Theoretical Studies of Near-Critical, Strongly Interacting Electron Systems

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TECHNICAL SUMMARY:This award supports theoretical research and education in the area of condensed matter physics with a focus on characterizing systems near quantum critical points (QCP), where an itinerant fermion system becomes unstable with respect to either spin or charge density wave ordering. Near a QCP, the interaction with fluctuations of a near-gapless, overdamped order parameter field destroys Fermi liquid behavior of electrons down to very low temperatures.The same interaction gives rise to pairing between incoherent, non-Fermi-liquid-like excitations. The PI will focus on the cuprate superconductors, itinerant ferromagnets, and heavy fermion materials. The ultimate goal of the studies is to understand the interplay between infrared divergences at criticality, Mott-Heisenberg physics, superconductivity, and the presence of the long-range dynamic component in the effective interaction.The proposed studies are divided into two classes. First, the PI plans to study itinerant fermionic systems near an antiferromagnetic instability. The PI aims to study the interplay between the superconductivity and the non-BCS pairing of incoherent fermions into bound spin singlets, and the emergence of the Mott-Heisenberg physics once the interaction becomes comparable to the fermionic bandwidth. The PI also proposes to test an electron-boson model on electron-doped cuprates, and to search for the fingerprints that qualitatively distinguish between magnetic and phononic mechanisms for the pairing. Second, the PI plans to study isotropic itinerant fermionic systems near a Pomeranchuk instability. His main goals are to determine the stability of the Eliashberg theory for incoherent fermions, to analyze under what conditions long-range dynamic interaction destroys a continuous QCP, to study a possible intermediate spiral order parameter configuration, and to study transport properties near QCP, particularly the role of umklapp scattering.The research will involve the training of graduate students at UW and international collaborations. Intellectual merit: The analysis of the system behavior near a QCP is of fundamental importance and of high experimental and theoretical interest. The research will advance our knowledge of system behavior near a QCP, and will also serve as a bridge between the studies of doped Mott insulator and the studies of a near-critical Fermi liquid.Broader impact: This award also supports the training of students in advanced theoretical condensed matter physics and international collaboration.NON-TECHNICAL SUMMARY:This award supports theoretical research and education in the area of condensed matter physics. The PI will study an interesting kind of phase transition that occurs at the absolute zero of temperature. The transformation between different states of matter is driven by fundamental quantum mechanical principles rather then temperature. Such phase transitions can profoundly affect way electrons behave at temperatures as high as room temperature and possibly higher. Quantum phase transitions may play a key role in understanding the phenomenon of high temperature superconductivity and other properties of a class of materials where strong interactions among electrons lead to correlations in their motion and new states of electronic matter. The PI's research aims to understand how the unusual behavior of electrons that occurs when a material is close to a quantum phase transition can lead to superconductivity. This award also supports the training of students in advanced theoretical condensed matter physics.