Slave-rotor mean field theories of strongly correlated systems and the Mott transition in finite dimensions

The multiorbital Hubbard model is expressed in terms of quantum phase variables (``slave rotors'') conjugate to the local charge, and of auxiliary fermions, providing an economical representation of the Hilbert space of strongly correlated systems. When the phase variables are treated in a local mean-field manner, similar results to the dynamical mean-field theory are obtained, namely a Brinkman-Rice transition at commensurate fillings together with a ``preformed'' Mott gap in the single-particle density of states. The slave- rotor formalism allows to go beyond the local description and take into account spatial correlations, following an analogy to the superfluid-insulator transition of bosonic systems. We find that the divergence of the effective mass at the metal- insulator transition is suppressed by short range magnetic correlations in finite-dimensional systems. Furthermore, the strict separation of energy scales between the Fermi- liquid coherence scale and the Mott gap found in the local picture, holds only approximately in finite dimensions, due to the existence of low-energy collective modes related to zero-sound.Comment: 16 pages, 12 figure

Affluence and Poverty in Old Age: New Evidence from the European Community Household Panel

The relative income positions and welfare of elderly persons in the EU are analysed using data from the European Community Household Panel. The elderly invariably receive lower average incomes than the non-elderly, but there are large variations across countries. Pensions represent the main source of income of the elderly, with labour and capital incomes contributing only marginally to total income. High replacement rates for old age pensions are not sufficient to guarantee protection against poverty in old age, as they fail to provide adequate support to vulnerable groups e.g., individuals with incomplete employment histories, or lone pensioners. A much stronger association exists between welfare or satisfaction in old age and income inequality than between welfare or satisfaction and the generosity of old age pensions. It is thus the inequality reduction potential of old age pensions, rather than their generosity, that appears to be the key element in determining the ability of a pension scheme to deliver an adequate standard of living to the elderly. Pension reforms that intend to separate the income redistribution and income insurance functions of old age pensions are likely to strengthen intra-generational income redistribution, and diversify the sources from which the elderly obtain their incomes. The multi-pillar scheme is thus likely to eliminate poverty in old age more effectively than current income financing.income distribution ; international comparisons ; pensions ; poverty

Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory

We examine whether the Mott transition of a half-filled, two-orbital Hubbard model with unequal bandwidths occurs simultaneously for both bands or whether it is a two-stage process in which the orbital with narrower bandwith localizes first (giving rise to an intermediate `orbital-selective' Mott phase). This question is addressed using both dynamical mean-field theory, and a representation of fermion operators in terms of slave quantum spins, followed by a mean-field approximation (similar in spirit to a Gutzwiller approximation). In the latter approach, the Mott transition is found to be orbital-selective for all values of the Coulomb exchange (Hund) coupling J when the bandwidth ratio is small, and only beyond a critical value of J when the bandwidth ratio is larger. Dynamical mean-field theory partially confirms these findings, but the intermediate phase at J=0 is found to differ from a conventional Mott insulator, with spectral weight extending down to arbitrary low energy. Finally, the orbital-selective Mott phase is found, at zero-temperature, to be unstable with respect to an inter-orbital hybridization, and replaced by a state with a large effective mass (and a low quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been extende

Coexistence of solutions in dynamical mean-field theory of the Mott transition

In this paper, I discuss the finite-temperature metal-insulator transition of the paramagnetic Hubbard model within dynamical mean-field theory. I show that coexisting solutions, the hallmark of such a transition, can be obtained in a consistent way both from Quantum Monte Carlo (QMC) simulations and from the Exact Diagonalization method. I pay special attention to discretization errors within QMC. These errors explain why it is difficult to obtain the solutions by QMC close to the boundaries of the coexistence region.Comment: 3 pages, 2 figures, RevTe

A brief review of recent advances on the Mott transition: unconventional transport, spectral weight transfers, and critical behaviour

Strongly correlated metals close to the Mott transition display unusual transport regimes, together with large spectral weight transfers in optics and photoemission. We briefly review the theoretical understanding of these effects, based on the dynamical mean-field theory, and emphasize the key role played by the two energy scales associated with quasiparticle coherence scale and with the Mott gap. Recent experimental results on two-dimensional organic compounds and transition metal oxides are considered in this perspective. The liquid-gas critical behaviour at the Mott critical endpoint is also discussed. Transport calculations using the numerical renormalization group are presented.Comment: Review article. 9 pages, 5 figures. Proceedings of the Vth International Conference on Crystalline Organic Metals, Superconductors and Magnets (ISCOM 2003

Is the Mott transition relevant to f-electron metals ?

We study how a finite hybridization between a narrow correlated band and a wide conduction band affects the Mott transition. At zero temperature, the hybridization is found to be a relevant perturbation, so that the Mott transition is suppressed by Kondo screening. In contrast, a first-order transition remains at finite temperature, separating a local moment phase and a Kondo- screened phase. The first-order transition line terminates in two critical endpoints. Implications for experiments on f-electron materials such as the Cerium alloy Ce$_{0.8}$La$_{0.1}$Th$_{0.1}$ are discussed.Comment: 5 pages, 3 figure

Work, authority and participation : the scenario of circular organizing

Two antagonistic tendencies in the way work is organized can be observed in Europe and the North-American continent. First, there is the growing tendency toward more efficient and flexible modes of organizing by way of a clear (re)distribution of authority. At the other hand, the importance of participation and cooperation in the workplace is growing, also in view of ideas about empowerment, organizational democracy, and self-management. The purpose of this paper is to explore the possibility of organizing work in a circular manner, based on a synthesis of traditional and participative work relationships. Circularity implies that an ultimate authority in the workplace is absent and each member of the organization can participate directly or through representation in decision-making. Design precepts for organizing in a circular manner are discussed, and some implications for the role of authority and participation in the (postmodern) workplace are explored.labour economics ;