Ordered states in the disordered Hubbard model

The Hubbard model is studied in which disorder is introduced by putting the on-site interaction to zero on a fraction f of (impurity) sites of a square lattice. Using Quantum Monte Carlo methods and Dynamical Mean Field theory we find that antiferromagnetic long-range order is initially enhanced at half-filling and stabilized off half-filling by the disorder. The Mott-Hubbard charge gap of the pure system is broken up into two pieces by the disorder: one incompressible state remains at average density n=1 and another can be seen slightly below n=1+f. Qualitative explanations are provided.Comment: 17 pages, including 8 figures. Paper for Festschrift in honor of Hans van Leeuwen's 65th birthda

Metal--Insulator Transitions in the Falicov--Kimball Model with Disorder

The ground state phase diagrams of the Falicov--Kimball model with local disorder is derived within the dynamical mean--field theory and using the geometrically averaged (''typical'') local density of states. Correlated metal, Mott insulator and Anderson insulator phases are identified. The metal--insulator transitions are found to be continuous. The interaction and disorder compete with each other stabilizing the metallic phase against occurring one of the insulators. The Mott and Anderson insulators are found to be continuously connected.Comment: 6 pages, 7 figure

Conducting phase in the two-dimensional disordered Hubbard model

We study the temperature-dependent conductivity $\sigma(T)$ and spin susceptibility $\chi(T)$ of the two-dimensional disordered Hubbard model. Calculations of the current-current correlation function using the Determinant Quantum Monte Carlo method show that repulsion between electrons can significantly enhance the conductivity, and at low temperatures change the sign of $d\sigma/dT$ from positive (insulating behavior) to negative (conducting behavior). This result suggests the possibility of a metallic phase, and consequently a metal-insulator transition,in a two-dimensional microscopic model containing both interactions and disorder. The metallic phase is a non-Fermi liquid with local moments as deduced from a Curie-like temperature dependence of $\chi(T)$.Comment: 4 pages; 4 postscript figures; added (1) a new figure showing temperature dependence of spin susceptibility; (2) more references. accepted for publication in Phys. Rev. Let

Disorder-enhanced delocalization and local-moment quenching in a disordered antiferromagnet

The interplay of disorder and spin-fluctuation effects in a disordered antiferromagnet is studied. In the weak-disorder regime (W \le U), while the energy gap decreases rapidly with disorder, the sublattice magnetization, including quantum corrections, is found to remain essentially unchanged in the strong correlation limit. Magnon energies and Neel temperature are enhanced by disorder in this limit. A single paradigm of disorder-enhanced delocalization qualitatively accounts for all these weak disorder effects. Vertex corrections and magnon damping, which appear only at order (W/U)^4, are also studied. With increasing disorder a crossover is found at W \sim U, characterized by a rapid decrease in sublattice magnetization due to quenching of local moments, and formation of spin vacancies. The latter suggests a spin-dilution behavior, which is indeed observed in softened magnon modes, lowering of Neel temperature, and enhanced transverse spin fluctuations.Comment: 12 pages, includes 8 postscript figures. To appear in Physical Review B. References adde

Constrained-path quantum Monte Carlo simulations of the zero-temperature disordered two-dimensional Hubbard model

We study the effects of disorder on long-range antiferromagnetic correlations in the half-filled, two dimensional, repulsive Hubbard model at T=0. A mean field approach is first employed to gain a qualitative picture of the physics and to guide our choice for a trial wave function in a constrained path quantum Monte Carlo (CPQMC) method that allows for a more accurate treatment of correlations. Within the mean field calculation, we observe both Anderson and Mott insulating antiferromagnetic phases. There are transitions to a paramagnet only for relatively weak coupling, U < 2t in the case of bond disorder, and U < 4t in the case of on-site disorder. Using ground-state CPQMC we demonstrate that this mean field approach significantly overestimates magnetic order. For U=4t, we find a critical bond disorder of Vc = (1.6 +- 0.4)t even though within mean field theory no paramagnetic phase is found for this value of the interaction. In the site disordered case, we find a critical disorder of Vc = (5.0 +- 0.5)t at U=4t.Comment: Revtex, 13 pages, 15 figures. Minor changes to title and abstract, discussion and references added, figures 5, 6, 8, 9 replaced with easier to read version

Particle-Hole Symmetry and the Effect of Disorder on the Mott-Hubbard Insulator

Recent experiments have emphasized that our understanding of the interplay of electron correlations and randomness in solids is still incomplete. We address this important issue and demonstrate that particle-hole (ph) symmetry plays a crucial role in determining the effects of disorder on the transport and thermodynamic properties of the half-filled Hubbard Hamiltonian. We show that the low-temperature conductivity decreases with increasing disorder when ph-symmetry is preserved, and shows the opposite behavior, i.e. conductivity increases with increasing disorder, when ph-symmetry is broken. The Mott insulating gap is insensitive to weak disorder when there is ph-symmetry, whereas in its absence the gap diminishes with increasing disorder.Comment: 4 pages, 4 figure

Similarities between the Hubbard and Periodic Anderson Models at Finite Temperatures

The single band Hubbard and the two band Periodic Anderson Hamiltonians have traditionally been applied to rather different physical problems - the Mott transition and itinerant magnetism, and Kondo singlet formation and scattering off localized magnetic states, respectively. In this paper, we compare the magnetic and charge correlations, and spectral functions, of the two systems. We show quantitatively that they exhibit remarkably similar behavior, including a nearly identical topology of the finite temperature phase diagrams at half-filling. We address potential implications of this for theories of the rare earth ``volume collapse'' transition.Comment: 4 pages (RevTeX) including 4 figures in 7 eps files; as to appear in Phys. Rev. Let

Pseudogap effects induced by resonant pair scattering

We demonstrate how resonant pair scattering of correlated electrons above T_c can give rise to pseudogap behavior. This resonance in the scattering T-matrix appears for superconducting interactions of intermediate strength, within the framework of a simple fermionic model. It is associated with a splitting of the single peak in the spectral function into a pair of peaks separated by an energy gap. Our physical picture is contrasted with that derived from other T-matrix schemes, with superconducting fluctuation effects, and with preformed pair (boson-fermion) models. Implications for photoemission and tunneling experiments in the cuprates are discussed.Comment: REVTeX3.0; 4 pages, 4 EPS figures (included

Disorder and Impurities in Hubbard-Antiferromagnets

We study the influence of disorder and randomly distributed impurities on the properties of correlated antiferromagnets. To this end the Hubbard model with (i) random potentials, (ii) random hopping elements, and (iii) randomly distributed values of interaction is treated using quantum Monte Carlo and dynamical mean-field theory. In cases (i) and (iii) weak disorder can lead to an enhancement of antiferromagnetic (AF) order: in case (i) by a disorder-induced delocalization, in case (iii) by binding of free carriers at the impurities. For strong disorder or large impurity concentration antiferromagnetism is eventually destroyed. Random hopping leaves the local moment stable but AF order is suppressed by local singlet formation. Random potentials induce impurity states within the charge gap until it eventually closes. Impurities with weak interaction values shift the Hubbard gap to a density off half-filling. In both cases an antiferromagnetic phase without charge gap is observed.Comment: 16 pages, 9 figures, latex using vieweg.sty (enclosed); typos corrected, references updated; to appear in "Advances in Solid State Physics", Vol. 3

Correlated hopping of electrons: Effect on the Brinkman-Rice transition and the stability of metallic ferromagnetism

We study the Hubbard model with bond-charge interaction (`correlated hopping') in terms of the Gutzwiller wave function. We show how to express the Gutzwiller expectation value of the bond-charge interaction in terms of the correlated momentum-space occupation. This relation is valid in all spatial dimensions. We find that in infinite dimensions, where the Gutzwiller approximation becomes exact, the bond-charge interaction lowers the critical Hubbard interaction for the Brinkman-Rice metal-insulator transition. The bond-charge interaction also favors ferromagnetic transitions, especially if the density of states is not symmetric and has a large spectral weight below the Fermi energy.Comment: 5 pages, 3 figures; minor changes, published versio