Self-doping instability of the Wigner-Mott insulator

We present a theory describing the mechanism for the two-dimensional (2D) metal-insulator transition (MIT) in absence of disorder. A two-band Hubbard model is introduced, describing vacancy-interstitial pair excitations within the Wigner crystal. Kinetic energy gained by delocalizing such excitations is found to lead to an instability of the insulator to self-doping above a critical carrier concentration $n=n_c$, mapping the problem to a density-driven Mott MIT. This mechanism provides a natural microscopic picture of several puzzling experimental features, including the large effective mass enhancement, the large resistivity drop, and the large positive magneto-resistance on the metallic side of the transition. We also present a global phase diagram for the clean 2D electron gas as a function of $n$ and parallel magnetic field $B_{\shortparallel}$, which agrees well with experimental findings in ultra clean samples.Comment: 5 pages, 2 figure

Non-Fermi Liquid Behavior and Double-Exchange Physics in Orbital-Selective Mott Systems

We study a multi-band Hubbard model in its orbital selective Mott phase, in which localized electrons in a narrow band coexist with itinerant electrons in a wide band. The low-energy physics of this phase is shown to be closely related to that of a generalized double-exchange model. The high-temperature disordered phase thus differs from a Fermi liquid, and displays a finite scattering rate of the conduction electrons at the Fermi level, which depends continuously on the spin anisotropy.Comment: 5 pages, minor typos correcte

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

Theory of Core-Level Photoemission and the X-ray Edge Singularity Across the Mott Transition

The zero temperature core-level photoemission spectrum is studied across the metal to Mott insulator transition using dynamical mean-field theory and Wilson's numerical renormalization group. An asymmetric power-law divergence is obtained in the metallic phase with an exponent alpha(U,Q)-1 which depends on the strength of both the Hubbard interaction U and the core-hole potential Q. For Q <~ U_c/2 alpha decreases with increasing U and vanishes at the transition (U -> U_c) leading to a symmetric peak in the insulating phase. For Q >~ U_c/2, alpha remains finite close to the transition, but the integrated intensity of the power-law vanishes and there is no associated peak in the insulator. The weight and position of the remaining peaks in the spectra can be understood within a molecular orbital approach.Comment: 5 pages, 6 figure

Investment Dynamics: Good News Principle

We study a dynamic Cournot game with capacity accumulation under demand uncertainty, in which the investment is perfectly divisible, irreversible, and productive with a lag. We characterize equilibrium investments under closed-loop and S-adapted open-loop information structures. Contrary to what is established usually in the dynamic games literature with deterministic demand, we find that the firms may invest at a higher level in the open-loop equilibrium (which in some cases coincides with Markov perfect equilibrium) than in the closed-loop Nash equilibrium. The rankings of the investment levels obtained in the two equilibria actually depend on the initial capacities and on the degree of asymmetry between the firms. We also observe, contrary to the bad news principle of investment, that firms may invest more as demand volatility increases and they invest as if high demand (i.e., good news) will unfold in the future.Capacity Investment, Dynamic Games, S-adapted Open-Loop Equilibrium, Closed-loop Equilibrium.

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

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

Heavy-fermion and spin-liquid behavior in a Kondo lattice with magnetic frustration

We study the competition between the Kondo effect and frustrating exchange interactions in a Kondo-lattice model within a large-${\cal N}$ dynamical mean-field theory. We find a T=0 phase transition between a heavy Fermi-liquid and a spin-liquid for a critical value of the exchange $J_c = T_{K}^0$, the single-impurity Kondo temperature. Close to the critical point, the Fermi liquid coherence scale $T^\star$ is strongly reduced and the effective mass strongly enhanced. The regime $T>T^\star$ is characterized by spin-liquid magnetic correlations and non-Fermi-liquid properties. It is suggested that magnetic frustration is a general mechanism which is essential to explain the large effective mass of some metallic compounds such as LiV$_2$O$_4$.Comment: 7 pages, 1 figure. Late

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