T. V. Ramakrishnan

Superfluid and Insulating Phases in an Interacting-Boson Model: Mean-Field Theory and the RPA

The bosonic Hubbard model is studied via a simple mean-field theory. At zero temperature, in addition to yielding a phase diagram that is qualitatively correct, namely a superfluid phase for non-integer fillings and a Mott transition from a superfluid to an insulating phase for integer fillings, this theory gives results that are in good agreement with Monte Carlo simulations. In particular, the superfluid fraction obtained as a function of the interaction strength U for both integer and non-integer fillings is close to the simulation results. In all phases the excitation spectra are obtained by using the random phase approximation (RPA): the spectrum has a gap in the insulating phase and is gapless (and linear at small wave vectors) in the superfluid phase. Analytic results are presented in the limits of large U and small superfluid density. Finite-temperature phase diagrams and the Mott-insulator-normal-phase crossover are also described.

Absorption of electromagnetic radiation by superconducting YBa2Cu3O7: an oxygen-induced phenomenon

In the superconducting state, YBa2Cu3O7 absorbs electromagnetic radiation over a wide range of frequencies (8 MHz-9 GHz). The absorption is extremely sensitive to temperature, particle size and the magnetic field and depends crucially on the presence of oxygen. A possible explanation for the phenomenon based on the formation of Josephson junctions is suggested.

Theory of insulator metal transition and colossal magnetoresistance in doped manganites.

The persistent proximity of insulating and metallic phases, a puzzling characteristic of manganites, is argued to arise from the self-organization of the twofold degenerate e(g) orbitals of Mn into localized Jahn-Teller (JT) polaronic levels and broad band states due to the large electron-JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence.

Theory of the liquid-solid transition

The liquid-solid transition is found by calculating the thermodynamic potential of a dense classical system as a function of order parameters which are proportional to the lattice Fourier components of the density. Properties of the fluid enter only through the direct two-particle correlation function near freezing. Calculated parameters for freezing into bcc, fcc and hexagonal structures are in good agreement with experimental results.

Scaling theory of localization and non-ohmic effects in two dimensions

A scaling argument for the conductance G of a disordered electronic system permits interpolation for the behavior of G between the localized and extended limits. For dimensionality d>2, there is a mobility edge at which the conductivity goes continuously to zero. At d=2, there is no true metallic conduction; the conductivity goes smoothly from logarithmic to exponential decrease with sample size L. A perturbation calculation confirms the ln L behavior for weak disorder. At finite temperature T and electric field E, effective length scales depending upon T and E are derived on the basis of relaxation and heating models for purposes of comparison with experiments on thin films. These show non-ohmic ln T and ln E contributions to the conductivity.

Systematics in the thermopower behaviour of several series of bismuth and thallium cuprate superconductors: an interpretation of the temperature variation and the sign of the thermopower

The thermopower,

Effective actions and phase fluctuations in d -wave superconductors

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Non-ohmic effects of anderson localization

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Tracking Operator State Fluctuations in Gene Expression in Single Cells

Bi_2Ca_{1-x}Y_xSr_2Cu_2O_{8+\delta}

Coulomb Interactions and Nanoscale Electronic Inhomogeneities in Manganites

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