Exploring the phase diagram of high-Tc superconductors

The metallic state of superconductors at temperatures above their transition temperature, Tc, is usually referred to as the “normal phase.”  The normal phase of conventional superconductors like Al or Pb is just that, normal.  High-Tc superconductors like the cuprates, on the other hand, have very unconventional normal states.  In some cases, the normal state manifests some type of fairly well understood broken symmetry, such as charge or spin-density wave order.  However, in many instances, particularly near regions of the phase diagram where Tc is highest, the order appears be either of some exotic form, or fluctuating in space and time.  Measurements of pump-probe reflectivity (described in more detail on the “optical techniques” page) are particularly sensitive to the appearance of these types of order.  Recently, we have studied the cuprate superconductor Hg-1201 in collaboration with Martin Greven’s research group at University of Minnesota that is revealing the details of the appearance of ordering in the normal state across the entire composition range from under to overdoped.  Currently, we are looking at the appearance of nematicity, or broken rotational symmetry, in the pnictide superconducting system P:Ba122, in collaboration with the Analytis research group at UC Berkeley and LBNL.