Overview

Condensed matter theory research contacts the materials research program at ANL through a mix of individual theoretical studies and collaborative studies with the experimental groups. These frequently involve detailed modeling of complex materials and phenomena for which new theoretical methods and concepts are developed as needed.

Superconductivity

Aspects of superconductivity are being investigated theoretically include:

• Collaboration with the Synchrotron Group to determine and understand details of the excitation spectra of high temperature superconductors in the normal and superconducting states.
• Investigation of physical properties of high temperature superconductivity (HTSC) within a model based on the dominant role of extended saddle point singularities in the electron spectrum, large long-range phonon attraction and small short-range Coulomb repulsion (ELU model).
• Further development of the idea of resonant tunneling through localized centers between the CuO2 planes in HTSC as a mechanism of electron transport along the c-axis and suppression of 2D fluctuations.
• Calculation of corrections to low-temperature conductivity due to quantum interference and to interaction of electrons for a quasi-2D metal, as functions of temperature and magnetic field, in order to explain the temperature dependence of resistance in layered cuprates when their superconductivity is suppressed by a strong magnetic field.
• Investigation of charge- and spin-density-wave formation and superconductivity in intercalated Bi-based HTSC and comparison with experiment.

Electronic Structure and Properties

Investigations currently underway include the following:

• Obtaining information from optical-x-ray probes on materials where a localized character enters into the behavior. A primary application would then be the oxide materials but one would also include selected transition, rare earth, and actinide materials as well. The broad applicability of the methods follows from recent experiments by the Synchrotron Radiation Group at the APS. Under-standing these experiments in a way that generalizes to other problems (XANES being high on the list) requires both the generalization of density functional theory and the determination of computationally tractable local orbitals. Both issues are important but the local orbital aspect has been chosen for first attention.
• Calculation of X-ray excitation properties by approximate density functional theory, utilizing local orbitals, as the basis.
• Computation of angle-resolved photoemission (ARPES) intensities in high-Tc cuprates: BSCCO, YBCO 123 and 124, Nd-Ce-Cu-O, ruthenates etc. from first principles.

Magnetism

Current studies of magnetism include:

• Development of understanding of various features of experiments ongoing at ANL in artificial magnetic multilayer materials. The primary interest in these materials is to understand the coupling between the ferromagnetic layers separated by various "non-magnetic" spacer layers. The primary problem is determining the repeat distance at which the coupling between the ferromagnetic layers switches back and forth between ferromagnetic and antiferromagnetic alignment with increasing thickness of the spacer layer. The mechanism is a quantum-well-like effect that involves both bulk-like effects of the spacer and surface-reflection effects at the interface.
• Development of the theory of quantum linear magnetoresistance in layered metals and semimetals
• Construction of a theory of antiferromagnetism in insulating layered copper oxides using the ideas of a spin-Peierls transition and the role of antiferromagnetic fluctuations in the metallic region above TN on the electron spectrum.
• Construction of a theory of quantum fluctuational corrections for resistivity and magnetoresistivity in layered metals

Relationships to Other Projects

This program involves strong collaborations with ANL Materials Science programs on Supercondcutivity and Magnetism (58906), Magnetic Thin Films (58918), Synchrotron Radiation Research (58926), and Neutron and X-Ray Scatteering (58701); and with university programs at the University of Chicago, University of Illinois at Chicago, and Notre Dame.