Dynamics of Quantum Magnets and Superconductors

Yale University, New Haven CT

Investigators

Abstract

This award supports theoretical research on fundamental questions raised by experiments on the magnetic and charge transport properties of complex oxide compounds. The aim of this work is to elucidate the competition between the many possible ground states of correlated electron systems in low dimensions, the quantum phase transitions between them, and the complex crossovers in their dynamic and transport properties at finite temperature. One focus of this work will be the response of correlated quantum systems to non-magnetic impurities like Zn or Li. Nuclear magnetic resonance, neutron scattering, and tunneling microscopy experiments have shown that such impurities can be sensitive probes of the quantum state of the electrons. The PI plans to explore whether these probes can distinguish among the different models proposed for high temperature superconductors. The PI's recent theories correlate the results of different experimental probes and suggest new experiments and open new theoretical issues that will be investigated. The spectrum of collective spin-singlet and spin-triplet excitations in d-wave superconductors, the manner in which these excitations couple to the fermionic S=1/2 quasiparticle excitations, and their possible interplay with "stripe" correlations will also be studied. The PI also plans to pursue related projects on spin-glass order in strongly disordered transition metal and rare earth compounds, and tunneling between lateral quantum Hall edge states. %%% This award supports theoretical research on fundamental questions raised by experiments on the magnetic and charge transport properties of high temperature superconductors and related transition metal oxide compounds. The proposal involves using impurities like Zn or Li as probes of the quantum state of the electrons in these systems. The response of the correlated electron system as revealed by a variety of experiments is expected to elucidate the fundamental nature of magnetic and superconducting states in these materials. These states are both a puzzle and a motivation for theoretical and experimental work that challenges our fundamental concepts of superconducting, magnetic, and metallic states. A unifying theme of the proposed research involves exploring the role of quantum phase transitions in determining macroscopic properties and the nature of electronic excitations in high temperature superconductors and related transition metal oxides at finite temperature. Quantum phase transitions occur at zero temperature in response to small changes in an external parameter. The proposed work also supports graduate education in state-of-the art methods in condensed matter theory. ***