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Electronic, magnetic, or structural inhomogeneities ranging in size from nanoscopic to mesoscopic scales seem endemic and are possibly generic to colossal magnetoresistance manganites and other transition metal oxides. They are hence of great current interest and understanding them is of fundamental importance. We show here that an extension, to include long-range Coulomb interactions, of a quantum two-fluid l-b model proposed recently for manganites [Phys. Rev. Lett. 92, 157203 (2004)] leads to an excellent description of such inhomogeneities. In the l-b model two very different kinds of electronic states, one localized and polaronic (l) and the other extended or broad band (b) coexist. For model parameters appropriate to manganites and even within a simple dynamical mean-field theory (DMFT) framework, it describes many of the unusual phenomena seen in manganites, including colossal magnetoresistance (CMR), qualitatively and quantitatively. However, in the absence of long-ranged Coulomb interaction, a system described by such a model would actually phase separate, into macroscopic regions of l and b electrons, respectively. As we show in this paper, in the presence of Coulomb interactions, the macroscopic phase separation gets suppressed and instead nanometer scale regions of polarons interspersed with band electron puddles appear, constituting a kind of quantum Coulomb glass. We characterize the size scales and distribution of the inhomogeneity using computer simulations. For realistic values of the long-range Coulomb interaction parameter V-0, our results for the thresholds for occupancy of the b states are in agreement with, and hence support, the earlier approach mentioned above based on a configuration averaged DMFT treatment which neglects V-0; but the present work has features that cannot be addressed in the DMFT framework. Our work points to an interplay of strong correlations, long-range Coulomb interaction, and dopant ion disorder, all inevitably present in transition metal oxides as the origin of nanoscale inhomogeneities rather than disorder frustrated phase competition as is generally believed. As regards manganites, it argues against explanations for CMR based on disorder frustrated phase separation and for an intrinsic origin of CMR. Based on this, we argue that the observed micrometer (meso) scale inhomogeneities owe their existence to extrinsic causes, e.g., strain due to cracks and defects. We suggest possible experiments to validate our speculation.

A strong-coupling expansion for the Green's functions, self-energies, and correlation functions of the Bose-Hubbard model is developed. We illustrate the general formalism, which includes all possible (normal-phase) inhomogeneous effects in the formalism, such as disorder or a trap potential, as well as effects of thermal excitations. The expansion is then employed to calculate the momentum distribution of the bosons in the Mott phase for an infinite homogeneous periodic system at zero temperature through third order in the hopping. By using scaling theory for the critical behavior at zero momentum and at the critical value of the hopping for the Mott insulator–to–superfluid transition along with a generalization of the random-phase-approximation-like form for the momentum distribution, we are able to extrapolate the series to infinite order and produce very accurate quantitative results for the momentum distribution in a simple functional form for one, two, and three dimensions. The accuracy is better in higher dimensions and is on the order of a few percent relative error everywhere except close to the critical value of the hopping divided by the on-site repulsion. In addition, we find simple phenomenological expressions for the Mott-phase lobes in two and three dimensions which are much more accurate than the truncated strong-coupling expansions and any other analytic approximation we are aware of. The strong-coupling expansions and scaling-theory results are benchmarked against numerically exact quantum Monte Carlo simulations in two and three dimensions and against density-matrix renormalization-group calculations in one dimension. These analytic expressions will be useful for quick comparison of experimental results to theory and in many cases can bypass the need for expensive numerical simulations.

We use first-principles density-functional theory calculations to determine the vibrational properties of ultrathin n(1,2,…,7) -layer graphene films and present a detailed analysis of their zone-center phonons. We demonstrate that a low-frequency (˜112cm‑1) optical phonon with out-of-plane displacements exhibits a particularly large sensitivity to the number of layers, although no discernible change in the interlayer spacing is found as n varies. Frequency shifts of the optical phonons in bilayer graphene are also calculated as a function of its interlayer separation and interpreted in terms of the interplanar interaction.

We study the spectral properties of the charge-density-wave (CDW) phase of the half-filled spinless Falicov-Kimball model within the framework of the dynamical mean field theory. We present detailed results for the spectral function in the CDW phase as a function of temperature and U. We show how the proximity of the non-Fermi liquid phase affects the CDW phase, and show that there is a region in the phase diagram where we get a CDW phase without a gap in the spectral function. This is a radical deviation from the mean-field prediction where the gap is proportional to the order parameter.

We present magnetization (M) and magnetoresistance (MR)data for a series of $Sr_2FeMoO_6$ samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in $Sr_2FeMoO_6$ is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.

We study the T=0 crossover from the BCS superconductivity to Bose-Einstein condensation in the attractive Hubbard Model within dynamical mean field theory (DMFT) in order to examine the validity of Hartree-Fock-Bogoliubov (HFB) mean field theory, usually used to describe this crossover, and to explore the physics beyond it. Quantum fluctuations are incorporated using iterated perturbation theory as the DMFT impurity solver. We find that these fluctuations lead to large quantitative effects in the intermediate coupling regime, leading to a reduction of both the superconducting order parameter and the energy gap relative to the HFB results. A qualitative change is found in the single-electron spectral function, which now shows an incoherent spectral weight for energies larger than three times the gap, in addition to the usual Bogoliubov quasiparticle peaks.

Manganites of the $LA_{1-x}Ca_xMnO_3$ family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn–Teller (JT)coupling with phonons the twofold degenerate $e_g$ states at the Mn sites dynamically reorganize themselves into localised, JT polarons l with exponentially small inter-site hopping, and band-like, nonpolaronic states b, leading to a new 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulator–metal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of the l and b states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.

We recently introduced the dynamical cluster approximation (DCA), a technique that includes short-ranged dynamical correlations in addition to the local dynamics of the dynamical mean-field approximation while preserving causality. The technique is based on an iterative self-consistency scheme on a finite-size periodic cluster. The dynamical mean-field approximation (exact result) is obtained by taking the cluster to a single site (the thermodynamic limit). Here, we provide details of our method, explicitly show that it is causal, systematic, Phi derivable, and that it becomes conserving as the cluster size increases. We demonstrate the DCA by applying it to a quantum Monte Carlo and exact enumeration study of the two-dimensional Falicov-Kimball model. The resulting spectral functions preserve causality, and the spectra and the charge-density-wave transition temperature converge quickly and systematically to the thermodynamic limit as the cluster size increases.

We present the exact solution of a model of interacting fermions in any dimension with a pure repulsive interaction projecting out a given Cooper channel. The solution rests upon the infinite ranged character of the interaction in real space, leading to a functional integral that is dominated by a Gaussian term. The solution produces strong superconducting enhancements and quasi-long-ranged order in a channel that is not present in the Hamiltonian explicitly, but of the form given by arguments from order by projection.

The transition-metal oxide La2NiO4 is studied using a multiband Hubbard model with a Hund coupling J. We obtain its mean-field phase diagram in the U- (correlation strength on Ni sites) Delta (charge-transfer energy) plane. For J=0 we have a continuous transition from a paramagnetic metal (PMM) to a paramagnetic insulator (PMI). On increasing J, the magnetic moment M and the band gap EG change discontinuously at J=Jc(U, Delta ). For J=1, the insulating phase (AFI) is antiferromagnetic, but we have, in addition, both paramagnetic (PMM) and antiferromagnetic (AFM) metallic phases. The insulator-metal (AFI-AFM and AFI-PMM) phase boundaries are first order, but the metal-metal (AFM-PMM) boundary is second order; and it seems to meet the first-order boundary at a critical end point. A reentrant PMM-AFM-PMM sequence is also seen.

The ground-state ‘phase’ diagrams and some low-energy properties of isotropic antiferromagnetic spin- 1/2 and spin-1 chains with a next-nearest-neighbour exchange $J_2$ and an alternation $\delta$ of the nearest-neighbour exchanges have been compared for the first time using the density matrix renormalization group method. In the spin- 1/2 chain, the system is gapless for $\delta=0$ and $J_2 < J_{2c} = 0.241$, and is gapped everywhere else in the $J_2–\delta$ plane. At $J_{2c}$, for small $\delta$, the gap increases as $\delta^\{alpha}$, where $\alpha = 0.667 \pm 0.001. 2J_2 + \delta = 1$ is a disorder line. To the left of this line, the structure factor S(q) peaks at $q_{max}=\pi$ (the N´eel ‘phase’), while to the right, $q_{max}$ decreases from $\pi$ to $\pi/2$ (the spiral ‘phase’) as $J_2$ increases. We also discovera '$\uparrow\uparrow\downarrow\downarrow$ phase’ for large values of both $J_2$ and $\delta$. In the spin-1 case, we find a line running from a gapless point at $(J_2,\delta) = (0,0.25 \pm 0.01)$ up to a ‘gapless’ point at $(0.73 \pm 0.005, 0)$ such that the open-chain ground state is fourfold degenerate below the line and is unique above it. There is a disorder line in this case also and it has the same equation as in the spin- 1/2 case, but the line ends at about $\delta = 0.136$. Similarly to the spin- 1/2 case, to the left of this line, the peak in the structure factor is at $\pi$ (the N´eel ‘phase’), while to the right of the line, it is at less than $\pi$ (the spiral ‘phase’). For $\delta = 1$, the system corresponds to a spin ladder and the system is gapped for all values of the interchain coupling for both spin- 1/2 and spin-1 ladders.

Flux-lattice melting in the mixed phase of anisotropic, layered superconductors is studied in the limit of infinite anisotropy. In this limit, if the external field is applied perpendicular to the layer plane, the problem reduces to that of the liquid-solid transition of an assembly of point vortices restricted to move on the superconducting layers and interacting through an anisotropic pair potential. We generalize approximate theories for correlations in the liquid-state to deal with the layering and study the melting of the vortex system using density-functional theory. Calculations of correlation functions measurable in neutron scattering are presented. The theory predicts a first-order melting transition. It has no adjustible parameters and the results are in good agreement with experiments on $Bi_2Sr_2CaCu_2O_8$.

The $C_{60}$ molecule and its fragments are studied using configuration interaction and variational Monte Carlo techniques, within the Hubbard model. Using benzene as a test case, we compare the results of the approximate calculations with exact calculations. The fragments of $C_{60}$ studied are pyracylene, fluoranthene, and corannulene. The energies, bond orders, spin-spin, and charge-correlation functions of these systems are obtained for various values of the Hubbard parameter, U. The analyses of bond orders and correlation functions of these individual molecules allow us to visualize pyracylene as a naphthalene unit with two ethylenic moieties and fluoranthene as weakly bridged benzene and naphthalene units. Corannulene is the largest fragment of $C_{60}$ that we have studied. The hexagon-hexagon (h-h) bond orders are slightly larger than those of the hexagon-pentagon (h-p) bonds, a feature also found in other fragments. We also find bonds between two coordinated carbon sites to be stronger than bonds involving three coordinated carbon sites. In $C_{60}$, the h-h bonds are stronger than in corannulene and the h-p bonds weaker than in corannulene for all correlation strengths. Introducing bond alternation in the buckyball enhances this difference.

We develop a symmetrized version of the projector quantum Monte Carlo SPQMC method which preserves the symmetries of the system by simultaneously sampling all symmetry-related Ising configurations at each MC step and use it to study the effect of electron correlations on a single $C_{60}$ molecule and its structural motifs, within the Hubbard model. This SPQMC method allows more accurate estimates of correlation functions as seen from calculations on small systems. The method applied to some molecular fragments of $C_{60}$ , viz., pyracylene, fluoranthene, and corannulene, gives a good chemical description of these systems. Analysis of the ground-state bond orders allows us to visualize pyracylene as a naphthalene moiety with weakly bridged ethylenic units, fluoranthene as weakly bridged naphthalene and benzene units, and corannulene as radialene with ethylenic bridges. We study the ground-state properties of a single fullerene molecule, with and without bond alternation. The bond orders in the ground state for the two types of nearest-neighbor bonds are unequal, even for uniform $C_{60}$ . The spin and charge correlations give a consistent picture of the interacting ground state in all these systems. We construct projections of the bond orders and spin-spin correlation functions on the space of irreducible representations of the icosahedral point group. These projections, analogous to the structure factors of translationally invariant systems, give the amplitudes for distortions which transform as the irreducible representations of the point group. The amplitude for the $H_g$ distortion is the largest, while the spin structure has large weights in the $T_{2g}$ and $G_u$ representations.

We use the density-matrix renormalization group to study the one-dimensional bosonic Hubbard model, with and without disorder. We obtain the gaps in all phases, certain correlation functions, and the superfluid density. A finite-size-scaling analysis enables us to obtain an accurate phase diagram and the beta function at the superfluid–Mott insulator transition and the Kosterlitz-Thouless essential singularity in the pure case. We also obtain coupling constants used in effective field theories for this system.

This review presents the current status of our understanding of the phase diagram of the charge stabilised polystyrene colloidal suspensions exhibiting fluid, body centered cubic and face centered cubic phases. Also discussed are the results of the present authors on density functional theory and Monte Carlo simulations of laser induced freezing, showing

Using the density-matrix renormalization-group technique, we study the ground-state phase diagram and other low-energy properties of an isotropic antiferromagnetic spin-1/2 chain with both dimerization and frustration, i.e., an alternation delta of the nearest-neighbor exchanges and a next-nearest-neighbor exchange J(2). For delta = 0, the system is gapless for J(2) < J(2c) and has a gap for J(2) > J(2c) where J(2c) is about 0.241. For J(2) = J(2c) the gap above the ground state grows as delta to the power 0.667 +/- 0.001. In the J(2)-delta plane, there is a disorder line 2J(2) + delta = 1. To the left of this line, the peak in the static structure factor S(q) is at q(max) = pi (Neel phase), while to the right of the line, q(max) decreases from pi to pi/2 as J(2) is increased to large values (spiral phase). For delta = 1, the system is equivalent to two coupled chains as on a ladder and it is gapped for all values of the interchain coupling.

We present detailed mean-field and random-phase-approximation studies of the negative-U, extended Hubbard model with a view to understanding the properties of the doped barium bismuthates. In particular, we obtain the phase diagram, the excitation spectrum, and the optical conductivity in the semiconducting phase of the bismuthates. We show by explicit calculations how this model leads to a natural explanation for the two, well-separated transport and optical gaps observed in the semiconducting phases of the bismuthates. We fix the parameters in our model by fitting these experimentally observed gaps; and with these parameter values we compute other properties of these systems. We also show how metallic screening and disorder can decrease the superconducting Tc dramatically. Our theory leads to an exotic charge-transport mechanism, dominated by charge ±2e bosons (cooperons), in the semiconducting phases of these systems.

We study the effects of lattice contraction on insulator-metal transitions in strongly correlated systems, in the context of the $d =\infty$, iterated perturbation theory approximation for the half-filled Hubbard model. We compute the phase diagram in the pressure-temperature plane for the paramagnetic metal to paramagnetic insulator transition. We present results for conductivity jumps across the transition and for the variation in conductivity with pressure and temperature in either phase. Our results agree qualitatively with experimental data on transition metal oxides.

We obtain metal-insulator phase diagrams at half-filling for the five-band extended Hubbard model of the square-planar CuO2 lattice treated within a Hartree-Fock mean-field approximation, allowing for spiral spin-density waves. We indicate the existence of an insulating phase (covalent insulator) characterized by strong covalency effects, not identified in the earlier Zaanen-Sawatzky-Allen phase diagram. While the insulating phase is always antiferromagnetic, we also obtain an antiferromagnetic metallic phase for a certain range of interaction parameters. Performing a nonperturbative calculation of J(eff), the in-plane antiferromagnetic interaction is presented as a function of the parameters in the model. We also calculate the band gap and magnetic moments at various sites and discuss critically the contrasting interpretation of the electronic structure of high-T(c) materials arising from photoemission and neutron-scattering experiments.

We point out possibilities for exotic physics in barium bismuthates, from a detailed study of the negative-U, extended-Hubbard model proposed for these systems. We emphasize the different consequences of electronic and phononic mechanisms for negative U. We show that, for an electronic mechanism, the semiconducting phases must be unique, with their transport properties dominated by charge ± 2e Cooperon bound states. This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport.

No abstract

We analyze the experimental core-level photoemission and x-ray absorption (XA) spectra pf $NaCuO_2$ in terms of an Anderson-impurity Hamiltonian including the band-structure effects of the oxygen sublattice and show that the XA results unambiguously indicate a negative value of the charge transfer energy \Delta . We also perform mean-field calculations for the electronic structure for the edge-shared one-dimensional $CuO_2$ lattice of $NaCuO_2$ within the multiband Hubbard Hamiltonian and show that the origin of the insulating nature lies in band-structure rather than correlation effects, and that the gap has a $pd-p$ character.

This paper contains a review of the physical properties of the undoped and alkali-doped C60 materials, including their crystal structure, electronic, optical and vibrational properties and the effect of pressure on the crystal and electronic structure. The mechanisms of superconductivity in alkali-doped C60 in terms of phonon mediated electron pairing vis-a-vis electronic interaction effects are discussed.

The structure of the unpinned vortex fluid in layered oxide high-temperature superconductors is investigated numerically using appropriate classical liquid-state theory, for magnetic fields B perpendicular to the layer plane. The direct correlation function so obtained is used as input for a first-principles,parameter-free calculation of the discontinuous fluid-to-Abrikosov-lattice transition boundary $T_f(B)$.This transition boundary is found to have a shape and location similar to that observed in strongly anisotropic materials, e.g., Bi-Sr-Ca-Cu-O.

Irreversible, Pressure induced, quasicrystal-to-crystal transitions are observed for the first time in melt spun alloys at 4.9 GPa for Al 78 Mn22 and 9.3 GPa for Al86 Mn14 by monitoring the electrical resistivities of these alloys as a function of pressure. Electron diffraction and x-ray measurements are used to show that these quasicrystalline phases have icosohedral point group symmetry. The crystalline phases which appear at high pressures are identified as h.c.p. for Al78 Mn22 and orthorhombic for Al86 Mn14.

This corrects the article DOI: 10.1103/PhysRevLett.102.136401.