Strange metals and the phases of quantum materials

Harvard University, Cambridge MA

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical research which will examine the so-called "strange metal" phase of quantum matter, which is ubiquitous in systems in which the electron-electron interaction is very strong. Quantum mechanics was initially developed in the early twentieth century as a theory of the motion of a single electron around the nucleus of a hydrogen atom, but it can also describe the motion of a large number of electrons in metals and semiconductors, and this description was vital to the electronics revolution. The past few decades have witnessed the discovery of numerous "quantum materials" in which the role of quantum mechanics is more profound than previously thought and not well understood. In quantum materials, it is important to treat the motion of electrons collectively, and account for the entanglement between the different electrons. Most prominent among these new materials are the high temperature superconductors found in a series of compounds which contain copper, oxygen, and numerous other transition metals, which can conduct electricity without any loss of energy above a relatively high temperature. In this project, the PI and his team will develop a theory of electronic motion in such materials focusing especially on a regime of temperatures and electron density where the motion of electrons is most unlike those in conventional materials. This regime is often called the "strange metal" phase, and it is clear that the mutual entanglement of electrons plays an important role in bringing out the peculiar properties of this phase. This project will build on the successes of a model previously developed by the PI, which provides a simple setting in which the entanglement between the electrons is, in a sense, maximal, and yet the equations of quantum mechanics can be solved exactly. The PI and his collaborators have recently extended this model to a more realistic setting which maintains its solvability, and this yields an encouraging correspondence with observations on strange metals. This project will extend this understanding of the strange metal to other phases of quantum materials, including those exhibiting superconductivity at high temperatures and microstructures that consist of one-atom-thick sheets of carbon atoms stacked in various configurations on top of each other. This award will also contribute to the development of the scientific workforce by supporting the training of graduate students and postdoctoral associates in topics at the forefront of theoretical condensed matter physics. Furthermore, the PI will continue to pursue an active program of public lectures, interviews, colloquia, and lectures at schools for advanced graduate students. TECHNICAL SUMMARY This award supports theoretical research which will examine the "strange metal" phase of quantum matter, which is ubiquitous in correlated electron systems, especially those that exhibit higher temperature superconductivity. A complete, quantitative understanding of the strange metal phase is essential to progress in the theory of quantum materials. The PI and his team have recently made progress on a theory of strange metals, which has the attractive features of being simple, universal, and broadly applicable across the range of correlated materials. In this project, the team will further develop the Sachdev-Ye-Kitaev model and related theories and compare the results quantitatively with numerous experimental probes. For the cuprate superconductors, the strange metal phase will be related to various neighboring phases, including the pseudogap, the superconductor, and the charge-ordered phases. Another focus area of this project is on graphene microstructures and will include studies of nanoscale graphene flakes and the manner in which they can exhibit signatures of the non-Fermi liquid of the Sachdev-Ye-Kitaev model. This award will also contribute to the development of the scientific workforce by supporting the training of graduate students and postdoctoral associates in topics at the forefront of theoretical condensed matter physics. Furthermore, the PI will continue to pursue an active program of public lectures, interviews, colloquia, and lectures at schools for advanced graduate students. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.