Controlled variations of quantum phases observed by NMR

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Non-technical abstract A wide range of phenomena of current interest in condensed matter physics are observed in electronic materials where interactions must be taken into account for a proper description. How those correlations are manifest depends on multiple parameters, which can be used to tune the complex systems. For example, the state of matter observed in very low temperatures can be controlled by an external parameter, like magnetic field, strain or carrier doping. Such tuning is performed to investigate new and uncharacterized states of matter, to control the physical properties within one state or another, to provide insight where prevailing understandings break down, and to investigate the potential for applications through optimization. In this project, nuclear magnetic resonance (NMR) is applied in versatile ways to selected systems subjected to variable magnetic fields and uniaxial strain. The specific objectives range from studies of the properties of otherwise inaccessible states of matter, to clarifying questions unresolved by other means. NMR is used because it provides a window into the local electronic environment. Both graduate and undergraduate students are trained in cryogenic, radiofrequency, and simulation technologies, with opportunities to train and collaborate with research groups located at user facilities in the United States and abroad, such as the National MagLab in Tallahassee. In addition to the transfer of science, technology and hardware between research groups initiated by these collaborations, the students benefit from exposure to complementary expertise by way of interactions with scientists working in other areas, and through extended visits to facilities. Technical abstract This project employs solid state magnetic resonance techniques to address questions associated with quasi-two dimensional superconductors and frustrated quantum magnets. The approach integrates nuclear magnetic resonance (NMR) with different combinations of extreme conditions, specifically uniaxial strain, high magnetic fields, and temperatures ranging to less than 1 K. The objective is to study the nature of induced phases and excitations. NMR is well-suited for this task since the technique is generally sensitive to the local magnetic and charge environment through the hyperfine interaction and nuclear quadrupolar coupling to the electric field gradient. The strain method has potential for broader application since it can be varied continuously in situ; here it will be employed in a compound long-considered a chiral p-wave superconductor. Through transport and susceptibility studies, dramatic, non-monotonic changes in the critical temperature were demonstrated. NMR experiments will inform as to strain-induced changes to both normal and superconducting states, including changes to Fermi surface topology and density of states, identification of an anticipated split superconducting transition, or a transition between different superconducting orders. High magnetic fields will be utilized to stabilize and study the physical properties of inhomogeneous superconductivity in a quasi-2D compound for which where orbital suppression of the superconducting state is avoided as a result of uncommonly large electronic structure anisotropy.