Nuclear Magnetic Resonance Studies of Quantum Criticality in Correlated Electron Materials

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Non-technical Abstract: Electrons in solids may exhibit a wide range of important phenomena, ranging from superconductivity to magnetism. Often this behavior can be tuned by controlling some external parameter such as temperature, pressure, or magnetic field. This project investigates several key materials through a set of controlled nuclear magnetic resonance (NMR) experiments with the aim of understanding how to tune and control their properties. Such tuning can lead to dramatic and unexpected changes in the material behavior, believed to emerge from a phenomenon called "quantum criticality". This work advances fundamental understanding of quantum criticality by probing the nature of exotic electronic states within a set of carefully selected materials, and provides modern physics training and mentoring to graduate and undergraduate students for the next generation of scientists and engineers.Technical Abstract: The objective of this proposal is to develop a deeper understanding of the behavior of strongly correlated electron materials near quantum phase transitions. Strongly correlated electron materials hold tremendous potential for new technologies, but a fundamental understanding of their relevant many-body physics remains one of the greatest challenges in condensed matter physics. Often the most interesting science arises at a quantum critical point, where unconventional superconductivity emerges and non-Fermi liquid behavior dominates the normal state behavior. Nuclear magnetic resonance (NMR) is a powerful tool to investigate the fluctuations that drive much of these effects, but requires controlled experiments to disentangle the effects of spin and charge degrees of freedom. This project utilizes new NMR techniques to probe these dynamics under both hydrostatic pressure and uniaxial strain in complementary systems, including a model heavy fermion system exhibiting a Fermi surface jump at a quantum critical point, and a metal with ferro-quadrupolar order of localized f-electrons. Several theoretical predictions are being tested in this project about the nature of the excitations in the vortex cores of a spin-triplet superconductor with a multicomponent chiral order parameter, and how the spin fluctuations in the normal state respond to uniaxial strain.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.