Over the past few decades, several families of superconductors have been discovered that do not fit
the conventional wisdom about superconductivity. Some, like the perovskite "high-temperature
superconductors," have unusually high transition temperatures.
Many have magnetic properties,
with magnetism even coexisting with superconductivity. This is surprising since magnetic impurities
quickly destroy superconductivity in conventional superconductors. As an extreme case, families
of iron-based superconductors were discovered a few years ago. Even more surprisingly,
the magnetic nature may well cause the electron pairing required for superconductivity.
Unconventional superconductors also have wave functions that differ from those of the conventional superconductors
known through most of the twentieth century. While various unconventional superconductors
share some properties, they have their own unique features as well, making the source
of the superconductivity a complex problem.
We explore the low-temperature phase diagrams of these systems under uniaxial pressure.
We can investigate the influence of geometry, for example by studying
samples with tetragonal crystal symmetry and a layered structure. Applying pressure perpendicular
to the layers or within them can have very different effects. The former squeezes the layers
closer together, while the latter pushes them slightly apart. This affects the dimensionality,
since widely separated layers have little interaction and mimic a two-dimensional system.
Reduced dimensionality is a common thread among many, although not all, of the new superconductors.
Another potentially important effect of in-plane pressure is that it breaks the
tetragonal symmetry of the crystal.
Some recent papers:
``Detection of high-field superconducting phase in CeCoIn$_5$ with magnetic susceptibility,"
S.D. Johnson, R.J. Zieve, and J.C. Cooley,
Phys. Rev. B 86, 144518 (2012);
also at arXiv:1206.4122
``Superconductivity in single-crystal YIn$_3$," S.D. Johnson, J.R. Young, R.J. Zieve, and
J.C. Cooley, Solid State Commun. 152, 513 (2012);
Solid State Commun. 152, 513 (2012);
also at arXiv:1112.3083
``Nonlinear effect of uniaxial pressure on superconductivity in CeCoIn$_5$,"
S.D. Johnson, R.J. Zieve, and J.C. Cooley
Phys. Rev. B 83, 144510 (2011);
also at arXiv:1010.4653
``Anisotropic dependence of superconductivity on uniaxial pressure in CeIrIn$_5$,"
O.M. Dix, A.G. Swartz, R.J. Zieve, J. Cooley, T.R. Sayles, and M.B. Maple,
Phys. Rev. Lett. 102, 197001 (2009);
also at arXiv:0908.4366