Anti-Localisation to Strong Localisation: The Interplay of Magnetic Scattering and Structural Disorder

We study the effect of magnetic scattering on transport in a system with strong structural disorder, using exact finite size calculation of the low frequency optical conductivity. At weak electron-spin coupling spin disorder leads to a decrease in resistivity by weakening the quantum interference precursors to Anderson localisation. However, at strong electron-spin coupling, the double exchange limit, magnetic scattering increases the effective disorder, sharply increasing the resistivity. We illustrate the several unusual transport regimes in this strong disorder problem, identify a re-entrant insulator-metal-insulator transition, and map out the phase diagram at a generic electron density.Comment: 7 pages, EPL style, with 4 embedded figs. To appear in Europhys. Let

Insulator-Metal Phase Diagram of the Optimally Doped Manganites from the Disordered Holstein-Double Exchange Model

We study the Holstein-Double Exchange model in three dimensions in the presence of substitutional disorder. Using a new Monte Carlo technique we establish the phase diagram of the clean model and then focus on the effect of varying electron-phonon coupling and disorder at fixed electron density. We demonstrate how extrinsic disorder controls the interplay of lattice polaron effects and spin fluctuations and leads to widely varying regimes in transport. Our results on the disorder dependence of the ferromagnetic T_c and metal-insulator transitions bear direct comparison to data on the `optimally doped', x=0.3-0.4, manganites. We highlight disorder induced polaron formation as a key effect in these materials, organise a wide variety of data into a simple `global phase diagram', and make several experimental predictions.Comment: Published versio

Anomalous pseudogap in population imbalanced Fermi superfluids

In a Fermi superfluid increasing population imbalance leads initially to reduction of the transition temperature, then the appearance of modulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, and finally the suppression of pairing itself. For interaction strength such that the `balanced' system has a normal state pseudogap, increasing imbalance reveals anomalous spectral behavior. At a fixed weak imbalance (small polarization) the stable homogeneous superfluid occurs only above a certain temperature. The density of states has a minimum at the Fermi level, then a weak peak {\it within the gap}, and then the large, gap edge, coherence features. On heating, this non monotonic energy dependence changes to a more conventional fluctuation driven pseudogap, with a monotonic energy dependence. At large imbalance the ground state is FFLO and `pseudogapped' due to the modulated order. It changes to a gapless normal state on heating, and then shows a pseudogap again at a higher temperature. These weak imbalance and strong imbalance features both involve effects well beyond mean field theory. We establish them by using a Monte Carlo technique on large lattices, motivate the results in terms of the pairing field distribution, and compare them to spectroscopic results in the imbalanced unitary Fermi gas.Comment: 7 pages, 5 figures. This version supercedes 1508.0039

Anomalous pseudogap in population imbalanced Fermi superfluids

In a Fermi superfluid increasing population imbalance leads initially to reduction of the transition temperature, then the appearance of modulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states, and finally the suppression of pairing itself. For interaction strength such that the `balanced' system has a normal state pseudogap, increasing imbalance reveals anomalous spectral behavior. At a fixed weak imbalance (small polarization) the stable homogeneous superfluid occurs only above a certain temperature. The density of states has a minimum at the Fermi level, then a weak peak {\it within the gap}, and then the large, gap edge, coherence features. On heating, this non monotonic energy dependence changes to a more conventional fluctuation driven pseudogap, with a monotonic energy dependence. At large imbalance the ground state is FFLO and `pseudogapped' due to the modulated order. It changes to a gapless normal state on heating, and then shows a pseudogap again at a higher temperature. These weak imbalance and strong imbalance features both involve effects well beyond mean field theory. We establish them by using a Monte Carlo technique on large lattices, motivate the results in terms of the pairing field distribution, and compare them to spectroscopic results in the imbalanced unitary Fermi gas.Comment: 7 pages, 5 figures. This version supercedes 1508.0039