Sudip Chakravarty

Hidden order in the cuprates

We propose that the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of

Interlayer tunneling and gap anisotropy in high-temperature superconductors

{d}_{{x}^{2}\ensuremath{-}{y}^{2}}

Electronic correlation effects and superconductivity in doped fullerenes.

-type. The transition to this state is rounded by disorder, but in the limit that the disorder is made sufficiently small, the pseudogap crossover should reveal itself to be such a transition. The ordered state breaks time-reversal, translational, and rotational symmetries, but it is invariant under the combination of any two. We discuss these ideas in the context of ten specific experimental properties of the cuprates, and make several predictions, including the existence of an as-yet undetected metal-metal transition under the superconducting dome.

Weak localization: The quasiclassical theory of electrons in a random potential

A quantitative analysis of a recent model of high-temperature superconductors based on an interlayer tunneling mechanism is presented. This model can account well for the observed magnitudes of the high transition temperatures in these materials and implies a gap that does not change sign, can be substantially anisotropic, and has the same symmetry as the crystal. The experimental consequences explored so far are consistent with the observations.

SCALING THEORY OF TWO-DIMENSIONAL METAL-INSULATOR TRANSITIONS

A theory of the electronic properties of doped fullerenes is proposed in which electronic correlation effects within single fullerene molecules play a central role, and qualitative predictions are made which, if verified, would support this hypothesis. Depending on the effective intrafulllerene electron-electron repulsion and the interfullerene hopping amplitudes (which should depend on the dopant species, among other things), the calculations indicate the possibilities of singlet superconductivity and ferromagnetism.

An explanation for a universality of transition temperatures in families of copper oxide superconductors

Abstract The recent subject of weak localization is an important area of research in condensed matter physics which has received a considerable amount of experimental and theoretical attention. In its existing form, the theory is founded entirely on the impurity technique of Greens function. In this paper, we present a theory of weak localization which is put rigorously on a quasiclassical basis thus providing a more intuitive understanding. For example, it becomes apparent that much of the underlying physics is a result of quantum mechanical interference effects in a very elementary sense. It is one of those unique cases where the superposition principle of quantum mechanics leads to observable consequences in the properties of macroscopic systems. In addition, all the important quantitative results obtained so far are recovered in the present quasiclassical approach.

Quantum criticality between topological and band insulators in

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.

(3+1)

A remarkable mystery of the copper oxide high-transition-temperature (Tc) superconductors is the dependence of Tc on the number of CuO2 layers, n, in the unit cell of a crystal. In a given family of these superconductors, Tc rises with the number of layers, reaching a peak at n = 3, and then declines: the result is a bell-shaped curve. Despite the ubiquity of this phenomenon, it is still poorly understood and attention has instead been mainly focused on the properties of a single CuO2 plane. Here we show that the quantum tunnelling of Cooper pairs between the layers simply and naturally explains the experimental results, when combined with the recently quantified charge imbalance of the layers and the latest notion of a competing order nucleated by this charge imbalance that suppresses superconductivity. We calculate the bell-shaped curve and show that, if materials can be engineered so as to minimize the charge imbalance as n increases, Tc can be raised further.

-dimensions

Four-component massive and massless Dirac fermions in the presence of long range Coulomb interaction and chemical potential disorder exhibit striking fermionic quantum criticality. For an odd number of flavors of Dirac fermions, the sign of the Dirac mass distinguishes the topological and the trivial band insulator phases, and the gapless semimetallic phase corresponds to the quantum critical point that separates the two. Up to a critical strength of disorder, the semimetallic phase remains stable, and the universality class of the direct phase transition between two insulating phases is unchanged. Beyond the critical strength of disorder the semimetallic phase undergoes a phase transition into a disorder controlled diffusive metallic phase, and there is no longer a direct phase transition between the two types of insulating phases.

Superconductivity of Doped Fullerenes

We consider the π-electrons on the surface of a single molecule of C60 (fullerene) interacting via an on-site repulsive interaction U. We show that, for a large range of parameters, the energy to add two electrons to the molecule is less than twice the energy to add one electron to each of two molecules. Thus, at scales greater than the molecular diameter, there is a strong effective attraction between electrons of purely electronic origin. This attraction can readily produce superconductivity. The effect appears to be insensitive to the details of the model considered.