Phase transitions: Hiding in plain sight

Phase transitions are amazing things.  As some knob is turned (temperature, pressure, magnetic field), a physical system undergoes a sudden, seemingly discontinuous change in material properties.  This doesn't happen when looking only at one or two of the constituents of the system - only when we have a big ensemble.  In our daily experience, we are used to the control parameter being temperature, and we take particular notice of phases that have dramatically different, obvious-to-the-naked-eye properties.  Solids and liquids have completely different responses to shear (or rather, liquids lack rigidity).  Liquids and gases have vastly different densities.

It turns out that there are many more phases out there, distinguished in ways that are more subtle and harder to see.  Gadolinium is a magnet below room temperature, and a nonmagnetic metal above room temperature, but to the naked eye looks the same in both phases.  We only know that the transition is there because it has measurable consequences (e.g., you could actually see magnetic forces from a hunk of cold gadolinium).

This week, there was some media attention paid to work from David Hsieh's group at Cal Tech, where they discovered an example of a particularly subtle transition in strontium iridate (Sr₂IrO₄).  In that stuff, similar in some ways to the copper oxide superconductors (based on CuO₄ motifs), there are unpaired electrons (and therefore unpaired spins) on the iridium atoms.  Below a critical temperature (near 200 K, or about -70 Celsius), these spins somehow spontaneously arrange themselves in a subtle way that picks out special directions in the crystal lattice and breaks mirror symmetry, but is not some comparatively well-known kind of magnetic ordering.  They are only able to identify this weird "hidden" ordered phase via a particular optical measurement, since the broken symmetry of the ordered state "turns on" some optical processes that would otherwise be forbidden.  The interest in this system stems from whether similar things may be happening in the cuprate superconductors, and whether the iridates could be tweaked and manipulated like the cuprates.  Neat stuff, and a reminder that sometimes nature can be subtle:  There can be a major change in symmetry properties (i.e., above the transition temperature, the x and y directions in the crystal are equivalent, but below the transition they aren't anymore) that shows up spontaneously, but is still hard to detect.