Ultrafast electron dynamics at metal surfaces: Competition between electron-phonon coupling and hot-electron transport

Abstract

An experimental scheme ͑double pump/reflectivity probe using femtosecond laser pulses͒ enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

Terahertz spin current pulses controlled by magnetic heterostructures

“…The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter 13 based on the inverse spin Hall effect 14,15 that converts the spin flow into a THz electromagnetic pulse. We find that the Ru cap layer yields a considerably longer spin-current pulse because electrons are injected in Ru d states that have a much smaller mobility than Au sp states 16 . Thus, spin current pulses and the resulting THz transients can be shaped by tailoring magnetic heterostructures, which opens the door for engineering high-speed spintronic devices as well as broadband THz emitters 7,8,9 , in particular covering the elusive range from 5 to 10THz.…”

“…We expect a very different transport dynamics in the Fe/Au and Fe/Ru structures because Au has a much higher electron mobility than Ru (Ref. 16). On a microscopic level, the non-equilibrium electrons arriving in the Au layer will occupy only sp states that exhibit high band velocity 19 (~1nm fs -1 ) and a long lifetime 19 (~100fs).…”

“…The electrons will thus reside in the Au layer for a relatively short while before they are reflected back into the Fe film. In Ru, on the other hand, the hot electrons will mainly occupy more localized d-band states with a lower band velocity 21 (~0.1nm fs -1 ), and they undergo more scattering, in part because of the stronger electron-phonon coupling 16 . Therefore, transport of the non-equilibrium electrons should occur much slower in the Ru than in the Au layer, accompanied by significantly more spin accumulation.…”

Separation of ultrafast spin currents and spin-flip scattering in Co/Cu(001) driven by femtosecond laser excitation employing the complex magneto-optical Kerr effect

“…We include electronic heat diffusion, which is driven by the spatial gradient in the electronic temperature [23], and account for the Co film with thickness d. We further include the Cu substrate in the calculation. We take the optical penetration depth δ skin of the pump pulse at 800 nm central wave length into account and include thereby the position dependent optical absorption in the calculation.…”

“…To estimate the spin-flip contribution we start with calculating the time-and spatially dependent electronic and lattice temperatures, T e (z, t) and T l (z, t), respectively, for different Co film thicknesses d on Cu(001) employing a two temperature model (2TM) [23]. We include electronic heat diffusion, which is driven by the spatial gradient in the electronic temperature [23], and account for the Co film with thickness d. We further include the Cu substrate in the calculation.…”