Quantum phase transition and von Neumann entropy of quasiperiodic Hubbard chains

The half-filled Hubbard chains with the Fibonacci and Harper modulating site potentials are studied in a selfconsistent mean-field approximation. A new order parameter is introduced to describe a charge density order. We also calculate the von Neumann entropy of the ground state. The results show that the von Neumann entropy can identify a CDW/SDW （charge density wave/spin density wave） transition for quasiperiodic models.     

Correlations in nanoclusters of narrow-band metals: Numerical simulation of Auger-electron spectra

Calculations of energy and wave functions of ground and low-lying excited states of small clusters of transition metals are carried out using the method of the exact diagonalization of Hamilton matrix of the extended Hubbard model. The binding energy of two holes in the cluster is calculated, the possibility of their effective attraction is shown in the wide range of the interaction parameters. One-particle and two-particle densities of states are analyzed, and the Auger-electron spectra confirming the presence of the bound state are obtained.     

A Rigorous Investigation on the Ground State of the Penson-Kolb Model

By using either numerical calculations or analytical methods, such as the bosonization technique, the ground state of the Penson-Kolb model has been previously studied by several groups. Some physicists argued that, as far as the existence of superconductivity in this model is concerned, it is canonically equivalent to the negative-U Hubbard model.However, others did not agree. In the present paper, we shall investigate this model by an independent and rigorous approach. We show that the ground state of the Penson-Kolb model is nondegenerate and has a nonvanishing overlap with the ground state of the negative-U Hubbard model. Furthermore, we also show that the ground states of both the models have the same good quantum numbers and may have superconducting long-range order at the same momentum q ＝ 0. Our results support the equivalence between these models.     

氮化硼纳米片的电子结构和自旋调控

单层氮化硼纳米材料具有与石墨烯相似的原子排列方式, 但是由于硼原子和氮原子之间的电荷转移, 两种材料的电子特性具有较大的差异. 本文采用Hubbard模型和量子力学第一性原理计算相结合的方法研究了具有氢原子饱和的锯齿型边界的三角形氮化硼纳米片(Nanoflake) 的电子结构, 发现：与相应的石墨烯纳米片不同, 出现在氮化硼纳米片费米能级附近的零能态(zero-energy-states)要么被电子完全占据, 要么是全空的, 表现出自旋简并的特点; 通过对氮化硼纳米片进行电子(或空穴)掺杂可以有效地调控"零能态"上的电子占据, 进而对氮化硼纳米片的自旋进行调控. 这将为氮化硼纳米材料在自旋电子学等领域的应用提供重要的理论依据. 关键词： 氮化硼纳米片 电子结构调控 Hubbard 模型 量子力学第一性原理     

Magnetism and phase separation in the ground state of the Hubbard model

We discuss the ground state magnetic phase diagram of the Hubbard model off half filling within the dynamical mean-field theory. The effective single-impurity Anderson model is solved by Wilson's numerical renormalization group calculations, adapted to symmetry broken phases. We find a phase separated, antiferromagnetic state up to a critical doping for small and intermediate values of U, but could not stabilize a Néel state for large U and finite doping. At very large U, the phase diagram exhibits an island with a ferromagnetic ground state. Spectral properties in the ordered phases are discussed. Received 9 January 2002 Published online 25 June 2002     

Hartree-Fock ground state of the three-dimensional electron gas

In 1962, Overhauser showed that within Hartree-Fock (HF) the electron gas is unstable to a spin-density wave state. Determining the true HF ground state has remained a challenge. Using numerical calculations for finite systems and analytic techniques, we study the unrestricted HF ground state of the three-dimensional electron gas. At high density, we find broken spin symmetry states with a nearly constant charge density. Unlike previously discussed spin wave states, the observed wave vector of the spin-density wave is smaller than 2k(F). The broken-symmetry state originates from pairing instabilities at the Fermi surface, a model for which is proposed.     

Nagaoka ferromagnetism in the two-dimensional infinite-U Hubbard model

We present different numerical calculations based on variational quantum Monte Carlo simulations supporting a ferromagnetic ground state for finite and small hole densities in the two-dimensional infinite-U Hubbard model. Moreover, by studying the energies of different total spin sectors, these calculations strongly suggest that the paramagnetic phase is unstable against a phase with a partial polarization for large hole densities delta approximately 0.40 with evidence for a second-order transition to the paramagnetic large doping phase.     

Ground states of extended Hubbard models in the atomic limit

The possible ground states of an extended Hubbard model in the atomic limit, augmented by an additional nearest neighbour Ising-like interaction and an external magnetic field, are rigorously determined for arbitrary values of the coupling parameters and arbitrary chemical potential. The method used requires only simple convexity arguments and the examination of all possible configurations of small clusters of lattice sites, which may be done by computer. The results are valid for all lattices ofAB type (two interpenetrating sublattices). The type of order found are ferromagnetic, antiferromagnetic, and charge density wave. Perturbation theory suggests that for finite band width there may be a state showing both a charge density wave and ferromagnetic order.     

Spin-charge density waves in the Hubbard model

The ground state of the Hubbard model in a square lattice is examined in the Hartree-Fock mean field approximation at zero temperature. At small finite hole doping, the system has periodically distributed soliton like structures whose modulations are incommensurate. In a self-consistent way, the Fermi energy can always be located in a gap. The incommensurate states have lower energies than the commensurate antiferromagnetic states calculated at the same filling. These soliton structures persist even when a sizeable nearest neighbor repulsive interaction is included.     

Magnetic ground state of coupled edge-sharing CuO2 spin chains

By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO2 spin chains, as found in the (La,Ca,Sr)14Cu24O41 system, for instance. Our data rely on spin-polarized electronic structure calculations including on-site interaction [local density approximation plus the multiorbital mean-field Hubbard model (LDA+U)] and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO2 spin chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.     

Stripes and superconductivity in a one-dimensional self-consistent model

We show that many observable properties of high-temperature superconductors can be obtained in the framework of a one-dimensional self-consistent model with included superconducting correlations. Analytical solutions for spin, charge, and superconductivity order parameters are found. The ground state of the model at low hole doping is a spin-charge solitonic superstructure. Increased doping leads to a transition to the superconducting phase. There is a region of doping where superconductivity, spin density wave, and charged stripe structure coexist. The charge density modulation appears in the vicinity of vortices (kinks in the 1D model) in the superconducting state.     

Slave-boson phase diagram of the two-dimensional extended Hubbard model: Influence of electron-phonon coupling

We present a detailed study of the extended Hubbard-Peierls model on a square lattice using the slave-boson method proposed by Kotliar and Ruckenstein. The emphasis is on the investigation of the ground state phase diagram. To compare the relative stability of several homogenous phases, the effective bosonized action was evaluated by means of a two-sublattice saddlepoint approximation which allows for the symmetry broken states compatible with the underlying bipartite lattice structure. Paying particular attention to the interplay of electron-electron and electron-phonon interaction, we take into account various types of magnetic ordered phases, i.e. para-, ferro-, ferri-, and antiferromagnetic states, as well as charge ordered phases, e.g. a static (, ) Peierls distorted state. Furthermore the approach has been applied to the following special cases: the Hubbard model, the extended Hubbard model, and the Hubbard-Peierls model. A careful numerical solution of the corresponding self-consistency equations enables us to map out the ground-state phase diagrams of the various models at arbitrary band filling over the whole range of interaction strength. In the phase diagram of the Hubbard model we found a large region with ferrimagnetic order away from half-filling. The phase diagram of the halffilled band extended Hubbard model shows a first-order transition from a spin-density-wave to a charge-density-wave state which is displaced from the mean-field lineU=4V towards largerV. At large negativeU andV we obtain a domain with charge separation. The phase compares favorably with earlier quantum Monte-Carlo results. Including the local electron-phonon coupling the charge-density-wave region is considerably enlarged. Away from half-filling the phase diagram becomes more complex: besides the pure magnetic phases we obtain ferri- and paramagnetic states which show additional charge-density order. Aspects of phase separation are discussed. Finally we investigate the variation of the different gap and order parameters along characteristic lines in the parameter space and determine the renormalized quasiparticle bands.     

Magnetic transitions in strong coupling expansions for nearly degenerate states

A strong coupling expansion for a two‐band Hubbard model on two sites with nearly degenerate states is considered. A comparative analysis is performed for different schemes of perturbation theory which are applicable to systems with nearly degenerate states. A fourth order approach which builds on a four‐dimensional low‐energy subspace with nearly degenerate states captures accurately the transition from an antiferromagnetic to a ferromagnetic ground state at large on‐site Coulomb interaction.     

On the coexistence of superconductivity and charge density waves

We study a simple tight-binding (Hubbard) model for electrons interacting via a short range attractive potential. We show that on a cubic lattice, for a half-filled band the ground state may feature both superconductivity and a charge density wave. We examine the response of such a ground state to an external magnetic field and describe the effect of the charge density wave on the Meissner effect.     

Electronic state in the 2D Hubbard model

Electronic state of the 2D Hubbard model near the half-filling is analyzed by use of the composite operator method. Doping and temperature dependence of density of states show similar behaviors obtained in numerical simulation. The weight of the upper and lower Hubbard bands at the half filling are not evenly distributed in the Brillouin zone, keeping roughly the original band distribution. With hole doping the lower Hubbard band spreads in the whole zone.     

Ground state of the half-filled extended Hubbard model beyond the Hartree-Fock approximation

The local approach for the intraatomic correlation is applied to study the ground state phase diagram of the extended Hubbard model with a half-filled band. The long-range orders are not destroyed by the correlation effect in the limit of weak interaction, though the values of the order parameters are reduced from those of the Hartree-Fock approximation, especially for the antiferromagnetic-state. We find that the antiferromagnetic-charge order phase boundary is only slightly shifted towards the charge ordered state, while the phase boundary between the singlet superconducting and the charge ordered phases remains the same as that derived from the Hartree-Fock approximation.     

Correlation effects in quantum spin-Hall insulators: a quantum Monte Carlo study

We consider the Kane-Mele model supplemented by a Hubbard U term. The phase diagram is mapped out using projective auxiliary field quantum Monte Carlo simulations. The quantum spin liquid of the Hubbard model is robust against weak spin-orbit interaction, and is not adiabatically connected to the spin-Hall insulating state. Beyond a critical value of U>U(c) both states are unstable toward magnetic ordering. In the quantum spin-Hall state we study the spin, charge, and single-particle dynamics of the helical Luttinger liquid by retaining the Hubbard interaction only on a ribbon edge. The Hubbard interaction greatly suppresses charge currents along the edge and promotes edge magnetism but leaves the single-particle signatures of the helical liquid intact.     

Magnetic and charge orders in zigzag nanographene ribbons

We theoretically study the electronic states in graphene ribbons which are strongly affected by the edge states, the peculiar non-bonding molecular orbitals localized along the zigzag edges of the ribbons. New kinds of edge localized electronic states with spin and charge polarizations are found in the mean field solutions of the extended Hubbard model with onsite and nearest-neighbor Coulomb repulsions. These novel states appear due to the interplay between the edge states and the Fermi instabilities. We also examine the competition between the charge polarized state and the spin polarized state to draw a phase diagram depending on Coulomb parameters. The results obtained by the mean field calculations with the extended Hubbard model modified to include Coulomb integrals provide useful insights to understand and functionalize the nanoscale materials.     

Origin of ferromagnetism via hole doping in SnO2: First-principles calculation

First-principles calculations are carried out in order to find the ferromagnetism dependence on the number of holes substituted for Sn sites. The results show that strong localization of defect states of the p bands of the oxygen atoms near the dopants favors high-spin states and local moment formation. These states appear to be ferromagnetically coupled with a rather long-range magnetic interaction, resulting in a half-metallic ferromagnetic ground state for the whole systems. Analysis of the total energies indicates that the induced well-confined ferromagnetism in the oxygen p orbitals due to hole doping is quite possible and easily controlled in these systems, which indicate a new way to develop a half-metallic ferromagnet in nonmagnetic d⁰ oxides.     

Variational approach to spin fluctuations in the Hubbard model

We study a one parameter variational wave function to improve the spin density wave ground state of the Hubbard model by inclusion of quantum spin fluctuations. Using a perturbative approach and novel lattice summation techniques we present analytical as well as numerical results for the correlation energies and the staggered magnetizations in one and two dimensions. We find ground state energies which are satisfyingly close to known exact results and are significantly lower than those of existing Gutzwiller and numerical treatments.