Critical slowing of the spin and charge density wave order in thin film Cr following photoexcitation*

Single-crystal chromium is an itinerant antiferromagnet, exhibiting a spin density wave (SDW) that is incommensurate with the lattice below its Néel temperature of 311 K. It also exhibits a periodic lattice distortion and charge density wave (CDW), which are coupled to the SDW and can be measured by X-ray diffraction as satellite peaks around the (002) Bragg peak. In a tin film, the SDW/CDW order is pinned at the interfaces and the ordering wavevector is quantized with hysteretic switches as a function of temperature. Here we conduct a time-resolved X-ray diffraction experiment at the X-ray free-electron laser at LCLS of a thin film of Cr to measure the dynamics of the periodic lattice distortion following photoexcitation from different temperatures. The evolution of the system back to the ground state occurs on different time scales at different temperatures, with a slowing down by more than two orders of magnitude as the ground state temperature approaches the hysteretic regions, indicative of an increasing activation energy for reorientation of the SDW. We extend existing phenomenological Landau modeling of the mean-field free energy of incommensurate SDW/CDW systems to include pinning of available wavevectors and reproduce the observed temperature dependence of the wavevector and energy barriers. Thus, we use confinement of the geometry and the unique capabilities of X-ray free electron lasers to assess the energy landscape of system the through the time domain.