Ultrafast electron dynamics excited and probed with X-rays

The Grüneisen parameter determines the magnitude of the system dimensional change in response to its thermal energy variation. Differentiating the various contributions to the Grüneisen parameter is of great importance to clearly understand the thermal expansion mechanism of solids. In particular, the electronic Grüneisen parameter (γe) characterizes the electronic contribution to the thermal expansion and is directly related to the electronic density of states at Fermi level. Here, we report the first measurement of electronic Grüneisen parameter γe of the ferromagnetic transition metal nickel using a novel approach of femtosecond electron diffraction (FED) [1]. In this measurement, the electronic thermal expansion was enhanced by ultrafast heating using femtosecond optical pulses. Then, its temporal evolution was differentiated from other thermal contributions by simultaneously monitoring the laserinduced ultrafast stress and structural dynamics in time domain with FED. This method overcomes the restriction of traditional lowtemperature methods and offers a unique path to study electronic thermal expansion in magnetic metals. The measured γe above the Cure temperature indicates that the local magnetic moment that largely persists in the paramagnetic state of nickel does not contribute significantly to the thermal expansion [2]. Most interestingly, this value is significantly different from that measured in the ferromagnetic state. This deference implies not only its dependency on the magnetic ordering but also the existence of the ultrafast demagnetization process. 1. S. H. Nie, X. Wang, H. Park, R. Clinite, and J. Cao, Phys. Rev. Lett. 96, Art. No. 025901 (2006). 2. Xuan Wang, et. al., Appl. Phys. Lett., 92, 121918 (2008).