Jaime Merino

Theoretical aspects of charge ordering in molecular conductors

This is the peer reviewed version of the following article: Journal of the Physical Society of Japan 75 (2006): 051009, which has been published in final form at http://dx.doi.org/10.1143/JPSJ.75.051009

Spectral broadening due to long-range Coulomb interactions in the molecular metal TTF-TCNQ

Laura Cano-Cortés, Andreas Dolfen, Jaime Merino, Jörg Behler, Bernard Delley, Karsten Reuter, and Erik Koch ∗ Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain Institut für Festkörperforschung, Forschungzentrum Jülich, 52425 Jülich, Germany Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Paul-Scherrer-Institut, HGA/123, CH-5232 Villigen PSI, Switzerland (Dated: February 6, 2008)

Quasiparticles at the Verge of Localization near the Mott Metal-Insulator Transition in a Two-Dimensional Material

The dynamics of charge carriers close to the Mott transition is explored theoretically and experimentally in the quasi-two-dimensional organic charge-transfer salt, kappa-(BEDT-TTF)_(2)Cu[N(CN)_(2)]Br_(x)Cl_(1-x), with varying Br content. The frequency dependence of the conductivity deviates significantly from simple Drude model behavior: there is a strong redistribution of spectral weight as the Mott transition is approached and with temperature. The effective mass of the quasiparticles increases considerably when coming close to the insulating phase. A dynamical mean-field-theory treatment of the relevant Hubbard model gives good quantitative description of experimental data.

Fluctuation Diagnostics of the Electron Self-Energy: Origin of the Pseudogap Physics

We demonstrate how to identify which physical processes dominate the low-energy spectral functions of correlated electron systems. We obtain an unambiguous classification through an analysis of the equation of motion for the electron self-energy in its charge, spin, and particle-particle representations. Our procedure is then employed to clarify the controversial physics responsible for the appearance of the pseudogap in correlated systems. We illustrate our method by examining the attractive and repulsive Hubbard model in two dimensions. In the latter, spin fluctuations are identified as the origin of the pseudogap, and we also explain why d-wave pairing fluctuations play a marginal role in suppressing the low-energy spectral weight, independent of their actual strength.

Non-Fermi Liquid Behavior in Nearly Charge Ordered Layered Metals

Non-Fermi liquid behavior is shown to occur in two-dimensional metals which are close to a charge ordering transition driven by the Coulomb repulsion. A linear temperature dependence of the scattering rate together with an increase of the electron effective mass occur above T*, a temperature scale much smaller than the Fermi temperature. It is shown that the anomalous temperature dependence of the optical conductivity of the quasi-two-dimensional organic metal alpha-(BEDT-TTF)2MHg(SCN)4, with M = NH4 and Rb, above T* = 50-100K, agrees qualitatively with predictions for the electronic properties of nearly charge ordered two-dimensional metals.

Quantum critical behavior of electrons at the edge of charge order.

We consider quantum critical points in which quantum fluctuations associated with charge rather than magnetic order induce unconventional metallic properties. Based on finite-T calculations on a two-dimensional extended Hubbard model, we show how the coherence scale T(*) characteristic of Fermi liquid behavior of the homogeneous metal vanishes at the onset of charge order. A strong effective mass enhancement reminiscent of heavy fermion behavior indicates the possible destruction of quasiparticles at the quantum critical points. Experimental probes on quarter-filled layered organic materials are proposed for unveiling the behavior of electrons across the quantum critical region.

Charge Fluctuations in Geometrically Frustrated Charge Ordering System

Effects of geometrical frustration in low-dimensional charge ordering systems are theoretically studied, mainly focusing on dynamical properties. We treat extended Hubbard models at quarter-filling...

Ferromagnetism, paramagnetism, and a Curie-Weiss metal in an electron-doped Hubbard model on a triangular lattice

Motivated by the unconventional properties and rich phase diagram of NaxCoO2 we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative intersite hopping amplitudes (t 0 a large enhancement of the effective mass, itinerant ferromagnetism, and a metallic phase with a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. The different behavior encountered is a consequence of the larger noninteracting density of states (DOS) at the Fermi level for t > 0 than for t 0 the energy cost of polarizing the system is much smaller than for t T-* and Fermi liquid behavior with enhanced parameters below T-*, where T-* is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis also shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of NaxCoO2 and is used to identify the simplest relevant model that captures the essential physics in NaxCoO2. We propose a model which allows for the Na ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.

Parquet decomposition calculations of the electronic self-energy

Photoemission spectroscopy is one of the most direct routes to investigate electronic systems. However, if electron correlations are strong, the identification of the predominant scattering processes through the analysis of the photoelectron spectra becomes a formidable challenge. The authors introduce here a decomposition of the electronic self-energy (see image), based on the so-called parquet equations, which, as suggested by their name, take into account all scattering channels and their interconnections. The application of this new parquet decomposition to self-energies computed in the case of local Coulomb interaction in two-dimensions (2D Hubbard model) yields several new results. Future investigations based on parquet decompositions and/or the fluctuation diagnostics of the self-energy could shed further light on the microscopic processes controlling the spectral functions in correlated electron models.

Nonlocal coulomb correlations in metals close to a charge order insulator transition

The charge ordering transition induced by the nearest-neighbor Coulomb repulsion V in the 1/4-filled extended Hubbard model is investigated using cellular dynamical mean-field theory. We find a transition to a strongly renormalized charge ordered Fermi liquid at V(CO) and a metal-to-insulator transition at V(MI)>V(CO). Short range antiferromagnetism occurs concomitantly with the CO transition. Approaching the charge ordered insulator, V approximately <V(MI), the Fermi surface deforms and the scattering rate of electrons develops momentum dependence on the Fermi surface.