Slave-boson Approach Research Articles

High-Tcphase diagram based on the SU(2) slave-boson approach to thet−JHamiltonian

Based on an improved SU(2) slave-boson approach showing coupling between the charge and spin degrees of freedom, we derive a phase diagram of high-${T}_{c}$ cuprates which displays both the superconducting and pseudogap phases in the plane of temperature vs hole doping rate. It is shown that phase fluctuations in the order parameters result in a closer agreement with the observed phase diagram of an arch shape, by manifesting the presence of an optimal doping rate closer to observation, compared to the U(1) slave-boson theory.

Mean-field approach for the B phase of (La,Ca)MnO3

I consider a simple cubic lattice of mixed valent Mn ions (Mn4+ and Mn3+) and calculate the ground state energy for the ferromagnetic B phase using a slave-boson mean-field approach. Each Mn ion has three localized t2g electrons with their spins ferromagnetically coupled to form a spin S=3/2. Ions in the Mn3+ configuration have an additional eg electron to form a total spin of (S+1/2). The eg electrons are allowed to hop between the Mn sites (giving rise to the double exchange), but the multiple occupancy of the eg levels is excluded at each site. Five slave bosons per site are introduced to take into account the correlations between the eg electrons.

Kondo-type transport through a quantum dot under magnetic fields

In this paper, we investigate the Kondo correlation effects on linear and nonlinear transport in a quantum dot connected to reservoirs under finite magnetic fields, using the slave-boson mean field approach suggested by Kotliar and Ruckenstein [Phys. Rev. Lett. >57, 1362 (1986)]. A brief comparison between the present formulation and other slave-boson formulation is presented to justify this approach. The numerical results show that the linear conductance near electron-hole symmetry is suppressed by the application of the magnetic fields, but an anomalous enhancement is predicted in the nonsymmetry regime. The effect of external magnetic fields on the nonlinear differential conductances is discussed for the Kondo system. A significant reduction of the peak splitting is observed due to the strong Kondo correlation, which agrees well with experimental data.

Holon pair bose condensation based on the SU(2) slave-boson approach to the t– J Hamiltonian; instability of the normal state against holon pairing

Based on the application of the SU(2) slave-boson theory to the Bethe–Salpeter equation for the boson (holon) sector, we derive the bose condensation temperature T c as a function of hole doping rate for high T c cuprates. It is shown that the computed results of the t-matrix yielded good agreement with the functional integral approach of the SU(2) slave-boson theory in reproducing the bose condensation temperature T c of an arch shape as a function of doping rate.

Mott metal-insulator transition in the half-filled Hubbard model on the triangular lattice

We investigate the metal-insulator transition in the half-filled Hubbard model on the two-dimensional triangular lattice using both the Kotliar-Ruckenstein slave-boson technique and an exact numerical diagonalization of finite clusters. Contrary to the case of a square lattice, where a perfect nesting of the Fermi surface leads to a metal-insulator transition at arbitrarily small values of U, always accompanied by antiferromagnetic ordering, on a triangular lattice, due to the lack of perfect nesting, the transition takes place at a finite value of U, and frustration induces a nontrivial competition among different magnetic phases. Indeed, within the mean-field approximation in the slave-boson approach, as the interaction grows the paramagnetic metal turns into a metallic phase with incommensurate spiral ordering. Increasing the interaction further, a linear spin-density wave is stabilized, and finally for strong coupling the latter phase undergoes a first-order transition toward an antiferromagnetic insulator. No trace of the intermediate phases is seen in the exact diagonalization results, indicating a transition between a paramagnetic metal and an antiferromagnetic insulator.

COMMENSURATE VERSUS INCOMMENSURATE SPIN-ORDERING IN THE TRIANGULAR HUBBARD MODEL

The presence of incommensurate spin structures in the half-filled triangular Hubbard model, where frustration leads to a competition among different magnetic phases, is investigated using both the slave-boson technique, and exact diagonalization of finite clusters. We also investigate the metal-insulator transition which, due to the lack of perfect nesting, takes place at a finite value of U. Within the slave-boson approach, as the interaction grows the paramagnetic metal turns into a metallic phase with incommensurate spiral ordering. Increasing further the interaction, a linear spin-density-wave is stabilized, and finally for strong coupling the latter phase undergoes a first-order transition towards an antiferromagnetic insulator. No trace of the intermediate phases is instead found in the exact diagonalization results.

Local quasiparticle states around an Anderson impurity in ad-wave superconductor: Kondo effects

The Kondo effects of an Anderson impurity embedded into a d-wave superconductor is studied. Within the slave-boson mean-field approach, the derived Bogoliubov-de Gennes equations are solved via exact diagonalization. We show that a critical coupling strength, above which the Kondo effect takes place, exists regardless of whether the band particle-hole symmetry is present or not. The resonant quasiparticle peaks are found in the local density of states (LDOS) both directly at the impurity and around its neighbors, which is in sharp contrast to the case of nonmagnetic unitary impurities, where the LDOS vanishes on the impurity site.

CHARGE AND SPIN MODULATION IN THE PRESENCE OF A DIMERIZED CRYSTAL FIELD

I study the evolution of the antiferromagnetic ground state of the half-filled Hubbard model in the presence of a charge modulation induced by the dimerization of the crystal field, making use of the slave-boson approach. I also discuss how the model can be extended to describe the neutral-to-ionic transition which occurs in some organic mixed-stack donor-acceptor crystals.

Phase separation based on a U(1) slave-boson functional integral approach to thet−Jmodel

We investigate the phase diagram of phase separation for the hole-doped two dimensional system of antiferromagnetically correlated electrons based on the U(1) slave-boson functional integral approach to the t-J model. We show that the phase separation occurs for all values of J/t, that is, whether $0 < J/t < 1$ or $J/t \geq 1$ with J, the Heisenberg coupling constant and t, the hopping strength. This is consistent with other numerical studies of hole-doped two dimensional antiferromagnets. The phase separation in the physically interesting J region, $0 < J/t \lesssim 0.4$ is examined by introducing hole-hole (holon-holon) repulsive interaction. We find from this study that with high repulsive interaction between holes the phase separation boundary tends to remain robust in this low $J$ region, while in the high J region, J/t > 0.4, the phase separation boundary tends to disappear.

Tunneling density of states of high T(c) superconductors: d-wave BCS model versus SU(2) slave-boson model

Motivated by recent experimental measurements of the tunneling characteristics of high T(c) materials using scanning tunneling spectroscopy, we have calculated the I-V and differential conductance curves in the superconducting state at zero temperature. Comparing BCS-like d-wave pairing and the SU(2) slave-boson approach, we find that the slave-boson model can explain the asymmetric background observed in experiments. The slave-boson model also predicts that the height of the conductance peak relative to the background is proportional to the hole doping concentration x, at least for underdoped samples. We also observe the absence of the van Hove singularity, and comment on possible implications.

Interacting-impurity Josephson junction: Variational wave functions and slave-boson mean-field theory

We investigate the Josephson coupling between two superconductors mediated through an infinite U Anderson impurity, adapting a variational wave-function approach which has proved successful for the Kondo model. Unlike the Kondo problem, however, a crossing of singlet and doublet state energies may be produced by varying the ratio of Kondo energy to superconducting gap, in agreement with recent work of Clerk and Ambegaokar. We construct the phase diagram for the junction and discuss properties of different phases. In addition, we find the singlet and doublet state energies within a slave-boson mean-field approach. We find the slave-boson mean-field treatment is unable to account for the level crossing.

Consistent picture for resonance-neutron-peak and angle-resolved photoemission spectra in high-Tcsuperconductors

The spectra observed in ARPES measurements are examined together with the resonance peak observed in neutron scatterings, based on the slave-boson approach to $t-t^{\prime}-J$ model. We show that the peak/dip/hump features arise from the scattering of electrons by collective spin excitations which, at the same time, give rise to the neutron resonance mode. The doping dependences and the dispersions of the peak/dip/hump positions are shown to be consistent with experiments. Our results indicate that recently observed $\cos (6\theta)$ deviation from the pure d-wave also result from the renormalization by spin fluctuations.

Study of hole pair condensation based on the SU(2) slave-boson approach to the t– J Hamiltonian

We present a phase diagram of high T c cuprates involving the holon pair condensation based on the SU(2) slave-boson approach to the t– J Hamiltonian. Owing to the proper account of low-energy phase fluctuations by the SU(2) theory, we find that the predicted holon pairing temperature (the superconducting transition temperature) is in better agreement with experiments, compared to the U(1) mean field theory.

Spin and charge ordering in the dimerized Hubbard model

We study the effect of a chemically or deformation-induced charge-density wave on the spin-density-wave ground state of the Hubbard model at half-filling. We also consider the effect of a lattice deformation associated with a dimerization of the hopping term, thus introducing a competition with a paramagnetic bond-alternating phase. The slave-boson approach is used as an interpolation scheme to treat the electronic correlations from weak to strong coupling and determine the phase diagram of the model. We also apply our results to describe the neutral-ionic transition in organic mixed-stack donor-acceptor crystals.

Electron correlations in double exchange model with orbital degeneracy

We study the effects of the Hubbard interaction among conduction electrons on the double exchange model with the orbital degeneracy. Employing a slave-boson approach to treat the correlation for conduction electrons and a coherent-potential approximation for the Hund coupling, we calculate the density of states and the optical conductivity. The Hund coupling splits the density of states into two parts, which are modified by the band narrowing factor arising from the Hubbard interaction. Identifying the decrease of the magnetization for the localized spin with the increase of the temperature, we discuss the temperature dependence of the optical conductivity.

Incommensurate structure of a spin density wave in the one-dimensional t− J model

We make use of the slave-boson approach to investigate the magnetic properties of the one-dimensional t− J model in the weak-coupling limit J< t(1− n), where n is the number of electrons per unit cell. In the mean-field approximation at zero temperature we find that the helix spin-density-wave phase has a lower energy than the Néel phase. This latter disappears below a critical electron density n c , whereas the helix phase persists for all n. The hole-doping dependence of the spin-density-wave amplitude Δ and of the helix wave-vector q 0 are calculated. The temperature vs hole doping phase diagram in the mean-field approximation is also determined. It is shown that for n above a certain value n ∗ the helix state transforms into the Néel state by increasing the temperature as q 0 is reduced to zero. The Néel phase evolves towards the paramagnetic phase at higher temperatures. For n below n ∗ the helix phase transforms directly into the paramagnetic phase by increasing the temperature.

Mean-field description of the phase string effect in thet−Jmodel

A mean-field treatment of the phase string effect in the $t\ensuremath{-}J$ model is presented. Such a theory is able to unite the antiferromagnetic (AF) phase at half-filling and the metallic phase at finite doping within a single theoretical framework. We find that the low-temperature occurrence of the AF long-range ordering (AFLRO) at half-filling and superconducting condensation in the metallic phase are all due to Bose condensations of spinons and holons, respectively, on the top of a spin background described by bosonic resonating-valence-bond pairing. The fact that both spinon and holon here are bosonic objects, as the result of the phase string effect, represents a crucial difference from the conventional slave-boson and slave-fermion approaches. This theory also allows an underdoped metallic regime where the Bose condensation of spinons can still exist. Even though the AFLRO is gone here, such a regime corresponds to a microscopic charge inhomogeneity with short-ranged spin ordering. We discuss some characteristic experimental consequences for those different metallic regimes. A perspective on broader issues based on the phase string theory is also discussed.

Weak antiferromagnetism in the Kondo lattice

The N-fold degenerate Anderson model for a lattice is considered in the Kondo-regime. The system of f-electrons is supposed to be extremely correlated ( U→∞). Using a slave boson approach and a 1/ N expansion a criterion of the spin density wave (SDW) instability in the system of itinerant electrons is obtained. An effective interaction resulting in the SDW being formed appears due to spin fluctuations in the system of localized electrons.

Study of Hole Pair Condensation based on the SU(2) Slave-Boson Approach to the t-J Hamiltonian; Temperature, Momentum and Doping Dependences of Spectral Functions

Based on the U(1) and SU(2) slave-boson approaches to the t-J Hamil-tonian, we evaluate the one electron spectral functions for the hole doped high Tccuprates for comparison with the angle resolved photoemission spectroscopy(ARPES) data. We find that the observed quasiparticle peak in the superconducting state is correlated with the hump which exists in the normal state. We find that the spectral weight of the quasiparticle peak increases as doping rate increases, which is consistent with observation. As a conse-quence of the phase fluctuation effects of the spinon and holon pairing order parameters the spectral weight of the predicted peak obtained from the SU(2) theory is found to be smaller than the one predicted from U(1) mean field theory.

Vortex, Skyrmion, and elliptical domain-wall textures in the two-dimensional Hubbard model

The spin and charge texture around doped holes in the two-dimensional Hubbard model is calculated within an unrestricted spin rotational invariant slave-boson approach. In the first part, we examine in detail the spin structure around two holes doped in the half-filled system where we have studied cluster sizes up to $10\ifmmode\times\else\texttimes\fi{}10.$ It turns out that the most stable configuration corresponds to a vortex-antivortex pair that has lower energy than the N\'eel-type bipolaron even when one takes the far field contribution into account. We also obtain Skyrmions as local minima of the energy functional but with higher total energy than the vortex solutions. Additionally, we have investigated the stability of elliptical domain walls for commensurate hole concentrations. We find that (i) these phases correspond to local minima of the energy functional only in case of partially filled walls, (ii) elliptical domain walls are only stable in the low-doping regime.