Non-Fermi Liquid Behavior, Superconductivity, and Magnetism in Strongly Correlated Electron Systems.

TECHNICAL SUMMARY

This award supports theoretical research and education on strongly correlated electron systems. The research is focused on three interrelated areas: 1.) the physics of cuprate superconductors, 2.) the physics of recently discovered iron-based pnictide superconductors, and 3.) the novel physics of an itinerant fermionic system near a quantum-critical point.

The PI will explore the idea that the physics of the cuprates is primarily driven by proximity to an antiferromagnetic quantum critical point, a spin-fluctuation scenario. At a quantum critical point, collective spin excitations become gapless, and the interaction with them destroys coherent Fermi liquid quasiparticles. The same interaction mediates non-BCS d-wave pairing between incoherent fermions. The PI plans to address (i) whether the interaction with spin fluctuations gives rise to the pseudogap physics and (ii) whether the approach can be extended to a truly strong coupling.

The PI aims to develop a fundamental understanding of the pnictide superconductors, including: the origin of superconductivity which develops despite strong Coulomb repulsion, the symmetry of the pairing gap, the interplay between superconductivity and magnetism, the implications of the gap symmetry for the experiments in the superconducting state, and the feedback from the pairing on electrons.

The PI plans to develop a Fermi liquid theory valid near a quantum critical point. A conventional Fermi liquid description does not work because the self-energy is frequency dependent but local. This is a general problem which should lead to new understanding of the low-energy physics of interacting electrons.

The results of this research may be applicable to a wider class of materials that include the heavy fermion materials and cobaltates.

The research provides good training for graduate students. The PI will involve undergraduate students in the research. The research also involves international collaborations.

NON-TECHNICAL SUMMARY

This award supports theoretical research and education to pursue a possible origin of the unusual properties of high temperature superconducting materials. At sufficiently low temperatures, these materials exhibit superconductivity - an electronic state of matter that exhibits no resistance to the flow of electricity. An unusual and exciting feature of these materials is that the temperature at which superconductivity first appears can be some 6 times or more higher than the highest temperature at which superconductivity had been observed to occur in all previously known materials. If new materials can be discovered that become superconducting near room temperature, then vast amounts of energy could be saved by using them in power transition lines and a whole new technology of superconducting electronics would become more practical. But how does superconductivity arise in these materials? The PI takes the view that magnetism plays a crucial role, particularly when near a new kind of phase transition that occurs at the absolute zero of temperature. Fluctuating wisps of magnetic order might provide the mechanism that causes electrons to pair up and transform to the superconducting state. This research project uses advanced theoretical methods to determine signatures of this mechanism and how they may appear in experiments on particular superconducting materials. The PI will advance the theory of this kind of superconductivity and the unusual kind of metallic state from which it might spring.

The research provides good training for graduate students. The PI will involve undergraduate students in the research. The research also involves international collaborations.