Dynamics of Correlated Materials
Dynamics of Correlated Materials
Emmy Noether Group Laurenz Rettig
Emmy Noether Group Laurenz Rettig

News Post
New paper published: Ultrafast spin density wave transition in Chromium governed by thermalized electron gas
Dec 2016

Time-resolved ARPES allows direct access to the electronic signatures of broken symmetry phases, as well as their femtosecond dynamics. By measuring the spin density wave (SDW) transition in thin films of Cr, we study the dynamics of a phase transition in which the role of the lattice is minimized, in contrast to conventional charge density wave or superconducting materials. This allows for a more stringent test of the role played by the electronic temperature in driving materials from one phase to another under non-equilibrium conditions. By comparison with a mean field model we are able to quantitatively extract the evolution of the SDW order parameter through the ultrafast phase transition, and show that it is governed by the transient temperature of the thermalized electron gas. This shows that for phases governed by the temperature of a single sub-system (e.g. electronic, phononic) concepts from thermodynamic equilibrium are still applicable. This is despite the fact that on such ultrafast timescales, sub-systems are out of equilibrium with each other. We expect that this concept to be applicable beyond purely electronic phases.

For more information see:
C. W. Nicholson, C. Monney, R. Carley, B. Frietsch, J. Bowlan, M. Weinelt, and M. Wolf:
Phys. Rev. Lett. 117, 136801 (2016), [DOI: 10.1103/PhysRevLett.117.136801]

Figure: (Upper panels) Band structure of Cr before and after excitation, from a base temperature of 100 K. The right hand panel shows the extracted electronic temperature and the corresponding value of the order parameter (Δ). The SDW gap is found to be defined by the electronic temperature. (Bottom panels) Snapshots from the mean field model simulation of the SDW dynamics showing the SDW bands before excitation, paramagnetic phase following excitation, and the SDW gap reopening.
Figure: (Upper panels) Band structure of Cr before and after excitation, from a base temperature of 100 K. The right hand panel shows the extracted electronic temperature and the corresponding value of the order parameter (Δ). The SDW gap is found to be defined by the electronic temperature. (Bottom panels) Snapshots from the mean field model simulation of the SDW dynamics showing the SDW bands before excitation, paramagnetic phase following excitation, and the SDW gap reopening.