New Probes of Strong Interactions in Quantum Matter

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NONTECHNICAL SUMMARYEliminating inefficiencies in electrical conduction in the power grid by the use of superconducting wires could result in the saving of billions of dollars. The AmpaCity project in Essen Germany is one such example. However, a major obstacle to achieving that goal is understanding how to design new superconductors that work increasingly close to room temperature. A surprise in recent years was the discovery of high-temperature superconductivity in materials that have nothing in common with good metals. Consequently, understanding how such materials superconduct cannot be done with the standard building blocks of solid-state physics. This award supports research and education towards developing theoretical models to understand such superconductors. The PI will focus on analyzing a new class of recent experiments that offer unprecedented insight into the fundamental building blocks of the cuprate superconductors.In addition to training and mentoring graduate students, the PI will continue broad educational and outreach activities that include giving lectures to local middle-schools and public lectures to raise public awareness of physics.TECHNICAL SUMMARYWhile the strange metal has always stood as the basic obstacle in developing a theory of superconductivity in the cuprates, experiments designed to reveal the universal excitations that underly this state of matter have been nonexistent until now. The new experiments, momentum-resolved electron-energy loss spectroscopy (MEELS), measure the two-particle susceptibility and hence have all the many-body information needed to construct the dielectric function and also the single-particle electron self-energy. This award supports research and education towards extracting such information from the MEELS data. Special emphasis will be placed on comparison with optical measurements. Optical conductivity experiments measure the transverse dielectric response, while MEELS measures the longitudinal dielectric function. A key unanswered question is whether the two are equal. Certainly, they are expected to be equal in the limit of zero-momentum transfer, as any distinction between longitudinal and transverse dielectric functions disappears in this limit. However, the PI's analysis of the experimental data shows that this is not the case. The same analysis on other materials such as tantalum-diselinide, ruthenates, and even iron-pnictide superconductors reveals complete agreement between zero-momentum limits of the transverse and longitudinal dielectric functions. As this is the first instance where these two limits disagree, the cuprates represent an unprecendeted class of materials. Understanding of this discrepancy is at the heart of this project. The key question that arises is: Does the discrepancy between these two limits stand at the heart of strange-metal physics? Specific models will be studied to obtain a discrepancy between the two limits, and these models will be used to examine if they simultaneously explain the key characteristic of the strange metal, namely T-linear resistivity.In addition to training and mentoring graduate students, the PI will continue broad educational and outreach activities that include giving lectures to local middle-schools and public lectures to raise public awareness of physics.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.