V. Dobrosavljevic

Disorder-Driven Non-Fermi-Liquid Behavior in Kondo Alloys

We show how a model of disordered Anderson lattices can account for many non-Fermi-liquid features observed in some Kondo alloys. Because of the exponential nature of the Kondo temperature scale T{sub K}, even moderate disorder leads to a rather broad distribution of Kondo temperatures, inducing strong {ital effective} disorder seen by the conduction electrons. Spins with very low T{sub K}`s remain unquenched and dominate the low-temperature properties.This single underlying mechanism leads to logarithmic divergences in thermodynamic quantities and a linear temperature dependence of the resistivity. {copyright} {ital 1997} {ital The American Physical Society}

SCALING THEORY OF TWO-DIMENSIONAL METAL-INSULATOR TRANSITIONS

We discuss the recently discovered two-dimensional metal-insulator transition in zero magnetic field in the light of the scaling theory of localization. We demonstrate that the observed symmetry relating conductivity and resistivity follows directly from the quantum critical behavior associated with such a transition. In addition, we show that very general scaling considerations imply that any disordered two dimensional metal is a perfect metal, but most likely not a Fermi liquid.

Kondo disorder: a possible route towards non-Fermi-liquid behaviour

We present a general model of disorder in Kondo alloys that, under certain conditions, leads to non-Fermi-liquid behaviour. The central underlying idea is the presence of a distribution of local Kondo temperature scales. If this distribution is broad enough, such that there are sites with arbitrarily low Kondo temperatures, a non-Fermi-liquid phase is formed. We analyse thermodynamics and transport in this approach and show it is consistent with a number of Kondo alloys. We also compare the predictions of this model with the measured dynamical magnetic response of these systems.

Disorder-driven non-Fermi liquid behaviour of correlated electrons

Systematic deviations from standard Fermi-liquid behaviour have been widely observed and documented in several classes of strongly correlated metals. For many of these systems, mounting evidence is emerging that the anomalous behaviour is most likely triggered by the interplay of quenched disorder and strong electronic correlations. In this review, we present a broad overview of such disorder-driven non-Fermi liquid behaviour, and discuss various examples where the anomalies have been studied in detail. We describe both their phenomenological aspects as observed in experiment, and the current theoretical scenarios that attempt to unravel their microscopic origin.

Conductor--insulator quantum phase transitions

PART I: METAL-INSULATOR TRANSITIONS 1. Introduction to Metal-Insulator Transitions 2. Anderson Localisation 3. Visualizing Critical Correlations near the Metal-Insulator Transition in Ga1??xMnxAs 4. Local approaches to strongly correlated disordered systems 5. Penultimate fate of a dirty-Fermi-liquid 6. Glassy dynamics of electrons near the metal-insulator transition 7. Phase Competition and Inhomogeneous States as a New Paradigm for Complex Materials 8. Numerical Studies of Metal-Insulator Transitions in Disordered Hubbard Models PART II: SUPERCONDUCTOR-INSULATOR TRANSITIONS 9. Superconductor-Insulator Transitions: Present Status and Open Questions 10. Scaling Analysis of Direct Superconductor-Insulator Transitions in Disordered Ultrathin Films of Metals 11. Spin Effects Near the Superconductor-Insulator Transition 12. Magnetic field-induced novel insulating phase in 2D superconductors 13. SITs in ultrathin a-Pb films: comparisons of disorder, magnetic field and magnetic impurity tuned 14. Evidence of Cooper Pairs on the Insulating Side of the SIT 15. Spectroscopic Imaging STM Studies of Electronic Structure in both the Superconducting and Pseudogap Phases of Underdoped Cuprates 16. Suppression of Tunneling of Superconducting Vortices Caused by a Remote Gate: Andersons orthogonality catastrophe and localization 17. Theoretical Studies of Superconductor-Insulator TransitionsIn this article we study superconductor-insulator transitions within the general framework of an attractive Hubbard model. This is a well-defined model of s-wave superconductivity which permits different tuning parameters (disorder and field). Furthermore, it allows a comparison of various analytical and computational approaches in order to gain a complete understanding of the various effects of amplitude and phase fluctuations. We present a systematic pedagogical approach, aiming to equip the lay reader with enough apparatus to be able to understand the numerical calculations, reproduce some of the simpler results, and be able to tackle future problems related to inhomogeneous phases. We go into considerable detail on mean-field theory (MFT) and the Bogoliubov-de Gennes (BdG) approach, as these are a first line of attack which can capture much of the physics, but we also outline cases where this fails to capture phase fluctuations and more sophisticated Quantum Monte Carlo (QMC) calculations are necessary. We discuss the behavior of many observables, including densities of states, superfluid stiffness, and dynamical conductivity, for the disorder-tuned superconductor-insulator transition. We also discuss SITs tuned by parallel magnetic field, which are quite different due to pairbreaking.

DESTRUCTION OF THE MOTT INSULATING GROUND STATE OF CA2RUO4 BY A STRUCTURALTRANSITION

We report a first-order phase transition at T_M=357 K in single crystal Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low temperature orthorhombic to a high temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at TM and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T_N=110 K.

The local field distribution in a fluid

The distribution of potentials or fields felt at any given point in a liquid (the local field distribution) ends up being the crucial element in calculating quantities ranging from the inhomogeneous broadening of spectral lines to the rates of irreversible electron transfer. Indeed, the usefulness of this distribution in even its simplest form, the version which assumes a completely uncorrelated environment, has long been appreciated. However, there are a number of difficulties with this version. When the fluid density is low enough to make a neglect of correlations reasonable, the distribution function can still be awkward to calculate numerically. Much more seriously, the omission of correlations among the surrounding atoms is totally unrealistic in a dense liquid. We show here that it is possible to arrive at expressions for the local field distribution that are both accurate under dense liquid conditions and are straightforward to evaluate numerically. The key to this development turns out to be the r...

Typical medium theory of Anderson localization: A local order parameter approach to strong-disorder effects

We present a self-consistent theory of Anderson localization that yields a simple algorithm to obtain the typical local density of states as an order parameter, thereby reproducing the essential features of a phase diagram of localization-delocalization quantum phase transition in the standard lattice models of the disordered electron problem. Due to the local character of our theory, it can easily be combined with dynamical mean-field approaches to strongly correlated electrons, thus opening an attractive avenue for a genuine non-perturbative treatment of the interplay of strong interactions and strong disorder.

Mechanism of hopping transport in disordered mott insulators.

By using a combination of detailed experimental studies and simple theoretical arguments, we identify a novel mechanism characterizing the hopping transport in the Mott insulating phase of Ca2-xSrxRuO4 near the metal-insulator transition. The hopping exponent alpha shows a systematic evolution from a value of alpha=1/2 deeper in the insulator to the conventional Mott value alpha=1/3 closer to the transition. This behavior, which we argue to be a universal feature of disordered Mott systems close to the metal-insulator transition, is shown to reflect the gradual emergence of disorder-induced localized electronic states populating the Mott-Hubbard gap.

Localization-Induced Griffiths Phase of Disordered Anderson Lattices

We demonstrate that local density of states fluctuations in disordered Anderson lattice models universally lead to the emergence of non-Fermi liquid (NFL) behavior. The NFL regime appears at moderate disorder ( W = Wc) and is characterized by power-law anomalies, e.g., C/T approximately 1/T((1-alpha)), where the exponent alpha varies continuously with disorder, as in other Griffiths phases. This Griffiths phase is not associated with the proximity to any magnetic ordering, but reflects the approach to a disorder-driven metal-insulator transition (MIT). Remarkably, the MIT takes place only at much larger disorder W(MIT) approximately 12Wc, resulting in an extraordinarily robust NFL metallic phase.