Speed of electronic phase propagation in BaFe2As2 revealed by dilatometry

Accurate measurement of a material's coefficient of thermal expansion is a potentially powerful probe for investigating phase transitions. Unfortunately, familiar techniques are either low-resolution or put stringent requirements on sample preparation. This study implements ultrastable optical-fiber interferometry for contactless optical measurement with picometer spatial resolution, and can measure fragile samples less than 100 {}m thick with millikelvin-level temperature resolution. Using their method on BaFe₂As₂, the authors discover hysteresis at the first-order phase transition between its antiferromagnetic and paramagnetic phases, with a boundary moving at about 188 {}m/s.