The Zieve lab works primarily in low-temperature experiment, but there are also projects involving higher temperatures and occasionally computational work. The lab has a dilution refrigerator that can cool below 50 milliKelvin. A piezo-activated pressure cell can be mounted on the cryostat, enabling us to apply uniaxial stress to solid samples and change that stress at milliKelvin temperatures. Present work is on unconventional superconductors, particularly those with topological influences on their band structure.

A separate experiment uses superfluid helium as a model for neutron star behavior. Neutron stars are so dense -- roughly twice the mass of the sun, but with a radius of only about 10 km -- that the interior is at high enough pressure for atoms and even atomic nuclei to dissociate. High pressure also favors neutrons over protons, thanks to the electrostatic self-energy of protons, so most of the star's mass is composed of neutrons. The stars rotate quickly, often making tens or hundreds of revolutions per second. The rotation rate is usually very stable, with the stars gradually slowing down. However, once in a while a star experiences a "glitch" (that is the technical term!) in which the rotation abruptly speeds up. This counterintuitive behavior happens because the interior of the star is a neutron superfluid. The outside of the neutron star slows down, while the superfluid interior maintains its faster rotation. When the mismatch becomes too large, a breakdown occurs which allows the superfluid to impart some of its angular momentum to the exterior crust; this is observed as a glitch. To the extent that this is a property of superfluids, we can observe similar behavior in superfluid helium. We use a home-built apparatus to cool to 1.3 Kelvin, levitate a bucket of helium, spin it, and monitor the decay of its rotation. We indeed observe glitches, and we can proceed to study how they change with different parameters: rotation speed, rate of rotational damping, container size and shape, wall roughness, etc. We are also searching for statistical patterns, something that is nearly impossible in neutron stars that may glitch only once every few decades.

At room temperature, we have begun measurements of bulk photovoltaics under uniaxial pressure. Modern solar cells are based on pn junctions, where an electric field drives the electrons and holes excited by photons in opposite directions, thereby creating a current. These cells have limitations on their efficiency: photons with too little energy to excite an electron-hole pair are wasted, and when photons have more than the necessary energy it goes partly to heating in the solar cell. Bulk photovoltaics are an intriguing alternative. They have inherent symmetry-breaking that drives electrons excited by light in a particular direction, without the need for fabricating a junction. For complicated reasons, they may also circumvent the inefficiency from high-energy photons. Some recent work has shown that uniaxial strain, which can enhance the broken symmetry of a photovoltaic, can also dramatically enhance the resulting photocurrent. Our setup allows much higher strain than in the previous experiments. So far we do see an increase in the photocurrent, although by nowhere near as much as suggested from the previous work. There are many options for pursuing this work by altering the material studied and the directions of strain, light, and measured photocurrent.