Role of oxygen-oxygen hopping in the three-band copper-oxide model: quasiparticle weight, metal insulator and magnetic phase boundaries, gap values and optical conductivity

We investigate the effect of oxygen-oxygen hopping on the three-band copper-oxide model relevant to high-$T_c$ cuprates, finding that the physics is changed only slightly as the oxygen-oxygen hopping is varied. The location of the metal-insulator phase boundary in the plane of interaction strength and charge transfer energy shifts by $\sim 0.5$eV or less along the charge transfer axis, the quasiparticle weight has approximately the same magnitude and doping dependence and the qualitative characteristics of the electron-doped and hole-doped sides of the phase diagram do not change. The results confirm the identification of La$_2$CuO$_4$ as a material with intermediate correlation strength. However, the magnetic phase boundary as well as higher-energy features of the optical spectrum are found to depend on the magnitude of the oxygen-oxygen hopping. We compare our results to previously published one-band and three-band model calculations.Comment: 13.5 pages, 16 figure

On the value of the Curie temperature in doped manganites

We have verified that the variational mean field theory approach suggested by Narimanov and Varma (preprint cond-mat/0002191) being applied to the realistic two-band model provides a good agreement with experimental data for the Curie temperature in doped manganites A$_{1-x}$B$_x$MnO$_3$ ($x\simeq{0.3}$). We have also considered the problem of an interplay between the ferromagnetic and antiferromagnetic interactions by using the same approach.Comment: ReVTeX, 4 pages, 2 figures. To appear in Solid State Com

Effect of Hetrovalent substitution at Mn site on the Magnetic and Transport Properties of La0.67_{0.67}Sr0.33_{0.33}MnO3_3

Magnetic and transport properties of Ti substituted La$_{0.67}$Sr$_{0.33}$MnO$_3$ are drastically affected with a change in preparation conditions. Low temperature infra-red absorption measurements reveal that this is perhaps due to inhomogeniety in substitution of Ti$^{4+}$ on Mn sites. It is found that, in the high temperature annealed samples, the substitution of Ti supresses the double exchange interaction due to the formation of Mn$^{3+}$-O-Ti$^{4+}$ chains. While in the low temperature annealed case substitution of Ti causes formation of isolated ferromagnetic clusters linked to each other by a variable range hopping polaron.Comment: 11 pages, 8 figures, accepted in J. Magn. Magn. Magn. Mate

Antiferromagnetism and the gap of a Mott insulator: Results from analytic continuation of the self-energy

Direct analytic continuation of the self energy is used to determine the effect of antiferromagnetic ordering on the spectral function and optical conductivity of a Mott insulator. Comparison of several methods shows that the most robust estimation of the gap value is obtained by use of the real part of the continued self energy in the quasiparticle equation within the single-site dynamical mean field theory of the two dimensional square lattice Hubbard model, where for U slightly greater than the Mott critical value, antiferromagnetism increases the gap by about 80%.Comment: 8 pages, 9 figures. An error in normalization of optical conductivity (Fig. 9) corrected. to appear in Phys. Rev.

Crystal Structure and Physical Properties of U3T3Sn4 (T = Ni, Cu) Single-Crystals

Heat capacity experiments, crystal structure determination and transmission electron microscopy have been carried out on U3Cu3Sn4 single-crystals. U3Cu3Sn4 was confirmed to be a heavy-fermion antiferromagnet (TN=13(1) K) with a low temperature electronic heat capacity coefficient gamma=390 mJ/molUK2. Low temperature heat capacity experiments on a U3Ni3Sn4 single-crystal indicate that below 0.4 K there is a crossover between the previously observed non-Fermi liquid behavior and a Fermi liquid state.Comment: 12 pages (incl. 2 tables & 4 figures), to appear in Physica

Hall Resistivity in Ferromagnetic Manganese-Oxide Compounds

Temperature-dependence and magnetic field-dependence of the Hall effect and the magnetic property in manganese-oxide thin films are studied. The spontaneous magnetization and the Hall resistivity are obtained for a various of magnetic fields over all the temperature. It is shown that the Hall resistivity in small magnetic field is to exhibit maximum near the Curie point, and strong magnetic field moves the position of the Hall resistivity peak to much high temperature and suppresses the peak value. The change of the Hall resistance in strong magnetic field may be larger than that of the diagonal ones. The abnormal Hall resistivity in the ferromagnetic manganese-oxide thin-films is attributed to the spin-correlation fluctuation scattering.Comment: Latex, 2 Figures, 10 Page

A continuous-time solver for quantum impurity models

We present a new continuous time solver for quantum impurity models such as those relevant to dynamical mean field theory. It is based on a stochastic sampling of a perturbation expansion in the impurity-bath hybridization parameter. Comparisons to quantum Monte Carlo and exact diagonalization calculations confirm the accuracy of the new approach, which allows very efficient simulations even at low temperatures and for strong interactions. As examples of the power of the method we present results for the temperature dependence of the kinetic energy and the free energy, enabling an accurate location of the temperature-driven metal-insulator transition.Comment: Published versio

Dynamic conductivity of semiconducting manganites approaching the metal-insulator transition

We report the frequency-dependent conductivity of the manganite system La1-xSrxMnO3 (x <= 0.2) when approaching the metal-insulator transition from the insulating side. Results from low-frequency dielectric measurements are combined with spectra in the infrared region. For low doping levels the behavior is dominated by hopping transport of localized charge carriers at low frequencies and by phononic and electronic excitations in the infrared region. For the higher Sr contents the approach of the metallic state is accompanied by the successive suppression of the hopping contribution at low frequencies and by the development of polaronic excitations in the infrared region, which finally become superimposed by a strong Drude contribution in the fully metallic state.Comment: 10 pages, 7 figure

Optical spectral weights and the ferromagnetic transition temperature of CMR manganites: relevance of double-exchange to real materials

We present a thorough and quantitative comparison of double-exchange models to experimental data on the colossal magnetoresistance manganese perovskites. Our results settle a controversy by showing that physics beyond double-exchange is important even in La$_{0.7}$Sr$_{0.3}$MnO$_3$, which has been regarded as a conventional double-exchange system. We show that the crucial quantity for comparisons of different calculations to each other and to data is the conduction band kinetic energy $K$, which is insensitive to the details of the band structure and can be experimentally determined from optical conductivity measurements. The seemingly complicated dependence of $T_c$ on the Hund's coupling $J$ and carrier concentration $n$ is shown to reflect the variation of $K$ with $J$, $n$ and temperature. We present results for the optical conductivity which allow interpretation of experiments and show that a feature previously interpreted in terms of the Hund's coupling was misidentified. We also correct minor errors in the phase diagram presented in previous work.Comment: 13 pages, 7 eps figure

Orbital dynamics: The origin of the anomalous optical spectra in ferromagnetic manganites

We discuss the role of orbital degeneracy in the transport properties of perovskite manganites, focusing in particular on the optical conductivity in the metallic ferromagnetic phase at low temperatures. Orbital degeneracy and strong correlations are described by an orbital t-J model which we treat in a slave-boson approach. Employing the memory-function formalism we calculate the optical conductivity, which is found to exhibit a broad incoherent component extending up to bare bandwidth accompanied by a strong suppression of the Drude weight. Further, we calculate the constant of T-linear specific heat. Our results are in overall agreement with experiment and suggest low-energy orbital fluctuations as the origin of the strongly correlated nature of the metallic phase of manganites.Comment: To appear in: Phys. Rev. B 58 (Rapid Communications), 1 November 199