The energy and momentum selectivity of time- and angle-resolved photoemissionspectroscopy is exploited to address the ultrafast dynamics of theantiferromagnetic spin density wave (SDW) transition photoexcited in epitaxialthin films of chromium. We are able to quantitatively extract the evolution ofthe SDW order parameter $\Delta$ through the ultrafast phase transition.$\Delta$ is defined by the transient temperature of the thermalized electrongas. The complete destruction of SDW order on a sub-100~fs time scale isobserved, much faster than for conventional charge density wave materials. Ourresults reveal that equilibrium concepts for phase transitions such as theorder parameter may be utilized even in the strongly non-adiabatic regime ofultrafast photo-excitation.
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