 quantum Hall effect is also discussed.
We study bosonic and fermionic quantum two-leg ladders with orbital magnetic flux. In such systems, the ratio $\ensuremath{\nu}$ of particle density to magnetic flux shapes the phase space, as in quantum Hall effects. In fermionic (bosonic) ladders, when $\ensuremath{\nu}$ equals one over an odd (even) integer, Laughlin fractional quantum Hall (FQH) states are stabilized for sufficiently long-ranged repulsive interactions. As a signature of these fractional states, we find a unique dependence of the chiral currents on particle density and on magnetic flux. This dependence is characterized by the fractional filling factor $\ensuremath{\nu}$, and forms a stringent test for the realization of FQH states in ladders, using either numerical simulations or future ultracold-atom experiments. The two-leg model is equivalent to a single spinful chain with spin-orbit interactions and a Zeeman magnetic field, and results can thus be directly borrowed from one model to the other.
We investigate certain generalized flux phases arising in a mean-field approach to the t-J model. First we establish that the energy of noninteracting electrons moving in a uniform magnetic field has an absolute minimum as a function of the flux at exactly one flux quantum per particle. Using this result we show that if the hard-core nature of the hole bosons is taken into account, then the generalized flux states in the mean-field approximation constitute a solution where both the spinons and the holons experience an average flux of one flux quantum per particle. We elaborate the relation to the integer and fractional quantum Hall effect. Based on this analogy one expects that only states with certain fractional fillings (ν=1/odd integer) are stable against the fluctuations of the (gauge) field. We speculate that at arbitrary fillings the system minimizes its energy by phase separating into these stable states. Finally we work out the ground state energy for nonuniform (staggered) flux phases and establish that the often claimed equivalence of frustration and doping does not hold for these states.