Defect dynamics and melting behavior of colloidal antiferromagnetic tetratic phase confined in quasi-2d

We experimentally realize an antiferromagnetic tetratic phase with short-range translation order and quasi-long-range four-fold bond orientational order. The experiments employed hard-sphere micron size colloidal particles confined between parallel plates. We verify theoretical predictions (PHYSICAL REVIEW LETTERS 128, 255501 (2022)) of topological order resulting in bound pairs of oppositely-oriented dislocations and free dislocation pairs (FDPs) oriented at 90 degrees. FDPs move predominantly in the direction of their (total) Burgers vector unless in contact with another perpendicularly-oriented FDP. FDPs spontaneously form and annihilate only in oppositely oriented sets, conserving total topological charge. Further, FDPs have a tendency to align along an axis perpendicular to their (total) Burger's vector. We identify a region with significant defect movement resembling a "free dislocation gas" as well as a region of less activity resembling a Low Angle Grain Boundary in which "cage-like" dynamics are observed. The melting behavior of the material is demonstrated trhough the use of NIPAM shape-changing colloids; quasi-long-range four-fold bond orientational order is observed to reduce to short range order as the fluid phase is entered.