Article

# Destruction of the Small Fermi Surfaces in Na x CoO 2 by Disorder

Department of Physics, University of California, Davis, Davis, California, United States

Physical Review Letters (Impact Factor: 7.51). 08/2006; 97(1):016404. DOI: 10.1103/PhysRevLett.97.016404 Source: PubMed

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Deepa Kasinathan, Available from:### Click to see the full-text of:

Article: Destruction of the Small Fermi Surfaces in Na x CoO 2 by Disorder

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**ABSTRACT:**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) and an on-site Coulomb repulsion, U, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for 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, which effectively enhances the mass and the scattering amplitude of the quasiparticles. The shape of the DOS is crucial for the occurrence of ferromagnetism as for t > 0 the energy cost of polarizing the system is much smaller than for t < 0. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e., ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on NaxCoO2. The transport and magnetic properties measured in NaxCoO2 are consistent with DMFT predictions of a metal close to the Mott insulator and we discuss the role of Na ordering in driving the system towards the Mott transition. We propose that the Curie-Weiss metal phase observed in NaxCoO2 is a consequence of the crossover from a bad metal with incoherent quasiparticles at temperatures 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.Physical review. B, Condensed matter 06/2006; 73(23). DOI:10.1103/PhysRevB.73.235107 · 3.66 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Dynamical mean field theory combined with finite-temperature exact diagonalization is shown to be a suitable method to study local Coulomb correlations in realistic multi-band materials. By making use of the sparseness of the impurity Hamiltonian, exact eigenstates can be evaluated for significantly larger clusters than in schemes based on full diagonalization. Since finite-size effects are greatly reduced this approach allows the study of three-band systems down to very low temperatures, for strong local Coulomb interactions and full Hund exchange. It is also shown that exact diagonalization yields smooth subband quasi-particle spectra and self-energies at real frequencies. As a first application the correlation induced charge transfer between t2g bands in Na_0.3CoO_2 is investigated. For both Hund and Ising exchange the small eg' Fermi surface hole pockets are found to be slightly enlarged compared to the non-interacting limit, in agreement with previous Quantum Monte Carlo dynamical mean field calculations for Ising exchange, but in conflict with photoemission data. Comment: 9 pages, 7 figuresPhysical Review B 09/2006; 75:045125. DOI:10.1103/PhysRevB.75.045125 · 3.74 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Shubnikov-de Haas oscillations for two well-defined frequencies, corresponding, respectively, to areas of 0.8 and 1.36% of the first Brillouin zone, were observed in single crystals of Na(0.3)CoO2. The existence of Na superstructures in Na0.3CoO2, coupled with this observation, suggests the possibility that the periods are due to the reconstruction of the large Fermi surface around the Gamma point. An alternative interpretation in terms of the long sought-after epsilon'(g) pockets is also considered but found to be incompatible with existing specific heat data.Physical Review Letters 10/2006; 97(12):126401. DOI:10.1103/PhysRevLett.97.126401 · 7.51 Impact Factor