Pressure Tuning of Competing Quantum States

California Institute Of Technology, Pasadena CA

Investigators

Abstract

NON-TECHNICAL ABSTRACT Transitions between different states - solid to liquid to gas, conductor to insulator - are common occurrences. At the absolute zero of temperature, different physics enters. Quantum mechanics plays a central role, altering the universal response that has been mapped out for classical transitions. This can lead to new possibilities in the optical response of materials, the electronic character of devices, or the magnetic capacity of storage materials. This project probes the quantum characteristics of model systems in three related arenas: parsing the cooperation and competition between magnetism and superconductivity, delineating the new states of matter accessed when sheets of electrons are placed in very high magnetic fields, and deciphering the way in which an insulator can be transformed into a metal. In each case, the research takes advantage of the ability of diamond anvil cell technology to access pressures outside of everyday techniques and to transmit high energy x-rays. The wide array of techniques, from x-ray scattering at a synchrotron to electrical measurements in the laboratory, and the necessity to apply physics, chemistry and materials science to these studies, trains students well for careers in industry, the national laboratories, or academia. Bringing research perspectives to education is another emphasis, including connections with STEM teachers and the general public. TECHNICAL ABSTRACT The effects of a quantum phase transition can be felt up to surprisingly high temperatures. Many materials of technological import demonstrate unusual electronic, optical, and magnetic properties that have been ascribed to the close proximity of quantum critical points. However, complex materials properties, conflated with competing ground states, have made it difficult to discern the essential physics. Moreover, without temperature as a variable, studies of quantum critical points are hard pressed to approach the exactitude that has become the hallmark of experiments on classical critical phenomena. This project combines high-resolution studies of pure materials tuned to accessible quantum critical points using pressure to reveal fundamental aspects of magnetism, disorder, correlated materials, and quantum phase transitions. These include the competition between modulated spin order and superconductivity, the fundamental quantum magnetism of spin singlets arranged on a square lattice, and the transition from insulator to metal in a correlated material. The wide array of techniques, from x-ray scattering to magnetotransport, trains students well for careers in industry, the national laboratories, or academia. Advances in diamond anvil cell technology are important for condensed matter physicists and geophysicists alike. Supporting STEM teachers and helping them develop curricula is a priority.