290S/290K Quantum Materials Seminar speaker Andrew Mackenzie (University of St. Andrews) Wednesday March 22 at 2 pm in 402 Physics South

Time/Venue Wednesday, March 22 at 2 pm in Pacific time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title When is a phase diagram not a phase diagram? Lessons learned from Sr₂RuO₄
Abstract I will review recent developments in the experimental study of the thermodynamic properties of the superconducting state of one of the cleanest of all known unconventional superconductors, Sr₂RuO₄.  After nearly two decades in which its superconducting state was thought to have a time-reversal symmetry breaking, odd parity order parameter of the form p_x ± ip_y, recent NMR work has shown beyond reasonable doubt that its parity is in fact even.  The focus of attention has now moved to whether or not it has a two-component order parameter and whether or not time reversal symmetry is broken.
Investigating these issues has raised a number of uncomfortable questions of a generality that goes beyond any physics specific to superconducting Sr₂RuO₄.  One of the most common experimental activities in the fields of correlated electron physics and unconventional superconductivity is the construction of ‘phase diagrams’.  What is often being done in reality is taking some two-dimensional cut through a higher-dimensional phase space and plotting on it the positions of a series of experimentally observed anomalies.  By referring to the results as ‘phase diagrams’, we make the implicit assumption that the boundaries of bulk thermodynamic phases have been identified.  Based on our findings in strain-tuned Sr₂RuO₄, I will show that this implicit assumption should be treated with extreme caution, in relation not only to this particular material but likely to many others as well.