Uniaxial Pressure Measurements Near Quantum Criticality

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Quantum critical points, phase transitions driven by quantum fluctuations at zero temperature, influence many unusual low-temperature behaviors of metallic systems. Samples near a quantum critical point exhibit complex phase diagrams including superconductivity, antiferromagnetism, ferromagnetism, spin glass behavior, and non-Fermi liquid regimes. Transitions among these phases are often controlled by parameters such as pressure, alloying, or magnetic field. Achieving a quantitative understanding of quantum critical phenomena will greatly improve our grasp of many-body systems. This project uses the special capabilities of a rare uniaxial pressure technique to study several heavy-fermion systems. Tuning sample properties by pressure and comparing with results from tuning by alloying will help distinguish the influence of a quantum critical point from consequences of disorder. Measuring inherently anisotropic samples will allow controlled variation of lattice constants and conduction electron/f-electron hybridization, key parameters in determining the ground state of a strongly correlated system. Finally, the ability to maintain the sample temperature below 300 mK during pressure changes allows study of hysteretic effects in low-temperature spin glass phases. Exposure to cryogenic equipment and pressure techniques will prepare graduate and undergraduate students for further work in academia, industry, or national laboratories.%%%The simplest theories of metals ignore interactions among the electrons in a material. As a result, these theories cannot explain the many interesting and important phenomena which depend on electron-electron correlations, among them superconductivity and magnetism. In the class of materials known as "heavy fermions," strong electron-electron interactions dominate the behavior. These compounds often exhibit more than one highly correlated phase, and can be tuned from one phase to another by using magnetic fields or pressure. This makes the heavy fermions an excellent testing ground for finding the precise conditions needed for superconducting or magnetic phases. Studies on heavy fermions will improve understanding of their relatives, the high-Tc superconductors, and may guide searches for other superconductors. This project investigates heavy-fermion systems through a rare pressure technique that provides a controlled means of tuning a sample among phases. Pressure is particularly useful for varying the interatomic distances, which in turn determine the electron interactions. This work will elucidate the role of crystal defects, the importance of planar structures within crystals, and memory effects upon phase changes. Exposure to techniques for measurements under pressure and at low temperature will prepare graduate and undergraduate students for further work in academia, industry, or national laboratories.