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arXiv:cond-mat/0409491 · Physical Review B

Theory of hybrid state in a metal with a small Fermi surface and strong collective excitations

Fabian H. L. Eßler, Alexei M. Tsvelik

Published May 26, 2005• 23 citations
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Abstract

We develop a theory of a hybrid state, where quasiparticles coexist with strong collective modes, taking as a starting point a model of infinitely many one-dimensional Mott insulators weakly coupled by interchain tunneling. This state exists at an intermediate temperature range and undergoes an antiferromagnetic phase transition at temperatures much smaller than the Mott-Hubbard gap. The most peculiar feature of the hybrid state is that its Fermi surface volume is unrelated to the electron density. We present a self-consistent derivation of the low-energy effective action for our model.

DOI
10.1103/physrevb.71.195116

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We develop a theory of a hybrid state, where quasiparticles coexist with strong collective modes, taking as a starting point a model of infinitely many one-dimensional Mott insulators weakly coupled by interchain tunneling. This state exists at an intermediate temperature range and undergoes an antiferromagnetic phase transition at temperatures much smaller than the Mott-Hubbard gap. The most peculiar feature of the hybrid state is that its Fermi surface volume is unrelated to the electron density. We present a self-consistent derivation of the low-energy effective action for our model.
We consider a model of one-dimensional Mott insulators coupled by a weak interchain tunneling ${t}_{\ensuremath{\perp}}.$ We first determine the single-particle Green's function of a single chain by exact field-theoretical methods and then take the tunneling into account by means of a random-phase approximation. In order to embed this approximation into a well-defined expansion with a small parameter, the Fourier transform ${T}_{\ensuremath{\perp}}(k)$ of the interchain coupling is assumed to have a small support in momentum space such that every integration over transverse wave vector yields a small factor ${\ensuremath{\kappa}}_{0}^{2}\ensuremath{\ll}1.$ When ${T}_{\ensuremath{\perp}}(0)$ exceeds a critical value, a small Fermi surface develops in the form of electron and hole pockets. We demonstrate that Luttinger's theorem holds both in the insulating and in the metallic phases. We find that the quasiparticle residue Z increases very fast through the transition and quickly reaches a value of about 0.4--0.6. The metallic state close to the transition retains many features of the one-dimensional system in the form of strong incoherent continua.
We investigate the Mott transition in weakly coupled one-dimensional (1D) fermionic chains. Using a generalization of dynamical mean field theory, we show that the Mott gap is suppressed at some critical hopping t{ perpendicular}{c2}. The transition from the 1D insulator to a 2D metal proceeds through an intermediate phase where the Fermi surface is broken into electron and hole pockets. The quasiparticle spectral weight is strongly anisotropic along the Fermi surface, both in the intermediate and metallic phases. We argue that such pockets would look like "arcs" in photoemission experiments.
The evolution from an anomalous metallic phase to a Mott insulator within the two-dimensional Hubbard model is investigated by means of the cellular dynamical mean-field theory. We show that approaching the density-driven Mott metal-insulator transition the Fermi surface is strongly renormalized and the quasiparticle description breaks down in a very anisotropic fashion. Regions where the quasiparticles are strongly scattered (hot spots) and regions where the scattering rate is relatively weak (cold spot) form irrespective of whether the parent insulator has antiferromagnetic long-range order, while their location is not universal and is determined by the interplay of the renormalization of the scattering rate and the Fermi surface shape.
The dynamical mean-field theory (DMFT) is employed to study the correlation-driven metal-insulator transition in the semi-infinite Hubbard model at half-filling and zero temperature. We consider the low-index surfaces of the three-dimensional simple-cubic lattice, and systematically vary the model parameters at the very surface, the intralayer and interlayer surface hopping, and the surface Coulomb interaction. Within the DMFT the self-energy functional is assumed to be local. Therewith, the problem is self-consistently mapped onto a set of coupled effective impurity models corresponding to the inequivalent layers parallel to the surface. Assuming that the influence of the high-energy Hubbard bands on the low-energy quasiparticle resonance can be neglected at the critical point, a simplified ``linearized DMFT'' becomes possible. The linearized theory, however, is formally equivalent to the Weiss molecular-field theory for the semi-infinite Ising model. This implies that qualitatively the rich phenomenology of the Landau description of second-order phase transitions at surfaces has a direct analog for the surface Mott transition. Motivated by this formal analogy, we work out the predictions of the linearized DMFT in detail. It is found that under certain circumstances the surface of a Mott insulator can be metallic, while a Mott-insulating surface of a normal metal is not possible. We derive the corresponding phase diagrams, the (mean-field) critical exponents and the critical profiles of the quasiparticle weight. The results are confirmed by a fully numerical evaluation of the DMFT equations using the exact-diagonalization (ED) method. By means of the ED approach, we especially investigate the noncritical parts of the phase diagrams and discuss the U and layer dependence of the quasi-particle weight. For strong modifications of the surface model parameters, the surface low-energy electronic structure dynamically decouples from the bulk.
An unusual metallic phase is argued to develop in the one-dimensional ionic Hubbard model, at half-filling and zero magnetization, at intermediate electron-electron repulsion UUU when second-neighbors hopping is allowed and tuned close to a topological Lifshitz transition (connected with a change of the Fermi surface in the noninteracting system). The metallic state lies between a band insulator phase at low repulsion and a correlated (Mott-like) insulator phase at high repulsion. In approaching the latter, the model supports short-range antiferromagnetic order and spontaneous dimerization of both bond charge and nearest-neighbor antiferromagnetic correlations. A combination of mean-field and effective-field theory (bosonization) provides an analytical understanding of the physical processes underlying the argued phase transitions. The ground and low-energy excited states of finite-length chains are explored by density-matrix renormalization-group (DMRG) calculations, providing numerical evidence for the intermediate gapless phase. Such finite systems are attainable by cold atoms in optical lattices for a wide range of the parameter UUU.
We study a three-dimensional Anderson-Hubbard model under the coexistence of short-range interaction and diagonal disorder within the Hartree-Fock approximation. We show that the density of states at the Fermi energy is suppressed in the metallic phases near the metal-insulator transition as a proximity effect of the soft Hubbard gap in the insulating phases. The transition to the insulator is characterized by a vanishing DOS in contrast to formation of a quasiparticle peak at the Fermi energy obtained by the dynamical mean field theory in pure systems. Furthermore, we show that there exist frozen spin moments in the paramagnetic metal.