Novel Effects of High Pressure on Superconductivity
Narelle J. Hillier
Department of Physics, Washington University
Superconductivity has been studied extensively since it was first discovered by Kamerlingh Onnes over 100 years ago. High pressure studies are vital in furthering our understanding of this novel state, as pressure allows researchers to enhance the properties of existing superconductors, to find new superconductors, and to test the validity of theoretical models. Following an introduction to superconductivity I will give a brief overview of high pressure techniques used to study materials at extreme pressures, including He-gas and diamond anvil cell (DAC) systems. The focus will then turn to the novel properties of various materials under extreme pressure. At high pressure Li shows a marked deviation from free-electron behavior, leading to a superconducting transition temperature that is among the highest of all elemental superconductors. Mg, however, has not been found to be superconducting. By substituting divalent Mg for monovalent Li to create Li(Mg) alloys, it is possible to explore the effect of increasing electron concentration on the phase diagram, Tc(P). Finally, the cuprate and iron-based materials are considered high-Tc superconductors. These layered compounds exhibit anisotropic behavior under pressure. Precise hydrostatic measurements of dTc/dP on HgBa2CuO4+d carried out in conjunction with uniaxial pressure experiments provide insight into the effect of each of the lattice parameters on Tc. Among the iron-based materials, the LnFePO (Ln = La, Pr, Nd) materials have not been studied extensively due to their relatively low transition temperatures. However, various high pressure measurements indicate a startling array of pressure dependences. For various pressure media, the superconducting transition temperature may either increase or decrease with increasing pressure! These findings, along with other high-pressure studies on iron-based compounds, show startling evidence that the superconducting state in the iron-based superconductors is highly sensitive to lattice strain. Hydrostatic and uniaxial measurements are therefore vital for understanding the nature of superconductivity in these materials and for determining how to enhance Tc.