Modelling colossal magnetoresistance manganites

I briefly survey here attempts to model the rich and strange behaviour of colossal magnetoresistance manganites, after outlining some of the phenomena observed in them, and describing the three relevant strong local interactions of the e_g electrons (in two different orbital states at each site), namely with Jahn-Teller phonon modes (strength g), with resident t_2g spins (ferromagnetic Hund's rule coupling J_H) and amongst each other (the Mott Hubbard correlation U) . A new two fluid model of nearly localized l polarons and band (b) electrons for low energy behaviour emerges for large g; some of its applications are mentioned here. I describe some results of strong coupling U, J_H calculations in single site DMFT (Dynamical Mean Field Theory), and show that in the wide orbital liquid regime many characteristic manganite phenomena such as an insulating ferromagnetic ground state, thermal insulator metal transition, colossal magnetoresistance (cmr), materials systematics and the observed low effective carrier density can all be understood qualitatively and quantitatively. We also discuss the two 'phase' coexistence frequently found in these systems, and show that electrostatic coulomb interactions mute lb phase separation into nanoscale electronic inhomogeneity with l regions and b puddles. Finally, some problems of current interest as well as general ones arising, eg polarons and the physics of large electron phonon coupling g in the adiabatic regime, are mentioned

Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites

We argue that many properties of the half-doped manganites may be understood in terms of a new two-(eg electron)-fluid description, which is energetically favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a competition between canting of the core spins of Mn promoting mobile carriers and polaronic trapping of carriers by JT defects, in the presence of CE, orbital and charge order. We show that this explains several features of the doping and magnetic field induced insulator-metal transitions, as the particle-hole asymmetry and the smallness of the transition fields.Comment: 4 pages, 4 figure

Instabilities and Insulator-Metal transitions in Half-Doped Manganites induced by Magnetic-Field and Doping

We discuss the phase diagram of the two-orbital model of half-doped manganites by calculating self-consistently the Jahn-Teller (JT) distortion patterns, charge, orbital and magnetic order at zero temperature. We analyse the instabilities of these phases caused by electron or hole doping away from half-doping, or by the application of a magnetic-field. For the CE insulating phase of half-doped manganites, in the intermediate JT coupling regime, we show that there is a competition between canting of spins (which promotes mobile carriers) and polaronic self-trapping of carriers by JT defects. This results in a marked particle-hole asymmetry, with canting winning only on the electron doped side of half-doping. We also show that the CE phase undergoes a first-order transition to a ferromagnetic metallic phase when a magnetic-field is applied, with abrupt changes in the lattice distortion patterns. We discuss the factors that govern the intriguingly small scale of the transition fields. We argue that the ferromagnetic metallic phases involved have two types of charge carriers, localised and band-like, leading to an effective two-fluid model.Comment: 22 pages, 28 figure

The Exotic Barium Bismuthates

We review the remarkable properties, including superconductivity, charge-density-wave ordering, and metal-insulator transitions, of lead- and potassium-doped barium bismuthate. We discuss some of the early theoretical studies of these systems. Our recent theoretical work, on the negative-U\/, extended-Hubbard model for these systems, is also described. Both the large- and intermediate-U\/ regimes of this model are examined, using mean-field and random-phase approximations, particularly with a view to fitting various experimental properties of these bismuthates. On the basis of our studies, we point out possibilities for exotic physics in these systems. We also emphasize the different consequences of electronic and phonon-mediated mechanisms for the negative U.\/ We show that, for an electronic mechanism, the \secin \,\,phases of these bismuthates must be unique, with their transport properties {\it dominated by charge $\pm 2e$ Cooperon bound states}. This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport and comment on the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To appear in Int. J. Mod. Phys. B as a review articl

Multi-wavelength observations of the gamma-ray flaring quasar S4 1030+61 in 2009-2014

We present a study of the parsec-scale multi-frequency properties of the quasar S4 1030+61 during a prolonged radio and gamma-ray activity. Observations were performed within Fermi gamma-ray telescope, OVRO 40-m telescope and MOJAVE VLBA monitoring programs, covering five years from 2009. The data are supplemented by four-epoch VLBA observations at 5, 8, 15, 24, and 43 GHz, which were triggered by the bright gamma-ray flare, registered in the quasar in 2010. The S4 1030+61 jet exhibits an apparent superluminal velocity of (6.4+-0.4)c and does not show ejections of new components in the observed period, while decomposition of the radio light curve reveals nine prominent flares. The measured variability parameters of the source show values typical for Fermi-detected quasars. Combined analysis of radio and gamma-ray emission implies a spatial separation between emitting regions at these bands of about 12 pc and locates the gamma-ray emission within a parsec from the central engine. We detected changes in the value and direction of the linear polarization and the Faraday rotation measure. The value of the intrinsic brightness temperature of the core is above the equipartition state, while its value as a function of distance from the core is well approximated by the power-law. Altogether these results show that the radio flaring activity of the quasar is accompanied by injection of relativistic particles and energy losses at the jet base, while S4 1030+61 has a stable, straight jet well described by standard conical jet theories.Comment: accepted by MNRAS, 16 pages, 14 figures, 8 tables, 5 pages of supplementary materia

Strongly correlated electrons in solids

In this review, phenomena and ideas connected with strongly interacting electrons in condensed matter systems will be outlined. A simple picture which regards electrons as basically a free gas of quantum particles is the basis of our understanding of all electronic behaviour of metals and insulators. In the last few decades, families of systems have been explored in which the interaction energy is much larger than the energy of free motion, or kinetic energy. These include transition metal oxides with unfilled d electron shells, rare earth intermetallics with f electrons, etc. Their behaviour is rich and not well understood in terms of existing paradigms. I point out some experimental features common to these, e.g. that they are ‘bad’ metals with unusually low quantum coherence temperatures. I also briefly give illustrative examples such as cuprates (which are high temperature superconductors), man-ganites (exhibiting colossal magnetoresistance) and heavy fermions (rare earth intermetallics). Some of the ideas, theories and methods available for describing these sysytems are outlined. It is suggested that a new general approach may be needed for this qualitatively different regime of many electron behaviour

Some open problems in the physics of disordered systems

Some problems in the physics of disordered systems are pointed out; most of these arise from experiments