Optical and magneto-optical response of a doped Mott insulator

We study the optical, Raman, and ac Hall response of the doped Mott insulator within the dynamical mean-field theory (d = infinity ) for strongly correlated electron systems. The occurrence of the isosbectic point in the optical conductivity is shown to be associated with the frequency dependence of the generalized charge susceptibility. We compute the Raman response, which probes the fluctuations of the "stress tensor," and show that the scattering is characterized by appreciable incoherent contributions. The calculated ac Hall constant and Hall angle also exhibit the isosbectic points. These results are also compared with those obtained for a non-FL metal in d = infinity. The role of low-energy coherence (FL) or incoherence (non-FL) in determining the finite frequency response of strongly correlated metals in d = infinity is discussed in detail. As an application of interest, we compute the dielectric figure-of-merit (DFOM), a quantity that is of potential importance for microwave device applications. We demonstrate explicitly that systems near the filling driven Mott transition might be good candidates in this respect, and discuss the influence of real-life factors on the DFOM.64

Electronic correlations in the iron pnictides

In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron's kinetic energy leading to remarkable experimental manifestations in optical spectroscopy. The high-Tc superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-Tc superconductivity in the iron pnictides puts a spotlight on the relevance of correlation effects in these materials. Here we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. Hallmarks of strong electronic many-body effects reported here are important because the iron pnictides expose a new pathway towards a correlated electron state that does not explicitly involve the Mott transition.Comment: 10 page

A POSSIBLE EXPLANATION OF THE NORMAL-STATE NUCLEAR-MAGNETIC-RESONANCE T(1)(-1) FOR THE CUPRATES

We seek to explain the large T = 0 intercept of the normal-state NMR relaxation rate of Cu nuclei in cuprate superconductors by T = 0 local quantum fluctuations in the S = 1/2 Heisenberg model. Good quantitative agreement is obtained at both low and high T with published experimental results

EXTENDED HUBBARD-MODEL IN 2 DIMENSIONS

The extended Hubbard model with nearest neighbour inter-site correlations has been studied by mapping it onto a spin-Hamiltonian. In the case of a half-filled band the problem is equivalent to that of an anisotropic Heisenberg model in the absence of a field. The phases in the U-Q plane obtained here are in good agreement with those obtained by Monte Carlo simulation. The antiferromagnetic XY-ordered phase of the model is shown to be very similar to an RVB phase. Extending the analogy away from half-filling, a possible dependence of the critical temperature t(c) with the doping concentration-delta in a high-T(c) system is suggested

Universal dielectric response across a continuous metal-insulator transition

A wide range of disordered materials, including disordered correlated systems, show universal dielectric response (UDR), followed by a superlinear power-law increase in their optical responses over exceptionally broad frequency regimes. While extensively used in various contexts over the years, the microscopics underpinning UDR remains controversial. Here, we investigate the optical response of the simplest model of correlated fermions, the Falicov-Kimball model, across the continuous metal-insulator transition (MIT) and analyze the associated quantum criticality in detail using cluster extension of dynamical mean-field theory. Surprisingly, we find that UDR naturally emerges in the quantum critical region associated with the continuous MIT. We tie the emergence of these novel features to a many-body orthogonality catastrophe accompanying the onset of strongly correlated electronic glassy dynamics close to the MIT, providing a microscopic realization of Jonscher's time-honored proposal as well as a rationale for similarities in optical responses between correlated electronic matter and canonical glass formers

Effect of strong correlations and static diagonal disorder in the d = infinity Hubbard model

We investigate the effects of static, diagonal disorder in the d = infinity Hubbard model by treating the dynamical effects of local Hubbard correlations and disorder on an equal footing. This is achieved by a proper combination of the iterated perturbation theory and the coherent-potential approximation. Within the paramagnetic phase, we find that the renormalized Fermi-liquid metal phase of the pure Hubbard model is stable against disorder for small disorder strengths. With increasing disorder, strong resonant scattering effects destroy low-energy Fermi-liquid coherence, leading to an incoherent metallic state off half-filling. Finally, for large enough disorder, a continuous transition to the disordered insulating phase occurs. The nature of the incoherent metallic phase, as well as the effects of the low-energy coherence (incoherence) on optical conductivity and electronic Raman spectra, are considered in detail.6419art. no.19511

Magnetic and structural properties of the iron oxychalcogenides La2 O2Fe2 OM2 (M= S,Se)

We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La2O2Fe2OM2(M=S,Se). Elastic neutron scattering reveals that M=S and Se have similar nuclear structures at room and low temperatures. Our neutron diffraction data reveals that both materials obtain antiferromagnetic ordering at a Néel temperature TN90.1±0.16 K and 107.2±0.06 K for M=Se and S, respectively. The magnetic arrangements for both M=S, Se compounds are obtained through Rietveld refinement. We find the order parameter exponent β to be 0.129±0.006 for M=Se and 0.133±0.007 for M=S. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen replacement, the nature of the magnetic interactions is similar in these materials

Magnetic and structural properties of the iron oxychalcogenides La2 O2Fe2 OM2 (M= S,Se)

We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La2O2Fe2OM2(M=S,Se). Elastic neutron scattering reveals that M=S and Se have similar nuclear structures at room and low temperatures. Our neutron diffraction data reveals that both materials obtain antiferromagnetic ordering at a Néel temperature TN90.1±0.16 K and 107.2±0.06 K for M=Se and S, respectively. The magnetic arrangements for both M=S, Se compounds are obtained through Rietveld refinement. We find the order parameter exponent β to be 0.129±0.006 for M=Se and 0.133±0.007 for M=S. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen replacement, the nature of the magnetic interactions is similar in these materials